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solar energetic particles (seps) are one of the extreme space weather phenomena. a huge sep event increases the radiation dose received by aircrews, who should be warned of such events as early as possible. we developed a warning system for aviation exposure to seps. this article describes one component of the system, which calculates the temporal evolution of the sep intensity and the spectrum immediately outside the terrestrial magnetosphere. to achieve this, we performed numerical simulations of sep transport in interplanetary space, in which interplanetary sep transport is described by the focused transport equation. we developed a new simulation code to solve the equation using a set of stochastic differential equations. in the code, the focused transport equation is expressed in a magnetic field line coordinate system, which is a non-orthogonal curvilinear coordinate system. an inverse gaussian distribution is employed as the injection profile of seps at an inner boundary located near the sun. we applied the simulation to observed sep events as a validation test. the results show that our simulation can closely reproduce observational data for the temporal evolution of particle intensity. by employing the code, we developed the warning system for aviation exposure to solar energetic particles (wasavies). | interplanetary particle transport simulation for warning system for aviation exposure to solar energetic particles |
we study the helicity density patterns which can result from the emerging bipolar regions. using the relevant dynamo model and the magnetic helicity conservation law we find that the helicity density patterns around the bipolar regions depend on the configuration of the ambient large-scale magnetic field, and in general they show a quadrupole distribution. the position of this pattern relative to the equator can depend on the tilt of the bipolar region. we compute the time-latitude diagrams of the helicity density evolution. the longitudinally averaged effect of the bipolar regions shows two bands of sign for the density distributions in each hemisphere. similar helicity density patterns are provided by the helicity density flux from the emerging bipolar regions subjected to surface differential rotation. | the magnetic helicity density patterns from non-axisymmetric solar dynamo |
we present in this letter the first global comparison between traditional line-tied steady-state magnetohydrodynamic models and a new, fully time-dependent thermodynamic magnetohydrodynamic simulation of the global corona. to approximate surface magnetic field distributions and magnitudes around solar minimum, we use the lockheed evolving surface-flux assimilation model to obtain input maps that incorporate flux emergence and surface flows over a full solar rotation, including differential rotation and meridional flows. each time step evolves the previous state of the plasma with a new magnetic field input boundary condition, mimicking photospheric driving on the sun. we find that this method produces a qualitatively different corona compared to steady-state models. the magnetic energy levels are higher in the time-dependent model, and coronal holes evolve more along the following edge than they do in steady-state models. coronal changes, as illustrated with forward-modeled emission maps, evolve on longer timescales with time-dependent driving. we discuss implications for active and quiet sun scenarios, solar wind formation, and widely used steady-state assumptions like potential field source surface calculations. | time-dependent dynamics of the corona |
most hot jupiters have extended envelopes of gas that spread beyond their roche lobes. a typical envelope is weakly gravitationally bound to the planet and, therefore, its structure and properties are strongly affected by stellar wind disturbances, for example, coronal mass ejections. earlier, we performed gas-dynamic modeling of the interaction of a narrow coronal mass ejection (cme) with the envelope of the hot jupiter hd 209458b. in this study, we investigate the influence of the planet's magnetic field and stellar wind on the structure and dynamics of the hd 209458b envelope exposed to a similar cme. for this, an mhd model of the cme interaction with the envelope was developed. it was assumed that the field of the planet had a generally accepted value and corresponded to 1/10 of the magnetic moment of jupiter. the field of the star was assumed to be weak (10-3 g), which ensured the super-alfvén flow of the stellar wind around the planet. the comparison of the mhd simulation with gas-dynamic calculations shows that, for the adopted values of the fields of the planet and the star, the effect of the magnetic field on the envelope is not decisive and the qualitative picture of the flow does not change. at the same time, taking magnetic fields into account leads to a change in the quantitative characteristics of the envelope and the mass loss rate, which can be important in determining the evolutionary status of the exoplanet. | mhd model of the interaction of a coronal mass ejection with the hot jupiter hd 209458b |
the expansion of hot electrons in flaring magnetic loops is crucial to understanding the dynamics of solar flares. in this paper we investigate, for the first time, the transport of hot electrons in a magnetic mirror field based on a 1d particle-in-cell simulation model. the hot electrons with small pitch angles transport into the cold plasma, which leads to the generation of langmuir waves in the cold plasma and ion acoustic waves in the hot plasma. the large pitch angle electrons can be confined by the magnetic mirror, resulting in the different evolution timescale between electron parallel and perpendicular temperatures. this will cause the formation of electron temperature anisotropy, which then generates the whistler waves near the interface between hot electrons and cold electrons. the whistler waves can scatter the large pitch angle electrons to smaller value through the cyclotron resonance, leading to electrons escaping from the hot region. these results indicate that the whistler waves may play an important role in the transport of electrons in flaring magnetic loops. the findings from this study provide some new insights to understand the electron dynamics of solar flares. | expansion of solar coronal hot electrons in an inhomogeneous magnetic field: 1d pic simulation |
aims: we investigate synthetic observational signatures generated from numerical models of transverse waves propagating in complex (braided) magnetic fields.methods: we consider two simulations with different levels of magnetic field braiding and impose periodic, transverse velocity perturbations at the lower boundary. as the waves reflect off the top boundary, a complex pattern of wave interference occurs. we applied the forward modelling code fomo and analysed the synthetic emission data. we examined the line intensity, doppler shifts, and kinetic energy along several line-of-sight (los) angles.results: the doppler shift perturbations clearly show the presence of the transverse (alfvénic) waves. however, in the total intensity, and running difference, the waves are less easily observed for more complex magnetic fields and may be indistinguishable from background noise. depending on the los angle, the observable signatures of the waves reflect some of the magnetic field braiding, particularly when multiple emission lines are available, although it is not possible to deduce the actual level of complexity. in the more braided simulation, signatures of phase mixing can be identified. we highlight possible ambiguities in the interpretation of the wave modes based on the synthetic emission signatures.conclusions: most of the observables discussed in this article behave in the manner expected, given knowledge of the evolution of the parameters in the 3d simulations. nevertheless, some intriguing observational signatures are present. identifying regions of magnetic field complexity is somewhat possible when waves are present; although, even then, simultaneous spectroscopic imaging from different lines is important in order to identify these locations. care needs to be taken when interpreting intensity and doppler velocity signatures as torsional motions, as is done in our setup. these types of signatures are a consequence of the complex nature of the magnetic field, rather than real torsional waves. finally, we investigate the kinetic energy, which was estimated from the doppler velocities and is highly dependent on the polarisation of the wave, the complexity of the background field, and the los angles. | forward modelling of mhd waves in braided magnetic fields |
we present 2.5d global, ideal magnetohydrodynamic (mhd) simulations of magnetically and rotationally driven protostellar jets from keplerian accretion discs, wherein only the initial magnetic field strength at the inner radius of the disc, bi, is varied. using the amr-mhd code azeus, we self-consistently follow the jet evolution into the observational regime (> 10^3 au) with a spatial dynamic range of ∼6.5 × 105. the simulations reveal a three-component outflow: (1) a hot, dense, super-fast, and highly magnetized `jet core'; (2) a cold, rarefied, trans-fast, and highly magnetized `sheath' surrounding the jet core and extending to a tangential discontinuity; and (3) a warm, dense, trans-slow, and weakly magnetized shocked ambient medium entrained by the advancing bow shock. the simulations reveal power-law relationships between bi and the jet advance speed, vjet, the average jet rotation speed, <vφ>, as well as fluxes of mass, momentum, and kinetic energy. quantities that do not depend on bi include the plasma-β of the transported material that, in all cases, seems to asymptote to order unity. jets are launched by a combination of the `magnetic tower' and `bead-on-a-wire' mechanisms, with the former accounting for most of the jet acceleration - even for strong fields - and continuing well beyond the fast magnetosonic point. at no time does the leading bow shock leave the domain and, as such, these simulations generate large-scale jets that reproduce many of the observed properties of protostellar jets including their characteristic speeds and transported fluxes. | mhd simulations of the formation and propagation of protostellar jets to observational length-scales |
magnetism defines the complex and dynamic solar corona. coronal mass ejections (cmes) are thought to be caused by stresses, twists, and tangles in coronal magnetic fields that build up energy and ultimately erupt, hurling plasma into interplanetary space. even the ever-present solar wind possesses a three-dimensional morphology shaped by the global coronal magnetic field, forming geoeffective corotating interaction regions. cme evolution and the structure of the solar wind depend intimately on the coronal magnetic field, so comprehensive observations of the global magnetothermal atmosphere are crucial both for scientific progress and space weather predictions. although some advances have been made in measuring coronal magnetic fields locally, synoptic measurements of the global coronal magnetic field are not yet available. we conclude that a key goal for 2050 should be comprehensive, ongoing 3d synoptic maps of the global coronal magnetic field. this will require the construction of new telescopes, ground and space-based, to obtain complementary, multiwavelength observations sensitive to the coronal magnetic field. it will also require development of inversion frameworks capable of incorporating multi-wavelength data, and forward analysis tools and simulation testbeds to prioritize and establish observational requirements on the proposed telescopes. | untangling the global coronal magnetic field with multiwavelength observations |
the collapse of the magnetic rotating protostellar cloud with mass of $10\,m_{\odot}$ is numerically studied. the initial ratios of the thermal, magnetic, and rotational energies of the cloud to the modulus of its gravitational energy are 0.3, 0.2 and 0.01, respectively. the emphasis is on the evolution and properties of the quasi-magnetostatic primary disk formed at the isothermal stage of the collapse. simulations show that the primary disk size and mass increase during evolution from $1500$ au to $7400$ au and from $0.3\,m_{\odot}$ to $5.2\,m_{\odot}$, respectively. magnetic field is quasi-radial in the cloud envelope and quasi-uniform within the primary disk. a toroidal magnetic field is generated behind the front of the fast shock mhd wave propagating from the primary disk boundary and in the region of the outflow formed near the first hydrostatic core. the hierarchical structure of collapsing protostellar clouds can be revealed in observations in terms of the magnetic field geometry and the angular momentum distribution. | primary disks and their observational appearance in collapsing magnetic rotating protostellar clouds |
it has been shown in a previous work that torsional alfvén waves can drive turbulence in nonuniform coronal loops with a purely axial magnetic field. here we explore the role of the magnetic twist. we modeled a coronal loop as a transversely nonuniform straight flux tube, anchored in the photosphere, and embedded in a uniform coronal environment. we considered that the magnetic field is twisted and control the strength of magnetic twist by a free parameter of the model. we excited the longitudinally fundamental mode of standing torsional alfvén waves, whose temporal evolution was obtained by means of high-resolution three-dimensional ideal magnetohydrodynamic numerical simulations. we find that phase mixing of torsional alfvén waves creates velocity shear in the direction perpendicular to the magnetic field lines. the velocity shear eventually triggers the kelvin-helmholtz instability (khi). in weakly twisted magnetic tubes, the khi is able to grow nonlinearly, and subsequently, turbulence is driven in the coronal loop in a similar manner as in the untwisted case. when the magnetic twist remains weak, it delays the onset of the khi and slows the development of turbulence down. in contrast, magnetic tension can suppress the nonlinear growth of the khi when the magnetic twist is strong enough, even when the khi has locally been excited by the phase-mixing shear. thus, turbulence is not generated in strongly twisted loops. movie associated to fig. 5 is available at https://www.aanda.org | transition to turbulence in nonuniform coronal loops driven by torsional alfvén waves. ii. extended analysis and effect of magnetic twist |
the evolution of collapsing clouds embedded in different star-forming environments is investigated using three-dimensional non-ideal magnetohydrodynamics simulations considering different cloud metallicities (z/z_⊙ = 0, 10-5, 10-4, 10-3, 10-2, 10-1, and 1) and ionization strengths (cζ = 0, 0.01, 1, and 10, where cζ is a coefficient controlling the ionization intensity and cζ = 1 corresponds to the ionization strength of nearby star-forming regions). with all combinations of these considered values of z/z_⊙ and cζ, 28 different star-forming environments are prepared and simulated. the cloud evolution in each environment is calculated until the central density reaches n≈ 10^{16} cm^{-3} just before protostar formation, and the outflow driving conditions are derived. an outflow appears when the (first) adiabatic core forms in a magnetically active region where the magnetic field is well coupled with the neutral gas. in cases where outflows are driven, their momentum fluxes are always comparable to the observations of nearby star-forming regions. thus, these outflows should control the mass growth of the protostars as in the local universe. roughly, an outflow appears when z/ z_⊙ > 10^{-4} and cζ ≥ 0.01. it is expected that the transition of the star formation mode from massive stars to normal solar-type stars occurs when the cloud metallicity is enhanced to the range of z/z_⊙ ≈ 10^{-4}-10-3, above which relatively low-mass stars would preferentially appear as a result of strong mass ejection. | driving conditions of protostellar outflows in different star-forming environments |
aims: magnetic fields play an important role during the formation and evolution of stars. of particular interest in stellar evolution is what effect they have on the transport angular momentum and mixing of chemical elements along the radial direction in radiative regions. current theories suggest a dynamo loop as the mechanism responsible for maintaining the magnetic field in the radiative zone. this loop consists of differential rotation on one side and magnetohydrodynamic (mhd) instability - the so-called tayler instability - on the other. however, how this might work quantitatively is still an unsettled question, largely because we do not yet understand all the properties of the instability in question. in this paper we explore some properties of the tayler instability.methods: we present 3d mhd simulations of purely toroidal and mixed poloidal-toroidal magnetic field configurations to study the behavior of the tayler instability. for the first time the simultaneous action of rotation and magnetic diffusion are taken into account and the effects of a poloidal field on the dynamic evolution of unstable toroidal magnetic fields is included.results: in the absence of diffusion, fast rotation (rotation rate, ω∥, compared to alfvén frequency, ωa,φ) is able to suppress the instability when the rotation and magnetic axes are aligned and when the radial field strength gradient p< 1.5 (where p ≡ ∂lnb/∂lnϖ and ϖ is the cylindrical radius coordinate). when diffusion is included, this system turns unstable for diffusion dominated and marginally diffusive dominated regions. if the magnetic and rotation axes are perpendicular to each other, ω⊥, the stabilizing effect induced by the coriolis force is scale dependent and decreases with increasing wavenumber. in toroidal fields with radial field gradients bigger than p> 1.5, rapid rotation does not suppress the instability but instead introduces a damping factor ωa/ 2ω∥ to the growth rate, in agreement with the analytic predictions. for the mixed poloidal-toroidal fields we find an unstable axisymmetric mode, not predicted analytically, right at the stability threshold for the non-axisymmetric modes; it has been argued that an axisymmetric mode is necessary for the closure of the tayler-spruit dynamo loop. | stability of toroidal magnetic fields in stellar interiors |
coronal jets are eruptions identified by a collimated, sometimes twisted spire. they are small-scale energetic events compared with flares. using multiwavelength observations from the solar dynamics observatory/atmospheric imaging assembly and a magnetogram from hinode/spectro-polarimeter (hinode/sp), we study the formation and evolution of a jet occurring on 2019 march 22 in noaa active region 12736. a zero-β magnetohydrodynamic simulation is conducted to probe the initiation mechanisms and appearance of helical motion during this jet event. as the simulation reveals, there are two pairs of field lines at the jet base, indicating two distinct magnetic structures. one structure outlines a flux rope lying low above the photosphere in the north of a bald patch region, and the other structure shows a null point high in the corona in the south. the untwisting motions of the observed flux rope were recovered by adding an anomalous (artificial) resistivity in the simulation. a reconnection occurs at the bald patch in the flux rope structure, which is moving upward and simultaneously encounters the field lines of the null point structure. the interaction of the two structures results in the jet, while the twist of the flux rope is transferred to the jet by the reconnected field lines. the rotational motion of the flux rope is proposed to be an underlying trigger of this process and responsible for helical motions in the jet spire. | simulation of a solar jet formed from an untwisting flux rope interacting with a null point |
in this paper, for a number of significantly different parameters of a localized plane layer of collisionless electron-proton plasma and an external magnetic field parallel to its surface, we perform a detailed numerical analysis of the evolution of the magnetic field structure and the dynamics of plasma expansion into vacuum from a region with initially isotropically heated electrons by means of the particle-in-cell simulation. the region has the form of a long semi-cylinder, the axis of which is located on the surface of the plasma layer. we reveal that the process of the decay of such an inhomogeneously heated strong "plasma-vacuum" discontinuity is largely controled by the anisotropy of the resulting electron-velocity distribution and the development of the weibel instability caused by it. we establish, under certain conditions, the formation and rapid expansion of strongly inhomogeneous electron currents in the form of filaments (similar to z-pinches) parallel to the external magnetic field, as well as the formation and slow evolution of current sheets oriented at various angles to the boundary between the plasma and the deforming magnetic field. it is shown that these currents can produce fields that are much larger than the external magnetic field, and the conditions required for this are qualitatively indicated. the discovered phenomena are possible in coronal loops, stellar wind, and explosive processes in planetary magnetospheres, as well as in laser plasma. the latter makes it possible to model similar phenomena in astrophysical plasma. | weibel instability and deformation of an external magnetic field in the region of decay of a strong discontinuity in a plasma with hot electrons |
context. several observations of stellar jets show evidence of x-ray emitting shocks close to the launching site. in some cases, including young stellar objects (ysos) at different stages of evolution, the shocked features appear to be stationary. we study two cases, both located in the taurus star-forming region. hh 154, the jet originating from the embedded binary class 0/i protostar irs 5, and the jet associated with dg tau, a more evolved class ii disk-bearing source or classical t tauri star (ctts).aims: we investigate the effect of perturbations in x-ray emitting stationary shocks in stellar jets and the stability and detectability in x-rays of these shocks, and we explore the differences in jets from class 0 to class ii sources.methods: we performed a set of 2.5d magnetohydrodynamic numerical simulations that model supersonic jets ramming into a magnetized medium. the jet is formed of two components: a continuously driven component that forms a quasi-stationary shock at the base of the jet and a pulsed component consisting of blobs perturbing the shock. we explored different parameters for the two components. we studied two cases: hh 154, a light jet (less dense than the ambient medium), and a heavy jet (denser than the ambient medium) associated with dg tau. we synthesized the count rate from the simulations and compared these data with available chandra observations.results: our model is able to reproduce the observed jet properties at different evolutionary phases (in particular, for hh 154 and dg tau) and can explain the formation of x-ray emitting quasi-stationary shocks observed at the base of jets in a natural way. the jet is collimated by the magnetic field forming a quasi-stationary shock at the base which emits in x-rays even when perturbations formed by a train of blobs are present. we found similar collimation mechanisms dominating in both heavy and light jets.conclusions: we derived the physical parameters that can give rise to x-ray emission consistent with observations of hh 154 and dg tau. we have also performed a wide exploration of the parameter space characterizing the model; this can be a useful tool to study and diagnose the physical properties of yso jets over a broad range of physical conditions, from embedded to disk-bearing sources. we show that luminosity does not change significantly in variable jet models for the range of parameters explored. finally, we provide an estimation of the maximum perturbations that can be present in hh 154 and dg tau taking into account the available x-ray observations. the movie is available at http://www.aanda.org | structure of x-ray emitting jets close to the launching site: from embedded to disk-bearing sources |
we present a new method to investigate the effective magnetic field decay of isolated neutron stars, from the analysis of the long-term timing data of a large sample of radio pulsars. there are some differences between the distributions of frequency’s second derivatives of the pulsar spins with different effective field decay timescales. kolmogorov-smirnov tests are performed to reexamine the consistency of distributions of the simulated and reported data for a series of values of decay timescales. we show that the timescale of the effective field decay exceeds ∼5 myr for pulsars with spindown age τ c < 107 yr or ∼100 myr for pulsars with 107 < τ c < 109 yr in the sample. the result does not depend on any specific theories of the field evolution, the inclination decay, or the variation in the moment of inertia. it is also found that the extent of the closed-line region of the magnetic field is close to the light cylinder r lc, i.e., the corotating radius r c ≈ r lc is a good approximation for the observed pulsar population. | the effective magnetic field decay of radio pulsars: insights from the statistical properties of their spin frequency's second derivatives |
we introduce a theory for the development of a transitional column density σtp between the lognormal and the power-law forms of the probability distribution function in a molecular cloud. our turbulent magnetohydrodynamic simulations show that the value of σtp increases as the strength of both the initial magnetic field and turbulence increases. we develop an analytic expression for σtp based on the interplay of turbulence, a (strong) magnetic field, and gravity. the transition value σtp scales with {{ \mathcal m }}02, the square of the initial sonic mach number, and β 0, the initial ratio of gas pressure to magnetic pressure. we fit the variation of σtp among different model clouds as a function of {{ \mathcal m }}02{β }0 or, equivalently, the square of the initial alfvénic mach number {{ \mathcal m }}{{a}0}2. this implies that the transition value σtp is an imprint of cloud initial conditions and is set by turbulent compression of a magnetic cloud. physically, the value of σtp denotes the boundary above which the mass-to-flux ratio becomes supercritical and gravity drives the evolution. | the transition from a lognormal to a power-law column density distribution in molecular clouds: an imprint of the initial magnetic field and turbulence |
we present a systematic study of periodic x-ray sources in the limiting window (lw), a ∼70 arcmin2 field representative of the inner galactic bulge and the target of ∼1 ms chandra observations. using the gregory-loredo algorithm, which applies bayes's theorem to the phase-folded light curve and is well suited for irregularly sampled x-ray data, we detect 25 periodic signals in 23 discrete sources, among which 15 signals are new discoveries and two sources show dual periods. the vast majority of the 23 periodic sources are classified as magnetic cataclysmic variables (cvs), based on their period range, x-ray luminosities, spectral properties, and phase-folded light curves that are characteristic of spin modulation. meanwhile, there is a paucity of non-magnetic cvs seen as periodic sources, which can be understood as due to a low detection efficiency for eclipsing sources. under reasonable assumptions about the geometry of magnetic cvs and a large set of simulated x-ray light curves, we estimate the fraction of magnetic cvs in the inner galactic bulge to be ≲23 per cent, which is similar to that in the solar neighbourhood. there is an apparent lack of long-period (≳3.3 h) cvs in the lw, when contrasted with the range of known cvs in the solar neighbourhood. we suggest that this might be an age effect, in the sense that cvs in the inner bulge are more evolved systems and have substantially shrunk their orbits. | periodic x-ray sources in the galactic bulge: application of the gregory-loredo algorithm |
the view on velocity structures in molecular clouds and their relationship with magnetic fields (b field) has evolved during the past decade from almost no correlation to highly parallel. our numerical simulations suggest a more nuanced picture: depending on whether the self-gravity is dynamically dominant, the velocity field can be governed by either contraction (at high densities) or turbulence (at low densities), and their anisotropies will tend to be either perpendicular or parallel, respectively, to the b fields. high-density regions are always embedded in the low-density fore/background, so the velocity behaviors from lines of sight (loss) with high column densities will be a mixture of orthogonal anisotropies, which can be hard to interpret and necessitates zooming in onto certain los scales to better characterize localized behaviors. we tested and confirmed the above prediction with co observations of the taurus molecular cloud. | velocity anisotropy in self-gravitating molecular clouds. ii. observation |
by means of the monte carlo method, we simulate the evolutionary distribution of accreting neutron stars (nss) in the magnetic field versus spin period (b-p) diagram where the accretion induced magnetic-field decay model is exploited. the simulated results show that by mass accretion the b-p distribution of the accreting ns would evolve along the equilibrium period line to a region with low field and short period. the b-p distributions of the simulated accreting nss are consistent with those of the observed millisecond pulsars (msps) after accretion of σm 0.1-0.2 m_{⊙}. we also test the effects of the initial magnetic field and the spin period on the evolved b-p distribution of the accreting nss. it is shown that the evolved distributions of the simulated samples are independent of the selection of the initial condition when the ns magnetic field decays to a value less than σm 10^{10}$ g | the simulation of the magnetic field and spin period evolution of accreting neutron stars |
we present hydrodynamical simulations of the interaction of coronal mass ejections (cme) in the interplanetary medium (ipm). in these events, two consecutive cmes are launched from the sun in similar directions within an interval of time of a few hours. in our numerical model, we assume that the ambient solar wind is characterized by its velocity and mass-loss rate. then, the cmes are generated when the flow velocity and mass-loss rate suddenly change, with respect to the ambient solar wind conditions during two intervals of time, which correspond to the duration of each cme. after their interaction, a merged region is formed and evolve as a single structure into the ipm. in this work, we are interested in the general morphology of this merged region, which depends on the initial parameters of the ambient solar wind and the cmes involved. in order to understand this morphology, we have performed a parametric study in which we characterize the effects of the initial parameters variations on the density and velocity profiles at 1 au, using as reference the well-documented event of july 25th, 2004. based on this parametrization we were able to reproduce the main features of the observed profiles ensuring the travel time and the speed and density magnitudes. then, we apply the parametrization results to the interaction events of may 23, 2010; august 1, 2010; and november 9, 2012. with this approach and varying the values of the input parameters within the cme observational errors, our simulated profiles reproduce the main features observed at 1 au. even though we do not take into account the magnetic field, our models give a physical insight into the propagation and interaction of icmes. | numerical simulations of icme-icme interactions |
coronal supra-arcade downflows (sads) are observed as dark trails descending toward hot turbulent-fan-shaped regions. due to the large temperature values and gradients in these fan regions, the thermal conduction (tc) should be very efficient. while several models have been proposed to explain the triggering and the evolution of sads, none of these scenarios address a systematic consideration of tc. thus, we accomplish this task numerically simulating the evolution of sads within this framework. that is, sads are conceived as voided (subdense) cavities formed by nonlinear waves triggered by downflowing bursty localized reconnection events in a perturbed hot fan. we generate a properly turbulent fan, obtained by a stirring force that permits control of the energy and vorticity input in the medium where sads develop. we include anisotropic tc and consider plasma properties consistent with observations. our aim is to study whether it is possible to prevent sads from vanishing by thermal diffusion. we find that this will be the case, depending on the turbulence parameters, in particular if the magnetic field lines are able to envelope the voided cavities, thermally isolating them from the hot environment. velocity shear perturbations that are able to generate instabilities of the kelvin-helmholtz type help to produce magnetic islands, extending the lifetime of sads. | mhd simulations of coronal supra-arcade downflows including anisotropic thermal conduction |
starting from an exact, steady-state, force-free solution of the magnetohydrodynamic (mhd) equations, we investigate how resistive current layers are induced by perturbing line-tied three-dimensional magnetic equilibria. this is achieved by the superposition of a weak perturbation field in the domain, in contrast to studies where the boundary is driven by slow motions, like those present in photospheric active regions. our aim is to quantify how the current structures are altered by the contribution of so-called quasi-separatrix layers (qsls) as the null point is shifted outside the computational domain. previous studies based on magneto-frictional relaxation have indicated that despite the severe field line gradients of the qsl, the presence of a null is vital in maintaining fast reconnection. here, we explore this notion using highly resolved simulations of the full mhd evolution. we show that for the null-point configuration, the resistive scaling of the peak current density is close to j ∼η−1, while the scaling is much weaker, i.e.j ∼η−0.4, when only the qsl connectivity gradients provide a site for the current accumulation. | simulations of 3d magnetic merging: resistive scalings for null point and qsl reconnection |
context. it is generally assumed that magnetic fields play an important role in the formation and evolution of protoplanetary disks. one way of observationally constraining magnetic fields is to measure polarized emission and absorption produced by magnetically aligned elongated dust grains. the fact that radiation also becomes linearly polarized by light scattering at optical to millimeter wavelengths complicates magnetic field studies.aims: we characterize the linear polarization of mid-infrared radiation due to scattering of the stellar radiation and dust thermal re-emission radiation (self-scattering).methods: we computed the radial polarization profiles at wavelengths across the n and q bands for a broad range of circumstellar disk configurations. these simulations served as a basis to analyze the correlations between selected disk parameters and the resulting linear polarization.results: we find that the thermal re-emission radiation is stronger than the scattered stellar radiation for disks with inner holes smaller than ~10 au within the considered parameter range. the mid-infrared polarization due to scattering shows several clear trends: for scattered stellar radiation only, the linear polarization degree decreases slightly with increasing radial distance, while it increases with radial distance for thermal re-emission radiation only and for a combination of scattered stellar radiation and thermal re-emission radiation. the linear polarization degree decreases with increasing disk flaring and luminosity of the central star. an increasing inner radius shifts the increase of the linear polarization degree further outside, while a larger scale height increases the linear polarization degree for small radial distances and decreases this degree further outside. for longer wavelengths, i.e., toward the q band in our study, the linear polarization degree converges more slowly.conclusions: we found several clear trends for polarization due to scattering. these trends are the basis to distinguish polarization due to scattering from polarization due to dichroic emission and absorption. | characterization of mid-infrared polarization due to scattering in protoplanetary disks |
context. rotation period measurements of low-mass stars show that the spin distributions in young clusters do not exhibit the spin-up expected due to contraction in the phase when a large fraction of stars is still surrounded by accretion discs. many physical models have been developed to explain this feature based on different types of star-disc interactions alone. in this phase, the stars accrete mass and angular momentum and may experience accretion-enhanced magnetised winds. the stellar structure and angular momentum content thus strongly depend on the properties of the accretion mechanism. at the same time, the accretion of mass and energy has a significant impact on the evolution of the stellar structure and the moment of inertia. our understanding of the spin rates of young stars therefore requires a description of how accretion affects the stellar structure and angular momentum simultaneously.aims: we aim to understand the role of accretion to explain the observed rotation-rate distributions of forming stars.methods: we computed evolution models of accreting very young stars and determined in a self-consistent way the effect of accretion on stellar structure and the angular momentum exchanges between the stars and their disc. we then varied the deuterium content, the accretion history, the entropy content of the accreted material, and the magnetic field as well as the efficiency of the accretion-enhanced winds.results: the models are driven alternatively both by the evolution of the momentum of inertia and by the star-disc interaction torques. of all the parameters we tested, the magnetic field strength, the accretion history, and the deuterium content have the largest impact. the injection of heat plays a major role only early in the evolution.conclusions: this work demonstrates the importance of the moment of inertia evolution under the influence of accretion for explaining the constant rotation-rate distributions that are observed during the star-disc interactions. when we account for rotation, the models computed with the recently calculated torque along with a consistent structural evolution of the accreting star are able to explain the almost constant spin evolution for the whole range of parameters we investigated, but it only reproduces a narrow range around the median of the observed spin rate distributions. further development, including for example more realistic accretion histories based on dedicated disc simulations, are likely needed to reproduce the extremes of the spin rate distributions. | effects of accretion on the structure and rotation of forming stars |
magnetars are a group of very young neutron stars with extremely strong magnetic fields. they provide a unique laboratory for discovering and testing physics in extreme conditions. magnetars exhibit relatively slow rotation compared with other young pulsars, and they undergo fast spin down. both properties suggest the existence of a very strong dipole magnetic field, of the order of 1014 g. including candidates, there are currently 29 known magnetars. they are discovered by detecting persistent/transient x-ray emission or soft gamma-ray bursts or flares. in general, the magnetar model can explain energetic phenomena associated with the strong magnetic field. however, many puzzles of magnetars remain: the energy budget of a burst-active period, the connection between the burst or flares and the outburst or glitches in the persistent emission, and the evolution between magnetars and other neutron stars. additional surprises come with the discovery of each new source. in order to answer these questions, a gamma-ray burst monitor with a full-sky field of view and an ability to locate a burst with adequate precision is required. the gravitational wave high-energy electromagnetic counterpart all-sky monitor (gecam) mission consists of two satellites operating at opposite ends of the diameter of a near-earth orbit. this design enables the field of view to cover the entire sky. moreover, about 40% of the sky is monitored by both satellites at the same time. localization of bursts can thus be improved by triangulation between the two satellites. from simulations, a typical short burst from a magnetar can be located to within a region that can be covered by about six pointed observations with x-ray telescopes (e.g., the xmm-newton). the localization can be improved even further if the burst is also detected by other gamma-ray monitors (e.g., the gamma-ray burst monitor on the fermi gamma-ray space telescope or the burst alert telescope on the neil gehrels swift observatory). therefore, gecam is a powerful instrument for discovering new magnetars; fully recording the burst-active episodes of magnetars; attempting to detect soft gamma-ray outbursts of magnetar persistent emission and studying their timing properties; and for guiding multi-wavelength follow-up observations. all these are important observational goals that can help us to understand the physical properties of magnetars and the new physics that underlies them. | observational prospects for magnetars with gecam |
the mhd version of the adaptive mesh refinement (amr) code, mg, has been employed to study the interaction of thermal instability, magnetic fields, and gravity through 3d simulations of the formation of collapsing cold clumps on the scale of a few parsecs, inside a larger molecular cloud. the diffuse atomic initial condition consists of a stationary, thermally unstable, spherical cloud in pressure equilibrium with lower density surroundings and threaded by a uniform magnetic field. this cloud was seeded with 10 per cent density perturbations at the finest initial grid level around n = 1.1 cm-3 and evolved with self-gravity included from the outset. several cloud diameters were considered (100, 200, and 400 pc) equating to several cloud masses (17 000, 136 000, and 1.1 × 106 m⊙). low-density magnetic-field-aligned striations were observed as the clouds collapse along the field lines into disc-like structures. the induced flow along field lines leads to oscillations of the sheet about the gravitational minimum and an integral-shaped appearance. when magnetically supercritical, the clouds then collapse and generate hourglass magnetic field configurations with strongly intensified magnetic fields, reproducing observational behaviour. resimulation of a region of the highest mass cloud at higher resolution forms gravitationally bound collapsing clumps within the sheet that contain clump-frame supersonic (m ∼ 5) and super-alfvénic (ma ∼ 4) velocities. observationally realistic density and velocity power spectra of the cloud and densest clump are obtained. future work will use these realistic initial conditions to study individual star and cluster feedback. | striations, integrals, hourglasses, and collapse - thermal instability driven magnetic simulations of molecular clouds |
models of highly inhomogeneous baryosynthesis of the baryonic asymmetric universe allow for the existence of macroscopic domains of antimatter, which could evolve in a globular cluster of antimatter stars in our galaxy. we assume the symmetry of the evolution of a globular cluster of stars and antistars based on the symmetry of the properties of matter and antimatter. such object can be a source of a fraction of antihelium nuclei in galactic cosmic rays. it makes possible to predict the expected fluxes of cosmic antinuclei with use of known properties of matter star globular clusters we have estimated the lower cutoff energy for the penetration of antinuclei from the antimatter globular cluster, situated in halo, into the galactic disk based on the simulation of particle motion in the large-scale structure of magnetic fields in the galaxy. we have estimated the magnitude of the magnetic cutoff for the globular cluster m4. | researching of magnetic cutoff for local sources of charged particles in the halo of the galaxy |
rapidly spinning magnetic grains can acquire large magnetic dipole moments due to the barnett effect. here we study the new effect of barnett magnetic dipole-dipole interaction on grain-grain collisions and grain growth, assuming that grains are spun up by radiative torques. for the ideal situation in which grains have parallel barnett dipole moments aligned with the ambient magnetic field, we find that the collision rate between grains having embedded iron inclusions can be significantly enhanced due to barnett magnetic dipole-dipole interaction when grains rotate suprathermally by radiative torques. we discuss the implications of enhanced collision rate for grain growth and destruction in the circumstellar envelope of evolved stars, photodissociation regions, and protostellar environments. our results first reveal the potential importance of the dust magnetic properties, magnetic fields, and the local radiation field for grain growth and destruction. detailed numerical simulations of grain dynamics that take into account the variation of barnett dipoles and grain alignment are required to quantify the exact role of barnett dipole-dipole interaction in grain evolution. | effects of barnett magnetic dipole-dipole interaction on grain growth and destruction |
we derive a corrected analytical solution for the propagation and enhanced phase mixing of torsional alfvén waves, in a potential magnetic field with exponentially divergent field lines, embedded in a stratified solar corona. further we develop a code named tawas that calculates the analytic solution describing torsional alfvén waves using idl software language. we then use tawas to demonstrate that both our correction to the analytic solution and the inclusion of wave reflection have a significant impact on alfvén wave damping. we continue to utilize tawas by performing a parameter study in order to identify the conditions under which enhanced phase mixing is strongest. we find that phase mixing is the strongest for high frequency alfvén waves in magnetic fields with highly divergent field lines and without density stratification. we then present a finite difference solver, wigglewave, which solves the linearized evolution equations for the system directly. comparing solutions from tawas and wigglewave we see that our analytical solution is accurate within the limits of the wkb approximation but under-reports the wave damping, caused by enhanced phase mixing, beyond the wkb limit. both tawas and wigglewave solve the linearized governing equations and not the complete non-linear magnetohydrodynamics (mhd) equations. paper ii will consider simulations that solve the full mhd equations including important non-linear effects. | enhanced phase mixing of torsional alfvén waves in stratified and divergent solar coronal structures - paper i. linear solutions |
using 2d particle-in-cell plasma simulations, we study electron acceleration by temperature anisotropy instabilities, assuming conditions typical of above-the-loop-top sources in solar flares. we focus on the long-term effect of te,⊥ > te,∥ instabilities by driving the anisotropy growth during the entire simulation time through imposing a shearing or a compressing plasma velocity (te,⊥ and te,∥ are the temperatures perpendicular and parallel to the magnetic field). this magnetic growth makes te,⊥/te,∥ grow due to electron magnetic moment conservation, and amplifies the ratio ω ce/ω pe from ~0.53 to ~2 (ω ce and ω pe are the electron cyclotron and plasma frequencies, respectively). in the regime ω ce/ω pe ≲ 1.2-1.7, the instability is dominated by oblique, quasi-electrostatic modes, and the acceleration is inefficient. when ω ce/ω pe has grown to ω ce/ω pe ≳ 1.2-1.7, electrons are efficiently accelerated by the inelastic scattering provided by unstable parallel, electromagnetic z modes. after ω ce/ω pe reaches ~2, the electron energy spectra show nonthermal tails that differ between the shearing and compressing cases. in the shearing case, the tail resembles a power law of index αs~ 2.9 plus a high-energy bump reaching ~300 kev. in the compressing runs, αs~ 3.7 with a spectral break above ~500 kev. this difference can be explained by the different temperature evolutions in these two types of simulations, suggesting that a critical role is played by the type of anisotropy driving, ω ce/ω pe, and the electron temperature in the efficiency of the acceleration. | stochastic electron acceleration by temperature anisotropy instabilities under solar flare plasma conditions |
context. relative magnetic helicity is conserved by magneto-hydrodynamic evolution even in the presence of moderate resistivity. for that reason, it is often invoked as the most relevant constraint on the dynamical evolution of plasmas in complex systems, such as solar and stellar dynamos, photospheric flux emergence, solar eruptions, and relaxation processes in laboratory plasmas. however, such studies often indirectly imply that relative magnetic helicity in a given spatial domain can be algebraically split into the helicity contributions of the composing subvolumes, in other words that it is an additive quantity. a limited number of very specific applications have shown that this is not the case.aims: progress in understanding the nonadditivity of relative magnetic helicity requires removal of restrictive assumptions in favor of a general formalism that can be used in both theoretical investigations and numerical applications.methods: we derive the analytical gauge-invariant expression for the partition of relative magnetic helicity between contiguous finite volumes, without any assumptions on either the shape of the volumes and interface, or the employed gauge.results: we prove the nonadditivity of relative magnetic helicity in finite volumes in the most general, gauge-invariant formalism, and verify this numerically. we adopt more restrictive assumptions to derive known specific approximations, which yields a unified view of the additivity issue. as an example, the case of a flux rope embedded in a potential field shows that the nonadditivity term in the partition equation is, in general, non-negligible.conclusions: the nonadditivity of relative magnetic helicity can potentially be a serious impediment to the application of relative helicity conservation as a constraint on the complex dynamics of magnetized plasmas. the relative helicity partition formula can be applied to numerical simulations to precisely quantify the effect of nonadditivity on global helicity budgets of complex physical processes. | additivity of relative magnetic helicity in finite volumes |
magnetic field generation and evolution models that are capable of describing a large body of observational material are currently available for different celestial bodies. despite recent decades of great success in numerical magnetic hydrodynamics and in detailed research into some specific problems, asymptotic methods still have to be used to clarify the magnetic field generation mechanism in dynamo theory. in this review, current asymptotic methods are presented together with the results of their application to the simulation of solar, stellar, and galactic magnetic activities. | current results on the asymptotics of dynamo models |
we simulate the evolution of the stellar wind and the supernova remnant (snr) originating from a runaway massive star in a uniform galactic environment based on three-dimensional magnetohydrodynamics models. taking the stellar wind into consideration, we can explain the radio morphologies of many snrs. the directions of the kinematic velocity of the progenitor, the magnetic field, and the line of sight are the most important factors influencing the morphologies. if the velocity is perpendicular to the magnetic field, the simulation will give us two different unilateral snrs and a bilateral symmetric snr. if the velocity is parallel to the magnetic field, we obtain a bilateral asymmetric snr and a quasi-circular snr. our simulations show the stellar wind plays a key role in the radio evolution of an snr, which implies that the galactic global density and magnetic field distribution play a secondary role. | how does the stellar wind influence the radio morphology of a supernova remnant? |
this paper presents the spatio-temporal evolution of magnetic field due to the nonlinear coupling between fast magnetosonic wave (fmsw) and low frequency slow alfvén wave (saw). the dynamical equations of finite frequency fmsw and saw in the presence of ponderomotive force of fmsw (pump wave) has been presented. numerical simulation has been carried out for the nonlinear coupled equations of finite frequency fmsw and saw. a systematic scan of the nonlinear behavior/evolution of the pump fmsw has been done for one of the set of parameters chosen in this paper, using the coupled dynamical equations. filamentation of fast magnetosonic wave has been considered to be responsible for the magnetic turbulence during the laser plasma interaction. the results show that the formation and growth of localized structures depend on the background magnetic field but the order of amplification does not get affected by the magnitude of the background magnetic field. in this paper, we have shown the relevance of our model for two different parameters used in laboratory and astrophysical phenomenon. we have used one set of parameters pertaining to experimental observations in the study of fast ignition of laser fusion and hence studied the turbulent structures in stellar environment. the other set corresponds to the study of magnetic field amplification in the clumpy medium surrounding the supernova remnant cassiopeia a. the results indicate considerable randomness in the spatial structure of the magnetic field profile in both the cases and gives a sufficient indication of turbulence. the turbulent spectra have been studied and the break point has been found around k which is consistent with the observations in both the cases. the nonlinear wave-wave interaction presented in this paper may be important in understanding the turbulence in the laboratory as well as the astrophysical phenomenon. | nonlinear effects associated with fast magnetosonic waves and turbulent magnetic amplification in laboratory and astrophysical plasmas |
resolution studies of test problems set baselines and help define minimum resolution requirements, however, resolution studies must also be performed on scientific simulations to determine the effect of resolution on the specific scientific results. we perform a resolution study on the formation of a protostar by modelling the collapse of gas through 14 orders of magnitude in density. this is done using compressible radiative non-ideal magnetohydrodynamics. our suite consists of an ideal magnetohydrodynamics (mhd) model and two non-ideal mhd models, and we test three resolutions for each model. the resulting structure of the ideal mhd model is approximately independent of resolution, although higher magnetic field strengths are realised in higher resolution models. the non-ideal mhd models are more dependent on resolution, specifically the magnetic field strength and structure. stronger magnetic fields are realised in higher resolution models, and the evolution of detailed structures such as magnetic walls are only resolved in our highest resolution simulation. in several of the non-ideal mhd models, there is an off-set between the location of the maximum magnetic field strength and the maximum density, which is often obscured or lost at lower resolutions. thus, understanding the effects of resolution on numerical star formation is imperative for understanding the formation of a star. | resolving numerical star formation: a cautionary tale |
the separation of a filament and sigmoid is observed during an x1.4 flare on 2012 july 12 in solar active region 11520, but the corresponding change in magnetic field is not clear. we construct a data-constrained magnetohydrodynamic simulation of the filament-sigmoid system with the flux rope insertion method and magnetic flux eruption code, which produces a magnetic field evolution that may explain the separation of the low-lying filament and high-lying hot channel (sigmoid). the initial state of the magnetic model contains a magnetic flux rope with a hyperbolic flux tube, a null-point structure, and overlying confining magnetic fields. we find that the magnetic reconnections at the null point make the right footpoint of the sigmoid move from one positive magnetic polarity (p1) to another (p3). the tether-cutting reconnection at the hyperbolic flux tube occurs and quickly cuts off the connection of the low-lying filament and high-lying sigmoid. in the end, the high-lying sigmoid erupts and grows into a coronal mass ejection, while the low-lying filament remains stable. the observed double j-shaped flare ribbons, semicircular ribbon, and brightenings of several loops are reproduced in the simulation, where the eruption of the magnetic flux rope includes the impulsive acceleration and propagation phases. | data-constrained mhd simulation for the eruption of a filament-sigmoid system in solar active region 11520 |
recent direct numerical simulations (dns) of large-scale turbulent dynamos in strongly stratified layers have resulted in surprisingly sharp bipolar structures at the surface. here, we present new dns of helically and non-helically forced turbulence with and without rotation and compare with corresponding mean-field simulations (mfs) to show that these structures are a generic outcome of a broader class of dynamos in density-stratified layers. the mfs agree qualitatively with the dns, but the period of oscillations tends to be longer in the dns. in both dns and mfs, the sharp structures are produced by converging flows at the surface and might be driven in non-linear stage of evolution by the lorentz force associated with the large-scale dynamo-driven magnetic field if the dynamo number is at least 2.5 times supercritical. | sharp magnetic structures from dynamos with density stratification |
in addition to the gamma-ray binaries which contain a compact object, high energy (he) and very high energy (vhe) gamma-rays have also been detected from colliding-wind binaries. the collision of the winds produces two strong shock fronts, one for each wind, both surrounding a shock region of compressed and heated plasma, where particles are accelerated to very high energies. magnetic field is also amplified in the shocked region, on which the acceleration of particles greatly depends. in this work we performed full three-dimensional magnetohydrodynamic simulations of colliding winds, coupled to a code that evolves the kinematics of passive charged test particles subject to the plasma fluctuations. after the run of a large ensemble of test particles with initial thermal distributions we show that such shocks produce a non-thermal population (nearly 1% of total particles) of few tens of gevs up to few tevs, depending on the initial magnetization level of the stellar winds. we were able to determine the loci of fastest acceleration, in the range of mev/s to gev/s, to be related to the turbulent plasma with amplified magnetic field of the shock. these results show that colliding wind binaries are indeed able to produce a significant population of high energy particles, in relatively short timescales, compared to the dynamical and diffusion timescales. | colliding-wind binaries as a source of tev cosmic rays |
the high resolution imager (hrieuv) telescope of the extreme ultraviolet imager (eui) instrument onboard solar orbiter has observed euv brightenings, so-called campfires, as fine-scale structures at coronal temperatures. the goal of this paper is to compare the basic geometrical (size, orientation) and physical (intensity, lifetime) properties of the euv brightenings with regions of energy dissipation in a nonpotential coronal magnetic-field simulation. in the simulation, hmi line-of-sight magnetograms are used as input to drive the evolution of solar coronal magnetic fields and energy dissipation. we applied an automatic euv-brightening detection method to euv images obtained on 30 may 2020 by the hrieuv telescope. we applied the same detection method to the simulated energy dissipation maps from the nonpotential simulation to detect simulated brightenings. we detected euv brightenings with a density of 1.41 ×10−3 brightenings/mm2 in the eui observations and simulated brightenings between 2.76 ×10−2 - 4.14 ×10−2 brightenings/mm2 in the simulation, for the same time range. although significantly more brightenings were produced in the simulations, the results show similar distributions of the key geometrical and physical properties of the observed and simulated brightenings. we conclude that the nonpotential simulation can successfully reproduce statistically the characteristic properties of the euv brightenings (typically with more than 85% similarity); only the duration of the events is significantly different between observations and simulation. further investigations based on high-cadence and high-resolution magnetograms from solar orbiter are under consideration to improve the agreement between observation and simulation. | a statistical comparison of euv brightenings observed by so/eui with simulated brightenings in nonpotential simulations |
magnetic field extrapolation is a fundamental tool to reconstruct the three-dimensional magnetic field above the solar photosphere. however, the prevalently used force-free field model might not be applicable in the lower atmosphere with non-negligible plasma β, where the crucial process of flux rope formation and evolution could happen. in this work, we perform extrapolation in active region 12158, based on a recently developed magnetohydrostatic (mhs) method that takes plasma forces into account. by comparing the results with those from the force-free field extrapolation methods, we find that the overall properties, which are characterized by the magnetic free energy and helicity, are roughly the same. the major differences lie in the magnetic configuration and the twist number of the magnetic flux rope (mfr). unlike previous works either obtained sheared arcades or one coherent flux rope, the mhs method derives two sets of mfr, which are highly twisted and slightly coupled. specifically, the result in the present work is more comparable to the high-resolution observations from the chromosphere, through the transition region to the corona, such as the filament fibrils, pre-eruptive braiding characteristics, and the eruptive double-j-shaped hot channel. overall, our work shows that the newly developed mhs method is more promising to reproduce the magnetic fine structures that can well match the observations at multiple layers, and future data-driven simulation based on such extrapolation will benefit in understanding the critical and precise dynamics of flux rope before eruption. | magnetic field extrapolation in active region well comparable to observations in multiple layers |
we explain some phenomena existing generally in the timing residuals: amplitude and sign of the second derivative of a pulsar's spin-frequency (ddot{ν }), some sophisticated residual patterns, which also change with the time span of data segments. the sample is taken from hobbs et al., in which the pulsar's spin-frequency and its first derivative have been subtracted from the timing solution fitting. we first classify the timing residual patterns into different types based on the sign of ddot{ν }. then we use the magnetic field oscillation model developed in our group to fit successfully the different kinds of timing residuals with the markov chain monte carlo method. finally, we simulate the spin evolution over 20 years for a pulsar with typical parameters and analyse the data with the conventional timing solution fitting. by choosing different segments of the simulated data, we find that most of the observed residual patterns can be reproduced successfully. this is the first time that the observed residual patterns are fitted by a model and reproduced by simulations with very few parameters. from the distribution of the different residual patterns in the p-dot{p} diagram, we argue that (1) a single magnetic field oscillation mode exists commonly in all pulsars throughout their lifetimes; (2) there may be a transition period over the lifetimes of pulsars, in which multiple magnetic field oscillation modes exist. | understanding the residual patterns of timing solutions of radio pulsars with a model of magnetic field oscillation |
the temporal evolution of a spectrum during a steeply rising submillimeter (thz) burst that occurred on 2003 november 2 was investigated in detail for the first time. observations show that the flux density of the thz spectrum increased steeply with frequency above 200 ghz. their average rising rates reached a value of 235 sfu ghz-1 (corresponding to spectral index α of 4.8) during the burst. the flux densities reached about 4 000 and 70 000 sfu at 212 and 405 ghz at the maximum phase, respectively. the emissions at 405 ghz maintained such a continuous high level that they largely exceeded the peak values of the microwave (mw) spectra during the main phase. our studies suggest that only energetic electrons with a low-energy cutoff of ∼1 mev and number density of ∼106-108 cm-3 can produce such a strong and steeply rising thz component via gyrosynchrotron radiation based on numerical simulations of burst spectra in the case of a nonuniform magnetic field. the electron number density n, derived from our numerical fits to the thz temporal evolution spectra, increased substantially from 8 × 106 to 4 × 108 cm-3, i.e., the n value increased 50 times during the rise phase. during the decay phase it decreased to 7 × 107 cm-3, i.e., it decreased by about five times from the maximum phase. the total electron number decreased an order of magnitude from the maximum phase to the decay phase. nevertheless, the variation in amplitude of n is only about one time in the mw emission source during this burst, and the total electron number did not decrease but increased by about 20% during the decay phase. interestingly, we find that the thz source radius decreased by about 24% while the mw source radius, on the contrary, increased by 28% during the decay phase. | study of temporal evolution of emission spectrum in a steeply rising submillimeter burst |
we discuss different exotic phases and components of matter from the crust to the core of neutron stars based on theoretical models for equations of state relevant to core collapse supernova simulations and neutron star merger. parameters of the models are constrained from laboratory experiments. it is observed that equations of state involving strangeness degrees of freedom such as hyperons and bose-einstein condensates are compatible with 2{m}_{solar} neutron stars. the role of hyperons is explored on the evolution and stability of the protoneutron star in the context of sn1987a. moment of inertia, mass and radius which are direct probes of neutron star interior are computed and their observational consequences are discussed. we continue our study on the dense matter under strong magnetic fields and its application to magnetoelastic oscillations of neutron stars. | neutron stars: laboratories for fundamental physics under extreme astrophysical conditions |
we have studied the large scale dynamo process forced with helical magnetic energy. the magnetically driven dynamo is not so well studied as kinetically forced dynamo. it has been thought to represent the amplification of magnetic field in the stellar corona, accretion disk, or plasma lab. however, the interaction between the helical magnetic field and plasma is a more fundamental phenomenon that can be extended to the early universe. the scale-invariant helical magnetic field not only explains the currently observed large scale astrophysical magnetic fields but also has information on the horizon scale in the early universe. the interaction between magnetic field and plasma is inherently non-linear, making its mechanism difficult to understand. but, if the plasma system is driven with helical field, the process can be linearized with alpha&betaand large scale magnetic field. conventionally, alpha effect is thought to transfer magnetic field to the large scale regime, and betaeffect diffuses magnetic field. however, these conclusions are based on the incompletely derived alpha&beta. to get the exact profiles of evolving alpha&\b{eta}, we solved a coupled semi-analytic equation set and applied the result to simulation data for the large scale magnetic helicity and magnetic energy. our result shows that the averaged alpha effect decreases before making a significant contribution to the amplification of large scale b field. rather, betaeffect, which keeps negative, de facto plays a key role in the amplification of large scale b field with laplacian. and, this negative diffusivity accounts for the attenuation of plasma kinetic energy | negative turbulent magnetic diffusivity $\\beta$ effect in a magnetically forced system |
the evolution of toroidal field instability is essential to understand the topology of the magnetic fields observed in early-type stars, or in the isothermal core of the red giants. we study the non-linear evolution of the instability of a predominant toroidal field stored in a stably stratified stellar interior in spherical symmetry. in particular we followed the non-linear phase in order to understand the role of stable stratification in suppressing the instability. we use the mhd equations as implemented in the anelastic approximation in the eulag code and perform a series of high-resolution numerical simulations of the instability exploring a large parameter space. we show that beyond a critical value gravity strongly suppress the instability, in agreement with the linear analysis. the intensity of the initial field also plays a crucial role, as weaker fields show much lower growth rates. moreover, the fastest growing modes have a very large characteristic radial scale, at variance with recent claims from the linear analysis. our results illustrate that the anelastic approximation can efficiently describe the evolution of toroidal field instability in realistic stellar interior. the suppression of the instability as consequence of stratification can likely play a role to explain the magnetic desert in ap/bp stars, since weak fields are only marginally unstable in the case of strong gravity. | non-linear simulations of the tayler instability |
previously derived faraday rotation constraints on the volume-filling intergalactic magnetic field (igmf) have used analytical models that made a range of simplifying assumptions about magnetic field evolution in the intergalactic medium and did not consider the effect of baryonic feedback on large-scale structures. in this work, we revise existing faraday rotation constraints on the igmf using a numerical model of the intergalactic medium from the illustristng cosmological simulation that includes a sophisticated model of the baryonic feedback. we use the illustristng model to calculate the rotation measure and compare the resulting mean and median of the absolute value of the rotation measure with data from the nrao vla sky survey (nvss). the numerical model of the intergalactic medium includes a full magnetohydrodynamic model of the compressed primordial magnetic field as well as a model of the regions where the magnetic field is not primordial, but is rather produced by the process of baryonic feedback. separating these two types of regions, we are able to assess the influence of the primordial magnetic field on the faraday rotation signal. we find that by correcting for regions of compressed primordial field and accounting for the fact that part of the intergalactic medium is occupied by magnetic fields spread by baryonic feedback processes rather than by the primordial field relaxes the faraday rotation bound by a factor of ≃3. this results in b0 < 1.8 × 10-9 g for large correlation length igmfs. | revision of faraday rotation measure constraints on the primordial magnetic field using the illustristng simulation |
it is generally believed that angular momentum is distributed during the gravitational collapse of the primordial star forming cloud. however, so far there has been little understanding of the exact details of the distribution. we use the modified version of the gadget-2 code, a three-dimensional smoothed-particle hydrodynamics simulation, to follow the evolution of the collapsing gas in both idealized as well as more realistic minihalos. we find that, despite the lack of any initial turbulence and magnetic fields in the clouds the angular momentum profile follows the same characteristic power-law that has been reported in studies that employed fully self-consistent cosmological initial conditions. the fit of the power-law appears to be roughly constant regardless of the initial rotation of the cloud. we conclude that the specific angular momentum of the self-gravitating rotating gas in the primordial minihalos maintains a scaling relation with the gas mass as l ∝ m^{1.125}. we also discuss the plausible mechanisms for the power-law distribution. | angular momentum distribution during the collapse of primordial star-forming clouds |
we model the evolution of the spin frequency's second derivative v̈ and the braking index n of radio pulsars with simulations within the phenomenological model of their surface magnetic field evolution, which contains a long-term power-law decay modulated by short-term oscillations. for the pulsar psr b0329+54, a model with three oscillation components can reproduce its v̈ variation. we show that the “averaged” n is different from the instantaneous n, and its oscillation magnitude decreases abruptly as the time span increases, due to the “averaging” effect. the simulated timing residuals agree with the main features of the reported data. our model predicts that the averaged v̈ of psr b0329+54 will start to decrease rapidly with newer data beyond those used in hobbs et al. we further perform monte carlo simulations for the distribution of the reported data in |v̈| and |n| versus characteristic age τc diagrams. it is found that the magnetic field oscillation model with decay index α = 0 can reproduce the distributions quite well. compared with magnetic field decay due to the ambipolar diffusion (α = 0.5) and the hall cascade (α = 1.0), the model with no long term decay (α = 0) is clearly preferred for old pulsars by the p-values of the two-dimensional kolmogorov-smirnov test. supported by the national natural science foundation of china. | modeling the evolution and distribution of the frequency's second derivative and the braking index of pulsar spin |
we use the 2.5-d electromagnetic particle-in-cell simulation code to investigate the acceleration of electrons in solar coronal holes through the interaction of alfvén waves with an interplume region. the interplume is modeled by cavity density gradients that are perpendicular to the background magnetic field. the aim is to help explain the observation of suprathermal electrons under a relatively quiet sun. simulations show that alfvén waves interacting with the interplume region give rise to a strong local electric field that accelerates electrons in the direction parallel to the background magnetic field. suprathermal electron beams and small-scale coherent structures are observed within interplume of strong density gradients. these features result from the nonlinear evolution of the electron beam plasma instability. | electron acceleration and small-scale coherent structure formation by an alfvén wave propagating in coronal interplume region |
we have constrained the charge-mass (ɛ -m ) phase space of millicharged particles through the simulation of the rotational evolution of neutron stars, where an extra slow-down effect due to the accretions of millicharged dark matter particles is considered. for a canonical neutron star of m =1.4 m⊙ and r =10 km with typical magnetic field strength b0=1012 g , we have shown an upper limit of millicharged particles, which is compatible with recently experimental and observational bounds. meanwhile, we have also explored the influences on the ɛ -m phase space of millicharged particles for different magnetic fields b0 and dark matter density ρdm in the vicinity of the neutron star. | constraints on millicharged particles by neutron stars |
we employ the supernova fallback disk model to simulate the spin evolution of isolated young neutron stars (nss). we consider the submergence of the ns magnetic fields during the supercritical accretion stage and its succeeding reemergence. it is shown that the evolution of the spin periods and the magnetic fields in this model is able to account for the relatively weak magnetic fields of central compact objects and the measured braking indices of young pulsars. for a range of initial parameters, evolutionary links can be established among various kinds of ns sub-populations including magnetars, central compact objects and young pulsars. thus, the diversity of young nss could be unified in the framework of the supernova fallback accretion model. | unifying neutron star sub-populations in the supernova fallback accretion model |
energy conversion via reconnecting current sheets is common in space and astrophysical plasmas. frequently, current sheets disrupt at multiple reconnection sites, leading to the formation of plasmoid structures between sites, which might affect energy conversion. we present in situ evidence of the firehose instability in multiple reconnection in the earth's magnetotail. the observed proton beams accelerated in the direction parallel to magnetic field and ion-scale fluctuations of whistler type imply the development of firehose instability between two active reconnection sites. the linear wave dispersion relation, estimated for the measured plasma parameters, indicates a positive growth rate of firehose-related electromagnetic fluctuations. simulations of temporal evolution of the observed multiple reconnection by using a 2.5d implicit particle-in-cell code show that, as the plasmoid formed between two reconnection sites evolves, the plasma at its edge becomes anisotropic and overcomes the firehose marginal stability threshold, leading to the generation of magnetic field fluctuations. the combined results of observations and simulations suggest that the firehose instability, operating between reconnection sites, converts plasma kinetic energy into energy of magnetic field fluctuations, counteracting the conversion of magnetic energy into plasma energy occurring at reconnection sites. this suggests that magnetic energy conversion in multiple reconnection can be less efficient than in the case of the single-site reconnection. | in situ evidence of firehose instability in multiple reconnection |
star formation involves gravity, turbulence, magnetic fields, and feedback from new stars through jets, radiation and winds. the evolution of the density probability distribution function (ρ-pdf) is directly related to the star formation rate (sfr), forming the basis of several star formation models. we utilize two runs from the starforge simulation suite that follow the evolution of molecular clouds, while resolving individual stars and including all gas and feedback physics. the two runs have different initial conditions, one is a periodic box with driven turbulence (box), while the other is an isolated cloud without turbulent driving (sphere). we find that the ρ-pdf for both runs is initially well-fit by a log-normal (ln) plus a power-law (pl) function. however, as the sfr peaks, the pdf for the sphere run becomes well-fit by just a wide ln. conversely, the box run pdf remains well-fit by a ln+pl function for the entirety of the run. | evolution of the gas density in a simulated star-forming cloud with stellar feedback |
using three-dimensional magnetohydrodynamics simulations, the driving of protostellar jets is investigated in different star-forming cores with the parameters of magnetic field strength and mass accretion rate. powerful high-velocity jets appear in strongly magnetized clouds when the mass accretion rate on to the protostellar system is lower than $\dot{m} \lesssim 10^{-4}\, {\rm m}_\odot$ yr-1. on the other hand, even at this mass accretion rate range, no jets appear for magnetic fields of prestellar clouds as weak as μ0 ≳ 5-10, where μ0 is the mass-to-flux ratio normalized by the critical value (2πg1/2)-1. for $\dot{m}\gtrsim 10^{-4}\, {\rm m}_\odot$ yr-1, although jets usually appear just after protostar formation independent of the magnetic field strength, they soon weaken and finally disappear. thus, they cannot help drive the low-velocity outflow when there is no low-velocity flow just before protostar formation. as a result, no significant mass ejection occurs during the early mass accretion phase either when the prestellar cloud is weaky magnetized or when the mass accretion rate is very high. thus, protostars formed in such environments would trace different evolutionary paths from the normal star formation process. | can high-velocity protostellar jets help to drive low-velocity outflow? |
i will review our current knowledge of magnetism in hot stars, in particular the origin and occurrence of magnetic fields in these stars with a radiative envelope, as well as the impact of the magnetic fields on many aspects such as the internal structure of the star, its circumstellar environment, and stellar evolution. these results have been achieved over the last few years thanks to large spectropolarimetric surveys (mimes, binamics, britepol, life...) combined with theoretical developments and simulations. it is now also possible to combine spectropolarimetry with photometry from space missions (e.g. tess) to perform magneto-asteroseismology and obtain additional insight on the physics of hot stars. | magnetism in hot stars |
the solar abundances of fe and of the cno elements play an important role in addressing a number of important issues such as the formation, structure, and evolution of the sun and the solar system, the origin of the chemical elements, and the evolution of stars and galaxies. despite the large number of papers published on this issue, debates about the solar abundances of these elements continue. the aim of the present investigation is to quantify the impact of photospheric magnetic fields on the determination of the solar chemical abundances. to this end, we used two 3d snapshot models of the quiet solar photosphere with a different magnetization taken from recent magneto-convection simulations with small-scale dynamo action. using such 3d models we have carried out spectral synthesis for a large set of fei, ci, ni, and oi lines, in order to derive abundance corrections caused by the magnetic, zeeman broadening of the intensity profiles and the magnetically induced changes of the photospheric temperature structure. we find that if the magnetism of the quiet solar photosphere is mainly produced by a small-scale dynamo, then its impact on the determination of the abundances of iron, carbon, nitrogen and oxygen is negligible. | the impact of surface dynamo magnetic fields on the chemical abundance determination |
the rotational evolution of an accreting pre-main-sequence star is influenced by its magnetic interaction with its surrounding circumstellar disk. using the pluto code, we perform 2.5d magnetohydrodynamic, axisymmetric, time-dependent simulations of star-disk interaction—with an initial dipolar magnetic field structure, and a viscous and resistive accretion disk—in order to model the three mechanisms that contribute to the net stellar torque: accretion flow, stellar wind, and magnetospheric ejections (periodic inflation and reconnection events). we investigate how changes in the stellar magnetic field strength, rotation rate, and mass accretion rate (changing the initial disk density) affect the net stellar torque. all simulations are in a net spin-up regime. we fit semi-analytic functions for the three stellar torque contributions, allowing for the prediction of the net stellar torque for our parameter regime, as well as the possibility of investigating spin evolution using 1d stellar evolution codes. the presence of an accretion disk appears to increase the efficiency of stellar torques compared to isolated stars, for cases with outflow rates much smaller than accretion rates, because the star-disk interaction opens more of the stellar magnetic flux compared to that from isolated stars. in our parameter regime, a stellar wind with a mass-loss rate of ≈1% of the mass accretion rate is capable of extracting <=50% of the accreting angular momentum. these simulations suggest that achieving spin equilibrium in a representative t tauri case within our parameter regime, e.g., bp tau, would require a wind mass-loss rate of ≈25% of the mass accretion rate. | magnetic braking of accreting t tauri stars: effects of mass accretion rate, rotation, and dipolar field strength |
compact objects, such as neutron stars and black holes, are characterized by the presence of strong magnetic fields that are crucial to explain the high-energy emission from these sources. such magnetic fields may be subject to complex evolution inside the hosting relativistic plasmas, like dynamo-like processes amplifying initial seed fields in accretion disks or in the early stages of neutron star formation, or dissipative reconnection events in thin current sheets, as believed to occur in the magnetospheres of magnetars. here we present a unified treatment of these non-ideal effects within the framework of general relativistic magnetohydrodynamics (grmhd). results of recent numerical simulations obtained with the echo code will be briefly discussed for selected test cases. | creation and dissipation of magnetic fields in non-ideal grmhd simulations |
stars and their corresponding protoplanetary disks form in diverse environments. to account for these natural variations, we investigate the formation process around nine solar mass stars with a maximum resolution of 2 au in a giant molecular cloud of (40 pc)$^3$ in volume by using the adaptive mesh refinement code \ramses. the magnetohydrodynamic simulations reveal that the accretion process is heterogeneous in time, in space, and among protostars of otherwise similar mass. during the first roughly 100 kyr of a protostar evolving to about a solar mass, the accretion rates peak around $10^{-5}$ to $10^{-4}$ m$_{\odot}$ yr$^{-1}$ shortly after its birth, declining with time after that. the different environments also affect the spatial accretion, and infall of material to the star-disk system is mostly through filaments and sheets. furthermore, the formation and evolution of disks varies significantly from star to star. we interpret the variety in disk formation as a consequence of the differences in the combined effects of magnetic fields and turbulence that may cause differences in the efficiency of magnetic braking, as well as differences in the strength and distribution of specific angular momentum. | accounting for the diversity in stellar environments |
we perform mhd simulations of a thin resistive and viscous accretion disk around a neutron star with the surface dipolar magnetic field of 108 gauss. the system evolution is followed during the interval of 500 millisecond pulsar rotations. matter is accreted through a stable accretion column from the disk onto the star. we also show propagation of the stellar wind through the corona. analysis of the mass accretion flux and torques on the star shows that the disk reaches the quasi-stationary state. | mhd simulations of resistive viscous accretion disk around millisecond pulsar |
context. previous studies show that the physical structures and kinematics of a region depend significantly on the ionisation fraction. these studies have only considered these effects in non-ideal magnetohydrodynamic simulations with microturbulence. the next logical step is to explore the effects of turbulence on ionised magnetic molecular clouds and then compare model predictions with observations to assess the importance of turbulence in the dynamical evolution of molecular clouds.aims: in this paper, we extend our previous studies of the effect of ionisation fractions on star formation to clouds that include both non-ideal magnetohydrodynamics and turbulence. we aim to quantify the importance of a treatment of the ionisation fraction in turbulent magnetised media and investigate the effect of the turbulence on shaping the clouds and filaments before star formation sets in. in particular, here we investigate how the structure, mass and width of filamentary structures depend on the amount of turbulence in ionised media and the initial mass-to-flux ratio.methods: to determine the effects of turbulence and mass-to-flux ratio on the evolution of non-ideal magnetised clouds with varying ionisation profiles, we have run two sets of simulations. the first set assumes different initial turbulent mach values for a fixed initial mass-to-flux ratio. the second set assumes different initial mass-to-flux ratio values for a fixed initial turbulent mach number. both sets explore the effect of using one of two ionisation profiles: step-like (sl) or cosmic ray only (cr-only). we compare the resulting density and mass-to-flux ratio structures both qualitatively and quantitatively via filament and core masses and filament fitting techniques (gaussian and plummer profiles).results: we find that even with almost no turbulence, filamentary structure still exists although at lower density contours. comparison of simulations shows that for turbulent mach numbers above 2, there is little structural difference between the sl and cr-only models, while below this threshold the ionisation structure significantly affects the formation of filaments. this holds true for both sets of models. analysis of the mass within cores and filaments shows that the mass decreases as the degree of turbulence increases. finally, observed filaments within the taurus l1495/b213 complex are best reproduced by models with supercritical mass-to-flux ratios and/or at least mildly supersonic turbulence, however, our models show that the sterile fibres observed within taurus may occur in highly ionised, subcritical environments.conclusions: from the analysis of the simulations, we conclude that in the presence of low turbulent velocities, the ionisation structure of the medium still plays a role in shaping the structure of the cloud, however, above mach 2, the differences between the two profiles become indistinguishable. however, differences may be present in the underlying velocity structure. kinematics studies will be the focus of the next paper in this series. regions with fertile fibres likely indicate a trans- or supercritical mass-to-flux ratio within the region while sterile fibres are likely subcritical and transient. | ionisation in turbulent magnetic molecular clouds. i. effect on density and mass-to-flux ratio structures |
the performed magnetohydrodynamic simulation examines the importance of magnetofluid evolution, which naturally leads to current sheets in the presence of three-dimensional (3d) magnetic nulls. the initial magnetic field is constructed by superposing a 3d force-free field on a constant axial magnetic field. the initial field supports 3d magnetic nulls having the classical spine axis and the dome-shaped fan surface and exerts non-zero lorentz force on the magnetofluid. importantly, the simulation identifies the development of current sheets near the 3d magnetic nulls. the morphology of the current sheets is similar to a cylindrical surface where the surface encloses the spine axis. the development is because of favorable deformation of magnetic field lines constituting the dome-shaped fan surface. the deformation of field lines is found to be caused by the flow generated by magnetic reconnections at current sheets which are located away from the cylindrically shaped current sheets. | simulated evolution of three-dimensional magnetic nulls leading to generation of cylindrically-shaped current sheets |
context. uranus is the only planet in the solar system whose rotation axis and orbital plane are nearly parallel to each other. uranus is also the planet with the largest angle between the rotation axis and the direction of its magnetic dipole (roughly 59°). consequently, the shape and structure of its magnetospheric tail is very different to those of all other planets in whichever season one may consider. the only in situ measurements were obtained in january 1986 during a flyby of the voyager ii spacecraft. at that date, uranus was near solstice time, but unfortunately the data collected by the spacecraft were much too sparse to allow for a clear view of the structure and dynamics of its extended magnetospheric tail. later numerical simulations revealed that the magnetic tail of uranus at solstice time is helically shaped with a characteristic pitch of the order of 1000 planetary radii.aims: we aim to propose a magnetohydrodynamic model for the magnetic tail of uranus at solstice time.methods: we constructed our model based on a symmetrised version of the uranian system by assuming an exact alignment of the solar wind and the planetary rotation axis and an angle of 90° between the planetary magnetic dipole and the rotation axis. we do also postulate that the impinging solar wind is steady and unmagnetised, which implies that the magnetosphere is quasi-steady in the rotating planetary frame and that there is no magnetic reconnection at the magnetopause.results: one of the main conclusions is that all magnetic field lines forming the extended magnetic tail follow the same qualitative evolution from the time of their emergence through the planet's surface and the time of their late evolution after having been stretched and twisted several times downstream of the planet. in the planetary frame, these field lines move on magnetic surfaces that wind up to form a tornado-shaped vortex with two foot points on the planet (one in each magnetic hemisphere). the centre of the vortex (the eye of the tornado) is a simple double helix with a helical pitch (along the symmetry axis z) λ = τ[vz+bz/(μ0ρ)1/2], where τ is the rotation period of the planet, μ0 the permeability of vacuum, ρ the mass density, vz the fluid velocity, and bz the magnetic field where all quantities have to be evaluated locally at the centre of the vortex. in summary, in the planetary frame, the motion of a typical magnetic field of the extended uranian magnetic tail is a vortical motion, which asymptotically converges towards the single double helix, regardless of the line's emergence point on the planetary surface. | a physical model for the magnetosphere of uranus at solstice time |
to understand how stars can form in the interstellar medium (ism), it has to be understood how cold (~ 10 k) and dense gas (> 10^{4} cm^{-3}) can emerge during the evolution of the ism. with the herschel telescope it was found that most of this dense star forming gas is organised in filamentary structures.to understand how this dense filamentary gas forms, multiple co transitions were observed towards the musca filament, which can form low-mass stars, using the apex telescope. these observations were complemented with [cii] and [oi] observations by the sofia telescope. the non-detection of [cii] demonstrates that the musca cloud is embedded in a weak fuv field (< 1 g0). however, the observed co(4-3) line with apex demonstrates the presence of warm (> 50 k) co gas around the musca filament which cannot be explained with heating by the fuv radiation field. a comparison of the observed co(4-3) emission with shock models shows that the emission can be the result of a low-velocity (< 4 km/s) j-type shock. further analysis of this emission demonstrates that this shock emission resembles the signature of a shock responsible for mass accretion on a filament. this suggests that a low-velocity shock as a result of continuous mass accretion is responsible for the formation of cold and dense gas that can form stars in the musca filament.the accretion scenario for musca is further analysed with low-j co observations from apex and nanten2 to study the large scale gas kinematics. these observations unveil a velocity gradient over the musca filament crest which is correlated with the velocity field of the nearby ambient gas. this suggests that the velocity gradient is the result of mass accretion from the ambient cloud. analysing the full musca cloud demonstrates a spatial and kinematic asymmetry from low- to high-density gas. this asymmetry is seen as a v-shape in the position-velocity (pv) diagram perpendicular to the musca filament. including atomic hydrogen (hi) observations in the analysis first of all confirms that musca is part of a larger hi cloud, the chamaeleon-musca complex. it also demontrates that the kinematic asymmetry is seen from the hi cloud down to the filament crest. furthermore, the co-hi asymmetry is found for basically all dense regions (cha i, cha ii, cha iii and musca) with archival data of chamaeleon-musca, while hi shows indications of more than one velocity component. this asymmetric accretion scenario is predicted by magnetised cloud-cloud collision simulations, where the bending of the magnetic field is responsible the observed asymmetric accretion scenario. the filament formation in musca is thus the result of two intersecting converging flows which are driven by the magnetic field bending due to a large-scale colliding hi flow that triggered the observed star formation in the full chamaeleon-musca complex.finally, the kinematics of the high-mass star forming ridge dr21 and its surrounding gas are studied to compare low- and high-mass star formation. this shows a similar spatial and kinematic asymmetry as in musca, which suggests that dr21 is formed by a giant molecular cloud (gmc) collision. however, it is also found for high-mass star formation in the dr21 cloud that gravity plays an important role on large scales (> 1 pc) while for musca gravity only starts to dominate locally (r < 0.1-0.2 pc). so, due to the high density in the dr21 cloud after the gmc collision, gravity eventually drives the evolution of the compressed cloud for high-mass star forming regions. kinematic observations of the full cygnus-x north region show further indications of two interacting velocity components over the entire region, which indicates that a high-velocity (> 10 km/s) gmc collision can result in the formation of an ob association similar to ob2. these ob stars then form in gravitationally collapsing hubs and ridges due to the compression by the gmc collision. | the formation of dense gas in low- and high-mass star forming regions |
while the sun is a quiet and well-balanced star now, during its first few million years it possessed a strong magnetic field and actively accreted material from its circumstellar environment. theoretical models predict that under certain circumstances the interaction of a strongly magnetic star and its circumstellar disk may lead to short bursts of increased accretion onto the star (d'angelo & spruit 2012). examples for this phenomenon may be the members of a group of young eruptive stars called exors. their prototype, ex lup, had its historically largest outburst in 2008. spectroscopic evidence suggests that the mass accretion proceeds through the same magnetospheric accretion channels both in quiescence and in outburst but with different mass flux (sicilia-aguilar et al. 2012). to characterize for the first time ex lup's magnetic field, we obtained spectropolarimetric monitoring for it with the cfht/espadons. we detected strong, poloidal magnetic field with a prominent cool polar cap and an accretion spot above it. we compared our results with numerical simulations, in order to check the applicability of the d'angelo & spruit model as an explanation of ex lup's accretion outbursts. if ex lup is a good proxy for the proto-sun, similar magnetic field-disk interactions and outbursts might have happened during the early evolution of the solar system as well. | magnetic field and accretion in ex lup |
in this lecture on stellar magnetism we discuss how the dynamo generated magnetic field shapes the extended hot atmosphere and how the feedback loop between rotation, convection, turbulence, dynamo action and braking by stellar wind influences the secular evolution and the rotational history of solarlike stars. we discuss each key physical mechanism such as dynamo action and wind dynamics and discuss angular momentum transport inside and outside the star. in order to illustrate these complex processes and their nonlinear interaction we use both pedagogical exercises and discuss more advanced agnetohydrodynamics numerical simulations. we propose seven problems and their solution to help getting a good first understanding of stellar magnetohydrodynamics. | stellar magnetism: bridging dynamos and winds |
stellar chromospheres and winds represent universal attributes of stars on the cool portion of h-r diagram. in this paper we derive observational constrains for the chromospheric heating and wind acceleration from cool evolved stars and examine the role of alfven waves as a viable source of energy dissipation and momentum deposition. we use a 1.5d magnetohydrodynamic code with a generalized ohm's law to study propagation of alfven waves generated along a diverging magnetic field in a stellar photosphere at a single frequency. we demonstrate that due to inclusion of the effects of ion-neutral collisions in magnetized weakly ionized chromospheric plasma on resistivity and the appropriate grid resolution, the numerical resistivity becomes 1-2 orders of magnitude smaller than the physical resistivity. the motions introduced by non-linear transverse alfven waves can explain non-thermally broadened and non-gaussian profiles of optically thin uv lines forming in the stellar chromosphere of α tau and other late-type giant and supergiant stars. the calculated heating rates in the stellar chromosphere model due to resistive (joule) dissipation of electric currents on pedersen resistivity are consistent with observational constraints on the net radiative losses in uv lines and the continuum from α tau. at the top of the chromosphere, alfven waves experience significant reflection, producing downward propagating transverse waves that interact with upward propagating waves and produce velocity shear in the chromosphere. our simulations also suggest that momentum deposition by non-linear alfven waves becomes significant in the outer chromosphere within 1 stellar radius from the photosphere that initiates a slow and massive winds from red giants and supergiants. | the magnetic coupling of chromospheres and winds from late type evolved stars: role of mhd waves |
recent results of magnetohydrodynamic (mhd) simulations are presented treating the launching of jets from young stellar object accretion disks. the simulations consider the evolution of magnetically diffusive disks that eject high-speed outflows. a few exemplary results are presented: (i) the disk structural evolution that leads to different launching conditions over time, (ii) a general interrelation between the disk magnetization at the outflow launching radius and the outflow asymptotic properties, (iii) jet launching within a self-generated disk magnetic field amplified by a disk mean field dynamo, and (iv) simulations of jets from orbiting jet sources. | modeling the magnetized accretion and outflows in young stellar objects |
we present an analytical model to investigate the production of pebbles and their radial transport through a protoplanetary disk (ppd) with magnetically driven winds. while most of the previous analytical studies in this context assumed that the radial turbulent coefficient is equal to the vertical dust diffusion coefficient, in the light of the results of recent numerical simulations, we relax this assumption by adopting effective parameterizations of the turbulent coefficients involved, in terms of the strength of the magnetic fields driving the wind. theoretical studies have already pointed out that even in the absence of winds, these coefficients are not necessarily equal, though how this absence affects pebble production has not been explored. in this paper, we investigate the evolution of the pebble production line, the radial mass flux of the pebbles, and their corresponding surface density as a function of the plasma parameter at the disk midplane. our analysis explicitly demonstrates that the presence of magnetically driven winds in a ppd leads to considerable reduction of the rate and duration of the pebble delivery. we show that when the wind is strong, the core growth in mass due to the pebble accretion is so slow that it is unlikely that a core could reach a pebble isolation mass during a ppd lifetime. when the mass of a core reaches this critical value, pebble accretion is halted due to core-driven perturbations in the gas. with decreasing wind strength, however, pebble accretion may, in a shorter time, increase the mass of a core to the pebble isolation mass. | on the dynamics of pebbles in protoplanetary disks with magnetically driven winds |
the dynamical coupling between solar chromospheric plasma and the magnetic field is investigated by numerically solving a fully self-consistent, two-dimensional initial-value problem for the nonlinear collisional mhd equations including electric resistivity, thermal conduction, and, in some cases, gas-dynamic viscosity. the processes in the contact zone between two horizontal magnetic fields of opposite polarities are considered. the plasma is assumed to be initially motionless and to have a temperature of 50,000 k uniform throughout the plasma volume; the characteristic magnetic field corresponds to a plasma β ≳1 . in a physical time interval of 17 seconds typically covered by a computational run, the plasma temperature gradually increases by a factor of two to three. against this background, an impulsive (in 0.1 seconds or less) increase in the current-aligned plasma velocity occurs at the site of the current-layer thinning (sausage-type deformation, or m =0 pinch instability). this velocity burst can be interpreted physically as an event of suprathermal-proton generation. further development of the sausage instability results in an increase in the kinetic temperature of the protons to high values, even to those observed in flares. the form of our system of mhd equations indicates that this kind of increase is a property of the exact solution of the system for an appropriate choice of parameters. magnetic reconnection does not manifest itself in this solution: it would generate flows forbidden by the chosen geometry. therefore, the pinch-sausage effect can act as an energiser of the upper chromosphere and be an alternative to the magnetic-reconnection process as the producer of flares. | numerical mhd simulation of the coupled evolution of plasma and magnetic field in the solar chromosphere. i. gradual and impulsive energisation |
we study the resistive evolution of a localized self-organizing magnetohydrodynamic equilibrium. in this configuration the magnetic forces are balanced by a pressure force caused by a toroidal depression in the pressure. equilibrium is attained when this low-pressure region prevents further expansion into the higher-pressure external plasma. we find that, for the parameters investigated, the resistive evolution of the structures follows a universal pattern when rescaled to resistive time. the finite resistivity causes both a decrease in the magnetic field strength and a finite slip of the plasma fluid against the static equilibrium. this slip is caused by a pfirsch-schlüter-type diffusion, similar to what is seen in tokamak equilibria. the net effect is that the configuration remains in magnetostatic equilibrium whilst it slowly grows in size. the rotational transform of the structure becomes nearly constant throughout the entire structure, and decreases according to a power law. in simulations this equilibrium is observed when highly tangled field lines relax in a high-pressure (relative to the magnetic field strength) environment, a situation that occurs when the twisted field of a coronal loop is ejected into the interplanetary solar wind. in this paper we relate this localized magnetohydrodynamic equilibrium to magnetic clouds in the solar wind. | resistive evolution of toroidal field distributions and their relation to magnetic clouds |
we use ensemble machine learning algorithms to study the evolution of magnetic fields in magnetohydrodynamic (mhd) turbulence that is helically forced. we perform direct numerical simulations of helically forced turbulence using mean field formalism, with electromotive force (emf) modeled both as a linear and non-linear function of the mean magnetic field and current density. the form of the emf is determined using regularized linear regression and random forests. we also compare various analytical models to the data using bayesian inference with markov chain monte carlo (mcmc) sampling. our results demonstrate that linear regression is largely successful at predicting the emf and the use of more sophisticated algorithms (random forests, mcmc) do not lead to significant improvement in the fits. we conclude that the data we are looking at is effectively low dimensional and essentially linear. finally, to encourage further exploration by the community, we provide all of our simulation data and analysis scripts as open source ipython notebooks. | exploring helical dynamos with machine learning: regularized linear regression outperforms ensemble methods |
the origin and evolution of magnetic fields in the universe is still an open question. observations of galaxies at high-redshift give evidence for strong galactic magnetic fields even in the early universe which are consistently measured at later times up to the present age. however, primordial magnetic fields and seed field generation by battery processes cannot explain such high field strengths, suggesting the presence of a rapid growth mechanism in those high-redshift galaxies and subsequent maintenance against decay. astrophysical dynamo theory provides efficient means of field amplification where even weak initial fields can grow exponentially on sufficiently fast timescales, driving the conversion of kinetic energy into magnetic energy. we investigate the role which feedback mechanisms play in the creation of the turbulence necessary for dynamos to operate. performing magnetohydrodynamic simulations of cooling halos of dwarf and milky way-like high-redshift progenitors, we compare the magnetic field evolution of weak seed fields with various topologies and stellar feedback mechanisms. we find that strong feedback can drive galactic gas turbulence which gives rise to velocity fields with fast exponential magnetic field growth. the simulations display a high gas fraction and a clumpy morphology with kinematics resembling kolmogorov turbulence and magnetic energy spectra as predicted by kazantsev dynamo theory. magnetic fields reach equipartition with $\mu$g field strength. in a final quiescent phase where feedback is turned off, gas turbulence is reduced and a quadrupole symmetry is observed in the magnetic field. these findings support the theory of rapid magnetic field amplification inside high-redshift galaxies, when the universe was still young. | astrophysical dynamos and the growth of magnetic fields in high-redshift galaxies |
the removal of ism of disk galaxies through ram pressure stripping (rps) has been extensively studied in numerous simulations (see roediger 2009 and references therein). the models show that rps has a significant impact on galaxy evolution (truncation of the ism will lead to a decrease in star formation and a change in galaxy color). nevertheless, the role of magnetic fields (mfs) on the dynamics of the gas in this process has not been sufficiently studied, although the influence of the mfs on the large scale structure is well established. this motivated us to perform a 3d mhd simulation of a disk galaxy with an isothermal, non-self gravitating and magnetized gaseous disk in equilibrium with a galaxy potential (allen & santillán, 1991). we model rps on the galactic disk under the wind-tunnel approximation with the use of the ramses code (teyssier, 2002) in order to understand the effects of mfs in rps. | mhd simulations of ram pressure stripping of disk galaxies |
the fermi gamma-ray space telescope greatly increased the number of known pulsars in the gamma-ray band. it was discovered that pulsars often emit a significant fraction of their spin-down energy in gamma-rays, offering a new powerful diagnostic tool of neutron star magnetic field structure. the modeling of gamma-ray data suggests that in order to interpret the observations one needs to understand the field geometry and the plasma state in the emission region. in recent years, significant progress has been achieved in understanding the magnetospheric structure in the limit of abundant plasma supply, the so-called force-free limit. i used force-free solutions to study the time evolution of the pulsar's obliquity angle. i showed that pulsar rotation and magnetic axes evolve toward alignment as a power-law in time, in broad agreement with observations. despite the success of the force-free approach for understanding the global field geometry, the location of the emission region and radiation mechanism remained a mystery. to address this from first principles, in this thesis i developed kinetic three dimensional simulations of pulsar magnetospheres which include the physics of plasma production and particle acceleration. i found that the state of the magnetosphere crucially depends on the way charges are introduced into the magnetosphere. in simulations with self-consistent pair production i found several solutions with qualitatively different plasma distribution in the open field line region depending on the inclination of the pulsar. in a surprising finding, i discovered that gr effects help to establish efficient particle acceleration and pair production near the surface of low obliquity pulsars. finally, i performed gr kinetic simulations of oblique rotators and showed that pulsar high-energy emission is dominated by synchrotron radiation emitted by particles that are energized by magnetic reconnection in the equatorial current sheet. the computed lightcurves and spectra of high-energy emission reproduce the observed morphology of gamma-ray pulsars. | physics of pulsar magnetospheres |
the common envelope (ce) phase is critical for the formation of close binary stars with at least one compact star. during this phase, a giant star engulfs its companion, which spirals into the giant's envelope. the envelope is ejected and a close binary system results. the ejection mechanism is unknown and predicting the final state of the system is a long-standing problem in binary stellar evolution. this work establishes the moving-mesh hydrodynamics code arepo as a new approach to model ce phases, going beyond the current state of the art. envelopes of giants are stabilized as initial conditions. then, the first ce simulation on a moving mesh demonstrates the occurence of dynamical instabilities. arepo's refinement capabilities allow high resolution around the point masses representing the core of the giant and the companion; this is essential for convergence. the first magnetohydrodynamic ce simulations show strong field amplifications possibly due to the magnetorotational instability. analyzing the transport of angular momentum and energy yields no significant contribution by magnetic fields; main drivers are gravitational torques and shocks. including the ionization state of the gas increases the unbound mass by releasing recombination energy, but still fails to completely eject the envelope during the simulation. | hydrodynamics of the common envelope phase in binary stellar evolution. |
we investigate equilibrium height of a flux rope, and its internal equilibrium in a realistic plasma environment by carrying out numerical simulations of the evolution of systems including a current-carrying flux rope. we find that the equilibrium height of a flux rope is approximately described by a power-law function of the relative strength of the background field. our simulations indicate that the flux rope can escape more easily from a weaker background field. this further confirms that a catastrophe in the magnetic configuration of interest can be triggered by a decrease in strength of the background field. our results show that it takes some time to reach internal equilibrium depending on the initial state of the flux rope. the plasma flow inside the flux rope due to the adjustment for the internal equilibrium of the flux rope remains small and does not last very long when the initial state of the flux rope commences from the stable branch of the theoretical equilibrium curve. this work also confirms the influence of the initial radius of the flux rope in its evolution; the results indicate that a flux rope with a larger initial radius erupts more easily. in addition, by using a realistic plasma environment and a much higher resolution in our simulations, we notice some different characteristics compared to previous studies in forbes. supported by the national natural science foundation of china. | numerical experiments on the evolution in coronal magnetic configurations including a filament in response to the change in the photosphere |
the dynamo action likely present within fully convective regions is explored through global-scale 3-d simulations. these simulations provide a contextual analog for the convective dynamos that are likely operating deep within the interiors of fully convective low mass stars. a logarithmic range of rotation rates is considered, thereby capturing both convection barely sensing the effects of rotation to others in which the coriolis forces are prominent. the vigorous dynamo action realized within all of these turbulent convective cores builds magnetic fields with peak strengths exceeding a megagauss, with the overall magnetic energy (me) in the faster rotators reaching super-equipartition levels compared to the convective kinetic energy (ke). such strong fields are able to coexist with the flows without quenching them through lorentz forces. this state is achieved due to the velocity and magnetic fields being nearly co-aligned, and with peak magnetic islands being somewhat displaced from the fastest flows as the intricate evolution of these mhd structures proceeds. as the rotation rate is increased, the primary force balance shifts from nonlinear advection balancing lorentz forces to a magnetostrophic balance between coriolis and lorentz forces. | the magnetic furnace: examining fully convective dynamos and the influence of rotation |
bow shock formation, magnetic reconnection and plasmoid ejection are thought to be present in most planetary environments. the aim of this work is to show that the presence of this kind of structures in the vicinity of planets is not restrictive to magnetized bodies. the presence of a interplanetary magnetic field carried with the stellar wind is responsible of the formation of a planetary magnetotail, ejection of plasmoids and magnetic reconnection events, even though the planet-obstacle is completely unmagnetized. we study the interaction of this magnetized winds coming from cool ms stars, solar analogues, with non-magnetized terrestrial planets provided of an extended earth-like exosphere thought 2.5d numerical simulations carried out with pluto. in this work, we show a preliminary study of the impact of stellar winds on the evolution and stability of earth-like atmospheres/exospheres, in order to determine the position of the bow shock and its properties, and the position of the x point of magnetic reconnection in the case of non-magnetized rocky planets, in order to determine their emission and possible detection through these structures in the stellar winds. | formation of structures in the wind of sun-like stars by the presence of unmagnetized terrestrial planets |
quantum simulation with ultracold gases provides a highly customizable platform for the study of many-body physics, shedding new light on important physical systems. magnetic fields or, equivalently in the case of a uniform, static field, rotation are especially important parameters for many-body systems including nuclear matter, neutron stars, superfluid helium, and clean conductive samples exhibiting the quantum hall effect. this thesis details the construction of a new experiment to study rapidly-rotating quantum gases, and two experimental results from the new apparatus.the procedure of geometric squeezing utilizes a rotating, elliptical harmonic trap to realize the squeezing hamiltonian for guiding center motion. we first outline the observation of a geometrically squeezed state of a rapidly-rotating bose-einstein condensate entering the lowest landau level. we also measure its hall response, analogous to the e x b drift of charged particles in crossed electric and magnetic fields.we then detail the realization of a geometrically squeezed state of a rapidly rotating, non-interacting atomic fermi gas and the measurement of its hall drift velocity. the fermi gas shrinks down in one direction to a size limited by the width of the highest occupied landau level. in the orthogonal direction it expands exponentially, at a local speed given by the local hall drift velocity. we examine the physics away from the rapidly rotating regime and find that it is well described by the phase space evolution of the fermi surface under the influence of coriolis and centrifugal forces, in direct analogy to the cranking model for rotating nuclei. | geometric squeezing of a degenerate fermi gas |
when solving mysteries about distant astronomical objects, sometimes it pays to take inspiration from sources closer to home. in todays example, strange fluid behavior in the earths oceans combined with a healthy helping of magnetic fields may provide the answer to a long-standing puzzle about the changing composition of red-giant stars.simulated salt fingers in fluids with decreasing rayleigh numbers. the rayleigh number determines whether heat in a system is transferred primarily through diffusion or convection. [fariarehman]a possibility for instabilityred giants undergo a process called dredge-up, during which their outer convective envelopes bring fusion products up to the surface, altering the chemical abundances there. after the dredge-up, surface abundances arent expected to change yet observations show that they continue to evolve long after the dredge-up is complete. what drives this unexpected late-stage mixing in red giants?one solution involves an instability called fingering convection. fingering convection occurs in fluids with vertical gradients in temperature and chemical composition a setup we see everywhere from the interiors of stars to earths oceans. when the equilibrium of such a fluid is perturbed, the temperature diffuses more quickly than the chemical composition as the system seeks to reestablish equilibrium, triggering a runaway effect.what does this look like in practice? take the ocean as an example. the density of saltwater is determined by temperature and salt content, and warm saltwater often lies atop denser, colder water that is less salty. when a bubble of warmer water is pushed into the colder water beneath it, it cools quickly, but the salt is slow to diffuse outward. the cold, salty water is now denser than the water surrounding it, causing it to sink deeper. as this process continues, salt-rich fingers dive downward, eventually depositing the saltier water deep in the ocean.the density of the material in stellar interiors depends on temperature, which diffuses rapidly, and chemical composition, which diffuses slowly the perfect setup for fingering convection.vertical velocity of fluid parcels for three values of the lorentz force coefficient, hb, which increases as the square of the magnetic field strength. click to enlarge. [harrington garaud 2019]chemical mixingpast modeling has shown that fingering convection does help mix chemical species in red-giant stars, but two orders of magnitude too slowly to explain observations. however, these simulations didnt consider what effect magnetic fields which are certainly present in the interiors of these stars have on convection. what happens when we throw magnetic fields into the mix?peter harrington and pascale garaud (university of california, santa cruz) used numerical models to explore the effect of magnetic fields on the rate of convection in stellar interiors. in their simulations, the authors apply a vertical background magnetic field of varying strength and randomly impose small perturbations in the temperature and composition. the perturbations grow as the instability takes hold, forming narrow fingers aligned with the magnetic field.evolution of the compositional nusselt number (a measure of the strength of the vertical compositional transport) over time. simulations with higher magnetic field strengths saturate more rapidly and reach higher rates of vertical transport. [harrington garaud 2019]implications for convectionthe authors find that including magnetic fields in their simulations increases the rate of convection, with stronger magnetic fields leading to more rapid convection. for a purely vertical magnetic field of 0.03 tesla (reasonable for stellar interiors), the convection rate increases by two orders of magnitude enough to resolve the disagreement between theory and observations.magnetized fingering convection should affect more than just red giants; the authors note that main-sequence stars and white dwarfs should also exhibit this behavior, which needs to be accounted for when interpreting observed surface abundances.citationenhanced mixing in magnetized fingering convection, and implications for red giant branch stars, peter z. harrington pascale garaud 2019apjl 870l5. doi:10.3847/2041-8213/aaf812 | solving a stellar abundance problem (with a little help from our oceans) |
star formation informs our understanding of astrophysical processes at many different scales ranging from galaxy formation and evolution and the formation of stellar systems and exoplanets. stars form in highly turbulent and magnetized molecular clouds. recent work has shown that self-gravity, turbulence, magnetic fields, and stellar feedback are all critical to understanding the observed star formation efficiencies. the standard analytic approach has been to assume a lognormal density probability distribution function (pdf) and which accounts for self-gravity and stellar feedback only in an ad-hoc way. in our current work, we investigate a recently proposed model that uses a piecewise lognormal plus power law density pdf and which can account for self-gravity and stellar feedback. to test this model, we compare it to a suite of simulations which alternately include or do not include stellar feedback. we find that the simulation with stellar feedback has a more realistic star formation efficiency than the other simulations and is well described by a piecewise lognormal plus power law density pdf. | investigating the impact of stellar feedback on models of star formation |
high resolution alma co (2-1) observations of the ram pressure stripped galaxies ngc 4402 in the virgo cluster, and ngc 4921 in the coma cluster, show some of the clearest evidence yet of the impact of ram pressure on the molecular ism of galaxies. the leading side of ngc 4402, upon which ram pressure is incident, has a large extraplanar plume of material, also seen in hst observations of dust extinction. molecular gas in the plume, which has a width and length of about 1 x 2 kpc, shows distinct non-circular motions as large as 80 km/s; these are the most extreme velocities in the galaxy and are likely the result of acceleration by ram pressure. we also detect some isolated clouds below the plume near the disk mid-plane. these clouds have velocities consistent with normal galactic rotation, and appear to be unaffected by ram pressure. we also find morphological and kinematic evidence of ram pressure compression of molecular gas in a region of intense star formation on the leading side just inside the plume, at slightly smaller galactocentric radius. it is possible star formation in this region is triggered by compression from the ram pressure. in ngc4921, the largest spiral in the coma cluster, we are able to study the morphological effects of ram pressure on the dense ism in even greater detail, due to the higher relative strength of ram pressure in coma, and the fact that ngc 4921 is oriented nearly face-on. we detect distinct filaments of dense ism in both co (2-1) and hst dust. these filaments can be as long as 2 kpc, but are only about 200 pc wide, and are projected outward into the otherwise gas stripped region on the leading side, indicating that they have resisted stripping and decoupled from the surrounding gas. they also appear to have young star clusters at the head, seen with hst, possibly the result of compressed gas at the tips of the filaments. along with the high density of the filaments, from comparison with structures in mhd simulations, we find evidence that these filaments may be supported against ram pressure by magnetic fields. in ngc 4402 we are able to study the large scale effects of ram pressure on the dense ism, and in ngc 4921 we investigate in higher resolution the evolution of the individual dense ism substructures under the force of compression and stripping. | alma evidence for the direct ram pressure stripping of molecular gas in two cluster galaxies |
bright-rimmed clouds (brcs), which are located at periphery of hii regions, are considered to be potential sites for induced star formation by uv radiation from nearby massive stars. many theorists have developed 2d/3d hydrodynamical models to understand dynamical evolution of such molecular clouds. most simulations, however, did not always include the magnetic field effect, which is of importance in the astrophysics. this is because that there are few observation results examining the magnetic field configuration of brcs in detail. in order to obtain information on magnetic field in and around brcs, we have made near-infrared (jhks) imaging polarimetry toward 24 brcs showing strong interaction with hii region (urquhart et al. 2009). we used the imaging polarimeter sirpol/sirius (fov ~7.7’ x 7.7’) mounted on irsf 1.4 m telescope at the south african astronomical observatory.we found that polarization vectors, i.e., magnetic fields inside the clouds, follow the curved bright rim just behind the bright rim for almost all of the observed brcs. our investigation into the relation between the ambient magnetic field direction and the uv radiation direction suggests a following tendency. in the case that the ambient magnetic field is perpendicular to the direction of incident uv radiation, the clouds are likely to have bright rims with small curvatures. on the other hand, in the case that the ambient field is parallel to the uv radiation, they would have those with larger curvatures. in this presentation, we will present the physical quantities for these brcs (i.e., magnetic field strength, the post shock pressure by the ionization front, etc.) as well as these morphological results. | a study of magnetic fields on bright-rimmed clouds |
giant molecular clouds (gmcs) are primary sites of star formation in the present-day universe. although scarce in numbers, newborn massive stars exert a disproportionate influence on the chemistry, dynamics, and structure of natal clouds and the surrounding interstellar medium (ism). in particular, copious ultraviolet (uv) photons emitted by massive stars create regions of warm ionized gas, h ii regions, which expel and erode cold molecular gas available for star formation. while the destructive effects of uv radiation feedback on birth clouds have long been recognized by theorists, quantitative understanding of how cloud dispersal actually occurs remains incomplete. to assess the impact of uv feedback on the evolution of gmcs and star formation therein, this thesis explores the dynamics of h ii regions and dispersal of gmcs in diverse star-forming environments using analytic calculations as well as numerical simulations. in chapter 2, we investigate the nature of small-scale dynamical instabilities that may occur at ionization fronts (ifs), sharp boundaries of h ii regions separating a warm ionized gas from a cold neutral gas. we carry out a linear stability analysis of a plane-parallel if threaded by magnetic fields parallel to the front. we find that the growth rate of the if instability is linearly proportional to the wavenumber of perturbation as well as the velocity of the upstream cold gas with respect to the if, completely analogous to the darrieus-landau instability of deflagration fronts in terrestrial combustion and thermonuclear supernovae. magnetic fields play a stabilizing role by reducing the density contrast across the if and by exerting magnetic pressure and tension forces. in strongly magnetized ifs, perturbations propagating parallel to the if are completely suppressed by magnetic tension. the if instability works together with the rayleigh-taylor instability for ifs accelerating away from an ionizing source. in chapter 3, we study spherical expansion of dusty h ii regions and related cloud disruption. we develop a semi-analytic model for expansion of dusty h ii regions due to thermal pressure of ionized gas and radiation pressure on dust grains, accounting for non-uniform internal structure as well as the inward gravity from gas and central star cluster. we confirm the semianalytic shell expansion solutions by comparing with numerical simulations. we calculate the minimum star formation efficiency (sfe) required for an expanding h ii region around a star cluster to disrupt the parent cloud. we find that while typical gmcs in normal disk galaxies are disrupted by gas pressure- driven expansion with sfe of less than about 10%, denser, more massive clouds in extreme environments are disrupted by radiation-pressure driven expansion with higher sfe. the disruption timescale is on the order of a free-fall timescale, suggesting that cloud disruption is rapid once sufficiently luminous h ii region is formed. in chapter 4, we turn to computational modeling of star cluster formation in turbulent gmcs. we implement a highly efficient and accurate adaptive ray tracing (art) method for describing propagation of uv radiation produced by multiple point sources in the magnetohydrodynamics code athena. we adopt a recently proposed parallel algorithm for communication of ray information between processors with additional new features that further improve parallel performance. we validate our implementation against a variety of test problems of expanding h ii regions. the results of scaling tests show that the art module has excellent strong and weak scaling up to ∼1000 processors. to demonstrate application of our art implementation, we conduct a simulation of star cluster formation allowing for self-consistent star formation as well as density inhomogeneities resulting from supersonic turbulence. we also make direct comparison of radiation fields computed from the art and the widely-used m1 closure scheme, and find that the latter is unable to accurately describe radiation field near individual point sources. finally, in chapter 5, we present the results of radiation hydrodynamic simulations of star cluster formation and ensuing cloud dispersal by photoionization and radiation pressure for a range of cloud parameters. our parameter study shows that the net sfe increases primarily with the initial surface density of the cloud and that clouds are destroyed within ∼ 2-10myr after the onset of radiation feedback. the importance of radiation pressure (relative to photoionization) increases with the initial surface density. the dominant mass loss mechanism is photoevaporation, although dynamical ejection also contributes significantly in low-mass and/or high surface density clouds. we show that the photoevaporation rate depends only on the ionizing photon rate and cloud size with a scaling relation consistent with theoretical expectations. however, the radial outflow momentum generated by thermal and radiation pressure forces is nearly an order of magnitude lower than the prediction based on spherical expansion of an embedded h ii region, due to escape of radiation and momentum cancellation. | dynamical evolution of giant molecular clouds driven by uv radiation feedback from massive stars |
it has been recognized that non-ideal mhd effects (ohmic diffusion, hall effect, ambipolar diffusion) play crucial roles for the circumstellar disk formation and evolution. ohmic and ambipolar diffusion decouple the gas and the magnetic field, and significantly reduces the magnetic torque in the disk, which enables the formation of the circumstellar disk (e.g., tsukamoto et al. 2015b). they set an upper limit to the magnetic field strength of ~ 0.1 g around the disk (masson et al. 2016). the hall effect notably changes the magnetic torques in the envelope around the disk, and strengthens or weakens the magnetic braking depending on the relative orientation of magnetic field and angular momentum. this suggests that the bimodal evolution of the disk size possibly occurs in the early disk evolutionary phase (tsukamoto et al. 2015a, tsukamoto et al. 2017). hall effect and ambipolar diffusion imprint the possibly observable characteristic velocity structures in the envelope of class 0/i ysos. hall effect forms a counter-rotating envelope around the disk. our simulations show that counter rotating envelope has the size of 100-1000 au and a recent observation actually infers such a structure (takakuwaet al. 2018). ambipolar diffusion causes the significant ion-neutral drift in the envelopes. our simulations show that the drift velocity of ion could become 100-1000 ms-1. | the impact of non-ideal effects on the circumstellar disk evolution and their observational signatures |
we investigate possible driving mechanisms of volcanic activity on large rocky super-earths with masses exceeding four earth masses. due to high pressures in the mantles of these planets, melting in deep mantle layers can be suppressed, even if the energy release due to tidal heating and radioactive decay is substantial in these areas of the mantles. we investigate if a newly identified heating mechanism, namely induction heating by the star"s magnetic field, can drive volcanic activity on these planets due to its unusual heating pattern close to the planet"s surface, which leads to heat production in the very upper part of the mantle. in this region the pressure is not yet high enough to preclude the melt formation. we use a model for induction heating we developed and apply it to the super-earth hd 3167b, which has a mass of approximately seven earth masses. we calculate induction heating in the planet"s interiors assuming an electrical conductivity profile of a hot rocky planet and a moderate stellar magnetic field typical of an old inactive star, which one can expect for hd 3167. then, we use a mantle convection code (chic) to simulate the evolution of volcanic outgassing with time.fig. 1. total outgassing of co2 , co, h2o, and h2 from hd 3167b assuming the magnetic field of the star of 0, 1, and 5 g. induction heating leads to a much earlier onset of volcanism on the planet and increases the outgassing by several tens of bar. the mantle viscosity is 10 times the mantle viscosity of the earth.according to our results, in most cases volcanic outgassing on hd 3167b is not very significant in the absence of induction heating, however, including this heating mechanism changes the picture and leads to a substantial increase in the outgassing from the planet"s mantle. evolution of volcanic outgassing is illustrated in fig. 1. induction heating also leads to a much earlier onset of volcanic activity on this planet. this result shows that induction heating combined with a high surface temperature is capable of driving volcanism on massive super-earths, which has very important observational implications. | interior heating by stellar magnetic fields as a driver of volcanic activity on massive rocky planets |
we present the results of the numerical simulations of the collapse of magnetic rotating protostellar clouds with mass 10 m⊙. the hierarchical structure of the cloud formed at the isothermal stage of the collapse is investigated for various initial ratios of the magnetic energy of the cloud to the modulus of its gravitational energy, εm. it is shown that the size of the quasi-magnetostatic primary disk and the region of efficient magnetic braking increase with εm. in the case εm ≥ 0.6, practically the entire cloud evolves into the state of quasi-magnetostatic equilibrium. a 'dead' zone forms inside the first hydrostatic core and in the area of the outflow formation, where ohmic dissipation and ambipolar diffusion become essential. | influence of the magnetic field on the formation of protostellar disks |
the presence of multiphase gas in the cgm plays an essential role in galactic evolution. specifically, multiphase outflows prevent pristine cosmic gas from accreting onto galaxies, regulating star formation in the disk. yet observations of co-spatial cold and hot outflowing gas are puzzling as the ablation time of the cold gas is much shorter than the acceleration time. previous work showed radiation cooling enables the survival of cold clouds in hot galactic winds and magnetism significantly alters the morphology of the cloud while negligibly affecting its mass accretion. we present a first-look at the dependence on plasma beta for the entrainment of cold magnetised clouds in magnetised hot winds using three-dimensional magnetohydrodynamic simulations. we find significantly smaller cold clouds can survive when magnetic fields are strong than in pure hydrodynamic simulations, as the magnetic energy density progressively gains relevance over its thermal counterpart between simulations. these changes in cloud radius by orders of magnitude contrasts with only small changes in the mass growth of cold gas, and are discussed in terms of magnetic draping and geometry of 104 k gas. our results help explain emission and absorption line measurements of neutral hydrogen in the circumgalactic medium. these results can aid in understanding rapidly outflowing gas in galactic winds, the cold gas fraction in galactic haloes and the problem of 'missing baryons'. progress in future simulations will depend on their ability to resolve significantly smaller scales in order to ensure convergence of cold gas masses for lower betas, or adoption of sub-grid models. | the interplay between radiative cooling and magnetic draping |
current sheets are localized regions of the plasma in which the current density can become singular in the zero-dissipation limit. while the critical role of such structures in mediating fast reconnection and turbulence has been recognized for some time, the universal nature of the plasmoid instability of these structures in high-lundquist-number plasmas and its role has become a subject of significant interest relatively recently. enabled by sophisticated computer simulations and analytical theory and tested by experiments, the plasmoid instability has transformed our understanding of magnetic reconnection and inspired new research in space, astrophysical, and laboratory (including fusion and high-energy-density) plasmas. in this talk, i will review the evolution of our understanding in systems where closed field lines exist (such as in a torus) and those where they may not (such as in stellar coronae and compact astrophysical objects where field lines may be line-tied). even the definition of magnetic reconnection in the latter class of systems remains a contested issue. in turbulent systems characterized by the formation of thin current sheets, the onset of the plasmoid instability is shown to interrupt the inertial range and introduce new power laws for the dissipation range and produce coherent structures that play an essential role in particle acceleration and heating in non-relativistic as well as relativistic regimes. exascale computers, exploited by state-of-the-art codes, hold the promise of breaking new ground in making predictions in plasma regimes that have been hitherto inaccessible. despite the progress made in theory and experiment, many open questions remain. some of them will be discussed. this research is supported by the doe, nasa, and nsf. | james clerk maxwell prize for plasma physics: current sheets and the plasmoid instability: mediators of fast magnetic reconnection and turbulence |
zonal flows in rotating systems have been previously shown to be suppressed by the imposition of a background magnetic field aligned with the direction of rotation. understanding the physics behind the suppression may be important in systems found in astrophysical fluid dynamics, such as stellar interiors. however, the mechanism of suppression has not yet been explained. in the idealized setting of a magnetized beta plane, we provide a theoretical explanation that shows how magnetic fluctuations directly counteract the growth of weak zonal flows. two distinct calculations yield consistent conclusions. the first, which is simpler and more physically transparent, extends the kelvin-orr shearing wave to include magnetic fields and shows that weak, long-wavelength shear flow organizes magnetic fluctuations to absorb energy from the mean flow. the second calculation, based on the quasilinear, statistical ce2 framework, is valid for arbitrary wavelength zonal flow and predicts a self-consistent growth rate of the zonal flow. we find that a background magnetic field suppresses zonal flow if the bare alfvén frequency is comparable to or larger than the bare rossby frequency. however, suppression can occur for even smaller magnetic field if the resistivity is sufficiently small enough to allow sizable magnetic fluctuations. our calculations reproduce the \eta/b02 = const. scaling that describes the boundary of zonation, as found in previous work, and we explicitly link this scaling to the amplitude of magnetic fluctuations. these results could provide a plausible explanation for why the zonal jets in jupiter go as deep as juno has discovered but not any deeper. | magnetic suppression of zonal flows on a beta plane |
observations of young open clusters have revealedabimodaldistribution of the rotation periods of solar-like starsthathas proven difficult to explain under the existing rubric ofmagnetic braking. recent studies suggest that magneticcomplexity can play an important role incontrollingstellar spin-down rates. in this talk i will discuss the missing term representing magnetic morphology in the context of stellar spin-down models. using state-of-the-artmagnetohydrodynamical magnetized wind simulations we have derived analytical expressions representing the magnetic field morphology dependence of mass and angular momentum loss rates. magnetic field complexity provides a natural physical basis for stellar rotation evolution models requiring a rapid transition between weak and strong spin-down modes. | the missing magnetic morphology term in stellar rotation evolution |
the bondi and bondi-hoyle-lytlleton formulas give the order of magnitude steady-accretion rate onto a point mass at rest or moving, respectively, in a uniform density gas in the limit of negligible gas self-gravity. this applies in star-forming clouds where self-gravity is negligible near protostars and new-born stars, but instead of being uniform the gas is supersonically turbulent and threaded by dynamically important (alven mach number ∼ 1) large-scale magnetic fields. to determine the bondi-like accretion rate in these environments, we used the orion2 code to carry out grid-based 3d adaptive mesh refinement (amr) magnetohydrodynamic (mhd) simulations of accretion onto sink particles embedded in an environment of fully developed, magnetized supersonic isothermal turbulence. we evolved the models until the median and mean accretion rates, over particles, became steady. we present a simple semi-analytic model that predicts the median and mean accretion rate from the turbulent properties of the background medium, such as the 3d mach number and rms plasma-β, and show that it is highly consistent with our simulations. numerical codes can use our semi-analytic model as an accurate sub-grid model for accretion in magnetized supersonic isothermal turbulence. | bondi-like accretion in magnetized supersonic isothermal turbulence |
in this work we will show the difference between stellar wind cavities of typical o and b-type stars and our local heliosphere. in particular we will show the affect of different cooling functions on stellar wind/astrosphere evolution when radiative cooling is taken into account. a study on the effect of an ism magnetic field on these simulations will also be presented and compared to the case for the heliosphere. | a comparison between different stellar wind cavities and the heliosphere |
millisecond pulsars (msps) are formed in the accreting binary neutron star systems, while the magnetic fields of msps decays to 10**8 gauss, and spin frequency is spun-up to several milliseconds, after the ns accretes about 0.2 solar masses. the distribution of msp spins imply that gravitational wave (gw) emission limits the maximum spin frequencies of msps. we simulated the msp spin evolution by accretion, while the gw contributions are considered. | the spin frequency distribution of millisecond pulsar and implication of gravitational wave emission |
the theory of the fossil magnetic field of young stars and their accretion disks has been verified by comparing the observational data with the results of numerical simulations of the collapse of protostellar clouds. a new model of dust evaporation has been proposed, in which the parameter is not the thickness of the mantles, but the initial ratio of the core radius to the mantle radius of a dust grain. a semi-analytical description of the evolution of the radius distribution of dust grains was constructed. on its basis, the variations in the relative number density of dust grains, as well as the average values of the radius, cross-sectional area, and mass of dust grains, were calculated. it was shown that at the stage of disappearance of dust cores, these averages reach their maxima, but this does not affect the interaction of dust with gas particles, since the dust becomes scarce. using cloud models w3 (main), ngc 2024, and dr 21 oh1, it has been demonstrated that neglecting dust evaporation underestimates the fossil magnetic field by several times. the possibility of formation of a magnetic compaction at the outer boundary of the zone of strong magnetic field diffusion (dead zone) has been confirmed. it is concluded that a correct calculation of dust evolution, ionization of the medium, and collapse anisotropy makes it possible to match the theoretical and observed magnetic fields of young stars and their accretion disks. | effect of dust evaporation on the fossil magnetic field of young stars and their accretion disks |
numerical studies of two-dimensional β-plane homogeneous decaying magnetohydrodynamic turbulence are presented. s tudy of fundamental properties of such turbulence allows understanding the evolution of various astrophysical objects from the sun and stars to planetary systems, galaxies, and galaxy clusters. q ualitative analysis of e nergy spectra in two-dimensional β-plane mhd allows understanding the cause of inverse cascade in two-dimensional β-plane magnetohydrodynamic turbulence and the formation and dynamics of zonal flows in astrophysical plasma. the equations of two-dimensional magnetohydrodynamics with the coriolis force in the β-plane approximation are used for the qualitative analysis and numerical simulation of processes in plasma astrophysics. the results of numerical simulation of two-dimensional β-plane mhd turbulence with a spatial resolution of 4096 × 4096 with different rossby parameters β are presented. the formation of iroshnikov-kraichnan spectrum is shown in the early stages of evolution of two-dimensional β-plane magnetohydrodynamic turbulence. the direct energy cascade, which is characteristic of the observed iroshnikov-kraichnan spectrum, is presented . the self-similarity of the decay of iroshnikov-kraichnan spectrum is studied. v iolation of the self-similarity of the decay on long time scales and formation of kolmogorov spectrum is discovered. the inverse cascade of kinetic energy, which is characteristic of the observed kolmogorov spectrum, provides the formation of zonal flows. this work was supported by the russian foundation for basic research (project s no. 19-02-00016 and no. 20-32-90001). | inverse and direct energy cascades in two-dimensional β-plane magnetohydrodynamic turbulence |
outflows and winds driven by magnetic fields play an important role in the evolution of protoplanetary disks via the removal of angular momentum and mass. in the context of star and planet formation, it is therefore important to understand how these outflows and winds function under realistic physical conditions. in this contribution, i will present our axisymmetric non-ideal magnetohydrodynamic models of outflows that now include radiative transfer and simplified thermochemistry. the models launch magneto-centrifugal outflows which are moderated by thermochemical effects. for example, we find that irradiation and thermochemical heating are more important than non-ideal magnetic dissipation effects. we also post-process our simulations with chemical modelling and non-lte radiative transfer to search for diagnostic spectral lines that can observationally distinguish between disks with magnetic winds versus those with purely photoevaporative outflows. in particular, we find that spectral line profiles and velocity asymmetries in first moment maps of certain molecular lines could be used to identify outflows from disks, and to distinguish between magnetic and photoevaporative launching mechanisms. | global non-ideal mhd simulations of protoplanetary disks with irradiation, thermochemistry, and synthetic observations |
magnetospheric accretion of circumstellar disc material is a mechanism that affects a variety of astrophysical objects, from young low-mass stellar objects to compact objects, such as white dwarves and neutron stars. within this picture, the accretion flow in their inner circumstellar disc is channeled along the stellar magnetic field lines and ultimately shocks the stellar surface, inducing an exchange of mass, angular momentum and energy that can spin up/down the star itself and modify its intrinsic evolution. we will look at preliminary results of non-ideal 2.5-dimensional simulations of the surroundings of magnetised rotating young stars and explore how different parameters of the problem can affect the accretion dynamics and change their spin evolution and emission properties. | mhd protostar-disc interaction. |
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