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a mechanism for direct acceleration of electrons to extremely high energy ina cold, collisional plasma has been observed. detailed time- and space-resolved measurements (including high-speed movies)on an experiment at caltech reveal the following sequence: a coaxial magnetized plasma gun forms a dense, cold mhd-drivencollimated plasma jet (length 10's of cm, duration 20-40 μ s, 100kiloamp current, t=2 ev). the jet becomes kink unstable when it attains a critical length (thishappens at about 20-30 μ s and the kink e-folding time is about 4 μ s). the kink lateral acceleration provides a large effective gravity (∼ 10^{10} m s^{-2}) that instigates a secondary rayleigh-taylor (rt)instability (the rt e-folding time is ∼1 μ s). the rt instability instigates a magnetic reconnection manifested by abreaking of the jet at the location of the rt instability and interruptionof the electric current associated with the reconnection are wave emission, particle heating,euv emission, and a hard x-ray pulse measured by a plastic scintillator. the x-ray pulse lasts a fraction of a microsecond, has about 6 kev energy,and coincides with the rt instability. because the \symbol{126}1 micronelectron collision mean free path is much shorter than the 10 cm long rtregion, electron acceleration to high energy was not expected. it isproposed that despite the short collision mean free path, a sub-dreicerinductive electric field \symbol{126}ldi/dt caused by the currentdisruption accelerates a small electron subgroup to 6 kev energy withoutthese electrons undergoing collisions and that after being accelerated tohigh energy, these fast electrons suddenly decelerate via collisions andradiate x-rays. although this subgroup constitutes only a small fraction ofall the electrons, because of the high energy per electron, the subgroupcontains substantial kinetic energy. this process could be important insolar corona situations where kinks provide sufficient lateral accelerationto drive secondary rayleigh-taylor instabilities. | hard x-rays observed associated with instability of a collisional mhd-driven collimated plasma jet |
it is shown that dissipation of the diamagnetic current heating the coronal plasma with the classical conductivity in the absence of a longitudinal current can increase significantly due to the cellular transverse microstructure of the flux tube, thereby compensating for intense radiative losses. | possible mechanism of enhancement of diamagnetic current dissipation during the heating of magnetic flux tubes in the solar corona |
the solar wind is driven by ion heating in the corona and inner heliosphere, presumably due to dissipation of a turbulent cascade. this heating is observed to be preferentially perpendicular to the large-scale magnetic field, and to act on heavy ions more than on protons. theoretical and observational studies indicate that small-scale turbulent fluctuations in the solar wind can be characterized by the dispersive properties of kinetic alfvén waves (kaws). however, linear dissipation of kaws is thought to be dominated by the landau resonance. this would lead to primarily parallel heating, and for ion beta < 1 would mostly heat electrons. we point out that highly oblique kaws can exist for frequencies passing through the proton cyclotron frequency, indicating that they can be cyclotron resonant with the entire core proton distribution. we present an illustrative calculation of quasilinear ion diffusion due to a steady, critically-balanced spectrum of kaws, using an analytical two-fluid dispersion relation at plasma beta = 0.1. we find that the landau heating is small, and that the protons are primarily heated in the perpendicular direction by the cyclotron interaction. we also find that alpha particles are preferentially heated by these fluctuations. we will discuss the significance of this mechanism to solar wind generation and heating, and outline future steps toward a realistic physical model. | perpendicular ion heating by cyclotron resonant dissipation of turbulently generated kinetic alfvén waves in the solar wind |
the interpretation of remote sensing data in the context of the underlying coronal heating mechanism is complicated by several factors, including limited instrument sensitivity, nonequilibrium ionization and multiple emitting structures along the line of sight. in this poster, we investigate observable signatures of impulsive heating in active region noaa 1158 through efficient hydrodynamic loop models, magnetic field extrapolations, and advanced forward modeling techniques. to compute synthetic observations for all euv channels of the atmospheric imaging assembly (aia), we calculate the emissivity for the relevant ions using chianti, fold this information through the appropriate instrument response functions, and then map it to the extrapolated field geometry. the timelag, the temporal offset which gives the maximum cross-correlation between two channels, is calculated in each pixel of our synthesized aia observations for several aia channel pairs. we investigate the impact of two different parameters on the simulated timelags: the frequency at which the loops are reheated and the orientation of the active region relative to the line of sight. additionally, using observations of noaa 1158 from aia, we perform our timelag analysis for these same channel pairs. we investigate several methods for making detailed comparisons between our synthetic and observational results in order to constrain the parameter space of possible heating frequencies. finally, our forward-modeling software has been developed to be both modular and generally applicable. we briefly discuss how this framework for synthesizing observations might be useful to the larger solar physics community. | using synthetic and observed timelags to constrain nanoflare heating frequency in active region cores |
the planets and rocky satellites of our solar system show very different evolutions and present-day dynamics. these result from the convective regimes prevailing in their mantle. however, the conditions necessary for convection to generate plate tectonics on earth, episodic resurfacing on venus, heat pipes on io, or no resurfacing on mars, remain strongly debated. the difficulty comes from the complexity of rocks rheology : viscous at high temperature and on long time-scale, brittle at low temperature and short time-scale. this " soft matter " behaviour can be recovered in the laboratory using aqueous colloidal dispersions, whose rheology varies from viscous to elasto-visco-plastic to brittle when their temperature, and/or their water or ionic content, vary. we therefore have investigated the physics of thermal and solutal convection in those systems. they show a diversity of convective regimes, including the ones encountered in rocky mantles; and the conditions under which heat pipes, plates, plumes, subduction and accretion develop self-consistently from convection, can now be systematically studied in a fish-tank. for exemple, we observed that one-sided subduction can be induced by lithosphere buckling, or by the impingement of a hot plume under the lithosphere. scaling laws show that this strong association between plumes and subduction initiation could explain on venus the association of large coronae (created by hot upwelling mantle plumes) with trenches that have topographic signatures similar to the earth's subduction zones. moreover, the same mechanism may have been instrumental in the nucleation and growth of cratons on earth and the onset of continuous plate tectonics. on the other hand, the characteristics of mid-ocean ridges (morphology, segmentation, mechanical instabilities such as transform faults, overlapping spreading centers or microplates) not only depend on spreading velocity but also on the mechanical properties and structure of the axial lithosphere. there again, scaling suggests that a hotter archean earth could have shown similar accretion structures to present-day venus. finally, every experiment always shows an evolution through time, passing through several convective regimes as it cools or dries: similarly a planet will not stay in the same convective regime for ever. these aqueous colloidal dispersions are so far the only laboratory fluids capable of producing the sufficient shear localization to generate self-consistently asymmetric subduction and/or plate tectonics. it is worth noting that such strong and rapid deformation localization is due to their biphasic nature (water and a skeletton of silica nano-particles). this suggests that fluids, and especially partial melting, may be a key-parameter to produce earth's dynamics. | thermal convection in a " soft " planetary mantle : plates, plumes, subduction and accretion, and their interactions. |
the physical mechanism that heats the solar corona is one of the still open science questions in solar physics. one of the proposed mechanism for coronal heating are nanoflares. to investigate their role in coronal heating we study the properties of the small-scale heating events in the solar atmosphere using 3d mhd simulations. we present a method to identify and track these heating events in time which allows us to study their life time, energy, and spectral signatures. these spectal signatures will be compared with available spectrosopic observations obtained with iris and sumer. ultimately, these results will be important for the coordinated scientific exploitation of spice and eui along with other instruments onboard solar orbiter to address the coronal heating problem. | characterisation of small-scale heating events in the solar atmosphere from 3d mhd simulations and their potential role in coronal heating |
thermal instability is a fundamental process of astrophysical plasmas. it is expected to occur whenever the cooling is dominated by radiation and cannot be compensated by heating. this mechanism has been invoked to explain structures at multiple scales in the universe, from the filamentary structure of the ism to the phenomenon of coronal rain in the solar corona. in this work we conduct 2.5-d radiation mhd simulations with the bifrost code of an enhanced activity network in the solar atmosphere. coronal loops are produced self-consistently, mainly through ohmic heating, which is stratified and of a high enough frequency as to produce thermal non-equilibrium. during the cooling and driven by thermal instability, coronal rain is produced along the loops. due to flux freezing, the catastrophic cooling process leading to a rain clump produces a local enhancement of the magnetic field, thereby generating a distinct magnetic strand within the loop up to a few gauss stronger than the ambient corona. the compression downstream leads to an increase in temperature that generates a strongly emitting spicule-like feature in the uv during the rain impact. the stronger magnetic field strength in the rarefied upstream region has a stronger ohmic heating, leading to a filamentary coronal strand with enhanced euv emission. thermal instability and _x0005_non-equilibrium can therefore be associated with localised and intermittent uv brightening in the transition region and chromosphere, as well as contribute to the characteristic filamentary morphology of the solar corona in the euv. an additional effect of a strand with enhanced magnetic field is to serve as a waveguide, which combined with the ohmic heating can act as a seed to sustain the coronal loop and the thermal non-equilibrium cycle. | thermal instability-induced fundamental magnetic strands in coronal loops |
the solar corona, the outermost layer of the sun's atmosphere, is heated to temperatures in excess of one million kelvin, nearly three orders of magnitude greater than the surface of the sun. this so-called "coronal heating problem" has occupied the field of solar astrophysics for over seventy years and is one of the most important open questions in astronomy as a whole. while it is generally agreed that the continually stressed coronal magnetic field plays a role in producing these million-degree temperatures, the exact mechanism responsible for transporting this stored energy to the coronal plasma is yet unknown. nanoflares, small-scale bursts of energy likely resulting from the frequent reconnection of twisted magnetic field lines, have long been proposed as a candidate for heating the non-flaring corona, especially in areas of high magnetic activity. however, a direct detection of heating by nanoflares has proved difficult due to their faint, transient nature and as such, properties of this proposed heating mechanism remain largely unconstrained. in this thesis, i use a hydrodynamic model of the coronal plasma combined with a sophisticated forward modeling approach and machine learning classification techniques to predict signatures of nanoflare heating and compare these predictions to real observational data. in particular, the focus of this work is constraining the frequency with which nanoflares occur on a given magnetic field line in non-flaring active regions. first, i give an introduction to the structure of the solar atmosphere and coronal heating, discuss the hydrodynamics of coronal loops, and provide an overview of the important emission mechanisms in a high-temperature, optically-thin plasma. then, i describe the forward modeling pipeline for predicting time-dependent, multi-wavelength emission over an entire active region. next, i use a hydrodynamic model of a single coronal loop to predict signatures of "very hot" plasma produced by nanoflares. i find that several effects, including flux limiting, nonequilibrium ionization, and nanoflare duration, are likely to affect the observability of this direct signature of nanoflare heating. then, i use the forward modeling code described above to simulate time-dependent, multi-wavelength aia emission from active region noaa 1158 for a range of nanoflare frequencies and find that signatures of the heating frequency persist in multiple observable quantities. finally, i use these predicted diagnostics to train a random forest classifier and apply this model to real aia observations of noaa 1158. in doing so, i am able to map the heating frequency, pixel by pixel, across the entire active region. altogether, this thesis represents a critical step in systematically constraining the frequency of energy deposition in active regions. a novel component of this thesis is the development of a modular forward modeling pipeline, written in the python programming language, that builds a "magnetic skeleton" from a three-dimensional field extrapolation, configures thousands of field-aligned hydrodynamic loop models, and computes arbitrary line-of-sight projections of the time-dependent, three-dimensional active region emission. the code is flexible and scalable and is openly-developed such that it may be used and improved by the larger solar physics community. another novel component of this thesis is the use of machine learning to compare real observations and model results. by training a random forest classifier on predicted diagnostics, i am able to systematically and quantitatively assess observations in the context of multiple diagnostics in order to make an accurate prediction of the properties of the heating. | diagnosing the frequency of energy deposition in the magnetically-closed solar corona |
motivated by recent rhessi observations that point to the existence of a mechanism that confines electrons to the coronal parts of flare loops more effectively than coulomb collisions, we consider the impact of pitch-angle scattering off turbulent magnetic fluctuations on the parallel transport of electrons in flaring coronal loops. it is shown that the presence of such a scattering mechanism in addition to coulomb collisional scattering can significantly reduce the parallel thermal and electrical conductivities relative to their collisional values. we provide illustrative expressions for the resulting thermoelectric coefficients that relate the thermal flux and electrical current density to the temperature gradient and the applied electric field. we then evaluate the effect of these modified transport coefficients on several items of interest to the modeling of flares, including: the peak flare coronal temperature that can be attained, the post-impulsive-phase cooling time of heated coronal plasma, and the importance of the beam-neutralizing return current on both ambient heating and the energy loss rate of accelerated electrons. we also discuss the ways in which anomalous transport processes have an impact on the required overall energy content of accelerated electrons in solar flares. | suppression of parallel transport in turbulent magnetized plasmas and its impact on non-thermal and thermal aspects of solar flares |
solar coronal loops are the building blocks of the solar corona. they can be observed in x-ray and extreme ultraviolet (euv), revealing the high plasma temperature (1 mk - 10 mk) of the corona. however, it is still a matter of debate how the magnetic energy is dissipated to heat the coronal plasma. in order to properly differentiate between heating mechanisms, the location and frequency of the energy deposition, in particular, must be properly constrained. we know from numerical simulations that a heating that is quasi-steady and concentrated toward the loop footpoints can lead to a state of thermal non-equilibrium. this physical process can lead to the formation of cool material in the hot solar corona, in the form of coronal condensations (t ~ 0,1 mk - 0,01 mk). the discovery of ubiquitous long-period euv pulsations in the solar corona and in particular in solar coronal loops, with soho and then sdo, have brought a renewed attention on the importance of thermal non-equilibrium in the solar atmosphere. i will give an overview of the latest developments, on both observations and modelling of long-period euv pulsations in coronal loops and their relationship with coronal rain events. i will show in particular that they are two aspects of the same phenomenon and why understanding the characteristics of thermal non-equilibrium cycles is essential to understand the circulation of mass and energy in the corona. | long-period euv pulsations & coronal rain: multi-scale manifestations of thermal non-equilibrium in the solar atmosphere |
the entire solar system including earth is enveloped in a region of space where the suns magnetic field dominates, this region is called the heliosphere. due to this position in the heliosphere, a strong coupling exists between the sun and our planet. the sun continuously ejects particles, the solar wind, which is composed mainly of protons, electrons as well as some helium and heavier elements. these high energetic particles then hit the earth and are partly deflected by the earths magnetosphere (the region around earth governed by the geomagnetic field). depending on the strength of the solar wind hitting our planet, the magnetosphere is disturbed and perturbations can be seen down to the lower atmosphere. the upper atmosphere is affected by short wave-length solar radiation that ionise the neutral atoms, this region is referred to as the ionosphere. in the ionosphere, some of the heavier ion populations, such as o+, are heated and accelerated through several processes and flow upward. in the polar regions (polar cap, cusp and plasma mantle) these mechanisms are particularly efficient and when the ions have enough energy to escape the earths gravity, they move outward along open magnetic field lines. these outflowing ions may be lost into interplanetary space. another aspect that influences o+ ions are disturbed magnetospheric conditions. they correlate with solar active periods, such as coronal holes or the development of solar active regions. from these regions, strong ejections emerge, called coronal mass ejections (cmes). when these cmes interact with earth, they produce a compression of the magnetosphere as well as reconnection between the terrestrial magnetic field lines and the interplanetary magnetic field (imf) lines, which very often leads to geomagnetic storms. the energy in the solar wind as well as the coupling to the magnetosphere increase during geomagnetic storms and therefore the energy input to the ionosphere. this in turn increases the o+ outflow. in addition, solar wind parameter variations such as the dynamic pressure or the imf also influence the outflowing ions. our observations are made with the cluster mission, a constellation of 4 satellites flying around earth in the key magnetospheric regions where we usually observe ion outflow. in this thesis, we estimated o+ outflow for different solar wind parameters (imf, solar wind dynamic pressure) and extreme ultraviolet radiations (euv) as well as for extreme geomagnetic storms. we found that o+ outflow increases exponentially with enhanced geomagnetic activity (kp index) and about 2 orders of magnitude during extreme geomagnetic storms compared to quiet conditions. furthermore, our investigations on solar wind parameters showed that o+ outflow increases for high dynamic pressure and southward imf, as well as with euv radiations. finally, the fate of o+ ions from the plasma mantle were studied based on cluster observations and simulations. these results confirm that ions observed in the plasma mantle have sufficient energy to be lost in the solar wind. | how does o+ outflow vary with solar wind conditions? |
the mechanisms that heat and accelerate the fast and slow wind have not yet been conclusively identified, and their understanding is one of the major science goals of the solar orbiter (so) and solar probe plus (spp) missions. helium abundance and properties in the solar wind are critical tracers for both processes so that understanding them is key towards gaining insight in the solar wind phenomenon, and being able to model it and predict its properties. we present a generalization of the recently developed global solar corona and inner heliosphere model with low-frequency alfven wave turbulence [van der holst et al. (2014)] to include alpha-particle dynamics. this new multi-fluid model uses the stochastic heating mechanism to partition the turbulence dissipation into coronal heating of the electrons and ions. the momentum and energy exchange rates due to coulomb collisions are accounted for. we discuss the feasibility for alfven wave turbulence to simultaneously address the coronal heating and proton-alpha particle differential streaming. | global multi-fluid solar corona model with alfven wave turbulence |
understanding the energy transfer between different scales is a major challenge in collisionless plasmas. kelvin-helmholtz instability (khi) is a universal shear flow instability taking place in many plasma systems such as magnetospheres of magnetized and non-magnetized planets, solar corona, coronal mass ejectas, astrophysical accretion disks, as well as in laboratory plasmas. during recent years it has been shown that khi can produce significant plasma transport via reconnection in the vortices as well as via secondary mechanisms such as kinetic wave activity and diffusion through thin boundaries created by the khi. likely sssociated with this transport, statistical studies have shown that cold-component ions are about 30-40 percent more abundant and hotter on the dawn side of the earth's plasma sheet. in this talk we discuss the origin of the asymmetric evolution of the khi and associated plasma heating via various physical mechanisms associated with the khi such as kinetic plasma waves and magnetic reconnection, and by using data from esas cluster mission, nasas themis and mms missions, as well as high-resolution numerical simulations that can address the generation of unstable ion velocity distribution functions. kelvin-helmholtz instability and associated non-adiabatic heating mechanisms play also an important if not a dominant role in jovian magnetospheric system. | on the cross-scale coupling from fluid-scale kelvin-helmholtz waves into ion and electron scales and associated plasma heating |
the parker solar probe and solar orbiter mark the start of a new era for research on the solar corona and the inner heliosphere. to take full advantage of the datasets that will be produced by these unprecedented missions in order to test the theories proposed to explain coronal heating, the origins of the solar winds or high-energy particles we must pursue the development of advanced data analysis techniques and numerical models. the solar atmosphere is a dynamic system where energy deposition, exchange, and loss mechanisms vary greatly in strength and nature with radial distance. i will briefly review current effort to create new models of the coronal and solar wind plasma that attempt to couple the different layers of the solar atmosphere, include major and minor species, account for anisotropies of the distribution functions, the formation and effects of non-thermal particle populations and dynamic solar magnetic fields. i will also review recent developments of tools that will help the analysis of multi-instrument and multi-spacecraft data. | a new generation of numerical models and tools for parker solar probe and solar orbiter |
the solar wind is a unique laboratory to study the turbulent processes occurring in a collisionless plasma with high reynolds numbers. a turbulent cascade—the process that transfers the free energy contained within the large scale fluctuations into the smaller ones—is believed to be one of the most important mechanisms responsible for heating of the solar corona and the solar wind. the paper analyzes power spectra of solar wind ion bulk velocity and magnetic field fluctuations that are computed with a time resolution of 32 ms in the frequency range around the break between inertial and kinetic scales. the study uses measurements of the bright monitor of the solar wind (bmsw) on board the spektr-r spacecraft that are limited to the 600 km/s bulk speed. since the spektr-r magnetometer is not in operation, the bmsw measurements are complemented with 10 hz magnetic field observations from the wind spacecraft propagated to the spektr-r location. the statistics based on more than 42.000 individual spectra show that: (1) the spectra of both quantities can be fitted by two power-law segments; (2) the median slopes of parallel and perpendicular fluctuation components are different; (3) the break between mhd and kinetic scales as well as the slopes are mainly controlled by the ion beta parameter. these experimental results are compared with high-resolution 2d hybrid particle-in-cell simulations, where the electrons are considered to be a massless, charge-neutralizing fluid with a constant temperature, whereas the ions are described as macroparticles representing portions of their distribution function. in spite of several limitations (lack of the electron kinetics, lower dimensionality), the model results agree well with the experimental findings. finally, we discuss differences between both observations and simulations in relation to the role of important physical parameters in determining the properties of the turbulent cascade. | power spectral density of magnetic field and ion velocity fluctuations from inertial to kinetic range |
solar flares are dramatic events in which magnetic reconnection in the corona leads to heating of plasma to tens of mk and acceleration of particles to high energies. they also centrally involve transport between the corona (where the magnetic reconnection occurs) and the lower solar atmosphere (where most energy is radiated from). there is substantial evidence for the presence of alfvénic waves/turbulence in solar flares, for example, in the ubiquitous nonthermal broadening of flare spectral lines. the physical role that alfvénic waves have in the flare has attracted considerable attention, especially since 2007-2010. this article reviews what spectroscopic observations reveal about the properties and importance of alfvénic waves, turbulence and transport in solar flares; mechanisms for wave excitation by magnetic reconnection at high lundquist numbers and braking of the sunward reconnection jet; and models of wave energy transport to the lower atmosphere and the resulting heating and dynamics. the article finishes with discussion of the outlook for new progress. | alfvén waves in solar flares |
state-of-the-art mhd calculations reveal acceptable agreement with observational data for the height profile of the temperature $t(h)$ in the transition region of solar corona. simultaneously, the velocity of the solar wind $u(h)$ has also been calculated. the developed method gives the possibility at given frequency dependent spectral density of alfvén waves (aw) coming from chromosphere $\mathcal{w}(\omega)$ to calculate both height profiles $t(h)$ and $u(h)$. in agreement with the concepts of the self-induced opacity of plasma with respect of aw, the narrow width $\lambda$ of the transition region is determined by the fast temperature increase of the viscosity $\eta(t,b)$. after more than 70 years of development of solar physics, the alfvén hypothesis of heating of the solar corona by aw has remained without alternatives; none of other mechanisms can explain $ab$ $initio$ the value of $\lambda$. the performed mhd analysis explains the height dependence of the non-thermal broadening of the chromospheric spectral lines and predicts angular dependence of this broadening with respect of position in solar disc. one can expect significant impact of mhd analysis in the interpretation of the long expected data from parker solar probe. | the theory of heating of the solar corona and launching of the solar wind by alfvén waves |
the steady, supersonic outflow from the sun we call the solar wind was first posited in the 1950s and initial theories rightly linked the acceleration of the wind to the existence of the million-degree solar corona. still today, the wind acceleration mechanisms and the coronal heating processes remain unsolved challenges in solar physics. in this work, i seek to answer a portion of the mystery by focusing on a particular acceleration process: alfven waves launched by the motion of magnetic field footpoints in the photosphere. the entire corona is threaded with magnetic loops and flux tubes that open up into the heliosphere. i have sought a better understanding of the role these magnetic fields play in determining solar wind properties in open flux tubes. after an introduction of relevant material, i discuss my parameter study of magnetic field profiles and the statistical understanding we can draw from the resulting steady-state wind. in the chapter following, i describe how i extended this work to consider time dependence in the turbulent heating by alfven waves in three dimensional simulations. the bursty nature of this heating led to a natural next step that expands my work to include not only the theoretical, but also a project to analyze observations of small network jets in the chromosphere and transition region, and the underlying photospheric magnetic field that forms thresholds in jet production. in summary, this work takes a broad look at the extent to which alfven-wave-driven turbulent heating can explain measured solar wind properties and other observed phenomena. | magnetic influences on the solar wind (ph.d. dissertation) |
two of the three gases that display isenthalpic joule-thomson (j-t) warming under laboratory conditions are hydrogen and helium, the main constituents of the solar plasma, but the temperatures that are attained by this route are at most a few hundred k. increases in ion temperature by several orders of magnitude are claimed for hydrogen plasmas subject to expansion into a vacuum; modest increases are reported for the shortlived tests of this effect that have been carried out in space in the wakes of artificial satellites and of the moon. attempts to calculate the j-t coefficient at very high temperatures using equations of state and thermodynamics remain very preliminary. the potential contribution of plasma expansion to heating of the solar corona must therefore be assessed empirically, but this is consistent with how the j-t effect was first identified. the sunspot record, euv measurements by the eve instrument on the sdo satellite, and solar wind fluctuations documented by the ace satellite indicate broadly coherent periodicity from the photosphere to the outer corona consistent with a non-pulsatory heating process. it comprises three successive stages characterised by induction, the j-t mechanism, and plasma expansion. astronomical data may therefore be used to derive rather than to test an extension of the j-t effect which could help to explain heating in other solar system bodies and other stellar coronae. | the contribution of the joule-thomson effect to solar coronal heating |
this article describes the life and work of french astrophysicist evry schatzman (1920-2010). he was a pioneer in the study of white dwarfs during the 1940s and was one of the proponents of the wave heating theory of the solar corona. he made important contributions to the fields of internal stellar structure, novae, mechanisms of acceleration of cosmic rays, the role of turbulent diffusion in stellar evolution and its consequences for the lithium abundance, and the rate of solar neutrinos. schatzman is mostly recognized as the creator of the french school of theoretical astrophysics. although he was not the first theoretician of astrophysics in his country, he was the first to have felt the need for a rapid development of this subject in france, and the first to teach it and to guide the path of many young researchers. many of them became involved, and some leaders, in space science. | evry leon schatzman |
the physics behind the heating of the solar corona and the acceleration of the fast solar wind from coronal holes (predominantly open field regions of the solar corona) is not well understood. recent observations of large-amplitude counter-propagating alfvén waves at the base of coronal holes suggest that the outward and the inward waves interact nonlinearly to generate turbulence which heats and accelerates the plasma. however, the mechanism of generating inward alfvén waves is yet to be fully established. most theories within the mhd framework invoke partial reflection of outward alfvén waves from gradients in the alfvén speed to explain the inward waves. however, to date, no experiment has reported the detection of a reflected alfvén wave in an experimental arrangement relevant to coronal holes. we have done new experiments to detect a reflected wave from an alfvén speed gradient under conditions scaled to match coronal holes. the experiments were conducted in the large plasma device at the university of california, los angeles. our results show that the reflected alfvén wave amplitude increases as the ratio of the wavelength to the gradient scale length increases. the results of the experiments are presented. | study of alfvén wave reflection to address the solar coronal heating problem |
a likely candidate mechanism to heat the solar corona and solar wind is low-frequency "alfvénic" turbulence sourced by magnetic fluctuations near the solar surface. depending on its properties, such turbulence can heat different species via different mechanisms, and the comparison of theoretical predictions to observed temperatures, wind speeds, anisotropies, and their variation with heliocentric radius provides a sensitive test of this physics. here we explore the importance of normalized cross helicity, or imbalance, for controlling solar-wind heating, since it is a key parameter of magnetized turbulence and varies systematically with wind speed and radius. based on a hybrid-kinetic simulation in which the forcing's imbalance decreases with time-a crude model for a plasma parcel entrained in the outflowing wind-we demonstrate how significant changes to the turbulence and heating result from the "helicity barrier" effect. its dissolution at low imbalance causes its characteristic features-strong perpendicular ion heating with a steep "transition-range" drop in electromagnetic fluctuation spectra-to disappear, driving a larger fraction of the energy into electrons and parallel ion heat, and halting the emission of ion-scale waves. these predictions seem to agree with a diverse array of solar-wind observations, offering to explain a variety of complex correlations and features within a single theoretical framework. | electron-ion heating partition in imbalanced solar-wind turbulence |
context. the corona of the sun is the part of the solar atmosphere with temperatures of over one million kelvin, which needs to be heated internally in order to exist. this heating mechanism remains a mystery; we see large magnetically active regions in the photosphere lead to strong extreme uv (euv) emission in the corona. on much smaller scales (on the order of tens of mm), there are bipolar and multipolar regions that can be associated with evenly sized coronal bright points (cbps).aims: our aim was to study the properties of cbps in a statistical sense and to use continuous data from the sdo spacecraft, which makes it possible to track cbps over their whole lifetime. furthermore, we tested various rotation-speed profiles for cbps in order to find out if the lower corona is co-rotating with the photosphere. then we compiled a database with about 346 cbps together with information of their sizes, shapes, appearance and disappearance, and their visibility in the euv channels of the aia instrument. we want to verify our methods with similar previous studies.methods: we used the high-cadence data of the largest continuous sdo observation interval in 2015 to employ an automated tracking algorithm for cbps. some of the information (e.g., the total lifetime, the characteristic shape, and the magnetic polarities below the cbps) still requires human interaction.results: in this work we present statistics on fundamental properties of cbps along with some comparison tables that relate, for example, the cbp lifetime with their shape. cbps that are visible in all aia channels simultaneously seem to be brighter in total and also have a stronger heating, and hence a higher total radiation flux. we compared the euv emission visibility in different aia channels with the cbp's shape and lifetime. from the tracking algorithm we confirm a strict co-rotation of the cbps with the photospheric differential rotation.conclusions: the tracked cbps have a typical lifetime of about 1-6 h, while the hottest and brightest ones seem to exist for significantly longer time, up to 24 h. furthermore, the merging of two cbps seems not to have an influence on the overall size of the persisting cbp. finally, fainter and cooler cbps tend to have only weaker magnetic polarities, which clearly supports a coronal bright point heating mechanism based on magnetic energy dissipation. movies are available at https://www.aanda.org. | coronal bright point statistics. i. lifetime, shape, and coronal co-rotation |
we present new x-ray observations of the optically-obscured protostar hl tau and the intermediate mass herbig be star hd 100546. both objects are surrounded by spectacular disks showing complex morphology including rings and gaps that may have been sculpted by protoplanets. hl tau was detected as a variable hard x-ray source by chandra, typical of late-type magnetically-active coronal sources. no extended x-ray emission was seen along the hl tau jet, or along the jet of the t tauri binary system xz tau located 23 arcsecs to its east. in contrast, hd 100546 was detected by xmm-newton as a soft x-ray source with no short-term (<1 day) variability. its x-ray properties are remarkably similar to the herbig stars ab aur and hd 163296, strongly suggesting that their x-ray emission arises from the same mechanism and is intrinsic to the herbig stars themselves, not due to unseen late-type companions. we consider several possible emission mechanisms and conclude that the x-ray properties of hd 100546 are consistent with an accretion shock origin, but higher resolution grating spectra capable of providing information on individual emission lines are needed to more reliably distinguish between accretion shocks and alternatives. we show that x-ray ionization and heating are mainly confined to the upper disk layers in both hl tau and hd 100546, and any exoplanets near the midplane at distances >1 au are well-shielded from x-rays produced by the central star. | x-ray emission and disk irradiation of hl tau and hd 100546 |
the solar atmosphere shows anomalous variation in temperature, starting from the 5500 k photosphere to the million-degree kelvin corona. the corona itself expands into the interstellar medium as the free streaming solar wind, which modulates and impacts the near-earth space weather. the precise source regions of different structures in the solar wind, their formation height, and the heating of the solar atmosphere are inextricably linked and unsolved problems in astrophysics. observations suggest correlations between coronal holes (chs), which are cool, intensity deficit structures in the solar corona, with structures in the solar wind. observations also suggest the local plasma heating in the corona through power-law distributed impulsive events. in this thesis, we use narrowband photometric, spectroscopic, and disc-integrated emission of the solar atmosphere ranging from near ultraviolet to x-rays along with in-situ solar wind measurements to understand (i). the source regions of the solar wind, (ii). the underlying mechanism of solar coronal heating, and (iii). the differentiation in dynamics of chs with the background quiet sun (qs) regions, which do not show any significant signature of the solar wind. we leverage machine learning and numerical modeling tools to develop solar wind forecasting codes using interpretable ai, inversion codes to infer the properties of impulsive events and to understand the differences in the thermodynamics of chs and qs regions. we finally present a unified scenario of solar wind emergence and heating in the solar atmosphere and discuss the implications of inferences from this thesis. | heating and dynamics of the solar atmosphere |
the magnetic network extending from the photosphere (solar radius ≃ r⊙) to the lower corona ( r_⊙ +10 mm) plays an important role in the heating mechanisms of the solar atmosphere. here we develop further the models of the authors with realistic open magnetic flux tubes, in order to model more complicated configurations. closed magnetic loops and combinations of closed and open magnetic flux tubes are modelled. these are embedded within a stratified atmosphere, derived from observationally motivated semi-empirical and data-driven models subject to solar gravity and capable of spanning from the photosphere up into the chromosphere and lower corona. constructing a magnetic field comprising self-similar magnetic flux tubes, an analytic solution for the kinetic pressure and plasma density is derived. combining flux tubes of opposite polarity, it is possible to create a steady background magnetic field configuration, modelling a solar atmosphere exhibiting realistic stratification. the result can be applied to the solar and heliospheric observatory michelson doppler imager (soho/mdi), solar dynamics observatory helioseismic and magnetic imager (sdo/hmi) and other magnetograms from the solar surface, for which photospheric motions can be simulated to explore the mechanism of energy transport. we demonstrate this powerful and versatile method with an application to hmi data. | modelling 3d magnetic networks in a realistic solar atmosphere |
data obtained in the framework of the interball-tail probe (1995-2000) and rhessi (from 2002 to the present) projects have revealed variations in the x-ray intensity of the solar corona in the photon energy range of 2-15 kev during the period of the quiet sun. previously, a hypothesis was proposed that this phenomenon could be associated with the effect of coronal heating. in the present study, a new mechanism of coronal plasma heating is proposed on the basis of the experimental data and the quantum theory of photon pairs that are produced from vacuum in the course of the universe's expansion. a similar mechanism based on the splitting of photon pairs in the interplanetary and intergalactic space is also proposed to explain the observed microwave background radiation. | mechanism of heating the solar corona in the splitting of massive photon pairs |
it is well established that transverse mhd waves are ubiquitous in the solar corona. one of the possible mechanisms for heating both open (e.g., coronal holes) and closed (e.g., coronal loops) magnetic field regions of the solar corona is mhd wave-driven turbulence. in this work, we study the variation of the filling factor of overdense structures in the solar corona due to the generation of transverse mhd wave-driven turbulence. using 3d mhd simulations, we estimate the density filling factor of an open magnetic structure by calculating the fraction of the volume occupied by the overdense plasma structures relative to the entire volume of the simulation domain. next, we perform forward modeling and generate synthetic spectra of fe xiii 10749 å and 10800 å density-sensitive line pairs using fomo. using the synthetic images, we again estimate the filling factors. the estimated filling factors obtained from both methods are in reasonable agreement. also, our results match fairly well with the observations of filling factors in coronal holes and loops. our results show that the generation of turbulence increases the filling factor of the solar corona. | how does transverse mhd wave-driven turbulence influence the density filling factor in the solar corona? |
in the context of the solar atmosphere, we re-examine the role of neutral and ionized species in dissipating the ordered energy of intermediate-mode mhd waves into heat. we solve conservation equations for the hydrodynamics and for hydrogen and helium ionization stages, along closed tubes of magnetic field. first, we examine the evolution of coronal plasma under conditions where coronal heating has abruptly ceased. we find that cool (<105k) structures are formed lasting for several hours. mhd waves of modest amplitude can heat the plasma through ion-neutral collisions with sufficient energy rates to support the plasma against gravity. then we examine a calculation starting from a cooler atmosphere. the calculation shows that warm (>104) k long (> several mm) tubes of plasma arise by the same mechanism. we speculate on the relevance of these solutions to observe properties of the sun and similar stars whose atmospheres are permeated with emerging magnetic fields and stirred by convection. perhaps this elementary process might help to explain the presence of 'cool loops' in the solar transition region and the production of broad components of transition region lines. the production of ionized hydrogen from such a simple and perhaps inevitable mechanism may be an important step towards finding the more complex mechanisms needed to generate coronae with temperatures in excess of 106k, independent of a star's metallicity. | inevitable consequences of ion-neutral damping of intermediate mhd waves in sun-like stars |
nearly all low-mass stars are believed to exhibit subsurface convection, some level of magnetic dynamo activity, and radiative emission from chromospheric (t = 10,000 k) and coronal (t > 1 million k) layers above their photospheres. linsky et al. (2020) highlighted the usefulness of comparing x-ray and h i lyman alpha flux trends from cool stars as a way of constraining how these atmospheres are produced and maintained. here, we seek to simulate chromospheric and coronal heating for a broad set of f, g, k, and m stars and investigate whether the observed trends in x-ray and lyman alpha emission can be reproduced. we also produce a new conversion of the sun's observed time-variable x-ray emission (from the goes 1-8 angstrom band) into the lower-energy rosat/pspc band more commonly used in studies of cool-star x-rays. because we have not yet conclusively solved our sun's own chromospheric and coronal heating problems, we parameterize the rate of simulated energy deposition using known expressions for the maximum available poynting flux and efficiencies of various proposed mechanisms (see, e.g., cranmer & winebarger 2019). a key input parameter turns out to be the driving velocity at the photospheric base of the coronal magnetic field lines. straightforward extrapolation from mixing-length convection theory drastically underestimates the velocity required to explain the emission from m dwarfs. however, empirical trends from spectroscopically inferred microturbulence velocities seem to do a better job, and we will explore why this may be an important clue to the underlying physics. lastly, we note that understanding the origins of x-ray and uv emission from cool stars will also help us better predict the present-day properties and long-term evolution of exoplanet atmospheres. | chromospheric and coronal heating in cool stars: constraints on physical processes from x-ray and lyman alpha observations |
most m dwarf flares exhibit a strong response in the x-ray and near-ultraviolet (nuv), which is in line with the neupert effect and the standard heating scenario for less energetic solar flares. however, some flares produce only bright x-rays (quasi-neupert) and others only a bright nuv response (non-neupert). our fundamental understanding of stellar flares and particle acceleration beyond the sun is hampered by the lack of multiwavelength observational data that can robustly constrain the physics in state-of-the-art models. a large xmm- newton flare campaign on au mic was executed over 7 days to determine the characteristics and heating origins of neupert versus non-neupert flares. the 8x increase in xmm coverage and first contemporaneous multi-wavelength study of the system resulted in ~40 x-ray flares and ~18 nuv flares at a time-resolution that is 100x better than before. the timing, amplitudes, and spectral characteristics of the x-ray, nuv, and optical radiation in these flares will determine whether heating differences are determined by variations in the low-energy cutoff of electron beams or if additional heating mechanisms (proton beams and magnetic trapping) are required to self-consistently explain the multiwavelength behaviors in dme flares. these new multi-wavelength flare observations challenge state-of-the-art radiative-hydrodynamic flare models in novel ways. we propose to analyze the xmm-newton and swift x-ray light curves and spectra to constrain the evolution of the flaring corona. we will calculate coronal temperatures, densities, emission measures, and velocities from the x-ray data and compare the detailed timing of the t > 10 mk emissions from the upper atmosphere to the white-light continuum radiation from the lower atmosphere. for the first time, nuv/v-band continuum ratios can be analyzed at high-time resolution in several flares. this ratio constrains the heating distribution in the deep stellar atmosphere where electron beams (or other heating sources) deposit their energy in the impulsive phase. the electron beam parameters will be constrained with unique vla and atca data in the optically thin part of the gyrosynchrotron spectrum. we will calculate white-light footpoint areas from the uvw2 (and ground-based data) to combine with the radio data for constraints on radiative-hydrodynamic flare models with the radyn code. we will use a precomputed grid of radiative-hydrodynamic model predictions for physical analysis of the x-ray spectra in order to explain the origin of the neupert effect in stellar flares. for the first time, we will analyze the observations in the context of a phenomenological model of flare loop development that is directly guided by solar flare observations. particle acceleration is thought to be important in many astrophysical systems, but its physics is only loosely tied to observables (e.g., spectra) of macroscopic systems. in most solar flares, relatively few electrons are accelerated to e > 100 kev. the neupert effect in high-energy m dwarf flares informs the greater importance of these particles in converting magnetic into thermal energy in stronger magnetic field environments than usually attained in the sun. the white-light flares of au mic and many other m dwarf flare stars have been observed with tess or kepler/k2; the panchromatic constraints on au mic's flares will be invaluable for comprehensive, accurate extrapolations of optical observations to the uv and x-ray regimes. a better understanding of stellar flares at wavelengths in the uvw2 band is important for robust identifications of kilonovae, which produce optical continuum colors that are nearly identical to m dwarf flares. | high-time resolution physics in stellar flares from a 7-day multi-wavelength campaign on au mic |
interplanetary coronal mass ejections (icmes) and high speed streams (hsss) are noteworthy drivers of disturbance of interplanetary space. interaction between them can cause several phenomena, such as; generation of waves, enhanced geo-effectiveness, particle acceleration, etc. however, how does thermodynamic properties vary during the icme-hss interaction remain an open problem. in this study, we investigated the polytropic behavior of plasma during an icme-hss interaction observed by stereo and wind spacecraft. we find that the icme observed by the stereo-a has polytropic index $\alpha = 1.0$, i.e., exhibiting isothermal process. moreover, wind spacecraft observed the hss region, non-interacting icme, and icme-hss interaction region. during each regions we found $\alpha$=1.8, $\alpha$=0.7, and $\alpha$=2.5, respectively. it implies that the hss region exhibits a nearly adiabatic behaviour, icme region is closely isothermal, and the icme-hss interaction region exhibits super-adiabatic behaviour. the insufficient expansion of the icme due to the interaction with hss triggers the system for heating and cooling mechanisms which dependent on the degrees of freedom of plasma components. | distinct polytropic behavior of plasma during icme-hss interaction |
m dwarf flares observed by the transiting exoplanet survey satellite (tess) sometimes exhibit a peak-bump light-curve morphology, characterized by a secondary, gradual peak well after the main, impulsive peak. a similar late phase is frequently detected in solar flares observed in the extreme ultraviolet from longer hot coronal loops distinct from the impulsive flare structures. white-light emission has also been observed in off-limb solar flare loops. here, we perform a suite of one-dimensional hydrodynamic loop simulations for m dwarf flares inspired by these solar examples. our results suggest that coronal plasma condensation following impulsive flare heating can yield high electron number density in the loop, allowing it to contribute significantly to the optical light curves via free-bound and free-free emission mechanisms. our simulation results qualitatively agree with tess observations: the longer evolutionary timescale of coronal loops produces a distinct, secondary emission peak; its intensity increases with the injected flare energy. we argue that coronal plasma condensation is a possible mechanism for the tess late-phase flares. | a possible mechanism for the "late phase" in stellar white-light flares |
determination of the three-dimensional (3d) thermodynamic structure of the solar corona by observational means is of great importance to advance the understanding of the physical mechanisms responsible for coronal heating, as well as for the development of global magnetohydrodynamic (mhd) models of the corona. oriented to this end, solar rotational tomography makes use of time series of images of the solar corona in visible light (vl) and in extreme ultraviolet, to determine the 3d distribution of the electron density and temperature of the solar corona. in this work we present a new tomographic technique that, based on coronal images in lyman-α and vl, allows 3d reconstruction of the lyman-α doppler dimming term. in combination with a global model of the magnetic field, this allows 3d reconstruction of the solar wind speed. the recently operational metis coronagraph, aboard the solar orbiter space mission, takes images in both spectral ranges, which will allow to apply the technique for the first time. we describe the methodology and present preliminary results based on numerical simulations. | reconstrucción tridimensional de la velocidad del viento solar mediante tomografía lyman-α |
the relationship between coronal mass ejections (cmes) and flares is an important issue in solar and stellar physics. the studies on the origination and generation mechanisms of interplanetary cme (icme) materials are crucial for understanding the connection between cmes and flares. the materials inside icmes can be classified into three types, coming from corona directly (corona materials), heated by magnetic reconnection in corona (heated corona materials), and generated by chromospheric evaporation (chromospheric evaporation materials). here the contribution and first ionization potential (fip) bias of three types of materials inside icmes associated with different flare intensities are analyzed and compared. we find that the speeds and scales of near-earth icmes both increase with flare intensities. the proportions of heated corona materials are nearly constant with flare intensities. the contributions of corona materials (chromospheric evaporation materials) are significantly decreased (increased) with flare intensities. more than two-thirds of materials are chromospheric evaporation materials for icmes associated with strong flares. the fip bias of corona materials and heated corona materials is almost the same. the fip bias of chromospheric evaporation materials is significantly higher than that of corona materials and heated corona materials, and it is increased with flare intensities. the above characteristics of fip bias can be explained reasonably by the origination and generation mechanisms of three types of icme materials. the present study demonstrates that the origination and generation mechanisms of icme materials are significantly influenced by flare intensities. the reasons for the elevation of fip bias, if icmes are regarded as a whole, are that the fip bias of chromospheric evaporation materials is much higher, and the chromospheric evaporation materials contributed significantly to the icmes that are associated with strong flares. | the contribution and fip bias of three types of materials inside icmes associated with different flare intensities |
the exploration of the inner heliosphere by the parker solar probe has revealed a highly structured solar wind with ubiquitous deflections from the parker spiral, known as switchbacks. interchange reconnection (ir) may play an important role in generating these switchbacks, by forming unstable particle distributions that generate wave activity that in turn may evolve to such structures. ir occurs in very low-beta plasmas and in the presence of strong guiding fields. although ir is unlikely to release enough energy to provide an important contribution to the heating and acceleration of the solar wind, it affects the way the solar wind is connected to its sources, connecting open field lines to regions of closed fields. this "switching on" provides a mechanism by which the plasma near coronal hole boundaries can mix with that trapped inside the closed loops. this mixing can lead to a new energy balance. it may significantly change the characteristics of the solar wind because this plasma is already preheated and can potentially have quite different density and particle distributions. it not only replenishes the solar wind, but also affects the electric field, which in turn affects the energy balance. this interpenetration is manifested by the formation of a bimodal ion distribution, with a core and a beam-like population. such distributions are indeed frequently observed by the parker solar probe. here we provide a first step toward assessing the role of such processes in accelerating and heating the solar wind. | ion kinetics of plasma interchange reconnection in the lower solar corona |
context. it is well known among the scientific community that solar flare activity often begins well before the main impulsive energy release. however, a consistent explanation for this phenomenon has not yet been established.aims: our aim is to investigate the earliest phase of four distinct flares observed by solar orbiter/stix and determine the relationships of the newly heated plasma to flare structure and dynamics.methods: the analysis focuses on four events that were observed from both earth and solar orbiter, which allows for a comparison of stix observations with those of goes/xrs and sdo/aia. the early phases of the events were studied using stix and goes spectroscopic analysis to investigate the evolution of the physical parameters of the plasma, including the isothermal temperature and emission measure. furthermore, to determine the location of the heated plasma, stix observations were combined with aia images.results: the events with clear emission prior to the impulsive phase show elevated temperatures (> 10 mk) from the very beginning, which indicates that energy release started before any detection by stix. although the temperature shows little variation during the initial phase, the emission measure increases by about two orders of magnitude, implying a series of incrementally greater energy releases. the spectral analysis of stix and goes from the very first time bins suggests that the emission has a multi-thermal nature, with a hot component of more than 10 mk. alternative heating mechanisms may be more significant during this phase, since nonthermal emission, as observed by stix, is only detected later. stix and aia images reveal the presence of more compact sources of hot plasma early in the flare that originate from different locations with respect to the standard loop-top source that is observed later in the flare. however, because extended bremsstrahlung sources are much more difficult to detect than compact sources, there might be additionally heated plasma in the loop-top during hot onsets.conclusions: this analysis confirms the existence of "hot onsets", with stix detecting the hot onset pattern even earlier than goes. these elevated temperatures imply that energy release actually begins well before any detection by stix. therefore, hot onsets may be significant in the initiation, early development, or even prediction of solar flares. | the existence of hot x-ray onsets in solar flares |
a large amount of the radiated energy during solar and stellar flares is emitted as white-light continuum emission, extending through the ultraviolet and optical wavelength regimes. broadband photometry and optical spectral observations of m dwarf flares suggest that the white-light peak is located in the near-ultraviolet wavelength regime similar to a blackbody with t ~ 10,000 k, whereas radiative-hydrodynamic models using a solar-type flare heating mechanism (nonthermal electrons with a flux ~ 1011 erg / s / cm2 accelerated in the corona) predict that the peak lies at redder wavelengths at the head of the balmer continuum. we have completed a successful flare monitoring campaign on the dm4e star gj 1243, in order to constrain the time-evolution of the peak of the white-light continuum. the campaign took place over 12 hours on aug 31/sept 1, 2014, and included optical monitoring from nine ground-based telescopes as the hubble space telescope recorded time-tagged spectra in the near-ultraviolet (2450-2840 å) with the cosmic origins spectrograph. two flares occurred during the hst observations, and we show preliminary results relating the continuum and line (fe ii and mg ii) emission to the simultaneous ground-based optical spectra and photometry. this dataset provides new constraints for radiative-hydrodynamic modeling of the lower flaring atmosphere in addition to input for models of the effects of flares on biomarkers and habitability around m dwarfs. | time-resolved near-ultraviolet flare spectra with the hubble space telescope / cosmic origins spectrograph |
phase mixing of plasma waves leads to the evolution of increasingly smaller-scale perturbations, eventually leading to viscous or resistive dissipation of energy (chen 2021). this has been suggested as a potential cause of solar coronal heating, though the details of the exact mechanisms involved are still unclear. we examine the effect of sheared magnetic fields on the phase mixing of alfven waves, which increases their wavenumber transverse to the magnetic field, and so transforms them into kinetic alfven waves that play a large role in electron heating. an ideal mhd model displays singularities at resonant alfven points; thus, to investigate these phenomena, we incorporate finite ion larmor radius effects into our gyro-rmhd model, allowing us to resolve the behavior of the plasma at those resonant layers. | energy dissipation by kinetic alfven waves in sheared magnetic fields |
one of the possible mechanisms for heating the solar atmosphere is the magnetic reconnection occurring at different spatiotemporal scales. the discovery of fast bursty nanojets due to reconnection in the coronal loops has been linked to nanoflares and is considered as a possible mechanism for coronal heating. the occurrence of these jets mostly in the direction inwards to the loop was observed in the past. in this study, we report 10 reconnection nanojets, four with directions inward and six moving outward to the loop, in observations from the high-resolution coronal imager 2.1 and the atmospheric imaging assembly (aia) on board the solar dynamics observatory. we determined the maximum length, spire width, speed, and lifetimes of these jets and studied their correlations. we found that outward jets with higher speeds are longer in length and duration while the inward jets show opposite behavior. the average duration of the outward jets is ≈42 s and that of inward jets is ≈24 s. we identified jets with subsonic speeds below 100 km s-1 to high speeds over 150 km s-1. these jets can be identified in multiple passbands of aia extending from the upper transition region to the corona suggesting their multithermal nature. | hi-c 2.1 observations of reconnection nanojets |
we'll review the current state-of-the-art for ground-based instrumentation and techniques to achieve high-resolution observations. we'll use the 4m daniel k. inouye solar telescope (dkist), the european solar telescope (est) and other ground-based instrumentation as examples to demonstrate instrument designs and observing techniques. using adaptive optics and post-facto image processing techniques, the recently completed dkist provides unprecedented resolution and high polarimetric sensitivity that enables astronomers to unravel many of the mysteries the sun presents, including the origin of solar magnetism, the mechanisms of coronal heating and drivers of flares and coronal mass ejections. versatile ground-based instruments provide highly sensitive measurements of solar magnetic fields, that in the case of dkist, also include measurements of the illusive magnetic field of the faint solar corona. ground-based instruments produce large and diverse data sets that require complex calibration and data processing to provide science-ready to a broad community. we'll briefly touch on ongoing and future instrumentation developments, including multi-conjugate adaptive optics. | ground-based instrumentation and observational techniques |
evidence for the presence of ion cyclotron waves (icws), driven by turbulence, at the boundaries of the current sheet is reported in this paper. by exploiting the full potential of the joint observations performed by parker solar probe and the metis coronagraph on board solar orbiter, local measurements of the solar wind can be linked with the large-scale structures of the solar corona. the results suggest that the dynamics of the current sheet layers generates turbulence, which in turn creates a sufficiently strong temperature anisotropy to make the solar-wind plasma unstable to anisotropy-driven instabilities such as the alfvén ion cyclotron, mirror-mode, and firehose instabilities. the study of the polarization state of high-frequency magnetic fluctuations reveals that icws are indeed present along the current sheet, thus linking the magnetic topology of the remotely imaged coronal source regions with the wave bursts observed in situ. the present results may allow improvement of state-of-the-art models based on the ion cyclotron mechanism, providing new insights into the processes involved in coronal heating. | does turbulence along the coronal current sheet drive ion cyclotron waves? |
the excess temperature of the solar corona over the photosphere poses a challenge. multiple energetic events contribute to maintaining the corona at such high temperatures. the energy released in different events can vary across several orders of magnitude. large energy events of geomagnetic importance like flares and coronal mass ejections (cmes) contribute little to the global energetics of the solar corona. therefore, events with several (9-10) orders of magnitudes of lower energy, with much higher frequency of occurrence, need to be studied in great detail. observations suggest that these impulsive events with different energies follow a power-law distribution, indicating a common underlying mechanism. we perform observation-motivated modeling of coronal loops (magnetic flux tubes) to understand the energetics of these small transient events and their similarity with impulsive events like flares. this thesis uses the ebtel code based on the 0d hydrodynamical description of coronal loops. this approach is appropriate for getting quick estimates of the energetics of the system over a wide range of parameters. we then discuss the improvement of ebtel to make it suitable over a broader range of parameters. this is followed by using improved ebtel to explore the possibility of simulating impulsive events of different energy generated using a single power-law distribution. comparison between observed emissions from various components of multi-thermal plasma and hydrodynamical models suggest the heating to be impulsive. since field-aligned flows induced due to impulsive events are a crucial part of our modeling of coronal loops, we discuss the implications of such flows in the context of transition region heating. | energetics of the solar atmosphere |
the solar corona is hundreds of times hotter than the visible surface below. the details of the physical mechanisms involved in energizing the solar atmosphere continue to elude scientists. this plasma flows outward from the sun, becoming the solar wind, where it reaches super-alfvénic speeds, filling the solar system and defining the heliosphere. there have been remote observations of the solar corona for centuries, and in situ measurements of the solar wind for almost 60 years. computer simulation capabilities have vastly improved, and simulation techniques of the coupling between the solar atmosphere and solar wind continue to advance. yet there are longstanding, major unsolved questions of how the corona is heated and the solar wind is formed that involve universal physical processes such as magnetic reconnection, turbulence, and waves. these questions remain because observations and simulations are limited to narrow aspects of physics and/or system, and thus cannot capture cross-scale and cross-region coupling. coronal heating is linked to the different pathways of solar wind formation, leaving imprints in solar wind structures that survive to earth, and ultimately affect the driving of geospace dynamics. we discuss how coronal heating and solar wind physics requires new observations and simulation capabilities to link the kinetic scales, through the mesoscales, to the global processes. | outstanding questions of coronal heating and solar wind physics |
context. coronal bright points (bps) are numerous, bright, small-scale dynamical features found in the solar corona. bright points have been observed to exhibit intensity oscillations across a wide range of periodicities and are likely an important signature of plasma heating and/or transport mechanisms.aims: we present a novel and efficient wavelet-based method that automatically detects and tracks the intensity evolution of bps using images from the atmospheric imaging assembly (aia) on board the solar dynamics observatory (sdo) in the 193 å bandpass. through the study of a large, statistically significant set of bps, we attempt to place constraints on the underlying physical mechanisms.methods: we used a continuous wavelet transform (cwt) in 2d to detect the bps within images. one-dimensional cwts were used to analyse the individual bp time series to detect significant periodicities.results: we find significant periodicity at 4, 8-10, 17, 28, and 65 min. bright point lifetimes are shown to follow a power law with exponent −1.13 ± 0.07. the relationship between the bp lifetime and maximum diameter similarly follows a power law with exponent 0.129 ± 0.011.conclusions: our wavelet-based method successfully detects and extracts bps and analyses their intensity oscillations. future work will expand upon these methods, using larger datasets and simultaneous multi-instrument observations. movies associated to figs. 5, 11-13 are available at https://www.aanda.org. | automated analysis of oscillations in coronal bright points |
solarc euvst is designed to comprehensively understand the energy and mass transfer from the solar surface to the solar corona and interplanetary space, and to investigate the elementary processes that take place universally in cosmic plasmas. the proposed mission is a fundamental step for answering how the plasma universe is created and evolves, and how the sun influences the earth and other planets in our solar system. the two primary science objectives for solarc euvst are : i) understand how fundamental processes lead to the formation of the solar atmosphere and the solar wind, ii) understand how the solar atmosphere becomes unstable, releasing the energy that drives solar flares and eruptions. solarc euvst will, a) seamlessly observe all the temperature regimes of the solar atmosphere from the chromosphere to the corona at the same time, b) resolve elemental structures of the solar atmosphere with high spatial resolution and cadence to track their evolution, and c) obtain spectroscopic information on the dynamics of elementary processes taking place in the solar atmosphere. in this talk, we will first discuss the science target of the solarc euvst, and discuss the science topic associated flare in detail. photospheric motions lead to the accumulation of free magnetic energy in the corona. this system eventually becomes unstable, releasing the energy through magnetic reconnection. this process of energy conversion heats the plasma to high temperatures and drives coronal mass ejections (cmes). by measuring the properties of multi-temperature flaring plasma, solarc euvst will investigate why the reconnection is fast despite the high magnetic reynolds number. it will also monitor the temporal evolution of solar active regions and identify the triggering mechanism for the flare and eruption. we will discuss the science objectives "understand the fast magnetic reconnection process" and "identify the signatures of global energy buildup and the local triggering of the flare and eruption". we also report the current status of solarc euvst. | science objectives and current status of solar-c_euvst |
the k-distribution function provides a microscopic description of statistical information on the energetic charged particle distributions and is an invaluable aid towards the determination of macroscopic parameters (energetic ion moments) that are fundamentally important for studying the source, loss, transport and acceleration of energetic plasma inside the magnetosphere. we use κ-distribution fits to combined chems (3 to 236 kev/e), lemms (0.024 < e < 18 mev), and inca (5.2 to >220 kev) h+ and o+ energetic ion spectra covering measurements made during 2004-2016 to calculate the >20 kev energetic ion moments inside saturn's magnetosphere. we focus on partial density (n), integral intensity (in), partial pressure (p), integral energy intensity (ie); as well as the characteristic energy (ec=ie/in), temperature and κ-index of these ions as a function of local time (00:00 to 24:00 hrs) and l-shell (5-20 rs). the roelof and skinner [2000] model, ie., a semi-empirical parametric model that uses a combination of gaussian and exponential decay functions, together with a two harmonic expansion that allows us to modulate the ion distributions in both l-shell and local time, is then utilized to retrieve the equatorial h+ and o+ density, pressure, and temperature, allowing also the determination of the polytropic index (γ; ratio between the specific heat at constant pressure and constant volume) for both h+ and o+. we find that (a) the 9<l<20 region corresponds to a local equatorial acceleration region, where sub-adiabatic transport of h+ (γ 1.25) and quasi-isothermal behavior of o+ (γ 0.95) dominate the ion energetics; (b) energetic ions are heavily depleted in the inner magnetospheric regions, and their behavior appears to be quasi-isothermal (γ <1); (c) the (quasi-) periodic energetic ion injections in the outer parts of saturn's magnetosphere (especially beyond 17-18 rs) produce durable signatures in the energetic ion moments; (d) the plasma sheet does not seem to have a "ground thermodynamic state", but the extended neutral gas distribution at saturn provides an effective cooling mechanism that does not allow the plasma sheet to behave adiabatically. | cassini/mimi measurements of energetic ion moments and the polytropic index in saturn's magnetosphere. |
we have measured the root-mean-square (rms) amplitude of intensity fluctuations in plume and interplume regions of a polar coronal hole. these intensity fluctuations correspond to density fluctuations. using data from the sun watcher using active pixel system detector and image processing (swap) on the project for onboard autonomy (proba2), our results extend up to a height of about 1.35 solar radii. at low heights, both the absolute intensity fluctuation and the amplitudes relative to the mean intensity decrease with height. however, starting at about 1.2 solar radii, the relative amplitude increases with height, reaching 20-40% by 1.35 solar radii. this corresponds to density fluctuations of 10-20%. these increasing relative amplitudes imply that the density fluctuations are generated in the corona itself. one possibility is that the density fluctuations are generated by an instability of alfven waves. this generation mechanism is consistent with some theoretical models and with observations of alfven wave amplitudes in coronal holes. these density fluctuations are likely to play an important indirect role in coronal heating by promoting the reflection of alfven waves and driving turbulence | density fluctuations in a polar coronal hole; no |
in the standard picture of plasma turbulence, energy injected at large scales cascades conservatively to small scales, eventually thermalizing to heat particles via a variety of mechanisms. this leads to the ''zeroth law of turbulence,'' whereby the macroscopic statistics of the flow and magnetic field are independent of the details of how energy is microscopically dissipated into heat. the helicity barrier is a newly discovered effect that upends this notion for low-beta imbalanced alfvenic turbulence, as occurs in the solar wind and corona. because of a conserved generalized helicity invariant, the energy cascade is ''stuck'' around the ion gyroscale, which causes the turbulent energy to build up in time (if it is continuously forced). the outcome is quite dramatic for the solar wind and for plasma heating in general because it changes the thermodynamics properties of the plasma based on how it is stirred at large scales. the effect explains a number of long-standing puzzles from in-situ solar-wind observations, including the steep ''transition range'' seen in magnetic spectra, magnetic helicity signatures, and ion distribution functions. it also links two previously well-studied coronal heating mechanisms (turbulence and ion-cyclotron waves), suggesting an important role of the helicity barrier in the coronal heating problem. more generally, it demonstrates how the complex, nonlinear microphysics of collisionless plasmas can have a strong influence on the macroscopic properties of astrophysical processes. | violation of the zeroth law of turbulence through the helicity barrier effect |
recent observations by the extreme ultraviolet imager (eui) onboard solar orbiter have revealed prevalent small-scale transient brightenings in the quiet solar corona termed campfires. to understand the generation mechanism of these coronal brightenings, we constructed a self- consistent and time-dependent quiet-sun model extending from the upper convection zone to the lower corona using a realistic 3d radiation mhd simulation. from the model we have synthesized the coronal emission in the eui 174 å passband. we identified several transient coronal brightenings similar to those in eui observations. the size and lifetime of these coronal brightenings are 2-4 mm and ∼2 min, respectively. these brightenings are located at a height of 2-4 mm above the photosphere, and the surrounding plasma is often heated above 1 mk. these findings are consistent with the observational characterisation of the campfires. through a comparison of the magnetic field structures before and after the occurrence of brightenings, we conclude that these coronal brightenings are generated by component magnetic reconnection between interacting bundles of field lines or the relaxation of highly twisted flux ropes. occurring in the coronal part of the atmosphere, these events show no measurable signature in the photosphere. these transient coronal brightenings may play an important role in heating of the local coronal plasma. | transient small-scale brightenings in the quiet sun corona: a model for "campfires" observed with solar orbiter |
the presence of condensations in the solar corona has the potential to be an important diagnostic of coronal heating. we present the results of models of nanoflare heated coronal loops using the 1-d hydrodynamic argos code. the nanoflares are modeled by discrete pulses of energy along the loop. we explore the occurrence of cold condensations due to the effective equivalent of thermal non-equilibrium (tne) in loops with steady heating, and examine its dependence on nanoflare timing and intensity and also nanoflares location along the loop, including randomized distributions of nanoflares. we find that randomizing nanoflare distributions, both in time/intensity and location, tends to diminish the likelihood of condensations compared to regularly occurring nanoflares with the same average properties, but that condensations can sometimes occur in regimes where regularly occurring nanoflares would not produce tne. also, the condensations stay in the loop for a shorter amount of time when the nanoflares distributions are random. these properties can be used in the future to investigate diagnostics of coronal heating mechanisms. | modeling of condensations in coronal loops produced by nanoflares with variable frequency and location |
context. the study explores the photospheric magnetic properties of bright and faint small-scale loop systems in the solar atmosphere of the quiet sun, also known as x-ray or coronal bright points.aims: to understand how plasma confined in small-scale loops is heated to million degrees, the loop-associated photospheric and coronal magnetic flux properties should be known because the magnetic field is generally assumed to be the main energy source or waveguide. this and follow-up studies aim to provide a qualitative and quantitative investigation of these magnetic properties and their impact on the heating of plasma to million degrees.methods: we used quasi-temporal imaging observations taken in the 193 å channel of the atmospheric imaging assembly (aia) and line-of-sight magnetograms from the helioseismic magnetic imager (hmi) on board the solar dynamics observatory. the observations cover 48 h of data at a 6 min cadence with a field of view of 400″ × 400″, from which 90 loop systems (of which 83 are cbps) were extracted and analysed in full detail.results: we obtain the evolution properties of both faint and bright small-scale loop systems (sslss) related to either magnetic flux emergence or magnetic flux coalescence and a chance encounter of magnetic fluxes. we estimate the lifetimes of the two loop systems and the impact of the magnetic flux evolution on their life span. the photospheric magnetic flux associated with sslss confining plasma heated to coronal temperatures is found to cover at least two orders of magnitude from 3.0 × 1018 mx to 1.8 × 1020 mx. the analysis of the maximum intensity of sslss during their lifetime shows numerous spikes of intensity that are identified as small (a few aia pixels) compact brightenings associated with cancelling magnetic fluxes. most of them are identified as microflares. the intensity flux range of these spikes is reported. the coronal intensity flux evolution of sslss is strongly correlated with the total unsigned photospheric magnetic flux evolution when there is little or no contamination in the selected field of view of the sslss by unrelated magnetic fluxes or intensity features. we report on the footpoint separation and change during the lifetime of the faint and bright sslss. the magnetic flux emergence and decay rates of some of the sslss are also provided in this study.conclusions: the power-law index α of the logarithm of the total unsigned magnetic flux and the total intensity for the full lifetime of sslss is 1.10 ± 0.02, compared with 1.14 ± 0.03 for a previous study of the whole disc in the same intensity range (fe xii 193-195 å). this indicates that the emission of the corona of the quiet sun at ∼1.25 mk is mostly confined to small-scale loops (some brighter, others fainter). therefore, it is imperative to understand the mechanism that heats the plasma in these loops. movies associated to figs. 3-11 are available at https://www.aanda.org. | photospheric magnetic flux and coronal emission properties of small-scale bright and faint loops in the quiet sun |
the solar ultraviolet intensities of spectral lines originating from li- and na-like ions have been observed to surpass the expectations derived from plasmas with coronal approximation. the violation of the coronal approximation can be partially attributed to non-equilibrium ionization (nei) due to dynamic processes occurring in the vicinity of the transition region. to investigate the impact of these dynamics in alfvén-wave-heated coronal loop, a set of equations governing nei for multiple ion species was solved numerically in conjunction with 1.5-dimensional magnetohydrodynamic equations. following the injection of alfvén waves from the photosphere, the system undergoes a time evolution characterized by phases of evaporation, condensation, and quasi-steady states. during the evaporation phase, the ionization fractions of li- and na-like ions were observed to increase when compared to the fractions in ionization equilibrium, which lead to the intensity enhancement of up to 1.6. this over-fractionation of li- and na-like ions was found to be induced by the evaporation process. while collisions between shocks and the transition region temporarily led to deviations from ionization equilibrium, on average over time, these deviations were negligible. conversely, under-fractions of the ionization fraction led to intensity reduction of down to 0.9 during the condensation phase and the quasi-steady state. given the dependency of the over/under-fractionation on mass circulations between the chromosphere and the corona, these observations will serve as valuable benchmarks to validate not only alfvén wave models but also other existing mechanisms on coronal heating. | anomalous emission from li- and na-like ions in the corona heated via alfvén wave |
in kagome metals, the chiral current order $\eta$ with time-reversal-symmetry-breaking is the source of various exotic electronic states, while the method of controlling the current order and its interplay with the star-of-david bond order $\phi$ are still unsolved. here, we reveal that tiny uniform orbital magnetization $m[\eta,\phi]$ is induced by the chiral current order, and its magnitude is prominently enlarged under the presence of the bond order. importantly, we derive the magnetic-field ($h$)-induced ginzburg-landau free energy expression $\delta f[h,\eta,\phi]$, which enables us to elucidate the field-induced current-bond phase transitions in kagome metals. the emergent current-bond-$h$ trilinear coupling term in the free energy, $-3m_1 h\eta\phi$, naturally explains the characteristic magnetic field sensitive electronic states in kagome metals, such as the field-induced current order and the strong interplay between the bond and current orders. furthermore, we present a natural explanation for the drastic strain-induced increment of the current order transition temperature t_{trsb} reported by a recent experiment. | drastic magnetic-field-induced chiral current order and emergent current-bond-field interplay in kagome metal av3sb5 (a=cs,rb,k) |
a new spin-dependent deflection mechanism is revealed by considering the spin-correlated radiation-reaction force during laser-electron collision. we found that such deflection originates from the non-zero work done by the radiation-reaction force along the laser polarization direction in each half-period, which is larger/smaller for spin-anti-paralleled/spin-paralleled electrons. the resulted anti-symmetric deflection is further accumulated when the spin-projection onto the laser magnetic field is reversed in adjacent half-periods. the discovered mechanism dominates over the stern-gerlach deflection for electrons of several hundreds of mev and 10 pw-level laser peak power. the results provide a new perspective to study the strong-field qed physics in quantum radiation-reaction regime and an approach to leverage the study of radiation-dominated and strong-field qed physics via particle spins. | spin-dependent radiative deflection in the quantum radiation-reaction regime |
we present the results of the pulse-amplitude-resolved spectroscopy of the accreting pulsar v 0332+53 using the nustar observations of the source in 2015 and 2016. we investigate the dependence of the energy of the cyclotron resonant scattering feature (crsf) as a function of x-ray luminosity on timescales comparable with the spin period of the pulsar within individual observations, and the behavior on longer timescales within and between the two observed outbursts. we confirm that in both cases the crsf energy is negatively correlated with flux at luminosities higher than the critical luminosity and is positively correlated at lower luminosities. we also confirm the recently reported gradual decrease in the line energy during the giant outburst in 2015. using the nustar data, we find that this decrease was consistent with a linear decay throughout most of the outburst, and flattened or even reversed at the end of the 2015 outburst, approximately simultaneously with the transition to the subcritical regime. we also confirm that by the following outburst in 2016 the line energy rebounded to previous values. the observed behavior of the crsf energy with time is discussed in terms of changes in the geometry of the crsf forming region caused by changes in the effective magnetospheric radius. | changes in the cyclotron line energy on short and long timescales in v 0332+53 |
this paper describes an analysis of the nustar data of the fastest-rotating magnetar 1e 1547 - 5408, acquired in 2016 april for a time lapse of 151 ks. the source was detected with a 1-60 kev flux of 1.7 × 10-11 erg s-1 cm-2, and its pulsation at a period of 2.086710(5) s. in 8-25 kev, the pulses were phase-modulated with a period of t = 36.0 ± 2.3 ks, and an amplitude of ∼0.2 s. this reconfirms the suzaku discovery of the same effect at $t=36.0 ^{+4.5}_{-2.5}$ ks, made in the 2009 outburst. these results strengthen the view derived from the suzaku data, that this magnetar performs free precession as a result of its axial deformation by ∼0.6 × 10-4, possibly caused by internal toroidal magneti fields (mfs) reaching ∼1016 g. like in the suzaku case, the modulation was not detected in energies below ∼8 kev. above 10 kev, the pulse-phase behaviour, including the 36 ks modulation parameters, exhibited complex energy dependencies: at ∼22 kev, the modulation amplitude increased to ∼0.5 s, and the modulation phase changed by ∼65° over 10-27 kev, followed by a phase reversal. although the pulse significance and pulsed fraction were originally very low in >10 kev, they both increased noticeably, when the arrival times of individual photons were corrected for these systematic pulse-phase variations. possible origins of these complex phenomena are discussed, in terms of several physical processes that are specific to ultrastrong mfs. | a nustar confirmation of the 36 ks hard x-ray pulse-phase modulation in the magnetar 1e 1547.0 - 5408 |
recent observations of frb 20190520b have revealed rapid (tens of s) fluctuation of its dispersion measure within apparently fixed bounds, as well as a reversal of its rotation measure. the fluctuations of dispersion measure are uncorrelated with the intervals between bursts, setting upper bounds $\sim 10\,$ s on any characteristic time-scale of the dispersing region; it must be very compact. measurements of the full dependence of the dispersive time delay on frequency may determine the actual electron density and the size of this region. it is possible to set a lower bound on the mass of the frb source from constraints on the size of the dispersing region and its time-scale of variation. comparison of the variations of dm and rm leads to an estimate of the magnetic field $\sim 500~ \mu$g. | the environment and constraints on the mass of frb 190520b |
transport in hot and dilute, i.e., collisionless, astrophysical and space, plasmas is called "anomalous." this transport is due to the interaction between the particles and the self-generated turbulence by their collective interactions. the anomalous transport has very different and not well known properties compared to the transport due to binary collisions, dominant in colder and denser plasmas. because of its relevance for astrophysical and space plasmas, we explore the excitation of turbulence in current sheets prone to component- or guide-field reconnection, a process not well understood yet. this configuration is typical for stellar coronae, and it is created in the laboratory for which a 2.5d geometry applies. in our analysis, in addition to the immediate vicinity of the x-line, we also include regions outside and near the separatrices. we analyze the anomalous transport properties by using 2.5d particle-in-cell code simulations. we split off the mean slow variation (in contrast to the fast turbulent fluctuations) of the macroscopic observables and determine the main transport terms of the generalized ohm's law. we verify our findings by comparing with the independently determined slowing-down rate of the macroscopic currents (due to a net momentum transfer from particles to waves) and with the transport terms obtained by the first order correlations of the turbulent fluctuations. we find that the turbulence is most intense in the "low density" separatrix region of guide-field reconnection. it is excited by streaming instabilities, is mainly electrostatic and "patchy" in space, and so is the associated anomalous transport. parts of the energy exchange between turbulence and particles are reversible and quasi-periodic. the remaining irreversible anomalous resistivity can be parametrized by an effective collision rate ranging from the local ion-cyclotron to the lower-hybrid frequency. the contributions to the parallel and the perpendicular (to the magnetic field) components of the slowly varying dc-electric fields, balanced by the turbulence, are similar. this anomalous electric field is, however, smaller than the contributions of the off-diagonal pressure and electron inertia terms of ohm's law. this result can now be verified by in-situ measurements of the turbulence, in and around the magnetic reconnection regions of the earth's magnetosphere by the multi-spacecraft mission mms and in laboratory experiments like mrx and vineta-ii. | turbulent transport in 2d collisionless guide field reconnection |
the outer-crust structure and composition of a cold, non-accreting magnetar are studied. we model the outer crust to be made of fully equilibrated matter where ionized nuclei form a coulomb crystal embedded in an electron gas. the main effects of the strong magnetic field are those of quantizing the electron motion in landau levels and of modifying the nuclear single-particle levels producing, on average, an increased binding of nucleons in nuclei present in the coulomb lattice. the effect of a homogeneous and constant magnetic field on nuclear masses has been predicted by using a covariant density functional in which induced currents and axial deformation due to the presence of a magnetic field that breaks time-reversal symmetry have been included self-consistently in the nucleon and meson equations of motion. although not yet observed, for b ≳1016 g both effects contribute to produce different compositions—odd-mass nuclei are frequently predicted—and to increase the neutron-drip pressure as compared to a typical neutron star. specifically, in such a regime, the magnetic-field effects on nuclei favor the appearance of heavier nuclei at low pressures. as b increases, such heavier nuclei are also preferred up to larger pressures. for the most extreme magnetic field considered, b =1018 g, and for the models studied, almost the whole outer crust is made of 4092zr52. | outer crust of a cold non-accreting magnetar |
the first confirmed periodically varying 6.031 and 6.035 ghz hydroxyl masers are reported here. they vary contemporaneously with the 6.7 ghz methanol masers in g323.459-0.079. the 1.665 ghz hydroxyl and 12.2 ghz methanol masers associated with g323.459-0.079 are also periodic. evidence for periodicity is seen in all features in all transitions save a single 1.665 ghz hydroxyl maser feature. historical excited-state hydroxyl maser observations set a stricter upper limit on the epoch in which a significant accretion event occurred. the associated burst in 6.7 ghz methanol maser activity has subsided significantly while the hydroxyl transitions are brightening possibly the result of changing physical conditions in the masing cloudlets. time lags in methanol are confirmed and may be the result of the periodic flaring propagating outward from the central region of maser activity. a possible magnetic field reversal occurred during the accretion event. | synchronized periodic maser flares of multiple oh and ch3oh lines in g323.459-0.079 |
vertically stratified shearing box simulations of magnetorotational turbulence commonly exhibit a so-called butterfly diagram of quasi-periodic azimuthal field reversals. however, in the presence of hydrodynamic convection, field reversals no longer occur. instead, the azimuthal field strength fluctuates quasi-periodically while maintaining the same polarity, which can either be symmetric or antisymmetric about the disc mid-plane. using data from the simulations of hirose et al., we demonstrate that the lack of field reversals in the presence of convection is due to hydrodynamic mixing of magnetic field from the more strongly magnetized upper layers into the mid-plane, which then annihilate field reversals that are starting there. our convective simulations differ in several respects from those reported in previous work by others, in which stronger magnetization likely plays a more important role than convection. | convective quenching of field reversals in accretion disc dynamos |
astrophysical flows exhibit rich behaviour resulting from the interplay of different forms of energy--gravitational, thermal, magnetic and radiative. for magnetic cataclysmic variable stars, material from a late, main sequence star is pulled onto a highly magnetized (b>10 mg) white dwarf. the magnetic field is sufficiently large to direct the flow as an accretion column onto the poles of the white dwarf, a star subclass known as am herculis. a stationary radiative shock is expected to form 100-1,000 km above the surface of the white dwarf, far too small to be resolved with current telescopes. here we report the results of a laboratory experiment showing the evolution of a reverse shock when both ionization and radiative losses are important. we find that the stand-off position of the shock agrees with radiation hydrodynamic simulations and is consistent, when scaled to am herculis star systems, with theoretical predictions. | laboratory analogue of a supersonic accretion column in a binary star system |
three high-mass x-ray binaries have been discovered recently exhibiting enormous spin-up rates. conventional accretion theory predicts extremely high-surface dipolar magnetic fields that we believe are unphysical. instead, we propose quite the opposite scenario; some of these pulsars exhibit weak magnetic fields, so much so that their magnetospheres are crushed by the weight of inflowing matter. the enormous spin-up rate is achieved before inflowing matter reaches the pulsar's surface as the penetrating inner disc transfers its excess angular momentum to the receding magnetosphere, which, in turn, applies a powerful spin-up torque to the pulsar. this mechanism also works in reverse; it spins a pulsar down when the magnetosphere expands beyond corotation and finds itself rotating faster than the accretion disc, which then exerts a powerful retarding torque to the magnetic field and to the pulsar itself. the above scenaria cannot be accommodated within the context of neutron-star accretion processes occurring near spin equilibrium, thus they constitute a step towards a new theory of extreme (far from equilibrium) accretion phenomena. | not an oxymoron: some x-ray binary pulsars with enormous spin-up rates reveal weak magnetic fields |
neutron stars are a prime laboratory for testing physical processes under conditions of strong gravity, high density, and extreme magnetic fields. among the zoo of neutron star phenomena, magnetars stand out for their bursting behavior, ranging from extremely bright, rare giant flares to numerous, less energetic recurrent bursts. the exact trigger and emission mechanisms for these bursts are not known; favored models involve either a crust fracture and subsequent energy release into the magnetosphere, or explosive reconnection of magnetic field lines. in the absence of a predictive model, understanding the physical processes responsible for magnetar burst variability is difficult. here, we develop an empirical model that decomposes magnetar bursts into a superposition of small spike-like features with a simple functional form, where the number of model components is itself part of the inference problem. the cascades of spikes that we model might be formed by avalanches of reconnection, or crust rupture aftershocks. using markov chain monte carlo sampling augmented with reversible jumps between models with different numbers of parameters, we characterize the posterior distributions of the model parameters and the number of components per burst. we relate these model parameters to physical quantities in the system, and show for the first time that the variability within a burst does not conform to predictions from ideas of self-organized criticality. we also examine how well the properties of the spikes fit the predictions of simplified cascade models for the different trigger mechanisms. | dissecting magnetar variability with bayesian hierarchical models |
context. the magnetic field is a crucial ingredient of neutron stars. it governs the physics of accretion and of the resulting high-energy emission in accreting pulsars. studies of the cyclotron resonant scattering features (crsfs) seen as absorption lines in the x-ray spectra of the pulsars permit direct measurements of the field strength.aims: from an analysis of a number of pointed observations with different instruments, the energy of crsf, ecyc, has recently been found to decay in her x-1 , which is one of the best-studied accreting pulsars. we present our analysis of a homogeneous and almost uninterrupted monitoring of the line energy with swift/bat.methods: we analyzed the archival swift/bat observations of her x-1 from 2005 to 2014. the data were used to measure the crsf energy averaged over several months.results: the analysis confirms the long-term decay of the line energy. the downward trend is highly significant and consistent with the trend measured with the pointed observations: decyc/ dt ~ -0.3 kev per year.conclusions: the decay of ecyc either indicates a local evolution of the magnetic field structure in the polar regions of the neutron star or a geometrical displacement of the line-forming region due to long-term changes in the structure of the x-ray emitting region. the shortness of the observed timescale of the decay, -ecyc/ėcyc ~ 100 yr, suggests that trend reversals and/or jumps of the line energy might be observed in the future. | swift/bat measurements of the cyclotron line energy decay in the accreting neutron star hercules x-1: indication of an evolution of the magnetic field? |
a joint analysis is done of the radio and x-ray observations of sn 1993j. it is argued that neither synchrotron cooling behind the forward shock nor thermal cooling behind the reverse shock is supported by observations. in order for adiabatic models to be consistent, a reinterpretation of the radius of the spatially resolved very long baseline interferometry-source (vlbi) is needed during the first few hundred days. instead of reflecting the position of the forward shock, it is then associated with the expansion of the rayleigh-taylor unstable region emanating from the contact discontinuity. although observations imply a constant ratio between the energy densities in magnetic fields and relativistic electrons, they do not appear to scale individually with the thermal energy density behind the forward shock; rather, in adiabatic models, the evolution of the magnetic field strength is best understood as scaling inversely with the supernova radius. | joining radio with x-rays: a revised model for sn 1993j |
we present a comprehensive timing and spectral analysis of the hmxb 4u 1538-522 by using the nuclear spectroscopic telescope array (nustar) observatory data. using three archived observations made between 2019 and 2021, we have detected ~ 526 s coherent pulsations up to 60 kev. we have found an instantaneous spin-down rate of $\dot{p} = 6.6_{-6.0}^{+2.4} \times 10^{-6}$ s s-1 during the first observation. the pulse profiles had a double peaked structure consisting of a broad primary peak and an energy dependent, weak secondary peak. we have also analysed the long-term spin period evolution of 4u 1538-522 from data spanning more than four decades, including the data from fermi/gbm. based on the recent spin trends, we have found that the third torque reversal in 4u 1538-522 happened around mjd 58800. the source is currently spinning up with $\dot{p} = -1.9(1) \times 10^{-9}$ s s-1. we also report a periodic fluctuation in the spin period of 4u 1538-522. the broad-band persistent spectra can be described with a blackbody component and either power law or comptonization component along with a fe kα line at 6.4 kev and a cyclotron absorption feature around 22 kev. we have also found a relatively weak absorption feature around 27 kev in the persistent spectra of 4u 1538-522 in all three observations. we have estimated a magnetic field strength of $1.84_{-0.06}^{+0.04} (1+z) \times 10^{12}$ and $2.33_{-0.24}^{+0.15} (1+z) \times 10^{12}$ g for the two features, respectively. | torque reversal and cyclotron absorption feature in hmxb 4u 1538-522 |
a simple model of chiral asymmetry is proposed to interpret the origin of the strong toroidal magnetic field. the electrons relevant to dynamics forming the the field are in a quantume degenerate state with ultra-relativistic fermi energy. the system is described by dirac hartree fock method using scaled h-bar method. neutron stars are rotating and have large angular momentum which is formed by cranking model and breaks time reversal. dirac current is decomposed into convection and spin currents due to clifford number. the strong toroidal magnetic field is formed by the spin like current resulted by the chiral asymmetry brought about electron capture caused by the parity-violating weak interaction. | origin of the strong toroidal magnetic field in magnetars |
models invoking magnetic reconnection as the particle acceleration mechanism within relativistic jets often adopt a gradual energy dissipation profile within the jet. however, such a profile has yet to be reproduced in first-principles simulations. here we perform a suite of 3d general relativistic magnetohydrodynamic simulations of post-neutron star merger disks with an initially purely toroidal magnetic field. we explore the variations in both the microphysics (e.g., nuclear recombination, neutrino emission) and system parameters (e.g, disk mass). in all of our simulations, we find the formation of magnetically striped jets. the stripes result from the reversals in the poloidal magnetic flux polarity generated in the accretion disk. the simulations display large variations in the distributions of stripe duration, τ, and power, <p φ>. we find that more massive disks produce more powerful stripes, the most powerful of which reaches <p φ> ~ 1049 erg s-1 at τ ~ 20 ms. the power and variability that result from the magnetic reconnection of the stripes agree with those inferred in short-duration gamma-ray bursts. we find that the dissipation profile of the cumulative energy is roughly a power law in both radial distance, z, and τ, with a slope in the range of ~1.7-3; more massive disks display larger slopes. | striped jets in post-neutron star merger systems |
an analytic model of the time-dependent electric and magnetic fields of an astrophysical jet is presented. these fields satisfy the time-dependent faraday’s law and describe a jet with increasing length. the electric field contains both electrostatic and inductive parts. the electrostatic part corresponds to the rate of injection of toroidal magnetic flux, while the sum of the electrostatic and inductive parts results in the electric field parallel to the magnetic field being zero everywhere. the pinch force associated with the electric current provides a peaked pressure on the jet axis and a pressure minimum at the radius where the poloidal magnetic field reverses direction. | analytic model for the time-dependent electromagnetic field of an astrophysical jet |
we show that the neutrino chirality flip, that can take place in the core of a neutron star at birth, is an efficient process to allow neutrinos to anisotropically escape, thus providing a to induce the neutron star kick velocities. the process is not subject to the {\it no-go theorem} since although the flip from left- to right-handed neutrinos happens at equilibrium, the reverse process does not take place given that right-handed neutrinos do not interact with matter and therefore detailed balance is lost. for simplicity, we model the neutron star core as being made of strange quark matter. we find that the process is efficient when the neutrino magnetic moment is not smaller than $4.7 \times 10^{-15}\mu_b$, where $\mu_b$ is the bohr magneton. when this lower bound is combined with the most stringent upper bound, that uses the luminosity data obtained from the analysis of sn 1987a, our results set a range for the neutrino magnetic moment given by $4.7 \times 10^{-15} \leq \mu_\nu/\mu_b \leq (0.1 - 0.4)\times 10^{-11}$. the obtained kick velocities for natal conditions are consistent with the observed ones and span the correct range of radii for typical magnetic field intensities. | lower bound for the neutrino magnetic moment from kick velocities induced at the birth of neutron stars |
during a one-hour interval of interplanetary magnetic field (imf) bz ≈ 0 nt, the equatorial spacecraft double star tc-1 encountered the dawn flank magnetopause many times at the magnetic local time (mlt) of about 08:00 and the latitude of about -27°. during each encounter, reconnection jets were observed with their velocities up to more than 500 km/s, significantly higher than the background flow in the magnetosheath. the fast flows match the theoretical prediction of alfvénic acceleration well. the medium temperature and density of ions in the boundary layer indicate the open magnetic field topology inside this layer. the mainly southward and tailward flows of the plasma jets alongside with the negative slopes of the walén test indicate that the spacecraft was located south of the reconnection site, consistent with both anti-parallel and component reconnection models. the accelerated flows were observed lasting for about one hour, with some modulations by the oscillations of the magnetopause, but no reversals in the direction of vz were found during the interval. the significantly enhanced flows in the boundary layer compared to the adjacent magnetosheath indicate that the reconnection was quasi-continuously active at the magnetopause northward of the spacecraft under such imf conditions. at the same time, the bipolar signatures in bn with enhancements of the magnetic field indicate the occurrence of the flux transfer events (ftes). the observed reconnection was quasi-continuous, whereas the simultaneously accompanied ftes were time-dependent under the imf bz ≈ 0 nt. for this event, however, it is not possible to identify whether the reconnection was anti-parallel or component because the tc-1 was far away from the reconnection site. | quasi-continuous reconnection accompanied by ftes during imf bz ≈ 0 nt observed by double star tc-1 at the dawnside magnetopause |
accretion shocks in young stellar objects (yso) are a subject of great interest in astrophysics; they exhibit intense magnetic activity and are surrounded by an accretion disk from which matter falls down onto the stellar surface in the form of columns following the magnetic lines (b ~ kg) at the free-fall velocity (100-500 km/s). as a column impacts the stellar surface, a radiative shock is created which heats up the infalling flow. as a consequence, a new reverse shock forms and some oscillations are expected in the emitted radiation as a proof of this periodic dynamic, but no periodicity has yet been detected in observations.to understand the reasons for this apparent inconsistency, we have recently developped an experimental setup [b. albertazzi et al. science 346, 325 (2014)] in which a plasma flow (generated by a high energy laser: 1013 w/cm2 - 0.6 ns pulse) is confined inside a poloidal magnetic field (20 t). this jet has an aspect ratio >10, a temperature of tens of ev, an electron density of 1018 cm-3 and propagates at 700 km/s as show by our previous numerical work [a. ciardi et al. physical review letters, 110 (2013)]. to investigate the accretion dynamics, the jet acts as the accretion column and hits a secondary target acting as the stellar surface. we will present the recent results on generation and dynamics of the jet and the new experimental results of this configuration, namely of a supersonic reverse shock traveling within the accretion column with a speed of 100 km/s, representing a mach number of ~ 30, and the observation of increased density structures along the edges of the interaction. this will be discussed in the light of 3d-magneto-hydrodynamic simulations which parametric variations allow to understand how the various plasma parameters affect the accretion. | laboratory formation of a scaled protostellar jet by coaligned poloidal magnetic field: recent results and new exeprimental studies |
star-like amphiphilic polyacrylamide (shpam), consisting of nano-sio2 as the core and a layer of amphiphilic chains as the shell, having two gradient molecular weights, was prepared via a facile method. to elucidate its enhanced oil recovery (eor) potential, the viscoelastic flow and displacement efficiency of shpam were studied using geological rock-core samples. the results showed that the core-shell microstructure and intermolecular interactions imparted viscoelasticity to shpam in a lower-concentration region. the elasticity mechanism of shpam solutions in porous media was determined by quantifying the relaxation time. low-field nuclear magnetic resonance experiments suggested that shpam flowed through the dominant porous media followed by intermediate porous media having radii of 0.08-10.0 μm. the thickness of the adsorbed layer was independent of the shear rate, demonstrating that shpam was compatible with porous media owing to the reversibly viscoelastic flow. the core flooding tests demonstrated that even after hydrolyzed polyacrylamide flooding, 3.0% oil saturation was further recovered by shpam flooding even in a region with low capillary number (<10-5). moreover, over 27% oil recovery with 70% cumulative oil recovery was achieved with a shpam pore volume of 0.3 and concentration of 1500 mg/l. the displacement efficiency tests confirmed that the high viscoelasticity of shpam imparted anomalous enhanced oil recovery efficiency to the hpam. | viscoelastic displacement and anomalously enhanced oil recovery of a novel star-like amphiphilic polyacrylamide |
exoplanets hat-p-7b and corot-2b have an unusual quirk: instead of having eastward equatorial winds, like the majority of hot jupiters, these two hot jupiters have westward winds. a new study explores whether magnetic fields cause this odd reversal.blowing the wrong wayartists impression of hat-p-7b, an inflated hot jupiter. [nasa, esa, and g. bacon (stsci)]you might think that the hottest and therefore brightest part of a tidally locked hot jupiter should be the part that directly faces its nearby host star. surprisingly, our observations of hot jupiters have generally revealed an offset for the peak brightness thats slightly east of the point directly facing the host. these observations suggest that hot jupiters host strong eastward-blowing winds near their equators that can displace their hottest point.two planets break this rule, however: hat-p-7b and corot-2b. observations of both of these hot jupiters instead reveal hotspots that lie west of the point facing the host. astronomers have generally interpreted this to imply that these two planets have westward-blowing equatorial winds but why?there are a number of proposed explanations for this odd apparent reversal:the planet may not be tidally locked as expected; if it rotates on its axis slightly slower than it orbits its host, this could drive westward winds.the apparent offset hotspot location could be an illusion caused by asymmetric cloud distribution.interactions of the planets magnetic field with its atmosphere could modify its wind pattern.in a new study led by alexander hindle (newcastle university, uk), a team of scientists explores the feasibility of this third option.magnetic wavesplot of the geopotential, which traces temperature, in the authors simulations, with (bottom) and without (top) the presence of magnetic fields. the hotspot (marked with a white cross) displaces to the east for the hydrodynamic case and to the west for the magnetohydrodynamic case. [hindle et al. 2019]hindle and collaborators use both analytic models and simulations to show what happens in the atmosphere of a planet with a strong magnetic field. they explore a layer of atmosphere that can behave like shallow water, developing planetary-scale waves. without a magnetic field, these waves will naturally travel eastward. but in the presence of a strong toroidal magnetic field, the wave shears as it travels, resulting in westward-tilting eddies. this drives the winds to switch direction to the west.the authors next calculate the minimum magnetic field strength needed to create this equatorial wind reversal for planets with the properties of hat-p-7b and corot-2b. they find that an inflated hot jupiter like hat-p-7b would need a field strength above just 6 gauss (for comparison, the earths magnetic field is 1 g). estimated field strengths for inflated hot jupiters lie in the 50100 g range, so attributing hat-p-7bs wind reversal to magnetic fields is well within reason.for an ordinary hot jupiter like corot-2b, however, a field strength of 3,000 g is needed. the maximum expected field strength for a hot jupiter like corot-2b is 250 g, which isnt sufficient to drive the reversal. hindle and collaborators conclude that a different mechanism is likely at work on this planet.more observations of hot jupiters in the future as well as three-dimensional simulations will help us to further understand the wind behavior in the atmospheres of these toasty planets.citationshallow-water magnetohydrodynamics for westward hotspots on hot jupiters, a. w. hindle et al 2019 apjl 872 l27. doi:10.3847/2041-8213/ab05dd | reversing winds on hot jupiters |
magnetic fields are ubiquitous in nature, with currents sheets found on numerous stellar bodies. here, we model plasma diamagnetism's affects in the current sheet in a earth-like magnetotail. a commonly used analytic model for magnetic field reversals is the harris equilibrium. in this model, the particle motion is completely integrable, and is characterized by a strongly peaked (in z) density profile that asymptoticly approaches zero, and a magnetic field that varies in z, but points in the x direction. when we allow for normal component to the current sheet of the magnetic field, the particle dynamics are nonintegrable, and frequently chaotic.we have developed a test-particle simulation method for calculating the self-consistent equilibrium of the earth's magnetotail that fully incorporates the nonlinear/chaotic charged particle dynamics of the ions. the equilibrium of the magnetic field is qualitatively similar to the harris model, but the density is asymptoticly constant and the current is created by a completely different mechanism. we show that the current density can be broken into a free current density and a bound current density. the free current is formed by the meandering motion of ions in the vicinity of the field reversal, and the bound current is caused by plasma diamagnetism. furthermore, the more field aligned the ion sources are in the asymptotic region, the thinner the current sheet, the more peaked the density profile, and the smaller the effects of diamagnetism. | effects of diamagnetism onmagnetotail current sheet equilibrium |
in this study, we investigated the time-independent dynamics (disc structure, forces and torques) of a quasi-keplerian disc around a millisecond pulsar (msp) with an internal dynamo. we considered the disc around a msp to be divided into the inner, middle and outer regions. by assuming that the disc matter flows in a quasi-keplerian motion, we derived analytical equations for a complete structure (temperature, pressure, surface density, optical depth and magnetic field) of a quasi-keplerian thin accretion disc, and the pressure gradient force (pgf). in our model, the msp-disc interaction results into magnetic and material torques, such that for a given dynamo (ɛ ) and quasi-keplerian (ξ ) parameter, we obtained enhanced spin-up and spin-down torques for a chosen star spin period. results obtained reveal that pgf results into episodic torque reversals that contribute to spinning-up or spinning-down of a neutron star, mainly from the inner region. the possibility of a quasi-keplerian disc is seen and these results can explain the observed spin variations in msps like sax j1808.4-3658 and xte j1814-338. | on the structure of quasi-keplerian accretion discs surrounding millisecond x-ray pulsars |
we present new results of fully general relativistic magnetohydrodynamic (grmhd) simulations of binary neutron star (bns) mergers performed with the whisky code. all the models use a piecewise polytropic approximation of the apr4 equation of state (eos) for cold matter, together with a ''hybrid'' part to incorporate thermal effects during the evolution. we consider both equal and unequal-mass models, with total masses such that either a supramassive ns or a black hole (bh) is formed after merger. each model is evolved with and without a magnetic field initially confined to the stellar interior. we present the different gravitational wave (gw) signals as well as a detailed description of the matter dynamics (magnetic field evolution, ejected mass, post-merger remnant properties, disk mass). our new simulations provide a further important step in the understanding of these gw sources and their possible connection with the engine of short gamma-ray bursts (both in the ``standard'' and in the ``time-reversal'' scenarios) and with other electromagnetic counterparts. | general relativistic magnetohydrodynamic simulations of binary neutron star mergers with the apr4 equation of state |
the influence of positronium photoionization rate on the heating of psr j0250+5854 polar cap is considered. it is assumed that the polar cap is heated only by reverse positrons accelerated in pulsar diode. it is supposed that pulsar diode is located near the star surface (polar cap model) and operates in the steady state space charge-limited flow regime. the influence of a small-scale magnetic field on the electric field inside the pulsar diode is taken into account. the reverse positron current is calculated in the framework of two models: rapid and gradually screening. to calculate the production rate of electron-positron pairs we take into account only the curvature radiation of primary electrons and its absorption in magnetic field. it is assumed that some fraction of electron-positron pairs may be created in bound state that can later be photoionized by thermal photons from star surface. | the influence of positronium photoionization rate on the heating of j0250+5854 polar cap |
a novel experiment resembling a planar plasma gun has been developed to produce magnetically driven high-energy-density (hed) plasma jets on the 1 ma, 220 ns rise time cobra generator at cornell university. the experimental setup consists of a central pin electrode that injects a single gas puff on axis, surrounded by a second annular electrode with a continuous gas injection slit. a permanent ring magnet is housed within the central electrode to provide an initial poloidal magnetic field which links the two electrodes. in this way, the experiment mimics a magnetized central object such as a star or blackhole surrounded by a rotating accretion disk. the resulting free-boundary, high aspect ratio plasma jets strongly resemble naturally occurring astrophysical jets. here, we investigate extended magnetohydrodynamic (xmhd) effects on jet dynamics and stability via the ability to have the cathode as the central pin electrode and anode as the annular electrode or vice versa with the added flexibility to reverse the polarity of the background poloidal magnetic field independently of the cathode and anode. measurements of densities, temperatures, velocities, and magnetic fields are collected using thomason scattering, b-dot probes, faraday rotation, laser interferometry, and optical spectroscopy. the experimental results are supported by 3d modelling using perseus an xmhd code; simulation results are used to study the evolution of canonical fields, flux tubes, and helicity in the plasma jets. this work was supported by the doe office of science grant no. de-sc0023238. | extended magnetohydrodynamic effects on dynamics and stability of magnetically driven high energy density plasma jets |
we present the results from the australian long baseline array (lba) observations of the ground- and excited-state oh masers at high resolutions towards the massive star-forming region g351.417+0.645 in 2012. we obtain the most accurate spatial gradient of magnetic fields at ground state transitions and verify the reliability of magnetic field strengths measured from previous lower resolution observations. in comparison with previous lba observations in 2001 at 6.0 ghz, we identified several matched zeeman pairs. we found that the oh maser features have no significant change of magnetic field strengths and directions with small internal proper motions, implying quite stable physical conditions. additionally, we found that 1665- and 6035-mhz oh maser features reveal the same trend of reversal of magnetic fields. moreover, we also analyzed the physical conditions at different locations from the coincidence of different oh maser transitions based on current oh maser models. | lba high resolution observations of ground- and excited-state oh masers towards g351.417+0.645 |
the influence of the positronium photoionization rate on the polar cap x-ray luminosity of old radio pulsars is considered. it is assumed that the polar cap is heated only by reverse positrons accelerated in the pulsar diode. it is supposed that the pulsar diode is in a stationary state with the lower plate located near the star surface (polar cap model) occupies all the pulsar tube cross section and operates in the regime of steady space charge by the limited electron flow. the influence of a small-scale magnetic field on the electric field inside the pulsar diode is taken into account. the reverse positron current is calculated in the framework of two models: rapid and gradual screening. to calculate the production rate of electron-positron pairs we take into account only the curvature radiation of primary electrons and its absorption in the magnetic field. it is assumed that some fraction of electron-positron pairs is created in a bound state (positronium). later, such positroniums are photoionized by thermal photons from the polar cap. | the influence of the positronium photoionization rate on the polar cap x-ray luminosity of radio pulsars |
scan quantum mechanics is a novel interpretation of some aspects of quantum mechanics in which the superposition of states is only an approximate effective concept. quantum systems scan all possible states in the superposition and switch randomly and very rapidly among them. a crucial property that we postulate is quantum inertia, that increases whenever a constituent is added, or the system is perturbed with all kinds of interactions. once the quantum inertia iq reaches a critical value icr for an observable, the switching among its different eigenvalues stops and the corresponding superposition comes to an end, leaving behind a system with a well defined value of that observable. consequently, increasing the mass, temperature, gravitational strength, etc. of a quantum system increases its quantum inertia until the superposition of states disappears for all the observables and the system transmutes into a classical one. moreover, the process could be reversible. entanglement can only occur between quantum systems because an exact synchronization between the switchings of the systems involved must be established in the first place and classical systems do not have any switchings to start with. future experiments might determine the critical inertia icr corresponding to different observables, which translates into a critical mass mcr for fixed environmental conditions as well as critical temperatures, critical electric and magnetic fields, etc. in addition, this proposal implies a new radiation mechanism from astrophysical objects with strong gravitational fields, giving rise to non-thermal synchrotron emission, that could contribute to neutron star formation. superconductivity, superfluidity, bose-einstein condensates, and any other physical phenomena at very low temperatures must be reanalyzed in the light of this interpretation, as well as mesoscopic systems in general. | quantum inertia stops superposition: scan quantum mechanics |
w49 a is a star-forming region (sfr) found in the constellation of aquila. it contains 3 active regions: w49 north (w49 n), w49 south west (w49 sw) and w49 south (w49 s). we present preliminary results from two epochs (e-)merlin observations of all ground-state oh masers towards the star-forming region (sfr) complex w49 a. the first epoch of observations was done in full-polarization mode with merlin in 2005 while the second epoch was obtained only in dual circular polarization during the test observations of the upgraded e-merlin in 2013. the overall maser spatial distributions in both epochs are in good agreement. we found several new high velocity maser features up to +34 km s-1 and -28 km s-1. the magnetic field strengths are between 1.1 to 10.8 mg. all three sources show evidence of magnetic field reversal. | full polarization analysis of oh masers at 18-cm toward w49 a star forming region |
radio pulsar j0250+5854 is the slowest pulsar among rotation powered pulsars. it is an old pulsar with spin-down age τ = 13.7 · 106 years, which rotates with period p = 23.54 s, the strength of dipolar magnetic field at pole estimated by pulsar spin-down rate is bdip = 5.1 · 1013 g [1]. such pulsars lie beyond conventional pulsar "death line" and its existence is usually explained by the presence of surface small-scale magnetic field (see, for example, [2]). the other explanation is presented by [3]. in this paper the influence of surface small-scale magnetic field on the heating of psr j0250+5854 polar cap is considered with assumption that the pulsar is close to aligned, i.e. the inclination angle is χ . 30°. it is assumed that the polar cap is heated only by reverse positrons accelerated in pulsar diode. it is supposed that pulsar diode is located near the star surface (polar cap model) and operates in the steady state space charge-limited flow regime. the reverse positron current is calculated in the framework of two models: rapid and gradually screening. to calculate the production rate of electron-positron pairs we take into account only the curvature radiation of primary electrons and its absorption in magnetic field. it is assumed that some fraction of electron-positron pairs may be created in bound state that can later be photoionized by thermal photons from star surface. we do not take into account the influence of polarization of curvature radiation on pair generation. also we do not taken into account positronium decay. it is shown that under this assumptions almost all electron-positron pairs are produced in bound state and the multiplicity of photoionized pairs may exceed 10−102 per primary electron | the influence of small-scale magnetic field on the heating of j0250+5854 polar cap |
masers provide an important, high-resolution probe of the near-circumstellar environment of late-type evolved stars. sio maser polarization may be key in characterizing their magnetic fields. however, the detailed mechanisms responsible for the polarization of sio masers in their near-circumstellar environments continue to be the subject of debate. primary sources may include the local magnetic field or anisotropic pumping, while additional polarization may arise due to conversion from linear to circular modes through scattering or faraday rotation. reducing uncertainties in maser polarization theory is critical to our understanding of the astrophysics of these regions. the linear polarization in some masers displays a rotation of ∼π/2 as a function of position within the feature; such features can provide robust constraints on sio maser polarization theories as they allow inference of the angle between the magnetic field and the line of sight, θ. we analyzed a single sio ν = 1, j = 1-0 maser feature displaying a linear polarization rotation of >π/2 toward the mira variable, tx cam, as observed by the very long baseline array in five epochs forming part of a prior larger imaging sequence. while we find that the fractional linear polarization across the feature is consistent the asymptotic theoretical solution for saturated masers in the limit of small zeeman splitting, with the rotation occurring as θ passes through a critical angle, the polarization angle itself rotates too smoothly to arise from this mechanism alone. possible causes of this discrepancy include a variation in θ along each sampled line of sight and faraday rotation. we provide the first quantitative estimate of the former, requiring a change in θ of ~9 degrees along each line of sight. to investigate the latter, we developed a new theoretical formalism for radiative transport of maser polarization more general than several previous approaches, and specifically including optional faraday rotation. preliminary results indicate that, while faraday rotation can increase the net change in the angle of linear polarization, it does little to smooth the instantaneous flip predicted by θ crossing the critical angle. in addition, our results constitute the first indication that two previous theoretical solutions for circular polarization generated under small zeeman splitting are consistent with each other, as well as with our new formalism. the two studies described here provide important new constraints on maser polarization theory and open new observational and theoretical avenues for further exploration of this area of research. | observational and theoretical studies of sio maser polarization toward late-type evolved stars: insights from evpa reversal features |
the afrl shiva star capacitor bank (1300 μf, up to 120 kv) used typically at 4 to 5 mj stored energy, 10 to 15 ma current, 10 μs current rise time, has been used to drive metal shell (solid liner) implosions for compression of axial magnetic fields to multi-megagauss levels, suitable for compressing magnetized plasmas to mif conditions. mif approaches use magnetic field to reduce thermal conduction relative to inertial confinement fusion (icf). mif substantially reduces required implosion speed and convergence. using profiled thickness liner enables large electrode apertures and field-reversed configuration (frc) injection. using a longer capture region, frc trapped flux lifetime was made comparable to implosion time and an integrated compression test was conducted. the frc was radially compressed a factor of ten, to 100x density >1018 cm-3 (a world frc record), but temperatures were only 300-400 ev, compared to intended several kev. compression to megabar pressures was inferred by the observed liner rebound, but the heating rate during the first half of the compression was less than the normal frc decay rate. principal diagnostics were soft x-ray imaging, soft x-ray diodes, and neutron measurements. this work has been supported by doe-ofes. | magnetized inertial fusion (mif) research at the shiva star facility |
the use of an induction motor requires the process of stopping the motor speed quickly, using mechanically and electrically generated braking torque. dynamic braking, which is done by making a magnetic field stationary motor. this condition is carried out by injecting a dc current into the three phase induction motor stator coil after the connection of the stator coil is released from the ac supply voltage source. the advantages of dynamic braking include ease of speed regulation of three-phase induction motors and mechanical losses can be reduced. a system for reversing the direction of star-delta three phase induction motor rotation with programmable logic control (plc) as a controller. requires an exchange of rotating directions from the direction of turning right to turn left. the process of changing the direction of rotation of the motor requires dynamic braking, the braking process is carried out by injecting a dc current into the stator coil of an induction motor, which is related to plc. by applying dynamic braking on a three phase induction motor it is obtained faster than without dynamic braking. the injection current is given to a 3-phase induction motor of 1 kw for dynamic braking of 2.2 a. the timing of the motor stops for a minimum load of 100 watt without braking at 6.2 seconds and with braking of 4.4 seconds, for a maximum load of 600 w, without braking. | dynamic braking application on three phase induction motor using plc |
the influence of surface small-scale magnetic field on the heating of psr j0250+5854 polar cap is considered. it is assumed that the polar cap is heated only by reverse positrons accelerated in pulsar diode. it is supposed that pulsar diode is located near the star surface (polar cap model) and operates in the steady state space charge-limited flow regime. the reverse positron current is calculated in the framework of two models: rapid and gradually screening. to calculate the production rate of electron-positron pairs we take into account only the curvature radiation of primary electrons and its absorption in magnetic field. it is assumed that some fraction of electron-positron pairs may be created in bound state that can later be photoionized by thermal photons from star surface. | the influence of small-scale magnetic field on the heating of j0250+5854 polar cap |
we show that the neutrino chirality flip, which can take place in the core of a neutron star at birth, is an efficient process to allow neutrinos to anisotropically escape, thus providing a to induce the neutron star kick velocities. the process is not subject to the no-go theorem since although the flip from left- to right-handed neutrinos happens at equilibrium, the reverse process does not take place given that right-handed neutrinos do not interact with matter and therefore detailed balance is lost. for simplicity, we model the neutron star core as being made of strange quark matter. we find that the process is efficient when the neutrino magnetic moment is not smaller than 4.7 × 10−15μ b, where μb is the bohr magneton. when this lower bound is combined with the most stringent upper bound, which uses the luminosity data obtained from the analysis of sn 1987a, our results set a range for the neutrino magnetic moment given by 4.7 × 10−15 ≤ μ ν/μb ≤ (0.1−0.4) × 10−11. the obtained kick velocities for natal conditions are consistent with the observed ones and span the correct range of radii for typical magnetic field intensities. | lower bound for the neutrino magnetic moment from kick velocities induced at the birth of neutron stars |
recently, some fast radio bursts (frbs) have been reported to exhibit complex and diverse variations in faraday rotation measurements (rm) and polarization, suggesting that dynamically evolving magnetization environments may surround them. in this paper, we investigate the faraday conversion (fc) effect in a binary system involving an frb source and analyze the polarization evolution of frbs. for a strongly magnetized high-mass companion binary, when an frb with ~100% linear polarization passes through the radial magnetic field of the companion star, the circular polarization (cp) component will be induced and oscillate symmetrically around the point with the degree of cp equal to zero, the rate and amplitude of the oscillation decrease as the frequency increases. the very strong plasma column density in the hmcbs can cause cp to oscillate with frequency at a very drastic rate, which may lead to depolarization. near the superior conjunction of the binary orbit, the dm varies significantly due to the dense plasma near the companion, and the significant fc also occurs in this region. as the pulsar moves away from the superior conjunction, the cp gradually tends toward zero and then returns to its value before incidence. we also investigate the effect of the rotation of the companion star. we find that a sufficiently significant rm reversal can be produced at large magnetic inclinations and the rm variation is very diverse. finally, we apply this model to explain some polarization observations of psr b1744-24a and frb 20201124a. | polarization evolution of fast radio burst sources in binary systems |
the scattering of neutral particles by an atomic nucleus can lead to electronic ionization and excitation through a process known as the migdal effect. we revisit and improve upon previous calculations of the migdal effect, using the dirac-hartree-fock method to calculate the atomic wave functions. our methods do not rely on the use of the dipole approximation, allowing us to present robust results for higher nuclear recoil velocities than was previously possible. our calculations provide the theoretical foundations for future measurements of the migdal effect using neutron sources, and searches for dark matter in direct detection experiments. we show that multiple ionization must be taken into account in experiments with fast neutrons, and derive the semi-inclusive probability for processes that yield a hard electron above a defined energy threshold. we present results for the noble elements up to and including xenon, as well as carbon, fluorine, silicon and germanium. the transition probabilities from our calculations are publicly available. | precise predictions and new insights for atomic ionization from the migdal effect |
due to the low nuclear recoils, sub-gev dark matter (dm) is usually beyond the sensitivity of the conventional dm direct detection experiments. the boosted and migdal scattering mechanisms have been proposed as two new complementary avenues to search for light dm. in this work, we consider the momentum-transfer effect in the dm-nucleus scattering to derive the new bounds on sub-gev dm for these two scenarios. we show that such an effect is sizable so that the existing bounds on the dm-nucleus scattering cross section can be improved significantly. | new strong bounds on sub-gev dark matter from boosted and migdal effects |
we provide a mathematica package, directdm, that takes as input the wilson coefficients of the relativistic effective theory describing the interactions of dark matter with quarks, gluons and photons, and matches it onto an effective theory describing the interactions of dark matter with neutrons and protons. the nonperturbative matching is performed at leading order in a chiral expansion. the one-loop qcd and qed renormalization-group evolution from the electroweak scale down to the hadronic scale, as well as finite corrections at the heavy quark thresholds are taken into account. we also provide an interface with the package dmformfactor so that, starting from the relativistic effective theory, one can directly obtain the event rates for direct detection experiments. | directdm: a tool for dark matter direct detection |
in most direct detection experiments, the free nuclear recoil description of dark matter scattering breaks down for masses ≲100 mev , or when the recoil energy is comparable to a few times the typical phonon energy for dark matter lighter than 1 mev, scattering via excitation of a single phonon dominates and has been computed previously, but for the intermediate mass range or higher detector thresholds, multiphonon processes dominate. we perform the first calculation of the scattering rate via multiphonon production for the entire kev-gev dark matter mass range, assuming a harmonic crystal target. we provide an analytic description that connects the single phonon, multiphonon, and the nuclear recoil regimes. our results are implemented in the public package darkelf. | dark matter direct detection from the single phonon to the nuclear recoil regime |
in this letter, we show that the wino-higgsino dark matter (dm) is detectable in near future dm direct detection experiments for almost all consistent parameter space in the spontaneously broken supergravity (sugra) if the muon g - 2 anomaly is explained by the wino-higgsino loop diagrams. we also point out that the present and future lhc experiments can exclude or confirm this sugra explanation of the observed muon g - 2 anomaly. | wino-higgsino dark matter in mssm from the g - 2 anomaly |
the low-background, vuv-sensitive 3-inch diameter photomultiplier tube r11410 has been developed by hamamatsu for dark matter direct detection experiments using liquid xenon as the target material. we present the results from the joint effort between the xenon collaboration and the hamamatsu company to produce a highly radio-pure photosensor (version r11410-21) for the xenon1t dark matter experiment. after introducing the photosensor and its components, we show the methods and results of the radioactive contamination measurements of the individual materials employed in the photomultiplier production. we then discuss the adopted strategies to reduce the radioactivity of the various pmt versions. finally, we detail the results from screening 286 tubes with ultra-low background germanium detectors, as well as their implications for the expected electronic and nuclear recoil background of the xenon1t experiment. | lowering the radioactivity of the photomultiplier tubes for the xenon1t dark matter experiment |
one of the next frontiers in dark-matter direct-detection experiments is to explore the mev to gev mass regime. such light dark matter does not carry enough kinetic energy to produce an observable nuclear recoil, but it can scatter off electrons, leading to a measurable signal. we introduce a semianalytic approach to characterize the resulting electron-scattering events in atomic and semiconductor targets, improving on previous analytic proposals that underestimate the signal at high recoil energies. we then use this procedure to study the time-dependent properties of the electron-scattering signal, including the modulation fraction, higher-harmonic modes and modulation phase. the time dependence can be distinct in a nontrivial way from the nuclear scattering case. additionally, we show that dark-matter interactions inside the earth can significantly distort the laboratory-frame phase-space distribution of sub-gev dark matter. | modulation effects in dark matter-electron scattering experiments |
the most sensitive haloscopes that search for axion dark matter through the two photon electromagnetic anomaly convert axions into photons through the mixing of axions with a large background direct current (dc) magnetic field. in this work we apply the poynting theorem to the resulting axion modified electrodynamics and identify two possible poynting vectors, one which is similar to the abraham poynting vector in electrodynamics and the other to the minkowski poynting vector. inherently the conversion of axions to photons is a nonconservative process with respect to the created oscillating photonic degree of freedom. we show that the minkowski poynting theorem picks up the added nonconservative terms while the abraham does not. the nonconservative terms may be categorized more generally as "curl forces," which in classical physics are nonconservative and nondissipative forces localized in space, not describable by a scalar potential and exist outside the global conservative physical equations of motion. to understand the source of energy conversion and power flow in the detection systems, we apply the two different poynting theorems to both the resonant cavity haloscope and the broadband low-mass axion haloscope. our calculations show that both poynting theorems give the same sensitivity for a resonant cavity axion haloscope, but predict markedly different sensitivity for the low-mass broadband capacitive haloscope. hence we ask the question, can understanding which one is the relevant one for axion dark matter detection be considered under the framework of the abraham-minkowski controversy? in reality, this should be confirmed by experiment when the axion is detected. however, many electrodynamic experiments have ruled in favor of the minkowski poynting vector when considering the canonical momentum in dielectric media. in light of this, we show that the axion modified minkowski poynting vector should indeed be taken seriously for sensitivity calculation for low-mass axion haloscopes in the quasistatic limit, and predict orders of magnitude better sensitivity than the abraham poynting vector equivalent. | poynting vector controversy in axion modified electrodynamics |
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