Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Plasma Physics
Volume 65, Number 11
Monday–Friday, November 9–13, 2020; Remote; Time Zone: Central Standard Time, USA
Session TO16: Space Physics: Sun and Solar WindLive
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Chair: Shan Wang, University of Maryland, College Park |
Thursday, November 12, 2020 9:30AM - 9:42AM Live |
TO16.00001: Narrowband large amplitude oblique whistler-mode waves in the solar wind and their interaction with electrons Cynthia Cattell, Benjamin Short, Tien Vo, Aaron Breneman, Peter Grul Large-amplitude (up to 70 mV/m) whistler-mode waves at frequencies of \textasciitilde 0.2 to 0.4 f$_{\mathrm{ce\thinspace }}$(electron cyclotron$_{\mathrm{\thinspace }}$frequency)are frequently observed in the solar wind. The waves are obliquely propagating at angles close to the resonance cone, with significant electric fields parallel to the background magnetic field, enabling strong interactions with solar wind electrons. Frequencies and/or propagation angles are distinctly different from whistler-mode waves usually observed in the solar wind, and amplitudes are 1 to 3 orders of magnitude larger. Waves occur most often in association with stream interaction regions, and are often `close-packed.' Wave occurrence as a function of normalized electron heat flux and beta is consistent with the whistler heat flux fan instability. The oblique propagation and large amplitudes of these whistlers enable resonant interactions with electrons over a broad energy range, and, unlike parallel whistlers, don't require that the electrons and waves counter-propagate. Therefore, they are much more effective in modifying solar wind electron distributions than parallel-propagating waves. We show results from a 3d particle tracing code showing the strong interactions of electrons, providing evidence for the importance of these waves for the evolution of solar wind electrons. [Preview Abstract] |
Thursday, November 12, 2020 9:42AM - 9:54AM Live |
TO16.00002: Alfv\'{e}nic turbulence in an expanding, collisionless, magnetized plasma Archie Bott, Lev Arzamasskiy, Matthew Kunz, Eliot Quataert, Jonathan Squire Using hybrid-kinetic particle-in-cell simulations, we study the evolution of an expanding, collisionless, magnetized plasma in which Alfv\'{e}nic turbulence is persistently driven. Pressure anisotropy generated adiabatically by the plasma expansion (and consequent decrease in the mean magnetic-field strength) gradually reduces the effective elasticity of the field lines, causing residual energy build-up in the turbulent fluctuations and modifying their spatial anisotropy. Critical balance is maintained even as the linear frequency of the Alfv\'{e}nic fluctuations is modified by this pressure anisotropy. For a sufficiently large plasma beta, the plasma eventually becomes unstable to the oblique firehose instability, which excites rapidly growing magnetic fluctuations at ion-Larmor scales. Through associated pitch-angle scattering of particles, the ion pressure anisotropy is maintained near marginal firehose stability, even as the plasma's expansion continues. The resulting evolution of parallel and perpendicular temperatures is non-adiabatic. Predictions may be tested by measurements of high-beta plasma in the near-Earth solar wind and have implications for understanding the interplay between macro- and micro-scale physics in hot, dilute, astrophysical plasmas. [Preview Abstract] |
Thursday, November 12, 2020 9:54AM - 10:06AM Live |
TO16.00003: On Stochastic Ion Heating in the Inner Heliosphere: Radial Trends and Parker Solar Probe Observations Mihailo Martinovic, Kristopher Klein, Benjamin Chandran, Jia Huang, Emily Lichko, Justin Kasper, Michael Stevens, Sofiane Bourouaine Stochastic heating (SH) is a non-linear plasma heating mechanism, frequently proposed as a candidate to explain the strong heating of the solar wind ions perpendicular to the magnetic field. It is driven by the violation of magnetic moment invariance due to large-amplitude, low-frequency Alfvenic turbulent fluctuations. Using Helios and Parker Solar Probe (PSP) observations, we track the radial variation of SH throughout the inner heliosphere from 0.16 to 1 au. We find that SH is increasingly important as one observes plasma closer to the Sun, specifically that the stochastic heating rate varies as $Q_{SH} \sim r^{-2.5}$. In accordance with theoretical predictions, observations of flattop shaped proton velocity distributions are characteristic for periods where SH is predicted to be a dominant heating mechanism. We also find that $Q_{SH}$ does not significantly vary inside intermittent structures, such as switchbacks regularly measured by PSP. [Preview Abstract] |
Thursday, November 12, 2020 10:06AM - 10:18AM Live |
TO16.00004: A New Model for Self-Consistent Simulations of Kinetic Dynamics in the Expanding Solar Wind Maria Elena Innocenti, Elisabetta Boella, Anna Tenerani, Marco Velli With the launch of Solar Orbiter, it is now possible to probe magnetically connected solar wind plasma across significantly separated heliocentric distances (at Parker Solar Probe, Solar Orbiter, Earth), and have a direct insight into the evolution of solar wind kinetic process with heliocentric distance. Kinetic features are ubiquitous in the young solar wind and rarer (but still non negligible) at 1 AU [Bale et al, 2019]. During propagation, kinetic processes constrain solar wind parameters [Stverak et al, 2008; Matteini et al, 2013] and regulate heat flux [Scime et al, 1994]. We simulate this evolution with the fully kinetic semi-implicit Expanding Box Model code EB-iPic3D [Innocenti et al, 2019a, b], which models kinetically a solar wind plasma parcel moving away from the Sun while expanding in the transverse direction. We investigate how plasma expansion triggers the onset and modifies the evolution of kinetic instabilities (eg, electron firehose and whistler instability) that constrain solar wind parameters and impact heat flux evolution with heliocentric distance. We then study the competition of expansion and turbulence in determining the solar wind temperature radial dependence. [Preview Abstract] |
Thursday, November 12, 2020 10:18AM - 10:30AM Live |
TO16.00005: Electron acceleration by pressure anisotropy instabilities under solar flare plasma conditions Mario Riquelme, Alvaro Osorio, Lorenzo Sironi, Daniel Verscharen We use particle-in-cell (PIC) simulations to show that pressure anisotropy instabilities can stochastically accelerate electrons in plasmas with temperatures, magnetic fields and densities suitable to solar/stellar flares. Using a setup where the global magnetic field grows, we self-consistently produce the growth of electron pressure anisotropy, driving different electron scale plasma modes unstable (whistler and z-modes). In the regime $\omega_{ce}/\omega_{pe} \sim 1$ (where $\omega_{ce}$ and $\omega_{pe}$ are the electron cyclotron and plasma frequencies, respectively), and after the instabilities have reached their non-linear, saturated regime (after the global magnetic field has been amplified by a factor $\sim 3$), the electron energy spectrum can develop a power-law tail with indices between $\sim 2$ and $3$, and reach $\sim$MeV energies. [Preview Abstract] |
Thursday, November 12, 2020 10:30AM - 10:42AM Live |
TO16.00006: Cross-Helicity Generation and Structure Formation in $\beta$-plane MHD Turbulence Robin Heinonen, Maya Katz, Patrick Diamond We study turbulence in the solar tachocline using the 2-D magnetohydrodynamic equations in the $\beta$-plane approximation. Cross-helicity conservation in this system is explicitly broken by the $\beta$ term. We use an analytical closure to study the nonlinear cross-helicity dynamics and relate the generation of finite cross-helicity to the induced electric field. Furthermore, we use a method based on deep supervised learning, previously introduced to study the Hasegawa-Wakatani system, to infer from numerical simulation a mean-field model for the turbulence dynamics. We use the inferred model to study structure formation in the limit of weak mean toroidal magnetic field. (In the presence of a stronger field, the fluctuations Alfv\'enize and zonal structures are destroyed.) Results are presented and compared to analytical calculations. [Preview Abstract] |
Thursday, November 12, 2020 10:42AM - 10:54AM Live |
TO16.00007: Electric Fields and Currents of the Sun and Solar Wind Charles Driscoll A simple model of solar electric fields explains the solar wind energetics and coronal "heating", invoking only thermo-electric and photo-electric forces. In the (collisional) solar interior, thermal electron pressure \textit{necessarily} generates a radial electric field, integrating to a surface field eE$_{\mathrm{th}}$(R$_{\mathrm{s}})\cong $1.4eV/Mm, comparable to the proton weight m$_{\mathrm{p}}$g$=$2.8eV/Mm. In the (less collisional) plasma "sheath" of the photosphere and corona, the outward photon flux $\Gamma _{\mathrm{\gamma }}=$60.MW/m$^{\mathrm{2}}$ causes additional electron displacement, giving eE$_{\mathrm{\gamma }}$(r) $= \quad \sigma _{\mathrm{\gamma e}} \quad \Gamma_{\mathrm{\gamma }}$ /c. Here, the main uncertainty is the photon cross-section $\sigma_{\mathrm{\gamma e}}$ for electrons \textit{correlated} with protons: H-minus and "rydberg" hydrogen states have $\sigma _{\mathrm{\gamma e}}\cong $0.5x10\textasciicircum -20m$^{\mathrm{2}}$, whereas \textit{isolated} electrons have Thompson cross-section $\sigma_{\mathrm{\gamma e}}\cong $0.7x10\textasciicircum -28m$^{\mathrm{2}}$. An average cross-section $\sigma_{\mathrm{\gamma e}}\cong $3x10\textasciicircum -24m$^{\mathrm{2}}$ can generate the observed solar wind, as "collisional runaway" protons accelerate out of the 2.keV gravity well and up to 1.3 keV kinetic energy within several R$_{\mathrm{s}}$. This coherent proton/electron flow will glow as the K-Corona, obviating the traditional T$=$100eV hydrostatic models. Fluctuating 3D electric fields and charge currents will arise from convective surface granulation ("roiling") and from "current pinch" propagation dynamics, generating the observed \textit{fluctuating} magnetic fields. Some characteristics of solar wind currents can be ascertained from the extensive databases of satellite magnetic field measurements. [Preview Abstract] |
Thursday, November 12, 2020 10:54AM - 11:06AM Live |
TO16.00008: A Kinetic Plasma Rosetta Stone for Understanding Plasma Heating and Particle Acceleration in Space and Astrophysical Plasmas Gregory G. Howes, Andrew J. McCubbin, Sarah A. Horvath, Peter Montag, Collin R. Brown, James Juno, Kristopher G. Klein, Jason M. TenBarge, James W. R. Schroeder The general question of how plasmas are heated and particles accelerated underlies many key challenges at the frontier of heliophysics and astrophysics, including solar coronal heating, particle acceleration in solar flares and supernova remnants, and auroral electron acceleration. The hot and diffuse plasmas in many space and astrophysical environments lead to weakly collisional conditions, so plasma kinetic theory is essential to understand both how particles are energized and whether that leads to heating of the bulk plasma or the directed energization of accelerated particles. The field-particle correlation technique is an innovative method to understand how the electromagnetic fields energize particles in weakly collisional plasmas, yielding a velocity-space signature that is characteristic of a given mechanism of energization. These signatures can be used both to distinguish and identify the mechanism at play and to determine the net rate of particle energization. I will present the construction of a "Rosetta Stone" of these velocity-space signatures that can be used to identify the mechanisms of energization in kinetic plasma turbulence, collisionless magnetic reconnection, and collisionless shocks. [Preview Abstract] |
Thursday, November 12, 2020 11:06AM - 11:18AM Live |
TO16.00009: Electron Landau Damping in Simulated Dissipation Range Turbulence Sarah Horvath, Gregory Howes, Andrew McCubbin Turbulence in astrophysical plasmas is thought to play a role in the heating of the solar wind, though many questions remain to be solved regarding the exact nature of the mechanisms driving this process in the heliosphere. In particular, the physics of collisionless interactions between particles and the electromagnetic fields in the dissipation range of the turbulent cascade remains incompletely understood. A recent analysis of an interval of Magnetosphere Multiscale (MMS) mission observations using the field-particle correlation technique found the first direct evidence for electron Landau damping in the dissipation range of the solar wind. Motivated by this discovery, we perform a high-resolution gyrokinetic simulation of the turbulence in the MMS interval to investigate the role of electron Landau damping in the dissipation of turbulent energy. We employ the field-particle correlation technique on our simulation data, compare the results to the known velocity-space signatures of Landau damping outside the dissipation range, and evaluate the net electron energization. We find qualitative agreement between the numerical and observational results for some key aspects of the energization, and speculate on the nature of disagreements in light of experimental factors, such as differences in resolution, and of developing insights into the nature of field-particle interactions in the presence of dispersive kinetic Alfv\'{e}n waves. [Preview Abstract] |
Thursday, November 12, 2020 11:18AM - 11:30AM Live |
TO16.00010: Particle-In-Cell simulations of the oblique whistler heat flux instability. Scattering of the strahl electrons into the halo and heat flux regulation in the solar wind near the Sun. Alfredo Micera, Andrei Zhukov, Rodrigo Alvaro Lopez, Elisabetta Boella, Miaria Elena Innocenti, Marian Lazar, Giovanni Lapenta The whistler heat flux instability is a collisionless kinetic process that is often invoked to explain the scattering of strahl electrons into halo (e.g. Maksimovic et al. 2005) and the resultant regulation of the heat-flux in the solar wind. However, its role in this respect is poorly understood. To shed light into this matter, we investigate the evolution of counter-streaming core and strahl electrons under conditions typically encountered in the solar wind near the Sun. We prove with two-dimensional PIC simulations that a parallel or oblique whistler heat flux instability can be excited, depending on the initial drift velocities of the two electron populations. We confirm that the impact of parallel whistler waves on the regulation of the electron heat flux and the pitch-angle scattering of strahl electrons is only marginal(e.g. Kuzichev et al. 2019). On the contrary, we observe for the first time that the oblique whistler waves produce enhanced pitch-angle scattering of suprathermal electrons resulting in the transfer of a significant fraction of strahl electrons into the halo. The process is accompanied by a substantial reduction of the heat flux carried by the field-aligned strahl electrons. [Preview Abstract] |
Thursday, November 12, 2020 11:30AM - 11:42AM Live |
TO16.00011: The Effects of Non-Equilibrium Features on Solar Wind Plasma Evolution Jada Walters, Kristopher Klein, Daniel Verscharen, Michael Stevens, Jenny Verniero Typical treatments of plasma waves assume a hot, drifting bi-Maxwellian distribution function, but distribution functions observed in the solar wind often contain significant departures from this idealized model.This project investigates how these departures from an idealized bi-Maxwellian distribution function affects the onset and evolution of microinstabilities. To this end we use the Arbitrary Linear Plasma Solver (ALPS) to find the parameters of the expected waves using actual proton distributions measured by the Wind spacecraft. These parameters are then compared to the results from a bi-Maxwellian fit of the same proton distributions. By doing this, we can quantify how important these non-equilibrium features are to the evolution of the plasma. We plan to apply this analysis to proton distributions observed by Parker Solar Probe to gain insight into the different plasmas observed closer to the Sun, but this method could be applied to determine the effects of non-equilibrium features of the distribution function in a variety of physical contexts. [Preview Abstract] |
Thursday, November 12, 2020 11:42AM - 11:54AM Live |
TO16.00012: Field-Particle Correlation Signatures of Magnetic Pumping Peter Montag The mechanisms of particle energization are crucial to understanding space and astrophysical plasmas. Previous work has shown correlations between the electric and magnetic fields and the plasma distribution function show distinct signatures for different mechanisms of particle energization. We show that a model of magnetic pumping which has been proposed as an energization mechanism in the solar wind predicts a distinctive correlation signature. We then test this prediction against data from magnetic pumping simulations. [Preview Abstract] |
Thursday, November 12, 2020 11:54AM - 12:06PM Live |
TO16.00013: Electron temperature of the solar wind Stanislav Boldyrev, Cary Forest, Jan Egedal The temperature of the solar wind plasma expanding from the hot solar corona does not decrease with the distance as fast as predicted by the adiabatic expansion law. The non-adiabatic solar-wind cooling is a long-standing problem of space plasma physics. We discuss the analytic results on the temperature of the electron component of the solar wind [1]. We argue that heating of the solar-wind electrons results from the energy exchange of the fast electrons propagating from the corona along the background magnetic field (the beam or strahl) and the electrons trapped between the electric potential and magnetic mirror walls (the core). An analogous mechanism was considered previously in relation to the electron heating in the expanders of the mirror machines (the regions between the mirror throat and the wall of the expanding chamber) [2]. We argue that due to weak Coulomb collisions, the temperature of the electrons declines with the heliospheric distance as $T(r)\sim r^{-2/5}$, in good agreement with the observations. [1] S. Boldyrev, C. Forest,J. Egedal, Electron temperature of the solar wind, Proceedings of the National Academy of Sciences, 117, 9232-9240 (2020). [2] D. D. Ryutov, Axial electron heat loss from mirror devices revisited, Fusion Sci. Technol. 47, 148–154 (2005). [Preview Abstract] |
Thursday, November 12, 2020 12:06PM - 12:18PM Not Participating |
TO16.00014: In-Situ Switchback Formation in the Expanding Solar Wind Jonathan Squire, Benjamin Chandran, Romain Meyrand Recent near-sun solar-wind observations from Parker Solar Probe have found a highly dynamic magnetic environment, permeated by abrupt radial-field reversals, or ``switchbacks.'' We show that many features of the observed turbulence are reproduced by a spectrum of Alfv\'enic fluctuations advected by a radially expanding flow. Starting from simple superpositions of low-amplitude outward-propagating waves, our expanding-box compressible MHD simulations naturally develop switchbacks because (i) the normalized amplitude of waves grows due to expansion and (ii) fluctuations evolve towards spherical polarization (i.e., nearly constant field strength). These results suggest that switchbacks form in-situ in the expanding solar wind and are not indicative of impulsive processes in the chromosphere or corona. [Preview Abstract] |
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