Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Plasma Physics
Volume 67, Number 15
Monday–Friday, October 17–21, 2022; Spokane, Washington
Session PO06: Plasma Astrophysics IILive Streamed
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Chair: Frederick Skiff, Univ. Iowa Room: Ballroom 111 C |
Wednesday, October 19, 2022 2:00PM - 2:12PM |
PO06.00001: Relativistic magnetic explosions Maxim Y Lyutikov We develop models of magnetically-driven relativistic explosions, with application to flares from Soft Gamma-Ray Repeaters and Fast Radio Bursts. Non-stationarity, and the conservation of magnetic flux make magnetized explosion qualitatively different from stationary MHD flows, as well as fluid explosions. We study generation of relativistic coronal ejection, conditions for generating causally-disconnected flows, and later dynamics of ejected structures. |
Wednesday, October 19, 2022 2:12PM - 2:24PM |
PO06.00002: 3D dynamics of magnetar magnetospheres: Kinked fluxtubes and global eruptions Jens F Mahlmann, Alexander A Philippov, Vassilios Mewes, Bart Ripperda, Elias R Most, Lorenzo Sironi The origin of outbursts of hard X-rays from highly magnetized neutron stars is still unknown, yet the release of magnetic energy through kink and tearing instabilities in magnetar magnetospheres can explain such flaring activity. Crustal surface motions twist the magnetar magnetosphere by shifting frozen-in footpoints of magnetic field lines. 2D axisymmetric magnetospheres develop catastrophic instabilities when their displacement angle along the magnetar surface exceeds 180 degrees. The release of strong toroidal fields buried deep inside the magnetosphere can power events up to the observed energetics of giant flares, $10^47$ erg. However, 2D models are dimensionally restricted, and important non-axisymmetric dynamics are not captured. Thus, we model the full 3D dynamics of twisted force-free flux bundles in dipolar magnetospheres. In this 3D setup, a slowly rotating disc on the stellar surface (hotspot) gradually twists an extended flux tube. For varying hotspot locations and extent, the flux bundle develops different instabilities. While twists applied at higher latitudes quickly open up the dipole magnetosphere, the kink instability can develop closer to the star and dissipate magnetic energy locally along closed field lines. We discuss criteria for the development of different instabilities and how they can be distinguished in X-ray flare observations by the amount of released energy. |
Wednesday, October 19, 2022 2:24PM - 2:36PM |
PO06.00003: Alfven-wave turbulence in neutron star and black hole magnetospheres Joonas Nättilä, Andrei M Beloborodov Relativistic turbulence can illuminate the magnetosphere of astrophysical compact objects, like pulsars, magnetars, neutron star mergers, and black hole accretion flows. This turbulence can be powered by "ringing" of these extreme magnetospheres --- propagation of magnetic shear waves on magnetic field line bundles. These Alfven waves are theoretically known to interact nonlinearly with each other and to produce wave turbulence that can forward cascade the energy from the largest scales to the smallest plasma scales. We use fully-kinetic 3D simulations to study the energization of magnetically-dominated plasmas by this wave turbulence. The turbulence is excited naturally by simulating collisions of large-scale Alfven waves and following their subsequent interaction. We demonstrate that the nonlinear interaction between the waves operates also in the relativistic/magnetically-dominated plasma regime, generates a wave cascade, and is capable of dissipating the magnetic energy of the initial waves. The resulting turbulence is observed to produce quasi-thermal heating that energizes particles only along the magnetic field lines. This can generate synchrotron-silent plasmas that have important implications for radiation signals originating from the compact object magnetospheres. |
Wednesday, October 19, 2022 2:36PM - 2:48PM |
PO06.00004: Particle-In-Cell Simulations of Pulsar Magnetospheres using WarpX Revathi Jambunathan, Hannah E Klion, Andrew Myers, Jean-Luc Vay, Michael Rowan, Ann S Almgren, John B Bell, Axel Huebl, Remi Lehe, Weiqun Zhang The plasma composition and structure of pulsar magnetospheres and the physical processes that drive particle acceleration are not well understood. Global pulsar magnetosphere simulations are required to answer these questions. However, resolving the current sheet skin-depth which is O(~106) smaller than the pulsar radius for realistic systems, is intractable even on large supercomputers. Thus, the magnetic field strength in global PIC simulations is typically scaled-down restricting the maximum energy of the charged particles. We will present the effect of scaling down the magnetic field on particle acceleration, energy dissipation, and Poynting flux. We use WarpX, a highly scalable, electromagnetic PIC code with advanced algorithms to mitigate numerical artifacts in mesh-refinement simulations. We will also present 3D simulations to study the effect of pulsar obliquity and plasma injection rate on the plasma structure and Poynting flux. Additionally, we explore the use of ultra high-order spectral methods (PSATD) to perform pulsar magnetosphere simulations and compare its accuracy and performance with traditional finite-difference methods. |
Wednesday, October 19, 2022 2:48PM - 3:00PM |
PO06.00005: General relativistic activation of the polar cap of compact neutron stars Rui P Torres, Fabio Cruz, Thomas Grismayer, Ricardo A Fonseca, Luis O Silva Magnetospheres of compact objects such as neutron stars and black holes are complex systems where quantum electrodynamic (QED) processes, kinetic-scale pair plasma physics and general relativity (GR) play all an important role. To study such intricate and exotic systems, advanced simulation techniques are required. In this work, we try to understand under which conditions the pulsar mechanism is possible. Previous works have shown that it is the appearance of a spacelike region on the polar cap, induced by the frame-dragging effect, that triggers this mechanism for low obliquity rotators [e.g. A.Philippov et al ApJ 2015 and S.Gralla et al ApJ 2016]. However, these works were either fluid models or neglected the GR effects on particle dynamics. Making use of a recently developed GR module for the particle-in-cell (PIC) code OSIRIS, we study the least favorable configuration, the aligned rotator, employing the slow rotation limit of the Kerr metric. We show that indeed it is possible to ignite the polar cap if GR effects are included. In addition, we demonstrate that the pulsar mechanism can only exist for stars with a Schwarzschild radius larger than 0.45 stellar radii. Finally, we characterize the new spacelike region and compare it to previous force-free models. |
Wednesday, October 19, 2022 3:00PM - 3:12PM |
PO06.00006: Global Kinetic Modeling of the Intrabinary Shock in Spider Pulsars Jorge I Cortes, Lorenzo Sironi Spider pulsars are compact binary systems composed of a millisecond pulsar and a low mass companion. The relativistic and strongly magnetized pulsar wind impacts onto the companion, ablating it and slowly consuming its atmosphere. This interaction gives rise to the formation of an intrabinary shock, a proposed site of nonthermal emission and particle acceleration. We perform global fully-kinetic particle-in-cell simulations of the interaction of a striped pulsar wind with a companion star. We present first-principles synchrotron spectra and light curves, which closely match the observations if the orbital angular momentum of the system is nearly aligned with the pulsar spin axis. |
Wednesday, October 19, 2022 3:12PM - 3:24PM |
PO06.00007: Electron heating in astrophysical blast waves Arno V Vanthieghem, Martin Lemoine, Laurent Gremillet, Vasileios Tsiolis, Anatoly Spitkovsky, Yasushi Todo, Frederico Fiuza Gamma-ray bursts, blazars, and supernovae provide ideal environments for efficient energy channeling between different plasma species through collective processes. Astrophysical shock waves are one of the most outstanding representatives of such complex many-body phenomena. Extensively studied in astrophysical and laboratory environments, observations and kinetic simulations indicate strong electron heating in the precursor of collisionless shock waves propagating in unmagnetized electron-ion plasmas. We outline a theoretical model accounting for the electron heating via a Joule-like process through the interplay between pitch-angle scattering in the microturbulence and the coherent electrostatic field induced by the difference in inertia between species. Using analytical kinetic estimates, semi-analytical Monte Carlo methods, and ab-initio Particle-In-Cell simulations, we first demonstrate the validity of this model in the relativistic regime relevant for the afterglow emission of gamma-ray burst [1] and then extend it to characterize the electron to ion temperature ratio in the downstream of nonrelativistic high-Mach numbers shock waves relevant for supernova remnants and laboratory experiments. |
Wednesday, October 19, 2022 3:24PM - 3:36PM |
PO06.00008: Cavitation and enhanced particle acceleration in relativistic shocks John R Peterson, Siegfried H Glenzer, Frederico Fiuza Relativistic collisionless shocks associated with gamma-ray bursts (GRBs) can accelerate charged particles to high energies. Central to this acceleration process is the formation of magnetic turbulence by plasma streaming instabilities such as the Weibel instability. We show that under certain conditions, a plasma cavitation instability can grow in the shock precursor, strongly amplifying both the strength and length scale of magnetic fields far beyond that produced via the Weibel instability. The cavities are driven by the shock-accelerated electron beam, which is charge- but not current-neutralized by the cold upstream ions. The growth rate and saturation size of these magnetic cavities are well described by analytical theory in both electron-ion and pair-loaded shocks. Cavitation is accompanied by strongly enhanced particle acceleration. |
Wednesday, October 19, 2022 3:36PM - 3:48PM |
PO06.00009: Electron and ion heating in high-Mach number collisionless shocks Alexis Marret, Frederico Fiuza Collisionless shocks are ubiquitous in astrophysical plasmas and play an important role in plasma heating, magnetic field amplification, and in accelerating high energy cosmic rays. A fundamental open question in collisionless shock physics is what are the mechanisms that control the difference in the temperature of ions and electrons in the downstream. Observations of high-Mach number shocks have shown that Te/Ti ≥ 0.1, indicating that electrons gain significantly more energy than that corresponding to simple thermalization of their initial kinetic energy. In this work we use fully-kinetic 3D simulations to investigate the electron and ion heating mechanisms in collisionless shocks. In the high Mach number regime, we find that Te/Ti is dictated by the interplay between the ion current-filamentation and the drift-kink instability. The connection of these results with recent experimental studies and their dependence on the Mach number will be discussed. |
Wednesday, October 19, 2022 3:48PM - 4:00PM |
PO06.00010: Parametric Decay Instability of Alfven Waves in Low-Beta Astrophysical Plasmas Hui Li, Zhaoming Gan, Xiangrong Fu In low-beta environments that often exist in the corona of stars (including our Sun), corona of accretion disks and astrophysical jets, Parametric Decay Instability (PDI) of Alfven waves provides a possible avenue of directly converting magnetic energy to compressible waves (slow modes) which could heat the plasmas. While the PDI is being actively investigated in the non-relativistic plasmas, its behavior in relativistic MHD is less well known. We will present results on the growth and saturation of such an instability in different parameter regimes. Applications for several astrophysical systems will be discussed as well. |
Wednesday, October 19, 2022 4:00PM - 4:12PM |
PO06.00011: Intermittency and electron heating in kinetic-Alfvén-wave turbulence Muni Zhou, Zhuo Liu, Nuno F Loureiro Recent high-resolution, in situ measurements of the electromagnetic fluctuations and plasma distribution functions have provided unprecedented opportunities to study the rich plasma dynamics in the kinetic range of the turbulence. We report analytical and numerical investigations of sub-ion scale turbulence in weakly collisional, low beta plasmas using a hybrid fluid-kinetic model, focusing on the spectral properties of the fluctuations and electron heating. In the isothermal limit, the numerical results strongly support a description of the turbulence as a critically-balanced Kolmogorov-like cascade of kinetic Alfvén wave fluctuations, as amended by Boldyrev & Perez (Astrophys. J. Lett. 758, L44 (2012)) to include intermittent effects. With the inclusion of electron kinetic physics, the energy spectrum is found to steepen due to electron Landau damping, which is enabled by the local weakening of nonlinearities in current sheets, and yields significant energy dissipation in the velocity space. The use of a Hermite formalism to express the velocity space dependence of the electron distribution function allows us to obtain an analytical, zeroth-order solution for the Hermite moments of the distribution, which is borne out by numerical simulations. |
Wednesday, October 19, 2022 4:12PM - 4:24PM |
PO06.00012: Electrical Resistivity of Collisionless, High-Beta Plasmas Himawan W Winarto, Matthew W Kunz The characteristic reversal scale of amplified magnetic fields during several stages of the fluctuation dynamo is set by the plasma resistivity. In weakly collisional high-beta plasmas, such as in the intracluster medium of galaxy clusters, the nature of this resistivity is likely to be influenced by Larmor-scale kinetic instabilities (firehose, mirror) that are excited by flow-driven pressure anisotropies. The magnetic structures created by these instabilities scatter and trap particles, modifying the plasma resistivity in a way that depends upon extrinsic quantities such as the magnetic-field strength. Using particle-in-cell simulations and scaling arguments, we investigate the changes in plasma resistivity caused by these micro-instabilities. By driving a current through the perturbed plasma, we solve the high-beta collisionless version of the Spitzer-Haerm problem. Our results can be implemented within fluid or hybrid-kinetic treatments to improve the fidelity of large-scale dynamo simulations. |
Wednesday, October 19, 2022 4:24PM - 4:36PM |
PO06.00013: Ion Heat Flux Instability in High-β, Weakly Collisional, Magnetized Plasma Evan L Yerger, Matthew W Kunz High-β plasmas can be highly magnetized (ρ/H ≪ 1) at the largest astro-physical scales, e.g., in the intracluster medium (ICM) of galaxy clusters. If the plasma is furthermore weakly collisional, the transport of momentum and heat is highly anisotropic with respect to the magnetic field direction. In thermally stratified plasmas at sufficiently high β, the parallel heat flux can be large enough to trigger a kinetic ion heat flux instability, which back-reacts on the transport by deforming the field lines on ion-Larmor scales. In this work, we use the hybrid particle-in-cell code PEGASUS++ to calculate the steady-state heat flux through a stratified, high-β, collisionless, magnetized plasma. By tracking a large sample of ions and simulating across a range of β and temperature gradient length scales, we calculate the effective collision operator for ion heat-flux-driven wave turbulence and use it to solve the Chapman-Enskog-Braginskii problem. We discuss the implications of our results for the electron heat flux whistler instability and magneto-thermal convection in general. |
Wednesday, October 19, 2022 4:36PM - 4:48PM |
PO06.00014: Turbulent Reacceleration of Streaming Cosmic Rays and Damping of Compressive Turbulence Chad Bustard, Peng Oh Subsonic, compressive turbulence transfers energy to cosmic rays (CRs), a process known as non-resonant reacceleration. It is often invoked to explain observed ratios of primary to secondary CRs at GeV energies, assuming wholly diffusive CR transport. However, such estimates ignore the impact of CR self-confinement and streaming. Furthermore, the back reaction of CRs, which damp compressive motions, on the turbulent flow has received scant attention. We study these issues in stirring box magnetohydrodynamic simulations using Athena++, with field-aligned diffusive and streaming CR transport. When streaming is included, reacceleration rates depend on plasma β. They are significantly slower than canonical reacceleration rates in low-β environments like the interstellar medium (ISM), but remain unchanged in high-β environments like the intracluster medium (ICM). Despite slow reacceleration rates, we show that CR "viscosity" can still significantly alter the turbulent cascade, steepening the turbulent power spectrum and damping small scale compressive turbulence. This "divergence-cleaning" renders small-scale turbulence largely solenoidal and could suppress fluctuations that are important for thermal instability and for resonant scattering of high-energy (E > 300 GeV) CRs. |
Wednesday, October 19, 2022 4:48PM - 5:00PM |
PO06.00015: Electron Re-acceleration via Ion Cyclotron Waves in the Intracluster Medium Aaron Tran, Lorenzo Sironi In galaxy clusters, the intracluster medium (ICM) is expected to host a diffuse, long-lived, and invisible population of "fossil" cosmic-ray electrons (CRe) with 1–100 MeV energies. These CRe, if re-accelerated 100x in energy, can contribute to diffuse low-frequency radio synchrotron emission. We study CRe scattering and energy gain due to ion cyclotron (IC) waves generated by continuously-driven compression in 1D particle-in-cell simulations. We find that pitch-angle scattering of CRe by IC waves induces energy gain via magnetic pumping. In an optimal range of IC-resonant momenta, CRe may gain up to ~10–30% of their initial energy in one compress/dilate cycle with magnetic field amplification ~3–6x, assuming adiabatic decompression without further scattering and averaging over initial pitch angle. |
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