Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session NO06: Astrophysical Shocks, HED, and Cosmic RaysOn Demand
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Chair: Manjit Kaur, University of California, Irvine Room: Rooms 310-311 |
Wednesday, November 10, 2021 9:30AM - 9:42AM |
NO06.00001: A novel class of solutions for the strong spherical shocks driven by expanding pistons Dmitri D Ryutov The shocks in this configuration appear in numerous problems of astrophysics and geophysics. Known are self-similar solutions for a piston expanding in a radially stratified medium with a power-law density distribution or for a power-law time-dependence of piston velocity. We show that it is possible to generalize these solutions by exploiting an effect of geometrical stretching of the shocked material in a spherical geometry. For not-too-stiff equation of state, e.g. that of a fully-ionized plasma (gamma=5/3), the flow within the shocked gas turns out to be nearly isobaric. This allows one to produce analytical solutions for radial density variation of an ambient gas of the form of bumps or dimples, and for temporal variations of the piston velocity of the form of bumps or dimples. In particular, it is possible to describe interaction of a supernova-driven shock with the layers of a lower (higher) density or their combinations. Work performed for U.S. DOE by LLNS LLNL under Contract No. DE-AC52-07NA27344. |
Wednesday, November 10, 2021 9:42AM - 9:54AM |
NO06.00002: A Revised Theory of Diffusive Shock Acceleration Rebecca Diesing, Damiano Caprioli Diffusive shock acceleration (DSA) is an efficient mechanism that produces power-law distributions of nonthermal particles and is responsible for the acceleration of Galactic cosmic rays (CRs) at the forward shocks of supernova remnants (SNRs). However, observations of nonthermal SNR emission imply CR energy distributions that are generally steeper than E-2, the standard DSA prediction. Recent results from kinetic hybrid simulations suggest that such steep spectra may arise from the drift of magnetic structures with respect to the thermal plasma downstream of the shock. Using a semi-analytic model of non-linear DSA, we generalize this result to a wide range of shocks. By accounting for the motion of magnetic structures in the downstream, we produce CR energy distributions that are substantially steeper than E-2 and consistent with observations. Our revised theory of DSA reproduces both modestly steep spectra of Galactic supernova remnants (∝E-2.2) and the very steep spectra of young radio supernovae (∝E-3). |
Wednesday, November 10, 2021 9:54AM - 10:06AM |
NO06.00003: On the Microphysics of Relativistic Collisionless Electron-ion-positron Shocks Daniel Groselj, Lorenzo Sironi, Andrei M Beloborodov We present a set of first-principles simulations to elucidate the microscale physics of relativistic, pair-plasma-loaded weakly magnetized shocks. We show that even moderate changes in the plasma composition significantly impact the shock dynamics. When the bulk of the upstream momentum is carried by ions we find that (i) the strength of the mean magnetic field required for the transition from a Weibel to a Larmor mediated shock drops as a function of the pair-loading factor Z, (ii) the energy fraction transferred from ions to pairs is only weakly dependent on Z , and (iii) pair-loaded shocks are efficient particle accelerators only in the limit of vanishing external magnetization. The acceleration is enhanced through the formation of intense magnetic cavities that populate the precursor during late stages of shock evolution. Alongside simulations, we develop a set of theoretical estimates which yield predictions consistent with numerical results. Our findings have important implications for the modeling of the early afterglow emission of both short and long GRBs and place additional constraints on the maximum value of external magnetization that allows for particle acceleration. |
Wednesday, November 10, 2021 10:06AM - 10:18AM |
NO06.00004: Hybrid Simulations of Acceleration of Heavy Nuclei at Non-Relativistic Shocks Damiano Caprioli, Colby C Haggerty We investigate the diffusive shock acceleration of particles with mass to charge ratio, A/Q > 1. We introduce He- and C-like particles at solar abundances into 2D hybrid (kinetic ions/fluid electrons) simulations of non-relativistic collisionless shocks. |
Wednesday, November 10, 2021 10:18AM - 10:30AM |
NO06.00005: Plasma wakes driven by Compton scattering: nonlinear regime and electron acceleration Thomas Grismayer, Fabrizio Del Gaudio, Luis O Silva Plasma wakes can be generated with a large variety of drivers such as particle or light beams. The light beam sees the plasma as a dielectric medium and excites plasma modes via its ponderomotive force. It has been recently shown that there is an alternative to ponderomotive drivers [1]. Photons with a wavelength smaller than the plasma inter-particle distance only interact with the electrons of the plasma through Compton scattering. A photon burst exerts a piston force on the electron of the plasma which can lead to plasma wakes. We focus on this fundamental process for different photon frequencies, photon flux, and plasma magnetization. The analytical findings are in very good agreement with the simulations performed with the particle-in-cell code OSIRIS coupled to a Compton scattering module [2]. For low photon energy densities, the amplitude of the wakes remains linear, and one observes the generation of Langmuir and extraordinary modes. For extreme photon energy densities, the amplitude of the wake becomes non-linear and becomes analogous to the blowout of the regime observed with intense lasers [3]. We will highlight the typical mechanisms that eventually limit electron acceleration in these wakes such as phase velocity of the wake, dephasing, driver collimation, and driver depletion. |
Wednesday, November 10, 2021 10:30AM - 10:42AM |
NO06.00006: Lepton-driven Bell Instability: linear growth and saturation of the magnetic field Siddhartha Gupta, Damiano Caprioli, Colby C Haggerty Streaming instabilities driven by energetic charged particles are very crucial for understanding the self-confinement of these particles near the sources in astrophysical environments. Using kinetic plasma simulations, we investigate non-resonant (Bell) streaming instability driven by energetic leptons. We identify the conditions to drive the instability, characterise the linear and saturated stages and compare the results with the proton-driven case. Our results are important for understanding the phenomenology of astrophysical environments where leptons are accelerated (e.g., oblique shocks and around pulsar wind nebulae), and also laboratory experiments. |
Wednesday, November 10, 2021 10:42AM - 10:54AM |
NO06.00007: Electron-foreshock instabilities in oblique high-Mach-number shocks Martin S Weidl, Artem Bohdan, Paul J Morris, Frank Jenko, Martin Pohl Motivated by simulations of oblique shocks in supernova remnants, we investigate which instabilities are excited by relativistic electron beams in the extended foreshock of non-relativistic high-Mach-number shocks. We analyze 2D PIC simulations to obtain phase-space distributions in various regions of the electron foreshock, where shock-reflected electrons interact with the incoming upstream flow and excite characteristic instabilities. Periodic PIC simulations and linear dispersion analyses are used to determine where and how these instabilities are generated. |
Wednesday, November 10, 2021 10:54AM - 11:06AM |
NO06.00008: The role of plasma instabilities in relativistic radiation mediated shocks Arno V Vanthieghem, Jens F Mahlmann, Amir Levinson, Alexander A Philippov, Frederico Fiuza Relativistic radiation mediated shocks (RRMS) dictate the early emission in numerous transient sources such as supernovae, low luminosity gamma-ray bursts, binary neutron star mergers, and tidal disruption events. These shock waves are mediated by Compton scattering and copious electron-positron pair creation. It has been pointed out recently that a high pair multiplicity inside the shock transition leads to a lepton-baryon velocity separation, prone to plasma instabilities [1]. The interaction of the different species with this radiation-mediated microturbulence can lead to coupling and heating that is unaccounted for by current single-fluid models. |
Wednesday, November 10, 2021 11:06AM - 11:18AM |
NO06.00009: Study of the non-resonant streaming instability including thermal effects and particle collisions Alexis Marret, Andrea Ciardi, Roch Smets, Julien Fuchs Streaming cosmic rays can power the exponential growth of a seed magnetic field by exciting a non-resonant instability that feeds on their bulk kinetic energy. In this work we investigate analytically and numerically the effects of the background plasma temperature and particle collisions on the instability. We find that increasing the temperature of the ambient plasma can substantially reduce the growth rate and the magnitude of the saturated magnetic field. In collisionless or poorly collisional plasmas, we demonstrate that the instability generates pressure anisotropies that act to reduce the magnetic field amplification. We show that when proton Coulomb collisions are sufficiently frequent, these pressure anisotropies are suppressed and the magnetic field energy density can be amplified to values 20% higher than in the collisionless case. In contrast, simulations of poorly ionized plasmas confirm the rapid damping of the instability with increasing proton-neutral collisions predicted by linear fluid theory calculations. In addition to astrophysical applications, these results pave the way for the design of laboratory experiments on the non-resonant streaming instability. |
Wednesday, November 10, 2021 11:18AM - 11:30AM Not Participating |
NO06.00010: ULF waves downstream of ICME driven shocks. Xochitl Blanco Cano, Luis Preisser, Primoz Kajdic, Diana Rojas-Castillo, Christopher T Russell, Lan Jian, Janet G Luhmann IP shocks can be driven in the solar wind by fast coronal mass ejections. Past studies have shown that a variety of ULF waves can be observed upstream of these shocks. Less attention has been given to understand what wave modes permeate the sheath regions downstream of these shocks. It is expected that due to temperature anisotropy, ion cyclotron and mirror mode waves can grow downstream of IP shocks in a similar way to planetary magnetosheaths. Understanding these waves is important because they contribute to shock acceleration processes. In this work we use STEREO data to study wave structure downstream of ICME driven shocks. The waves observed downstream of IP quasi-parallel shocks have larger amplitudes than waves in the regions downstream of quasi-perpendicular shocks. A variey of waves can be found in the sheath regions of ICMEs, even when IP shocks are weak, mostly for quasi-perpendicular shocks. These include ion cyclotron waves (ICW) with well defined peaks in frequency, broad spectra ICW and mirror mode storms, which tend to occur for higher plasma beta. |
Wednesday, November 10, 2021 11:30AM - 11:42AM |
NO06.00011: Energy partition between ions and electrons in high Mach number nonrelativistic collisionless shocks Vasileios Tsiolis, Anatoly Spitkovsky, Patrick Crumley Astrophysical environments such as supernova remnants and galaxy clusters host nonrelativistic collisionless shocks which can accelerate and heat protons and electrons. These shocks are capable of equilibrating the temperature between the particle species. We use two-dimensional particle-in-cell simulations in order to investigate the structure of magnetized nonrelativistic shocks and to study the downstream electron-to-ion temperature ratio, Te/Ti, over a wide range of sonic, Ms, and Alfvénic Mach numbers, MA. We find that for Ms < 2, the electron-to-ion temperature ratio shows weak dependence on MA, and the two species are close to energy equipartition. At higher Ms, the temperature ratio is predominately determined by MA, experiencing a minimum value of Te/Ti ~ 0.1 at MA ~ 10 and reaching an asymptotic value of Te/Ti ~ 0.3 at higher Mach numbers, 60 < MA < 140. In general, the downstream electron temperature is of order of 10% of the upstream ion kinetic energy. This is considerably larger than the adiabatic prediction of the 1/1836 electron-ion temperature ratio in an electron-ion collisionless shock. High MA shocks are susceptible to the Weibel instability, which creates filamentary structures at the shock foot and ramp. The presence of filaments in density and magnetic field at the high MA shock precursor is responsible for the super-adiabatic heating of electrons, through energization from parallel electric field oscillations associated with the screening of the cross-shock potential electric field. We focus on the details of this heating mechanism and its dependence on shock parameters. |
Wednesday, November 10, 2021 11:42AM - 11:54AM |
NO06.00012: Particle energization in solenoidal and compressive relativistic plasma turbulence Vladimir V Zhdankin Turbulence may be driven in high-energy astrophysical plasmas through a broad variety of mechanisms, which are generally classified by their degree of compressibility. The influence of the large-scale driving mechanism on kinetic turbulence and the associated energy dissipation has not yet been established for the relativistic collisionless plasma regime relevant to these systems. To address this, I will describe recent results from particle-in-cell simulations of turbulence with solenoidal and compressive driving. These simulations cover the relativistic regime (where electrons and ions are both relativistically hot) and the semi-relativistic regime (where electron temperature is relativistic but ion temperature is sub-relativistic). I will summarize the properties of turbulent fluctuations, the electron-ion energy partition, and nonthermal particle acceleration for both types of driving. These results have implications for modeling emission and cosmic ray acceleration in high-energy astrophysical systems such as hot accretion flows onto black holes. |
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