Electron Pre-acceleration at Nonrelativistic High-Mach-number Perpendicular Shocks

Abstract

We perform particle-in-cell simulations of perpendicular nonrelativistic collisionless shocks to study electron heating and pre-acceleration for parameters that permit the extrapolation to the conditions at young supernova remnants. Our high-resolution large-scale numerical experiments sample a representative portion of the shock surface and demonstrate that the efficiency of electron injection is strongly modulated with the phase of the shock reformation. For plasmas with low and moderate temperature (plasma beta 5 10, we explore the nonlinear shock structure and electron pre-acceleration for various orientations of the large-scale magnetic field with respect to the simulation plane, while keeping it at 90°to the shock normal. Ion reflection off of the shock leads to the formation of magnetic filaments in the shock ramp, resulting from Weibel-type instabilities, and electrostatic Buneman modes in the shock foot. In all of the cases under study, the latter provides first-stage electron energization through the shock-surfing acceleration mechanism. The subsequent energization strongly depends on the field orientation and proceeds through adiabatic or second-order Fermi acceleration processes for configurations with the out-of-plane and in-plane field components, respectively. For strictly out-of-plane field, the fraction of suprathermal electrons is much higher than for other configurations, because only in this case are the Buneman modes fully captured by the 2D simulation grid. Shocks in plasma with moderate p b provide more efficient pre-acceleration. The relevance of our results to the physics of fully 3D systems is discussed.

Observations of Magnetic Reconnection in the Transition Region of Quasi‐Parallel Shocks

“…We show that reconnecting current sheets are present within a disordered transition region close to the shock ramp, which is consistent with the appearance of these structures in recent hybrid and kinetic shock simulations (Bohdan et al, 2017;Gingell et al, 2017;Matsumoto et al, 2015). We show that reconnecting current sheets are present within a disordered transition region close to the shock ramp, which is consistent with the appearance of these structures in recent hybrid and kinetic shock simulations (Bohdan et al, 2017;Gingell et al, 2017;Matsumoto et al, 2015).…”

“…Magnetic reconnection, for which localized changes in magnetic topology result in rapid transfer of energy from fields to particles, has been observed in detail by Magnetospheric Multiscale (MMS) at Earth's magnetopause and more recently in the turbulent magnetosheath (Phan et al, 2018). In the standard model, reconnection occurs within an electron-scale diffusion region (Bohdan et al, 2017;Vasyliunas, 1975), while at ion scales coupled ions are ejected from the diffusion region as bidirectional jets (Gosling et al, 2005;Paschmann et al, 1979;Phan et al, 2000). In the standard model, reconnection occurs within an electron-scale diffusion region (Bohdan et al, 2017;Vasyliunas, 1975), while at ion scales coupled ions are ejected from the diffusion region as bidirectional jets (Gosling et al, 2005;Paschmann et al, 1979;Phan et al, 2000).…”

“…In contrast to magnetosheath observations reported by Phan et al (2018) and global hybrid simulations by Karimabadi et al (2014), structures discussed here appear within seconds of crossing the bow shock, suggesting a close association with shock processes and a rapid evolution. In the standard model, reconnection occurs within an electron-scale diffusion region (Bohdan et al, 2017;Vasyliunas, 1975), while at ion scales coupled ions are ejected from the diffusion region as bidirectional jets (Gosling et al, 2005;Paschmann et al, 1979;Phan et al, 2000). Reconnection exhausts then extend to much larger scales.…”

Magnetic Reconnection in a Quasi‐Parallel Shock: Two‐Dimensional Local Particle‐in‐Cell Simulation

“…Karimabadi et al () demonstrated by both hybrid simulations and fully kinetic simulations that reconnection occurs in turbulence in the Earth's magnetosheath. Matsumoto et al () and Bohdan et al () studied high Alfvén Mach number ( M A ~30 to 40) perpendicular shocks, and reconnection occurs in the transition region due to Weibel instability. Gingell et al () demonstrated by hybrid simulations that in the Earth's quasi‐parallel bow shock, reconnection occurs in both the transition and downstream regions.…”

PIC simulation methods for cosmic radiation and plasma instabilities

“…Only if the large-scale magnetic field is oriented strictly out of the simulation plane, the Larmor orbit of reflected ions, and hence the streaming velocity relative to the incoming electrons, δv, is contained in the simulation plane, and the Buneman waves with k δv are fully captured. Bohdan et al [34] present modified versions of equation 35 and 36 that are supposed to account for the projection effect but do not work perfectly, possibly on account of a modified alignment of the motional electric field and the equipotential surfaces of the Buneman waves in simulations with homogeneous magnetic field in the simulation plane.…”