Kinematic Collisionless Relaxation and Time Dependence of Supercritical Shocks with Alpha Particles

Abstract

Upon crossing, the shock front ions begin to strongly gyrate due to the effect of the macroscopic magnetic and electric field. The strongly nongyrotropic downstream distributions gradually gyrotropize due to the gyrophase mixing. Kinematic collisionless relaxation has been successful in explaining the downstream magnetic oscillations in time-independent planar laminar shocks. Time dependence of supercritical shocks becomes more pronounced with the increase of the Mach number. While downstream gyrating distributions are still formed due to macroscopic fields, pressure balance can no longer be maintained throughout, causing time dependence. The presence of alpha particles enhances the destabilization of the shock.

How non-stationary are moderately supercritical shocks?

“…In other words, the ramp position is time dependent in the shock frame and weakly oscillates. Such weak time dependence of moderately supercritical shocks has been found recently in other hybrid simulations (Gedalin 2019 a ; Ofman et al. 2019).…”

“…In other words, the ramp position is time dependent in the shock frame and weakly oscillates. Such weak time dependence of moderately supercritical shocks has been found recently in other hybrid simulations (Gedalin 2019a;Ofman et al 2019). It is worth noting that hybrid simulations show that shock formation and stable existence do not require the presence of microscopic fields (Burgess, Wilkinson & Schwartz 1989;Sckopke et al 1990;Scholer & Matsukiyo 2004;Moullard et al 2006;Comişel et al 2011;Ofman & Gedalin 2013a,b;Gedalin 2019a;Ofman et al 2019).…”

“…2006; Burgess & Scholer 2007; Scholer & Burgess 2007; Comişel et al. 2011; Ofman & Gedalin 2013 a , b ; Gedalin 2019 a ; Ofman et al. 2019).…”

Oblique High Mach Number Heliospheric Shocks: The Role of α Particles

“…In the hybrid model ions are modeled as large ensemble of particles embedded in the magnetic field and electrons are fluid. Previous models focused primarily on electron‐proton plasma (e.g., Burgess & Scholer, 2015; Burgess et al., 2016; Ofman & Gedalin, 2013a), while recently the models were extended by adding α particles in particle trajectory tracing model (Gedalin, 2017a, 2017b), in nonlinear one‐dimensional (1D) hybrid models (Gedalin, 2019a, 2019b; Gedalin et al., 2018) and in 2.5D hybrid models of quasi‐perpendicular shocks (Hao et al., 2014; Ofman et al., 2019; Preisser et al., 2020). Observations of surface irregularities of high‐ M interplanetary shocks at ion scales were reported (e.g., Wilson et al., 2017; Kajdič et al., 2019), and supplemented by two‐dimensional (2D) hybrid modeling of the e − p shock plasma.…”

“…In particular, rippling in Earth's quasi-perpendicular bow shock was detected by the four-constellation spacecraft of the Magnetospheric Multiscale mission and reported by Johlander et al (2016) in agreement with Ofman and Gedalin (2013a) 2.5D hybrid modeling prediction. It was realized early on after the discovery of the solar wind that the α particles are energetically important components of the SW plasma (Borrini et al, 1983;Geiss et al, 1970;Neugebauer & Snyder, 1962;Snyder & Neugebauer, 1964), and it was shown the criticality of the heliospheric shocks is affected by α particles (e.g., Gedalin, 2019b;Burgess, 1989), while their presence can vary considerably with the SW type, speed and the phase of the solar cycle with typical average 1%-5% relative abundance (Alterman & Kasper, 2019;Kasper et al, 2007Kasper et al, , 2012.…”