Effect of alpha particles on the shock structure

Abstract

The magnetic profile of a collisionless shock is shaped by ions. Dynamics of the ions is governed by the fields in the shock front. Species with different charge‐to‐mass ratio have different gyroradii; thus, their distributions upon crossing the shock became substantially different. Accordingly, the dependencies of the density and temperature on the coordinate along the shock normal should differ. In a stable one‐dimensional stationary shock the total pressure must remain constant throughout. Therefore, the magnetic pressure should anticorrelate with the particle pressure. Since the particle pressure is not uniform, downstream spatial magnetic oscillations should arise. Each of the species results in its own dependence of the pressure on the coordinate along the shock normal; therefore, the oscillations are not spatially periodic. The magnetic compression in the shocks with alpha particles is higher than the magnetic compression in purely proton shocks, while the amplitude of the downstream magnetic oscillations is lower.

Reformation of rippled quasi‐parallel shocks: 2‐D hybrid simulations

“…Particles show different behaviors at a quasi‐perpendicular shock and at a quasi‐parallel shock. At a quasi‐perpendicular shock, which has a distinguishable magnetic structure including foot, ramp and overshoot, a part of upstream incident particles can be reflected and quickly transmit into the downstream [ Leroy et al ., ; Sckopke et al ., ; Hada et al ., ; Lembège et al ., ; Ofman et al , ; Yang et al ., , , ; Guo and Giacalone , ; Ofman and Gedalin , ; Hao et al ., ; Gedalin , , , ; Johlander et al ., ; Tsubouchi et al ., ]; also, some reflected particles may travel along the background magnetic field to further upstream and lead to an ion foreshock [ Meziane et al ., ; Mazelle et al ., ; Savoini et al ., ]. At a quasi‐parallel shock, backstreaming particles generated by reflection can move far upstream along the background magnetic field, and then they will interact with upstream incident particles, resulting in the excitation of ultralow frequency (ULF) waves with oblique propagation [ Burgess , ; Scholer and Burgess , ; Lucek et al ., , , ; Tsubouchi and Lembège , ; Eastwood et al ., , ; Omidi et al ., , ; Wilson et al ., ; Shan et al ., , ; Wu et al ., ; Hao et al ., , ; Johlander et al ., ].…”

Understanding the Role of α Particles in Oblique Heliospheric Shock Oscillations

“…Most previous theoretical and modeling studies have considered electron‐proton plasma; however, α particles may have significant effects on the structure and downstream oscillations of magnetic field in oblique shocks due to their significant mass flux in the solar wind, in particular at times of increased solar activity, as evident from Wind data (e.g., Kasper et al, ). Test particle methods were used previously to model the acceleration of heavy ions by oblique shocks (e.g., Gedalin, ; Yang et al, ) and recently applied in combination with electron‐proton hybrid models in low Mach number shocks (e.g., Gedalin et al, ). The effects of various degree of α relative abundance on reformation of perpendicular shocks were studied with 1‐D PIC models (Rekaa et al, ).…”

Rankine‐Hugoniot Relations in Multispecies Plasma With Gyrotropic Anisotropic Downstream Ion Distributions

“…It was shown that even in collisional shocks different species are heated differentially (Bellei et al, ). Recently, it was shown that protons and α particles experience kinematic relaxation differently, and this affects the downstream magnetic field (Gedalin, ). Thus, it is necessary to establish new RH in a multispecies plasma to relate the upstream region to the downstream region where the distributions have already kinematically relaxed to the gyrotropic shape but are not isotropic yet.…”

“…The profile is simplified since whistler waves can be present in the upstream region and superimposed on the ramp (Wilson et al, 2012;Wilson III et al, 2017). For ions the effect of the macroscopic profile strongly dominates (Gedalin, 2017;Ofman et al, 2009;Oka et al, 2005). The shock ramp is narrow so that in the lowest approximation ions are decelerated by the cross-shock potential and only weakly deflected by the magnetic field.…”