Ion leakage, ion reflection, ion heating and shock‐front reformation in a simulated supercritical quasi‐parallel collisionless shock

Abstract

Ion reflection, ion leakage, ion heating and shock‐front reformation are studied based on the simulation results of a supercritical quasi‐parallel shock. The backstreaming ions upstream of the simulated shock are predominantly leakage ions. The leakage of downstream ions is regulated by the large‐amplitude waves in the shock transition region. These waves not only can act as a filter to reduce the leakage ion number density, but also can energize the backstreaming ions to result in the suprathermal upstream ions. The average temperature of the leakage ions is about 100 times that of the upstream incoming ions. The average leakage ion density is about 2% ∼ 3% of the upstream ion density. Ion reflections occur intermittently due to coherent ion scattering by the large‐amplitude whistler waves near the shock front. Most of the coherently reflected ions are subsequently scattered back toward the downstream to contribute to the ion heating. Only a small fraction of the reflected ions can escape upstream with an average number density about 0.1% ∼ 0.2% of the upstream ion density. The shock‐front appears to reform as characterized by the decay and reappearance of the large‐amplitude whistler waves near the shock ramp. We wish to propose a possible explanation for the shock‐front reformation process. Insufficient dissipation in the supercritical shock can lead to a highly‐steepened shock ramp, which in turn can emit large‐amplitude whistler waves to result in the ion reflection event. Since ion reflections can provide additional dissipation, to counter balance the nonlinear steepening and thereby reduce the wave amplitude and turn off the ion reflection event. The cycle of under‐dissipation followed by over‐dissipation is proposed to be a possible cause for the shock‐front reformation process.

Effect of strong thermalization on shock dynamical behavior

“…This difference between hybrid and PIC simulations have been clarified only recently by Hellinger et al [2002]. The shock self‐reformation is also observed at quasi‐parallel shocks conspicuously [ Burgess , 1989; Lyu and Kan , 1990; Winske et al , 1990; Onsager et al , 1991; Scholer et al , 1993]. The different mechanisms responsible for shock front nonstationarity have been recently reviewed by Lembège et al [2004].…”

Short large‐amplitude magnetic structures and whistler wave precursors in a full‐particle quasi‐parallel shock simulation

“…This so‐called interface mode was suggested to be the main dissipation mechanism during the reformation process. Lyu and Kan [1990] found in their quasi‐parallel shock simulations that the incident ion beam can be decelerated and reflected at the higher‐frequency whistler waves immediately upstream of the shock ramp. The incident ions are ultimately thermalized in the whistler waves, and a new shock forms upstream.…”

Global‐scale simulation of foreshock structures at the quasi‐parallel bow shock

“…Hybrid simulations have been carried out for the nonlinear structure and ion heating of quasi‐parallel shocks [e.g., Goodrich , 1985; Onsager et al , 1991; Thomas et al , 1990; Winske et al , 1990; Scholer and Terasawa , 1990; Lyu and Kan , 1990; Scholer , 1993; Scholer et al , 1993; Krauss‐Varban , 1995]. The hybrid codes include full ion kinetics and have been used to study various collisionless shocks.…”