Critical Mach Numbers for Magnetohydrodynamic Shocks with Accelerated Particles and Waves

TL;DR

This paper revisits the critical Mach number for MHD shocks, incorporating effects of accelerated particles and waves, highlighting the significant role of waves in shock dissipation and variability in particle acceleration events.

Contribution

It introduces the influence of wave amplification on the critical Mach number, extending traditional models by including wave effects in shock dissipation mechanisms.

Findings

Waves can significantly increase the critical Mach number at quasi-parallel shocks.

Accelerated particles have a negligible effect on the critical Mach number.

Wave amplification downstream enhances shock dissipation and variability in SEP acceleration.

Abstract

The first critical fast Mach number is defined for a magnetohydrodynamic shock as the Mach number where the shock transitions from subcritical, laminar, behavior to supercritical behavior, characterized by incident ion reflection from the shock front. The ensuing upstream waves and turbulence are convected downstream leading to a turbulent shock structure. Formally this is the Mach number where plasma resistivity can no longer provide sufficient dissipation to establish a stable shock, and is characterized by the downstream flow speed becoming subsonic. We revisit these calculations, including in the MHD jump conditions terms modeling the plasma energy loss to accelerated particles and the presence of waves associated with these particles. The accelerated particle contributions make an insignificant change, but the associated waves have a more important effect. Upstream waves can be strongly amplified in intensity on passing through the shock, and represent another means of shock dissipation. The presence of such waves therefore increases the first critical fast Mach number, especially at quasi-parallel shock where wave excitation is strongest. These effects may have significance for the solar regions where shock waves accelerate particles and cause Type II and Type III radio bursts, and also contribute to the event-to-event variability of SEP acceleration.