A mechanism that could stop the acceleration process within a collisionless shock

TL;DR

This paper analyzes the conditions under which particle acceleration in unmagnetized collisionless shocks halts, revealing that acceleration stops when about 30% of upstream particles are accelerated, based on instability analysis.

Contribution

It introduces a novel instability-based framework linking shock width, particle spectrum, and density ratio to the cessation of acceleration in collisionless shocks.

Findings

Acceleration stops at ~30% upstream particles accelerated.

Applicable only to unmagnetized shocks.

Provides a theoretical criterion for shock acceleration termination.

Abstract

Collisionless shocks are complex nonlinear structures that are not yet fully understood. In particular, the interaction between these shocks and the particles they accelerate remains elusive. Based on an instability analysis that relates the shock width to the spectrum of the accelerated particle and the shock density ratio, we find that the acceleration process could come to an end when the fraction of accelerated upstream particles reaches about 30\%. Only unmagnetized shocks are considered.