Shock acceleration in partially neutral plasmas

TL;DR

This paper develops a non-linear theory of shock acceleration in partially neutral plasmas, linking Balmer line observations from young supernova remnants to shock efficiency in cosmic ray production, considering charge-exchange effects.

Contribution

It introduces a new non-linear model for shock acceleration in partially neutral plasmas, incorporating charge-exchange effects and predicting observable Balmer line broadening.

Findings

Even with low CR energy conversion, Balmer line broadening is detectable.

Charge-exchange influences ion-neutral temperature equilibration.

The theory provides a method to test shock acceleration efficiency via Balmer lines.

Abstract

We present the non-linear theory of shock acceleration applied to SNRs expanding into partially neutral plasma. Using this theory we show how the Balmer lines detected from young SNRs can be used to test the efficiency of shocks in the production of cosmic rays. In particular we investigate the effect of charge-exchange between protons and neutral hydrogen occurring in the precursor formed ahead of the shock. In this precursor the CR pressure accelerate the ionized component of the plasma and a relative velocity between protons and neutral hydrogen is established. On the other hand the charge-exchange process tends to equilibrate ions and neutrals resulting in the heating of both components. We show that even when the shock converts only a few per cent of the total bulk kinetic energy into CRs, the heating is efficient enough to produce a detectable broadening of the narrow Balmer lines emitted by the neutral hydrogen.