Electron Heating, Magnetic Field Amplification, and Cosmic Ray Precursor Length at Supernova Remnant Shocks

TL;DR

This paper explores the magnetic field amplification and shock precursor length at supernova remnant shocks, analyzing how different saturation mechanisms affect electron heating and the potential observability of these precursors.

Contribution

It provides a theoretical estimate of precursor lengths under nonresonant and resonant amplification, linking these to electron heating and observational signatures in supernova remnants.

Findings

Precursor length can reach 10^{17} - 10^{18} cm and may be detectable.

Nonresonant saturation leads to higher magnetic fields and longer precursors.

Observational data from SN 1006 and Cas A support the theoretical predictions.

Abstract

We investigate the observability, by direct and indirect means, of a shock precursor arising from magnetic field amplification by cosmic rays. We estimate the depth of such a precursor under conditions of nonresonant amplification, which can provide magnetic field strengths comparable to those inferred for supernova remnants. Magnetic field generation occurs as the streaming cosmic rays induce a plasma return current, and may be quenched either by nonresonant or resonant channels. In the case of nonresonant saturation, the cosmic rays become magnetized and amplification saturates at higher magnetic fields. The precursor can extend out to $10^{17} - 10^{18}$ cm and is potentially detectable. If resonant saturation occurs, the cosmic rays are scattered by turbulence and the precursor length will likely be much smaller. The dependence of precursor length on shock velocity has implications for electron heating. In the case of resonant saturation, this dependence is similar to that in the more familiar resonantly generated shock precursor, which when expressed in terms of the cosmic ray diffusion coefficient $\varkappa $ and shock velocity $v_s$ is $\varkappa /v_s$. In the nonresonantly saturated case, the precursor length declines less quickly with increasing $v_s$. Where precursor length proportional to $1/v_s$ gives constant electron heating, this increased precursor length could be expected to lead to higher electron temperatures for nonresonant amplification. This should be expected at faster supernova remnant shocks than studied by previous works. Existing results and new data analysis of SN 1006 and Cas A suggest some observational support for this idea.