Dynamical effects of self-generated magnetic fields in cosmic ray modified shocks

TL;DR

This paper investigates how self-generated magnetic fields influence cosmic ray modified shocks, revealing that magnetic pressure significantly reduces shock modification and must be considered alongside turbulent heating in models.

Contribution

It demonstrates the dynamical importance of magnetic fields generated by cosmic rays, showing they reduce shock modifications more than previously assumed in unmagnetized models.

Findings

Magnetic fields with observed strengths have a significant dynamical effect on shocks.

Magnetic pressure can be comparable to thermal gas pressure, altering shock structure.

Neglecting magnetic feedback leads to overestimating shock modifications.

Abstract

Recent observations of greatly amplified magnetic fields ($\delta B/B\sim 100$) around supernova shocks are consistent with the predictions of the non-linear theory of particle acceleration (NLT), if the field is generated upstream of the shock by cosmic ray induced streaming instability. The high acceleration efficiencies and large shock modifications predicted by NLT need however to be mitigated to confront observations, and this is usually assumed to be accomplished by some form of turbulent heating. We show here that magnetic fields with the strength inferred from observations have an important dynamical role on the shock, and imply a shock modification substantially reduced with respect to the naive unmagnetized case. The effect appears as soon as the pressure in the turbulent magnetic field becomes comparable with the pressure of the thermal gas. The relative importance of this unavoidable effect and of the poorly known turbulent heating is assessed. More specifically we conclude that even in the cases in which turbulent heating may be of some importance, the dynamical reaction of the field cannot be neglected, as instead is usually done in most current calculations.