Magnetic and density spikes in cosmic ray shock precursors

TL;DR

This paper presents exact nonlinear MHD solutions describing magnetic and density spikes in cosmic ray shock precursors, revealing propagating solitons supported by CR return currents, with potential links to observed supernova remnant features.

Contribution

It introduces fully nonlinear ideal MHD solutions for magnetic spikes in cosmic ray precursors, highlighting their propagation and structural properties, which differ from linear wave predictions.

Findings

Localized magnetic spikes propagate ahead of shocks.

Soliton trains resemble observed X-ray stripes in Tycho SNR.

Magnetic field amplification varies with SNR evolution stages.

Abstract

In shock precursors populated by accelerated cosmic rays (CR), the CR return current instability is believed to significantly enhance the pre-shock perturbations of magnetic field. We have obtained fully-nonlinear exact ideal MHD solutions supported by the CR return current. The solutions occur as localized spikes of circularly polarized Alfven envelopes (solitons, or breathers). As the conventional (undriven) solitons, the obtained magnetic spikes propagate at a speed $C$ proportional to their amplitude, $C=C_{A}B_{{\rm max}}/\sqrt{2}B_{0}$. The sufficiently strong solitons run thus ahead of the main shock and stand in the precursor, being supported by the return current. This property of the nonlinear solutions is strikingly different from the linear theory that predicts non-propagating (that is, convected downstream) circularly polarized waves. The nonlinear solutions may come either in isolated pulses (solitons) or in soliton-trains (cnoidal waves). The morphological similarity of such quasi-periodic soliton chains with recently observed X-ray stripes in Tycho supernova remnant (SNR) is briefly discussed. The magnetic field amplification determined by the suggested saturation process is obtained as a function of decreasing SNR blast wave velocity during its evolution from the ejecta-dominated to the Sedov-Taylor stage.