Influence of the back-reaction of streaming cosmic rays on magnetic field generation and thermal instability

TL;DR

This paper investigates how streaming cosmic rays influence magnetic field generation and thermal instability in electron-ion plasmas, revealing that cosmic rays can significantly enhance certain plasma instabilities.

Contribution

It introduces a multi-fluid model that accounts for cosmic ray back-reaction, showing enhanced growth rates of streaming instabilities and clarifying conditions for thermal instability types.

Findings

Short wavelength perturbations grow faster than previously known cosmic-ray streaming instabilities.

Thermal instability is unaffected by cosmic rays in the studied model.

Derived dispersion relation links plasma and cosmic-ray parameters to instability types.

Abstract

Using a multi-fluid approach, we investigate streaming and thermal instabilities of the electron-ion plasma with homogeneous cold cosmic rays propagating perpendicular to the background magnetic field. Perturbations are considered to be also across the magnetic field. The back-reaction of cosmic rays resulting in strong streaming instabilities is taken into account. It is shown that for sufficiently short wavelength perturbations, the growth rates can exceed the growth rate of cosmic-ray streaming instability along the magnetic field found by Nekrasov & Shadmehri (2012), which is in its turn considerably larger than the growth rate of the Bell instability (2004). The thermal instability is shown not to be subject to the action of cosmic rays in the model under consideration. The dispersion relation for the thermal instability has been derived which includes sound velocities of plasma and cosmic rays, Alfv\'{e}n and cosmic-ray streaming velocities. The relation between these parameters determines the kind of thermal instability ranging from the Parker (1953) to the Field (1965) instabilities. The results obtained can be useful for a more detailed investigation of electron-ion astrophysical objects such as supernova remnant shocks, galaxy clusters and others including the dynamics of streaming cosmic rays.