The Role of Pressure Anisotropy in Cosmic Ray Hydrodynamics

TL;DR

This paper explores how pressure anisotropy instability could serve as an alternative mechanism for cosmic ray self confinement and heating, especially on mesoscales near cosmic ray sources, complementing the traditional streaming instability.

Contribution

It introduces pressure anisotropy instability as a potential self confinement mechanism for cosmic rays, particularly relevant on mesoscales near injection sites.

Findings

Pressure anisotropy is less effective globally than streaming instability.

Pressure anisotropy may be significant on mesoscales near cosmic ray sources.

Alignment of pressure and density gradients affects self confinement efficiency.

Abstract

Cosmic ray propagation in the Milky Way and other galaxies is largely diffusive, with mean free path determined primarily by pitch angle scattering from hydromagnetic waves with wavelength of order the cosmic ray gyroradius. In the theory of cosmic ray self confinement, the waves are generated by instabilities driven by the cosmic rays themselves. The dominant instability is due to bulk motion, or streaming, of the cosmic rays, parallel to the background magnetic field B, and transfers cosmic ray momentum and energy to the thermal gas as well as confining the cosmic rays. Classical arguments and recent numerical simulations show that self confinement due to the streaming instability breaks down unless the cosmic ray pressure and thermal gas density gradients parallel to B are aligned, a condition which is unlikely to always be satisfied We investigate an alternative mechanism for cosmic ray self confinement and heating of thermal gas based on pressure anisotropy instability. Although pressure anisotropy is demonstrably less effective than streaming instability as a self confinement and heating mechanism on global scales, it may be important on mesoscales, particularly near sites of cosmic ray injection.