Hybrid Simulations of the Resonant and Non-Resonant Cosmic Ray Streaming Instability

TL;DR

This paper uses hybrid simulations to test and confirm theoretical predictions of cosmic ray streaming instabilities, exploring both resonant and non-resonant types with different CR distributions.

Contribution

It provides the first detailed simulation-based validation of linear theory predictions for both resonant and non-resonant cosmic ray streaming instabilities across various CR distributions.

Findings

Simulation results match theoretical growth rates.

Magnetic helicity evolution agrees with predictions.

Identified non-linear features in instability development.

Abstract

Using hybrid simulations (kinetic ions--fluid electrons), we test the linear theory predictions of the cosmic ray (CR) streaming instability. We consider two types of CR distribution functions: a "hot" distribution where CRs are represented by a drifting power law in momentum and an anisotropic "beam" of monochromatic particles. Additionally, for each CR distribution we scan over different CR densities to transition from triggering the resonant to the non-resonant (Bell) streaming instability. We determine the growth rates of these instabilities in simulations by fitting an exponential curve during the linear stage, and we show that they agree well with the theoretical predictions as a function of wave number agree. We also examine the magnetic helicity as a function of time and wave number, finding a general good agreement with the predictions, as well as some unexpected non-linear features to the instability development.