Modeling the Saturation of the Bell Instability using Hybrid Simulations

TL;DR

This paper uses hybrid simulations to investigate how the Bell instability saturates and determines the final magnetic field amplification, providing insights relevant for cosmic ray acceleration in astrophysical settings.

Contribution

It offers a first-principles analysis of the saturation mechanism of the Bell instability using hybrid simulations, revealing the magnetic pressure at saturation.

Findings

Magnetic pressure at saturation equals initial CR anisotropic pressure.

Final magnetic field amplification can be predicted from CR parameters.

Results applicable to various astrophysical environments.

Abstract

The nonresonant streaming instability (Bell instability) plays a pivotal role in the acceleration and confinement of cosmic rays (CRs); yet, the exact mechanism responsible for its saturation and the magnitude of the final amplified magnetic field have not been assessed from first-principles. Using a survey of hybrid simulations (with kinetic ions and fluid electrons), we study the evolution of the Bell instability as a function of the parameters of the CR population. We find that, at saturation, the magnetic pressure in the amplified field is comparable with the initial CR anisotropic pressure, rather than with the CR energy flux as previously argued. These results provide a predictive prescription for the total magnetic field amplification expected in the many astrophysical environments where the Bell instability is important.