A Kinetic Study of the Saturation of the Bell Instability

TL;DR

This paper investigates the nonlinear evolution and saturation mechanism of the Bell instability, a key process in cosmic ray acceleration near supernova remnants, through hybrid simulations.

Contribution

It provides the first detailed kinetic study of the saturation process of the Bell instability using self-consistent hybrid simulations.

Findings

Saturation occurs when magnetic pressure matches initial CR momentum flux.

The study links the saturation level to the CR population parameters.

Provides insights into magnetic field amplification in astrophysical shocks.

Abstract

The nonresonant cosmic ray instability, predicted by Bell (2004), is thought to play an important role in the acceleration and confinement of cosmic rays (CR) close to supernova remnants. Despite its importance, the exact mechanism responsible for the saturation of the instability has not been determined, and there is no first-principle prediction for the amplitude of the saturated magnetic field. Using a survey of self-consistent hybrid simulations (with kinetic ions and fluid electrons), we study the non-linear evolution of the Bell instability as a function of the parameters of the CR population. We find that saturation is achieved when the magnetic pressure in the amplified field is comparable to the initial CR momentum flux.