Modeling the Saturation of the Bell Instability using Hybrid Simulations

Georgios Zacharegkas, Damiano Caprioli, Colby Haggerty

Published 2022-10-14Version 1

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 momentum flux (i.e., the anisotropic CR pressure). These results provide a predictive prescription for the total magnetic field amplification expected in the many astrophysical environments where the Bell instability is important.