Saturated magnetic field amplification at supernova shocks

TL;DR

This paper analyzes the nonresonant cosmic-ray streaming instability at supernova shocks, deriving the saturation level of magnetic field amplification through kinetic theory and turbulence feedback.

Contribution

It provides a kinetic theory-based analysis of the nonresonant instability, including the nonlinear development and saturation of magnetic field amplification.

Findings

Nonresonant instability is driven by a compensating current in the plasma.

Saturation of magnetic fields can exceed the background magnetic field.

Resonant diffusion limits the streaming motion and influences saturation.

Abstract

Cosmic-ray streaming instabilities at supernova shocks are discussed in the quasilinear diffusion formalism which takes into account the feedback effect of wave growth on the cosmic ray streaming motion. In particular, the nonresonant instability that leads to magnetic field amplification in the short wavelength regime is considered. The linear growth rate is calculated using kinetic theory for a streaming distribution. We show that the nonresonant instability is actually driven by a compensating current in the background plasma. The nonresonant instability can develop into a nonlinear regime generating turbulence. The saturation of the amplified magnetic fields due to particle diffusion in the turbulence is derived analytically. It is shown that the evolution of parallel and perpendicular cosmic-ray pressures is predominantly determined by nonresonant diffusion. However, the saturation is determined by resonant diffusion which tends to reduce the streaming motion through pitch angle scattering. The saturated level can exceed the mean background magnetic field.