Influence of Ion-Neutral Damping on the Cosmic-Ray Streaming Instability: Magnetohydrodynamic Particle-in-cell Simulations

TL;DR

This study uses hybrid MHD-PIC simulations to analyze how ion-neutral damping influences the growth, spectrum, and saturation of the cosmic-ray streaming instability in partially ionized interstellar media, affecting cosmic-ray transport.

Contribution

It provides the first detailed numerical analysis of ion-neutral damping effects on CRSI growth, spectrum, and saturation phases using hybrid simulations.

Findings

Ion-neutral damping suppresses wave growth at certain wavelengths.

Higher ion-neutral collision rates lower the unstable bandwidth.

Wave amplitudes and cosmic-ray isotropization decrease with increased damping.

Abstract

We explore the physics of the gyro-resonant cosmic ray streaming instability (CRSI) including the effects of ion-neutral (IN) damping. This is the main damping mechanism in (partially-ionized) atomic and molecular gas, which are the primary components of the interstellar medium (ISM) by mass. Limitation of CRSI by IN damping is important in setting the amplitude of Alfv\'en waves that scatter cosmic rays and control galactic-scale transport. Our study employs the MHD-PIC hybrid fluid-kinetic numerical technique to follow linear growth as well as post-linear and saturation phases. During the linear phase of the instability -- where simulations and analytical theory are in good agreement -- IN damping prevents wave growth at small and large wavelengths, with the unstable bandwidth lower for higher ion-neutral collision rate $\nu_{\rm in}$. Purely MHD effects during the post-linear phase extend the wave spectrum towards larger $k$. In the saturated state, the cosmic ray distribution evolves toward greater isotropy (lower streaming velocity) by scattering off of Alv\'en waves excited by the instability. In the absence of low-$k$ waves, CRs with sufficiently high momentum are not isotropized. The maximum wave amplitude and rate of isotropization of the distribution function decreases at higher $\nu_{\rm in}$. When the IN damping rate approaches the maximum growth rate of CSRI, wave growth and isotropization is suppressed. Implications of our results for CR transport in partially ionized ISM phases are discussed.