The Microphysics of Gyroresonant Streaming Instabilities and Cosmic Ray Self-Confinement

Abstract

The self-regulation of cosmic ray (CR) transport in the interstellar and intracluster media has long been viewed through the lenses of linear and quasilinear kinetic plasma physics. Such theories are believed to capture the essence of CR behavior in the pres- ence of self-generated turbulence, forming the basis for the so-called “self-confinement paradigm,” which has been proposed to explain the isotropic propagation of CRs in the interstellar medium. However, the coupled nonlinear equations that describe the time-dependent system of CRs and electromagnetic fields are analytically intractable in the general case. Thus, obtaining analytical solutions has always relied on simplify- ing assumptions that remove potentially critical details arising from the nonlinearities of the problem.

We utilize the Particle-in-Cell (PIC) numerical method to study the time- dependent nonlinear behavior of the gyroresonant streaming instabilities, self- consistently following the combined evolution of particle distributions and self- generated wave spectra in one-dimensional periodic and aperiodic simulations. In the periodic case, we demonstrate that the early growth of instability conforms to the predictions from linear physics, but that the behavior can vary depending on the properties of the initial CR distribution. We emphasize that the nonlinear stages of instability depend strongly on the initial anisotropy of CRs. We derive estimates for the wave amplitudes at saturation and the time scales for relaxation of the CR distribution. In the aperiodic case, we show that the expansion of CRs from small injection regions naturally induces highly anisotropic CR distributions. Pitch-angle diffusion of CRs is then limited by the predominantly right-handed circularly polarized self-generated turbulence, allowing bulk CR drift velocities of ∼0.5c. We compare against a set of analytical solutions to the CR expansion problem and find that they do not accurately predict the time-dependent properties of the

CR population. We briefly study the wave damping processes of nonlinear Landau iii

damping and ion-neutral friction in order to assess the viability of performing damped CR streaming simulations in the future with the PIC method.