Cosmic-ray kinetic physics with the MHD-PIC method: applications and extension

I will first give an overview of the MHD-PIC method, which is developed to study the kinetic physics of cosmic-ray (CR) particles interacting with a background thermal plasma. It treats the background plasma as a fluid described by MHD, while treats the cosmic-rays as particles using the conventional PIC approach, with backreaction to conserve momentum and energy. It greatly alleviates the issue of scale separation encountered in conventional PIC approach for CR-related problems, such as particle acceleration in shocks. We have recently implemented this method to the Athena++ MHD code, with mesh refinement capabilities to further boost the accessible scale hierarchy. I will then focus on simulations of the CR gyro-resonant instabilities, being the key microphysical mechanism behind CR feedback that involves substantial scale separation. By designing a streaming box and an expanding box framework, our simulations achieve the steady-state balance between wave growth and damping, as well as between driving CR streaming/anisotropy and isotropization via wave scattering. It allows us to measure the CR transport coefficients from first principles as a function of background environment, which can be eventually incorporated into subgrid prescriptions for studies of CR feedback. Finally, I will describe the extension of the method to MHD-gPIC, where particles are integrated under the guiding center approximation with drift motion, with the potential to study particle acceleration in magnetic reconnection.