### Abstract

Contributed Talk - Splinter Solar

Friday, 15 September 2023, 17:00 (H 3005)

## A new Cosmic-ray driven instability

Mohamad Shalaby

Leibniz Institute for Astrophysics Potsdam

In this talk, I discuss the linear instabilities driven by cosmic rays (CRs) with a gyrotropically cold momentum distribution. Linear analysis reveals that CRs with a finite pitch angle drive a dominant intermediate-scale instability between the ion and electron gyroradii, if the CRs' drifting velocity, $v_{dr}$, is less than $v_{A}^{e}/2$, where $v_{A}^{e}/2$ represents the Alfvén speed of electrons. This instability triggers comoving ion-cyclotron wave modes of CRs (along the background magnetic field). The growth rate is typically more than an order of magnitude faster than that at the gyroscale of CR ions and is artificially suppressed in the solution of the dispersion relation of gyrotropically cold CRs when the popular assumption $omega ll Omega_i$ is adopted. We establish that both the intermediate and gyroscale instabilities have a resonant origin and show that these resonances can be understood through a simple graphical interpretation. This suggests that by considering the contribution of CRs with a power-law momentum distribution without such an assumption, a similar instability would arise. Through an ab initio kinetic simulation, we confirm the growth of the dominant intermediate-scale instability and explore its non-linear saturation. Furthermore, we demonstrate that approximating the electron-ion background plasma with either magnetohydrodynamics (MHD) or Hall-MHD fails to capture the fastest-growing instability in the linear regime, namely the intermediate-scale instability. This finding highlights the importance of accurately characterizing the background plasma to resolve the most unstable wave modes. Further work is needed to investigate the relative importance of these two instabilities in the non-linear, saturated regime and to develop a physical understanding of the effective CR transport coefficients in large-scale CR hydrodynamics theories.