Deciphering the physical basis of the intermediate-scale instability

TL;DR

This paper investigates the physics of cosmic-ray driven instabilities across scales, revealing their resonant origins, limitations of common plasma approximations, and implications for cosmic-ray transport.

Contribution

It identifies the resonant nature of intermediate and gyroscale instabilities and shows that standard MHD models fail to capture the fastest-growing instability.

Findings

Both instabilities have a resonant origin.

MHD and Hall-MHD approximations miss the intermediate-scale instability.

Different phase speeds affect particle-wave scattering.

Abstract

We study the underlying physics of cosmic-ray (CR) driven instabilities that play a crucial role for CR transport across a wide range of scales, from interstellar to galaxy cluster environments. By examining the linear dispersion relation of CR-driven instabilities in a magnetised electron-ion background plasma, we establish that both, the intermediate and gyroscale instabilities have a resonant origin and show that these resonances can be understood via a simple graphical interpretation. These instabilities destabilise wave modes parallel to the large-scale background magnetic field at significantly distinct scales and with very different phase speeds. Furthermore, we show 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 characterising the background plasma for resolving the most unstable wave modes. Finally, we discuss the implications of the different phase speeds of unstable modes on particle-wave scattering. 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.