Hear Me Roar: Whistler Lion Roars in high-β plasmas of galaxy clusters

Lion roars are short-lived, low frequency, right-hand polarized whistler waves generated by anisotropic electrons trapped in low magnetic field regions of nonlinear mirror modes. They are observed in low-beta plasmas near the Earth and the solar-wind. In the intracluster medium (ICM) of galaxy clusters the mirror instability can also be excited by turbulent shearing and compression, given the high values of plasma β, and we found preliminary indications of lion roars also present in those ambient conditions (Ley et al. 2022). We perform fully kinetic particle-in-cell (PIC) simulations of a high-β plasma subject to a continuous amplification of B(t) to study the nonlinear stages of the mirror instability and the ensuing excitation of whistler lion roars. We see that, as mirror modes reach nonlinear amplitudes, whistler lion roars start to emerge with sub-dominant amplitudes and quasi-parallel propagation. We characterize their wave properties and the population of electrons that excite them, with results consistent with previous works. Within our parameter space, we argue that whistler lion roars are a concomitant feature of mirror instability even at high-β, and therefore expected to be present in astrophysical environments such as the ICM. We discuss the implications of our results on electron thermal conduction models in the ICM such as whistler-regulated MHD (Drake et al. 2021).