Whistler Lion Roars within Mirror Modes in Galaxy Clusters

Lion roars are bursts of whistler waves associated with low magnetic field regions of mirror modes. They are observed in plasmas near Earth, Saturn and the solar wind. In the intracluster medium (ICM) of galaxy clusters, mirror instability is also expected to be excited, but it is not yet clear whether whistler lion roars can also be present in this high-β environment. In this work, we perform fully kinetic particle-in-cell simulations of a plasma subject to a continuous amplification of the mean magnetic field to study the nonlinear stages of the mirror instability and the ensuing excitation of whistler lion roars under ICM conditions. Once mirror modes reach nonlinear amplitudes, a simultaneous excitation of whistler lion roars and ion-cyclotron waves (IC) is observed, with sub-dominant amplitudes and quasi-parallel propagation. We show that the underlying mechanism of excitation is the pressure anisotropy of electrons and ions trapped in mirror modes with loss-cone type distributions. We observe that IC waves play an essential role in regulating the global ion pressure anisotropy at nonlinear stages. We argue that lion roars are a concomitant feature of mirror instability even at high β, therefore expected to be present in the ICM. We discuss the implications of this work on the ICM turbulent heating via magnetic pumping.