Kinetic mirror instability in the magnetosphere

Recent THEMIS observations in Earth's magnetotail have found magnetic holes, which are conjectured to be the late stage mirror-mode structures. The satellite data show no clear ion anisotropy remaining in those structures, however, electrons are seen to be anisotropic with T_⊥ < T_∥ outside of the magnetic holes while being isotropic inside. We investigate the formation and nonlinear evolution of mirror-mode structures using particle in cell simulations. We study kinetic instabilities in a homogeneous magnetized plasma in which both ions and electrons are initialized with anisotropic, bi-Maxwellian distribution functions. The competition of two instabilities, mirror and ion cyclotron instability, determines the nonlinear evolution. We examine the importance of helium ions and electron anisotropy in suppressing ion cyclotron waves and favoring the mirror mode. The evolution of anisotropy in ion and electron components is analyzed and compared to the satellite observations.