Mirror instability and L-mode electromagnetic ion c yclotron instability: Competition in the Earth's magnetosheath

摘要

We performed both two- and three-dimensional hybrid simulations of the competing processes between the L-mode electromagnetic ion cyclotron (EMIC) and mirror instabilities, assuming anisotropic energetic ions with 71/TH = 4.0. In the twodimensional model, the energy of the EMIC waves is higher at the linear growth phase because its growth rate is larger than that of the mirror mode. In the three-dimensional model, however, the energy of the mirror mode waves is larger than that of the EMIC waves for all times because the wave number spectra of mirror mode waves form torus-like structures. We also theoretically derived a necessary condition for the dominance of the mirror instability. As the mirror mode waves relax the temperature anisotropy effectively, the linear growth rates of the EMIC waves become smaller before saturation. The EMIC waves cause heating of protons trapped by the nonlinear potentials due to coexistence of forward and backward propagating waves and nverse cascading. They terminate the linear growth of EMIC waves. Because of theseparallel heatings, the temperature anisotropy decreases to the threshold of the mirrorinstability and thus the mirror mode wave saturates. At the nonlinear stage,coalescence of the mirror mode waves takes place in both models. The quick dissipation of the EMIC waves occurs due to the heating by the nonlinear processes. On the other hand, the coalescence is a much slower process than the nonlinear processes of EMIC waves, and thus the mirror mode waves remain in the three-dimensional model.