Instability of the parallel electromagnetic modes in Kappa distributed plasmas – II. Electromagnetic ion–cyclotron modes

The low-frequency fluctuations of the interplanetary magnetic field are frequently attributed to electromagnetic ion-cyclotron (EMIC) waves generated either locally and self-consistently by the temperature anisotropy of ions, or in the corona and transported by the super-Alfvénic solar wind. This paper conducts a refined analysis of the EMIC instability in the presence of suprathermal populations. The anisotropic distributions are modelled with two different power-law distributions functions, the additive bi-Kappa (BK) and the more general productbi- Kappa (PBK) distribution function. EMIC solutions are derived exactly numerically for the full range of the plasma parameters, including conditions relevant for the solar wind and magnetospheric plasmas. Accurate physical correlations are provided between the maximum growth rates and the instability threshold conditions. The expectation that the instability might be stimulated by the suprathermals is confirmed by both Kappa models, but in a complementary way: while the instability thresholds are lowered by the BK model, at higher anisotropies the growth rates are enhanced only by the PBK model.