Investigation of EMIC instability in the inner magnetosphere using observed ion velocity distributions from cluster observations

This study investigates the generation and evolution of electromagnetic ion cyclotron (EMIC) waves in the inner magnetosphere by integrating in situ observations with a kinetic theoretical framework based on a kappa-distributed plasma model. Using Cluster spacecraft data from July 18, 2005, the occurrence of EMIC waves is correlated with enhanced cold plasma density and localized magnetic field variations. Ion velocity distribution functions (VDFs) observed during EMIC activity exhibit significant suprathermal tails, justifying the application of kappa distribution fits to model nonthermal proton populations. A dispersion relation for obliquely propagating EMIC waves is derived for such non-Maxwellian conditions, incorporating observed plasma parameters. Numerical solutions of the dispersion relation reveal temporal variations in wave growth rates, with initial intervals showing rapid instability, indicative of local wave generation, and later intervals exhibiting diminished growth, suggestive of wave propagation from remote sources. These results highlight the sensitivity of EMIC wave dynamics to variations in ion anisotropy and suprathermal features of the observed VDFs, thereby contributing to a more comprehensive understanding of wave–particle interactions and energy redistribution processes in Earth's magnetospheric plasma.