Wave-particle interaction in a plasmaspheric plume observed by a Cluster satellite

The wave-particle interaction is a possible candidate for the energy coupling between the ring current and plasmaspheric plumes. In this paper, we present wave and particle observations made by the Cluster C1 satellite in a plasmaspheric plume in the recovery phase of the geomagnetic storm on 18 July 2005. Cluster C1 simultaneously observed Pc1-2 waves and extremely low frequency (ELF) hiss in the plasmaspheric plume. Through an analysis of power spectral density and polarization of the perturbed magnetic field, we identify that the observed Pc1-2 waves are linearly polarized electromagnetic ion cyclotron (EMIC) waves and show that the ELF hiss propagates in the direction of the ambient magnetic field in whistler mode. In the region where the EMIC waves were observed, the pitch angle distribution of ions becomes more isotropic, likely because of the pitch angle scattering by the EMIC waves. It is shown that the ELF hiss and EMIC waves are spatially separated: The ELF hiss is located in the vicinity of the electron density peak within the plume while the EMIC waves are detected in the outer boundary of the plume because of the different propagation characteristics of the ELF hiss and EMIC waves.