EMIC waves and plasmaspheric and plume density: CRRES results

Electromagnetic ion cyclotron (EMIC) waves frequently occur during geomagnetic storms, specifically during the main phase and 3-6 days following the minimum Sym - H value. EMIC waves contribute to the loss of ring current ions and radiation belt MeV electrons. Recent studies have suggested that cold plasma density structures found inside the plasmasphere and plasmaspheric plumes are important for the generation and propagation of EMIC waves. During the CRRES mission, 913 EMIC wave events and 124 geomagnetic storms were identified. In this study we compare the quiet time cold plasma density to the cold plasma density measured during EMIC wave events across different geomagnetic conditions. We found statistically that EMIC waves occurred in regions of enhanced densities. EMIC waves were, on average, not associated with large local negative density gradients.