EMIC Wave Occurrence During Geomagnetic Storm Events: CRRES Results

Many geomagnetic storms have been observed to have electromagnetic ion cyclotron (EMIC) waves present, but not all storms have EMIC waves associated with them. The accepted generation mechanism for these waves is a temperature anisotropy in the magnetosphere, which can occur through the expansion of the ring current (hot plasma population) during a geomagnetic storm into the region of the plasmasphere (cold plasma population) or during the recovery phase of a storm when the plasmasphere expands back out to its quiet time position. Radially extended plasma drainage plumes may also play a role. The CRRES mission gave us a unique chance to look at magnetic field, electric field, plasma, and particle data that we can use to identify and analyse EMIC wave events. Here we study storms occurring when the Kyoto SYM-H index is less than - 40 nT, and identify each phase: onset, main phase, and recovery. We have also catalogued EMIC wave occurrences throughout the CRRES mission. Statistical tests are performed to assess whether there is a preference for EMIC wave events to occur during any of the storm phases, and/or during specific sections of the phases (e.g. between 80 - 90% of the recovery phase). The orbit of CRRES also allows us to determine where in the magnetosphere EMIC wave events are most likely to occur. This can then be combined with analysis of wave properties to help determine the generation region and conditions needed for EMIC wave events to occur at storm times.