EMIC wave activity during geomagnetic storm and nonstorm periods: CRRES results

Electromagnetic ion cyclotron (EMIC) waves have been observed during geomagnetic storms and are thought to contribute to ring current and radiation belt particle loss during the main phase. Ground-based storm time studies alternatively observe the majority of storm time Pc1-2 pulsations during the recovery phase. In this study we look at the occurrences of EMIC waves during 119 storms occurring throughout the CRRES mission. The storms were defined using the Sym-H index. The storm was divided into three phases: pre-onset, main, and recovery (80% of minimum Sym-H value). The majority, 56.25%, of storm time EMIC waves were found to occur during the main phase, while 35.57% were observed in the recovery phase. The recovery phase definition was then extended to 6 days after the minimum in order to compare with past ground studies. Although more EMIC waves were observed during this extended recovery phase, the maximum occurrence rate of EMIC waves remains in the main phase. A slight increase does appear in the 4-6 days after the minimum Sym-H value. The mean occurrence location of EMIC waves was at L = 6.07, MLT = 15.12. This suggests that EMIC waves may have been generated when the ring current and plasmasphere or plasma plume particle populations overlap, often observed during the main phase due to storm dynamics. The rise in the occurrence rate in the extended recovery phase potentially is dominated by the process of plasmaspheric expansion after the end of the storm.