Abstract:
Geomagnetic indices have been used as a proxy for studying electromagnetic ion cyclotron (EMIC) wave occurrences under di erent geomagnetic conditions. However, the drivers of EMIC waves are di erent during non-storm, storm time and during individual storm phases. Using 7 years of data from the twin Van Allen Probes, we demonstrate that the occurrence probability of EMIC waves are not well captured by a speci c
geomagnetic activity index alone, but is rather well manifested by considering individual storm phases. We show EMIC wave occurrence statistics during di erent storm phases (preonset, main and recovery) for geomagnetic activity indices Sym-H, AE, and Kp and
solar wind dynamic pressure Pdyn, illustrating that the occurrence rates vary signi cantly
during di erent storm phases even for a given geomagnetic index. We also utilize this
large database to show EMIC wave occurrence distribution, and how various wave and
plasma parameters behave under di erent geomagnetic conditions. EMIC waves occur
2.9 times more often during geomagnetic storms than during non-storm times. The
majority (72%) of storm time EMIC waves occur during the recovery phase due to long
recovering time, while the highest occurrence rates are in the pre-onset phase, followed by main and recovery phases. EMIC waves in the main phase have occurrence peaks in the dusk to pre-midnight sectors while recovery phase events spread to more MLT sectors
with peaks in the morning sector. Wave amplitudes are found to be evenly distributed
across di erent MLT sectors during all geomagnetic conditions.
