Global characteristics of electromagnetic ion cyclotron waves: Occurrence rate and its storm dependence

We statistically examine the occurrence rate of electromagnetic ion cyclotron (EMIC) waves observed by Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE). We use the 8Hz magnetic field data set that covers the whole CCE mission period of nearly 4.5years from August 1984 to January 1989, which is more than three times the period studied by Anderson et al. [] (similar to 452days). The large data volume allows us to evaluate the storm phase dependence of the spatial occurrence pattern of EMIC waves. The major results of this study are summarized as follows. (a) The occurrence rate is below 5% on the nightside at all L. On the dayside, the rate is <5% in the inner magnetosphere (L<6), while it is higher than 5% in the outer magnetosphere (L6), up to 25%. The highest rate appears in the afternoon sector. (b) The overall occurrence rate is higher for H-band events than He-band events, except for the opposite feature seen in the inner magnetosphere on the early afternoon-to-post midnightside (L<6, 14h<22h). (c) H-band events occur frequently in the outer magnetosphere (L7) in the afternoon sector, regardless of geomagnetic activity. Under quiet conditions, H-band events also occur in the outer magnetosphere on the morningside (4hMLT<8h). (d) He-band events frequently occur in the inner magnetosphere (L<7) on the prenoon to duskside (10hMLT<19h) under disturbed conditions (Dst-50nT). (e) The storm time He-band waves are generated more frequently during the main phase than the recovery phase, with the main-phase wave excitation seen toward the afternoonside outer magnetosphere (L>7). The results indicate two independent major processes that cause EMIC wave excitation in the Earth's magnetosphere: one externally triggers H-band waves on the dayside, and the other internally excites He-band waves on the dusk-to-afternoonside. We suggest that the former is due to solar wind compression which leads to perpendicular adiabatic ion heating and in turn an increase in temperature anisotropy, and that the latter is caused by injections of new, highly energetic ion population from the plasma sheet, with its velocity distributions becoming pancake-like on the dusk-to-afternoonside. The frequent occurrence seen on the afternoonside, at a wide L range, and during the main/development phase, strongly suggests the significant role of the sunward surge of the plasmasphere and plasma plumes in the injection-associated (i.e., storm time) EMIC wave generation.