Geomagnetic storms and EMIC waves: Van Allen Probe observations

Utilizing the data from magnetometer instrument of Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) suite on board Van Allen Probe A, the occurrences of electromagnetic ion cyclotron (EMIC) waves during geomagnetic storms and nonstorm periods are investigated. The 270 EMIC wave events and 76 geomagnetic storms were identified during the period under research, from 8 September 2012 to 30 April 2014, when the apogee of Van Allen Probe A covered all the magnetic local time (MLT) sectors. Fifty of the 76 storms observed 124 EMIC wave events, of which 80 are found in the recovery phase, more than those observed in the main phase. Majority EMIC wave events (similar to 54%) were observed during the nonstorm periods. Occurrence rates of EMIC waves as a function of L and MLT during different geomagnetic conditions are also examined, whose peaks in main phase are higher than those in recovery phase. However, occurrences of EMIC waves in recovery phase distribute more uniformly than those do in main phase. Evolution of the distribution characteristics of EMIC waves respect to L and MLT in different geomagnetic phases is investigated, consistent with that of the plasmasphere during geomagnetic storms, implying that the cold and dense plasma in the plasmasphere or plasmaspheric plume play a significant role in the generation of EMIC waves in the inner magnetosphere. Few EMIC waves in the dayside sector during the preonset periods are observed, suggesting that the effect of solar wind dynamic pressure on the generation of EMIC waves in the inner magnetosphere in those periods is not so significant as expected.