Ion cyclotron waves during a great magnetic storm observed by Freja double-probe electric field instrument

Evolution of the great magnetic storm in April 1993 is studied using observations of electromagnetic ion cyclotron (EMIC) waves by the F1 double-probe electric field instrument onboard the Freja satellite. The almost continuous operation of the F1 instrument in the overview mode allowed us to follow the global EMIC wave activity at low altitudes above the ionosphere during several subsequent days covering the initial (compression), main, and recovery phases of the storm. During the initial phase of the storm the spatial occurrence of EMIC waves has a postnoon high-latitude maximum, in agreement with earlier statistical results. A sudden and dramatic change of this pattern was observed with the start of the storm main phase. During the main phase, wave amplitudes were greatly enhanced and the active wave region moved to considerably lower latitudes to the late evening MLT sector. Also, the existence of heavy ions in the later main phase changed the distribution of wave frequencies dramatically. Most interestingly, a number of oxygen band EMIC waves were observed during a limited period of about 7 hours in the later main phase. The observed asymmetric MLT distribution of these oxygen band waves implies that the oxygen loss rate is faster than the drift rate. The results suggest that the EMIC waves play a crucial role in the main and early recovery phase of a great storm.