Statistical analysis of Pi2 pulsations inside and outside the plasmasphere observed by the polar orbiting DE-1 satellite

摘要

We have statistically analyzed Pi2 pulsations, using the 6-second magnetic field data acquired by the polar orbiting DE-1 satellite (a perigee of about 500 km altitude and an apogee of about 3.6 R E altitude). From February 1983 to January 1991, 746 events were found in the H component of the geomagnetic field data from Kakioka or Hermanus, which were located near midnight at substorm onsets determined with the AL index. Out of these 746 events, 91 events had high coherence (>0.6) between DE-1 and ground stations. We examined the spatial distributions of the occurrence probability and the L dependence of the power ratio to the ground and phase difference. The following observational results are obtained: (1) There are many high-coherence events in the compressional component at high latitude as well as at low latitude on the nightside; (2) In the compressional component, the power ratio of DE-1 to ground stations has a weak peak around L ～ 5 on the nigthside and flankside; and (3) the phase difference between DE-1 and ground stations is changed by ～180° around L ～ 5 on the nightside and flankside. Some high-coherence events were found in the compressional component in the polar region. We concluded Pi2 pulsations might be excited by plasmaspheric virtual resonance (PVR) mode, in which compressional waves are produced inside the plasmasphere and oscillate the ambient magnetic field at high latitude.