Observations of magnetospheric high- m poloidal waves by ST-5 satellites in low Earth orbit during geomagnetically quiet times

Abstract

The poloidal waves with large azimuthal wave numbers (m) in the magnetosphere are known to be generated by drift or drift-bounce resonance with energetic ring current particles, and these waves may play a role in modulating the energetic particles in the inner magnetosphere. When examining the magnetic field data collected by the NASA Space Technology 5 (ST-5) satellites in the low Earth orbit, Le et al. (2011) discovered many wave events with frequencies of 30-200mHz (in the Pc2 and Pc3 bands), and they proposed that these waves should, in fact, be Doppler-shifted high-m poloidal waves in the magnetosphere with frequencies at only a few millihertz (in the Pc5 band). Using a new method that examines the differences in wave phase detected by the three ST-5 satellites, we confirm that the frequencies in the Earth frame for the poloidal waves observed are mainly between 3 and 5mHz. Not only were poloidal waves observed frequently by ST-5 in the dayside magnetosphere but they were also occasionally seen in the nightside when the satellites passed through the same L shells. In each wave event, the azimuthal wave number may change with L, but the wave frequency in the Earth frame remains the same. We also find that poloidal waves can last more than 9h during geomagnetically quiet conditions, suggesting that even a very weak ring current can supply enough energetic particles to excite poloidal waves.