A Statistical Study of EMIC Waves Observed at THEMIS Probes in the Outer (L > 7) Magnetosphere Under Quiet Geomagnetic Conditions

Abstract:

Electromagnetic ion cyclotron (EMIC) waves can be generated in the equatorial magnetosphere by an anisotropic temperature distribution (T_⊥>T_∥) of energetic (10-100 keV) ions. They are typically observed in the afternoon sector with a wide L range (L > 7) and occur near the plasmapause. Thus, it has been suggested that the distribution of EMIC wave occurrence in the afternoon sector is associated with the plasmaspheric expansion (i.e., plasmapsheric bulge or plume). Since the plasmapause is further away the earth during times of low geomagnetic activity, it is expected that EMIC wave’s spatial distribution under quiet geomagnetic conditions could be different from that under averaged geomagnetic conditions. In our study we investigate the spatial distribution of EMIC waves during quiet geomagnetic conditions (Kp ≤ 1) in the outer magnetosphere (L > 7) using the magnetic field data from Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft from 2008 to 2009. We also examine the spatial distribution of cold plasma density estimated from THEMIS spacecraft potential data and compare it with EMIC wave occurrence during quiet geomagnetic conditions.