Global distribution of EMIC waves derived from THEMIS observations

Electromagnetic ion cyclotron (EMIC) waves play an important role in magnetospheric dynamics and their global distribution has been of great interest. This paper presents the distribution of EMIC waves over a broader range than ever before, as enabled by observations with the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft from 2007 to 2010. Our major findings are: (1) There are two major peaks in the EMIC wave occurrence probability. One is at dusk and 8-12 R_E where the helium band dominates the hydrogen band waves. The other is at dawn and 10-12 R_E where the hydrogen band dominates the helium band waves. (2) In terms of wave spectral power the dusk events are stronger (≈10 nT ²/Hz) than the dawn events (≈3 nT²/Hz). (3) The dawn waves have large normal angles (>45°) in the hydrogen band and even larger normal angles (>60°) in the helium band. The dusk waves have small normal angles (≤30°) in both the hydrogen and helium bands. (4) The hydrogen band waves at dawn are weakly left-hand polarized near the equator, become linearly polarized with increasing latitude and eventually weakly right-hand polarized at high latitudes whereas the helium band waves at dawn are linearly polarized at all latitudes. Dusk waves in both bands are strongly left-hand polarized over a wide range of latitude. Based on the linear EMIC instability model presented by Horne and Thorne (1994), we suggest that the main underlying factor for the observed spatial variations of these wave properties would be local density of cold plasma and chemical abundance. In addition, the distinct properties of H and He band waves found in this study would deserve a new attention in relation to EMIC wave generation mechanisms.