Statistical study of ULF waves in the magnetotail by THEMIS observations

Abstract

Ultra-low-frequency (ULF) waves are ubiquitous in the magnetosphere. Previous studies mostly focused on ULF waves in the dayside or near-Earth region (with radial distance R<12 RE). In this study, using the data of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission during the period from 2008 to 2015, the Pc5–6 ULF waves in the tail region with X∗GSM<0, 8 RE<R<32 RE (mostly on the stretched magnetic field lines) are studied statistically. A total of 1089 azimuthal oscillating events and 566 radial oscillating events were found. The statistical results show that both the azimuthal and radial oscillating events in the magnetotail region (12 RE<R<32 RE) are more frequently observed in the post-midnight region. The frequency decreases with increasing radial distance from Earth for both azimuthal oscillating events (8 RE<R<16 RE) and radial oscillating events (8 RE<R<14 RE), which is consistent with the field line resonances theory. About 52 % of events (including the azimuthal and radial oscillating events) are standing waves in the region of 8–16 RE, while only 2 % are standing waves in the region of 16–32 RE. There is no obvious dawn–dusk asymmetry of ULF wave frequency for events in 8 RE<R<32 RE, which contrasts with the obvious dawn–dusk asymmetry found by previous studies in the inner magnetosphere (4 RE<R<9 RE). An examination for possible statistical relationships between the ULF wave parameters and substorm occurrences is carried out. We find that the wave frequency is higher after the substorm onset than before it, and the frequency differences are more obvious in the midnight region than in the flank region.