Statistical Results of the Power Gap Between Lower-Band and Upper-Band Chorus Waves

Abstract

The primary power gap of chorus waves is statistically investigated using 7-year waveform data from THEMIS probes for the first time. Overall, ~2/3 of chorus events have a power gap between the lower and upper bands. Both the gap frequency and the frequency bandwidth of the gap have a broad distribution, which peaks at frequencies of ~0.49fce and ~0.07fce, respectively. The gap frequency tends to increase with increasing |MLAT| (magnetic latitude), while the frequency width gradually increases with L-shell. In most of banded events, the peak frequency of upper band is roughly twice that of lower band. Two types of events are studied. For Type I events, one population is located around Pup/Plow = 10−1, and the other is around Pup/Plow = 10−3. However, Type II events are roughly concentrated around Pup/Plow = 10−3. The gap frequency positively correlates with the frequency of upper band. Our study provides detailed observational constraints on potential mechanisms of the power gap formation.