Statistical Study on the Azimuthal Mode Number of Pc5 ULF Wave in the Inner Magnetosphere

The azimuthal mode number, m, of ultra-low frequency (ULF) waves is a significant contributing factor for radiation belt electron energization, because it determines the conditions for resonant interaction between waves and particles. Based on multi-point magnetic field measurements of GOES satellites from January to September of 2011, we statistically analyze the distributions of the characteristics of m of Pc5 ULF waves. In the dayside, the local peaks in the distributions of wave power spectra density locate at ∼10 and ∼13 MLT for m < 0 (westward propagation) and m > 0 (eastward propagation) waves respectively, suggesting the waves generally propagate anti-sunward. In the nightside, the local peaks are at 22–23 MLT for both m < 0 and m > 0 waves, suggesting possible relation to substorm activities. Further investigation shows that, with increasing solar wind activities, the enhancements of dayside peaks are primarily contributed by |m| ≤ 3 waves, whereas the enhancements of nightside peak are contributed by both |m| ≤ 3 and |m| > 3 waves. With increasing AE index, the enhancements are more significant for the nightside peaks comparing to dayside peaks, and for |m| > 3 waves comparing to |m| ≤ 3 waves. The results of this study provide inputs for further investigation on the radial diffusion coefficient of radiation belt electrons with considering mode number information.