Statistical results describing the bandwidth and coherence coefficient of whistler-mode waves using THEMIS waveform data

Abstract:

The bandwidth and coherence coefficient of lower-band whistler mode waves are analyzed using THEMIS waveform data for rising tone, falling tone, and hiss-like emissions separately. We also evaluate their dependences on the spatial location, electron density, the ratio of plasma frequency to local electron gyrofrequency, and the wave amplitude. Our results show that the bandwidth normalized by the local electron gyrofrequency (f_ce) of rising and falling tones is very narrow (~0.01 f_ce), much smaller than that of the hiss-like emissions (~0.025 f_ce). Meanwhile, with the increasing wave amplitude, the normalized bandwidth of discrete emissions gradually decreases, whereas that of hiss-like emissions increases slowly. The coherence coefficient of rising and falling tones is extremely large (~1), while the coherence coefficient of hiss-like emissions is a little smaller, but is still larger than 0.5. For all categories of whistler-mode waves, the normalized bandwidth increases at larger L-shells. Furthermore, the normalized bandwidth is positively correlated with the ratio of plasma frequency to local electron gyrofrequency, but is inversely correlated with the electron density. Interaction between radiation belt electrons and whistler mode waves has been widely described within quasi-linear diffusion theory. Our observations show that the quasi-linear theory is not entirely applicable for modeling electron interaction with rising and falling tones due to their narrow bandwidth and high coherence coefficient. However, it is suitable to simulate wave-particle interaction between electrons and hiss-like emissions. Moreover, the correlations between the normalized bandwidth of chorus waves (especially the discrete emissions) and other parameters may provide insights for the generation mechanism of chorus waves.