Statistical analysis on the validity of the cold plasma approximation for Chorus waves based on Van Allen Probes Observations and their effects on radiation belt electrons

Theoretical analysis has demonstrated that the dispersion relation of the chorus waves plays an essential role in the resonant interaction and energy transformation between the waves and magnetospheric electrons. Previous quantitative analysis often simplified the chorus dispersion relation using the cold plasma assumption. However, the applicability of the cold plasma assumption is doubtful, especially during geomagnetic disturbance. We here present a systematic statistic on the validity of the cold plasma dispersion relation of chorus waves based on the observations from the Van Allen Probes over the period of 2012 to 2018. The statistical results show that observed magnetic field intensities significantly deviate from those calculated from the cold plasma dispersion relation, which is more pronounced with the increase of geomagnetic activity or the decrease of background plasma density. The region with large deviations is mainly concentrated in the nightside and expands both in the radial and azimuthal direction as the geomagnetic activity increases or the background plasma density decreases. In addition, the bounce-averaged electron scattering rates are computed using the observed and cold plasma dispersion relation of chorus waves. Compared with the usage of the cold plasma dispersion relation, the usage of the observed dispersion relation considerably lowers the minimum resonant energy of electrons, and lowers the scattering rates of electrons above tens of keV but enhances those below. As well, these differences are more pronounced with the enhancement of geomagnetic activity or the decrease of background plasma density.