Wave normal distribution of whistler-mode chorus waves and their relation to energetic electron distributions in Earth’s inner magnetosphere

Abstract:

Whistler-mode chorus waves are known to play a fundamental role in energetic electron dynamics in Earth's inner magnetosphere, and their wave normal distribution is critical in evaluating the rate of electron pitch angle scattering and energy diffusion. In this study, we analyze chorus wave properties measured by the EMFISIS instrument on the twin Van Allen Probes over the past two years. Our statistical results show that wave normal distributions exhibit two peaks, one in the quasi-parallel direction, and the other at very oblique angles. Their wave normal angle distribution is also clearly dependent on the location and geomagnetic activity. Interestingly, wave normal angles tend to be more quasi-parallel at larger L-shells and under stronger geomagnetic activity, whereas they become more oblique at lower L-shells and during relatively quiet periods. Since the generation of whistler-mode waves is closely related to 1-100 keV electrons, energetic electron distributions measured by the HOPE and MagEIS instrument are analyzed to evaluate the key parameters leading to either quasi-parallel or very oblique waves.