The Inward Radial Diffusion and Slow Decay of Energetic Electrons in the Earth's Radiation Belts

Abstract:

We investigate the inward intrusion of energetic electrons in the Earth’s radiation belts observed by the Van Allen probes during a 10-day quiet period in March 2013. The electron flux measurements from Mageis and REPT instruments on the Van Allen probes show the clear radial diffusion and slow decay of ~300 keV to ~4.5 MeV electrons. The energetic electrons are injected at L ~ 4.75 on March 06, and gradually diffuse inward at each energy channel to L ~ 4 until interrupted by a strong geomagnetic disturbance on March 17. Meanwhile, the differential energy flux of the energetic electrons decreased by about 1 order in 10 days. The electrons exhibit flattened pitch angle distributions above ~40°. We adopt a 3 dimensional radiation belt model which incorporates radial and local diffusion processes to simulate this event. The empirical radial diffusion rates provide reasonable agreement with the observed inward diffusion profile. The hiss wave amplitudes are observed by the THEMIS spacecraft on the dayside and by the Van Allen probes on the nightside. The electrons with energies lower than ~1 MeV are effectively scattered by hiss waves, causing the slow decay in consistent with observations. The higher energy electrons are effectively scattered by EMIC waves near the loss cone, and by hiss waves at higher pitch angles. The decaying timescale and the pitch angle distribution caused by the pitch angle scattering in the simulation are consistent with the observation at each energy channel. Our study demonstrates that the quiet time energetic electron dynamics are effectively controlled by the radial diffusion and pitch angle scattering processes in the Earth’s radiation belts.