Chorus induced rapid evolution of relativistic electron pitch angle distributions in the Earth's outer radiation belt

This paper reports an event of rapid evolution of relativistic electron differential fluxes in the outer radiation belt. Using Van Allen Probe-B data, we examined electron fluxes at L≈5.8, MLT≈2, and Mlat≈1.7 during the period rm 15: 19–15: 49 UT on September 7, 2016. The data show that relativistic electron pitch angle distributions transformed from butterfly-shaped to nearly flat-topped patterns within 30 minutes. Due to the slow orbital speed and minimal position changes of the satellite during this period, this observation can be regarded as an observation at a fixed spatial position. Numerical simulations indicate that the interaction with whistler-mode chorus waves is the primary mechanism for the pitch angle distribution evolution in this event.