Pitch-angle distribution evolution of energetic electrons in the inner radiation belt and slot region during the 2003 Halloween storm

In this study, we report dynamic evolutions of 30-500 keV energetic electrons (in flux and pitch-angle distribution) in the radiation belt region with 1.6 < L < 6.2. Those evolutions were observed by the IES instrument on board the Polar spacecraft during the Halloween storm period on October 31, 2003 when the radiation belt was strongly distorted. This injection of energetic electrons into the slot region may be associated with the plasmapause movement and Hiss/Chorus enhancement. This flux enhancement is possibly associated with convective transport from the plasma sheet, enhanced radial diffusion and local wave-particle interaction acceleration. By adopting a fitting parameter of loss time tau(L) we solved the bounce-averaged pitch angle diffusion equation driven by field-aligned whistler-mode waves (including chorus and hiss). We show that pitch-angle scattering can account for the pitch-angle distribution evolution in 30-500 keV electrons in the innermost radiation belt near L = 1.7 (as observed by Polar satellite) and the slot region 2 < L < 3. In particular, simulated results indicate that the loss-cone region is almost empty, and outside the loss-cone region both flux and anisotropy of energetic electrons are reduced with the gyroresonant time. The obtained time scale for the pitch-angle distribution evolution is found to be approximately tens of hours, consistent with observation.