Identifying the Cause of Outer Radiation Belt Enhancements: The Importance of Accounting for the K-Dependence of and uncertainties in the Electron Phase Space Density.

Abstract:

We present results showing the temporal evolution of electron Phase Space Density (PSD) in the outer radiation belt during the most intense geomagnetic storm of the last decade which occurred on March 17^th 2015. The PSD profiles as a function of L* at fixed first, M, and second, K, adiabatic invariants are derived from Van Allen Probe ECT electron flux data as well as the Tsyganenko magnetic field model. Based on growing local PSD peaks at M=1000 MeV/G and K=0.18 G^1/2Re respectively, previous studies argued that the outer radiation belt flux enhancement that occurred during this storm resulted from local acceleration driven by VLF waves. However, these PSD profiles at fixed first and second invariants were determined using the global Tsyganenko magnetic field model. Moreover, large discrepancies between the observed and model magnetic field occurred during this event. Here we show that if data points along the PSD profiles are removed where the model and measured magnetic fields are in poor agreement, there is no clear evidence of growing local PSD peaks. In addition, we also show that the majority of the outer radiation belt consisted of electrons with much lower K-values than 0.18 G^1/2Re, and that at these lower K-values there is also no clear evidence of growing local PSD peaks. Contrary to prior studies we show that the outer radiation belt flux enhancement is consistent with inward radial diffusion and that the observed outer radiation belt flux enhancement during this geomagnetic storm can be reproduced using a radial diffusion model driven by measured ULF waves without including any local acceleration. These results highlight the importance of careful analysis of the electron PSD profiles over a range of K-values accounting for uncertainties in the magnetic field model to correctly identify the mechanism responsible for the electron flux enhancements observed in the outer radiation belt.