On the Importance of Gradients in the Low-Energy Electron Phase Space Density for Relativistic Electron Acceleration

Observations of the electron radiation belts have shown links between increases in the low-energy seed population and enhancements in the >1-MeV flux. During active times, low-energy electrons are introduced to the radiation belt region before being accelerated to higher energies via a range of mechanisms. The impact of variations in the seed population on the 1-MeV flux level were explored using the British Antarctic Survey Radiation Belt Model. We find that, for a period from the 21 April to 9 May 2013, the increase in the low-energy electron flux was vital to recreate the observed 1-MeV flux enhancement on the 1 May but was less important for the 1-MeV enhancement on the 27 April 2013. To better understand the relationships between the different energy populations, a series of idealized experiments with the 2-D British Antarctic Survey Radiation Belt Model were performed, which highlight a careful balance between losses and acceleration from chorus waves. Seed population enhancements alter this balance by increasing the phase space density gradient, and consequently, the rate of energy diffusion, allowing acceleration to surpass loss. Additionally, we demonstrate that even with the same chorus diffusion coefficients and the same low-energy boundary condition, the flux of similar to 500-keV to 1-MeV electrons increased when starting with a hard spectrum but decreased for a soft initial spectrum. This suggests that initial energy gradients in the phase space density were important to determine whether >500-keV electrons were enhanced due to chorus wave acceleration.