Storm-dependent radiation belt electron dynamics

Using recently published electron phase space densities (PSD) as a function of L* (L* is approximately the radial distance in Earth radii at the equator) and time, energization and loss in the Earth's outer electron radiation belt were studied quantitatively and numerically using a radial diffusion model that included finite electron lifetimes and an internal source parameterized as a function of geomagnetic indices. We used PSD data at fixed values of the first and second adiabatic invariants, corresponding to electrons mirroring near the Earth's equator with an energy of similar to 2.7 MeV at L* = 4. Model results for the second half of 2002 reproduced the average variations of the radiation belt electron PSD between L* = 2.5 and L* = 6 but with overprediction and underprediction at different times, implying that the same set of parameters cannot be applied to all storms. A detailed analysis of four individual storms showed that while electrons in three storms could be well simulated by energization from either radial diffusion only or internal heating only, incorporating both yielded the best results. For the other storm, an additional source of electrons was required to account for the enhanced PSD. The model results indicated that each storm is best simulated when a combination of radial diffusion and internal heating is used. Different storms required different magnitudes of radial diffusion and internal heating, and the relative contributions of these two acceleration mechanisms varied from storm to storm. A comparison of the results from different runs for the four storms and an analysis of the radial diffusion coefficients further suggest that internal heating contributes more to the enhancement of 2.7 MeV electrons at L* = 4 than radial diffusion.