Adiabatic effects on radiation belt electrons at low altitude

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[1] The storm-time adiabatic effects of radiation belt electrons mirroring at low altitude are not analogous to those of equatorially mirroring electrons. During the main phase of a geomagnetic storm the adiabatic effects on low-altitude electrons include the expansion of the drift shell, the rise of the mirror point in altitude that is unique for electrons mirroring off-equator, and the shift in the energy spectrum. Calculations of the adiabatic flux change at low altitudes using a modified dipole model demonstrate that the storm-time adiabatic effects on electron flux are both altitude- and storm-dependent. The rise of the electron mirror points can lead to a null flux region at the low altitudes. A satellite in the null flux region sees zero flux during the storm time due solely to adiabatic effects, which can persist when the nonadiabatic pitch angle diffusion is very slow. A low-altitude satellite above the null flux region will see a fractional flux drop due to the adiabatic effects. For example, for the March 2008 geomagnetic storm with minimum Dst of −72 nT, there would be a factor of 2.4–2.8 decrease in the flux of relativistic electrons mirroring at 700 km and L* = 4.5, compared to a decrease of a factor of ∼15 for equatorially mirroring electrons due to adiabatic effects. We propose that the resulting adiabatic change in the electron pitch angle distribution can cause increased electron precipitation without changing the pitch angle diffusion rate by exciting higher-order eigenmodes of the bounce-averaged pitch angle diffusion. This work is the first quantitative analysis combining both observation and modeling for the adiabatic effects on the variation of outer radiation belt electrons at low altitude.