Understanding how low energy electrons control the variability of the Earth's electron radiation belts
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The electron radiation belts are regions of geomagnetically trapped electrons, surrounding the Earth, presenting hazards to operational satellites. On the timeframe of hours, both the energy and particle flux of the radiation belts can change by orders of magnitude. Variations in the high energy relativistic electron flux depend on transport, acceleration, loss processes, and importantly, on the lower energy seed (10s – 100s keV) population. Seed population electrons are supplied to the radiation belt region during geomagnetically active periods and can be accelerated to higher energies via a range of processes. Unlike the higher energy, $>$1 MeV electrons, the azimuthal drift of the seed population is strongly affected by the convection electric field.
Using fourteen years of electron flux data from low Earth orbit (LEO) satellites, a statistical
study was performed on the magnetic local time distribution of three seed population
energies, across a range of activity levels, defined by the geomagnetic indices AE, AE*,
Kp, the solar wind velocity, and V
A novel method was developed that utilizes measurements from low altitude, polar
orbiting POES and MetOp satellites to retrieve the seed population at a pitch angle of 90
The impact of variations in the seed population on the 1 MeV flux level was explored using the 3-D BAS-RBM to solve a diffusion equation for the electron phase space density. For some periods, an enhancement in the seed population was vital to recreate observed 1 MeV flux enhancements. A series of idealised experiments with the 2-D BAS-RBM were performed which highlight a careful balance between losses and acceleration from chorus waves. Our results show that 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, pre-existing energy gradients in the phase space density and the duration of chorus wave activity determine whether $>$500 keV electrons were enhanced due to local acceleration.
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Horne, Richard B.
Glauert, Sarah A.