Electron transport, acceleration and loss in Earth's outer radiation belt

Abstract

Earth's radiation belts are comprised of high energy charged particles trapped by Earth's magnetic field. The outer radiation belt is highly variable, with the trapped electron flux varying by orders of magnitude on timescales of hours. The radiation belts are a hazard for satellite operations as high energy charged particles can damage electrical components. For this reason it is essential to be able to understand, model and predict the processes driving the variability of the outer radiation belt. This is of particular importance during geomagnetically active times, when the variance in the Earth's magnetic field, the plasma wave distribution and the background plasma properties is highest. This thesis investigates the behaviour of relativistic electron flux in the outer radiation belt, and attempts to reproduce rapid storm time flux variations using the British Antarctic Survey radiation belt model (BAS-RBM). A set of events involving rapid drops in electron flux across a range of energies at high $L$ shells near the magnetopause were analysed. The observations were compared to event-specific last closed drift shell models. The results indicate that the magnetic local time at the satellite and its position relative to the last closed drift shell are both important when reproducing observations with radiation belt models. Simulations with the BAS-RBM determined that different last closed drift shell models can significantly alter the reproduction of flux dropouts, but that the current outer boundary conditions and statistical wave models are unable to recreate the inwards extent of flux dropouts in $L^*$. Chorus wave power near the strong diffusion limit was investigated as an alternate driver for electron flux dropouts. Previous analyses have treated strong diffusion primarily as a loss process. Satellite data was used to verify the existence of high chorus wave power leading to strong diffusion in the outer radiation belt. BAS-RBM simulations unexpectedly demonstrated that chorus waves causing strong diffusion led to an increase, not a decrease in the net flux as predicted when treating strong diffusion as a loss process. The results show the existence of a tipping point in chorus wave power between net acceleration and net loss. Due to this, radiation belt models using averaged chorus wave power will misrepresent the effects of chorus when chorus wave power is high. In order to better represent the variation in chorus wave power during geomagnetically active times, a plasma density dependent chorus model was developed for the BAS-RBM. The measurement of the background plasma density was determined to be biased away from low densities. This was corrected for in the chorus model. In combination with an event-specific density model, BAS-RBM simulations using this chorus model are capable of reproducing observations of enhancements of ultra-relativistic electron flux. These simulations confirm that chorus waves are able to directly accelerate electrons to multi-MeV energies with and without including the effects of radial diffusion.