Walton, Samuel David; (2022) The Evolution of the Electron Radiation Belt During Geomagnetic Storms. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The outer Van Allen radiation belt is an extremely variable region of highly energetic electrons in Earth's magnetosphere, providing a harsh environment for man-made satellites. The complex dynamics, governed by the competition between electron enhancement and loss processes, make the understanding of these dynamics challenging. In this thesis, we investigate typical outer radiation belt behaviour during geomagnetic storms, carried out using in-situ spacecraft observations from both polar and equatorial orbits. We initially examine the cross-L* coherence of radiation belt electron variations using particle observations from the SAMPEX/PET instrument during 15 geomagnetic storms. We find that >0.63 MeV electron flux variations are typically coherent inside and outside the minimum plasmapause location. However, variations inside the plasmapause show little correlation with those outside. After the main phase of storms, variations are coherent across all L*, regardless of plasmapause location. The temporal evolution of the outer radiation belt during storms is analysed statistically using SAMPEX/PET and Van Allen Probes particle data. We show that loss process effectiveness is enhanced during storms outside the plasmapause and during periods of high geomagnetic activity, exhibiting an MLT and L-shell dependence consistent with known causal wave modes. Pitch angle distributions (PADs) show a systematic, energy-dependent broadening across the recovery phase for ≲800 keV electrons following main phase acceleration. ≳800 keV fluxes become strongly peaked at 90° during the main phase, concurrent with the enhanced loss shown in the SAMPEX data. Finally, we present strong evidence for the importance of the Kennel-Petschek process in not only limiting electron flux, but also in controlling the temporal evolution of electron PADs during storms. These results provide new insights into radiation belt dynamics using both modern and long-lasting datasets, enhancing our understanding of the outer radiation belt, raising further questions for future studies and missions to this region of space.

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