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Title: Understanding how low energy electrons control the variability of the Earth's electron radiation belts
Author: Allison, Hayley Jane
ISNI:       0000 0004 7968 5195
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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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 VswBz. During periods of high activity, dawn-dusk flux asymmetries of over an order of magnitude were observed for > 30 and > 100 keV electrons, due to increased flux in the dawn sector. For > 300 keV electrons, magnetic local time asymmetries were also present, but arose primarily due to a decrease in the average dusk-side flux beyond L* ~ 4.5. 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 resulting dataset offers a high time resolution, across multiple magnetic local time planes, and was used to formulate event-specific low energy boundary conditions for the British Antarctic Survey Radiation Belt Model (BAS-RBM). This new low energy boundary condition from LEO data has a higher spatial and temporal resolution, and a broader L* coverage, than previous work. 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.
Supervisor: Del Zanna, Giulio ; Horne, Richard B. ; Glauert, Sarah A. Sponsor: Cambridge University Earth System Science Doctoral Training Partnership (ESS DTP)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Radiation belts ; electrons ; wave particle interactions ; seed population electrons