Staples, Frances Allana; (2021) The Role of the Magnetopause in Outer Radiation Belt Flux Dropouts. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The radiation belts are highly dynamic regions of relativistic particles trapped in the Earth’s magnetic field. A delicate balance between particle acceleration and loss mechanisms results in the waxing and waning of radiation belt flux. Electrons may be lost by precipitation into the Earth’s atmosphere, or through the magnetopause into interplanetary space. Periods of enhanced loss often results in flux dropouts, the near total drainage of electrons from the outer radiation belt. This thesis investigates the role of magnetopause shadowing in flux dropout events, carried out through large multi-satellite analysis of the magnetopause boundary and relativistic electron dynamics. A database of ~20,000 magnetopause crossings is developed to assess the accuracy of a statistical magnetopause model. The measured magnetopause is found to be systematically closer to the radiation belts under compression than the statistical model suggests. Differences between measurements and model are greater still during interplanetary shock events. The results demonstrate that empirical magnetopause models should be used cautiously to interpret energetic electron losses by magnetopause shadowing. New, multi-mission electron phase space density calculations are used to decipher electron dynamics during a flux dropout case study. Features of magnetopause shadowing are identified during the net-loss and the net-acceleration storm phases. Observations show that magnetopause shadowing may occur in two distinct ways; through direct intersection of electron drift path, or indirectly by radial transport towards the magnetopause. These observations demonstrate the importance of considering electron dynamics in multi-dimensional adiabatic coordinates. The characteristics of electron loss through atmospheric precipitation are also observed. Consequently, this method of phase space density analysis showed promise for differentiating loss processes. It is demonstrated how the multi-mission dataset improves time resolution of phase space density observations, without which electron dynamics would be misinterpreted. Methods of further work are recommended to verify the multi-dimensional characteristics of different loss processes.

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