Plasma flow boundaries in the high-latitude ionosphere

Laurens, Hannah and Grocott, Adrian (2019) Plasma flow boundaries in the high-latitude ionosphere. PhD thesis, Lancaster University.

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Abstract

The main component of this thesis is to investigate ionospheric flow boundaries derived from plasma convection observations and their relationship to magnetospheric boundaries. Observations of ionospheric boundaries can provide useful information on the time-dependant dynamics of the coupled solar wind-magnetosphere-ionosphere system. Ionospheric convection boundaries were inferred using an archive of data from the SuperDARN HF radar network which operates in the auroral regions of both hemispheres. Regions where the ionospheric flow reveals a change in the east-west direction, the convection reversal boundary (CRB), is a common and widely-used criterion to identify boundaries in the convection data. However, this approach can be limited in certain regions. This thesis firstly presents a new boundary identification technique, the convection curvature boundary (CCB) that is derived using a more physical interpretation of where boundaries in the convection data should lie. A statistical analysis is performed between the two different ionospheric boundaries and their applicability as a proxy for the open closed field line boundary (OCB) is tested. The CCB is found to be more robust than the CRB which tends to identify boundaries at unphysical latitudes, for example, close to the pole and at noon and midnight magnetic local times (MLTs) where it becomes less well defined. The noon region in particular shows that the CCB identifies boundaries at latitudes more consistent with theoretical considerations, especially when asymmetries are introduced into the ionospheric convection as a result of a By-dominated interplanetary magnetic field (IMF). Results from a statistical analysis comparing the poleward auroral boundary revealed that the CCB aligned more closely at almost all MLTs. It was found that CRB and CCB location is dependant on the amount of local radar coverage at certain MLT sectors but that with sufficient coverage the CCB can provide an extremely good proxy for the poleward auroral boundary to within 1◦ magnetic latitude. Finally, a discussion of further work is given following some of the implications of the statistical results. In particular, the relationship between the effect of radar coverage and magnetotail phenomena (such as substorms), both of which have an effect on the resulting convection pattern, was investigated. Initial results suggest that measurements of the nightside ionospheric convection were modified by the location of radar coverage across the map and variations in the amount of radar coverage over the substorm period were ordered relative to the substorm onset location.