Dusk-Dawn Convection Asymmetries in the Earth's Magnetotail : The Influence of IMF By and Transient Dynamics

Lane, James and Grocott, Adrian (2022) Dusk-Dawn Convection Asymmetries in the Earth's Magnetotail : The Influence of IMF By and Transient Dynamics. PhD thesis, Lancaster University.

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Abstract

When a significant By (dusk-dawn) component exists in the Interplanetary Magnetic Field (IMF), magnetic reconnection with the terrestrial field introduces asymmetries into the Earth’s magnetosphere on a global scale, resulting in a ‘twisting’ of the magnetotail. The fast earthward convective flows which subsequently ‘untwist’ the tail are expected to have a dusk-dawn component which is inherently dependent on the sign of IMF By, giving rise to the ‘untwisting hypothesis’. This expected dependence is, however, only observed around ∼70% of the time. In this thesis, data is used from magnetospheric spacecraft missions, such as Cluster, and the radar system SuperDARN, to attempt to understand why the remaining ∼30% of flows do not exhibit this expected direction and to refine our understanding of the drivers of dusk-dawn asymmetry in the Earth’s magnetosphere. In Chapter 4, we derive a statistical dataset of magnetotail fast flow ‘detections’ and investigate a number of parameters associated with only those flows which explicitly demonstrate a dusk-dawn asymmetry. An overview of our 1639 asymmetric flows suggests differences between flows which agree and disagree with the expected dusk-dawn asymmetry. In Chapters 5 and 6, we present case studies containing instances of ‘disagree’ flow where the expected asymmetry is not always observed. We find that transient, dynamic phenomena, including a localised ‘flapping’ of the magnetotail current sheet (Chapter 5) and strong dipolarisation (Chapter 6) appear to be associated with flows which are independent of the large-scale asymmetry in the convection. In Chapter 7, we present further statistics intended to elucidate the ∼30% of disagree flows. We find that the expected IMF By control of the dusk-dawn flow is strongest (≥ 90%) when the possibility that any transient, dynamic phenomena (including current sheet flapping and strong dipolarisation) are less likely to be occurring. Collectively, this thesis presents evidence to suggest that transient, localised dynamics can override or prevent the expected IMF By-control of convective dusk-dawn magnetotail flows, notably when |IMF By| is weak, explaining at least two thirds (i.e. 20%) of the ∼30% disagreement.