The Earth's Magnetotail during Varying Levels of Geomagnetic Activity

This thesis describes work that has employed both ground- and space-based data to investigate the magnetotail and its response to differing levels of geomagnetic activity. The first study of current sheet motion involved analysing the number of crossings that the Cluster 3 spacecraft made of the cross-tail current sheet, as a measure of current sheet dynamics. Geomagnetic conditions measured during the crossings allowed a comparison of the current sheet dynamics during substorms, magnetic storms and quiet times. The results indicated that more motion of the current sheet is seen during substorms compared to quiet times. In addition, there was evidence that current sheet motion is suppressed when the ring current is enhanced. An analysis of current sheet structure is presented in the next study, which examines the orientation, current density and thickness of the current sheet. The current sheet was found to be more tilted during substorms than quiet times and when magnetic storms were occurring the tilt was reduced. The current density analysis showed larger values during storms compared to quiet and substorm times. The final study investigated how the lobe magnetic field varies according to geomagnetic activity. Results are presented showing evidence of a larger lobe magnetic field during magnetic storms, compared to quiet times and substorms. The thesis work provides a more complete picture of the current sheet and lobes, during different levels of geomagnetic activity, than has been previously shown. There is evidence that substorm occurrence is related to current sheet motion and a highly tilted orientation. The increased lobe field found during magnetic storms may cause a rigidity to the magnetotail that suppresses this motion and its tilting in the YZ plane. The increased lobe field also results in an increased current density in the current sheet during storms.