Magnetotail current sheet structure and dynamics from multi-point spacecraft measurements

We present results of studies examining the structure and dynamics of the magnetotail current sheet with the Cluster spacecraft. After an introduction to the magnetosphere, we present the technique for calculating current densities with Cluster magnetometer data and a comparison of those current densities for different spacecraft separations. We show that current densities inferred for 150 km spacecraft separation tend to be larger than those for 1500 km and 5000 km separations. Fluctuations in the current density are also largest for 150 km spacings, revealing that small spatial scale current structures with large current densities are present in the current sheet. A comparison of integrated current densities to the total change in magnetic field through a plasma sheet crossing is presented to distinguish differences between current densities for the three spacecraft separation distances. We find that integrated current densities sometimes equal the total change in magnetic field but there is no clear dependence on separation distance. The average structure of the current sheet is surveyed through both static and time-varying fits of the Harris current sheet model to Cluster measurements. Current density time series provide a more accurate constraint on such descriptions than was previously available. We find that a time-varying description often provides a reasonable description of current sheet dynamics during the phases of a substorm. Superposed epoch analysis of 21 substorm onsets identified at Cluster further supports the time-varying Harris model. We studied bifurcated current sheets with both an MHD simulation and Cluster measurements. Our MHD simulation, using solar wind conditions present during a previously published event, reveals that current sheet bifurcations are a few RE in scale size, are present over time scales of several minutes, and are magnetically linked to neutral lines in the tail. We developed search criteria to automatically identify bifurcated current sheets in Cluster data for 1500 km and 5000 km spacings using the MHD simulation and previously published events as a guide. We show that bifurcated current sheets are typically ≳ 1 RE in scale size and can often be associated with local neutral lines near Cluster, consistent with our MHD modeling results.