Cluster investigations of magnetotail processes
Physical processes in the Earth's magnetotail are investigated using data principally from the Plasma Electron And Current Experiment onboard the European Space Agency's 4 spacecraft Cluster mission. Three topics are presented: the expansion and thinning of the plasma sheet during the course of a substorm field-aligned current generation during intervals of flow shears in the plasma sheet and observations of flux-ropes and travelling compression regions. Multipoint observations of the boundary between the plasma sheet and the lobe have allowed the determination of the speed and direction of propagation of the boundary. Using the direction of the boundary motion we looked for a trend of Earthward or tailward directed motion that might be indicative of whether near-Earth neutral line formation or current disruption occurred first at substorm onset. Given the time resolution of the observations, we did not find compelling evidence to confirm either initiator of the substorm process. The occurrence of flow shears in the plasma sheet, associated with phenomena such as bursty bulk flows, may result in the twisting of magnetic field lines. The twisted field should support a field-aligned current and this mechanism has been proposed to be significant at the flow-braking region, where the substorm current wedge is formed. We examine flow shears in the magnetotail and check for the presence of field-aligned currents during these intervals. The variations observed in the field-aligned current can be explained by changes in the position of the greatest flow shear with respect to the spacecraft tetrahedron.