Cluster observations of a magnetic field cavity in the plasma sheet

During the recovery phase of a substorm occurring on 1 September 2002 the four Cluster spacecraft crossed from the northern tail lobe into the plasma sheet. While the spacecraft were located in the plasma sheet boundary layer, the magnetic field data from the four spacecraft detected a cavity of close to zero magnetic field. The plasma in this cavity had characteristics similar to that of the central plasma sheet, possibly implying that the central plasma sheet had expanded over and subsequently receded back over the spacecraft. However, the unique four-spacecraft tetrahedral configuration of the Cluster spacecraft shows that this is not a valid scenario as the cavity passed over the four spacecraft, travelling continuously equatorwards and Earthwards. Our analysis is based on data from the Fluxgate Magnetometer, Cluster Ion Spectrometer, Plasma Electron and Current Experiment, and Research with Adaptive Particle Imaging Detectors instruments on board the Cluster spacecraft.