Cluster Observations of the Postmidnight Plasma Sheet at 18 Re during Substorms

Abstract. On August 15, 2001 the Cluster spacecraft passed through the plasma sheet at 0100 LT at a distance of 18 Re downtail. During the passage, three substorms were observed. Each substorm was characterized by an increase in Bz or dipolarization. The first and last substorms were also accompanied by strong earthward flows. We utilize a variety of ground and spacecraft observations to establish the circumstances of these correlated changes. For the entire interval, the IMF By was strongly positive. For the first two substorms, the IMF Bz was significantly negative prior to the onset, but the third substorm occurred during very weak southward field. The Cluster field data indicate that IMF By significantly affected the magnetic field at 18 Re causing it to pass through the current sheet at an angle to the Z-axis. A principal axis system that best organizes the magnetic field is rotated in the opposite direction expected for IMF By effects in the tail. In two substorms, the arrival of flows at Cluster was associated with an intensification of an ongoing substorm expansion. The first substorm provides a clear example of a steep front of thickening of the plasma sheet passing over the spacecraft followed by several oscillations in vertical thickness. The third substorm took place as the spacecraft wer