Cluster four spacecraft measurements of small traveling compression regions in the near-tail

Cluster observations taken during a substorm on September 19, 2001 have revealed the presence of small traveling compression regions (TCRs) in the near tail. These measurements are used to determine directly the speed and direction of TCR propagation and the amplitude of the underlying bulge in the plasma sheet. The time-of-flight speeds derived from the arrival times of the magnetic perturbations at the different Cluster s/c yielded a mean speed of 413 km/s. For 2 of the TCRs s/c 1, 2 and 4 were located sufficiently close to the plasma sheet that they were immersed in the central plasma sheet plasma as the TCR swept over s/c 3. In this manner the Cluster measurements directly demonstrated that these small TCRs are caused by moving bulges in the plasma sheet-lobe interface. In summary, our analysis of the Cluster measurements has directly demonstrated the existence of moving bulges in the north-south thickness of the plasma sheet, most probably due to the formation of flux ropes, and their role in producing traveling compression regions