Braking of high-speed flows in the magnetotail: THEMIS joint observations

Abstract

The motion and deceleration processes of plasma sheet high-speed flows have great significance to magnetospheric particle acceleration, magnetic field perturbation, magnetic flux transport, triggering of substorm, and the current system formation in the magnetotail. From February to April 2009, two satellites of the Time History of Events and Macroscale Interactions during Substorms mission, THA and THE, were often separated largely in Z direction, but had small X and Y separations. Such special configuration allows simultaneous observations of high-speed flows at the center and boundary of the plasma sheet. Based on selected case study and statistical analysis, it is found that for about 89 % of the events we selected, the probe further away from the neutral sheet observed the high-speed flow earlier than the one close to the center, and the flow is mainly field aligned. And for about 95 % events the probe further away from the neutral sheet observed higher X component of the plasma flow. With the hypothesis that parallel flow keeps the same speed during its earthward propagation while central plasma sheet stream uniformly or suddenly brakes on its way to the earth, we deduced the position where the deceleration begins to be between 13 Re and 17 Re downtail, where the near-earth reconnection is supposed to occur. In addition, our statistical results show that dipolarization fronts observed in the central plasma sheet are more prominent than those observed in the plasma sheet boundary layer ahead of the high-speed flow.