Plasma Sheet Thickness During A Bursty Bulk Flow Reversal

On 17 March 2008 around 9:12 UT the five THEMIS spacecraft P1-P5 were in the plasma sheet between 22 and 23 hours magnetic local time (MLT), covering radial distances between 15 Earth radii (Re) (P1) and 9 Re (P5). All the spacecraft consecutively observed a bursty bulk flow (BBF) that traveled earthward, slowed down from ~400 km/s to 50 km/s between P1 and P5, and then turned in the opposite direction. The most tailward-located spacecraft, P1 and P2, detected thinning and then thickening of the plasma sheet around the time of the flow direction change. Meanwhile, the other three THEMIS spacecraft, which were located in a more dipolar region, observed plasma sheet thickening and then thinning. Observations indicated that the thinning/thickening was stronger around the BBF funnel. Further, during the interaction of the earthward-flowing BBF plasma with the Earth's dipolar field lines, the BBF was deflected by about 70 degrees at a scale of about 5 Re. The radial pressure gradient was substantially increased when the BBF reached the shortest radial distance to the Earth and substantially decreased after the tailward plasma flow. We conclude that the tailward pressure pulse produced by the enhanced radial pressure gradients after the earthward BBF stopped could be responsible for the observed tailward plasma flows.