Motion of reconnection region in the Earth's magnetotail

We obtain the X-line velocity in the Earth's magnetotail for 24 reconnection events observed by the Cluster spacecraft between 2001 and 2005. The data set consists of 10 single X-lines and 14 X-lines from the time intervals of multiple reconnection. Except for two X-lines from multiple reconnection, all the X-lines move tailward (radial outward) along the current sheet, which is consistent with the direction of the pressure gradient. The X-lines also propagate outward from the midnight sector in the dawn-dusk direction. The X-line speed in the Earth-tail direction is comparable and proportional to the reconnection inflow speed and approximately 0.1 of the reconnection outflow speed. These results suggest that both the global pressure distribution and the local reconnection physics may affect the motion of the X-line.