Three-dimensional Transient Magnetic Reconnection and Dipolarization Fronts in the Terrestrial Magnetotail

Abstract:

We report a Cluster observation of transient magnetic reconnection in the Earth’s magnetotail at the location of [X_gsm ~ -17.2 R_E, Y_gsm ~ -4.5 R_E and Z_gsm ~ 0]. The reconnection X-line retreated tailward with a speed of 34 km/s. An ion diffusion region with a weak guide field (~10% of the lobe field) was encountered during the flow reversal. Transient energetic electron beams, which directed away from the X-line, were detected repeatedly around the current sheet boundary layer with periods of about 60s during the tailward flow burst. On the earthward side of X-line, multiple earthward-propagating dipolarization fronts were observed quasi-periodically with time period of 60s-90s. Surprisingly the cross-tail sizes of some observed dipolarization fronts are only 1-2 ion inertial lengths. The X-line expanded across the tail in the electron flow direction with a speed about 0.03V_A, here V_A is the Alfven speed. Two spacecraft, which were separated in the cross-tail direction by 1 R_E, observed distinct structures and different tones of reconnection, which implies that reconnection proceeds drastically different in different segments of X-line.