THEMIS Observation of a Substorm Event on 4:35, Feb 22, 2008

We report on THEMIS observations during a substorm between 0430~0450 UT on Feb 22, 2008. The spacecrafts were aligned along the tail from X=-5 RE to -25 RE GSM. The most distant probe P1 (X=-24.5 RE) detected two successive tailward moving bipolar magnetic structures. P2 (X=-18 RE), P3 (X=-11 RE), P4 (X=-10.5 RE) and P5 (X=-6 RE) all captured signatures related to the earthward movement of a magnetic structure. THEMIS ground stations and all-sky imagers also recorded Pi2 pulsations and an aurora sudden brightnening. We perform a detailed timing analysis of spacecraft and ground-based data and reconstruct the time sequence of phenomena during this substorm. The earliest sign of substorm onset was the bipolar perturbation in the Northward component of the magnetic field (interpreted as the result of reconnection onset) at P1 at 04:35:16UT and corresponding magnetic perturbation at P2 at 04:35:14UT. Auroral onset was seen at or before 04:36:18UT, consistent with the onset of Pi2 pulsations at 04:36:10 UT. Earthward flows at P3 and P4 seen at ~04:36:03UT, and dipolarization onset at ~04:36:50UT, were observed slightly after the ground onset signature, implying that near-Earth dipolarization happened in the aftermath of both tail reconnection and auroral intensification. Reconnection in the tail preceded ground onset by 60~80 seconds, and also preceded near-Earth dipolarization (current disruption) by ~2 minutes. Two reconnection pulses were observed and a direct association with two auroral intensifications is possible, suggesting that tail reconnection, like the auroral expansion, advance in steps rather than continuously.