Timing of a Substorm Event at ~07:13UT, Jan 29, 2008

We report a timing analysis on a substorm around ~07:10UT on Jan 29, 2008 captured by the THEMIS spacecraft. The spacecraft (probes) were aligned along the tail between XGSM=-7 RE to -30 RE. During the time of the substorm, the solar wind had a vz component of ~50 km/s corresponding to a 6.5o southward tilt of the magnetotail. Thus we rotate the GSM coordinate system 6.5o anticlockwise around the Y axis to form a more physical coordinate system and investigate the satellite data in this coordinate system. The most distant probe P1 (XGSM=-29.5 RE) detected a bipolar magnetic signature corresponding to a tailward moving structure. P2 (XGSM=-18.5 RE) also saw magnetic signatures indicating tailward moving structure, while P3 (XGSM=-10.8 RE) and P4 (XGSM=-10.6 RE) captured dipolarization fronts and Earthward flows. THEMIS ground stations and all-sky imagers recorded Pi2 pulsations and brightening in a white-light auroral imager. We perform a detailed timing analysis of probe and ground-based data and reconstruct the time sequence of phenomena during this substorm. The earliest event related to the substorm was the Earthward beams on P3 and P4, followed by the aurora intensification at 07:12:22UT. The first magnetic deflection was detected by P2 and then bipolar perturbation in the north-south component of the magnetic field was captured by P1 at 07:12:32UT. Dipolarization fronts as well as convective flows arrived at P3 and P4 later at ~07:13:35UT accompanied by sudden increase of the cumulative magnetic flux transferred Earthward, and ground magnetic pulsations happened about the same time or later. We consider this substorm to be generated by tail reconnection at ~18 RE, different from the interpretation by Lui et al. [2008] on the same event. It is inferred that reconnection in the tail preceded the aurora intensification by ~2 minutes and preceded the ground magnetic perturbation and near-Earth dipolarization by ~3 minutes.