A magnetoseismic investigation of substorm onsets and its validation by satellite observations

We applied the travel-time magnetoseismic technique on the Pi~2 pulsations associated with the January 29, 2008 substorm to estimate the onset time and location in the magnetotail. The arrival time of Pi~2, which refers to the first peak in Pi~2 amplitude, observed by McMAC, THEMIS, and CARISMA ground magnetometers presents a strong function of latitude. Using the Tamao travel time as the forward model, the inversion from the observed Pi~2 arrival time estimates that the onset in the magnetotail started at X = -17.9 Re at 0713:25 UT, or approximately one minute earlier than the Pi~2 onset time identified in ground data. The estimated onset location in the magnetotail is consistent with the in situ observations by the THEMIS satellites, among which P1 and P2, located at X = -30 and -18 Re, observed negative Bz, while P3 detected earthward flows at X = -11 Re. The estimated start time in the magnetotail is at least 30 sec earlier than the auroral brightening seen by THEMIS all-sky imagers. Our magnetoseismic analysis and observations of this event are both consistent with reconnection model of substorm triggering.