Substorm onset timing via traveltime magnetoseismology

This letter introduces a traveltime magnetoseismic technique that can estimate the substorm onset time in the magnetotail from multi-point observations of Pi 2 pulsations. The arrival time of Pi 2, which is defined as the first peak in Pi 2 amplitude, observed by ground magnetometers presents a strong function of latitude. Using the Tamao traveltime as the forward model, the magnetoseismic inversion from the observed Pi 2 arrival time infers that the onset of the 22 July 1998 substorm in the magnetotail started at X similar to -20 RE at 6: 53: 10 UT, or 1-3 minutes earlier than the Pi 2 onset time conventionally identified in ground records. The magnetoseismic result is further compared with the observations of auroral brightening, revealing a clear sequence of events during this substorm. Citation: Chi, P. J., C. T. Russell, and S. Ohtani (2009), Substorm onset timing via traveltime magnetoseismology, Geophys. Res. Lett., 36, L08107, doi: 10.1029/2008GL036574.