Timing of substorm signatures during the November 24, 1996, Geospace Environment Modeling event

An excellent data set has been gathered for the November 24, 1996, Geospace Environment Modeling substorm interval that included an similar to 95 min interval of strongly southward interplanetary magnetic field, There were tw o expansion phase onsets and a pseudobreakup during this period. For both o nsets the classical signatures of onset in the auroral ionosphere (ground m agnetometer signatures of electrojet formation, ground observations of Pi2 pulsations, and auroral brightening) all occurred within less than a minute of each other, indicating consistency between these onset indicators and g iving reliable identification of onset times. On the other hand, low-latitu de positive bay observations that were most likely made near or within the longitude range of substorm onset did not provide accurate or consistent on set timing. Low-latitude Pi2s identified both onsets, though one was delaye d by 1 min from the auroral zone onset. Our most important and unexpected r esult is that particle injection at synchronous orbit was observed to initi ate similar to2.5 min prior to both expansion onsets in the auroral ionosph ere. We suggest that the early detection of substorm onset at synchronous o rbit was at least in part due to the unusually low latitude (63 degrees mag netic) of the auroral zone onsets, which maps closer to synchronous orbit t han usual. Onsets more often occur at somewhat higher latitudes, which map to a few R-E beyond synchronous orbit. As a result, particle injections at synchronous orbit are generally delayed with respect to the time of current wedge initiation within the plasma sheet. We do not currently know whether or not onset within the near-Earth plasma sheet generally occurs similar t o2.5 min prior to expansion onset in the auroral ionosphere. While we have no reason to believe that the timing we have observed is unique, other stud ies of low-latitude substorm onsets will be needed to ascertain the general ity of this result.