Evaluation of low-latitude Pi2 pulsations as indicators of substorm onset using Polar ultraviolet imagery

Impulsive Pi2 pulsations have long been recognized as one of the key signat ures of magnetic activity during substorm periods due to their wide observa ble range both in latitudes and longitudes. It is well documented that ther e is usually more than one Pi2 wave burst associated with a substorm and on ly one of them corresponds to the onset of the substorm. This observational fact poses obstacles to determining substorm onsets with Pi2 signals. Alth ough the Pi2 have become one of the most popular indicators for substorm on sets, the reliability of using the Pi2 in this fashion has not been serious ly investigated. In this paper we address this question with a statistical approach by using similar to 650 auroral substorm onsets identified with Po lar ultraviolet images for a time interval from April 1996 to May 1997. A c omparison of the low-latitude Pi2 pulsation onsets from Kakioka (L = 1.07) with the auroral breakups indicates that identifying substorm onset with th e Pi2 alone is often ambiguous. Of a total of 119 isolated (defined as simi lar to 10 min of quiet time preceding the onset) Pi2 bursts seen within sim ilar to 10 min from a magnetic positive bay, there were 65 events (similar to 55%) taking place within 3 min from breakups and 34 events (29%) indicat ing no sign of an auroral breakup within similar to 10 min of the Pi2 burst . This result suggests that Pi2 may not be as a good indicator of the subst orm onset as it was thought to be. Interestingly, it is always possible to associate Pi2 pulsations with some forms of auroral intensification. When c ompared to auroral breakups, Pi2 onsets are subject to a small delay of 1 - 3 min, with a peak around 1 min. Delays of Pi2 onsets are revealed to be a function of location relative to auroral breakup. This dependence is found to be consistent with the time of flight for a fast-mode wave, in a plasma pause cavity mode model, propagating in the magnetosphere.