Substorm onset timing: The December 31, 1995, event

The objective of the present study is to examine the timing of various onse t-associated signatures and address the cause-and-effect relationship betwe en the formation of a near-Earth neutral line (NENL) and the trigger of tai l current disruption. An event selected for this study took place on Decemb er 31, 1995. In this event the Geotail satellite was located at X = -30.3 R -E in the midnight sector at a local time between the GOES 8 and 9 geosynch ronous satellites. The timing of the Geotail observation of a fast (950-km/ s) tailward convection flow accompanied with southward B-z (< -10 nT) indic ates that the near-Earth reconnection process started at least 4 min before the ground substorm onset, which was identified by various signatures such as an auroral expansion, a Pi2 onset, a positive bay onset, and a negative bay onset. Both GOES satellites observed dipolarization. GOES 9 was locate d closer to the onset meridian and observed a sudden recovery (dipolarizati on) of the local magnetic field but with a noticeable (approximate to 1 min ) delay from the ground onset. This delay can be interpreted in terms of th e earthward expansion of tail current disruption initiated outside of geosy nchronous orbit. The timing of all these features is consistent with the id ea that dipolarization is a pileup of magnetic flux conveyed from the NENL. However, a sharp decrease in the H component at GOES 9 prior to the local dipolarization onset and the sudden start of a substorm are difficult to ex plain in terms of this idea. It is asserted that tail current disruption is a unique process rather than a direct consequence of the NENL formation, a lthough it is possible that the reconnection process sets up a favorable co ndition for triggering tail current disruption. The fast plasma flow in the plasma sheet ceased soon after the substorm onset, suggesting that during the expansion phase, the tail current disruption took over the near-Earth r econnection process as a major role in the substorm dynamics.