The ninth Coordinated Data Analysis Workshop (CDAW 9) focused upon several intervals within the PROMIS period (March--June 1986). Event interval C comprised the period 0000--1200 UT on May 3, 1986, which was a highly disturbed time near the end of a geomagnetic storm interval. A very large substorm early in the period commenced at 0111 UT and had a peak AE index value of ~1500 nT. Subsequent activity was lower, but at least three other substorms occurred at 2--3 hour intervals. The substorms on May 3 were well observed by a variety of satellites including ISEE 1, 2, and IMP 8 in the magnetotail plus SCATHA, GOES, GMS, and LANL spacecraft at or near geostationary orbit. A particularly important feature of the 0111 UT substorm was the simultaneous imaging of the southern auroral oval by DE 1 and of the northern auroral oval by Viking. The excellent constellation of spacecraft near local midnight in the radial range 5--9 RE made it possible to study the strong cross-tail current development during the substorm growth phase and the current disruption and current wedge development during the expansion phase. We use a time-evolving magnetic field model to map observed auroral features out into the magnetospheric equatorial plane. There was both a dominant eastward and a weaker westward progression of activity following the expansion phase.

A clear latitudinal separation (>10¿) of the initial region of auroral brightening and the region of intense westward electrojet current was identified. The combined ground, near-tail, and imaging data for this event provided an unprecedented opportunity to investigate tail current development, field line mapping, and substorm onset mechanisms. We find evidence for strong current diversion within the near-tail plasma sheet during the late growth phase and strong current disruption and field-aligned current formation from deeper in the tail at substorm onset. We conclude that these results are consistent with a model of magnetic neutral line formation in the late growth phase which causes plasma sheet current diversion before the substorm onset. The expansion phase onset occurs considerably later due to reconnection of lobelike magnetic field lines and roughly concurrent cross-tail disruption in the inner plasma sheet region. ¿ American Geophysical Union 1993