Special features of a substorm during high solar wind dynamic pressure

A substorm on July 24, 1986, exhibiting a rather unusual auroral morphology is analyzed with data from 10 spacecraft (Viking; DMSP F6 and F7; GOES 5 and 6; three LANL geosynchronous satellites; CCE; and IMP 8). This substorm occurred during high solar wind dynamic pressure (>5 nPa). Several notable features for this substorm are (1) the substorm onset activity was preceded by prominent auroral activations in the morning sector with spatial separations between adjacent bright regions ranging from ∼160 to 640 km, and their intensity was modulated at ∼3.2‐min intervals; (2) the initial substorm activity was concentrated in the morning sector, followed by a sudden activation in the dusk sector, leaving the midnight sector relatively undisturbed, in sharp contrast to the traditional substorm development in which the major activity occurs in the midnight sector; (3) while a substorm injection was observed at a geocentric distance of ∼8.4 RE by CCE in association with the substorm onset, particle injections (detectable with three LANL geosynchronous satellites) and dipolarization signatures (detectable by the two GOES satellites) were not observed until subsequent intensifications; (4) timing subsequent substorm intensifications from injections at the geosynchronous altitude differed from timing intensifications based on Viking auroral images by as much as ∼3 min even when the auroral activity occurred in the same magnetic local time sector as the geosynchronous satellite; (5) the polar cap boundary was at a significantly higher latitude than the poleward boundary delineated by detectable auroral luminosity in the auroral oval, supporting the previous result that auroral activity at substorm onset occurs substantially equatorward of the boundary between open and closed magnetic field lines. Detailed timing analysis suggests the substorm onset to be associated with southward interplanetary magnetic field (IMF), possibly with the crossing of an IMF sector boundary (interplanetary current sheet). The dimming of auroral luminosity in the midnight region was associated with a sudden northward turning of the IMF during high solar wind dynamic pressure condition.