RADAR OBSERVATIONS OF AURORAL-ZONE FLOWS DURING A MULTIPLE-ONSET SUBSTORM

We present an analysis of ground magnetic field, ionospheric flow, geo synchronous particle, and interplanetary data during a multiple-onset substorm on 12 April 1988. Our principal results concern the modulatio ns of the ionospheric flow which occur during the impulsive electrojet activations associated with each onset. During the first hour of the disturbance these take place every similar to 12.5 min and involve the formation of a new intense westward current filament in the premidnig ht sector, just poleward of the preexisting extended current system dr iven by the large-scale flow. These filaments are similar to 1 h MLT w ide (similar to 600 km), and initially expand poleward to a width of s imilar to 300 km before contracting equatorward and coalescing with th e preexisting current, generally leaving the latter enhanced in magnit ude and/or expanded in latitude. Within the impulsive electrojets the flow is found to be suppressed to values 50-100 m s(-1) or less during the first few minutes, before surging equatorward at 0.5-1.0 km s (-1 ) during the phase of rapid coalescence. The implication is that the p recipitation-induced Hall conductivity within the impulsive electrojet initially rises to exceed similar to 100 mho, before decaying over a few minutes. This value compares with Hall conductivities of similar t o 20 mho in the quasi-steady current regions, and a few mho or less in the regions poleward of the electrojets and in the preonset ionospher e. Preliminary evidence has also been found that the flow surges propa gate from midnight to the morning sector where they are associated wit h arrested equatorward motion or poleward contractions of the current system. These observations are discussed in terms of present theoretic al paradigms of the global behaviour of fields and flows which occur d uring substorms.