A study of a magnetospheric substorm recorded by the Viking auroral imager

A substorm, recorded at 80 second time resolution in images from the Viking auroral imager, has been studied using data from ground observations, from a geosynchronous satellite and from ISEE 1 in the magnetotail plasma sheet. Substorm onset, marked by negative bays at the ground, particle injection at synchronous orbit and plasma sheet dropout at ISEE 1, appeared to produce only local brightening of the night side oval. A rapid poleward expansion of the auroras twenty minutes later was accompanied by a major reduction of tail energy density and swelling of the inner plasma sheet. Recovery, 15 minutes later, of auroral zone bays and the plasma sheet was accompanied by fading of the equatorward auroras that left a bright poleward arc as the primary auroral emission.