THE RESPONSE OF IONOSPHERIC CONVECTION IN THE POLAR-CAP TO SUBSTORM ACTIVITY

We report multi-instrument observations during an isolated substorm on 17 October 1989. The EISCAT radar operated in the SP-UK-POLI mode mea suring ionospheric convection at latitudes 71 degrees Lambda-78 degree s Lambda. SAMNET and the EISCAT magnetometer Cross provide information on the timing of substorm expansion phase onset and subsequent intens ifications, as well as the location of the field aligned and ionospher ic currents associated with the substorm current wedge. IMP-8 magnetic field data are also included. Evidence of a substorm growth phase is provided by the equatorward motion of a flow reversal boundary across the EISCAT radar field of view at 2130 MLT, following a southward turn ing of the interplanetary magnetic field (IMF). We infer that the pola r cap expanded as a result of the addition of open magnetic flux to th e tail lobes during this interval. The flow reversal boundary, which i s a lower limit to the polar cap boundary, reached an invariant latitu de equator-ward of 71 degrees Lambda by the time of the expansion phas e onset. A westward electrojet, centred at 65.4 degrees Lambda, occurr ed at the onset of the expansion phase. This electrojet subsequently m oved poleward to a maximum of 68.1 degrees Lambda at 2000 UT and also widened. During the expansion phase, there is evidence of bursts of pl asma flow which are spatially localised at longitudes within the subst orm current wedge and which occurred well poleward of the westward ele ctrojet. We conclude that the substorm onset region in the ionosphere, defined by the westward electrojet, mapped to a part of the tail radi ally earthward of the boundary between open and closed magnetic flux, the ''distant'' neutral line. Thus the substorm was not initiated at t he distant neutral line, although there is evidence that it remained a ctive during the expansion phase. It is not obvious whether the electr ojet mapped to a near-Earth neutral line, but at its most poleward, th e expanded electrojet does not reach the estimated latitude of the pol ar cap boundary.