INSTANTANEOUS MAPPING OF HIGH-LATITUDE CONVECTION WITH COHERENT HF RADARS

Coherent HF radars at Goose Bay (Labrador) and Schefferville (Quebec) are used to study plasma convection in the high-latitude ionosphere. M aps of the two-dimensional flow pattern are obtained by merging simult aneous sets of radial velocity data, each with a time resolution of a few minutes. From a time sequence of such maps we have separated the c hanges in flow due to magnetic local time (MLT) variations over the re gion of observation, from those due to UT time variations. We study in detail the convection in the early morning sector observed on October 15, 1989, when the interplanetary magnetic field (IMF) reversed from southward to northward. This IMF reversal was not associated with a cl ear response in the nightside convection but rather with several sudde n changes, some of which anticipated the B(z) reversal. We suggest tha t these changes are associated with delayed and superposed ionospheric responses to previous IMF perturbations, or to local effects. After t he IMF reversal from south to north our observations of westward and s outhwestward velocities in the 71-degrees-77-degrees invariant latitud e range are consistent with the earlier simulations for B(z) > 0 and B (y) < 0. During the period of steady northward IMF after the reversal the convection pattern was observed to reconfigure slowly: a region of large westward velocities progressively moved poleward, while convect ion in the low-latitude part of the field of view faded away. The time constant of this slow reconfiguration was about 1 hour and varied wit h MLT, such that it was larger closer to midnight. These data, combine d with particle data from successive passes of the DMSP satellites, pr ovide information on the contraction of the polar cap after the IMF B( z) reversal and on the MLT dependency of the velocity at which this co ntraction occurs. They show that the polar cap contracts more rapidly in the daytime than in the nightime and more rapidly in the postmidnig ht sector than in the premidnight sector.