THE RESPONSE OF THE HIGH-LATITUDE DAYSIDE IONOSPHERE TO AN ABRUPT NORTHWARD TRANSITION IN THE IMF

We examine the response of the high-latitude ionosphere in the prenoon sector to a northward turning of the IMF. The event was observed in the 11-13 UT interval (approximately 8-10 MLT) on 1 June 1987, in the, course of a multi-day SUNDIAL campaign. The transition in the IMF was observed by the IMP-8 satellite which was located upstream of Earth at a distance of 36 R(E). The ionospheric response in the 70-degrees-80-degrees invariant latitude interval was monitored by the Johns Hopkins University/Applied Physics Laboratory HF coherent scatter radar located at Goose Bay, Labrador, and by the SRI International incoherent scatter located at Sondrestrom, Greenland. The preexisting plasma convection observed by the two radars exhibited large velocities (500-1000 m s-1) and stable long-term trends, consistent with the inertial rotation of the convection pattern expected of the conditions then prevailing, B(z)<0, B(y)>0. The plasma flow rapidly abated in response to the IMF transition. The electron density measurements made by the Sondrestrom radar in the meridional plane showed that the ionosphere had been rich in structure with the active deposition of ionization by particle precipitation. Subsequently it resembled an inactive, unstructured mid-latitude configuration. There was a dramatic decrease in the amount of backscatter observed by the HF radar. We analyze the times of transition in the various data sets and show that the ionosphere began to show the effects of the IMF transition about 2 min after its probable arrival at the magnetopause boundary.