NIGHTTIME PATTERNS OF IONOSPHERIC CONVECTION, CONDUCTANCE, HORIZONTALAND FIELD-ALIGNED CURRENTS DURING A STEADY MAGNETOSPHERIC CONVECTION EVENT

On 21 April 1988 the EISCAT radar carried out measurements in the CP-3 mode (large meridional scan) in the dusk to midnight MLT sector durin g a Steady Magnetospheric Convection (SMC) event lasting for 8 hours. This rare occasion made it possible to study for the first time the 2- dimensional distribution of convection velocities, conductances, elect ric potential, equivalent and real hoizontal currents of the auroral i onosphere during SMC conditions. In spite of a modest level of magneti c activity (AE-index between 200 and 250 nT) many characteristics rese mbled those of high magnetic activity. The auroral oval was at the nig htside about 10 degrees of latitude wide and the cross polar cap poten tial reached values up to 80 kV. Dominating features of the nightside convection pattern were the formation of a convection throat (with 15k V potential difference per 1 h of LT) centered at 23 MLT and the absen ce of the Harang Discontinuity (HD). The HD is traditionally understoo d as the overlapping region between two electrojets generated by a par tial intrusion of the west-ward electrojet into the northern flank of the eastward electrojet. Intense (similar to 1p mu A m(-2)) field-alig ned currents were generated at the large north-south conductance gradi ents in the convection throat region mainly due to the divergence of H all currents. We suggest that the so generated upward flowing current in the middle part of the auroral zone can provide the ionospheric clo sure of the field-aligned current generated in the magnetosphere due t o the dawn-dusk asymmetry of the cross-tail current and it also offers an explanation for the absence of the HD during SMC events.