Nighttime patterns of ionospheric convection, conductance, horizontal and field-aligned currents during a steady magnetospheric convection event

Abstract

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 during 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, electric potential, equivalent and real hoizontal currents of the auroral ionosphere during SMC conditions. In spite of a modest level of magnetic activity (AE-index between 200 and 250 nT) many characteristics resembled those of high magnetic activity. The auroral oval was at the nightside about 10° of latitude wide and the cross polar cap potential reached values up to 80 kV. Dominating features of the nightside convection pattern were the formation of a convection throat (with 15kV potential difference per 1 h of LT) centered at 23 MLT and the absence of the Harang Discontinuity (HD). The HD is traditionally understood as the overlapping region between two electrojets generated by a partial intrusion of the west-ward electrojet into the northern flank of the eastward electrojet. Intense ∼ 1pμA m −2) field-aligned currents were generated at the large north-south conductance gradients in the convection throat region mainly due to the divergence of Hall currents. We suggest that the so generated upward flowing current in the middle part of the auroral zone can provide the ionospheric closure of the! field-aligned current generated in the magnetosphere due to the dawn-dusk asymmetry of the cross-tail current and it also offers an explanation for the absence of the HD during SMC events.