Field-aligned currents of postnoon auroral arcs

On 9 January 2008, postnoon (at about 15-16 magnetic local time [MLT]) poleward moving auroral arcs were observed over Svalbard during slightly negative interplanetary magnetic field (IMF) Bz and the IMF By gradually changing from near zero to about -3 nT. The arcs appeared from the western (dayside) horizon and developed toward the east coming to the postnoon sector. The steerable 32-m antenna of the European Incoherent Scatter (EISCAT) radar was directed at an elevation angle of 45 degrees to the north-west at about 45 degrees to geomagnetic west, allowing for measurement of azimuthal (along east-west) plasma flow in a range of latitudes. On the basis of this radar configuration, a new method has been developed and applied for calculating the field-aligned currents (FACs) in the vicinity of the arcs. The arcs were located in the region of the postnoon convection reversal, and the background upward FAC had a density of about 1 mu A/m(2). The optical arcs were of the order of 30-40 km wide and coincided with peaks of upward FAC. The return downward FAC and the southward electric field were observed poleward of the arcs. Equatorward of the arcs, secondary weaker arcs were detected so that the postnoon arcs appear to have a double structure. These features are surprisingly similar to the electric field and current patterns of morning Sun-aligned arcs. We suggest that the double arc structure can be explained by the nonlinear ionospheric feedback that results in splitting of the elongated magnetospheric plasma structures associated with the arcs.