On the field-aligned currents in the vicinity of prenoon auroral arcs

[1] The EISCAT Svalbard radar (at 75° MLAT) was used to investigate the ionospheric plasma flows along auroral arcs in the 7–11 MLT sector with the radar beam directed along the L-shell. In nine cases auroral arcs appeared in the immediate vicinity of the radar beam allowing for observations of plasma flow simultaneously at different distances across the arc. In eight cases the arcs were associated with a plasma flow shear indicating sheets of upward field-aligned current (FAC) over the arcs. However, in one case the data indicate a downward net current in the region of the arc, with a flow shear and, especially, a downward FAC sheet adjoining the arc on the equatorial side. We suggest that in the downward FAC sheet, the Kelvin-Helmholtz instability can generate a series of auroral spots forming an arc, if the current sheet is co-located with a plasma boundary inside the magnetosphere.