Abstract
On 13 December 2004, morningside Sun‐aligned auroral arcs were observed at about 8 MLT over Svalbard (around 75 MLAT) during about 1.5 hours after the IMF Bz component had turned to strongly northward (+10 nT). The arcs appeared periodically (6.7 min) and moved poleward at a velocity of about 700 m/s. The arcs occurred during substorm recovery and appeared to develop from the area of enhanced luminosity seen in the western (nightside) horizon toward east. The electric fields and currents associated with the arcs have been calculated from the EISCAT Svalbard radar observations. The arcs were associated with periodic spatial structures of 270 km in latitudinal width. Each of the 270‐km wide structure consists of four specific FAC regions: the upward FAC region 75‐km wide containing the optical arc, the return downward FAC region 100‐km wide poleward of the arc; and a secondary weaker arc equatorward of the main arc with a pair of FACs similar to the main arc, but narrower in width. The interchange instability with the ionospheric feedback is suggested as a suitable generation mechanism for such kind arcs.
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