A STATISTICAL STUDY OF THE MOTIONS OF AURORAL ARCS IN THE HIGH-LATITUDE MORNING SECTOR

Motions of auroral arcs in the high-latitude morning sector (0300-0900 magnetic local time) have been studied statistically for magnetically quiet periods, using the data from all-sky images for two winters fro m two ground-based stations at magnetic latitudes (MLAT) of 84 degrees and 78 degrees. Most of the observed arcs are in a sun-aligned direct ion. Most of the arcs observed at 84 degrees MLAT move duskward with a typical velocity of similar to 500 m/s. A similar duskward motion of the arcs with a slightly higher velocity is dominant at 78 degrees MLA T, though some arcs move antisunward at this latitude. It is suggested that the observed duskward auroral motion is related to poleward shri nkage of the morningside oval on closed field lines during magneticall y quiet conditions.