Eastward Propagating Auroral Vortices Observed in the Post-Midnight Sector

Abstract:

We report a case study of eastward-propagating auroral vortices detected by the ground-based network of all-sky imagers and magnetometers in the northern Scandinavia. The three eastward-propagating mesoscale aruroal vortices were observed intermittently at about 15-minute intervals in the post-midnight sector (0:00-0:40 UT; 2:30-3:10 magnetic local time). Their properties are analogous to those of the omega bands or torches, which propagate typically eastward with a similar recurrence period in the post-midnight sector during the recovery phase of the substorm. In addition, the vortices were accompanied by characteristic magnetic variations corresponding to eastward-drifting pairs of downward and upward field-aligned currents (FACs).

On the other hand, we found some distinct differences from the omega bands and torches, described as follows: (1) The auroral vortices occurred during the expansion phase of the substorm, almost simultaneously with Pi 2 pulsations at the magnetic equator. (2) The drift velocity of the vortices was approximately 3 - 10 km/s at 100 km altitude, which is much faster than the typical velocity of the omega bands (V = 0.4 - 2 km/s). (3) The vortices were accompanied by the magnetic variations with periods of 3 - 5 min, which were shorter than periods of Ps 6 pulsations (T = 5 - 40 min) that typically coincide with the omega bands. (4) The ionospheric equivalent current systems derived from the magnetometer data indicated the upward FAC in the dark region inside the vortices, whereas the downward FAC is typically detected in the dark region between the torches. We interpret the observed aurora vortices as the transient phenomena associated with the generation process of the omega bands.