Drift motion of ionospheric arc-like absorption regions observed with a 256-beam imaging riometer in Alaska

We studied the local time dependence of the direction of cosmic noise absorption drift using a large data set obtained with a 16 × 16-antenna imaging riometer in Poker Flat, Alaska (65.1°N, 147.5° W in geographic coordinates; 65.4°N, 100.7° in geomagnetic coordinates). The drift velocities were determined using regression lines for the absorption peaks, which were automatically detected. Based on the characteristics of the motion of the drift region, the events were categorized into three magnetic local time (MLT) groups: I (13–19 MLT), II (21-01 MLT) and III (01–07 MLT). Most events in group I move southwestward with steady velocities. Events in group II had various velocities and directions. Events in group III also had various velocities, but the tilt angles of their vector velocities from the eastward axis were between −90° and 90°, meaning that they moved eastward. These results are consistent with some found in previous studies. A polar plot of the drift had a pattern with features similar to that of magnetospheric convection, which has been reported on several studies: equatorward in the pre-midnight sector and sunward in the morning and evening sectors.