Localized absorption events in the afternoon sector

Abstract The westward expansion of a substorm is accompanied by IPDP magnetic pulsations which appear in the afternoon sector shortly after the onset of a substorm in the midnight sector. Observations suggest a close correlation between some localized riometer absorption events and IPDP pulsations. The size and movement of absorption events has been studied using imaging riometer measurements at Kilpisjarvi (69.05 °N, 20.79 °E, L = 5.9), which give much better spatial and time resolution than earlier riometer measurements using broad antenna beams. The absorption occurs in a rather small spot-like area varying in size from 30 km by 30 km to 180 km by 130 km. In all cases the absorption area was smaller than the field of view of the imaging riometer, which covers a 240 km square at height 90 km. These afternoon side absorption events are the most localized events yet observed by imaging riometer. Sometimes southward movement was seen, at about 12–22 km/min (200–367 m/s). Small scale structure was also observed within the events.