LOCALIZED ABSORPTION EVENTS IN THE AFTERNOON SECTOR

The westward expansion of a substorm is accompanied by IPDP magnetic p ulsations which appear in the afternoon sector shortly after the onset of a substorm in the midnight sector. Observations suggest a close co rrelation between some localized riometer absorption events and IPDP p ulsations. The size and movement of absorption events has been studied using imaging riometer measurements at Kilpisjarvi (69.05 degrees N, 20.79 degrees E, L = 5.9), which give much better spatial and time res olution 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 absorpti on events are the most localized events yet observed by imaging riomet er. Sometimes southward movement was seen, at about 12-22 km/min (200- 367 m/s). Small scale structure was also observed within the events. ( C) 1997 Elsevier Science Ltd.