Development of an auroral absorption substorm: Studies of substorm related absorption events in the afternoon-early evening sector

Abstract We discuss the effects in ionospheric absorption of particle precipitation observed in the afternoon-early evening sector during substorms with onset in the midnight sector. All events considered here occurred during magnetically disturbed periods, K p > 3. For many of the substorm events a smooth southward moving absorption bay is seen in the midnight and evening sectors about 1 h preceeding the onset. The magnetic pulsation activity is low during this preceding bay. After substorm onset near magnetic midnight the precipitation region may expand with a sharp onset at the front towards the West in spatially confined regions at high and low L -values separately with about equal velocities. The observations are consistent with a model of westward expansion of the energetic electron precipitation in two regions, aligned parallel to the auroral oval, at high and low L -values of about L ⩾ 6 and L ⩽ 4.8. The westward expanding absorption activity correlates well with local magnetic variations. In magnetic pulsations PiB events are seen at high latitudes simultaneously with the westward moving onsets while at low latitudes IPDP pulsations are observed during the active part of the absorption events. Later in the substorm event a slowly varying absorption event (SVA) is sometimes observed at the lower L -values, L ∼ 3–4.