The changing polar cap - Interball-2 and ground observations

A joint ground-spacecraft study of the polar cap from January to March, 1997 shows that the dawn and dusk boundaries identified on the meridian scanning photometer scans from Eureka, in the central polar cap, agree well with the boundaries seen by the Interball-2 UVAI images for northward interplanetary magnetic field (IMF) B-z conditions. These boundaries are displaced duskward when B-y is positive and dawnward when B-y is negative. There are observable discrete emissions (above airglow) from the polar cap during much of the time. From four periods of extended observations, two contrasting types of polar caps emerge. For northward IMF B-z the polar cap is constricted in the dawn-dusk direction. Sun-aligned arcs emerge from the auroral oval (from the dawn flank with B-y positive and the dusk flank with B-y negative) and drift across the polar cap. In some cases, but not all, they appear following transients in the IMF B-y or B-z. For southward IMF B-z the central polar cap frequently has F-region plasma patches extending across much of the polar cap and drifting antisunward. Two auroral substorms occur, the first one (10 February 1997) showing a poleward auroral expansion along much of the oval and followed by a series of elongated, periodic F-layer patches. The second substonn (02 March 1997) has a poleward expansion confined to the midnight sector and the F-layer patches which subsequently appear are quite different (fainter and less extended), perhaps reflecting differing stability of ion flow patterns across the polar cap. While the extended views of the polar cap from the ground reveal much of the dynamics of polar arcs and patches in response to solar wind IMF changes, there are still unanswered questions as to the triggering mechanisms for both. And the onset and progress of auroral substorms in relation to F-layer patch formation and motion requires further investigation. One polar arc on January 22 recorded by the UVAI and the Eureka photometer permitted a determination of the UVAI sensitivity, indicating 1 CCD count corresponded to 1.5R of 630 nm emission. This calibration value is specific to this type of aurora (low energy with a 1630/1558 ratio of > 1). The UVAI images showed that the arc was a theta aurora, indicating that such auroras can be excited by either energetic electrons or soft electrons. (C) 2002 Elsevier Science Ltd. All rights reserved.