The influence of IMF By on the nature of the nightside high-latitude ionospheric flow during intervals of positive IMF Bz

This paper further addresses the issue of nightside flow bursts which occur during intervals of northward but strongly BY -influenced IMF. Recent discussions of such bursts concerned intervals during which the IMF BY component was negative. The present study concerns an interval of BY -positive IMF which occurred on 20 March 2002 (01:00– 12:00 UT). During the interval BY increased steadily from ∼2 to 12 nT, whilst the BZ component decreased steadily from ∼10 to 0 nT. There was thus a ∼6-h sub-interval during which the IMF clock angle remained between 30 and 60, such that moderate dayside reconnection and open flux production was maintained. It is found that flow bursts of a similar size and speed to those observed under BY negative (∼1000 m s−1, spanning 2–3 h of MLT in the midnight sector) also occur when BY is positive. However, the direction of east-west flow is reversed, indicating that they are driven by processes in the magnetosphere which are directly related to the orientation of the IMF. It is suggested that they are caused by a reconfiguration of an asymmetric tail resulting from prolonged dayside reconnection with a BY -dominated IMF. This is consistent with previous suggestions that they are associated with convective transport following reconnection in the more distant tail. Analysis of ground magnetic data, auroral images and geosynchronous particle data also show associated features, but indicate that the flow bursts are not directly associated with substorms.