Interhemispheric observations of the ionospheric signature of tail reconnection during IMF-northward non-substorm intervals

This paper presents the first interhemispheric radar observations interpreted as the ionospheric response to tail reconnection during IMF-northward non-substorm intervals. SuperDARN measurements of plasma convection in the nightside ionospheres of both hemispheres, taken on 21–22 February and 26–27 April 2000, show bursts of flow in the midnight sector which are understood to be characteristic of such phenomena. Upstream interplanetary magnetic field data confirm that the field orientation at the dayside magnetopause was northwards, but with a significant IMF Bγ component (negative during the first interval, positive during the second), for many hours prior to the bursts being observed. During the Bγ-negative interval the bursts were directed westwards in the Northern Hemisphere and eastwards in the Southern Hemisphere; during the Bγ-positive interval their directions were reversed. These two asymmetries between the different orientations of IMF Bγ and between the two hemispheres are key to our understanding of the magnetospheric phenomenon responsible for generating the bursts. They provide further evidence in support of the idea that the bursts are a result of reconnection in an asymmetric tail under the prolonged influence of IMF Bγ. Concurrent data from ground magnetometers and geosynchronous satellites confirm that the bursts have no associated substorm characteristics, consistent with previous studies.