Interplanetary magnetic field control of fast azimuthal flows in the nightside high‐latitude ionosphere

SuperDARN radar data from both hemispheres for the interval January 1999 to November 2006 have been inspected for evidence of high‐speed azimuthal flows in the nightside high‐latitude ionosphere. In the northern hemisphere, 88% of the westward directed fast flows occurred during IMF B Y negative intervals and 83% of the eastward flows occurred during IMF B Y positive intervals. In the southern hemisphere 82% of the westward flows occurred during IMF B Y positive intervals and 74% of the eastward flows occurred during IMF B Y negative intervals. ∼74% of these fast flows occurred when the IMF was within the 30° < ∣ θ ∣ < 100° clock angle range and the AE index was below 150 nT. These results support the hypothesis that highly dynamic magnetospheric activitycan be associated with predominantly northward, but B Y ‐dominated IMF driving conditions and subsequent non‐substorm reconnection in a twisted magnetotail.