Polar convection and Birkeland Currents during strongly positive IMF By

Abstract

The TRIAD satellite passed directly over the field of view of the Scandinavian Twin Auroral Radar Experiment (STARE) radar on May 1, 1978, at approximately 0618 UT when the By component of the interplanetary magnetic field (IMF) was unusually large in comparison to the other two components. The average IMF values in the interval from 0600 to 0700 UT on this day were Bx = −1.8 nT, By = 8.7 nT, and Bz = −0.2 nT (King, 1979). The TRIAD magnetic field perturbations revealed the presence of a very intense (ΔB ∼ 500 nT) region 1 downward‐flowing Birkeland current near 0930 MLT and poleward of a much weaker upward‐flowing region 2 current. This “net” (unbalanced) Birkeland current was observed at a 67° magnetic latitude in this morning sector (near 0930 MLT). The net Birkeland current was located equatorward of a region of intense westward (antisunward) ionospheric convection flow detected by the STARE radar. We conclude that the polar cap convection flow is distorted and displaced to low latitudes (< 67° magnetic latitude) in the morning sector during this period of strongly positive By and almost negligible Bx and Bz. The large net region 1 Birkeland current is associated with the convection flow reversal. Convection velocity data acquired by the AE‐C satellite on October 29, 1978, near 1723 UT when Bx = −0.5 nT, By = 8.3 nT, and Bz = −2.7 nT (average values during the 1700 to 1800 UT interval) show the convection reversal near 67° invariant latitude and 0800 MLT (King, 1979). These two examples support previous suggestions for the important influence that the By component of IMF has on the intensity and location of high‐latitude convection and Birkeland currents.