LARGE-SCALE CONVECTION PATTERNS OBSERVED BY DMSP

We present a comprehensive compilation of the average distribution of the electrostatic potential across the high-latitude ionosphere. The averages are compiled from potential along the satellite path calculated from thermal ion drift data from instrumentation on the Defense Meteorological Satellite Program (DMSP) flights 8 and 9 satellites. Data were collected from the DMSP F8 satellite during the period September 1987 to December 1990 and from the DMSP F9 satellite during the period March 1988 to December 1990. The potential distributions are separated by geomagnetic position, season, and orientation of the interplanetary magnetic field (IMF), and then averages of the distributions are calculated. The average potential distributions clearly show the displacement of polar cap convection contours to the dusk or dawn flanks under the influence of the IMF B(y) component. The cross-cap potential decreases as IMF B(z) changes from southward to northward. The average distributions indicate that the development of more than two convection cells for northward IMF is either uncommon or nonexistent. For IMF B(z) > 0 and B(z) > \B(y)\, a distorted pattern is observed in the average potential distribution, not a four-cell pattern as some previous studies suggest it should be. For all orientations of the IMF, the convection reversal boundary at the poleward edge of the auroral zone is observed in the average distributions to be a rotational boundary. It is not a shear boundary as suggested by some previous investigations. On average, the Harang discontinuity (convection reversal in the auroral zone near midnight) is observed to exist weakly or not at all. When examining individual passes, a strong eastward flow is present sometimes in the region of the Harang discontinuity, especially on the poleward boundary, but not at all times as implied by the Heppner-Maynard model.