Thermospheric wind response to driving forces in the vicinity of the Harang discontinuity

Fabry-Perot interferometer Doppler measurements of the 6300 Å oxygen emission line have been used to determine the neutral thermospheric wind over Halley, Antarctica. Simultaneous measurements of the local magnetic field have been used to generate a time frame with respect to the passage of the Harang discontinuity across the observing station. This identifies the night-time divergence in the dawn and dusk ion convection cells. Neutral wind measurements in this “Harang” time frame have shown a significantly higher degree of ordering than when considered in Universal Time. In particular, a reduction in the meridional component of the neutral wind is observed near the time of the discontinuity. This may be explained by a local reduction in Joule heating or through coupling to divergent convecting ions at the Harang discontinuity. The zonal component wind data show a marked transition from eastward to westward near 23:00 U.T., but there are no effects attributable to the Harang discontinuity. This transition arises because Halley moves from a regime where solar e.u.v. driven winds dominate to one where ion-drag is the most important driving force. This is supported by the dependence of the transition time on the level of local geomagnetic activity.