Climatologies of nighttime upper thermospheric winds measured by ground‐based Fabry‐Perot interferometers during geomagnetically quiet conditions: 2. High‐latitude circulation and interplanetary magnetic field dependence

Abstract

We analyze upper thermospheric (∼250 km) nighttime horizontal neutral wind patterns, during geomagnetically quiet (Kp < 3) conditions, over the following locations: South Pole (90°S), Halley (76°S, 27°W), Millstone Hill (43°N, 72°W), Søndre Strømfjord (67°N, 51°W), and Thule (77°N, 68°W). We examine the wind patterns as a function of magnetic local time and latitude, solar cycle, day of year, and the dawn‐dusk and north‐south components of the interplanetary magnetic field (IMF By and Bz). In magnetic coordinates, the quiet time high‐latitude wind patterns are dominated by antisunward flow over the polar cap, with wind speeds that generally increase with increasing solar extreme ultraviolet (EUV) irradiation. The winds are generally stronger during equinox than during winter, particularly over the South Pole in the direction of eastern longitudes. IMF By exerts a strong influence on the wind patterns, particularly in the midnight sector. During winter, By positive winds around midnight in the northern (southern) hemisphere are directed more toward the dusk (dawn) sector, compared to corresponding By negative winds; this behavior is consistent with the By‐dependence of statistical ionospheric convection patterns. The strength of the wind response to By tends to increase with increasing solar EUV irradiation, roughly in proportion to the increased wind speeds. Quiet time By effects are detectable at latitudes as low as that of Millstone Hill (magnetic latitude 53°N). Quiet time Bz effects are negligible except over the magnetic polar cap station of Thule.