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

[1] 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 B y and B z ). 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 B y exerts a strong influence on the wind patterns, particularly in the midnight sector. During winter, B y positive winds around midnight in the northern (southern) hemisphere are directed more toward the dusk (dawn) sector, compared to corresponding B y negative winds; this behavior is consistent with the B y -dependence of statistical ionospheric convection patterns. The strength of the wind response to B y tends to increase with increasing solar EUV irradiation, roughly in proportion to the increased wind speeds. Quiet time B y effects are detectable at latitudes as low as that of Millstone Hill (magnetic latitude 53°N). Quiet time B z effects are negligible except over the magnetic polar cap station of Thule.

An empirical model of the Earth's horizontal wind fields: HWM07

“…Although HWM07 shows dramatic improvement over HWM93, there are several factors that motivate categorization of HWM07 as provisional. The most important is the potentially poor representation of thermospheric high‐latitude circulation: neither HWM93 nor HWM07 appear to sufficiently describe the ion‐convection‐driven patterns that are observed in high‐latitude wind measurements [e.g., Hernandez et al , 1991; Thayer and Killeen , 1993; Emmert et al , 2006b]. Ion drag is a dominant momentum source for the high‐latitude thermosphere and ion convection patterns are organized by the geomagnetic field.…”

The spatial distribution of region 2 field-aligned currents relative to subauroral polarization stream

“…It is shown that both SAPS and zonal wind peak at almost the same position, representing strong effects of SAPS on the zonal wind. Regarding the timescale of ion-neutral coupling, Emmert et al (2006) found that the response of high-latitude wind to plasma convection is broadly maximized at a time lag of 1 h at around 250 km altitude during quiet time. They suggest that the time delay can be further reduced at higher altitude and on the nightside.…”

DWM07 global empirical model of upper thermospheric storm‐induced disturbance winds

“…A significant dependence on these parameters is likely, given that drag from enhanced ion convection is a major source of the disturbance winds. Systematic IMF orientation effects have been detected in quiet time winds [e.g., Sica et al , 1989; Emmert et al , 2006b] and in total winds under disturbed conditions [e.g., Hernandez et al , 1991].…”