While the ionospheric plasma flow is characterized by the dawn-dusk asymmetry in geomagnetic direction and magnitude, as mentioned above, it is still unclear how much the asymmetric ionospheric pattern is projected on the thermospheric wind pattern. Ion drag at high latitudes leaves traces of the ion flow in the thermospheric wind field, but the level is variable because force balance with other terms depends on local time and latitude. This tendency has also been substantiated by the localization of the SAPS in the dusk sector (Wang et al. As illustrated in previous works on the potential pattern (e.g., Ruohoniemi and Greenwald 2005 Thomas and Shepherd 2018), the horizontal potential gradient tends to be steeper at dusk than at dawn that is, the perpendicular electric field or ion velocity at dusk tend to be larger than that at dawn. This morphological difference may be attributed to steep gradients in the ionospheric conductivity near the solar terminator line (Atkinson and Hutchison 1978) or in the auroral zone (Vasyliunas 1970). This convection pattern tends to have more crescent-shaped cell at dawn and more round-shaped cell at dusk (Ruohoniemi and Greenwald 2005 Thomas and Shepherd 2018). Statistical analyses of the ionospheric ion velocity have revealed the dawn-dusk asymmetry of the ionospheric convection pattern. In keeping the wind streamline, the thermospheric stream crosses over the auroral latitudes in the midnight-dawn sector and transfers molecular-rich air from auroral latitudes to lower latitudes (Prölss 1993). In the polar cap region, both ionospheric and thermospheric flows are approximately anti-sunward. This is because equatorward flows are dominant in the thermosphere from midnight to dawn under a geostrophic force balance (Förster et al. 1987 Killeen and Roble 1988 Förster et al. In the dawn sector, the ionospheric plasma flow at auroral latitudes is mainly eastward, but the eastward thermospheric wind is hardly seen in measurements, even during periods of high geomagnetic activity (Thayer et al. Westward ion flows in the dusk cell of an ionospheric plasma convection at auroral latitudes and the subauroral polarization streams (SAPS) induce westward acceleration of the thermospheric wind by ion drag (Conde et al. Changes in the wind pattern from the geomagnetically quiet conditions become clearer with enhancements in geomagnetic activity. The wind patterns at F-region altitudes during periods of geomagnetically quiet conditions can be represented by the pressure gradient associated with solar radiation and geostrophic force balance induced by the Coriolis and centrifugal forces. While ions are minor species in the upper atmosphere or partially ionized plasma, the presence of ionized gas substantially affects the thermospheric wind. Thermospheric wind dynamics at high latitudes are significantly affected by ionospheric plasma motions through particle collisions between ions and neutrals, which is known as ion drag. This is the first study to report a dawn-dusk asymmetry of the thermospheric wind acceleration feature and signatures of the eastward wind acceleration at dawn by ion drag. Deviations from the quiet-time wind presented similar patterns in the direction with the ionospheric plasma convection but were larger in magnitude at dusk than at dawn. ![]() With an increase in the geomagnetic activity level, the thermospheric wind turned over from eastward to westward at dusk and increased the equatorward magnitude from midnight to dawn. The quiet-time wind pattern can be expected to be represented by a pressure gradient associated with the solar radiation and a geostrophic force balance. The measurements were sorted by a SuperMAG (SME) index, and a quiet-time wind pattern was defined as an hourly mean under SME ≤ 40 nT. We performed a statistical analysis of the thermospheric wind measurements with a Fabry–Perot interferometer (FPI 630 nm wavelength) in Tromsø, Norway, in the winter months for 9 years. Ion drag associated with the ionospheric plasma convection plays an important role in the high-latitude thermospheric dynamics, yet changes in the thermospheric wind with geomagnetic activity are not fully understood.
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