Upper Thermospheric Winds in the Dayside and Nightside Auroral Zone

Thermospheric wind circulation is important for the composition and dynamics of the neutral and ionized components of the upper atmosphere, and the circulation is often described as having a two-cell pattern as driven by large-scale auroral forcing. However, auroral forcing is variable and structured, and whether and how winds respond to such complex forcing is controversial. The controversy partly stems from the limited knowledge about the efficiency in ion drag, and the relative importance of this drag to other forces. In this presentation, we address the upper thermospheric winds in the dayside and nightside auroral zone and how they compare with the ionosphere convection. Winds are primarily measured by Scanning Doppler Imagers located at South Pole (for dayside), and Poker Flat (for nightside). Our results show that, on the Southern Hemisphere dayside, while the thermospheric winds resemble plasma convection under -By IMF conditions, they show a significant deflection under +By conditions, exhibiting a preference to flowing westward regardless of IMF. On the other hand, on the nightside, winds follow plasma convection closely even when the convection changes rapidly over time, such as during substorms. The winds are either strengthened or weakened depending the latitude and local time relative to the substorm onset. Our results illuminate the various wind response to plasma convection, where the ion drag appears to be more efficient on the nightside than dayside.