Strong high‐latitude zonal wind gradient observed by CHAMP and simulated by TIEGCM

Using 6 days cross-track wind data from CHAMP, we investigate the high-latitude westward wind gradient and reversal that occurs at fixed longitudes (-120 degrees similar to -30 degrees GLon in the Northern Hemisphere, 150 degrees similar to 210 degrees GLon in the Southern Hemisphere). The driver of the wind gradient at 60 degrees similar to 70 degrees GLat and in the noon sector is investigated via TIEGCM. The wind gradient is driven by the combined effects of the Earth's geomagnetic field configuration and the energy deposition from the solar wind. The temporal variations of the neutral wind cell in afternoon sector are critical to the formation of wind gradient. The development of wind gradient is due to the equatorward expansion of the afternoon westward wind cell from the geographic high-latitudes to 60 degrees similar to 70 degrees GLat. Whereas the disappearance is associated with the poleward contraction from 60 degrees to similar to 70 degrees GLat to higher latitudes. A further diagnostic analysis of model results shows that the primary internal driver of the wind gradient is a balance between ion drag and pressure gradient. In the acceleration phase, ion drag causes the formation of the strong gradient of westward wind, whereas pressure gradient decelerates it. A similar result is found in the deceleration phase. The ion drag is controlled predominantly by both the electron density and the relative motion between the ions and neutrals. Plain Language Summary At high latitudes, the neutrals often are dragged with ions due to the strong ion drag effects, introducing a similar two-cell pattern from the convection field into thermospheric winds. The tow-cell pattern of thermospheric winds is defined as "wind cell" in our work. During the Earth's rotation, the two-cell pattern of thermospheric winds will vary in latitudes and local times, causing the equatorward expansion or poleward contraction in geographic coordinates. This periodic expansion and contraction each day seem like a breath of Earth's thermosphere-ionosphere coupled system, as an interesting phenomenon. At 15-21 UT, the strong westward wind cell during the moderate geomagnetic period can arrive at fixed latitudes of 60 degrees similar to 70 degrees GLat and local times of 10-14 LT, promoting the strong wind gradient at fixed longitudes of -120 degrees similar to -30 degrees GLon.