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° ∼ −30° GLon in the Northern Hemisphere, 150° ∼ 210° GLon in the Southern Hemisphere). The driver of the wind gradient at 60° ∼ 70° 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° ∼ 70° GLat. Whereas the disappearance is associated with the poleward contraction from 60° to ∼70° 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.