Vertical gradients of neutral winds observed by ICON and HWM estimation during the geomagnetic storm on 26-28th August 2021

The Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) onboard the Ionospheric Connection Explorer (ICON) mission provides a good opportunity to investigate the altitude profile of thermospheric wind. In this study, we used the red line measurements of MIGHTI to compare with the estimated results by the Horizontal Wind Model (HWM-14). Data selected include both the geomagnetic quiet period (December 2019 to August 2022) and the geomagnetic storm on 26-28th of August 2021. During geomagnetic quiet periods, the estimations of neutral winds from HWM-14 show relatively good agreement with the observations from ICON. According to ICON’ s observation, near the equator, zonal winds reverse from westward to eastward at around 06:00 LT at higher altitudes and the stronger westward winds appear at later local time at lower altitudes. At around 16:00 LT, eastward winds at 300 km reverse to westward and vertical gradients of zonal winds similar to the sunrise hours can be observed. In the middle latitudes, zonal winds reverse about 2-4 hours earlier. Compared with zonal winds, meridional wind varies more significantly with season and latitude variations. According to ICON’ s observation, in the northern low latitudes, vertical reversals of meridional winds are found during 08:00-13:00 LT, from 300 km to 160 km and at around 18:00 LT, from 300 km to 200 km in June solstice. Similar reversals of meridional winds are found during 04:00-07:00 LT, from 300 km to 160 km and during 22:00-02:00 LT, from 270 km to 200 km in December solstice. In the southern low latitudes, meridional wind reversals occur during 08:00-11:00 LT, from 200 km to 160 km and during 21:00-02:00 LT, from 300 km to 200 km in June solstice. In December solstice, reversals of meridional wind appear during 20:00-01:00 LT, below 200 km and during 06:00-11:00 LT, from 300 km to 160 km. In the northern middle latitudes, the northward winds are dominant during 08:00-14:00 LT at 230 km during June solstice. Northward winds exist until 16:00 LT at 160 km and 300 km. During December solstice the northward winds are dominant from 06:00 to 21:00 LT. The vertical variation of neutral winds during the geomagnetic storm on 26-28th August have been analyzed in detail. Both meridional and zonal winds during active geomagnetic period observed by ICON show distinguishable vertical shear structures at different stages of the storm. On day-side, during the main phase, the peak velocities of westward winds extended from higher altitude to lower altitude. While during the recovery phase, the peak velocities of westward winds extended from lower altitude to higher altitude. The velocities of southward winds are stronger at lower altitudes during the storm. These vertical structures of horizontal winds during storm can not be reproduced by the HWM-14 wind estimations. And the overall response of the horizontal winds in low and middle latitudes to the storm has been underestimated by HWM-14. The ICON observations provide good dataset for improving the HWM wind estimations in the middle and upper atmosphere, especially the vertical variations.