Dayside neutral wind vertical shear at low F region altitude observed by the ICON satellite

The Michelson Interferometer for Global High-Resolution Thermospheric Imaging (MIGHTI) onboard the Ionospheric Connection Explorer (ICON) mission measures the neutral winds from 88 km to 310 km altitudes, which provides a good opportunity to investigate the vertical distribution of neutral winds, especially their vertical shears. Based on over two years data from MIGHTI/ICON, we focused on the wind vertical shears at the low F region (from 88 km to 245 km) in this study. As the green line of MIGHTI works only on the dayside, only the dayside from 0600 to 1800 local time (LT) has been considered. As a result, there were 206 and 96 orbits were identified with clear vertical shears for the meridian and zonal wind components, respectively. Interestingly, such wind vertical shears occurred not only during magnetically disturbed periods, as there were 75.73% and 78.12% orbits with vertical shears identified in the meridional and zonal wind with Kp less than 2. Dependences of the wind vertical shears on local time (LT), geographic latitude (Glat) and longitude (Glon) have been further checked, and we found that the wind vertical shears have different trends for the LT dependence, e.g., the mean altitude of meridional (zonal) wind reversed from southward to northward (westward to eastward)  is higher at dawn, and then slowly decreases toward dusk; or the mean altitude of wind reversal is higher at dusk and slowly increases towards dusk. Such two kinds of altitude trends of the wind vertical shears were also found for the dependence on Glat, but not on Glon. The mean altitudes of wind reversal are around 160 km for most of the Glon sectors, with only a slight decrease or increase at the sector where the magnetic equator is far away from the geographic equator, indicating that the ion drag should also play a role in causing the wind vertical shears. The relation between the wind vertical shears at E (England et al., 2022) and low F region altitudes have been further investigated. From the orbits with wind vertical shears at low F region identified, there were about 90% orbits with also wind vertical shears simultaneously observed at the E region, but the altitude trends can be the same or opposite for the vertical shears at the two regions, even for the same orbit. Such a relation suggests that the causes for wind vertical shears at the E and low F region altitudes could be different.