Climatology of the nighttime thermospheric winds over Sutherland, South Africa

We present the first climatological study of nighttime thermospheric neutral winds between February 2018 and January 2019 in Sutherland, South Africa (32.2S, 20.48E; geomagnetic latitude: 40.7S). This climatology is based on measurements of the nighttime oxygen airglow emission at 630.0 nm, taken by the Fabry-Perot interferometer (FPI). The study found that the annual meridional and zonal winds at this location vary between -100 and 120 m/s and show typical midlatitude seasonal variations. During the local summer months (December-February), the meridional wind is predominantly equatorward from dusk to predawn. However, during the winter months, the meridional wind is poleward from dusk, turns equatorward around midnight, and either remains in this direction for the rest of the night (e.g., June) or turns poleward again just before dawn (e.g., July and August). The zonal wind velocity is generally eastward during the evening until just before midnight, changing westward post-midnight. The zonal wind peaks at higher velocities during the winter months compared to the summer months. The eastward-to-westward transition occurs later during the winter months compared to the summer months. We compare results from HWM14 with the FPI measurements and find a better agreement between FPI measured winds and HWM14 modeled winds for the meridional component compared to the zonal component. In addition, the HWM14 zonal wind consistently peaked 3 hours prior to the measured wind. Furthermore, the HWM14-modeled eastward-to-westward transitions times were 2 hours earlier during the months of November to January. These time discrepancies appeared to be a phase shift which was apparent in all months, and we suggest this can be attributed to a phase shift of the terdiurnal tide between the model and measurements.