First results on characteristics of nighttime MSTIDs observed over South Africa: Influence of thermospheric wind and sporadic E

This paper reports the climatology of medium-scale traveling ionospheric disturbances (MSTIDs) observed between September 2018 and August 2019 in the nighttime redline airglow intensity measurements from an all-sky imager located in Sutherland, South Africa (32.4 degrees S, 20.8 degrees E; magnetic latitude: 40.7 degrees S). The nighttime MSTIDs appeared as either single or multiple dark bands, commenced mostly before midnight, and predominantly occurred during the southern winter solstice. They primarily propagated westward with speeds of 31-127 m/s. The periods and wavelengths of the multi-band MSTIDs were in the ranges of 26-155 min and 134-578 km, respectively, while the temporal and spatial scales of the single band MSTIDs were 11-30 min and 43-143 km, respectively. Analysis of neutral wind measurements from a co-located Fabry-Perot interferometer showed that the thermospheric wind was mostly northeastward in the evenings of MSTID occurrence and that the favored westward propagation of the MSTIDs was the least restricted propagation direction by the background wind. Based on data from nearby ionosondes, the MSTIDs were mostly accompanied by sporadic E and spread-F structures. From the seasonal occurrences of MSTIDs and sporadic E as well as the MSTID propagation and prevalent wind directions, it is concluded that Perkins instability is the most likely source of the majority of the observed nighttime MSTIDs, while the remainder may be seeded by gravity waves.