Ionosonde observations of daytime spread F at middle latitudes during a geomagnetic storm

Recently, Jiang et al., (2016) reported daytime spread F at Puer station (22.7oN, 101.05oE, Dip Lat 12.9oN). As a continual work, daytime spread F at middle latitudes was investigated in this study through ionograms recorded by an ionosonde installed at Zhangye (ZHY, 39.4oN, 100.0oE, Dip Lat 29.6oN) in the Northwest of China. The ionosonde at ZHY station recorded daytime spread F three times on 6 January, 2017. The first case occurred at approximately 06:45 LT and disappeared at about 08:35 LT. The second and third cases lasted from 09:50 LT to 10:30 LT and from 11:10 LT to 11:30 LT, respectively. In addition, Swarm satellite observations were used to reveal the possible mechanism in this study. Results show that daytime spread F observed in this study might be attributed to Traveling Ionospheric Disturbances (TIDs)/atmospheric gravity waves induced by a geomagnetic storm. However, the physical processes might be different for these cases. Ionospheric instabilities produced by local F region electric field might be contributed to the first case. The second and third cases might be attributed to off-vertical reflections from wave-like ionospheric structure generated by TIDs/atmospheric gravity waves.