Observations of Unusual Daytime Range Spread F at Middle Latitude During the Afternoon Hours

In this study, an unusual daytime range spread F event recorded by the ionosonde installed at Zhangye (ZHY, 39.4 degrees N, 100.13 degrees E, dip latitude 29.65 degrees N) station during the afternoon of December 23, 2016 was presented. Daytime spread F during the afternoon hours have seldom been reported at midlatitude regions. In addition, observations of daytime spread F are mostly the frequency spread F. In this case, daytime range spread F and spread Es in the midlatitude region were simultaneously observed during the afternoon hours (15:15-16:00 LT). Moreover, some distorted traces of the main traces occurred on ionograms during the morning. The result of these observations seems to indicate that these extra traces and daytime diffused echoes on ionograms might be attributed to the same ionospheric disturbances phenomenon. Analysis of the iso-frequency of ionospheric virtual height from ZHY ionosonde data during this event revealed the presence of wave-like disturbances consistent with traveling ionosphere disturbances (TIDs)/atmospheric gravity waves (AGWs). The periods of TIDs on this day are in a range of 20-70 min that can be roughly classified into MSTIDs. We found that the daytime range spread F might be induced by the passage of ionospheric wave-like structures caused by TIDs/AGWs. In addition, the seeding of TIDs/AGWs might be attributed to the stratosphere gravity waves over the Qinghai-Tibetan Plateau, which causes the coupled of the atmosphere-ionosphere-thermosphere to significantly impact the ionosphere.