Fig. 1

a A 630.0 nm airglow image captured in Tainan at 12:01 UT on March 13, 2015 at 12:01 UT, mapped onto the geographical coordinates under the assumption of an emission altitude of 250 km. b The locations of the transmitting (Tx)/receiving (Rx) stations and the intermediate reflection points of the HFD sounding system in Taiwan. The black triangles represent the Tx and Rx stations, the red circles indicate the intermediate reflection points between the Rx1 and the three Tx stations, and the blue circles represent those between the Rx0 and the three Tx stations