Occurrence of Postmidnight Bottom‐Type Scattering Layer Over Low Latitude

Abstract

The bottom‐type scattering layer (BSL) that occurs at postsunset or presunset has been reported in previous studies. In this work, two cases of postmidnight BSL occurring on geomagnetic quiet days (12 and 29 April 2015) were investigated by using the observational data obtained by beam 4 and 5 of Sanya VHF radar. These BSLs initiate at 00:06–00:18 LT within an altitude range of 220–240 km. Their thickness are relatively uniform, about 10 km. The altitude and Doppler velocities of the BSLs oscillate with time and the oscillation period of Doppler velocities is about 1 hr. During the presence of postmidnight BSL, spread echoes and satellite traces (STs) appeared in the ionograms recorded by Sanya ionosonde. The oscillation of Doppler velocities and the presence of STs suggest that the postmidnight BSL may be related to the F region bottomside large‐scale wave structure. Amplitude scintillations (S4) larger than 0.2 were also recorded by Sanya GPS scintillation receiver. These observations could imply an important role of gravity waves in the generation of the postmidnight BSL.