[1] The altitude of the sodium layer in the mesosphere and lower thermosphere is usually from 80 km to 105 km. In this paper, we report a set of double sodium layer (DSL) events observed by sodium lidar over Beijing, China. In these DSL events, the normal sodium layer and secondary sodium layer (SeSL) present separately. There were about 17 DSL events occurred in 319 observation nights during 2009$2011. All DSL events were observed in spring and summer. The SeSL appeared independently within the altitude range from 105 km to 130 km. The density of the SeSL is very high. The maximum ratio of peak density and the ratio of column density for the SeSL to the normal sodium layer are up to $60% and $47%, respectively. The SeSL lasted several hours, and then merged into the normal sodium layer. After the SeSL, a sporadic sodium layer occurred in the normal sodium layer.

The bottom‐type irregularity scattering layer (BSL) that can appear in the ionospheric F region bottomside has been observed generally after sunset, serving as a possible telltale of equatorial spread F (ESF). Using simultaneous multibeam radar measurements over two low‐latitude stations, Sanya (18.3°N, 109.6°E; dip latitude 13°N) and Fuke (19.3°N, 109.1°E; dip latitude 14°N) in China, we report, for the first time, a thin BSL that initially occurred at presunset (~1720 LT), much earlier than the occurrence of BSL generated from the equatorial plasma shear vortex‐driven instability. The presunset BSL was situated around 225 km altitude and continued to exist until the appearance of ESF plumes after sunset (~1930 LT). Interestingly, the Doppler velocities of the presunset BSL echoes measured by the radar and the F layer virtual heights obtained from the collocated Digisonde measurements over Sanya both show oscillations with a period of about 1 h, suggesting a close link between the occurrences of the BSL and of F region plasma density large‐scale wave structure before sunset. These observations could imply an important role of gravity waves in the generation of the presunset F region bottom‐type irregularities.

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