Peculiar Wave Structure of the Mesospheric Sporadic Sodium Layer Observed by Lidars in Hefei (31.8 degrees N, 117.3 degrees E) and Wuhan (30.5 degrees N, 114 degrees E), Central China

Lidar observations are an effective tracker for identifying atmospheric wave signals. In this research, observations through three lidars are utilized to study the wave structures. A broadband sodium fluorescence lidar and a narrowband Temperature/Wind (T/W) lidar located at Hefei (31.8 degrees N, 117.3 degrees E), while the third lidar system, a broadband lidar system, is located in Wuhan (30.5 degrees N, 114.4 degrees E), about 310 km from Hefei. The three lidars can observe sodium density, temperature and wind profiles simultaneously. Joint sodium density measurements by the fourth lidar beams from Wuhan and Hefei yield a peculiar NaS with distinguished C-structure and a large horizontal scale of wave structure over more than 310 km, on 21 December 2014. The Na-S event occurring at 14:20 to 17:50 UT corresponds well to the region where the Richardson Number (Ri) is larger than 10, which in turn indicates that the NaS was formed in an extremely stable atmosphere. Results from the Lomb-Scargle periodogram analysis suggest that the occurrence and evolution of this NaS event may have been caused by a gravity wave with a period of about 1.25 hr, and its end is closely related to the gravity wave fragmentation caused by convective instability. Plain Language Summary The sporadic sodium layer (SSLs or Na-S) is the most fascinating phenomenon observed from the mesospheric layer. On 21 December 2014, a peculiar Na-S, exhibiting distinguished C-structure and wave characteristics, was observed by four lidar beams, which came from a sodium fluorescence lidar and a temperature and wind (T/W) lidar at Hefei and a sodium fluorescence lidar at Wuhan. The Na-S event corresponds well to the region where the Richardson Number (Ri) is larger than 10, which indicates that this event was formed in an extremely stable atmosphere. Results from the Lomb-Scargle spectral analysis and instability analysis suggest that the occurrence of this Na-S is closely related to the propagation and breaking of gravity waves.