Identification of large-scale billow-like structure in the neutral sodium layer over Arecibo

We present a rare event that was recorded over Arecibo using sodium (Na) lidar. Billow-like structures with periods of similar to 60 min were seen in the Na layer above 102 km on the night of 7-8 July 2010. The absence of any high-altitude structure was noted on the following night. Spectral analysis using the Lomb-Scargle technique reveals periods with larger power on the first night as compared to the adjacent one. The keograms derived from a sequence of 557.7 nm airglow images show the passage of a large frontal wave on 7-8 July 2010. Further investigation on the occurrence of neutral instabilities was carried out using mesospheric temperature and horizontal wind obtained from the Sounding of Atmosphere using Broadband Emission Radiometry (SABER) and TIMED Doppler Interferometer (TIDI) instruments onboard the TIMED satellite. A good agreement is observed between the temperatures derived from SABER and ground-based airglow instruments. The mesospheric temperature and horizontal wind profiles allowed the determination of square of the Brunt-Vaisala frequency and Richardson number to quantitatively evaluate the possible role of different instabilities in generating this structure. The profile of the former entity reveals that the region is convectively stable during the time when the event was observed. However, the presence of strong shears in the region, where billow-like structures are observed on 7-8 July 2010, is noted along with a Richardson number of 0.22, indicating the likely occurrence of dynamical instability. Thus, the present work suggests that dynamical instability make conditions conducive for billow-like structures in the Na layer above 102 km. The possible role of plasma processes in generating these structures is also discussed.