A case study on occurrence of an unusual structure in the sodium layer over Gadanki, India

Abstract

The height-time-concentration map of neutral sodium (Na) atoms measured by a Na lidar during the night of 18 to 19 March 2007 over Gadanki, India (13.5°N, 79.2°E) reveals an unusual structure in the Na layer for around 30 min in the altitude range of 92 to 98 km which is similar to the usual 'C' type structures observed at other locations. In order to understand the physical mechanism behind the generation of this unusual event, an investigation is carried out combining the data from multiple instruments that include the meteor wind radar over Thiruvananthapuram, India (8.5°N, 77°E) and the SABER instrument onboard the TIMED satellite. The temperature and wind profiles from the data set provided by these instruments allow us to infer the Richardson number which is found to be noticeably less than the canonical threshold of 0.25 above 92 km over Thiruvananthapuram suggesting the plausible generation of Kelvin-Helmholtz (KH) billows over southwestern part of the Indian subcontinent. Based on the average wind speed and direction over Thiruvananthapuram, it is proposed that the KH-billow structure was modified due to the background wind and was advected with it in nearly 'frozen-in' condition (without significant decay) in the northeastward direction reaching the Na lidar location (Gadanki). This case study, therefore, presents a scenario wherein the initially deformed KH-billow structure survived for a few hours (instead of a few minutes or tens of minutes as reported in earlier works) in an apparently 'frozen-in' condition under favorable background conditions. In this communication, we suggest a hypothesis where this deformed KH-billow structure plays crucial role in creating the abovementioned unusual structure observed in the Na layer over Gadanki.

On the Long Lasting “C‐Type” Structures in the Sodium Lidargram: The Lifetime of Kelvin‐Helmholtz Billows in the Mesosphere and Lower Thermosphere Region

“…The C‐type structures observed in the lidargram on several occasions could be caused by wave breaking or KH billows and may be generated in the Na layer due to strong wind shear (Kane et al, ). As described in Sarkhel et al (), a KH billow structure can imprint a C‐type structure in the lidargram. Therefore, in order to judge the persistence of the C‐type in the lidargram, the evolution of KH billows needs to be investigated.…”

“…Therefore, Sridharan et al () supported the hypothesis, proposed by Kane et al (), of KH instability for the formation of complex sporadic structures. Sarkhel et al (, ) have examined the Na lidargram from Gadanki, India, on a night using a meteor radar from Thiruvananthapuram, India, and satellite borne measurements. They suggested that the KH billow formation took place over Thiruvananthapuram, southern part of India.…”

A case study of a ducted gravity wave event over northern Germany using simultaneous airglow imaging and wind-field observations

“…However, the ducting inhibits the vertical propagation of GWs and confines the major flow of wave energy and momentum to a rather limited altitude region (Chimonas and Hines, 1986). The earlier imaging studies using airglow emissions originating in the MLT region revealed different types of dynamical events like quasi-monochromatic GWs, ripples, and mesospheric fronts (Taylor et al, 1995;Walterscheid et al, 1999;Hecht et al, 2001;Smith et al, 2003;Makhlouf et al, 1995;Bageston et al, 2011;Lakshmi Narayanan et al, 2012;Sarkhel et al, 2012Sarkhel et al, , 2015aSarkhel et al, , b, 2019Hozumi et al, 2019;Mondal et al, 2021;Guharay et al, 2022).…”

C-Structures in Mesospheric Na and K Layers and Their Relations with Dynamical and Convective Instabilities

“…Several authors have suggested dynamical instability to be related to these complex structures e.g., [5][6][7]. Dynamical instability refers to a process in which the atmosphere becomes unstable and its movement cannot be considered a laminar fluid.…”