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

James M Russell,Geetha Ramkumar,Martin G Mlynczak,Qian Wu,Shikha Raizada,Jeong-Han Kim,Ramanathan Sekar,Dibyendu Chakrabarty,Amitava Guharay,Geonhwa Jee,Sundararajan Sridharan,Sumanta Sarkhel,Robert B Kerr,John D Mathews

doi:10.1186/s40623-015-0183-5

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.

Highlights

Mesospheric neutral sodium (Na) was discovered via measurements of nighttime spectral emissions at the wavelength corresponding to the NaD2 (589.0 nm) emission resonance line (Slipher 1929)
Role of dynamical instability and advection In order to understand the occurrence of the ‘λimage’ structure in the lidargram shown in Figure 3, we assembled all available data to determine if we could identify a possible source of this structure
We suggest that this deformed KH-billow structure, generated during 18:00 to 19:00 IST over Thiruvananthapuram region, was advected in the northeastward direction having developed more folder that was revealed subsequently as it crossed the field-of-view of the Na lidar over Gadanki

Summary

Introduction

Mesospheric Na was discovered via measurements of nighttime spectral emissions at the wavelength corresponding to the NaD2 (589.0 nm) emission resonance line (Slipher 1929). The Na concentration profiles show enhancement by a factor of 2 or more over the usual background layer in a narrow altitude region of up to a few kilometer thickness. These layers are known as sporadic Na layer (NaS) Kane et al 1991; Fan et al 2007; Dou et al 2009) wherein they deal with the correlation of NaS events with ES events In this context, it is to be noted that high altitude sporadic metal layers (or high altitude metal layers) have been reported from several observational sites It is to be noted that high altitude sporadic metal layers (or high altitude metal layers) have been reported from several observational sites (e.g. Friedman et al 2013, Höffner and Friedman 2004; Chu et al 2011; Xue et al 2013). Gao and Mathews (2014a,b) and references therein report on high altitude radar and optical meteors indicative of sputtering as a source of metal ions above the traditional meteor zone

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Conclusion