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

Abstract

[1] We present a rare event that was recorded over Arecibo using sodium (Na) lidar. Billow-like structures with periods of $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-Väisälä 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.

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

“…Sarkhel et al (2012a) observed similar billow-like structures in the Na layer over Arecibo, Puerto Rico. They concluded that strong wind shear made the region dynamically unstable and was likely responsible for the observed structure in the Na layer that were observed in the lidargram for about 3 h. This structure -while still evolving -would likely be transported some distance by the overall background wind.…”

“…That is, any structure related to Na + ions will map into the neutral Na layer on a few hour time scale. Sarkhel et al (2012a) observed high altitude (>102 km) billows in the neutral Na layer wherein they discussed that such structures in the neutral Na layer can be associated with the underlying ion layer during strong E S events for which ion concentration often exceeds 10 4 ions.cm −3 . In the intermediate altitude region (90 to 100 km), we encounter the most complex interaction with both neutral Na and Na ions playing comparable roles.…”

“…The lidargram of course includes the effects of neutral atmosphere dynamics on the Na layer as these atoms have a long lifetime relative to the evolution of atmospheric dynamics and they are collisionally wellembedded in the atmosphere. Based on typical electron concentration (approximately 3,000 electrons cm −3 ), Sarkhel et al (2012a) estimated the rate at which the Na atoms ionize would be approximately 3 × 10 −6 s −1 , which corresponds to a lifetime of neutral Na atoms of more than a day. This is supported by Xu and Smith (2003) wherein they have shown that the chemical lifetime of mesospheric Na atoms is more than a day in the MLT region, which is much larger than the transport time considered in our hypothesis.…”

“…They concluded that these complex structures could be due to the KH instability occurring in a region of strong wind shear. Of late, Sarkhel et al (2012a) observed a frozen-in billow-like structure in the Na layer over Arecibo and found that it was likely created via dynamical instability processes. As this topic remains open and with the prior observations of these unusual structures over Gadanki and Arecibo, we are motivated to carefully examine additional Na lidargrams, in this case from Gadanki on the night of 18 to 19 March 2007.…”

“…Lidar measurements of Na atom concentration over low latitude stations have been carried out for several decades (e.g. Clemesha et al 1979;Taylor et al 1995;Collins et al 2002;Clemesha 2004;Sarkhel et al 2009Sarkhel et al , 2010Sarkhel et al , 2012a. On occasion, 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.…”