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

Abstract

In order to understand the characteristics of long-lasting "C-type" structure in the Sodium (Na) lidargram, six cases from different observational locations have been analyzed. The Na lidargram, collected from low-, middle-, and high-latitude sites, show long lifetime of the C-type structures which is believed to be the manifestation of Kelvin-Helmholtz (KH) billows in the Mesosphere and Lower Thermosphere (MLT) region. In order to explore the characteristics of the long-lasting C-type structures, the altitude profile of square of Brunt-Väisälä frequency in the MLT region has been derived using the temperature profile collected from the Na lidar instruments and the SABER instrument onboard TIMED satellite. It is found to be positive in the C-type structure region for all the six cases which indicates that the regions are convectively stable. Simultaneous wind measurements, which allowed us to calculate the Richardson numbers and Reynolds numbers for three cases, suggest that the regions where the C-type structure appeared were dynamically stable and nonturbulent. This paper brings out a hypothesis wherein the low temperature can increase the magnitude of the Prandtl number and convectively stable atmospheric region can cause the magnitude of Reynolds number to decrease. As a consequence, the remnant of previously generated KH billows in nearly "frozen-in" condition can be advected through this conducive region to a different location by the background wind where they can sustain for a long time without much deformation. These long-lived KH billows in the MLT region will eventually manifest the long-lasting C-type structures in the Na lidargram.

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

“…In Figure 1, we can see the well-defined C-structures, highlighted with a red line at the top of the Na layer for all the cases which are very similar to those reported by other works e.g., [3,4,24]. Besides the data interruptions, we can identify two C-structures in case 1, on the night of 28-29 March 2008.…”

“…Unlike the two previous cases, in case 3, only one structure can be identified on 4 September 2008, around 98 km at 23:30 LT, lasting for one hour and with a 7 km vertical range. In Figure 1, we can see the well-defined C-structures, highlighted with a red line at the top of the Na layer for all the cases which are very similar to those reported by other works e.g., [3,4,24]. Besides the data interruptions, we can identify two C-structures in case 1, on the night of 28-29 March 2008.…”

“…The authors concluded that the long-lasting C-type in Na layers was a manifestation of a KH billow in MLT, which was generated in other locations and advected to the Na lidar stations by a "frozen-in" condition. Simultaneous wind and temperature measurements presented by [24] revealed convective and dynamic stabilities in the region of C-type structures. The results presented here partially agree with them, in which our findings showed positive values of N 2 in all events.…”

In Situ Observations of Neutral Shear Instability in the Statically Stable High‐Latitude Mesosphere and Lower Thermosphere During Quiet Geomagnetic Conditions

“…Mondal et al. (2019) reported lidar observations of “C‐type” structure and attributed it to “frozen‐in” KHI billows advecting with the background wind, although the time scale of two or more hours for the development of the structures reported by those authors argues for the convective roll instability rather than the KHI, for the reasons discussed in detail by Larsen (2004).…”

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

“…Meanwhile, the fine structures of Na S manifest distinct characters related to waves on short time scale (Chen & Yi, 2011; Liu & Yi, 2009; Liu et al., 2013). The bursts of the sodium atoms show a pulse period of 30 s (Liu & Yi, 2009), indicating some kind of small‐scale atmospheric perturbation effects on the evolution of Na S (Liu & Yi, 2009; Sarkhel et al., 2019).…”