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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  23 Sep 2020

23 Sep 2020

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This preprint is currently under review for the journal ACP.

Formation of a bottomside secondary sodium layer associated with the passage of multiple mesospheric frontal systems

Viswanathan Lakshmi Narayanan1, Satanori Nozawa2, Shin-Ichiro Oyama2,3,4, Ingrid Mann1, Kazuo Shiokawa2, Yuichi Otsuka2, Norihito Saito5, Satoshi Wada5, Takuya D. Kawahara6, and Toru Takahashi7,8 Viswanathan Lakshmi Narayanan et al.
  • 1Department of Physics and Technology, UiT – The Arctic University of Norway, Tromsø, Norway
  • 2Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, Japan
  • 3Space Physics and Astronomy Research Unit, University of Oulu, Finland
  • 4National Institute of Polar Research, Japan
  • 5RIKEN Center for Advanced Photonics, RIKEN, Saitama, Japan
  • 6Shinshu University, Japan
  • 7Department of Physics, University of Oslo, Norway
  • 8Electronic Navigation Research Institute, National Institute of Maritime, Port, and Aviation Technology, Tokyo, Japan

Abstract. We present a detailed investigation of the formation of a secondary sodium layer at altitudes of 79–85 km below the main sodium layer based on sodium lidar and airglow imager measurements made at Ramfjordmoen near Tromsø, Norway on the night of 19 December 2014. The airglow imager observations of OH emission revealed four passing frontal systems that resembled mesospheric bores which typically occur in ducting regions of the upper mesosphere. For about 1.5 hours, the lower altitude sodium layer had densities similar to that of the main layer with a peak around 90 km. The lower altitude sodium layer weakened and disappeared soon after the fourth front had passed. The fourth front had weakened in intensity by the time it approached the region of lidar beams and disappeared soon afterwards. The column integrated sodium densities increased gradually during formation of the lower altitude sodium layer. Temperatures measured with the lidar indicate that there was a strong thermal duct structure between 87 and 93 km. Furthermore, the temperature was enhanced below 85 km. Horizontal wind magnitudes estimated from the lidar showed strong wind shears above 93 km. We conclude that the combination of an enhanced stability region due to the temperature profile and intense wind shears have provided ideal conditions for evolution of multiple mesospheric bores revealed as frontal systems in OH images. The downward motion associated with the fronts appeared to have brought air rich in H and O from higher altitudes into the region below 85 km wherein the temperatures were also relatively high. This would have liberated sodium atoms from the reservoir species and suppressed the re-conversion of atomic sodium into reservoir species so that the lower altitude sodium layer could form and the column abundance could increase. The presented observations also reveal the importance of mesospheric frontal systems in bringing about significant variation of minor species over shorter temporal intervals.

Viswanathan Lakshmi Narayanan et al.

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Latest update: 19 Oct 2020
Publications Copernicus
Short summary
In the past, additional sodium layers occurring above the main sodium layer of the upper mesosphere were discussed. Here, formation of an additional sodium layer below the main sodium layer is discussed in detail. The event coincided with passage of multiple mesospheric bores which are step-like disturbances occurring in the upper mesosphere. Hence, this work highlights the importance of such mesospheric bores in causing significant changes to the minor species concentration in short time.
In the past, additional sodium layers occurring above the main sodium layer of the upper...