Unusual vertical oscillations in sodium density and the formation of sporadic sodium layer over the Zhongshan station

Hiyadutuje, Alicreance and Chen, Xiangcai and Habarulema, John Bosco and Kosch, Michael J. and Stephenson, Judy A. E. and Matamba, Tshimangadzo Merline and Huang, Wentao and Ban, Chao (2025) Unusual vertical oscillations in sodium density and the formation of sporadic sodium layer over the Zhongshan station. Annals of Geophysics, 68. ISSN 1593-5213

Abstract

Unusual vertical oscillations with some wave structures were observed in the sodium (Na) density layers (SLs) in Antarctica on 1 September 2019, by means of the lidar located at Zhongshan (69°S, 76°E). There was a clear vertical convection of the wavy sodium density layers which populated the region at an altitude of between ~90 and ~102 km producing the Sporadic/Sudden Sodium Layers (SSLs). The vertical oscillations had an average wavelength, average speed and period of ∼3.0‑4.5 km, ∼7.8 m/s and ∼7.3‑8.5 min, respectively. The possible cause of these vertical oscillations, as well as the mechanisms that could be behind the generation of these oscillations and wavy SSLs, were investigated. The Global Positioning System (GPS) satellite receiver located at Davis (68.6°S, 77.9°E), 116 km away from Zhongshan, was used to derive the Total Electron Content (TEC) perturbations in the region surrounding Zhongshan. SuperDARN HF radar at Zhongshan also showed some waves in the first 10 range gates (180‑800 km away), suggesting that the Traveling Ionospheric Disturbances (TIDs) were propagating in the d438;‑region. The cross‑correlation between GPS and lidar wave structures was computed. A good to strong correlation of –0.6‑–0.9 was found between waves observed by GPS and lidar. Additionally, a moderate correlation was found between the SuperDARN radar and lidar wave structures. The lidar neutral temperature showed upward Atmospheric Gravity Waves (AGWs), while SuperDARN and GPS showed the downward TIDs. Based on the polar cap (PC) index, TIDs could have been generated by Joule heating due to geomagnetic storm effects in the region. The estimated Richardson number values between 80 and 105 km at 16:00‑24:00 UT suggest that convective and dynamic instabilities could have generated the observed SSLs and AGWs. Vertical oscillation of the sodium density layers could have taken place because of waves breaking and interference from the downward TIDs and upward AGWs.