The sun finally sets, and the beauty of the polar night sky is revealed. The green lights of the aurora dance across the sky, accompanied by a bright green laser beam directed upwards. The stars revolve around it, because this is a special place: Earth’s southern pole (Fig. 1).
Fig. 1: Laser beam of the TELMA lidar at Amundsen-Scott South Pole Station in May 2023 with aurora and star trails in the background. Photograph by Marc Jacquart from NSF-IceCube with a 3 hour exposure.
The green laser beam belongs to TELMA, our Rayleigh lidar dedicated to observing dynamics in Earth’s middle atmosphere. When night falls in Antarctica, dramatic changes in atmospheric circulation occur. Driven by dropping temperatures, a circumferential jet known as the polar night jet is induced. It shapes the stratospheric polar vortex, setting Antarctica apart from the rest of the world even more.
At the vortex edge, atmospheric waves excited in the troposphere find optimal conditions to propagate upwards and grow – we study these dynamical processes in the framework of our SOUTHTRAC campaign with the CORAL and ALIMA Raleigh lidars. But what happens at the very core of the polar vortex?
Honestly, we don’t really know. Even though many Earth-observing satellite are polar-orbiting, meaning they are on a north-south track to achieve global coverage of the rotating Earth, they only approach the poles but do not cross directly over them. The temporal evolution of atmospheric dynamics at high vertical resolution is captured best by a ground-based lidar instrument. This is why we brought TELMA to Amundsen-Scott South Pole Station in January 2023.
The permanent darkness is of advantage, because no daylight filters are required, and measurements can be carried out for days. The extreme temperature differences that affect the finely tuned instrument, on the other hand, are demanding. TELMA’s laser and electronics must be held at room temperature. TELMA’s telescope, however, is exposed to the sky, cooling down to below -60°C. Ice crystals slowly accumulate on it.
Started by a station technician and supervised remotely at IPA, TELMA measures atmospheric density and temperature up to an altitude of almost 100 km. The variations in temperatures indicate atmospheric dynamics, among them gravity waves, planetary waves, tides, instabilities and, possibly, phenomena yet unidentified. The dynamics of the middle atmosphere extend to the regions above, the thermosphere and ionosphere, and play a role in determining space weather. To determine exactly what type of dynamics arise at this special place and what their characteristics are, is the assignment of TELMA. The first soundings within the forming polar vortex show mesospheric gravity waves whose excitation mechanism is yet to be investigated (Fig. 2).
Fig. 2: One of the first TELMA soundings at South Pole showing mesospheric gravity waves. (Graphics: DLR / B. Kaifler, CC-BY-ND-NC 3.0)