Lithium Observations in the Mesosphere: Seasonal Variability and the Impact of a Falcon 9 Re-entry

Ablation of re-entering satellites and rocket stages is expected to become a significant source of metals in the mesosphere, yet systematic observations remain limited so far. We present our initial Li atom observations between about 80 km and 100 km altitude using our lidar at Kühlungsborn, Germany (54°N, 12°E), covering a period between August 2024 and February 2025. The main source of the Li layer is still thought to be cosmic dust ablation. However, lithium is a crucial species for investigating anthropogenic impacts on the middle atmosphere because of its extensive use in the space industry. Our measurements revealed a column abundance mostly between 1x10⁶ cm^-2 and 5x10⁶ cm^-2. Initial simulations using the WACCM-Li model are in reasonable agreement with the observations, suggesting natural seasonal variability as the primary driver for the changes in Li abundance. Some of the observations in early 2025 showed, however, an unusually high abundance that cannot yet be explained by natural variation. A notable event occurred on February 19-20, 2025, at 00:21 UTC, with the detection of a Li cloud exhibiting densities ten times higher than typical, reaching up to ~30 atoms/cm³. Back-trajectory analysis with UA-ICON indicated the probed air mass originated from a location west of Ireland, coinciding with the atmospheric re-entry of a Falcon 9 upper stage. Simulations of the re-entry process revealed a beginning metal ablation of this rocket stage already around 100 km altitude due to its shallow entry angle. We will present the details of this case study as well as our observations of the typical Li layer. Furthermore, we will show first results of our new 3-channel multi-species lidar (MSL) that is set up to search for different species expected to be ablated by re-entering space debris, like Cu, Hf, AlO, etc., along observations of Li and (purely natural) Na.