Seasonal and Diurnal Variability of Thermospheric Metastable Helium Observed by Multi‐Platform Observations and Numerical Modeling
Zhenwei Liu, Chuan Huang, Jinru Cheng, Tangsheng Cheng, Hang Zhou, Chaomei Wu, Chen Liang, Tingdi Chen, Weilong Luo, Chong Wang, Zhaofeng Wang, Ruocan Zhao, Xianghui XueAbstract
Metastable helium () in the thermosphere emits at 1,083 nm and provides a powerful diagnostic of thermospheric composition, solar radiation, and transport. Its detection remains challenging because of its extremely low density and high‐altitude. Here, we present the first multi‐platform study of thermospheric variability using a density model, ground‐based LiDAR observations over Danzhou, China, and SPHEREx 1,083 nm measurements from May 2025 to April 2026. The three data sets show consistent seasonal variability, with reduced ‐related signals from late spring to midsummer and enhanced values in autumn and winter. Model results associate this pattern with changes in background ground‐state helium reservoir. A representative diurnal case shows the strongest response at 500–700 km, with enhanced values after 04:50 local time. An additional LiDAR‐observed layer above approximately 950 km is not fully captured by the model, highlighting the need for further model development. These results provide observational guidance for helium models.