Study of Atmospheric Depth Profiles at LHAASO Using the MSISE-90 Model

Abstract

High Altitude Cosmic Ray Observatory (LHAASO) is located at Haizi Mountain in Daocheng County, Sichuan Province. Its Wide Field of view Cherenkov Telescope Array (WFCTA) primarily studies cosmic rays by observing the Cherenkov light signals produced during extensive air showers. Calibration, simulation, and reconstruction of WFCTA are all related to atmospheric depth. The currently used atmospheric depth model is the US Standard Atmosphere Depth Profile Model. In this study, the US Standard Atmosphere Depth Profile Model is compared with the atmospheric depth profile recorded by the infrared radiometer SABER carried by the satellite TIMED at LHAASO from 14 km to 50 km, as well as with the atmospheric depth recorded by the ground meteorological station at LHAASO. The atmospheric depth obtained from the US Standard Atmosphere Model is consistently smaller. The MSISE-90 atmospheric model describes the neutral temperature and density from the Earth's surface to the thermosphere. Further research shows good consistency between the MSISE-90 atmospheric model and the atmospheric depth recorded by TIMED/SABER and the ground standard meteorological station at LHAASO. According to the MSISE-90 atmospheric model, the mean atmospheric depth profile at LHAASO is lowest in January, followed by February, March, April, November, and December, which are also the optimal observation months for WFCTA operation. The atmospheric boundary layer is highest in April, and there is about 2% diurnal variation in atmospheric depth. Using the functional form of the US Standard Atmosphere Model, the atmospheric depth profiles from 4.4 to 100 kilometers were fitted for each month, obtaining monthly atmospheric depth profile models at the LHAASO site. And compared the lateral distribution of the Cherenkov photons produced by 100 TeV cosmic-ray protons incident at a zenith angle of 30° in the MSISE-90 atmospheric model and the US Standard Atmosphere Model,with the maximum difference reaching approximately 20%.