基于MSISE-90研究高海拔宇宙线观测站处的大气深度廓线模型

Study of atmospheric depth profiles at large high altitude air shower observatory using MSISE-90 model

高海拔宇宙线观测站(LHAASO)位于四川省稻城县海子山,它的广角切伦科夫望远镜阵(WFCTA)主要是通过观测广延大气簇射过程中产生的切伦科夫光信号对宇宙线进行研究.WFCTA的标定、模拟和重建都和大气深度有关,目前使用的大气深度模型是美国标准大气深度廓线模型.本研究中将美国标准大气深度廓线模型与卫星TIMED搭载的红外辐射计SABER记录到的LHAASO处14-50 km处的大气深度廓线进行比较,同时也与LHAASO处地面气象站记录的大气深度进行比较,美国标准大气模型的大气深度均偏小.MSISE-90大气模型描述了地球大气中从地面到热层的中性温度和密度,进一步研究发现MSISE-90大气模型与TIMED/SABER和LHAASO处地面标准气象站记录的大气深度的一致性较好.根据MSISE-90大气模型计算得到LHAASO处的大气深度均值廓线在1月最低,其次是2月、3月、4月、11月和12月,这也是WFCTA运行的最佳观测月份.4月份的大气边界层最高,其大气深度存在约2%的日变化.利用美国标准大气模型的函数形式,拟合每月的4.4-100 km处的大气深度廓线,得到了LHAASO处的每月的大气深度廓线模型,并比较了30°天顶角入射的100 TeV的宇宙线质子在MSISE-90大气模型和美国标准大气模型中产生的切伦科夫光的横分布的差异,二者最大差异约可以达到20%.

High altitude cosmic ray observatory(LHAASO)is located at Haizi Mountain in Daocheng county,Sichuan province,China.Its wide field of view Cherenkov telescope array(WFCTA)is primarily used to study cosmic rays through observing the Cherenkov light signals produced during extensive air showers.Calibration,simulation,and reconstruction of WFCTA are all related to atmospheric depth.The atmospheric depth model currently used 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,and also 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 average atmospheric depth profile at LHAASO is lowest in January,followed sequentially 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,with the diurnal variation of atmospheric depth being about 2%.Using the functional form of the US standard atmosphere odel,the monthly atmospheric depth profile of the LHAASO site is obtained by fitting an atmospheric depth profile of 4.4 to 100 km per month.And the comparison between 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 that in the US standard atmosphere model shows that their maximum difference reaches about 20%.