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

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%.

青藏高原东坡大气边界层高度变化观测研究

利用2021年2—5月和8—11月青藏高原东坡高海拔宇宙线观测站(Large High Altitude Air Shower Observatory,LHAASO)多通道微波辐射计观测的高时空分辨率温度廓线数据,并结合2021年ERA5再分析资料,分析大气边界层高度(Atmospheric Boundary Layer Height,ABLH)的日、月和季平均日变化规律。结果表明:(1)晴朗天气的ABLH具有明显的波峰波谷变化,日出后地表温度升高,ABLH也随之持续升高,最大值通常出现在午后,至18时左右迅速降低,日落之后降至最低。(2)一年之中,4月平均ABLH最大,约为1200 m,11月只有600 m。ABLH白天波动大,夜晚稳定,平均降至400 m左右。基于ERA5再分析资料反演的ABLH整体结果偏小,但具有与微波辐射计一致的变化趋势。(3)ABLH最大值出现在春季,夏季和秋季次之,冬季最小,且均在14:00—15:00达到峰值。

成像激光雷达标定系统中三维升降转台的性能研究

三维升降转台是高海拔宇宙线观测站(Large High Altitude Air Shower Observatory,LHAASO)广角切伦科夫望远镜阵列(Wide Field of View Cherenkov Telescope Array,WFCTA)成像激光雷达标定系统的核心组成部分,主要由慢控子系统(Programmable Logic Controller,PLC)、机械子系统和标准零点子系统构成,用于精确控制激光束的出射方向。采用高精度数字图像处理技术对三维升降转台的性能进行研究,结果表明,旋转和升降定位精度分别为0.005°和0.056 mm,重复精度分别为0.003°和0.075 mm,满足成像激光雷达标定系统的实验要求。三维升降转台的自动控制巡航功能有效减少了人工干预,提高了系统远程使用的可靠性和效率。标准零点的校准功能可以有效监测三维升降转台的定位精度,并且在失准的情况下能够实现远程校准,确保三维升降转台长期高精度稳定运行。

Optimization of performance of the KM2A full array using the Crab Nebula

The full array of the Large High Altitude Air Shower Observatory(LHAASO)has been in operation since July 2021.For its kilometer-square array(KM2A),we optimized the selection criteria for very high and ultrahigh energyγ-rays using data collected from August 2021 to August 2022,resulting in an improvement in significance of the detection in the Crab Nebula of approximately 15%,compared with that of previous cuts.With the implementation of these new selection criteria,the angular resolution was also significantly improved by approximately 10%at tens of TeV.Other aspects of the full KM2A array performance,such as the pointing error,were also calibrated using the Crab Nebula.The resulting energy spectrum of the Crab Nebula in the energy range of 10-1000 TeV are well fitted by a log-parabola model,which is consistent with the previous results from LHAASO and other experiments.

Flux variations of cosmic ray air showers detected by LHAASO-KM2A during a thunderstorm on June 10,2021

The Large High Altitude Air Shower Observatory(LHAASO)has three sub-arrays,KM2A,WCDA,and WFCTA.The flux variations of cosmic ray air showers were studied by analyzing the KM2A data during a thunderstorm on June 10,2021.The number of shower events that meet the trigger conditions increases significantly in atmospheric electric fields,with a maximum fractional increase of 20%.The variations in trigger rates(increases or decreases)were found to be strongly dependent on the primary zenith angle.The flux of secondary particles increased significantly,following a trend similar to that of shower events.To better understand the observed behavior,Monte Carlo simulations were performed with CORSIKA and G4KM2A(a code based on GEANT4).We found that the experimental data(in saturated negative fields)were in good agreement with the simulations,assuming the presence of a uniform electric field of-700 V/cm with a thickness of 1500 m in the atmosphere above the observation level.Due to the acceleration/deceleration by the atmospheric electric field,the number of secondary particles with energy above the detector threshold was modified,resulting in the changes in shower detection rate.

Performance of LHAASO-WCDA and observation of the Crab Nebula as a standard candle

The first Water Cherenkov detector of the LHAASO experiment(WCDA-1)has been operating since April 2019.The data for the first year have been analyzed to test its performance by observing the Crab Nebula as a standard candle.The WCDA-1 achieves a sensitivity of 65 mCU per year,with a statistical threshold of 5 cr.To accomplish this,a 97.7%cosmic-ray background rejection rate around 1 TeV and 99.8%around 6 TeV with an ap proximate photon acceptance of 50%is achieved after applying an algorithm to separate gamma-induced showers.The angular resolution is measured using the Crab Nebula as a point source to be approximately 0.45°at 1 TeV and better than 0.2°above 6 TeV,with a pointing accuracy better than 0.05°.These values all match the design specifications.The energy resolution is found to be 33%for gamma rays around 6 TeV.The spectral energy distribution of the Crab Nebula in the range from 500 GeV to 15.8 TeV is measured and found to be in agreement with the results from other TeV gamma ray observatories.