Thunderstorms are common weather phenomena at high altitudes, accompanying with lightning, strong winds and other disasters. During thunderstorms, the strength of atmospheric electric fields could be up to 1000 V/cm or even higher. The intensity fluctuates violently and the polarity could change multiple times. So, direct measurement of the thunderstorm electric field is a quite challenging work. The Large High Altitude Air Shower Observatory (LHAASO), under the construction of a project at Daocheng (4410 m a.s.l., Sichuan, China), is featured with frequent thunderstorms, especially in summer. The distribution of thunderstorm parameters is presented by analyzing the near-earth atmospheric electric field of the LHAASO station in this work. The polarity and intensity variation characteristics of the electric field in the cumulus, mature and dissipating stages of thunderstorm are also discussed. The results show that the thunderstorms mainly occur in the period of a time from late afternoon to evening. They are more frequent and stronger in summer. During the mature stage, the field changes more dramatically. Our results could be helpful in understanding the variations of cosmic rays at LHAASO during thunderstorms, and provide valuable information for studying global thunderstorm activity.

For a high altitude experiment, the lateral distribution (particle density distribution) of secondary particles at detector level is an important parameter to get the information of primary shower event. During thunderstorms, the charged particles in extensive air showers will be accelerated or decelerated in strong atmospheric electric field. The intensity and the position of ground cosmic rays could be influenced. In this work, Monte Carlo simulations are performed by using CORSIKA to study the effect of thunderstorms electric field on the lateral distribution of charged particles at YBJ (4300 m a.s.l., Tibet, China). We found that the lateral distribution will be changed significantly in thunderstorm field. And the variation amplitude is found to be highly dependent on the electric field, the zenith angle of the primary shower event and the distance to the shower core. Our simulation results could be helpful in understanding the phenomena observed in ground based experiments (such as ARGO-YBJ) and the physical mechanism of secondary charged particles accelerated by thunderstorm electric field.

Daocheng in Sichuan province of China with an altitude up to 4410 m above the sea level, takes the function of hybrid technology to detect cosmic rays. As the major array of LHAASO, KM2A is composed of 5195 electromagnetic particle detectors (EDs) and 1188 muon detectors (MDs). In the ground-based experiments, there are two common independent data acquisition systems, corresponding to the scaler and shower operation modes. In order to learn more about the scaler mode in LHAASO-KM2A, we adopt the CORSIKA to study the shower development and employ the G4KM2A (based on Geant4) to simulate the detector responses. For one cluster (composed of 64 EDs) in the array of KM2A-ED, the event rates of showers having a number of fired EDs ≥ 1, 2, 3 and 4 (in a time coincidence of 100 ns) are recorded. The average rates of the four multiplicities are ∼ 88 kHz, ∼ 1.4 kHz, ∼ 210 Hz and ∼ 110 Hz, respectively. For the array of KM2A-MD, there are 16 MDs in one cluster. The average rates with multiplicities ≥ 1 and 2 are ∼ 84 kHz and ∼ 890 Hz, respectively. The corresponding primary energies are also given. According to our simulations, the energy threshold of the scaler mode can be lowered to ∼ 100 GeV. At the same time, the energy threshold of LHAASO-KM2A in shower mode is presented for comparison. The simulation results in this work are beneficial for the online trigger with scaler mode, and also be useful in understanding the experiment results in LHAASO-KM2A.

The ARGO-YBJ detector was operated for 5 years at the YangBaJing Cosmic Ray Observatory (4300 m a. s. l., Tibet, China). The high altitude location and the high granularity of the "full coverage" detector (central carpet with an active area of ∼ 93 %), allow a detailed space-time reconstruction of cosmic ray showers, making ARGO-YBJ well suitable to study the effects of thunderstorm electric field on the shower development in atmosphere. An accurate Monte Carlo simulation has been implemented to interpret the ARGO-YBJ data. CORSIKA is used to simulate the evolution and properties of extensive air showers in atmosphere. The G4argo code takes into account the response of the ARGO-YBJ detector. The correlation between the reconstructed shower event rate and the near-earth thunderstorm electric field has been studied in detail. We found that the variation of the shower rate depends on the strength and polarity of the electric field, and is also highly dependent on the zenith angle of the primary cosmic ray. These results could be helpful in understanding the effects of the electric fields on cosmic rays, and provide important information for other cosmic ray studies with ground-based detectors (such as LHAASO).

Study on the intensity changes of the cosmic rays during thunderstorms is very important for understanding the acceleration mechanism of secondary charged particles caused by an atmospheric electric field. In this work, Monte Carlo simulations were performed with CORSIKA to study the intensity changes of positrons, electrons and photons in thunderstorms electric field. We found that the number of these secondary particles changes significantly. The variation amplitude is closely related to the strength and polarity of the electric field. A series fields were chosen in our simulations, ranging from-1000 V/cm to 1000 V/cm. The correlations of the intensity variations between positrons, electrons and photons were also discussed. The LHAASO (Large High Altitude Air Shower Observatory) project is located at Daocheng, Sichuan province, China, at an attitude of 4400 m above the sea level. As the main part of this project, the electromagnetic particle detector (ED) in the kilometer-square array (KM2A) and the water Cherenkov detector array (WCDA) are sensitive to the secondary positrons, electrons and photons in extensive air shower (EAS). Our simulation results may provide important information for giving reasonable explanations to the forthcoming experimental data of LHAASO.