Effect of near-earth thunderstorm electric field on the flux of cosmic ray air showers in LHAASO-KM2A

The Large High Altitude Air Shower Observatory (LHAASO) is located at Haizi Mountain, Daocheng, Sichuan province, China. Due to its high-altitude location with frequent thunderstorm activities, the LHAASO is suited for studying the effects of near-earth thunderstorm electric fields on cosmic ray air showers. In this paper, Monte Carlo simulations are performed with CORSIKA and G4KM2A to analyze the flux variations of cosmic ray air showers detected by the kilometer-square array of LHAASO (LHAASO-KM2A) during thunderstorms. The strength, polarity, and layer thickness of atmospheric electric field (AEF) during thunderstorm are found to be associated with the shower rate variations. The flux of shower events satisfying trigger conditions of the KM2A increases with field intensity, particularly within negative fields, and the enhanced amplitude is more than 5% in -600 V/cm and 12% in -1000 V/cm, whereas it increases by only 1% and 7% in equivalent positive fields, respectively. While in positive fields ranging from 0 to 400 V/cm, the shower rate decreases with smaller amplitudes. Furthermore, the shower rate increases dramatically with the AEF layer thickness until a certain value, above which the variation trend slows down. The dependence of the trigger rate variation on the primary zenith angle has also been revealed, increasing in lower zenith angle ranges and showing opposite behaviors in higher ones. Additionally, we study that the relationship between the trigger rate variations and the primary energies, and find the enhanced amplitude of the shower rate decreases with increasing primary energy. Simultaneously, the shower events with lower primary energy show a significant increase, whereas events with higher primary energy are hardly affected during thunderstorms. Our simulations offer insights into the variation of the trigger rate detected by LHAASO-KM2A during thunderstorms.