Flux variations of cosmic ray air showers detected by LHAASO-KM2A during thunderstorms

Abstract

The Large High Altitude Air Shower Observatory (LHAASO) has three sub-arrays, KM2A, WCDA and WFCTA, located at 4410 m above sea level in Sichuan Province, China. The high-altitude location and the frequent occurrence of thunderstorms make LHAASO suitable to study the effects of atmospheric electric fields (AEFs) on cosmic ray air showers. By analyzing the data of KM2A, the flux variations of cosmic ray air showers during thunderstorms are studied. The total number of shower events that meet the KM2A trigger conditions increases significantly during thunderstorms, with the maximum value exceeding 20%. The variations of trigger rates are found to be strongly dependent on the primary zenith angle. To understand the shower rate changes, the flux variations of ground-level secondary particles are analyzed. We find the average number of particles per shower event increases significantly in strong AEFs. Due to the acceleration by AEFs, the number of secondary particles with energy above the detector threshold increases, and then more shower events satisfy the trigger conditions, resulting in the shower rate increases. At the same time, the secondary particles carrying positive and negative charges will be deflected in opposite direction in AEFs, and this effect increases with the zenith angle. As a result, the flux variations of shower events detected by KM2A are correlated with the primary direction.