  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 the thunderstorm on 10 June 2021. The
number of shower events that meet the trigger conditions increases
significantly in atmospheric electric fields, with maximum fractional increase
of 20%. The variations of trigger rates (increases or decreases) are found to
be strongly dependent on the primary zenith angle. The flux of secondary
particles increases significantly, following a similar trend with that of the
shower events. To better understand the observed behavior, Monte Carlo
simulations are performed with CORSIKA and G4KM2A (a code based on GEANT4). We
find that the experimental data (in saturated negative fields) are in good
agreement with simulations, assuming the presence of a uniform upward electric
field of 700 V/cm with a thickness of 1500 m in the atmosphere above the
observation level. Due to the acceleration/deceleration and deflection by the
atmospheric electric field, the number of secondary particles with energy above
the detector threshold is modified, resulting in the changes in shower
detection rate.
