Effects of atmospheric electric fields on cosmic rays

The electric fields associated with thunderclouds change the intensity of secondary cosmic rays observed on the ground. This effect has been investigated using several detectors located at the Mount Norikura Cosmic Ray Observatory where excesses of 1% and more of the average counting rate may be observed when the Observatory is covered by thunderclouds. A frequency analysis of the time series of days with such excesses for the period 26 October 1990 to 15 January 2002 shows the expected summer maximum in the rate of occurrence and, more surprisingly, a 26-day variation. An electric field mill was installed to help determine the relationship between the intensity variations and the strength and direction of the field near the detector system: the excess is usually observed when a negative electric field (accelerating negative charges downward) greater than 10 kV/m is present in the atmosphere above the observatory. Based on Monte Carlo simulations we predict that excess counting rates measured without charge discrimination will be expected as a consequence of the excess of positive muons among the secondary cosmic rays.