Abstract
Physikalisches Institut, Universitaet Bern, CH-3012, Bern, Switzerland~Received 18 December 2003; published 30 June 2004!The electric fields associated with thunderclouds change the intensity of secondary cosmic rays observed onthe ground. This effect has been investigated using several detectors located at the Mount Norikura CosmicRay Observatory where excesses of 1% and more of the average counting rate may be observed when theObservatory is covered by thunderclouds. A frequency analysis of the time series of days with such excessesfor the period 26 October 1990 to 15 January 2002 shows the expected summer maximum in the rate ofoccurrence and, more surprisingly, a 26-day variation. An electric field mill was installed to help determine therelationship 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! greaterthan 10 kV/m is present in the atmosphere above the observatory. Based on Monte Carlo simulations wepredict that excess counting rates measured without charge discrimination will be expected as a consequenceof the excess of positive muons among the secondary cosmic rays.DOI: 10.1103/PhysRevD.69.123010 PACS number~s!: 96.40.Kk, 52.80.Mg, 92.60.Pw, 96.50.PwI. INTRODUCTION
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