  A correlation between the secondary cosmic ray flux and the near-earth
electric field intensity, measured during thunderstorms, has been found by
analyzing the data of the ARGO-YBJ experiment, a full coverage air shower array
located at the Yangbajing Cosmic Ray Laboratory (4300 m a. s. l., Tibet,
China). The counting rates of showers with different particle multiplicities,
have been found to be strongly dependent upon the intensity and polarity of the
electric field measured during the course of 15 thunderstorms. In negative
electric fields (i.e. accelerating negative charges downwards), the counting
rates increase with increasing electric field strength. In positive fields, the
rates decrease with field intensity until a certain value of the field EFmin
(whose value depends on the event multiplicity), above which the rates begin
increasing. By using Monte Carlo simulations, we found that this peculiar
behavior can be well described by the presence of an electric field in a layer
of thickness of a few hundred meters in the atmosphere above the detector,
which accelerates/decelerates the secondary shower particles of opposite
charge, modifying the number of particles with energy exceeding the detector
threshold. These results, for the first time, give a consistent explanation for
the origin of the variation of the electron/positron flux observed for decades
by high altitude cosmic ray detectors during thunderstorms.
