The EAS counting rate during thunderstorms

Some episodes in which perturbed atmospheric conditions during thunderstorms affect the counting rate of Extensive Air Showers have been observed by the EAS-TOP array. The most significant of them are discussed. They consist of increasing EAS counting rates of duration ∼10-20 minutes accompanied by similar effects in the single ionizing particle rate. The entity of the effect is ∼10-15%, and is larger for showers hitting a large number of detectors. The effect is compatible with an additional acceleration of secondary shower electrons by strong atmospheric electric fields.