Observation of thundercloud-related gamma rays and neutrons in Tibet

Abstract

During the 2010 rainy season in Yangbajing (4300 m above sea level) in Tibet, China, a long-duration count enhancement associated with thunderclouds was detected by a solar-neutron telescope and neutron monitors installed at the Yangbajing Comic Ray Observatory. The event, lasting for $\sim 40\min$, was observed on July 22, 2010. The solar-neutron telescope detected significant $\gamma$-ray signals with energies $>40\mathrm{MeV}$ in the event. Such a prolonged high-energy event has never been observed in association with thunderclouds, clearly suggesting that electron acceleration lasts for 40 min in thunderclouds. In addition, Monte Carlo simulations showed that $>10\mathrm{MeV}$ $\gamma$ rays largely contribute to the neutron monitor signals, while $>1\mathrm{keV}$ neutrons produced via a photonuclear reaction contribute relatively less to the signals. This result suggests that enhancements of neutron monitors during thunderstorms are not necessarily clear evidence for neutron production, as previously thought.