Observation of thundercloud-related gamma rays and neutrons in Tibet

TL;DR

This study reports the first observation of long-duration high-energy gamma rays and neutrons associated with thunderclouds in Tibet, indicating prolonged electron acceleration and challenging previous assumptions about neutron monitor signals during thunderstorms.

Contribution

First detection of a 40-minute high-energy gamma-ray event linked to thunderclouds, with simulations clarifying neutron monitor signals' origins, thus advancing understanding of thundercloud particle acceleration.

Findings

Prolonged gamma-ray emission lasting about 40 minutes.

Monte Carlo simulations show gamma rays dominate neutron monitor signals.

Neutron enhancements during thunderstorms may not always indicate neutron production.

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 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-MeV $\gamma$ rays largely contribute to the neutron monitor signals, while $>$1-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 a clear evidence for neutron production, as previously thought.