Mapping Observation Project of High-Energy Phenomena during Winter Thunderstorms in Japan

TL;DR

This study presents a new multi-site gamma-ray observation network in Japan that detects high-energy bursts during winter thunderstorms, revealing their connection to lightning and atmospheric nuclear reactions.

Contribution

The paper introduces a novel portable gamma-ray detector network and demonstrates its effectiveness in mapping high-energy phenomena during winter thunderstorms in Japan.

Findings

Detected 27 long-duration gamma-ray bursts

Detected 8 short-duration bursts

Linked short bursts to atmospheric photonuclear reactions

Abstract

The Gamma-Ray Observation of Winter Thunderclouds (GROWTH) collaboration has been performing observation campaigns of high-energy radiation in coastal areas of Japan Sea. Winter thunderstorms in Japan have unique characteristics such as frequent positive-polarity discharges, large discharge current, and low cloud bases. These features allow us to observe both long-duration gamma-ray bursts and lightning-triggered short-duration bursts at sea level. In 2015, we started a mapping observation project using multiple detectors at several new observation sites. We have developed brand-new portable gamma-ray detectors and deployed in the Kanazawa and Komatsu areas as well as the existing site at Kashiwazaki. During three winter seasons from 2015, we have detected 27 long-duration bursts and 8 short-duration bursts. The improved observation network in Kashiwazaki enables us to discover that the short-duration bursts are attributed to atmospheric photonuclear reactions triggered by a downward terrestrial gamma-ray flash. Collaborating with electric-field and radio-band measurements, we have also revealed a relation between abrupt termination of a long-duration burst and a lightning discharge. We demonstrate that the mapping observation project has been providing us clues to understand high-energy atmospheric phenomena associated with thunderstorm activities.