Thundercloud Project: Exploring high-energy phenomena in thundercloud and lightning

Abstract

We designed, developed, and deployed a distributed sensor network aiming at observing high-energy ionizing radiation, primarily gamma rays, from winter thunderclouds and lightning in coastal areas of Japan. Starting in 2015, we have installed, in total, more than 15 units of ground-based detector system in Ishikawa Prefecture and Niigata Prefecture, and accumulated 551 days of observation time in four winter seasons from late 2015 to early 2019. In this period, our system recorded 51 gamma-ray radiation events from thundercloud and lightning. Highlights of science results obtained from this unprecedented amount of data include the discovery of photonuclear reaction in lightning which produces neutrons and positrons along with gamma rays, and deeper insights into the life cycle of a particle-acceleration and gamma-ray-emitting region in a thundercloud. The present paper reviews objective, methodology, and results of our experiment, with a stress on its instrumentation.

Multiple Gamma‐Ray Glows and a Downward TGF Observed From Nearby Thunderclouds

“…One is that there is a single gamma‐ray glow that is so wide that it covers two detectors 1.35 km apart at the same time. Previous observations of gamma‐ray glows during winter thunderstorms in Japan suggested that gamma‐ray glows are moving with thunderclouds (precisely, with rainfall regions; see Torii et al., 2011; Wada, Enoto, Nakamura, et al., 2019; Yuasa et al., 2020). However, in the present case, since the ambient wind is heading from Detector A to B, Detector A should observe the gamma‐ray enhancement 110 s earlier than Detector B if the gamma‐ray glow has a small variance of the flux in time.…”

Hazards to Aircraft Crews, Passengers, and Equipment from Thunderstorm-Generated X-rays and Gamma-Rays

“…One more high-energy phenomenon has been discovered to be associated with thunderstorms. It is often referred to as "gamma-ray glows" [5,[13][14][15][16][17][18] or "thunderstorm ground enhancements" [19,20], although no single term has been agreed on yet to describe it (see, e.g., [4,[21][22][23][24][25]). Enoto et al [25] use the term "γ-ray glow" only to describe "bremsstrahlung radiation from electrons accelerated in thunderclouds".…”

“…They term the latter emission "prolonged line emission." The term "prolonged emission" can be considered as a universal one describing any flux of high-energy photons not associated with rain washout of radioactive isotopes (see, e.g., [15][16][17][18]20,21,23,25]), with a duration of about one second or longer.…”

“…Gamma-ray fluxes, described as gamma-ray glows, etc., can last from seconds to minutes, and can produce gamma-ray photons with numbers exceeding the normal background level in some spectral ranges at least by one to two decades, while situations when the intensity of the glow is of the order of 10% of the background are also described [4,5,[13][14][15][16][17][18][19][20][21]26,27]. In principle, the generation of several subsequent and/or time-overlapping glows seems to be possible (see, e.g., [20,28]).…”

Collaboration between high schools in Japan and Argentina for cosmic-ray research using CosmicWatches

“…In Japan, citizen science projects have been conducted in recent years, along with intelligent devices and network communication technologies. One recent and successful example involving particle measurements is the Thundercloud project [4] to observe gamma rays generated in thunderclouds. In these cases, scintillation detectors are provided by scientists and installed in schools for observation together with students.…”