[Paper Review] Catalog of Gamma-ray Glows during Four Winter Seasons in Japan

Yuuki Wada, Takahiro Matsumoto+13 more|arXiv (Cornell University)|Aug 4, 2021

Lightning and Electromagnetic Phenomena57 references4 citations

TL;DR

This paper presents the first catalog of gamma-ray glows—minute-long bursts of high-energy gamma rays from thunderclouds—detected during four winter seasons (2016–2020) in Japan’s Hokuriku region using a network of 10 radiation monitors. The study identifies 70 glows with an average duration of 58.9 seconds, predominantly at night (77%), and reveals a power-law energy spectrum with exponential cutoff (photon index: 0.613 ± 0.009, cutoff energy: 4.68 ± 0.04 MeV, flux: 1.013 × 10⁻⁵ erg cm⁻² s⁻¹ in 0.2–20.0 MeV).

ABSTRACT

In 2015 the Gamma-Ray Observation of Winter Thunderstorms (GROWTH) collaboration launched a mapping observation campaign for high-energy atmospheric phenomena related to thunderstorms and lightning discharges. This campaign has developed a detection network of gamma rays with up to 10 radiation monitors installed in Kanazawa and Komatsu cities, Ishikawa Prefecture, Japan, where low-charge-center winter thunderstorms frequently occur. During four winter seasons from October 2016 to April 2020, in total 70 gamma-ray glows, minute-lasting bursts of gamma rays originating from thunderclouds, were detected. Their average duration is 58.9 sec. Among the detected events, 77% were observed in nighttime. The gamma-ray glows can be classified into temporally-symmetric, temporally-asymmetric, and lightning-terminated types based on their count-rate histories. An averaged energy spectrum of the gamma-ray glows is well fitted with a power-law function with an exponential cutoff, whose photon index, cutoff energy, and flux are $0.613\pm0.009$, $4.68\pm0.04$ MeV, and $(1.013\pm0.003) imes10^{-5}$ erg cm$^{-2}$ s$^{-1}$ (0.2-20.0MeV), respectively. The present paper provides the first catalog of gamma-ray glows and their statistical analysis detected during winter thunderstorms in the Kanazawa and Komatsu areas.

Motivation & Objective

• To systematically catalog gamma-ray glows during winter thunderstorms in Japan, a region with frequent low-charge-center storms.
• To analyze the temporal, spectral, and spatial characteristics of these high-energy atmospheric phenomena using a distributed network of radiation detectors.
• To investigate the statistical properties of gamma-ray glows, including duration, occurrence rate, and energy spectra, to understand their physical origin.
• To compare detection frequency and spectral features with previous studies in different climatic and altitudinal conditions, such as at Aragats (3200 m, summer storms).

Proposed method

• Deployed a network of up to 10 radiation monitors (BGO and CsI scintillators) across Kanazawa and Komatsu, Ishikawa Prefecture, Japan, during four winter seasons (Oct 2016–Apr 2020).
• Monitored gamma-ray count rates above 3 MeV continuously, identifying transient bursts (glows) lasting seconds to minutes.
• Classified glows into three types based on count-rate history: temporally-symmetric (Gaussian-like), temporally-asymmetric, and lightning-terminated.
• Fitted the energy spectra of 28 bright glows with a power-law function with exponential cutoff: dN/dE ∝ E^(-Γ) × exp(-E/E₀).
• Used response functions to unfold energy spectra and validated results with bootstrapping and data validation techniques.
• Correlated glow occurrences with meteorological data (wind speed/direction, temperature) and lightning data from XRAIN.

Experimental results

Research questions

• RQ1 What is the statistical distribution of gamma-ray glow durations and occurrence times during winter thunderstorms in Japan?
• RQ2 How do the temporal profiles of gamma-ray glows (symmetric, asymmetric, lightning-terminated) relate to underlying acceleration mechanisms?
• RQ3 What is the average energy spectrum of gamma-ray glows in the 0.2–20.0 MeV range, and how does it compare to other regions?
• RQ4 Why is the detection rate of gamma-ray glows lower in Hokuriku (0.93 events/month per site) than at Aragats (8.8 events/month), despite similar energy thresholds?
• RQ5 How do atmospheric conditions (temperature, wind, cloud base height) influence the detectability and characteristics of gamma-ray glows?

Key findings

• A total of 70 gamma-ray glows were detected over four winter seasons (Oct 2016–Apr 2020), with 77% occurring at night, indicating strong correlation with nocturnal thunderstorm activity.
• The average duration of gamma-ray glows is 58.9 seconds, with 50% showing temporally-symmetric (Gaussian-like) count-rate profiles, suggesting stable electron acceleration.
• The energy spectrum of the glows is best described by a power-law with exponential cutoff, with a photon index of 0.613 ± 0.009, cutoff energy of 4.68 ± 0.04 MeV, and flux of (1.013 ± 0.003) × 10⁻⁵ erg cm⁻² s⁻¹ in the 0.2–20.0 MeV range.
• A quarter of the glows exhibit temporally-asymmetric profiles, possibly due to multiple or complexly shaped acceleration regions, while another quarter terminate abruptly with lightning discharges.
• Average wind speed during glows is 4.6 m/s, flowing eastward, and average temperature is 4.6 °C, with no significant correlation to glow duration or flux.
• The detection rate (0.93 events/month per site) is significantly lower than at Aragats (8.8 events/month), likely due to lower lightning frequency and greater atmospheric absorption, as detectors in Hokuriku are farther from the acceleration region than at high-altitude Aragats.