Long-duration gamma-ray emissions from 2007 and 2008 winter thunderstorms

TL;DR

This study reports on long-duration gamma-ray emissions detected during winter thunderstorms in Japan, analyzing their spectra, source distance, and electron acceleration mechanisms, with implications for understanding terrestrial gamma-ray flashes.

Contribution

First detailed analysis of long-duration gamma-ray emissions from winter thunderstorms in Japan, linking observed spectra to relativistic electron acceleration models.

Findings

Gamma-ray emissions lasted over 1 minute without lightning association.

Spectra extended to 10 MeV, indicating bremsstrahlung from relativistic electrons.

Estimated source distances ranged from 110 to 690 meters.

Abstract

The Gamma-Ray Observation of Winter THunderclouds (GROWTH) experiment, consisting of two radiation-detection subsystems, has been operating since 2006 on the premises of Kashiwazaki-Kariwa nuclear power plant located at the coastal area of Japan Sea. By 2010 February, GROWTH detected 7 long-duration $\gamma$-rays emissions associated with winter thunderstorms. Of them, two events, obtained on 2007 December 13 and 2008 December 25, are reported.On both occasions, all inorganic scintillators (NaI, CsI, and BGO) of the two subsystems detected significant gamma-ray signals lasting for >1 minute. Neither of these two events were associated with any lightning. In both cases, the gamma-ray energy spectra extend to 10 MeV, suggesting that the detected gamma-rays are produced by relativistic electrons via bremsstrahlung. Assuming that the initial photon spectrum at the source is expressed by a power-law function,the observed photons can be interpreted as being radiated from a source located at a distance of 290-560 m for the 2007 event and 110-690 m for the 2008 one, both at 90% confidence level.Employing these photon spectra, the number of relativistic electrons is estimated as 10^9 - 10^{11}. The estimation generally agrees with those calculated based on the relativistic runaway electron avalanche model. A GROWTH photon spectrum, summed over 3 individual events including the present two events and another reported previously, has similar features including a cut-off energy, to an averaged spectrum of terrestrial gamma-ray flashes.