Thunderstorm ground enhancements in Finland: observations using spectroscopic radiation detectors

Abstract This study reports observations of Thunderstorm Ground Enhancements (TGE) that occurred in Southern Finland in the Municipality of Vantaa (60° 18’ N 24° 58’ E at 55 m a.m.s.l) on May 17th, 2020 between 10:23 and 10:28 UTC. The TGEs occurred when a storm front moved across the Helsinki region from roughly a southwest to northeast direction. The TGEs were caused by a Runaway Relativistic Electron Avalanche (RREA) occurring inside a thundercloud. Three independent measuring units of high-volume NaI(Tl) γ spectrometers were used to record ambient dose rate and γ spectra. The observed TGEs manifested themselves as sudden increases in γ radiation on the ground with a wide range of energies from 100 keV up to the maximum of the detector system of 8.8 MeV. The first event lasted 50 s and produced an increase in γ radiation of about 7–12% in the spectra measured with a ten-second collection time. The enhancement in γ radiation was terminated suddenly and simultaneously with an in-cloud lightning strike. This was followed by an 88–100 s long period when the level of γ radiation returned to normal values. After this, a second enhancement was observed to increase the level of γ radiation by 20–50%. The second enhancement lasted 100 s and it was also terminated simultaneously with an in-cloud lightning flash. The second enhancement appears to be stronger, but the exact locations of the thundercloud emitting the γ radiation were not known. From the duration of the second enhancement and the prevailing wind speed, the size of the RREA was estimated to be 400–1000 m. In the second enhancement, the most intense increase was between 100 and 1000 keV, although very-high-energy γ rays was observed up to 8.8 MeV. The shape of the background-subtracted γ spectra agreed well with the observations and predictions of the shape of the bremsstrahlung spectrum emitted by an RREA. Additionally, in the background-subtracted spectra, a minor enhancement of the 511 keV peak was also observed where the RREA also increased the number of positrons which annihilated and increased the emission of the 511 keV γ rays. The results of these high-latitude, low-altitude TGE events are the first ones report in Finland.