Terrestrial Gamma Glow (TGG) was detected for the first time in high latitudes outside the latitudinal band of ± 38° where Terrestrial Gamma Flashes (TGF) are typically observed. This study reports the observations of a TGG events that occurred in Southern Finland at the municipality of Vantaa (60° 18’ N 24° 58’ E at 55 m a.m.s.l) on May 17th, 2020 between 10:15 and 10:30 UTC. Three independently measuring units of NaI(Tl) γ spectrometers were used when a thundercloud emitting the γ radiation passed over or near the location of the detectors. The events occurred when a storm front moved across the Helsinki region in Finland in a roughly northeast direction. Two separate the γ radiation enhancements were observed. The first event lasted 52 s. and produced an increase in γ radiation of about 7 -12 %, after a break of about 85 -100 s., a second event occurred lasting about 100 s increasing the γ radiation by 20-50 % during the 10-second collection time. The exact locations of the thunderclouds emitting the γ radiation were not known and it is possible that the first event occurred farther away from the detectors and was hence seen as a weaker enhancement. The horizontal size was estimated by using the average wind speed and the duration of the second enhancement. The size estimate was around 400-500 m with maximum upper limit of 1000 m. In the second enhancement most visible increase in the γ radiation was in the 100 keV - ~800 keV energy range where a uniform increase was observed. In addition, a clear enhancement in the high energy γ rays, with energies from 3000 keV up to the maximum of the detector system of 8900 keV, was also observed. The presence of high energy γ rays is a clear signals of a TGG event. The shape of the background subtracted γ spectrum has a power law shape and agreed well with that of a predicted photon spectrum produced by a Runaway Relativistic Electron Avalanche.
