Multi-parametric analysis of thunderstorm ground enhancements (TGE) and associated gamma-ray emissions on Mount Hermon, Israel

Thunderstorm Ground Enhancements (TGEs), sometimes referred to as gamma-ray glows, are transient increases in surface-level gamma-ray flux driven by thunderstorm electric fields. This study investigates two TGE events observed on January 4–6 and January 14, 2018, at the Emilio Segre Cosmic-Ray Observatory on Mount Hermon in northern Israel. Both cases were under rain-free conditions, thus excluding radon washout effects. Gamma-ray counts, vertical electric field strength, neutron flux, and lightning data were continuously monitored, enabling a detailed analysis of possible TGE mechanisms. Both TGEs exhibited significant gamma-ray enhancements coinciding with electric field fluctuations. The January 4–6 event displayed a higher intensity (peak 3500 cpm) and longer duration than the January 14 event (peak 1600 cpm). While gamma-ray enhancements were accompanied with negative electric fields, an exception was observed on January 14, where a positive electric field coincided with increased gamma-ray counts, likely due to a dominant lower positive charge region (LPCR) of the thundercloud near the mountain. Spearman correlation analysis revealed substantial differences in the relationships between gamma rays, neutron flux, and atmospheric pressure during TGEs compared with fair weather conditions. Notably, the decay of gamma-ray counts followed a prolonged exponential trend with a mean half-life time of 54.05 min, strongly suggesting a significant contribution from radon progeny lifted and suspended by storm-induced circulation, along with Modification of Spectra (MOS) as a secondary gamma-ray production mechanism through the acceleration process. Lightning discharges, while present, showed no direct correlation with gamma-ray enhancements, reinforcing the dominant role of thunderstorm electric fields in TGE production. This study provides additional insights into TGE mechanisms, emphasizing the influence of electric field dynamics and cloud charge structures. These findings highlight the importance of multi-parametric, high-resolution measurements in advancing the understanding of gamma-ray production during thunderstorms.