Genesis of thunderstorm ground enhancements

TL;DR

This study analyzes the conditions leading to large thunderstorm ground enhancements (TGEs), focusing on electric fields, graupel fall, and lightning, and estimates particle flux intensities during a storm on 22 September 2022.

Contribution

It provides a detailed analysis of the physical conditions and particle fluxes associated with TGEs, highlighting the role of electric fields and graupel in their origination.

Findings

Large particle fluxes can reach up to 1.25 million gamma rays per square meter.

Only one TGE met the conditions to recover the electron energy spectrum.

The lower dipole responsible for electron acceleration decays with graupel fall coinciding with TGE termination.

Abstract

Proceeding from a stormy day of 22 September 2022, when 7 thunderstorm ground enhancements occurred (TGEs, 3 of them very large), we perform an analysis of the most important conditions, on which depend the origination of the large particle fluxes in the thunderous atmosphere. Among these conditions are the near-surface electric field (NSEF), graupel fall, and lightning activity. We estimate the intensity of the largest particle flux of 1,25 mln gamma rays hitting each square meter of surface on Aragats with energy spectra extended up to 70 MeV. Only one TGE from 7 meets the conditions to recover the electron energy spectrum; the fraction of electrons with energies above 10 MeV relative to the gamma ray flux reaches 45%. By carefully examining the graupel fall, we demonstrate that the lower dipole, which accelerates electrons, is formed by the main negative and lower positively charged regions. The lower dipole decays with a graupel fall that coincides with TGE terminations (usually by a lightning flash).