Investigating Cosmic-Ray Extensive Air Showers as a Source of Weak TGFs Using GEANT4 and CORSIKA

During the ALOFT flight campaign in July 2023, we discovered a population of weak TGFs in the range of 10^12 to 10^15 source photons in brightness (at a reference altitude of 15 km), not detectable by space-based instruments (such as ASIM on the ISS or Fermi) [Bjørge-Engeland 2024; Fuglestad 2025]. While extensive air showers (EAS) were previously discarded as a seeding source for space-observed TGFs (i.e., with source brightness above 10^16 photons) [Dwyer 2008; Carlson 2008], in principle, this does not exclude the possibility that the same mechanism could generate TGFs that are orders of magnitude weaker, like ALOFT’s weak TGFs. This hypothesis consists of EAS generating a large number of seed particles in a very short time, which are then multiplied by the RREA process by a few orders of magnitude. It could also involve some level of relativistic feedback. Furthermore, ALOFT observations suggest that weak TGFs may be due to an abrupt increase in the seed population rather than an increase in the electric field, given the fast rise time (too fast for relativistic feedback) and the fact that the TGF precedes the radio signal.

EAS originate from highly energetic cosmic-ray protons and nuclei showering in the atmosphere. Because Geant4 cannot simulate initial proton energies above 100 TeV, and such energies may be required, we will also use the CORSIKA code (high-energy part) with the FLUKA model (low-energy part), both of which are well-established reference models. A key here to evaluate this hypothesis, is the trade-off between initial cosmic proton fluxes (e.g., per hour per square kilometer) and their energies, as higher energies generate more seed electrons but are less frequent.

In this presentation, we will show a comprehensive evaluation of the possibility of generating weak TGFs via EAS energetic electron seeding in a realistic large-scale thunderstorm electric field close to the RREA threshold.

 

References:

I. Bjørge-Engeland et al. Evidence of a New Population of Weak Terrestrial Gamma-Ray Flashes Observed From Aircraft Altitude. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024GL110395

A. Fuglestad et al. The source brightness distribution of Terrestrial Gamma-ray Flashes from the ALOFT flight campaign.

J. R. Dwyer. Source mechanisms of terrestrial gamma-ray flashes. https://doi.org/10.1029/2007JD009248

Carlson, B. E., N. Lehtinen et al. (2008). Runaway relativistic electron avalanche seeding in the Earth’s atmosphere. https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2008JA013210