[Paper Review] Relativistic runaway electron breakdown and Terrestrial Gamma ray Flashes: GEANT4 simulation

Yu. A. Tsalkou, В. В. Тихомиров+1 more|arXiv (Cornell University)|Jan 23, 2013

Lightning and Electromagnetic Phenomena23 references3 citations

TL;DR

This study uses GEANT4 simulations to model relativistic runaway electron avalanche (RREA) in the upper atmosphere, demonstrating that RREA produces high-energy gamma-ray spectra with a hard spectral index close to the observed value of ~2.7. The results show that bremsstrahlung is the dominant mechanism for hard photon production, not Comptonization, and that spectral indices vary with altitude, supporting RREA as a viable source for Terrestrial Gamma-ray Flashes (TGFs).

ABSTRACT

Numerical simulation of a relativistic runaway electron breakdown in the upper atmosphere is performed using GEANT4 simulation toolkit. General features of a relativistic runaway electron avalanche are reconstructed and properties of radiations accompanying breakdown are obtained. It is demonstrated dependance of the high energy branch of photon spectra with respect to an altitude and shown what at the reasonable parameters hard photons have spectral index close to the observed value.

Motivation & Objective

• To simulate relativistic runaway electron breakdown (RREA) in the upper atmosphere using the GEANT4 toolkit.
• To investigate the spectral properties of high-energy photons produced during RREA and their dependence on altitude.
• To assess the role of different radiation mechanisms—particularly bremsstrahlung versus Comptonization—in generating the hard photon component observed in TGFs.
• To evaluate the potential implications of RREA-generated radiation for high-altitude aircraft and satellite systems.
• To provide a foundation for estimating TGF generation altitudes through spectral analysis of future high-precision TGF data.

Proposed method

• Simulated a 2000 m high, 1000 m radius cylindrical volume of atmosphere at 5000 m altitude with a uniform electric field of 2Ec (twice the critical field for RREA).
• Used GEANT4 to model particle interactions, including electron and positron production via pair creation, ionization, and bremsstrahlung radiation.
• Tracked particle energy spectra and radiation output using a thin 10 cm detection slice placed near the top of the simulation volume.
• Analyzed the energy spectra of photons and electrons, focusing on the high-energy branch (Eγ > 10 MeV) and its spectral index s.
• Evaluated the contribution of inverse Compton scattering by comparing predicted spectra from electron distributions with observed TGF spectra.
• Assessed the consistency of simulated spectra with observational data, particularly the hard excess above 10 MeV reported by AGILE.

Experimental results

Research questions

• RQ1 What is the spectral shape of high-energy photons produced during RREA in realistic atmospheric conditions?
• RQ2 How does the spectral index of the hard photon branch vary with altitude in the RREA process?
• RQ3 Is Comptonization on relativistic electrons a significant contributor to the observed hard TGF spectrum, or is bremsstrahlung the dominant mechanism?
• RQ4 Can the simulated RREA photon spectra reproduce the observed spectral index of ~2.7 in TGFs under realistic electric field and atmospheric parameters?
• RQ5 To what extent do simulated spectra support the use of spectral analysis as a method to estimate the generation altitude of TGFs?

Key findings

• The simulated high-energy photon spectra exhibit a spectral index s ≈ 2.7 at altitudes above h ≳ 5 km, closely matching the observed value for TGFs.
• The spectral index decreases with increasing altitude, indicating a flattening of the spectrum at higher altitudes, consistent with reduced absorption and scattering.
• At energies above 100 MeV, hard photons are definitively produced, with electron number density dominating the relativistic cascade at energies >1 MeV.
• Comptonization via single inverse scattering is ruled out as a major source of hard photons, as it would require an electron spectrum with p ≈ 6.4, which contradicts the observed p ≈ 2.7.
• Bremsstrahlung is identified as the dominant mechanism for hard photon production in RREA, given the consistency of simulated spectra with observations.
• The simulation supports the feasibility of using spectral index measurements to estimate the generation altitude of TGFs in future high-precision observations.