MOS and RREA Processes in Thunderclouds: Intensities and Spectral Shapes

Abstract The Relativistic Runaway Electron Avalanche (RREA) is the primary mechanism for enhancing atmospheric electron and gamma‐ray fluxes when the electric field exceeds a density‐dependent threshold. Another, non‐threshold process—Modification of the Electron Energy Spectrum (MOS)—occurs when subcritical fields energize ambient electrons, shifting their spectrum to higher energies and increasing bremsstrahlung probability. MOS becomes dominant at high energies, where the RREA flux rapidly decreases, explaining the persistent detections of gamma rays above 50–60 MeV. We simulate gamma‐ray yield over a wide range of Atmospheric Electric Field (AEF) to delineate MOS and RREA regimes and quantify spectral evolution with field strength. Experimental data from two Thunderstorm Ground Enhancements (TGEs) observed on 2 October 2024, are analyzed. By matching the exponential growth of measured count rates to modeled RREA yield, we derive the temporal evolution of the AEF during both TGEs, revealing the rate and magnitude of field strengthening that drive particle bursts and bridge the MOS–RREA transition in natural thunderstorms.