Combining Cherenkov and scintillation detector observations with simulations to deduce the nature of high-energy radiation excesses during thunderstorms

We present co-observations of three strong count-rate enhancements associated with thunderstorms observed over 17 April 2015 to 23 September 2015 by the High Altitude Water Cherenkov (HAWC) array, and a suite of small scintillation detectors comprising the Gamma-ray Observations During Overhead Thunderstorms (GODOT) instrument. Because the HAWC array is most sensitive to ionizing radiation at high energies (>100 MeV), and the small scintillation detectors are most sensitive to ionizing radiation at low energies (3–20 MeV), we investigate using the ratio of these detector counting rates variations to infer spectral characteristics of these enhancements, and understand the physics behind these thunderstorm accelerator mechanisms. We consider two extreme mechanisms that can produce these enhancements: a point source of relativistic runaway electron avalanches (RREA), and modification of the background cosmic-ray spectrum (MOS) from an electric field profile that is everywhere below the RREA threshold. We simulate the responses of HAWC and the GODOT 12.7 cm×⊘12.7 cm NaI(Tl) scintillator and show that their ratio can discern between the two models, and that the observed thunderstorm rate enhancements are incompatible with the spectra from a point source of RREA, but consistent with our model of MOS for thunderstorm potentials within the range of −250 to 250 MV.