Energetic Particles Produced by Thunderstorm Electric Fields

In this work, detailed Monte Carlo simulations were performed to test whether terrestrial gamma-ray flashes (TGFs) may be produced by thunderstorm electrification alone, without the presence of lightning, by the relativistic feedback mechanism. TGFs are powerful bursts of gamma rays produced by thunderstorms, often in association with lightning. They are produced by bremsstrahlung interactions of relativistic runaway electrons with air, typically requiring >10(17) energetic electrons to account for the gamma-ray emissions. These energetic electrons may be produced in the high-field regions near lightning leaders, or they may be produced by the self-sustained production of runaway electrons by relativistic feedback involving backward propagating runaway positrons and backscattered x-rays. It has been challenging to tell which mechanism is correct when lightning is present, since both may occur in the fields generated by the propagating leaders. Prior to 2023, the gamma-ray emissions from thunderstorms were broadly classified as either long-lasting gamma-ray glows or TGFs. However, recent aircraft observations by ALOFT are upending this distinction, showing that gamma-ray glows are highly dynamic and often morph into intense pulses that resemble TGFs observed from space. It is shown here that rapid thunderstorm charging produces strong gamma-ray glows with oscillating gamma-ray rates. These oscillations develop into intense pulse trains that closely resemble the multi-pulsed TGFs observed by CGRO/BATSE and ALOFT. Furthermore, the ionization produced by the high-energy particles partially discharges the electric field in some regions while strongly amplifying it in other regions, potentially leading to the initiation of narrow bipolar events and/or lightning.