Lightning Precursor Discharges and Terrestrial Gamma Ray Flashes

Spectacular naturally occurring bursts of X-rays with tens of mega electron-volt energies are observed in association with lightning activity in the Earth's atmosphere. These events were first discovered in 1994 by NASA's Compton Gamma Ray Observatory [Fishman et al., 1994] and are commonly referred to as terrestrial gamma-ray flashes (TGFs). Although since then many other orbital observatories captured these high energy events [Smith et al., 2005; Marisaldi et al., 2010; Briggs et al., 2011; Neubert et al., 2020] and these events have been clearly linked to very intense but short (approximately 10–50 microseconds) bursts of electromagnetic radiation [Ostgaard et al., 2021], the physical nature of these events remains unclear. It is widely recognized that these electromagnetic bursts are also the strongest naturally occurring sources of terrestrial very high frequency (VHF) radiation [Jacobson et al., 2012]. The most puzzling unexplained aspect of these events is that the extreme gamma ray and electromagnetic energy fluxes originate from very compact regions with spatial extent on the order of 100 meters [Lu et al., 2010; Rison et al., 2016; Tilles et al., 2019; Tilles et al., 2020; Kohn et al., 2020; Heumesser et al., 2021], yet they are light-deficient and often even optically undetectable compared to ordinary lightning discharges [Jacobson et al., 2012]. In this work we report on the mechanism that allows precise quantitative explanation of the vast majority of currently accumulated observations of these events at different altitudes in the Earth's atmosphere. The specifics includes TGF fluences and durations, optical darkness, magnitudes and specific waveform characteristics of low and very low frequency radio emissions termed narrow bipolar events and energetic in cloud pulses that are associated with TGFs, the peculiar observations of very fast (typically one tenths of the speed of light) spatial propagation features termed fast positive breakdown in the present literature, pulsing of TGFs, and low fluence TGFs not observable from the orbit.