Neither the exact mechanisms through which lighting is initially triggered and propagates through the free atmosphere nor the relationship of these processes to recently emerging observations of high-energy emissions associated with active thunderstorms are well understood. This research will apply state-of-the-art numerical simulation methods to investigate the emission of energetic electrons from lightning-related charge-streamer and stepped-leader discharges and the propagation of the associated energetic radiation (from X-rays to hard X-rays and terrestrial gamma-ray flashes) through the Earth's atmosphere. The only known mechanism capable of explaining generation of associated high-energy electrons is the "runaway process" by which energy gained from an imposed electric field exceeds that lost to collisions, thus accelerating electrons to very high energies (>1 MeV). To-date, two scenarios for initially "seeding" this runaway breakdown process have been proposed: (1) cosmic rays, and (2) "thermal" runaway induced by extremely high electric fields. Theoretical and experimental studies have shown the paramount importance of the coupling of streamer and leader discharge dynamics to the thermal runaway process, which is in-turn now thought to be inseparable from emission of X-rays by lightning or production of terrestrial gamma-ray flashes. Targeted observations of high-energy radiative emissions by thunderstorms and associated electrical activity have recently been obtained from ground-based, airborne and satellite platforms, but the detailed nature of relationships among these previously uncorrelated phenomena have not previously been quantified as this project sets out to do.
The Intellectual Merit of this activity is defined by efforts to develop a unifying theoretically-sound explanation for the location, timing and specific nature of observed high-energy radiation originating in the Earth's atmosphere. The importance of related questions is emphasized by the need to quantify the intensity of radiation emitted from lightning and to understand its relationship to the formation and propagation of lightning channels, and extends to the possibility of adverse health impacts for crewmembers and passengers of aircraft transiting regions of strong thunderstorm activity. Broader impacts of this effort will come through development of an interdisciplinary research program, via graduate and undergraduate student education and training, and through dissemination of results using illuminating graphical methods.
