The Principal Investigator will theoretically investigate thermal runaway breakdown of air and the associated energetic radiation from X-rays to hard X-rays/lower-energy gamma rays. Energetic radiation originating in the Earth's atmosphere has been recently detected from both space and ground based platforms, and intensive research efforts have been carried out in order to investigate its origin. The X-ray and gamma ray emissions are believed to originate from interactions between high-energy electrons and atomic particles through bremsstrahlung. The only existing mechanism capable of explaining the generation of these high-energy electrons in air is electron runaway. This phenomenon is a result of a decreasing probability of electron interactions with atomic particles for electrons with energies in the range from about 100 eV to about 1 MeV. For a high-energy electron, it is possible that the electron gains more energy from an external electric field than it loses to collisions, and continues to accelerate to very high (>1 MeV) energies. Thus far two scenarios of runaway breakdown have been proposed: runaway breakdown initiated by relativistic runaway electrons generated by cosmic rays, and initiated by thermal runaway electrons (cold electrons accelerated to high energies by extremely large electric fields).
The initial research work to explain the observed X-ray and gamma ray emissions focused on studies of relativistic runaway electron avalanche and the resulting breakdown of air. However, recent studies show that relativistic runaway electron theory has difficulty in explaining some features of observations, for example, the spectrum of X-ray and gamma ray emissions and the correlation between the X-ray bursts with the stepping processes of lightning leaders.
A critical condition for the occurrence of thermal runaway breakdown is an extremely large field sustained over a finite space during a relatively extended time period. This condition was a major obstacle for applying the thermal runaway breakdown to interpretation of the observed energetic emissions during early research work. However, recent progress in understanding of streamer discharges predicts possible realization of this condition during this type of discharge. It is the purpose of the research to investigate the runaway breakdown of air initiated by thermal runaway electrons and quantify the associated energetic emissions.
Intellectual merit of the research is defined by the need to develop a theory to explain the energetic radiation originating in the Earth's atmosphere. To advance current understanding of thermal runaway breakdown of air and its induced energetic emissions, three outstanding scientific questions will be addressed: (1) What is the electric field configuration during a lightning leader stepping process? (2) How does the thermal runaway breakdown initiate and develop? What is the energy distribution of the resulting runaway electrons? (3) What are the emissions from the thermal runaway breakdown of air, and how do these emissions link to the recently-discovered X-ray and gamma-ray flashes associated with thunderstorm activity? The importance of addressing these questions is underscored by the need to understand the role of thermal runaway breakdown of air in the production of observed X-ray and gamma-ray emissions, and in the formation and propagation of lightning. This study is important for interpreting recent observations and experiments on lightning discharges, and for establishing of the exact physical mechanism of the energetic radiation originating from the Earth's atmosphere.
Broader impacts of the research include: (1) the development of interdisciplinary research program involving studies relevant to both conventional and runaway breakdown theories, and energetic radiation generated in electron-atom interactions, (2) the education and training of a graduate student, (3) the support and further training of a post-doctoral scholar, (4) the involvement of undergraduate students in research activities during summertime periods, (5) the dissemination of results in the form of refereed publications and conference presentations.
Terrestrial gamma-ray flashes are bursts of high-energy photons (energies up to 100 mega electron volts have been reported to date) originating from the Earth's atmosphere in association with thunderstorm activity. Terrestrial gamma-ray flashes were serendipitously discovered by BATSE detector aboard the Compton Gamma-Ray Observatory originally launched to perform observations of celestial gamma-ray sources. More recently, these events have also been detected by the RHESSI satellite, the AGILE satellite, and the Fermi Gamma-ray Space Telescope. Moreover, electromagnetic remote sensing measurements have correlated terrestrial gamma-ray flashes with initial development stages of normal polarity intra-cloud lightning that transports negative charge upward. In this project we developed Monte Carlo models of electron acceleration (from several electron volts to hundreds of mega electron volts) and their transport in air under influence of strong electric fields typically realized in tips of lightning leaders. We applied these models to study the photon spectra at low-orbit satellite altitudes associated with energetic electrons produced during the negative corona flashes of stepping negative leaders. We have discovered that the photon spectrum, the photon fluence (i.e., number of photons crossing unit area of the detector) and photon beam geometry, produced by acceleration of thermal electrons by high-potential lightning leaders during stepping processes can explain the properties terrestrial gamma-ray flashes observed from satellites. This discovery defines a quantitative link between terrestrial gamma ray flashes and the physical properties of causative lightning discharges. It is therefore possible to use the measured terrestrial gamma ray flash characteristics as a diagnostic tool to analyze lightning discharges.
