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Comparison of Terrestrial Gamma Ray Flash Simulations with Observations by Fermi

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Technical Report,05 Aug 2015,04 Aug 2016

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University of New Hampshire Durham United States

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For this project Monte Carlo simulations of terrestrial gamma rays flashes TGFs were preformed and results compared with spacecraft observations, including results from Fermi, and other simulations. As a result of this work, a paper has been written and will be submitted to the Journal of Geophysical Research-Space Dwyer et al. 2016. For this paper, Monte Carlo simulations are used to determine source properties of terrestrial gamma-ray flashes TGFs as a function of atmospheric column depth and beaming geometry. The total mass per unit area traversed by all the runaway elections i.e., the total grammage during a TGF, Xi, is introduced, defined to be the total distance traveled by all the runaway electrons along the electric field lines multiplied by the local air mass density along their paths. It is shown that key properties of TGFs may be directly calculated from Xi and its time derivative, including the gamma-ray emission rate, the current moment, and the optical power of the TGF. For the calculations presented in this paper, the standard TGF gamma-ray fluence F0 0.1 cm -2 for a spacecraft altitude of 500 km and the standard total grammage Xi0 1018 gcm2 are introduced, and results are presented in terms of these values. In particular, the current moments caused by the runaway electrons and their accompanying ionization are found for a standard TGF fluence, as a function of source altitude and beaming geometry, allowing a direction comparison between the gamma rays measured in low-Earth orbit and the VLF-LF radio frequency emissions recorded on the ground. Such comparisons should help test and constrain TGF models and help identify the roles of lightning leaders and streamers in the production of TGFs.

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  • Atmospheric Physics

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