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. 2015 Dec 14;120(23):12143–12156. doi: 10.1002/2015JD023704

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©2015. The Authors.

This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made.

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The curves show the estimated number of initial photons, with energy above 1 MeV, needed in our MC simulation as a TGF to escape atmospheric attenuation as a function of atmospheric column density. The various curves are for weak TGFs (∼10 photons measured by RHESSI), average (∼25 photons), and bright (∼100 photons). A column mass density of 500 g/cm² corresponds to approximate 6 km altitude and a column mass density of 100 g/cm² corresponds to approximate 17 km. The number of initial photons is approximately proportional to exp(ρ _tot/45 g/cm²) from 250–100 g/cm².