Transport of Ionizing Radiation in Terrestrial-like Exoplanet Atmospheres

(Abridged) Propagation of ionizing radiation, as from parent star flares, supernovae, or gamma-ray bursts, is studied for a suite of simple model atmospheres of terrestrial-like exoplanets covering a large range of column densities and incident photon energies. We developed a Monte Carlo code to treat the Compton scattering and photoabsorption. Atmospheres thinner than about 100 g cm^-2 transmit a significant fraction of incident gamma-rays, but even the thinnest atmospheres are essentially opaque to X-rays below about 30 keV. For thicker atmospheres, the incident ionizing radiation is efficiently blocked, but most of the incident energy is redistributed via secondary electron excitation into diffuse UV and visible aurora-like emission, increasing the atmospheric transmission by many orders of magnitude; in some cases the transmission can be up to 10%, depending on the intervening UV opacity. For Earth, between 2 x 10^-3 and 4 x 10^-2 of the incident flux reaches the ground in the 200-320 nm range, depending on the presence of the current ozone shield. We discuss effects of angle of incidence, quenching of reemission by collisional deexcitation, and the creation of transient secondary ionospheres. Comparison with flare statistics suggests that frequent fluctuations in the UV flux received at the surface can be produced by stellar flares, supernovae, and gamma-ray bursts, especially for planets whose parent stars are less massive and/or younger than the Sun