Hard X-ray emission of the Earth’s atmosphere: Monte Carlo simulations

We perform Monte Carlo simulations of cosmic-ray induced hard X-ray emission in the Earth’s atmosphere. We find that the shape of the spectrum emergent from the atmosphere in the energy range ∼25–300 keV is determined by Compton scattering and photoabsorption and is virtually independent of the incident cosmic ray spectrum. We provide a fitting formula for the spectral intensity as a function of energy, solar modulation level, geomagnetic cutoff rigidity and zenith angle. A recent measurement of the atmospheric hard X-ray emission together with the cosmic X-ray background by the INTEGRAL observatory agrees with our prediction within 10%. This suggests a possibility of using Earth observations for in-orbit calibration of future hard X-ray telescopes. We also demonstrate that the hard X-ray spectra generated by cosmic rays in the crusts of the Moon, Mars and Mercury are expected to significantly differ from that emitted by the Earth’s atmosphere.