Abstract: A variety of processes in the Earth’s atmosphere generate X-rays and even gamma-rays. In the troposphere, the initial development of lightning discharges, known as leaders, emit X-rays and sometimes gamma-rays depending on the altitude and orientation of the leader. X-rays emitted by lightning leaders can be detected on the ground, and gamma-rays are now regularly detected on low-Earth-orbit (LEO) spacecraft. In the upper layers of the atmosphere, X- and gamma-rays are produced by energetic particles (electrons and protons) colliding with the atmospheric constituents. The energetic particles can include solar energetic particles (SEP), galactic cosmic rays (GCR), and radiation belt electrons and protons. The physics that produces X- and gamma-rays from these different sources is the same; but the resulting photon spectra, and their propagation in the atmosphere, can be very different. These photons provide a unique method of indirectly measuring the effects of the particles themselves on the atmosphere.

In this talk I will describe each of these sources of X- and gamma-rays, presenting an overview of naturally-occurring high-energy photons in Earth’s atmosphere. I will describe X-ray and gamma-ray production in lightning leaders, including recent observations of X-ray images on the ground. The majority of the talk will focus on modeling X- and gamma-ray production in the upper atmosphere due to energetic particle precipitation. We use a Monte Carlo model of electron and photon production in the atmosphere, together with a model of bremsstrahlung radiation production, to calculate X-ray fluxes and spectra resulting from different precipitating particle spectra. These modeling results can be used to relate measured X-ray fluxes and spectra, either on high-altitude balloons or LEO spacecraft, back to the precipitating particle distributions.