| Summary: | The radiation exposure in the lower atmosphere caused by extraterrestrial sources is increasing with altitude. The origin of this phenomenon is high energy particles penetrating the atmosphere and producing secondary particles in interactions with atoms in the air. The main sources of the primary radiation are atomic nuclei contained in the galactic cosmic rays arriving at Earth from outside the solar system after traversing the heliosphere. The galactic cosmic rays are subject to a quasi-periodic modulation leading to maximum intensity during solar minimum and minimum intensity during solar maximum. Additionally, during relatively short time periods particles may be accelerated to relativistic energies during solar events causing increases in the particle intensities on ground which are measurable by Neutron Monitor stations in so-called Ground Level Enhancements (GLE). Investigating such GLEs based on Neutron Monitor data and the related increase of the radiation exposure in the atmosphere and especially at aircraft altitudes is the main goal of the present work. For this purpose detailed calculations of the transport of relevant primary particles through the Earth's magnetosphere and their interactions with the atmosphere based on a Monte-Carlo technique were performed, and the resulting secondary particle fluxes and radiation exposures were determined. In order to verify the results of these calculations the secondary particle intensities induced by galactic cosmic rays were calculated and detailed comparisons with existing measurements were performed. Contrary to the isotropically incident galactic cosmic rays, the solar energetic particles posses variable angular distributions. The deviation of the charged primary particles in the magnetosphere of the Earth leads to a sensitivity of the Neutron Monitors to the incoming direction of the solar particles. Additionally, the comparison of the count rate increases during a GLE of Neutron Monitor stations at different locations provides information about the energy spectrum of the primary particles. These properties were exploited to determine the temporal evolution of the primary particle spectrum, the angular distribution and the incoming direction of the solar particles. This provided important insights into the anisotropy and the temporal variation of the influence of the incoming particles on the radiation environment in the atmosphere. Based on these results the secondary particle uxes at typical aircraft altitudes at 9 km to 12 km above sea-level and the related increase of the radiation exposure at arbitrary positions and times were determined. The anisotropic angular distribution which is present during the initial phase of the solar events resulted in a strong dependency of the radiation exposure on the geographic position. Additionally, the effective doses for typical north Atlantic and polar ights were calculated and increases of up to several hundred percent depending on the strength of the event were estimated.
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