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Abstract

Energetic particles enter Earth’s atmosphere at the poles. The charged particles are either from solar or magnetospheric origin and alter the chemistry of the middle and upper atmosphere. Most importantly, they enhance the production of nitrogen oxides (NOx) and hydrogen oxides (HOx) in the winter mesosphere and lower thermosphere. Both components are powerful ozone destroyers. The impact of HOx on ozone is limited to the mesosphere, because HOx has a short chemical lifetime (up to hours). In contrast, NOx can persist up to several months in the winter polar middle atmosphere and can be transported downward to the stratosphere. Models covering the middle and upper atmosphere underestimate this downward transport. This may lead to an underestimation of potential climate effects from energetic particle precipitation. This thesis investigates the polar winter transport from the lower thermosphere to the stratosphere. Several observational studies confirmed the downward transport (e.g., Randall et al. 2009; Semeniuk et al. 2005). However, it remains unclear which processes cause the transport from the lower thermosphere to the mesosphere. This thesis quantifies, for the first time, the contribution of advection, eddy diffusion and molecular diffusion for the transport through the mesopause. Advection and molecular diffusion dominate the transport through the mesopause. Eddy diffusion has a negligible impact on the transport. However, if eddy diffusion is enhanced as suggested by observations, it can significantly contribute to the transport. This leaves advection being responsible for the underestimation of the downward transport. Gravity waves are the key driver for the advective downwelling in the polar winter mesosphere. This thesis shows that weakening gravity waves enhances the mesospheric transport bringing it close to satellite observations. The altitude of the mesospheric momentum deposition is identified to be key for the polar downwelling. In addition to the analysis of the winter polar downward transport, climate effects of energetic particles are studied. Energetic particle precipitation reduces significantly ozone in the mesosphere and stratosphere. An ozone loss potentially influences the atmospheric temperature and the strength of the polar vortex. It has been shown that large variations in the polar vortex strength can propagate from the stratosphere down to the surface and force the surface temperature (Baldwin and Dunkerton 2001). This thesis presents the climate impact of a mesospheric and of a stratospheric ozone loss. No statistically significant changes in atmospheric winds are found neither for a mesospheric ozone loss nor for a stratospheric ozone loss. Hence, the influence of energetic particles is too weak to force significantly changes in the surface temperature. In summary, this thesis advances the understanding of energetic particle precipitation. Processes relevant for the winter polar downward transport from the lower thermosphere to the stratosphere are identified. Two novel findings are the importance of advection in the thermosphere and the impact of weaker gravity waves on the dynamics of the middle and upper atmosphere. Based on this thesis, large climate effects of energetic particles seem unlikely.