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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.
