Response of Atomic Hydrogen and Oxygen to Solar Radiation Changes: Measurements and Simulations

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The scientific objective of the next part of the HYDOX project is to distinguish between the direct (photochemical) and indirect (dynamical) solar cycle effects in the upper mesosphere and lower thermosphere (UMLT). Our tools are global fields of atomic oxygen, hydrogen, and chemical heating rates, and the NCAR ROSE general circulation model. During the first part of the project, we derived these fields from measurements of the Sciamachy and GOMOS instruments on board the Envisat satellite. We identified clear solar cycle dependencies and performed first simulations of the data by means of the ROSE model. The discrimination of photochemical and dynamical solar cycle effects in the data by means of a model requires the best possible representation of the lower atmosphere, because the dynamics in the UMLT region is highly non-local. Therefore, we propose to take active part in the advancement of the ROSE model recently started at NCAR to improve the tropospheric forcing of the model. This will give a better representation of solar tides, which dominate atmospheric variability in the UMLT region. We plan to perform several model sensitivity studies with the improved version of the model to study the role of solar forcing for tidal sources and wave filtering. The simulations will be compared to the measured data to discriminate the dynamical effects from the photochemical solar forcing in the UMLT region.