Solar Mid-Term Periodicities

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The investigations in this thesis are motivated by the discovery of a quasi- biennial fluctuation of the sunspot umbra-penumbra area ratio (U/P) in the Debrecen Photoheliographic Data (DPD), a detailed catalogue of sunspot activity. A summary of the phenomenon and the analyses of its periodic behaviour are followed by a presentation of some ideas of possible physical interpretations. The emphasis of the thesis lies in a wide survey of solar mid-term periodicities and their possible connections to the U/P fluctuation. When relevant data have been accessible, the periodic behaviours have been explored by filtering methods and spectral analyses. Several separate quantitative comparisons to the U/P fluctuation are given, illustrated by composite figures. No firm conclusion about the origin of the U/P fluctuation can be drawn at present, but several interesting similarities of the phenomenon and other mid-term periodicities...