Analysis and modeling of solar irradiance variations

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A prominent manifestation of the solar dynamo is the 11-year activity cycle, evident in indicators of solar activity, including solar irradiance. Although a relationship between solar activity and the brightness of the Sun had long been suspected, it was only directly observed after regular satellite measurements became available with the launch of Nimbus-7 in 1978. The measurement of solar irradiance from space is accompanied by the development of models aimed at describing the apparent variability by the intensity excess/deficit effected by magnetic structures in the photosphere. The more sophisticated models, termed semiempirical, rely on the intensity spectra of photospheric magnetic structures generated with radiative transfer codes from semiempirical model atmospheres. An established example of such models is SATIRE-S (Spectral And Total Irradiance Reconstruction for the Satellite era). One key limitation of current semiempirical models is the fact that the radiant properties of network and faculae are not adequately represented due to the use of plane-parallel model atmospheres (as opposed to threedimensional model atmospheres). This thesis is the compilation of four publications, detailing the results of investigations aimed at setting the groundwork necessary for the eventual introduction of three-dimensional atmospheres into SATIRE-S and a review of the current state of the measurement and modelling of solar irradiance. Also presented is an update of the SATIRE-S model. We generated a daily reconstruction of total and spectral solar irradiance, covering 1974 to the present, that is more reliable and, in most cases, extended than similar reconstructions from contemporary models.