Solar activity has been observed to vary at all measured time scales, of which the most prominent feature is the 11-year solar cycle. The latter was first noticed in the sunspot number, but is actually shown by almost all solar variables. Variations of the solar activity are of high importance for the Earth’s climate and thus the prediction of past and future solar activity is key for climate studies. In this dissertation both aspects are studied, where a new proxy for the prediction of future solar activity and a method to reconstruct irradiance in the past are presented.
A record of sunspot group areas, positions, and tilt angles covering almost 7 solar cycles has permitted us to analyse cycle-to-cycle variations of the sunspot group tilt angles and to compare these with global parameters of the solar cycle, namely the length, strength, and amplitude. This analysis has shown the potential of the sunspot group tilt angles to forecast the amplitude of upcoming solar cycles.
Another important measure of solar activity is the total solar irradiance (TSI). TSI is the total energy in the Sun’s radiation received per unit area and time at the top of the Earth’s atmosphere and is thus directly related to the Earth’s climate. It has only been measured for the last 33 years. Models of the TSI assuming that its variations are due to changes in the solar surface magnetic fields have been remarkably successful in reproducing the observed changes. For comparisons with climate records, longer term reconstructions are, however, needed. Therefore we have reconstructed TSI back to 1878. Our TSI reconstructions thus cover cycles 12–23, i.e. the period for which a high-quality sunspot area and position record is available. To describe the evolution of the solar surface magnetic fields we use a surface flux transport model which calculates daily synthetic full-disc magnetograms at the solar surface starting from sunspot positions and areas. These synthetic data are then used as input data in the SATIRE-S (Spectral And Total Irradiance REconstructions for the Satellite era) model to reconstruct the TSI.