Description
(Text)
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 Earths 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 Suns radiation received per unit area and time at the top of the Earths atmosphere and is thus directly related to the Earths 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 1223, i.e. the period for which a high-quality sunspot area and position record is available.
(Extract)
Of all the stars in the Universe, the Sun occupies a vital position for us, as it is the closest star to our planet Earth. It has been long observed and studied. Already by the year 800 B.C. the Chinese had their eyes on the Sun, and have the first written record of an observed sunspot. However, it was not until the time of Galileo Galilei and Christoph Scheiner at the beginning of the 17th century when the first telescopic observations of sunspots began. With these observations also came the notion that the sunspots are actually features on the solar surface and a continuous record of the sunspot number exists since then. Later on in 1843, Schwabe published the first work arguing that the sunspots on the solar surface varied in number with a period of approximately 11 years. Nowadays, the modern space instrumentation has proved the Sun to vary on even shorter time scales down to minutes through phenomena such as flares, coronal mass ejections and other transient events, the 5 minute oscillations (p-modes), or the changing granulation patterns. On the other hand, other proxies like cosmogenic isotope concentrations have implied variations of solar activity on even longer time scales up to centuries and millennia. 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.
