Analysis of historical solar observations and long-term changes in solar irradiance

The Sun has the potential to affect Earth’s climate. However, the short time span covered by direct measurements of the solar irradiance makes any meaningful conclusions about solar influence on climate difficult. Models have been developed that reconstruct the past solar irradiance from the available long-term proxies of solar magnetic activity. The most widely used dataset is the sunspot number series. However, it only gives information on the dark regions on the solar disc. Another type of data that might provide information critical for past irradiance reconstructions are the Ca II K spectroheliograms. Until now they have not been used due to the various artefacts plaguing them. In this thesis we present a new method to automatically process and photometrically calibrate the historical Ca II K spectroheliograms. We also reassess the relation between the Ca II K brightness and the magnetic field strength. This opens prospects for these data to be used to derive information about the evolution of the solar magnetic field and the solar irradiance over the whole 20th century. We also recalibrate the historical record of the group sunspot number. In particular, we show that the assumption of the linear relation between records by different observers is not valid in general and produce a new compilation with a direct non-parametric and non-linear cross-calibration of the individual records.