The centennial evolution of geomagnetic activity and its driving mechanisms This cumulative thesis is concerned with the evolution of geomagnetic activity since the beginning of the 20th century that is the time dependent response of the geomagnetic field to solar forcing. The focus lies on the description of the magnetospheric response field at ground level which is particularly sensitive to the ring current system and an interpretation of its variability in terms of the solar wind driving. Thereby this work contributes to a comprehensive understanding of long term solar terrestrial interactions. The common basis of the presented publications is formed by a reanalysis of vector magnetic field measurements from geomagnetic observatories located at low and middle geomagnetic latitudes. In the first two studies new ring current targeting geomagnetic activity indices are derived the Annual and Hourly Magnetospheric Currents indices (A HMC). Compared to existing indices (e.g. the Dst index) they do not only extend the covered period by at least three solar cycles but also constitute a qualitative ...