Past Levels of Solar Activity and Irradiance
S.K. Solanki [email@example.com], Max-Planck-Institute for Solar System Research, Katlenburg-Lindau, Germany.Direct observations of solar activity (as indicated by the number of sunpots on the solar surface) have been carried out since 1610. These show a cyclic variation having a period of roughly 11 years, but also longer term modulation of the amplitudes of these cycles. This record shows that the current period of strong activity is extraordinary. It also displays an example of the other extreme, with almost no sunspots during the Maunder minimum. The number of sunspots is not always the most useful measure, however, in particular when comparing with climate. Techniques are now available for computing the open and total magnetic flux starting from the sunspot number. In a further step the total and spectral solar irradiance can be determined from the open and total magnetic flux. Such results for the telescopic period are presented and discussed. The main disadvantage of the sunspot data set is its limited length. In order to extend this or other records of solar activity further back in time we need to make use of cosmogenic isotopes, such as 14C or 10Be. From the measured concentrations or computed production rates of these isotopes the sunspot number (and intermediate quantities, such as the solar modulation strength and strength of the open magnetic flux) is computed. The obtained results are discussed.