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Long-term synoptic observations in the resonance line of Ca II K and
measurements of the solar magnetic flux over several decades constitute a fundamental database for a variety of
retrospective analyses of the state of the solar magnetism. These data archives may also hold the key for
untangling some of the mysteries behind the solar dynamo, which in turn could result in a better predictive
capability of current dynamo models. Synoptic Ca II K observations began in 1907 at the Kodaikanal observatory (India) and
in 1915 at the Mt Wilson (California, USA) solar observatory. By the early 1970s a number of synoptic programs for
solar magnetic observations were established that provided full disk magnetograms. These programs include
measurements carried out at the Mt Wilson 150-foot solar tower, Wilcox observatory (California, USA), and by the
National Solar Observatory (NSO, USA). Today the NSO is continuing these observations through its
Synoptic Optical Long-term Investigations of the Sun (SOLIS) facility. I will review some of these historical observations,
their properties, and their importance for understanding the behavior of the solar magnetic field over multidecadal time
scales. I will also show recent results about using Ca II K spectroheliograms and sunspot magnetic field measurements
to reconstruct homogeneous series of pseudo-magnetograms prior the magnetograph era. |
