Abstract:
Long-term synoptic observations in the resonance line of Ca II K constitute a fundamental database for a variety of retrospective analyses of the state of the solar magnetism.
Synoptic Ca II K observations began in late 1904 at the Kodaikanal Observatory in India. In
the early 1970s, the National Solar Observatory (NSO) at Sacramento Peak (USA) started a
new program of daily Sun-as-a-star observations in the Ca II K line. Today the NSO is continuing these observations through its Synoptic Optical Long-term Investigations of the Sun
(SOLIS) facility. These different data sets can be combined into a single disk-integrated Ca
II K index time series that describes the average properties of the chromospheric emission
over several solar cycles. We present such a Ca II K composite and discuss its correlation
with the new entirely revised sunspot number data series. For this preliminary investigation,
the scaling factor between pairs of time series was determined assuming a simple linear
model for the relationship between the monthly mean values during the duration of overlapping observations.
