The historical record of sunspot areas is a valuable and widely used proxy of
solar activity and variability. The Royal Greenwich Observatory (RGO) regularly
measured this and other parameters between 1874 and 1976. After that time
records from a number of different observatories are available. These, however,
show systematic differences and often have significants gaps. Our goal is to
obtain a uniform and complete sunspot area time series by combining different
data sets. A homogeneus composite of sunspot areas is essential for different
applications in solar physics, among others for irradiance reconstructions.
Data recorded simultaneously at different observatories are statistically
compared in order to determine the intercalibration factors. Using these data
we compile a complete and cross-calibrated time series. The Greenwich data set
is used as a basis until 1976, the Russian data (a compilation of observations
made at stations in the former USSR) between 1977 and 1985 and data compiled by
the USAF network since 1986. Other data sets (Rome, Yunnan, Catania) are used
to fill up the remaining gaps. Using the final sunspot areas record the
Photometric Sunspot Index is calculated. We also show that the use of
uncalibrated sunspot areas data sets can seriously affect the estimate of
irradiance variations. Our analysis implies that there is no basis for the
claim that UV irradiance variations have a much smaller influence on climate
than total solar irradiance variations.Comment: 40 pages, 8 figures; JGR - Space Physics, publishe