A homogeneous database of sunspot areas covering more than 130 years

Abstract

[1] The historical record of sunspot areas is a valuable and widely used proxy of solar activity and variability. The Royal Greenwich Observatory 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 significant gaps. Our goal is to obtain a uniform and complete sunspot area time series by combining different data sets. A homogeneous 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) are used between 1977 and 1985, and data compiled by the USAF network are used since 1986. Other data sets (Rome, Yunnan, and 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.

Sunspot Numbers and Areas from the Madrid Astronomical Observatory (1876 – 1986)

“…We shall now compare the MSA series with that constructed by Balmaceda et al (2009), the BSA series. To examine the reliability of the MSA, we made a linear leastsquares fit taking as dependent variable the monthly MSA and as independent variable the monthly BSA (Figure 11).…”

“…The following are the equations of the linear fits corresponding to the different periods (and the total period): Lastly, we calculated the scaling factor for each period using the "bisector line" method described by Balmaceda et al (2009) (Table 3). Thus, each value of the MSA series can be compared with its corresponding value in the BSA series (or even fill in the gaps in this latter series).…”

Sunspot group tilt angles and the strength of the solar cycle

“…In the present study we also use the sunspot area data set compiled by Balmaceda et al (2009) using observations from a number of different observatories that were carefully crosscalibrated in order to reduce, as much as possible, systematic and other differences between observations at different sites as well as the number of data gaps. The combined data set goes back to 1874.…”

Are the strengths of solar cycles determined by converging flows towards the activity belts?

“…(1) represents the emergence of magnetic flux at the surface in the form of bipolar magnetic regions appearing at the latitudes and longitudes of sunspot groups taken from the combined RGO/SOON sunspot record (Balmaceda et al 2009) at the time of their maximum surface area. The sunspot group size was converted to magnetic flux flux as described in Cameron et al (2010, and references therein), the calibration being based upon the total unsigned magnetic flux derived from the synoptic magnetic maps 1 taken with the Michelson Doppler Imager (MDI) instrument onboard of the Solar Heliospheric Observatory (SoHO) spacecraft.…”

“…For instance, the minimum area required for a sunspot group to be included changed from 1 millionth of a hemisphere in the RGO data to 10 millionths of a hemisphere in the SOON data. In order to account for this change, Balmaceda et al (2009) used other datasets overlapping both the RGO and SOON data to determine a correction factor of approximately 1.4 for the sunspot group areas in the SOON data (see also Wilson & Hathaway 2005). While a global correction factor might be adequate for the total sunspot areas, applying it uniformly to individual sunspot groups of all sizes (as used in the source term of SFT simulations) is hardly justified since it probably grossly overestimates the areas of the large sunspot groups.…”