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Astron. Astrophys. 332, 1082-1086 (1998)

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1. Introduction

Space-borne experiments reveal that solar brightness varies due to the evolution of the Sun's magnetic field (Willson et al. 1981; Eddy et al. 1982; Hudson et al. 1982; Willson & Hudson 1991; Lean 1997; Fröhlich et al. 1997). The variations result from the competing influence of dark and bright magnetic features, i.e. sunspots and faculae, respectively (e.g. Solanki & Unruh 1997).

Current reconstructions of the contributions of solar active phenomena to solar irradiance variations separately model sunspot dimming and facular brightening. Time-series of sunspot area measurements allow for a relatively accurate reconstruction of sunspot darkening on both, long (inter-cycle) and short (inner-cycle) time-scales by means of the photometric sunspot index (PSI; Foukal 1981; Hudson et al. 1982, Fröhlich et al. 1994).

Faculae cannot be dealt with in the same manner. On short time-scales, i.e. shorter than the solar cycle, global indices like 10.7cm radio flux or Ly [FORMULA] flux measurements correlate well with irradiance residuals corrected for sunspot dimming and provide a reliable measure of solar brightening due to faculae and enhanced network. However, long-term (inter-cycle) variations of solar brightening are much harder to track due to the absence of a reliable proxy available over a sufficiently long time span (Schatten et al. 1985; Pap et al. 1994).

Zürich relative sunspot number, [FORMULA], is often used to track facular brightening on time-scales of decades to centuries (Foukal & Lean 1990; Lean et al. 1995), whereby the relation between facular emission and sunspot number is assumed to remain the same from one cycle to the next. It has been shown, however, that on a time-scale of decades the relation between two other possible proxies of facular emission, namely sunspot areas and 10.7cm flux, and sunspot number is not constant (Fligge & Solanki 1997; henceforth referred to as Paper I). The quantitative relationship between [FORMULA] and other possible indices, such as Ca K plage areas, or the white light facular area measurements of the Royal Greenwich Observatory, has so far not been investigated. In the present paper we compare such relationships and attempt to construct a better long-term proxy of facular emission than each individual proxy by combining them in a suitable manner. The time appears ripe for such an analysis because digitized Ca plage areas obtained from Mt. Wilson plates have only recently become available (Foukal 1996). We stress that in this paper we are only interested in variations from one cycle to the next.

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© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998