Solar irradiance variations are thought to affect the Earth's climate system, although it is still controversial to what extent (Eddy 1977; Kelly & Wigley 1992). Current models of past solar irradiance on time-scales longer than the solar cycle generally use as a proxy of facular emission (Foukal & Lean 1990, Zhang et al. 1994, Lean et al. 1995). Our analysis, however, indicates that may not be the most appropriate parameter to track facular variations on time scales longer than the solar cycle. Other possible proxies of facular brightening show significant inter-cycle variations relative to . In particular, they all exhibit a prominent increase of facular emission during cycle 16 and to a lesser extent cycle 17. Consequently, the above models tend to underestimate the contribution of facular emission to total irradiance during the 1920s and 1930s.
This may have important consequences, since on time-scales longer than months, facular emission is the major contributor to irradiance variations over a solar cycle (Fröhlich & Pap 1989). In addition, it has been proposed that gradual brightness variations of the quiet Sun also follow a facular index (Zhang et al. 1994; Lean et al. 1995).
In this context it is interesting that lags the global land/sea surface temperature (as provided by the International Panel on Climate change IPCC), leading to a problem of causality between Earth's climate change and solar activity. Increased facular emission during cycle 16 (and 17) may conceivably contribute to resolving this dilemma. Detailed reconstructions of solar brightness over the past century, taking inter-cycle variations into account, are needed to answer this question in a satisfactory manner, however. This is because the contribution of active regions to total irradiance variations is dictated by a delicate balance between the darkening caused by sunspots and the brightening due to faculae. Any change in this balance can cause disproportionately large changes in irradiance. Reconstructions of solar irradiance variations taking our composite facular index into account are beyond the scope of this paper and are presented by Solanki & Fligge (1998).
Finally, we wish to point out that since all the proxies considered here only or mainly provide information on active regions, our final facular proxy is not a priori a direct indicator of the long-term behavior of the network or possible brightenings caused by intra-network magnetic fields. This proxy is also not sensitive to changes in the brightness of the quiet Sun due to, e.g., variations in convective properties. There is indirect evidence, however, that a facular proxy such as ours may track such quiet-Sun contributions to long-term irradiance variations (Zhang et al. 1994; Lean et al. 1995; Baliunas & Soon 1995).
© European Southern Observatory (ESO) 1998
Online publication: March 30, 1998