The influence of an inclined rotation axis on solar irradiance variations

Compared with Sun-like stars, the irradiance variations of the Sun over the solar cycle appear to be relatively small for its average activity level ( Lockwood et al. 1992; Radick et al. 1998). It has been proposed that the sp ecial position of Earth-based observers in the ecliptic plane may give the impression of a subdued solar photometric variability (Schatten 1993). The aim of the present paper is to examine the influence on irradiance variatio ns of a solar rotation axis inclined towards the observer. A three-componen t model is used to calculate relative flux variations of a given active-reg ion distribution on the surface of the Sun as a function of inclination and wavelength. Wavelength-dependent intensity spectra are used to describe th e contributions of the undisturbed photosphere, sunspots and faculae. The s pectra result from models that have successfully been used to reproduce a h ost of solar data and thus represent realistic estimates of the radiative o utput from these solar features. We find that an inclined rotation axis inc reases the total solar irradiance variations maximally by 40%. The most pro bable value is approximately 6%. This is much less than that suggested by f ormer studies, which were based on simple contrast functions. In the averag ed Stromgren filters we estimate a most probable increase of the solar vari ability of 30%. In addition, we estimate the dependence of the flux in the chromospheric Ca II H&K lines on inclination. We find that the average chro mospheric activity level depends only slightly on the inclination angle. Th e chromospheric variability of Sun-like stars, however, is significantly af fected. Nonetheless, our results indicate that a different average inclinat ion of stellar rotation axes relative to the observer cannot explain the di screpancy between the brightness variations of the Sun and Sun-like stars.