The influence of an inclined rotation axis on solar irradiance variations

Citation
R. Knaack et al., The influence of an inclined rotation axis on solar irradiance variations, ASTRON ASTR, 376(3), 2001, pp. 1080-1089
Citations number
25
Categorie Soggetti
Space Sciences
Journal title
ASTRONOMY & ASTROPHYSICS
ISSN journal
14320746 → ACNP
Volume
376
Issue
3
Year of publication
2001
Pages
1080 - 1089
Database
ISI
SICI code
1432-0746(200109)376:3<1080:TIOAIR>2.0.ZU;2-9
Abstract
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.