A&A 376, 1080-1089 (2001)
DOI: 10.1051/0004-6361:20011007

The influence of an inclined rotation axis on solar irradiance variations

R. Knaack¹, M. Fligge¹, S. K. Solanki² and Y. C. Unruh<sup>3

1</sup>  Institute of Astronomy, ETH-Zentrum, 8092 Zürich, Switzerland
    e-mail: rknaack@astro.phys.ethz.ch; fligge@astro.phys.ethz.ch
²  Max-Planck-Institut für Aeronomie, 37191 Katlenburg-Lindau, Germany
    e-mail: solanki@linmpi.mpg.de
³  Institute of Astronomy, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
    e-mail: ycu@astro.univie.ac.at

(Received 10 April 2001 / Accepted 5 July 2001 )

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 special 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 variations of a solar rotation axis inclined towards the observer. A three-component model is used to calculate relative flux variations of a given active-region distribution on the surface of the Sun as a function of inclination and wavelength. Wavelength-dependent intensity spectra are used to describe the contributions of the undisturbed photosphere, sunspots and faculae. The spectra result from models that have successfully been used to reproduce a host of solar data and thus represent realistic estimates of the radiative output from these solar features. We find that an inclined rotation axis increases the total solar irradiance variations maximally by . The most probable value is approximately . This is much less than that suggested by former studies, which were based on simple contrast functions. In the averaged Strömgren filters we estimate a most probable increase of the solar variability of . In addition, we estimate the dependence of the flux in the chromospheric Ca II H& K lines on inclination. We find that the average chromospheric activity level depends only slightly on the inclination angle. The chromospheric variability of Sun-like stars, however, is significantly affected. Nonetheless, our results indicate that a different average inclination of stellar rotation axes relative to the observer cannot explain the discrepancy between the brightness variations of the Sun and Sun-like stars.

Key words: Sun: activity -- Sun: sunspots -- Sun: faculae, plages -- Sun: photosphere -- stars: activity

Offprint request: R. Knaack, rknaack@astro.phys.ethz.ch

© ESO 2001

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

• If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
• You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
• You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.