Photometric properties of facular features over the activity cycle

¹ INAF – Osservatorio astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italia e-mail: ermolli@oaroma.inaf.it
² Dipartimento di Fisica, Università di Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italia
³ High Altitude Observatory, 80307-3000 Boulder, CO, USA

Received: 7 July 2006
Accepted: 18 November 2006

Abstract

Aims.We have analyzed the contrast of facular features identified in a large dataset of PSPT full-disk photometric images and SoHO/MDI magnetograms, obtained from 1998 to 2005. The aim of this work is to contribute to the improvement of semi-empirical atmospheric models and of irradiance studies and to understand the reasons for the controversial results of facular contrast already presented in the literature.

Methods.We used different identification methods to analyze their effects upon the results obtained. We also analyzed the effects of the limited information content in the analyzed images.

Results.We show that selection effects associated with the identification method may produce significant differences in the results. The facular contrast is not only a function of both selection methods and the heliocentric angle, but also of feature size, activity level, and content of the analyzed images. Comparisons of the results obtained with computations of the most recent semi-empirical atmospheric models of facular features show that these models reproduce limb-angle corrected contrast measurements with an offset up to ≈1% from the disk center to μ = 0.3.