Solar cycle dependence of the apparent radius of the Sun

Free access article

Issue		A&A Volume 413, Number 2, January II 2004


Page(s)		725 - 732
Section		Diffuse matter in space
DOI		http://dx.doi.org/10.1051/0004-6361:20031573

A&A 413, 725-732 (2004)
DOI: 10.1051/0004-6361:20031573

Solar cycle dependence of the apparent radius of the Sun

F. Noël

Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
(Received 19 May 2003 / Accepted 4 September 2003 )

Abstract
Visual astrometric observations of the Sun covering the second and first part of solar cycles 22 and 23 respectively, have been carried out during the last 13 years with the Danjon astrolabe of Santiago, Chile. These observations give, among other solar parameters, an absolute value of the Sun's apparent radius. We report here the results obtained from 4092 homogeneous radius measurements at $\mathrm{30^\circ}$ and $\mathrm{60^\circ}$ zenith distances. The data set shows at both zenith distances a significant radius variation in phase with magnetic activity. Moreover, the observations at $\mathrm{30^\circ}$ , which are less affected by atmospheric noise, give a significantly higher correlation coefficient between radius variation and sunspot numbers. Other investigations of solar radius variations during the last decades based on different observing techniques, as well as two analyses of historical data, are commented. Most of them show also positive correlations between radius variation and solar activity. With the noted exception of Calern, France, the results obtained at other astrolabe stations during recent years are in agreement with Santiago. The discrepancy between Calern and Santiago and its probable cause are discussed.

Key words: astrometry -- Sun: general

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