Spectral analysis of solar diameter measurements recorded at Calern Observatory astrolabe during two solar cycles

¹ CRAAG - Observatoire d'Alger, BP 63, Bouzaréah, Alger, Algeria e-mail: irbah@unice.fr
² Université de Nice-Sophia Antipolis, UMR n 6525 Astrophysique, Parc Valrose, 06108 Nice Cedex 2, France
³ Observatoire de la Côte d'Azur, Département CERGA, Avenue Copernic, 06130 Grasse, France

Corresponding author: J. Borgnino, borgnino@unice.fr

Received: 31 January 2000
Accepted: 3 May 2001

Abstract

Solar diameter measurements performed with the Calern Observatory astrolabe (O.C.A - France) during more than two solar cycles show temporal variations. Due to the weather, seasonal effects and instrumental characteristics, recorded solar data are non uniformly sampled and present temporal gaps. Thus, to analyze these data, diameter measurements averaged over one or more months have generally been considered. This limits the accessible harmonic terms to a low frequency range. To determine short-term periodicities from the observed variations, all daily solar data need to be considered and also corrected from the zenithal distance. In the present work, we use two methods to analyze solar diameter measurements recorded at Calern Observatory astrolabe during the observation period 1975-1996. They are based on least square fits and deconvolution of the observation window function. Results deduced from the analysis confirm harmonic terms already found by other authors but also reveal new higher frequencies.