Comparison of CCD astrolabe multi-site solar diameter observations

¹ GEA - Observatório Nacional/MCT e Observatório do Valongo/UFRJ, Ladeira Pedro Antônio 43, Rio de Janeiro, Brasil e-mail: oat1@ov.ufrj.br
² Observatório do Valongo/UFRJ, Brasil
³ Tubitak National Observatory, Turkey
⁴ Observatoire de Paris, France
⁵ Dptm. GEMINI UMR 6203-Observatoire de la Côte d'Azur, France
⁶ Akdeniz University, Turkey
⁷ Observatório Nacional/MCT, Brasil
⁸ Pulkovo Observatory, Russia

Received: 21 May 2004
Accepted: 5 July 2004

Abstract

Results are presented of measured variations of the photospheric solar diameter, as concurrently observed at three sites of the R2S3 (Réseau de Suivi au Sol du Rayon Solaire) consortium in 2001. Important solar flux variations appeared in that year, just after the maximum of solar activity cycle 23, make that time stretch particularly promising for a comparison of the multi-site results. The sites are those in Turkey, France and Brasil. All observations are made with similar CCD solar astrolabes, and at nearby effective wavelengths. The data reductions share algorithms, that are alike, the outcomes of which are here treated after applying a normalization correction using the Fried parameter. Since the sites are geographically quite far, atmospheric conditions are dismissed as possible causes of the large common trend found.
Owing to particularities of each site, the common continuous observational period extends from April to September. The standard deviation for the daily averages is close to 047 for the three sites. Accordingly, the three series are smoothed by a low-pass-band Fourier filter of 150 observations (typically one month). The main common features found are a declining linear trend, of the order of 0.7 mas/day, and a relative maximum, around MJD 2120, of the order of 100 mas. Standard statistical tests endorse the correlation of the three series.