Abstract. Solar diameter observations are presented covering a period close to the maximum of the solar activity cycle 23, from July 1999 to December 2000, at the Observatório Nacional, Rio de Janeiro. The observations were made at λ = 5635Å, with a CCD Solar Astrolabe, thus timing the limb crossing through a fixed zenith distance. The formal precision of a single measurement is typically 0. 18. The results represent daily means from 4625 independent measurements. The series obtained has 225 homogeneously distributed points with a mean precision of 0. 12. The average value obtained for the solar semi-diameter within the analyzed period is 959. 01 ± 0. 02. The average of the observations, made under a wide range of different geometric, instrumental and environmental conditions, aims to provide results representing global variations of the solar radius. From this data, variations with amplitude equal to or larger than 0. 02 can be investigated. No statistically significant linear trend is obtained for the examined series. There is, however, some indication of an increasing trend for 1999 followed by a stabilization during 2000. Periodic variations are obtained with amplitudes between 0. 02 and 0. 07. Periods related to the solar rotation and the sunspot characteristic lifetime can be recognized.
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 0. 47 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.
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