Astron. Astrophys. 319, 673-682 (1997)

2. Observational data and tracking procedure

Our study deals with faculae displayed on the Meudon spectroheliograms (1.5 Å off the Ca II K line center with a 0.14 Å passband), which show faculae and sunspots at the photospheric level. The height of formation of is approximately 500 km (Vernazza et al. 1981). This is close to the height of about 425 km for a 1000-gauss field and of 300 km for a 2000-gauss field above the = 1 level, which are commonly used values (Nesme-Ribes et al. 1996a).

spectroheliograms have been digitized with 1.8 arcsec pixel size, which is the average spatial resolution obtained at Meudon. The solar radius is determined as follows. An approximated limb is first detected with a fixed brightness threshold. Then an inflection point is found by interpolating the radial intensity profile at each point of the approximated limb, yealding a better definition of the limb. Inherent scanning distortions are corrected (Mein & Ribes 1990) using the resulting radius versus polar angle: spectroheliograms are selected whose radius dispersion is less than one pixel.

The procedure for tracking the bright points forming a facula was described in Collin et al. (1995). We visually associate if possible each facular point present in a given image with the point corresponding to it in the following image. From this, we get the rotation rate and meridional motion for each facular point. Each bright point selected has an average size of 2 to 6 arcseconds. The number of points selected within a given facula depends on the facula size, its compactness, and the seeing conditions. This differs from studies in which the rotation is determined by the displacement of the faculae barycenter (Belvedere et al. 1977). In practice, our selection applies to well-defined faculae, or parts of faculae only, and discards the rest. The rotation rate corresponding to a given facula contains an additional component due to the time variation of the facula. We could identify about 160 points per image pair without ambiguity when the Sun was active. As we restricted our study to high-quality images, an average of 66 pairs were selected each year. A total of 47434 points were tracked over the period 1957-1964.

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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