Astron. Astrophys. 321, 323-329 (1997)

2. Observational data

Sunspots are tracked on the Meudon spectroheliograms (1.5 Å off the Ca II K line center), which display faculae and sunspots at the photospheric level, about 500 km above the level. The images have been digitized with 1.8 arcsec resolution, which corresponds to the average spatial resolution obtained at Meudon. Image processing and tracking procedures are described and discussed at length in Mein & Ribes (1990) and in Paper I. The random error on each sunspot velocity measurement is of the order of 15 m/s. The number of sunspots per hemisphere and per year, and for the whole period, is shown in Table 1.

Table 1. Sunspot number in the Meudon sample

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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