Variations of the granulation related to the solar cycle and with respect to its position on the solar disk

Abstract

Aims.We investigate variations in the scale and contrast of the solar granulation related to the solar cycle during the period 1978-1993. Furthermore, as a by-product, we have detected a variation with respect to the solar longitude, along the solar equator. Methods: All images were taken on film with the 50 cm refractor of the Pic du Midi Observatory, under excellent seeing conditions. Scale and contrast were derived from power spectra computed with digitized images. This simple and robust statistical method allowed us to get reliable results, independent of any image-processing parameter. Results: The contrast of the solar granulation varies nearly in phase with the solar cycle, being smaller at the periods of solar maximum. But we detected no corresponding variation in the scale; if there is one, it must be of low amplitude and masked by a spatial variation in the scale with respect to the position on the solar equator, which amounts to 3% rms. The contrast also varies with the position on the equator, in phase with the scale: where the scale is larger, the contrast is higher too. The amplitude of this spatial variation in the granulation scale is 9%; large photometric uncertainties did not allow us to quantify the amplitude of the contrast variation.