11-Year cyclic variation of the solar granulation

A homogeneous (in terms of spatial resolution and optical set-up) set of images of the solar granulation, obtained regularly between 1978 and 1991 with the 50-cm refractor of the Pic du Midi Observatory, has been analyzed to seek for possible variations related with the solar activity cycle. A simple, but reliable and objective, statistical analysis was performed (autocorrelation and power spectrum), which provides results unaffected by any processing parameter. The main uncertainties come from the statistical variation of the granulation pattern in the analyzed area (70″ × 70″), from the film calibration and from the noise produced by film granularity. Despite these uncertainties, the solar granulation appears to vary in phase with the solar cycle as follow: the contrast decreases with increasing activity, while the horizontal scale seems to increase. The uncertainty of the film calibrations prevents us, at this point of our investigation, to quantity the amplitude of the contrast variation.