Analysis of Supergranulation Pattern: the Void Probability Function of Solar Magnetograms

The appearance of large scale magnetic fields on the non-active solar surface is dominated by the so-called magnetic network. In full disk images, the reticulated pattern of such a network outlines the boundaries of the supergranular cells and, presumably, it results from the organization of tiny magnetic elements caused by horizontal convective flows of photospheric plasma. The magnetic network, when observed at high spatial resolution, reveals its spatially intermittent nature. As a matter of fact, the reticulated pattern is substituted by a collection of, more or less, aligned or clustered magnetic features producing a highly branched and fractal pattern embodying isolated magnetic elements. Recent spectropolarimetric observations, from space and ground-based telescopes, revealed the presence in solar magnetograms of multiscale underdense magnetic regions, commonly called voids, which may be considered a relevant signature of the processes occurring on the surface of our star. A void searching algorithm is used to study the statistical properties of such voids as observed in MDI and HINODE magnetograms. A clear discrepancy is observed between distributions of voids diameters for solar magnetograms and for a pseudo-random distribution having comparable particle densities.