Statistical properties of magnetic structures: Their dependence on scale and solar activity

Abstract

A long data set of MDI/SOHO full-disk magnetograms is analyzed in order to provide a large sample of network features as well as active regions associated with their magnetic properties (magnetic flux and magnetic flux density for example). The main objective of the paper is to study the variations of these properties along the solar cycle, and to compare the behavior of the magnetic features at various scales (covering three orders of magnitude in size). I also investigate the dependence of these properties on the activity level in their neighborhood and in the context of their global configuration on the solar disk. This study is important for a better understanding of the emergence and decay of active regions, as well as of the role of the network in the removal of the magnetic flux and in solar irradiance variations. The main results are as follows. (i) I observe a characteristic scale in the range 400–800 Mm2 (close to the supergranular scale) below which the size and magnetic flux distributions are power laws down to 40 Mm2 (corresponding to the range –1021 Mx). This scale increases from solar minimum to solar maximum. The shape of the distributions also suggests a strong similarity between small active regions (below the supergranular scale) and network patches. (ii) The North-South asymmetry in the number of structures, compatible with easier-to-detect leading structures in most cases, surprisingly exhibits a sign reversal for the weakest structures, suggesting a different origin, while the East-West asymmetry is compatible with a predominantly eastward inclination of magnetic flux tubes, including network structures and not only for emerging active regions as suggested by models. (iii) The dependence of the magnetic characterictics on the size of the structures is found to be variable with the activity level, the magnetic flux beeing more concentrated at solar maximum (especially for small features). Small-scale properties are also dependent on the proximity of an active region. (iv) Latitudinal distributions and flux variations along the solar cycle suggests a significant role of the meridional flows in the properties of the very small network features as well as an influence of supergranulation properties. (v) The variations of the magnetic properties of network elements and active regions with time are strongly size-dependent. The results also show a correlation between the number (or magnetic flux) of network structures and the sunspot cycle, with an amplitude larger than expected from previous results.