Scaling laws for magnetic fields on the quiet Sun

Abstract

The Sun's magnetic field is structured over a range of scales that span approximately seven orders of magnitudes, four of which lie beyond the resolving power of current telescopes. Here we have used a Hinode SOT/SP deep mode data set for the quiet-Sun disk center in combination with constraints from the Hanle effect to derive scaling laws that describe how the magnetic structuring varies from the resolved scales down to the magnetic diffusion limit, where the field ceases to be frozen-in. The focus of the analysis is a derivation of the magnetic energy spectrum, but we also discuss the scale dependence of the probability density function for the flux densities and the role of the cancellation function for the average unsigned flux density. Analysis of the Hinode data set with the line-ratio method reveals a collapsed flux population in the form of flux tubes with a size distribution that is peaked in the 10-100 km range. Magnetic energy is injected into this scale range by the instability mechanism of flux tube collapse, which is driven by the external gas pressure in the superadiabatic region at the top of the convection zone. This elevates the magnetic energy spectrum just beyond the telescope resolution limit. Flux tube decay feeds an inertial range that cascades down the scale spectrum to the magnetic diffusion limit, and which contains the tangled, "hidden" flux that is known to exist from observations of the Hanle effect. The observational constraints demand that the total magnetic energy in the hidden flux must be of the same order as the total energy in the kG flux tubes. Both the flux tubes and the hidden flux are found to be preferentially located in the intergranular lanes, which is to be expected since they are physically related. © 2012 ESO.