Dispersion of small magnetic elements inside active regions on the Sun

Abstract

A process of diffusion of small-scale magnetic elements inside four active regions (ARs) was analyzed. Line-of-sight magnetograms acquired by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamic Observatory (SDO) during a two-day time interval around the AR culmination time were utilized. Small magnetic elements of size of 3-100 squared HMI pixels with the field strength above the detection threshold of 30 Mx cm$^{-2}$ were detected and tracked. The turbulent diffusion coefficient was retrieved using the pair-separation technique. Comparison with the previously reported quiet-sun (QS) diffusivity was performed. It was found that: i) dispersion of small-scale magnetic elements inside the AR area occurs in the regime close to normal diffusion, whereas well-pronounced super-diffusion is observed in QS; ii) the diffusivity regime operating in an AR (the magnitude of the spectral index and the range of the diffusion coefficient) does not seem to depend on the individual properties of an AR, such as total unsigned magnetic flux, state of evolution, and flaring activity. We conclude that small-scale magnetic elements inside an AR do not represent an undisturbed photosphere, but they rather are intrinsic part of the whole coherent magnetic structure forming an active region. Moreover, turbulence of small-scale elements in an AR is not closely related to processes above the photosphere, but it rather carries the footprint of the sub-photospheric dynamics.