Solar Photospheric Flows and the Origin of Active Regions

Abstract

The early evolutionary stages of some active regions (ARs) are analysed with the aim of resolving one of the paramount-importance problems of heliophysics: what plays a primary, active role in the formation of active regions and sunspots---magnetic fields or motion of solar plasma? In particular, the first view underlies the popular model of rising $Ω$-shaped, twisted flux-tube loop of strong magnetic field. In this study, optical images obtained by the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory are used along with full-vector maps of magnetic field based on HMI data. Maps of fluid-particle tracks are constructed using local correlation tracking of photospheric brightness elements. Comparisons of optical and magnetic maps for three developing active regions confirm the conclusion made by Bumba and Howard in the mid-1960s that the supergranulation pattern is not destroyed by the emerging magnetic field of the developing AR, which would be inevitable if a loop of the strong-magnetic-field flux tube rose through the subphotospheric layers. There are no signs of spreading flows on scales comparable to the size of the whole active region. Various forms of dissymmetry between the leading and trailing parts of ARs are revealed. Next, an example of a well-developed sunspot group is considered, which demonstrates that the twist of the magnetic configuration may develop much later than the group starts growing. It is concluded that plasma flows appear to be determinant with respect to magnetic fields. The considered scenarios of the origin of ARs are in sharp contradiction with the rising-tube model.