Abstract
Aims: We aim to determine the effect of converging flows on the evolution of a bipolar magnetic region (BMR), and to investigate the role of these inflows in the generation of poloidal flux. We also discuss whether the flux dispersal due to turbulent flows can be described as a diffusion process. Methods: We developed a simple surface flux transport model based on point-like magnetic concentrations. We tracked the tilt angle, the magnetic flux and the axial dipole moment of a BMR in simulations with and without inflows and compared the results. To test the diffusion approximation, simulations of random walk dispersal of magnetic features were compared against the predictions of the diffusion treatment. Results: We confirm the validity of the diffusion approximation to describe flux dispersal on large scales. We find that the inflows enhance flux cancellation, but at the same time affect the latitudinal separation of the polarities of the bipolar region. In most cases the latitudinal separation is limited by the inflows, resulting in a reduction of the axial dipole moment of the BMR. However, when the initial tilt angle of the BMR is small, the inflows produce an increase in latitudinal separation that leads to an increase in the axial dipole moment in spite of the enhanced flux destruction. This can give rise to a tilt of the BMR even when the BMR was originally aligned parallel to the equator.
Highlights
Surface flux transport (SFT) simulations have been used with considerable success to describe the evolution of the large-scale photospheric magnetic field
The bipolar magnetic region (BMR) has been sheared by the differential rotation, while the random walk has dispersed the flux concentrations
The dispersal produced by the random walk is slightly lower than expected from a Fickian diffusion process when the step size is comparable to the typical size of an active region
Summary
Surface flux transport (SFT) simulations have been used with considerable success to describe the evolution of the large-scale photospheric magnetic field (see, e.g. DeVore et al 1984; Wang et al 1989; Mackay et al 2002a,b; Baumann et al 2004). The scalar quantity Br is advected by the large-scale flows (differential rotation and meridional flow) and the variable patterns of convection The latter have the effect of dispersing the magnetic field, and have commonly been modeled as a Fickian diffusion process (Leighton 1964), some authors prefer a less parametrized treatment of the turbulent dispersal. Hathaway (2010) uses an observation-based, timeevolving spectrum of spherical harmonics to produce random patterns of turbulent flows that advect magnetic concentrations This approach recovers some of the observed characteristics of the evolution of the photospheric field, such as the accumulation of flux in the network and the dispersal on multiple scales.
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