Testing solar surface flux transport models in the first days after active region emergence

¹ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: gottschling@mps.mpg.de
² School of Mathematical and Physical Sciences, University of Newcastle, Callaghan, New South Wales, Australia
³ Institut für Astrophysik, Georg-August Universität Göttingen, 37077 Göttingen, Germany

Received: 23 August 2021
Accepted: 1 November 2021

Abstract

Context. Active regions (ARs) play an important role in the magnetic dynamics of the Sun. Solar surface flux transport models (SFTMs) are used to describe the evolution of the radial magnetic field at the solar surface. The models are kinematic in the sense that the radial component of the magnetic field behaves as passively advected corks. There is, however, uncertainty about using these models in the early stage of AR evolution, where dynamic effects might be important.

Aims. We aim to test the applicability of SFTMs in the first days after the emergence of ARs by comparing them with observations. The models we employ range from passive evolution to models where the inflows around ARs are included.

Methods. We simulated the evolution of the surface magnetic field of 17 emerging ARs using a local surface flux transport simulation. The regions were selected such that they did not form fully fledged sunspots that exhibit moat flows. The simulation included diffusion and advection by a velocity field, for which we tested different models. For the flow fields, we used observed flows from local correlation tracking of solar granulation, as well as parametrizations of the inflows around ARs based on the gradient of the magnetic field. To evaluate our simulations, we measured the cross correlation between the observed and the simulated magnetic field, as well as the total unsigned flux of the ARs, over time. We also tested the validity of our simulations by varying the starting time relative to the emergence of flux.

Results. We find that the simulations using observed surface flows can reproduce the evolution of the observed magnetic flux. The effect of buffeting the field by supergranulation can be described as a diffusion process. The SFTM is applicable after 90% of the peak total unsigned flux of the AR has emerged. Diffusivities in the range between D = 250–720 km² s⁻¹ are consistent with the evolution of the AR flux in the first five days after this time. We find that the converging flows around emerging ARs are not important for the evolution of the total flux of the AR in these first five days; their effect of increasing flux cancellation is balanced by the decrease in flux transport away from the AR.