Abstract
Context.Solar active regions are associated with Evershed outflows in sunspot penumbrae, moat outflows surrounding sunspots, and extended inflows surrounding active regions. Extended inflows have been identified around established active regions with various methods. The evolution of these inflows and their dependence on active region properties as well as their effect on the global magnetic field are not yet understood.Aims.We aim to understand the evolution of the average inflows around emerging active regions and to derive an empirical model for these inflows. We expect that this can be used to better understand how the inflows act on the diffusion of the magnetic field in active regions.Methods.We analyzed horizontal flows at the surface of the Sun using local correlation tracking of solar granules observed in continuum images of the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We measured average flows of a sample of 182 isolated active regions up to seven days before and after their emergence onto the solar surface with a cadence of 12 h. About half of the active regions in the sample developed sunspots with moat flows in addition to the surrounding inflows. We investigated the average inflow properties with respect to active region characteristics of total flux and latitude. We fit a model to these observed inflows for a quantitative analysis.Results.We find that converging flows of about 20–30 m s−1are first visible one day prior to emergence, in agreement with recent results. These converging flows are present regardless of the active region properties of latitude or flux. We confirm a recently found prograde flow of about 40 m s−1at the leading polarity during emergence. We find that the time after emergence when the latitudinal inflows increase in amplitude depends on the flux of the active region, ranging from one to four days after emergence and increasing with flux. The largest extent of the inflows is up to about 7 ± 1° away from the center of the active region within the first six days after emergence. The inflow velocities have amplitudes of about 50 m s−1.
Highlights
Active regions are patches of magnetic field at the surface of the Sun
In Appendix B, we find that in regions within 2◦ of sunspots, the LCT flows are weaker than the direct Solar Dynamics Observatory (SDO)/HMI Doppler velocity measurements
We find that a moat flow is present in our flow data for regions with a clear sunspot, with typical velocities of about 150 m s−1
Summary
Active regions (hereafter ARs) are patches of magnetic field at the surface of the Sun. Theories of active region formation include a buoyant rise of magnetic flux tubes from the bottom of the convection zone (see the review by Fan 2009), as well as a formation in the bulk of the convection zone (Nelson et al 2013) or in the near-surface layers (Brandenburg 2005). Active regions are advected on large spatial scales by differential rotation (Snodgrass 1983) and the meridional flow (Duvall 1979). Methods for inferring the large-scale flows include tracking the motions of small-scale convective or magnetic features, which are displaced by the larger-scale motions, as well as a variety of tools for the analysis of solar oscillations, which are known as local helioseismology (see the review by Gizon & Birch 2005)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
