Evolution of solar surface inflows around emerging active regions

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−1 are 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−1 at 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.

Contribution of flows around active regions to the north-south helioseismic travel-time measurements

“…These smoothed background flows exhibit a modulation on the timescale of a solar cycle. This modulation is consistent with what is described by Gottschling et al (2021), although we computed the background flows in a different way. They are studied in more detail in Appendix B.…”

Solar surface flows during active region emergence

“…Observed flows (u = u obs ). In this model, we used the flow maps from Gottschling et al (2021) for the ARs in the sample. The flows stem from local correlation tracking (LCT, November and Simon 1988) of solar continuum intensity images, and they are based on the data processing by Löptien et al (2017), who used the Fourier local correlation tracking code (Welsch et al 2004;Fisher and Welsch 2008) on full-disk continuum intensity images from SDO/HMI.…”

“…The flows stem from local correlation tracking (LCT, November and Simon 1988) of solar continuum intensity images, and they are based on the data processing by Löptien et al (2017), who used the Fourier local correlation tracking code (Welsch et al 2004;Fisher and Welsch 2008) on full-disk continuum intensity images from SDO/HMI. Several changes in the data processing were made by Gottschling et al (2021) in order to correct for additional systematic effects in the data from Löptien et al (2017). Gottschling et al (2021) describe these changes in detail.…”

“…Cameron and Schüssler (2012) propose these inflows as a possible mechanism for a nonlinearity that regulates the solar cycle strengths in the solar dynamo. Recently, Gottschling et al (2021) measured the evolution of the flows around emerging ARs from before to up to seven days after emergence, finding that the time between the AR emergence and the time at which inflows set in after the emergence increases with the total magnetic field of the AR. These inflows have velocities of about 50 m s −1 as well, but they appear to be smaller in extent than the inflows around evolved ARs.…”

“…VargasDomínguez et al 2008), as the moat flow would interfere with the vortical flows investigated in this study. The assessment whether an EAR develops a spot with a clear penumbra in the observed time frame is based onGottschling et al (2021), see Sect. 2.6.4.…”