Average surface flows before the formation of solar active regions and their relationship to the supergranulation pattern

Abstract

Context. The emergence of solar active regions is an important but poorly understood aspect of the solar dynamo. Aims. Knowledge of the flows associated with the rise of active-region-forming magnetic concentrations through the near-surface layers will help determine the mechanisms of active region formation. Methods. We used helioseismic holography and granulation tracking to measure the horizontal flows at the surface that precede the emergence of active regions. We then averaged these flows over about sixty emerging active regions to reduce the noise, selecting active regions that emerge into relatively quiet Sun. To help interpret the results, we constructed a simple model flow field by generating synthetic “emergence locations” that are probabilistically related to the locations of supergranulation-scale convergence regions in the quiet Sun. Results. The flow maps obtained from helioseismology and granulation tracking are very similar (correlation coefficients for single maps around 0.96). We find that active region emergence is, on average, preceded by converging horizontal flows of amplitude about 40 m s−1. The convergence region extends over about 40 Mm in the east-west direction and about 20 Mm in the north-south direction and is centered in the retrograde direction relative to the emergence location. This flow pattern is largely reproduced by a model in which active region emergence occurs preferentially in the prograde direction relative to supergranulation inflows. Conclusions. Averaging over many active regions reveals a statistically significant pattern of near-surface flows prior to emergence. The qualitative success of our simple model suggests that rising flux concentrations and supergranule-scale flows interact during the emergence process.

Evolution of solar surface inflows around emerging active regions

“…3.5. This converging flow is consistent with that reported by Birch et al (2019). For the two higher-flux subsamples, the converging flow is spatially more extended than for the lower-flux ones.…”

“…Their work confirms inflows of about 20-30 m s −1 that extend about 10 • from the AR. Birch et al (2019) found a compact (less than 2 • ) converging flow of about 40 m s −1 in the day before active region emergence, using both LCT and helioseismic holography as independent methods for flow inferences on the surface. Comparisons of flows derived with LCT and local helioseismology showed that the two methods agree well (Švanda et al 2013;Birch et al 2016).…”

Average motion of emerging solar active region polarities

“… Birch et al (2016) found by averaging over the emerging active regions in the SDO/HEAR survey that there are no significant outflows during emergence, although these surface outflows are predicted by thin flux tube theory. Birch et al (2019) did, however, find an average east-west elongated prograde flow just prior to emergence. We now turn our attention to the evolution of the tilt angle and Joy’s law.…”

“…It is known that there is a relationship between the supergranulation pattern and where flux emergence occurs ( Birch et al 2019 ). We speculate that if the supergranulation is guiding the initial emergence process an alternative explanation for our results is that the emergence into predominantly east-west aligned north-south converging flows ( Birch et al 2019 ) leads to a preference for east-west alignment of the polarities. Why the emergence location is preferentially in east-west aligned inflows is not clear.…”

“…An alternative possibility is the motion of the turbulent convection (e.g . Parker 1955;Choudhuri & D'Silva 1990;Brandenburg 2005), with Schmidt (1968) having suggested that active region bipoles emerge in upwelling supergranular cells with an east-west orientation, and that the surface flows in the cell move the polarities outwards, away from one another.…”