A low upper limit on the subsurface rise speed of solar active regions

Average motion of emerging solar active region polarities: II. Joy's law

CONTEXT

The tilt of solar active regions described by Joy's law is essential for converting a toroidal field to a poloidal field in Babcock-Leighton dynamo models. In thin flux tube models the Coriolis force causes what we observe as Joy's law, acting on east-west flows as they rise towards the surface.

AIMS

Our goal is to measure the evolution of the average tilt angle of hundreds of active regions as they emerge, so that we can constrain the origins of Joy's law.

METHODS

We measured the tilt angle of the primary bipoles in 153 emerging active regions (EARs) in the Solar Dynamics Observatory Helioseismic Emerging Active Region survey. We used line-of-sight magnetic field measurements averaged over 6 h to define the polarities and measure the tilt angle up to four days after emergence.

RESULTS

We find that at the time of emergence the polarities are on average aligned east-west, and that neither the separation nor the tilt depends on latitude. We do find, however, that EARs at higher latitudes have a faster north-south separation speed than those closer to the equator at the emergence time. After emergence, the tilt angle increases and Joy's law is evident about two days later. The scatter in the tilt angle is independent of flux until about one day after emergence, when we find that higher-flux regions have a smaller scatter in tilt angle than lower-flux regions.

CONCLUSIONS

Our finding that active regions emerge with an east-west alignment is consistent with earlier observations, but is still surprising since thin flux tube models predict that tilt angles of rising flux tubes are generated below the surface. Previously reported tilt angle relaxation of deeply anchored flux tubes can be largely explained by the change in east-west separation. We conclude that Joy's law is caused by an inherent north-south separation speed present when the flux first reaches the surface, and that the scatter in the tilt angle is consistent with buffeting of the polarities by supergranulation.

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

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 ms^-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.

Flows around Averaged Solar Active Regions

We explore the general properties of near-surface flows around solar active regions. Helioseismic holography is applied to HMI Dopplergrams yielding nearly 5000 flow measurements of 336 unique active regions observed by the Solar Dynamics Observatory between 2010 and 2014. Ensemble averages of the flows, over subsets of regions sorted on the basis of magnetic flux, are performed. These averages show that converging flows, with speeds of about 10 m s-1 and extending up to 10° from the active-region centers, are prevalent and have similar properties for all regions with magnetic flux above 1021 Mx. Retrograde flows are also detected, with amplitudes around 10 m s-1, which predominantly, but not exclusively, flank the polar side of the active regions. We estimate the expected contribution of these active-region flows to longitudinal averages of zonal and meridional flows and demonstrate the plausibility that they are responsible for at least some component of the time-varying global-scale flows. The reliability of our flow determination is tested using publicly available MHD simulations of both quiet-Sun convection and of a sunspot. While validating the overall methodology in general, the sunspot simulation demonstrates the presence of artifacts that may compromise quantitative flow inferences from some helioseismic measurements.