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.

Asteroseismic detection of latitudinal differential rotation in 13 Sun-like stars.

The differentially rotating outer layers of stars are thought to play a role in driving their magnetic activity, but the underlying mechanisms that generate and sustain differential rotation are poorly understood. We report the measurement using asteroseismology of latitudinal differential rotation in the convection zones of 40 Sun-like stars. For the most significant detections, the stars' equators rotate approximately twice as fast as their midlatitudes. The latitudinal shear inferred from asteroseismology is much larger than predictions from numerical simulations.

Wave-like properties of solar supergranulation.

Supergranulation on the surface of the Sun is a pattern of horizontal outflows, outlined by a network of small magnetic features, with a distinct scale of 30 million metres and an apparent lifetime of one day. It is generally believed that supergranulation corresponds to a preferred 'cellular' scale of thermal convection; rising magnetic fields are dragged by the outflows and concentrated into 'ropes' at the 'cell' boundaries. But as the convection zone is highly turbulent and stratified, numerical modelling has proved to be difficult and the dynamics remain poorly understood. Moreover, there is as yet no explanation for the observation that the pattern appears to rotate faster around the Sun than the magnetic features. Here we report observations showing that supergranulation undergoes oscillations and supports waves with periods of 6-9 days. The waves are predominantly prograde, which explains the apparent super-rotation of the pattern. The rotation of the plasma through which the pattern propagates is consistent with the motion of the magnetic network.