Photospheric flows around sunspots and pores

1. Photospheric flows around sunspots and pores Michal Sobotka Astronomical Institute, Academy of Sciences of the Czech Republic, Ondřejov IHY General Assembly, Paris, 10 - 13 January 2006

2. Introduction • The interaction of moving plasma with magnetic fields in the photosphere influence strongly the activity processes in the chromosphere and corona. • Sunspots and pores are the largest concentrations of magnetic flux on the solar surface and are ranked among the basic phenomena of solar activity. • Sunspots and pores are dynamical systems accompanied by specific surface and sub-surface flows.

3. Two basic models of magnetic structure of sunspots

4. Horizontal motions around pores Method: LCT Convergent motions of granules in a 1500 km wide zone toward pores (Wang & Zirin 1992, Sobotka et al. 1999). These motions are driven by exploding granules and mesogranules.

5. Motions of granules toward the pore sometimes result in a penetration of bright features into the pore. Small granules or fragments of granules can penetrate up to 700 km into the umbra (Sobotka et al. 1999)

6. Horizontal motions around sunspots • Sunspot moat = annular region around a sunspot, free of static magnetic fields (Sheeley 1969) • Horizontal outward motions of magnetic elements, facular points and granules in the moat (Muller & Ména 1987, Brickhouse & Labonte 1988, Shine et al. 1987) • Speeds in the range 0.5 - 1 km/s, roughly twice of the supergranular outflow speed

7. Examples of moats defined as areas with outward radial motion of granules - TRACE WL series, LCT (Roudier & Sobotka) old stable spot growing spot

8. Nearly all spots have moats, also the young ones. The moats are mostly asymmetrical. decaying spots

9. High-resolution study of horizontal motions in the moat - SVST series, LCT, feature tracking (Bonet et al. 2005) 1. Local divergent motions of granules, reflecting mostly the expansion and fragmentation (0.64 km/s, tracking period 5 min).

10. 2. Large-scale regular outflow, which carries granules and centres of divergent motions away from the spot (0.51 km/s, tracking period 2 h) - the “net” moat flow.

11. 3. Radial outflow of G-band facular points in the moat through “channels” between the local divergent motions; the speeds are similar to those of granules. Feature tracking, 2 h.

12. Subphotospheric flows around sunspots • Time-distance helioseismology applied to sunspots (review by Kosovichev 2004) • Acoustic waves (p-modes) are used to map deep layers (2–50 Mm below the surface) • Surface gravity waves (f-modes) are used to map shallow sub-surface layers • Maps of subphotospheric flow velocities • Maps of subphotospheric variations of sound speed caused simultaneously by temperature and magnetic field inhomogeneities

14. Discussion • Sheeley (1972) suggested that a sunspot occupies the centre of a supergranular cell and the moat outflow is of a supergranular type. Some models (e.g. Meyer 1977, Parker 1979) require strong converging flows in deep layers (and outflows at the surface) to maintain the sunspot stable. • Helioseismic results are somewhat contradictory, confining the outflows to a very thin superficial layer and localizing the inflows (and downflows) also near the surface, to the depths of 2 - 3 Mm. • What is the nature of moat outflow?

15. Hurlburt & Rucklidge (2000) simulated flows around monolithic flux tubes representing spots and pores. These flows are driven by cooling of plasma near the flux tube, leading to downflows around the tube and hence inflows near the surface. Pores: The inflows are observed, but they might be caused by exploding granules. Sunspots: The inflow (stabilizing collar) may be hidden below the penumbra and only the counter-cell, the moat, is visible. pore sunspot

16. Thank you for attention So, how does it work in fact…?