Photospheric flows around sunspots and pores

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Photospheric flows around sunspots and pores

• Michal Sobotka
• Astronomical Institute, Academy of Sciences of
  the Czech Republic, Ondrejov

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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.

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Two basic models of magnetic structure of sunspots
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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.
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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)
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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

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Examples of moats defined as areas with outward
radial motion of granules - TRACE WL series, LCT
(Roudier Sobotka)
old stable spot
growing spot
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Nearly all spots have moats, also the young
ones. The moats are mostly asymmetrical.
decaying spots
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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).
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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.
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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.
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Subphotospheric flows around sunspots

• Time-distance helioseismology applied to sunspots
  (review by Kosovichev 2004)
• Acoustic waves (p-modes) are used to map deep
  layers (250 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

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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?

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