Magnetic Flux Transport and the Hemispheric Pattern of Filaments

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Magnetic Flux Transport and the Hemispheric
Pattern of Filaments
Anthony Yeates1
with D. H. Mackay1 and A. A. van Ballegooijen2
1 School of Mathematics Statistics, University
of St Andrews 2 Harvard-Smithsonian Centre for
Astrophysics
PROM Workshop, George Mason University 30/10/06
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Magnetic Flux Transport and the Hemispheric
Pattern of Filaments

• Newly developed global simulations of the coronal
  magnetic field over multiple rotations.
• No resetting of the field to potential at each
  rotation.
• Used to test a theory for the hemispheric pattern
  of filaments against observations.

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BACKGROUND
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The hemispheric pattern

• Filaments can be assigned a chirality

• Leroy, Bommier Sahal-Bréchot (1983) and Martin,
  Bilimoria Tracadas (1994) find that for
  quiescent filaments
• dextral filaments dominate in N hemisphere,
• sinistral filaments dominate in S hemisphere.
• Same pattern every solar cycle.
• Pattern doesn't hold for active region filaments.

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Photospheric flux transport

• Normal magnetic field at photosphere evolves
  according to surface flux transport model (Wang,
  Sheeley, DeVore)

differential rotation
supergranular diffusion
meridional flow
contours of Br
red positive Br
blue negative Br
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Previous work

• Mackay van Ballegooijen (2005)
• systematic study of pair of idealised bipolar
  regions
• using flux transport and magneto-frictional model
  - coronal field responds to photospheric motions
  by relaxing toward series of force-free
  equilibria.

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Hypothesis

• The hemispheric pattern of quiescent filament
  chirality is caused by
• photospheric transport of magnetic flux
• dominant helicities (Pevtsov, Canfield Metcalf
  1995)
• dominant bipole tilt angles (Joys Law).

tilt angle
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OBSERVATIONS
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Observed filaments
08/10/99

• Sample of 254 filaments over 5 month period from
  daily Big Bear H? images.
• Positions identified on Kitt Peak synoptic
  magnetograms (CR1949 to CR1954).

08/11/99
08/12/99
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Observed filament chiralities

• 99 filaments have definite chirality
  (statistically significant) based on observations
  of barbs, 64 dextral and 35 sinistral.

dextral

sinistral
.
undetermined
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Aim of this project

• (1) Model observed evolution of surface magnetic
  field over many rotations, including emerging
  flux with correct helicity.
• (2) Simulate coupled evolution of 3D coronal
  field through sequence of nonlinear force-free
  equilibria.
• (3) Compare chirality of flux rope structures
  with corresponding observed filaments.

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SIMULATION OF SURFACE MAGNETIC FIELD
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Application to real photosphere

• Attempt to simulate evolution of real Br on
  photosphere with spherical flux transport code.
• Start from observed magnetogram (corrected for
  differential rotation) ...

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Previous work

• Mackay, Gaizauskas van Ballegooijen (2000)
  applied the model to filament formation in an
  observed activity complex.

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Evolution over longer periods

• Example evolution with no resetting to potential
  at each rotation.
• Accuracy in strong field regions is lost after
  one rotation due to newly emerging flux.

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Emerging flux

• Developed a semi-automated procedure
• compare successive magnetograms
• find new bipolar regions
• measure key properties
• insert as ideal bipoles into simulation.

CR1948
CR1948 rotated
CR1949
Total 118 bipolar regions
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Simulation with emerging flux
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Where next?

• We have developed a new technique for simulating
  the global coronal magnetic field evolution over
  an extended period, with 3D non-potential
  modelling based on observed photospheric
  magnetograms.
• We apply the model to a specific 5 month period
  to study the origin of the hemispheric pattern of
  filaments
• dynamic photospheric boundary conditions
  complete
• data on observed filaments is collected