Determining the 3D Magnetic Field Geometry

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Determining the 3D Magnetic Field Geometry
A. A. van Ballegooijen, E. E. DeLuca, M.
Bobra Smithsonian Astrophysical
Observatory Cambridge, MA
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Field Extrapolation
AIA Objective 1 (energy input, storage, and
release) requires knowledge of 3D coronal
magnetic structure and dynamics. One approach is
to extrapolate observed vector fields into the
corona, using nonlinear force free field (NLFFF)
modeling

• Practical difficulties
• nonlinear problem
• limited sensitivity of Btrans
• 180-degree ambiguity
• photosphere is not force free
• (Bt-phot ? Bt-corona)
• At present, there is no good method for
  extrapolating strongly sheared fields.

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Developing Extrapolation Methods

• Extrapolating photospheric vector fields does
  not work well for
• filament channels where sheared field is
  elevated into chromosphere.
• Need observational constraints on chromospheric
  fields.
• Need to include non-force-free photospheric
  conditions into models.
• Test case (see poster)
• -- Add vertical force at z 2 Mm to simulate
  non-force-free condition.
• -- Construct 3D model (A) with the highly
  sheared field.
• -- Extract observed chromospheric vector
  field (z 4 Mm).
• -- Reconstruct 3D field (B) by extrapolating
  chromospheric field
• (uses magneto-frictional relaxation and
  injection of horizontal field
• at z 0 based on the error ?B Bobs
  Bmodel in chromosphere).
• -- Method gives a good fit to observed vector
  field.

4
Developing Extrapolation Methods
Test case chromospheric vector
field (blueobserved, blackmodel)
A
B
BBSOMDI, 2005/10/10, 1730UT
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Use H-alpha Fibrils?
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NLFFF Modeling Tasks

• Develop methods/codes for extrapolating vector
  fields in spherical
• geometry, covering large area on the Sun
  (reduce the effects of side
• boundaries) models must extend to large height
  2.5 Rsun.
• Develop methods for including observational
  constraints on
• chromospheric fields and effects of
  non-force-free photosphere.
• Test NLFFF methods/codes using synthetic
  vector-field data.
• Test NLFFF models using TRACE data. Can models
  reproduce the
• observed coronal loop structures?
• Develop quantitative methods for comparing 3D
  models with observed
• coronal loops and Ha filaments.