Review of Conditions for the Formation and Maintenance of Filaments

1
Review of Conditions for the Formation and
Maintenance of Filaments
http//spaceweather.com/images2002/18feb02/

• Paper by Sara F. Martin, 1998
• Review presented by Samuel Tun October 13, 2005

2
Introduction and overview

• The paper reviews the
• the filament environment
• dynamic conditions related to the formation of
  filaments, and
• Additional clues from the features of filaments
  and their surroundings

Typical thickness, height, and length are 5000
km, 50000 km, and 200000 km, respectively (Stix,
Ch. 9). Filaments and prominences are the same
phenomenon. Heres a movie of an erupting one
showing that.
All movies kindly provided by Dr. C. Denker, BBSO
/ NJIT
3
Conditions of the Filament environment

• Filaments and photospheric B-fields
• filaments occur along boundaries of opposite
  polarity line of sight magnetic fields, or
  neutral lines

Figure 1, Martin 1998
4

• above condition applies to quiescent and active
  region filaments, and of all scales (temporal and
  spatial)
• temporal and spatial scales hold the relation big
  and slowly evolving, small and quickly evolving

5

• although the boundary containing large-scale
  filaments can be all within one bipolar region,
  about twice as many filaments form between ARs
  than within them. Filaments can form between ARs
  of different ages.
• the above preference points to a possible
  quadrupolar magnetic configuration. Models
  abound, and evidence from magnetograms seems to
  indicate that this is so.

• Overlying arcades
• seems all filaments have overlying arcades

Figure 2, Martin 1998
6

• overlying coronal loops connect regions of
  opposite polarity unless they have different
  helicities.
• northern hemisphere dominated by regions of
  negative helicity, while the reverse is true for
  the southern one
• arcades are a necessary but insufficient
  condition for filaments

• The Filament Channel
• In medium to strong fields, filaments form only
  where there are no fibril bridges across the
  neutral line (across polarity inversion, region
  of maximum magnetic shear).

From http//www-solar.mcs.st-and.ac.uk/duncan/pro
minence/
7

• examples of fibril alignment (channel formation)
  preceding filament formation are presented
• some channels remain filament-less, and the
  mechanism for fibril alignment is yet to be
  determined.

Figure 11, Martin 1998
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• photospheric magnetograms show field is aligned
  along the filament channel. However, this is
  confirmed only for strong fields and of rough
  spatial resolution due to instrumental
  limitations.
• There is a permeating "background of
  mixed-polarity, small-scale fields" in quiet Sun.
  However, filament formation in ARs occurs between
  the greater plage areas, indicating that the
  formation of the channels depends on the
  large-scale AR magnetic fields

From http//helios.obspm.fr/observat/pages/galerie
/
9

• barbs have mass flows in different directions,
  ending in points of "minority polarity
• almost complete absence of barbs in ARs

Figure 5, Martin 1998
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Dynamic conditions for the formation of filaments

• Convergence of B-fields and canceling magnetic
  flux, the movie.

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• there is evidence that in order for filaments to
  form one must have merging areas of opposite
  polarities (flux convergence)
• opposite fields cancel, but the mechanism is
  still debated

Figure 7, Martin 1998
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• Changing fine structure

• high-resolution spectra show a constant movement
  in the fine structure, and the highest resolution
  images show thread lifetimes of about 10 minutes
• mass usually moves along the observable threads,
  indicating field alignment with threads

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• vector magnetograms give evidence that, in AR,
  filaments form in regions of maximum magnetic
  shear (low and parallel to polarity inversion
  line)
• some threads of large filaments appear to be
  inclined to the filament axis, demonstrating that
  there are fields pointing in those large angles

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• Field direction from plasma flows
• BBSO images were used to show that mass flows at
  different optical depths move in different
  directions in the filament and barbs

• Detection of counter streaming can
  compliment magnetograms in determining the
  filament magnetic fields

Figure 9, Martin 1998
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Discussion-pieces of prominence puzzle in broader
context

• Chirality patterns in channels, filaments, and
  overlying arcades

• coronal loop chirality is defined such that
  left-skewed arcades always lie over dextral
  filaments.

Figure 10, Martin 1998
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• dextral chirality dominates in Northern
  hemisphere, while the inverse is true for the
  South.This preference is sometimes violated, but
  chirality relationships (dextral with right
  bearing barbs) always hold.
• Confirmation of inverse magnetic component
  filaments
• there exists a component of B-field perpendicular
  to the field of the coronal loops above the
  filament. This had previously been modeled as an
  x-type neutral point
• Martin argues that since the footpoints of all
  the barbs in her figure 5 were found to be in the
  minority polarity, that their magnetic fields
  have a large component opposite that of the
  overlying loops

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• Barbs to fibril relations
• barbs and fibril pattern is aligned as viewed
  from above, but they are not so if viewed along
  the filament axis
• on positive side barbs reach down into minority
  polarity plagettes, while fibrils have upward
  reaching components going into the
  corona...separate fields
• channel-filament relation indicates that the
  channel magnetic field will dictate the filament
  properties
• The Cavity
• Martin suggested that the cavity exists because
  the fields of the filaments and the overlying
  arcades have different helicities (already saw
  these do not interconnect)

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Summary and conclusions

• Filaments exist between fields of opposite
  polarity at sides and ends
• 2 .a. all filaments have overlying arcade of
  coronal loops
• b. arcade is skewed with respect to polarity
  boundary
• 3 .a. filament channel exists below, and fibrils
  aligned
• b. no fibrils cross the neutral line
• c. magnetic field of the channel has inverse
  component
• 4 . Opposing magnetic flux move towards each
  other so as to bring the regions unto an overlap
• Converging magnetic fields cancel
• Barbs require continuous mass supply
• Minority polarity fields must exist nearby
• Chirality relations exist and must be satisfied
  by any model

Reference (paper discussed) Martin, S.F., 1998
Conditions for the Formation and Maintenance of
Filaments, Solar Physics 182, 107-137