Flux Collision Models of Prominence Formation

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Flux Collision Models of Prominence Formation
Brian Welsch (UCB-SSL), Rick DeVore Spiro
Antiochos (NRL-DC)
Filament imaged by NRLs VAULT II (courtesy
A.Vourlidas)
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Essentials of prominence field

• Sheared field parallel to PIL.
• Dipped or helical field lines, to support mass.
  (But cf. Karpen, et al., 2001!)
• Overlying field restraining sheared field.
• Q Does the topological structure of prominences
  form above photosphere?

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Previously, DeVore Antiochos (2000) sheared a
potential dipole, and got a prominence-like
field.

• Requires shear along PIL.
• Velocity efficiently injects helicity.
• No eruption not quadrupolar.
• Q Where does shear originate?

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Following MacKay et al. (1999), Galsgaard and
Longbottom (2000) collided two flux systems
and got reconnection some helical field lines
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Initial Topology in Galsgaard Longbottoms Model
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The Martens Zwaan Model

• Initially, bipoles do not share flux.
• Diffl Rotn in, e.g., N.Hemisphere drives
  reconnection between bipoles flux systems.
• Reconnection converts weakly sheared flux to
  strongly sheared flux

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But there are two ways the field can reconnect!
Left strapping field restrains prominence
field. Right underlying field subducted?
(Martens Zwaan)
Q What determines how the field reconnects?
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A Helicity! Reconnection preserves H, so initial
reconnected fields have same helicity.
H lt 0
H gt 0
For config. at left, start w/negative helicity ,
etc.
Q Which config matches the Sun?
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Shearing adds positive helicity!

• With potential initial fields, shearing-induced
  reconnection leads to H gt 0 state.
• To get H lt 0 state, try twisting fields prior to
  shearing, to model interaction of fields that
  emerged with H lt 0.

Two types of runs A) Sheared B) Twisted, then
sheared.
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Plan A Given two initially unconnected A.R.s,
shear to drive reconnection.

• DeVores ARMS code NRLs LCPFD FCT MHD code
• Two horizontal dipoles.
• Plane of symmetry ensures no shared flux
• Linear shear profile
• Reconnection via num. diffusion, so only two
  levels of grid refinement.

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Easier said than done!

• 1st run Reconnection not seen! Lacked
  sufficient topological complexity?
• 2nd run, four dipoles, w/nulls bald patch
  reconnected well! dips/ helical field lines
  but contrived config.

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3rd, 4th runs weak reconnection

• Realistic BC six dipoles required
• For untwisted runs,
• H gt 0 state results.()
• Tilt, after Joys Law, helps reconnection. ()
• Twisting fields prior to shearing enhances
  reconnection. () (Resulting H unclear!)

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Added background field,

• Without
• reconnection occurs higher up
• reconnected field exits top of box
• Might keep flux systems separate when twisting
  (prior to shearing). ()

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Added converging flow to shear
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Evolution of
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Results

• Reconnected fields not prom-like no dips,
  helices
• Sigmoids of both types, N S. Handedness of
  higher sigmoids does not correspond to SXT
  sigmoids.

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Conclusions

• Topological complexity needed for reconnection!
• Prominence-like configs not yet found!
• Role of twist present in pre-sheared fields still
  under investigation.

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References
ApJ, v. 539, 954-963, Dynamical Formation and
Stability of Helical Prominence Magnetic Fields
", DeVore, C. R. and Antiochos, S. K. (2000)
ApJ, v. 553, L85-L88, "Are Magnetic Dips
Necessary for Prominence Formation?", Karpen, J.
T., et al. (2001)
ApJ, v. 575, 578-584, "Coronal Magnetic Field
Relaxation by Null-Point Reconnection,
Antiochos, S.K., Karpen, J. T., and DeVore, C.R.
(2002)
ApJ, v. 558, 872-887, "Origin and Evolution of
Filament-Prominence Systems , Martens, P.C. and
Zwaan, C. (2001)
ApJ, v. 510, 444-459, "Formation of Solar
Prominences by Flux Convergence , Galsgaard,
K. and Longbottom, A. W. (1999)
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Run with Joys Law Tilt
()
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Post-reconnection topology
()
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Post-twist field, prior to shearing

• Bipole systems reconnect at twisting onset.
• Bipole spacing and strength might allow
  flux between flux systems.
• Converging flow might sweep flux out of the
  way to allow reconnection between bipole systems.
• ()

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H gt 0 State
()
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Hemispheric Patterns of Chirality
Phenomenon Property N(S) Hemisph.
Filament Channel Dextral(Sinistral)
Filament Barbs Right(Left)-bearing Filament
X-ray Loops Axes CCW(CW) Rotate
w/Height A.R. X-ray Loops Shape
(sigmoid) N(S)-shaped A.R. vector Current
Helicity Neg. (Pos.) Magnetograms Magnetic
Clouds Twist Left(Right)-Handed
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VAULT II Filament Image, w/axes (courtesy, A.
Vourlidas)