Flux Collision Models of Prominence Formation or, Breaking Up is Hard to Do

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Flux Collision Models of Prominence Formationor,
Breaking Up is Hard to Do
Brian Welsch (UCB-SSL), Rick DeVore Spiro
Antiochos (NRL-DC)
Filament imaged by NRLs VAULT II (courtesy
A.Vourlidas)
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Q How might a prominence-like topological
structure form in already-emerged fields?

• Essentials of prominence field
• Low-lying, sheared field, paralleling PIL.
• Long, essentially flat field lines only weakly
  arched, dipped, or helical.
• Less-sheared field lines overlying the low-lying
  sheared field.

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

• Requires localized, sustained shear along PIL.
• (flows of sufficient duration not observed!)
• Velocity efficiently injects helicity.
• No eruption not quadrupolar.
• Q Where else can 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 (2001) 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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Plan A Given two initially unconnected A.R.s,
shear to drive reconnection.

• DeVores ARMS code, 3-D, Cartesian, FCT,AMR,MHD
• Plane of symmetry ensures no shared flux two
  distinct flux systems.
• Linear shear profile
• We want reconnection (via numerical diffusion),
  so only two levels of grid refinement.

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Easier said than done! Reconnection not seen.

• Lacked sufficient topological complexity?
• Reconnection seen in other runs, with bald patch
  nulls.
• Flux systems not sufficiently distinct ?
• Field direction essentially continuous across
  weak quasi-separatrix layer (QSL) no current
  sheet.
• Didnt shear long enough?
• PIL rotates by gt 60o, over 10o in heliocentric
  latitude, implying greater than five rotations!

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A modified boundary condition resulted in
stronger QSLs, and weak reconnection was seen
but with wrong topology! ()
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Added background field, B0

• B0 is weak B0 0.1, while peak Bz is 4
• Reconnection occurs lower in box!
• B0 keeps reconnected field lines from rising
  through top of box.

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Follow Martens ZwaanIgnore shear, just
converge!
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Reconnection from Cancellation
Note that reconnecting field lines become more
sigmoidal as cancellation proceeds.
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Promising Results!

• Reconnection occurs at low X point, by numerical
  diffusion, at grid scale.
• Reconnection forms low-lying, flat and
  weakly-dipped and weakly-helical field lines!
• Higher field lines, reconnected first, are less
  sheared!

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Of note

• Helicity/twist present in post-reconnection
  fields was, of course, present in
  pre-reconnection fields, which were
  non-potential.
• Lowest-lying field lines are sigmoidal and are
  the most recently reconnected, and might
  therefore appear bright in X-rays.
• The most recently reconnected fields become more
  sigmoidal as convergence proceeds implying
  sigmoidal fields and filament growth are linked.
• This convergence/filament/sigmoid scheme matches
  fish hooks scenario advanced by Pevtsov et al.
  (1995)

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What about the mass?
Isosurface of mass enhancement.
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Next, study role of twist in initial
fieldsImitate Amari et al., twist, then
converge!
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Spin-Up Fluxes by ½ Turn in Each Footpoint
Next, converge!
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Reconnected fields are more dipped/helical!
(Essentially, twist combines additively,
increasing the net twist but twist per unit
length increases only weakly.)
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Conclusions

• Breaking Up is Hard to Do Reconnection does not
  readily occur.
• Shearing footpoints does not lead to strong
  coronal reconnection! Twisting footpoints does
  better, but not by much
• Converging and cancelling footpoints is by far
  the most efficient way to induce reconnection.
• Cancellation-induced reconnection can add twist
  to reconnected field lines, with implications for
  stability of post-reconnection flux system.
• Post-reconnection fields resemble prominences!

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Topology Over vs. Under
Bad post-reconnection topology observed.
Good post-reconnection topology in a prominence.
()
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Weakly-dipped post-reconnection fields
()
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Weakly-helical post-reconnection fields
()
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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)