Post on 07-Jan-2016
description
Flux Collision Models of Prominence Formation
Filament imaged by NRL’s VAULT II (courtesy A.Vourlidas)
Brian Welsch (UCB-SSL), Rick DeVore & Spiro Antiochos (NRL-DC)
Essentials of prominence field:
1. Sheared field parallel to PIL.
2. Dipped or helical field lines, to support mass. (But cf. Karpen, et al., 2001!)
3. Overlying field restraining sheared field.
Q: Does the topological structure of prominences form above photosphere?
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?
Following MacKay et al. (1999), Galsgaard and Longbottom (2000) collided two flux systems…
…and got reconnection & some helical field lines
Initial Topology in Galsgaard & Longbottom’s Model
The Martens & Zwaan Model
• Initially, bipoles do not share flux.
• Diff’l Rot’n in, e.g., N.Hemisphere drives reconnection between bipoles’ flux systems.
• Reconnection converts weakly sheared flux to strongly sheared flux
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?
A: Helicity! Reconnection preserves H, so initial & reconnected fields have same helicity.
H < 0 H > 0For config. at left, start w/negative helicity , etc.
Q: Which config matches the Sun?
Shearing adds positive helicity!
• With potential initial fields, shearing-induced reconnection leads to H > 0 state.
• To get H < 0 state, try twisting fields prior to shearing, to model interaction of fields that emerged with H < 0.
Two types of runs: A) Sheared; B) Twisted, then sheared.
Plan A: Given two initially unconnected
A.R.’s, shear to drive reconnection. • DeVore’s ARMS code:
NRL’s 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.
yv 0x
• 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.
Easier said than done!
3rd, 4th runs: weak reconnection• Realistic BC: six
dipoles required• For untwisted runs, H > 0 state results.(*)• Tilt, after Joy’s Law,
helps reconnection. (*)• Twisting fields prior
to shearing enhances reconnection. (*) (Resulting H unclear!)
Added background field, :• Without :
– reconnection occurs higher up
– reconnected field exits top of box
• Might keep flux systems separate when twisting (prior to shearing). (*)
0B
0B
Added converging flow to shear:
XVYV
Evolution of :ZB
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.
Conclusions:
• Topological complexity needed for reconnection!
• Prominence-like configs not yet found!
• Role of twist present in pre-sheared fields still under investigation.
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. 510, 444-459, "Formation of Solar Prominences by Flux Convergence ,” Galsgaard, K. and Longbottom, A. W. (1999)
ApJ, v. 558, 872-887, "Origin and Evolution of Filament-Prominence Systems ,” Martens, P.C. and Zwaan, C. (2001)
References:
Run with Joy’s Law Tilt:
(*)
Post-reconnection topology:
(*)
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.(*)
0B
0B
0B
H > 0 State
(*)
Phenomenon Property N(S) Hemisph.
Filament Channel Dextral(Sinistral) Filament Barbs Right(Left)-bearingFilament 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
Hemispheric Patterns of Chirality
VAULT II Filament Image, w/axes (courtesy, A. Vourlidas)