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)