Helicity as a Component of Filament Formation

D.H. Mackay

University of St. Andrews

Solar Theory Group

Contents.

• Previous Results :

Observations of Filament Formation.

Comparison of Theory with Observations.

• The Model.

• Simulations.

• Conclusions.

Filament Formation Observations• Gaizauskas et al. 2001 followed the formation and chirality of 10 large stable

filaments over 7 Carrington Rotations (1723-1727, June-October 1982). 2 large activity complexes with majority of flux

emergence in CR1723.

From 1724-1727 surface effects transport flux eastward and pole-ward. Very little change due to new emerging flux.

First filament formed after convergence and interaction of the activity complexes 27days after emergence.

Flux Transport Simulations.

• Mackay et al. 2000 carried out a comparison between theory and observations to determine the origin of the axial fields of the filaments.

• Evolved observed synoptic magnetograms with an initial potential coronal

field for 27 days under the surface effects of:

Differential rotation, Meridional Flows and Supergranular Diffusion.

• Flux transport effects only produced correct axial field in 5/10 cases.

• 27 days was insufficient time for surface effects “alone” to shear up the initial potential field and produce IP dipped field lines.

• Simulations missed an initial key ingredient from the observations: -ve helicity built up in the converging activity complexes during their formation as seen in chromospheric fibril structure.

Aims.• Reconsider the formation of the first filament with helicity included in the flux

transport model.• Show that with helicity included a magnetic structure that resembles the

filament can be produced along the PIL within observed time limits.

• Dextral Chirality

The Model• Use flux transport code to evolve large-scale photospheric/coronal

field.• Evolve, B through the induction equation.

• At the photosphere the field is subject to differential rotation, meridonal flows and supergranular diffusion.

• Reconnections only occur in the photosphere. In the corona use ideal induction equation with magneto-frictional method.

Configuring Simulations

• Consider idealised surface distributions which reproduce the main features of the observations :

Axisymmetric polar field.

3 bipolar regions.

Simulation 1: Initial Potential Field.

Initial Configuration 27 days of evolution

• Results agree with previous simulations : flux transport effects alone cannot produce the filament within 27 days from initial

potential magnetic fields.

Simulation 2: Untwisted Bipoles.

Initial Configuration. 27 days of evolution

• Results similar to that of the potential field simulations.

Simulation 3: Bipoles with -ve Helicity.

Initial Configuration. 27 days of evolution.

• When –ve helicity included in the initial configuration an IP

dipped flux tube is produced within 27 days.

Graph of Skew Angle

β =-0.6

β =-0.1

• Structures that resemble the filament may be produced for a wide range of the helicity parameter β < -0.3 .

Conclusions

• Have applied theory to observations where a filament formed 27 days after major flux emergence when only surface effects are acting on the flux.

• Shown that the inclusion of helicity (for which there is observational evidence) allows formation within allotted time (27 days).

• Model suggests that the axial component of filaments may results from a combination of:

Surface effects (flux transport)

Sub-surface effects (helicity)

• Surface effects convert large-scale helicity emerging in activity complexes into a smaller scale component parallel to PIL.