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Session 2 - Everything Else.
Display session, Friday, June 27
Ballroom C, Chair: Richard Canfield

[2.49] Why and Where do Filaments Form in Active Regions?

V. Gaizauskas (HIA/NRCC, Ottawa), C. Zwaan (Sterrekundig Instituut, Utrecht)

New prominence models based on recent observations depend upon magnetic reconnection between small-scale magnetic elements converging at a polarity inversion (PI). How then to explain active-region filaments where magnetic flux diverges over much of the lifetime of the region? A partial answer is that still-growing active regions containing filaments are not simple bipolar entities. They are instead multipolar activity complexes (`sunspot nests') wherein magnetic flux can be compressed along a meandering PI wherever new bipolar units emerge near old ones. A complete answer requires particulars about the distribution and motions of magnetic fields internal and external to the sunspot nests. We therefore surveyed over 150 active regions photographed on a large spatial scale at ORSO during 5 successive solar rotations in 1979, an epoch of rapid emergence and decay.

Of the total number of regions: - 5% are simple decaying bipolar plages with filaments on the PI; - 5% are ambiguous cases with sometimes a filament and field transition arches (FTA) sharing adjacent parts of a PI in a bipolar plage; -70% have boundary filaments exterior to the concentrations of magnetic flux around sunspots; - 61% are single bipoles of which 84% have no internal filament on their PI; - 52% are activity complexes (on at least one day, otherwise they are single bipoles) of which 60% have one or more filaments inside the complex.

We find that filaments inside sunspot nests mark off bipolar entities from one another, thus fulfilling the role of boundary filaments on the inside of the nests. We conclude that the boundary filament is the quintessential active- region filament. Examination of specific cases leads to the further conclusion that force-free fields together with cancelling flux play a critical role in forming boundary filaments.

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