L. Yelles Chaouche, C. Kuckein, V. Martínez Pillet, F. Moreno-Insertis
The 3D structure of an active region (AR) filament is studied using nonlinear
force-free field (NLFFF) extrapolations based on simultaneous observations at a
photospheric and a chromospheric height. To that end, we used the Si I 10827
\AA\ line and the He I 10830 \AA\ triplet obtained with the Tenerife Infrared
Polarimeter (TIP) at the VTT (Tenerife). The two extrapolations have been
carried out independently from each other and their respective spatial domains
overlap in a considerable height range. This opens up new possibilities for
diagnostics in addition to the usual ones obtained through a single
extrapolation from, typically, a photospheric layer. Among those possibilities,
this method allows the determination of an average formation height of the He I
10830 \AA\ signal of \approx 2 Mm above the surface of the sun. It allows, as
well, to cross-check the obtained 3D magnetic structures in view of verifying a
possible deviation from the force- free condition especially at the
photosphere. The extrapolations yield a filament formed by a twisted flux rope
whose axis is located at about 1.4 Mm above the solar surface. The twisted
field lines make slightly more than one turn along the filament within our box,
which results in 0.055 turns/Mm. The convex part of the field lines (as seen
from the solar surface) constitute dips where the plasma can naturally be
supported. The obtained 3D magnetic structure of the filament depends on the
choice of the observed horizontal magnetic field as determined from the
180\circ solution of the azimuth. We derive a method to check for the
correctness of the selected 180\circ ambiguity solution.
View original:
http://arxiv.org/abs/1201.2456
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