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A Nonlinear Force-Free Magnetic Field Approximation Suitable for Fast Forward-Fitting to Coronal Loops. II. Numeric Code and Tests
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Markus J. Aschwanden Submitted: 2012-07-11 14:05
Based on a second-order approximation of nonlinear force-free
magnetic field solutions in terms of uniformly twisted field lines
derived in Paper I, we develop here a numeric code that is capable
to forward-fit such analytical solutions to arbitrary magnetogram
(or vector magnetograph) data combined with (stereoscopically
triangulated) coronal loop 3D coordinates. We test the code here
by forward-fitting to six potential field and six nonpotential field
cases simulated with our analytical model, as well as by forward-fitting
to an exactly force-free solution of the Low and Lou (1990) model.
The forward-fitting tests demonstrate: (i) a satisfactory convergence
behavior (with typical misalignment angles of mu approx 1°-10°),
(ii) relatively fast computation times (from seconds to a few minutes),
and (iii) the high fidelity of retrieved force-free α -parameters
(α
m fitα
m model approx 0.9-1.0$ for simulations
and α
m fitα
m model approx 0.7pm0.3$ for the Low and Lou
model). The salient feature of this numeric code is the relatively
fast computation of a quasi-forcefree magnetic field, which closely
matches the geometry of coronal loops in active regions, and
complements the existing {sl nonlinear force-free field (NLFFF)}
codes based on photospheric magnetograms without coronal constraints.
Authors: Markus J. Aschwanden and Anna Malanushenko
Projects: STEREO
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Publication Status: Solar Physics (in press)
Last Modified: 2012-07-12 09:38
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