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First 3D Reconstructions of Coronal Loops with the STEREO A+B Spacecraft: IV. Magnetic Modeling with Twisted Force-Free Fields
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Markus J. Aschwanden Submitted: 2012-07-11 14:09
The three-dimensional (3D) coordinates of stereoscopically triangulated
loops provide strong constraints for magnetic field models of
active regions in the solar corona. Here we use STEREO/A and B data from
some 500 stereoscopically triangulated loops observed in four active
regions (2007 Apr 30, May 9, May 19, Dec 11), together with SOHO/MDI
line-of-sight magnetograms. We measure the average misalignment angle
between the stereoscopic loops and theoretical magnetic field models,
finding a mismatch of mu=19°-46° for a potential field model,
which is reduced to mu=14°-19° for a non-potential field model
parameterized by twist parameters.
The residual error is commensurable with stereoscopic measurement
errors (muSE approx 8°-12°). We developed a potential
field code that deconvolves a line-of-sight magnetogram into three
magnetic field components (B_x, B_y, B_z), as well as a non-potential
field forward-fitting code that determines the full
length of twisted loops (L approx 50-300 Mm),
the number of twist turns (median Ntwist=0.06),
the nonlinear force-free α -parameter
(median α approx 4 x 10-11 cm-1$),
and the current density (median j_z approx 1500 Mx cm-2 s-1).
All twisted loops are found to be far below the critical value
for kink instability, and Joule dissipation of their currents
is found be be far below the coronal heating requirement.
The algorithm developed here, based on an analytical solution of
nonlinear force-free fields that is accurate to second order
(in the force-free parameter α ),
represents the first code that enables fast forward-fitting
to photospheric magnetograms and stereoscopically triangulated loops
in the solar corona.
Authors: Markus J. Aschwanden, Jean-Pierre Wuelser, Nariaki V. Nitta, James R. Lemen, Marc L. DeRosa, and Anna Malanushenko
Projects: STEREO
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Publication Status: The Astrophysical Journal (in press)
Last Modified: 2012-07-12 09:38
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