Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007apj...654..675l&link_type=abstract
The Astrophysical Journal, Volume 654, Issue 1, pp. 675-686.
Astronomy and Astrophysics
Astronomy
12
Magnetohydrodynamics: Mhd, Sun: Magnetic Fields
Scientific paper
A well-known problem in solar physics is that solutions for the transverse magnetic field direction are ambiguous with respect to a 180° reversal in the field direction. In this paper we focus on three methods for the removal of the 180° ambiguity applied to three MHD models. These methods are (1) the reference field method, (2) the method of magnetic pressure gradient, and (3) the magnetic field divergence-free method. All three methods are noniterative, and methods 2 and 3 are analytical and fast. We apply these methods to three MHD equilibrium model fields: (1) an analytical solution of a nonlinear force-free magnetic field equilibrium from Low, (2) a simulation of an emerging twisted flux tube from Fan & Gibson, and (3) a pre-eruptive twisted magnetic flux rope equilibrium reached by relaxation from Amari et al. We measure the success of methods within ``inverse horizontal field'' regions in the boundary, which are mathematically defined by B⊥˙∇⊥Bz>0. When such regions overlap with the magnetic field neutral lines, they are known as ``bald patches'' (BPs) or inverse topology. Our most important conclusion is that the magnetic divergence-free method is far more successful than the other two methods within BPs. This method requires a second level of measurements of the vertical magnetic field. As high-quality multilevel magnetograms will come online in the near future, our work shows that multilayer magnetic field measurements will be highly desirable to objectively and successfully tackle the 180° ambiguity problem.
Amari Tahar
Fan Yuhong
Li Jing
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