Separator current sheets: Generic features in minimum-energy magnetic fields subject to flux constraints
Abstract
Equations are found for force-free magnetic equilibria in the ``coronal half-space'' z>0, subject to boundary conditions on the normal magnetic field at z=0. The distribution of normal field is assumed to be composed of NS isolated unipolar source regions of arbitrary shape, arranged arbitrarily on the plane. The equilibria are found by minimizing the magnetic energy subject to constraints on the total flux interconnecting pairs of source regions. For NS source regions interconnected in ND ways, there are Nc=ND-NS+1 distinct constraints on the field. Minimization subject to these constraints leads to an Nc-dimensional space of equilibria, for given boundary data. All field-lines connecting source regions are current-free, but the equilibrium contains Nc current-sheets lying at certain interfaces. In a two-dimensional magnetic field current sheets occur at points topologically equivalent to X-type neutral points in the potential field. In a three-dimensional field current sheets occur at points topologically equivalent to separators in the potential field. The free magnetic energy is a function of the Nc fluxes used to constrain the variation.
- Publication:
-
Physics of Plasmas
- Pub Date:
- December 2001
- DOI:
- 10.1063/1.1418431
- Bibcode:
- 2001PhPl....8.5277L
- Keywords:
-
- 96.60.Pb;
- 96.60.Mz;
- 96.60.Hv;
- 95.30.Qd;
- Photosphere;
- Electric and magnetic fields solar magnetism;
- Magnetohydrodynamics and plasmas