Magnetic helicity in astrophysics
Abstract
Magnetic helicity, which is conserved in ideal MHD, measures the linkage of magnetic lines of force. After a brief review, I consider the helicity of a stellar magnetic field and show that a modified definition of helicity, called relative helicity, enables one to compute the relative amount of linkage of two field configurations above the photosphere even though the subphotospheric field is unobservable. I also show how the relative helicity changes as the result of motions of photospheric material. Then I consider the dissipation of helicity and show that in certain situations it is dissipated much more slowly than magnetic energy. Finally, I propose a variational principle for static problems, and show that in the presence of finite pressure, helicity conservation yields the usual equation of magnetostatic equilibrium. Woltjer's conclusion that the result is a forcefree field is a result of his neglect of pressure.
 Publication:

Magnetospheric Phenomena in Astrophysics
 Pub Date:
 1986
 DOI:
 10.1063/1.35665
 Bibcode:
 1986AIPC..144..324F
 Keywords:

 Astrophysics;
 Conservation Laws;
 Lines Of Force;
 Magnetic Field Configurations;
 Magnetohydrodynamics;
 Stellar Magnetic Fields;
 Energy Dissipation;
 ForceFree Magnetic Fields;
 Magnetic Flux;
 Pressure Effects;
 Topology;
 Astrophysics;
 97.10.Ld;
 95.30.Qd;
 Magnetic and electric fields;
 polarization of starlight;
 Magnetohydrodynamics and plasmas