Twist, writhe and energy from the helicity of magnetic perturbed vortex filaments

Abstract

The twist and writhe numbers and magnetic energy of an orthogonally perturbed vortex filaments are obtained from the computation of the magnetic helicity of geodesic and abnormal magnetohydrodynamical (MHD) vortex filament solutions. Twist is computed from a formula recently derived by Berger and Prior [J. Phys. A 39 (2006) 8321] and finally writhe is computed from the theorem that the helicity is proportional to the sum of twist and writhe. The writhe number is proportional to the total torsion and to integrals of the vector potential. The magnetic energy is computed in terms of the integral of the torsion squared which allows us to place a bound to energy in in the case of helical filaments due to a theorem by Fukumoto [J. Phys. Soc. Japan,(1987)] in fluid vortex filaments. It is also shown that filament torsion coincides with the magnetic twist in the case under consideration, where a small orthogonal magnetic field exist along the thin filament. A new Aharonov-Bohm (AB) phase term is obtained in the writhe number expression which is not present in the Moffatt-Ricca (Proc Roy Soc London A,1992) computation.