2020/04/18 by Kai Yang, Kai E. Yang, Michael S. Wheatland +3
Biochemistry, Genetics and Molecular Biology · Mathematics · Physics and Astronomy · #Classical mechanics #Condensed matter physics #Field (mathematics) #Geomagnetism and Paleomagnetism Studies #Helicity #Ionosphere and magnetosphere dynamics #Magnetic energy #Magnetic field #Magnetic helicity #Magnetization #Magnetohydrodynamics #Mathematics #Physics #Quantum electrodynamics #Quantum mechanics #Solar and Space Plasma Dynamics #astro-ph.SR
paper · pdf · doi:10.3847/1538-4357/ab8810
33 pages, 7 figures, accepted by ApJ
arxiv created 2020/04/18 · openalex publication_date 2020/05/01 · arxiv updated 2020/05/27 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
Abstract Magnetic helicity is conserved under ideal magnetohydrodynamics and quasi-conserved even under a resistive process. The standard definition for magnetic helicity cannot be applied directly to an open magnetic field in a volume, because it is gauge-dependent. Instead, the relative magnetic helicity is widely used. We find that the energy of a potential magnetic field in a rectangular domain with periodic lateral boundary conditions is less than that of the field with a fixed normal component on all six boundaries. To make use of this lower energy potential field in the analysis of relative magnetic helicity, we introduce a new definition for magnetic helicity for the magnetic field, which involves the periodic potential field. We apply this definition to a sequence of analytic solutions and a numerical simulation. The results show that our new gauge-invariant helicity is very close to the current-carrying part of the relative magnetic helicity of the original magnetic field. We find also that the ratio between the current-carrying helicity and the relative magnetic helicity for the original and our defined relative helicity show different behavior. It seems that the new helicity is more sensitive to the component of the field due to the electric current in the volume, which is the source for instabilities and solar eruptive phenomena.