2008/05/13 by Mei Zhang, Natasha Flyer · 44 citations
Biochemistry, Genetics and Molecular Biology · Physics and Astronomy · #Boundary (topology) #Boundary value problem #Geomagnetism and Paleomagnetism Studies #Helicity #Instability #Ionosphere and magnetosphere dynamics #Magnetic field #Magnetic helicity #Solar and Space Plasma Dynamics #Upper and lower bounds #astro-ph
paper · pdf · doi:10.1086/589993
published in The Astrophysical Journal 683(2), 1160-1167 (IOP Publishing) · accepted by ApJ
arxiv created 2008/05/13 · openalex publication_date 2008/08/19 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/06
This paper follows up on a previous study showing that in an open atmosphere such as the solar corona, the total magnetic helicity of a force-free field must be bounded, and the accumulation of magnetic helicity in excess of its upper bound would initiate a nonequilibrium situation resulting in an expulsion such as a coronal mass ejection (CME). In the current paper, we investigate the dependence of the helicity bound on the boundary condition for several families of nonlinear force-free fields. Our calculation shows that the magnitude of the helicity upper bound of force-free fields is nontrivially dependent on the boundary condition. Fields with a multipolar boundary condition can have a helicity upper bound 10 times smaller than those with a dipolar boundary condition when helicity values are normalized by the square of their respective surface poloidal fluxes. This suggests that a coronal magnetic field may erupt into a CME when the applicable helicity bound falls below the already accumulated helicity as the result of a slowly changing boundary condition. Our calculation also shows that a monotonic accumulation of magnetic helicity can lead to the formation of a magnetic flux rope applicable to kink instability. This suggests that CME initiations by exceeding helicity bound and by kink instability can both be the consequences of helicity accumulation in the corona. Our study gives insights into the observed associations of CMEs with the magnetic features at their solar surface origins.