A morphological study of helical coronal magnetic structures

Magnetostatic solutions describing magnetic flux ropes in realistic geometry are used to study solar coronal structures observed to have sigmoidal forms in soft X-rays. These solutions are constructed by embedding a rope of helically symmetric force-free magnetic fields in an external field such that force balance is assured everywhere. The two observed sigmoidal shapes, the S shapes and the mirror-reflected S shapes referred to as Z shapes in this paper, are found in both hemispheres of the solar corona, but observations made over the last two solar cycles suggest that the Z and S shapes occur preferentially in the northern and southern solar hemispheres, respectively. Our study makes an identification of the sigmoidal high-temperature coronal plasmas with heating by the spontaneous formation of current sheets described by the theory of Parker. This process involves a tangential discontinuity developing across a ribbon-like, twisted flux surface through an interaction between a magnetic flux rope and the photosphere, under conditions of high electrical conductivity. In this identification, Z- and S-shaped sigmoids are associated with flux ropes with negative and positive magnetic helicities, respectively. This association is physically consistent with the conclusion, based independently on measurements of prominence magnetic fields, that magnetic flux ropes occur preferentially with negative and positive helicities in the northern and southern solar hemispheres, respectively.