“False” Sigmoids in the Solar Corona

Abstract

As a rule, sigmoidal structures in the colar corona, i.e., structures having the appearance of an S or reversed S, are taken to be evidence for twisting of the magnetic-field lines and the presence of electric currents in the corona. However, these formations can also exist in a potential field. The S-like shapes of solar filaments, for example, are determined by the distribution of photospheric magnetic fields, which do not depend on coronal currents. Sigmoidal field lines can appear in a fairly complex potential field in the vicinities of null singular points. Such field lines can display shear, which is also often taken as manifestation of a non-potential field. Thus, one should treat some evidence for non-potential magnetic fields in the corona cautiously, and rule out other origins for the appearance of “suspicious” structures before drawing conclusions about possible non-potentiality of the fields.