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.
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Acknowledgments
We thank colleagues at the Big Bear Solar Observatory and the scientific groups of the SDO project for the possibility of accessing their observational material.
Funding
This work was partially supported by the Ministry of Education and Science of the Russiand Federation, project KP 19-270.
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Russian Text © The Author(s), 2019, published in Astronomicheskii Zhurnal, 2019, Vol. 96, No. 9, pp. 776–784.
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Filippov, B.P. “False” Sigmoids in the Solar Corona. Astron. Rep. 63, 778–785 (2019). https://doi.org/10.1134/S1063772919090026
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DOI: https://doi.org/10.1134/S1063772919090026