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The Minimal Helicity of Solar Coronal Magnetic Fields

Yeates, AR

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Potential field extrapolations are widely used as minimum-energy models for the Sun's coronal magnetic field. As the reference to which other magnetic fields are compared, they have—by any reasonable definition—no global (signed) magnetic helicity. Here we investigate the internal topological structure that is not captured by the global helicity integral, by splitting it into individual field line helicities. These are computed using potential field extrapolations from magnetogram observations over Solar Cycle 24, as well as for a simple illustrative model of a single bipolar region in a dipolar background. We find that localized patches of field line helicity arise primarily from linking between strong active regions and their overlying field, so that the total unsigned helicity correlates with the product of photospheric and open fluxes. Within each active region, positive and negative helicity may be unbalanced, but the signed helicity is only around a tenth of the unsigned helicity. Interestingly, in Cycle 24, there is a notable peak in unsigned helicity caused by a single large active region. On average, the total unsigned helicity at the resolution considered is approximately twice the typical signed helicity of a single real active region, according to non-potential models in the literature.


Yeates, A. (2020). The Minimal Helicity of Solar Coronal Magnetic Fields. Astrophysical Journal Letters, 898(2), Article L49.

Journal Article Type Article
Acceptance Date Jul 16, 2020
Publication Date 2020-08
Deposit Date Jul 21, 2020
Publicly Available Date Jul 21, 2020
Journal Astrophysical Journal Letters
Print ISSN 2041-8205
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 898
Issue 2
Article Number L49
Publisher URL https:/


Accepted Journal Article (6.4 Mb)

Copyright Statement
© 2020. The American Astronomical Society. All rights reserved.

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