Handle URI:
http://hdl.handle.net/10754/599068
Title:
On the Shape of Force-Free Field Lines in the Solar Corona
Authors:
Prior, C.; Berger, M. A.
Abstract:
This paper studies the shape parameters of looped field lines in a linear force-free magnetic field. Loop structures with a sufficient amount of kinking are generally seen to form S or inverse S (Z) shapes in the corona (as viewed in projection). For a single field line, we can ask how much the field line is kinked (as measured by the writhe), and how much neighbouring flux twists about the line (as measured by the twist number). The magnetic helicity of a flux element surrounding the field line can be decomposed into these two quantities. We find that the twist helicity contribution dominates the writhe helicity contribution, for field lines of significant aspect ratio, even when their structure is highly kinked. These calculations shed light on some popular assumptions of the field. First, we show that the writhe of field lines of significant aspect ratio (the apex height divided by the footpoint width) can sometimes be of opposite sign to the helicity. Secondly, we demonstrate the possibility of field line structures which could be interpreted as Z-shaped, but which have a helicity value sign expected of an S-shaped structure. These results suggest that caution should be exercised in using two-dimensional images to draw conclusions on the helicity value of field lines and flux tubes. © 2012 Springer Science+Business Media B.V.
Citation:
Prior C, Berger MA (2012) On the Shape of Force-Free Field Lines in the Solar Corona. Sol Phys 278: 323–345. Available: http://dx.doi.org/10.1007/s11207-012-9938-3.
Publisher:
Springer Nature
Journal:
Solar Physics
KAUST Grant Number:
KUK-C1-013-04
Issue Date:
2-Feb-2012
DOI:
10.1007/s11207-012-9938-3
Type:
Article
ISSN:
0038-0938; 1573-093X
Sponsors:
C.P. wishes to thanks John Brooke for his comments and observations on the manuscript. C.P. was supported by the EPSRC and Award No. KUK-C1-013-04 made by King Abdullah University of Science and Technology (KAUST). M.B. was partially supported by an STFC grant and a Leverhulme grant.
Appears in Collections:
Publications Acknowledging KAUST Support

Full metadata record

DC FieldValue Language
dc.contributor.authorPrior, C.en
dc.contributor.authorBerger, M. A.en
dc.date.accessioned2016-02-25T13:52:16Zen
dc.date.available2016-02-25T13:52:16Zen
dc.date.issued2012-02-02en
dc.identifier.citationPrior C, Berger MA (2012) On the Shape of Force-Free Field Lines in the Solar Corona. Sol Phys 278: 323–345. Available: http://dx.doi.org/10.1007/s11207-012-9938-3.en
dc.identifier.issn0038-0938en
dc.identifier.issn1573-093Xen
dc.identifier.doi10.1007/s11207-012-9938-3en
dc.identifier.urihttp://hdl.handle.net/10754/599068en
dc.description.abstractThis paper studies the shape parameters of looped field lines in a linear force-free magnetic field. Loop structures with a sufficient amount of kinking are generally seen to form S or inverse S (Z) shapes in the corona (as viewed in projection). For a single field line, we can ask how much the field line is kinked (as measured by the writhe), and how much neighbouring flux twists about the line (as measured by the twist number). The magnetic helicity of a flux element surrounding the field line can be decomposed into these two quantities. We find that the twist helicity contribution dominates the writhe helicity contribution, for field lines of significant aspect ratio, even when their structure is highly kinked. These calculations shed light on some popular assumptions of the field. First, we show that the writhe of field lines of significant aspect ratio (the apex height divided by the footpoint width) can sometimes be of opposite sign to the helicity. Secondly, we demonstrate the possibility of field line structures which could be interpreted as Z-shaped, but which have a helicity value sign expected of an S-shaped structure. These results suggest that caution should be exercised in using two-dimensional images to draw conclusions on the helicity value of field lines and flux tubes. © 2012 Springer Science+Business Media B.V.en
dc.description.sponsorshipC.P. wishes to thanks John Brooke for his comments and observations on the manuscript. C.P. was supported by the EPSRC and Award No. KUK-C1-013-04 made by King Abdullah University of Science and Technology (KAUST). M.B. was partially supported by an STFC grant and a Leverhulme grant.en
dc.publisherSpringer Natureen
dc.subjectCorona, structuresen
dc.subjectHelicity, magnetic, observationsen
dc.subjectMagnetic fields, coronaen
dc.titleOn the Shape of Force-Free Field Lines in the Solar Coronaen
dc.typeArticleen
dc.identifier.journalSolar Physicsen
dc.contributor.institutionUniversity of Oxford, Oxford, United Kingdomen
dc.contributor.institutionUniversity of Exeter, Exeter, United Kingdomen
kaust.grant.numberKUK-C1-013-04en
All Items in KAUST are protected by copyright, with all rights reserved, unless otherwise indicated.