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The Writhe of Helical Structures in the Solar CoronaContext. Helicity is a fundamental property of magnetic fields, conserved in ideal MHD. In flux rope topology, it consists of twist and writhe helicity. Despite the common occurrence of helical structures in the solar atmosphere, little is known about how their shape relates to the writhe, which fraction of helicity is contained in writhe, and how much helicity is exchanged between twist and writhe when they erupt. Aims. Here we perform a quantitative investigation of these questions relevant for coronal flux ropes. Methods. The decomposition of the writhe of a curve into local and nonlocal components greatly facilitates its computation. We use it to study the relation between writhe and projected S shape of helical curves and to measure writhe and twist in numerical simulations of flux rope instabilities. The results are discussed with regard to filament eruptions and coronal mass ejections (CMEs). Results. (1) We demonstrate that the relation between writhe and projected S shape is not unique in principle, but that the ambiguity does not affect low-lying structures, thus supporting the established empirical rule which associates stable forward (reverse) S shaped structures low in the corona with positive (negative) helicity. (2) Kink-unstable erupting flux ropes are found to transform a far smaller fraction of their twist helicity into writhe helicity than often assumed. (3) Confined flux rope eruptions tend to show stronger writhe at low heights than ejective eruptions (CMEs). This argues against suggestions that the writhing facilitates the rise of the rope through the overlying field. (4) Erupting filaments which are S shaped already before the eruption and keep the sign of their axis writhe (which is expected if field of one chirality dominates the source volume of the eruption), must reverse their S shape in the course of the rise. Implications for the occurrence of the helical kink instability in such events are discussed.
Document ID
20100027447
Acquisition Source
Headquarters
Document Type
Preprint (Draft being sent to journal)
Authors
Toeroek, T.
(Observatoire de Paris-Meudon France)
Berger, M. A.
(Mullard Space Science Lab. Dorking, United Kingdom)
Kliem, B.
(Mullard Space Science Lab. Dorking, United Kingdom)
Date Acquired
August 24, 2013
Publication Date
April 23, 2010
Publication Information
Publication: Astronomy and Astrophysics
Volume: 516
Subject Category
Astrophysics
Report/Patent Number
ARXIV-1004.3918V1
AD-A522691
Funding Number(s)
CONTRACT_GRANT: NNH06AD58I
CONTRACT_GRANT: NNX08AG44G
Distribution Limits
Public
Copyright
Other
Keywords
WRITHE
HELICITY
PROMINENCES
FOREIGN REPORTS

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