Formation of current sheets and sigmoidal structure by the kink instability of a magnetic loop

We study dynamical consequences of the kink instability of a twisted coronal flux rope, using the force-free coronal loop model by [CITE] as the initial condition in ideal-MHD simulations. When a critical value of the twist is exceeded, the long-wavelength ($m = 1$) kink mode develops. Analogous to...

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Bibliographic Details
Published in:Astronomy and astrophysics (Berlin) 2004-01, Vol.413 (3), p.L23-L26
Main Authors: Kliem, B., Titov, V. S., Török, T.
Format: Article
Language:English
Subjects:
Astronomy
Corona. Coronal loops, streamers, and holes
Coronal mass ejection
Earth, ocean, space
Exact sciences and technology
instabilities
magnetic fields
MHD
Solar physics
Solar system
Sun: corona
Sun: coronal mass ejections (CMEs)
Sun: flares
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Summary:We study dynamical consequences of the kink instability of a twisted coronal flux rope, using the force-free coronal loop model by [CITE] as the initial condition in ideal-MHD simulations. When a critical value of the twist is exceeded, the long-wavelength ($m = 1$) kink mode develops. Analogous to the well-known cylindrical approximation, a helical current sheet is then formed at the interface with the surrounding medium. In contrast to the cylindrical case, upward-kinking loops form a second, vertical current sheet below the loop apex at the position of the hyperbolic flux tube (generalized X line) in the model. The current density is steepened in both sheets and eventually exceeds the current density in the loop (although the kink perturbation starts to saturate in our simulations without leading to a global eruption). The projection of the field lines that pass through the vertical current sheet shows an S shape whose sense agrees with the typical sense of transient sigmoidal (forward or reverse S-shaped) structures that brighten in soft X rays prior to coronal eruptions. The upward-kinked loop has the opposite S shape, leading to the conclusion that such sigmoids do not generally show the erupting loops themselves but indicate the formation of the vertical current sheet below them that is the central element of the standard flare model.
ISSN:0004-6361
1432-0746
DOI:10.1051/0004-6361:20031690