Formation of current sheets and sigmoidal structure by the kink instability of a magnetic loop
Bernhard Kliem Submitted: 2003-11-06 17:30
We study dynamical consequences of the kink instability of a
twisted coronal flux rope, using the force-free coronal loop model by
Titov & D'emoulin (1999) 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 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.
Authors: B. Kliem, V. S. Titov, and T. Toeroek
Publication Status: Astron. Astrophys. Lett., accepted
Last Modified: 2003-11-07 15:37