The evolution of writhe in kink-unstable flux ropes and erupting filaments

Török, T. and Kliem, B. and Berger, M. A. and Linton, M. G. and Démoulin, P. and Van Driel Gesztelyi, Lídia (2014) The evolution of writhe in kink-unstable flux ropes and erupting filaments. PLASMA PHYSICS AND CONTROLLED FUSION, 56 (6). ISSN 0741-3335

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Abstract

The helical kink instability of a twisted magnetic flux tube has been suggested as a trigger mechanism for solar filament eruptions and coronal mass ejections (CMEs). In order to investigate if estimations of the pre-eruptive twist can be obtained from obser- vations of writhe in such events, we quantitatively analyze the conversion of twist into writhe in the course of the instability, using numerical simulations. We consider the line tied, cylindrically symmetric Gold–Hoyle flux rope model and measure the writhe using the formulae by Berger and Prior which express the quantity as a single integral in space. We find that the amount of twist converted into writhe does not simply scale with the initial flux rope twist, but depends mainly on the growth rates of the insta- bility eigenmodes of higher longitudinal order than the basic mode. The saturation levels of the writhe, as well as the shapes of the kinked flux ropes, are very similar for considerable ranges of initial flux rope twists, which essentially precludes estimations of pre-eruptive twist from measurements of writhe. However, our simulations suggest an upper twist limit of ∼ 6π for the majority of filaments prior to their eruption.

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