Contributed Talk - Splinter Solar

Wednesday, 23 September 2020, 14:00   (virtual room L)

Parametric Study of Torus Instability Threshold

Bernhard Kliem, Jun Chen, Tibor Torok, Riu Liu
University of Potsdam; USTC Hefei, China; Predictive Science, Inc., San Diego, USA

A parametric numerical study of the torus instability threshold for the Titov-Demoulin (TD) (tokamak) equilibrium of a toroidal line-tied, force-free current channel and flux rope is presented, addressing the scatter about the nominal critical value of 3/2 for the decay index n_{cr}, which characterizes the equilibrium external poloidal field (the so-called strapping field). Values scattering in the range n_{cr} ~ 1--2 are typically found in numerical and observational studies of flux rope eruptions on the Sun. For vanishing external toroidal (shear, or guide) field, we find the threshold to lie in the theoretically expected range of ~ 1--3/2 (albeit with a reversed dependence on the minor radius, which stems from limitations of the TD equilibrium). An external toroidal field introduces a strong stabilizing effect on the instability, raising the threshold up to and even somewhat above 2, which explains observational and numerical results in this range. The potential field, used in practice, is found to yield a very good approximation to the threshold, provided its horizontal component perpendicular to the polarity inversion line is used to compute the decay index.