The Counter-kink Rotation of a Non-Hale Active Region

TL;DR

This paper studies the long-term evolution of a non-Hale active region, revealing its magnetic twist and flux tube deformation, and discusses whether kink instability explains its peculiar behavior.

Contribution

It provides a detailed analysis of the magnetic configuration and evolution of a non-Hale active region, challenging the role of kink instability in its development.

Findings

Magnetic twist sense aligns with hemispheric helicity expectations.

Relative motions suggest flux-tube deformation by external forces, not kink instability.

The region's evolution is consistent with external deformation rather than kink instability.

Abstract

We describe the long-term evolution of a bipolar non-Hale active region which was observed from October, 1995, to January, 1996. Along these four solar rotations the sunspots and subsequent flux concentrations, during the decay phase of the region, were observed to move in such a way that by December their orientation conformed to the Hale-Nicholson polarity law. The sigmoidal shape of the observed soft X-ray coronal loops allows us to determine the sense of the twist in the magnetic configuration. This sense is confirmed by extrapolating the observed photospheric magnetic field, using a linear force-free approach, and comparing the shape of computed field lines to the observed coronal loops. This sense of twist agrees with that of the dominant helicity in the solar hemisphere where the region lies, as well as with the evolution observed in the longitudinal magnetogram during the first rotation. At first sight the relative motions of the spots may be miss-interpreted as the rising of an $\Omega$-loop deformed by a kink-instability, but we deduce from the sense of their relative displacements a handedness for the flux-tube axis (writhe) which is opposite to that of the twist in the coronal loops and, therefore, to what is expected for a kink-unstable flux-tube. After excluding the kink instability, we interpret our observations in terms of a magnetic flux-tube deformed by external motions while rising through the convective zone. We compare our results with those of other related studies and we discuss, in particular, whether the kink instability is relevant to explain the peculiar evolution of some active regions.