On the origin of peculiar active regions

MANDRINI, C.H.; LÓPEZ FUENTES, M.C.; DÉMOULIN, P.; VAN DRIEL-GESZTELYI, L.; PEVTSOV, A.A.

Santorini

Congreso; SOLMAG 2002 - Magnetic Coupling of the Solar Atmosphere Euroconference y IAU Colloquium 188; 2002

ESA - IAU

We study the long term evolution of a set of bipolar active regions (ARs) in which the main photospheric polarities are seen to rotate one around the other during several solar rotations. After showing that differential rotation cannot produce this large change in the tilt angle, we interpret this peculiar evolution as being the result of the emergence of magnetic flux tubes that are distorted with respect to the classical Omega-loop shape. A possible origin of this distortion is the nonlinear development of a kink-instability. Flux tubes deformed by this mechanism must have the same sign of twist and writhe. From the observed evolution of the tilt of the bipoles, we derive the sign of the writhe of the tube forming each AR; while we compute the sign of the twist from transverse field measurements. Comparing the handedness of the magnetic twist and writhe, we find that the presence of kink-unstable flux tubes is coherent with no more than 32% of the studied cases; so, a small fraction of these peculiar ARs can be explained by this process. Then we discuss the role that other mechanisms may play inducing the inferred deformation, such as the Coriolis force or external rotational motions of the plasma as the tube ascends in the convection zone.