Evidence of Twisted Flux Tube Emergence in Active Regions

M. POISSON; C. H. MANDRINI; P. DÉMOULIN; M. LÓPEZ FUENTES

SOLAR PHYSICS

SPRINGER

Lugar: Berlin; Año: 2015 vol. 290 p. 727 - 751

0038-0938

Elongated magnetic polarities are observed during the emergencephase of bipolar active regions (ARs). These extended features, called magnetic ?tongues?, are interpreted as a consequence of the azimuthal component of the magnetic flux in the toroidal flux tubes that form ARs. We develop a new systematic and user-independent method to identify AR tongues. Our method is based on the determination and analysis of the evolution of the AR main polarity inversion line (PIL). The effect of the tongues is quantified by measuring the acute angle, τ , between the orientation of the PIL and the direction orthogonal to the AR main bipolar axis. We apply a simple model to simulate the emergence of a bipolar AR. This model lets us interpret the effect of magnetic tongues on parameters that characterize ARs (e.g., the PIL inclination and the tilt angles and their evolution). In this idealized kinematic emergence model, τ is a monotonously increasing function of the twist and has the same sign as the magnetic helicity. We systematically apply our procedure to a set of bipolar ARs (41 ARs) that were observed emerging in line-of-sight magnetograms along eight years. For the majority of the studied cases, the presence of tongues has a small influence on the AR tilt angle since tongues have much lower magnetic flux than the more concentrated main polarities. From the observed evolution of τ , corrected by the temporal evolution of the tilt angle and its final value when the AR is fully emerged, we estimate the average number of turns present in the sub-photospheric emerging flux rope. These values for the 41 observed ARs, except one, are below one. This indicates that sub-photospheric flux ropes have typically a low amount of twist, i.e. highly twisted flux tubes are rare. Our results demonstrate that the evolution of the PIL is a robust indicator of the presence of tongues and constrain the amount of twist present in emerging flux tubes.