Evidence of Twisted Flux-Tube Emergence in Active Regions

Abstract

Elongated magnetic polarities are observed during the emergence phase 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 monotonically 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 over eight years. For the majority of the cases studied, 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 subphotospheric emerging flux-rope. These values for the 41 observed ARs, except one, are below unity. This indicates that sub-photospheric flux-ropes typically have 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 constrains the amount of twist present in emerging flux-tubes.

Evolution of the magnetic field distribution of active regions

“…Figure 6 shows that larger ARs generally do take a longer time to emerge, but that the relationship between emergence time and peak flux is not a simple one, with some regions taking a long time to emerge without reaching a particularly high peak flux. The rates of flux emergence were seen to vary, both during the emergence of single regions and from one AR to another in agreement with the results of Poisson et al (2015Poisson et al ( , 2016 obtained on other sets of ARs. Some regions were seen to have an initial gradual emergence phase followed by a more rapid phase, while others emerged rapidly at the start and had a decreasing emergence rate later on.…”

The Origin of Net Electric Currents in Solar Active Regions

“…This is unlikely for both emerging and evolved/decaying ARs. Indeed, several observational and numerical studies show that newly emerged ARs generally possess a strongly sheared PIL (e.g., Manchester et al 2004;Canou et al 2009;Georgoulis et al 2012;Toriumi et al 2013;Poisson et al 2015).…”

Successive injection of opposite magnetic helicity in solar active region NOAA 11928

“…The flux of both polarities predominantly increases in time owing to the emergence of these flux regions. This AR has an almost constant rate of flux emergence which is not so frequent (e.g., see examples in Poisson et al 2015). The observed activity is limited to GOES class C without coronal mass ejections (CMEs).…”