A Model for the Emergence of a Twisted Magnetic Flux Tube

Abstract

Observations have shown that active region flux tubes often emerge in a twisted state and that the active region formed has magnetic helicity of the same sense as the flux tube that forms it. Separate theoretical models have been developed for coronal magnetic fields with helicity and for flux tubes with twist. Here we present a dynamical model that connects a twisted subphotospheric flux tube to a force-free coronal field. With this model it is possible to explore the emergence of a flux tube into the corona and its effect on both the coronal field and the subphotospheric flux tube. In particular, the model shows that only a fraction of the current carried by the twisted flux tube will pass into the corona. As a consequence of this ``mismatch,'' a torsional Alfvén wave is launched downward along the flux tube at the instant of emergence. As the flux tube continues to emerge, the helicity of the coronal field increases owing to rotation of the footpoints. Our model predicts that the level of rotation will depend upon the rapidity of flux emergence. After this transient period the helicity of the active region will reflect the twist in its parent tube.