June 14, 2010, 2:00 pm - 3:00 pm

June 14, 2010, 2:00 pm - 3:00 pm

Twist in Coronal Magnetic Fields

Anna Malanushenko (Montana State University)

Twist of a magnetic field is an important property that is believed to play a role in evolution of coronal magnetic fields and to be one of the possible drivers of magnetic instabilities. Twist is well studied in thin flux tube approximation, where it has a simple geometrical meaning of the amount of full turns that field lines make about the axis. Solar active regions can not, in general, be modeled as thin flux tubes with definite axes, yet there are observations of instabilities similar to those described for thin flux tubes. In this talk I am going to present the results of my dissertation work: the generalization of twist for structures of arbitrary shapes and the ways to measure it in the solar corona. I will demonstrate that such generalized twist sets a threshold on kink instability similar to the classical twist. I will further discuss the ways of estimating it in the solar corona, where measurements of volume-filling magnetic field are not routinely performed at the present state. I describe a method of inferring twist in coronal fields that makes quantitative use of the morphology of coronal field evident in extreme ultraviolet. This method is different from non-linear force-free models, and it does not rely on vector magnetograms, yet, I demonstrate that it is capable of resolving spatially varying twist. I use the developed methodology to study an emerging active region and observe how the coronal twist changes in correspondence with its injection through the photosphere, obtained from helicity flux, making evident on how changes in the photosphere drive changes in coronal field.