The Global Distribution of Magnetic Helicity in the Sun’s Corona

Modern observations are revealing the Sun’s large-scale magnetic field to have a complex, non-potential structure. Moreover, it is now widely believed that the loss of equilibrium of twisted magnetic flux ropes is responsible for many (if not all) coronal mass ejections. But predicting where these flux ropes will form, and in particular whether or when they might erupt, remains a challenge for model reconstructions of the coronal magnetic field. In this talk, I will introduce the “field line helicity” as an appropriate and practical diagnostic for identifying twisted structures in coronal models. Since field lines are magnetic sub-domains, this is a more meaningful measure than the density of magnetic helicity at individual points. On the other hand, it provides local information that the globally-integrated magnetic helicity cannot. I will illustrate the power of this diagnostic on my own numerical non-potential evolution model of the global corona.