Twisted coronal magnetic loops

Observed coronal loops have a surprisingly uniform thickness that cannot be easily understood in terms of standard coronal magnetic field models. We investigate the possibility that the uniform thickness can be explained by locally enhanced twist in the field, so that observed loops correspond to twisted coronal flux tubes. Our approach is to construct numerical models of fully three-dimensional force-free magnetic fields. To resolve the internal structure of an individual loop embedded within a much larger dipole configuration, we use a nonuniform numerical grid of size 609 x 513 x 593, the largest ever applied to a solar problem to our knowledge. Our models show that twist promotes circular cross sections in loops. Such cross sections are typically assumed, and have recently been verified from observations, but their physical cause has been heretofore unexplained.