Reconstructing Highly-twisted Magnetic Fields

Abstract

We investigate the ability of a nonlinear force-free code to calculate highly-twisted magnetic field configurations using the Titov and D\'{e}moulin (1999) equilibrium field as a test case. The code calculates a force-free field using boundary conditions on the normal component of the field in the lower boundary, and the normal component of the current density over one polarity of the field in the lower boundary. The code can also use the current density over both polarities of the field in the lower boundary as a boundary condition. We investigate the accuracy of the reconstructions with increasing flux-rope surface twist number $N_{\textrm{t}}$, achieved by decreasing the sub-surface line current in the model. We find that the code can approximately reconstruct the Titov-D\'{e}moulin field for surface twist numbers up to $N_{\textrm{t}} \approx 8.8$. This includes configurations with bald patches. We investigate the ability to recover bald patches, and more generally identify the limitations of our method for highly-twisted fields. The results have implications for our ability to reconstruct coronal magnetic fields from observational data.