A nonsingular model of the open magnetosphere

We present a modified version of the Toffoletto and Hill (1989) open magnetosphere model that incorporates a tail‐like interconnection field with a discontinuity to represent the slow‐mode expansion fan that defines the high‐latitude tail magnetopause. (The interconnection field is defined as the perturbation on an initially closed magnetosphere model to make it open.) The expansion fan controls the open field line region in the tail, and the intersection of the fan with the tail current sheet is, by design, the x line. The new interconnection field allows greater control of the tail field structure; in particular, it enables us to eliminate the nightside mapping singularity that occurs in previous models when the interplanetary magnetic field is nonsouthward. Also, in contrast to earlier models, the far tail x line extends farther downstream on the flanks than in the center of the tail, consistent with observations.