SURFACE CURRENTS ON MODELS OF FORCE-FREE SOLAR MAGNETIC-FLUX TUBES

A model of a cylindrically symmetric, force-free magnetic field consis ting of a sequence of concentric layers with piecewise-constant alpha is used to construct models of the surface currents on isolated, force -free magnetic flux tubes. Two boundary conditions are considered: a c urrent-neutralized flux tube (B(phi) = 0, B(z) not-equal 0 at r > r0), and an isolated current-carrying flux tube (B(phi) not-equal 0, B(z) = 0 at r > r0). A single-alpha model that is current-neutralized is a reverse-field pinch, and is unacceptable as a model for a solar flux t ube. Examples of two-alpha models for a current-neutralized flux tube are presented. The models of the surface currents satisfying either bo undary condition are shown to simplify considerably When the surface l ayer is thin. A model consisting of several layers, with piecewise-con stant alpha, may be used to find an approximate solution for a force-f ree flux tube with an arbitrarily specified current profile. 4 103