The dynamical properties of twisted ropes of magnetic field and the vigor of new active regions on the Sun

The equilibrium configuration following expansion of all, or part, of a force-free magnetic rope is worked out formally from the hydromagnetic equilibrium equations. It is shown that expansion of a portion of the length of a long magnetic rope beyond a certain radius causes the expanded portion to be wrapped with an azimuthal field composed of lines of force from the surface layer of the unexpanded portion. This singular redistribution takes an extended period of time to achieve. Further, it automatically causes the expanded portion to become unstable to buckling. We suggest that the restless behavior of the magnetic fields in new active regions in the solar photosphere is largely a result (a) of the buckling brought about by the expansion of the fields as they emerge through the photosphere, and (b) of the long-term redistribution of the surface lines of force onto the expanded portion of the rope. Subject headings: hydromagnetics - magnetic fields, solar - solar activity