NONLINEAR DYNAMICS OF TWISTED MAGNETIC-FLUX TUBES

The nonlinear response of an axisymmetric magnetic flux tube under the influence of an azimuthal twist force is studied. For a constant twist force, the tube approaches an inertial collapse phase in finite time. The competing pinch and magnetic pressure forces cause radial oscillation during the collapse. The plasma pressure is negligible in the process. When the tube is subject to a random twist, each end of the tube absorbs angular momentum preferentially in the direction of the initial field line winding. Thus again, the tube is twisted up and collapses in finite time. For typical solar parameters, the collapse time is a few tens of the coronal Alfven time. Both magnetic and kinetic energy increase explosively near the collapse as a result of the twist. The tube contracts to about one-tenth of its original size before reaching the kink threshold twist angle. (C) 1995 American Institute of Physics.