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 twis t force, the tube approaches an inertial collapse phase in finite time . The competing pinch and magnetic pressure forces cause radial oscill ation during the collapse. The plasma pressure is negligible in the pr ocess. When the tube is subject to a random twist, each end of the tub e absorbs angular momentum preferentially in the direction of the init ial field line winding. Thus again, the tube is twisted up and collaps es in finite time. For typical solar parameters, the collapse time is a few tens of the coronal Alfven time. Both magnetic and kinetic energ y increase explosively near the collapse as a result of the twist. The tube contracts to about one-tenth of its original size before reachin g the kink threshold twist angle. (C) 1995 American Institute of Physi cs.