NONLINEAR MAGNETOHYDRODYNAMIC EVOLUTION OF LINE-TIED CORONAL LOOPS

Simulations of the nonlinear evolution of the m = 1 kink mode in magne tic flux tubes with line-tying boundary conditions are presented, The initial structure of the flux tube is intended to model a solar corona l loop that either has evolved quasi-statically through sequences of e quilibria with increasing twist due to the application of localized ph otospheric vortex flows or has emerged with a net current through the photosphere. It is well known that when the twist exceeds a critical v alue that depends on its radial profile and on the loop length, the lo op becomes kink unstable, The nonlinear evolution of the instability i s followed using a three-dimensional MHD code in cylindrical geometry, in different types of magnetic field configurations, with the common property that the current is confined within the same radius, so that the magnetic field is potential in the external regions. The differenc es reside in the net axial current carried by the structure, ranging f rom a vanishing current (corresponding to an outer axial potential fie ld) to a high current (corresponding to an outer almost azimuthal pote ntial field). It is shown that, during the nonlinear phase of the inst ability, loops develop current sheets and, consequently, their evoluti on becomes resistive with the occurrence of magnetic reconnection, The dependence of the topology of the currents at saturation on the initi al magnetic structure, the details of the reconnection phenomenon, and the resistive dissipation mechanism are examined. Finally, the impact of the results on the understanding of coronal activity is discussed.