DYNAMIC EVOLUTION OF TWISTED MAGNETIC-FLUX TUBES .1. EQUILIBRIUM AND LINEAR-STABILITY

The three-dimensional dynamical evolution of twisted magnetic flux tubes is studied using a time-dependent magnetohydrodynamic (MHD) model. The flux tubes are intended to model solar coronal loops, and include the stabilizing effect of photospheric line tying. The model permits the complete evolution of flux tubes to be followed self-consistently, including the formation, equilibrium, linear instability, and nonlinear behavior. Starting from an initial uniform background magnetic field, a twisted flux tube is created by the application of slow, localized photospheric vortex flows. The flux tube evolves quasi-statically through sequences of equilibria with increasing twist, until it becomes linearly unstable to an ideal MHD kink mode. In this paper the equilibrium properties and the linear stability behavior are discussed. The nonlinear behavior will be treated in the second paper of this series. The application of the method to the uniform-twist, Gold-Hoyle field confirms the previous stability threshold for kink instability and provides estimates of the resulting growth rate. When the method is applied to a loop with a localized twist profile, it is found that the loop becomes kink unstable when the twist exceeds 4.8π on axis.