ON THE INTERCHANGE INSTABILITY OF SOLAR MAGNETIC-FLUX TUBES .1. THE INFLUENCE OF MAGNETIC TENSION AND INTERNAL GAS-PRESSURE

Small magnetic flux tubes at the solar surface are known to be interchange unstable. Previous studies of this instability have focussed on evacuated tubes in the thin tube approximation, thereby neglecting magnetic tension forces and internal atmospheres. We have extended the analysis to study the influence of these two missing components. An internal atmosphere can either have a stabilizing or destabilizing effect, depending upon the details of the prescribed gas pressure stratification. Magnetic tension forces in general stabilize the tubes by reducing the curvature of the boundary. For evacuated numerical models we find that the instability can be suppressed by a critical whirl flow of 2.2 km/s surrounding the tubes. This is considerably lower than previously found for evacuated thin tube structures (5.2 km/s). For tubes in temperature equilibrium with their surroundings this critical whirl velocity is even lower, while temperature differences at equal geometric height increase its value. The sharp interface between the magnetic field and the whirl flow is liable to the Kelvin-Helmholtz instability (KHI). However, the boundary layer of finite width at the tube surface might suppress the KHI, and hence make the whirl flow mechanism feasible, if the tube is highly evacuated at the relevant height of the atmosphere. We also discuss the stabilizing effect of a magnetic twist component on both instabilities.