ON THE INTERCHANGE INSTABILITY OF SOLAR MAGNETIC-FLUX TUBES .2. THE INFLUENCE OF ENERGY-TRANSPORT EFFECTS

We examine the interchange instability of thin photospheric magnetic flux tube models which satisfy both force and energy balance with their surroundings. The stability of the tubes is independent of the efficiency of internal convective energy transport and shows only a weak dependence on the plasma beta. The structures are susceptible to the instability in a layer 200 - 300 km deep immediately below optical depth unity in the quiet photosphere. The presence of an internal atmosphere reduces the magnetic field strength in comparison with that of an evacuated tube. While this has a stabilizing effect on the tube surface, temperature differences between interior and exterior are usually destabilizing. We find that the two effects approximately cancel each other for tubes with radii R less than or similar 200 km for which the stability properties are very similar to those of completely evacuated structures. For larger tubes, the temperature contrast with respect to the surroundings begins to dominate and destabilizes the tubes. Thus, despite the inclusion of energy transport effects on the tube structure, the stability problem of small tubes (with magnetic fluxes PHI < 10(19) - 10(20) Mx) remains. Consequences for photospheric magnetic fields are discussed.