ON THE CONVECTIVE STABILITY OF SOLAR PHOTOSPHERIC FLUX TUBES

One-dimensional magnetohydrodynamic (MHD) numerical simulations were performed in order to investigate the convective stability of an intense photosheric flux tube, adopting the thin flux-tube approximation. The radiative heat exchange between the flux tube and its surroundings was also taken into account using Newton's law of cooling. To study the stability, we examined the temporal evolution of a small perturbation (downflow) added to an initial atmosphere. When a closed-boundary condition is adopted at the lower boundary, the perturbation evolves into a longitudinal overstable oscillation. However, an open (how-through) condition at the lower boundary should be more suitable for representing the actual solar conditions, because a closed condition excludes the leakage of perturbations through the boundary. We thus repeated the simulation while adopting an open condition for the lower boundary. As a result, it was shown that the perturbation does not evolve into an overstable oscillation, but, rather, leaks out completely through the lower boundary. Our results imply that the actual photospheric flux tubes are in a convectively stable state, consistent with the recent observational results.