Hydraulic concentration of magnetic fields in the solar photosphere. VI - Adiabatic cooling and concentration in downdrafts

Abstract

The remarkable concentration of the general field of the Sun into isolated intense flux tubes at the visible surface must be a direct consequence of conditions immediately beneath the surface. It is pointed out that the convective heat transport in the magnetic field swept into the downdrafts in the junctions of supergranule boundaries is strongly suppressed by the magnetic field. The net heat transport is reduced to such a degree that the temperature of the downdraft within the field increases nearly adiabatically below the visible surface, and hence is significiantly cooler than the surrounding ambient gas. The reduced temperature enhances the downdraft within the field and permits the gravitational field to evacuate the flux tube. The magnetic field is then strongly compressed by the external gas pressure, leading to the extraordinary observed strengths of 1500 gauss or more. It is suggested that the magnetic knots found in active regions are formed wholly or partly by the same effect.The general occurrence of superadiabatic temperature gradients and convective heat transport in the dwarf and subdwarf main-sequence stars suggests that their magnetic fields are generally broken up and concentrated by this effect, much as in the Sun.