PHOTOSPHERIC CONVECTION IN STRONG MAGNETIC-FIELDS

The effect of magnetic fields on convection at the surfaces of cool st ars can be explored by comparing the results of detailed numerical exp eriments with high-resolution solar observations. We have investigated non-linear three-dimensional magnetoconvection in a fully compressibl e perfect gas. In this paper we study the effect of an imposed magneti c field on the pattern of convection in a deep stratified layer. When the field is strong enough to dominate the motion we find steady conve ction with rising plumes on a deformed hexagonal lattice, and a magnet ic network at the upper boundary. This gives way to spatially modulate d oscillations for weaker fields. As the field strength is further red uced the oscillations become more violent and irregular, and their hor izontal scale increases. Magnetic flux moves rapidly along the network that encloses the ephemeral plumes; when the imposed field is relativ ely weak, intense fields appear at junctions in the network, where the magnetic pressure is comparable to the gas pressure and an order of m agnitude greater than the dynamic pressure. This behaviour is related to convection in sunspots and plages and to the structure of intergran ular magnetic fields on the Sun.