RADIATIVE MAGNETOHYDRODYNAMIC SIMULATION OF SUNSPOT STRUCTURE

Abstract

Results of a 3D MHD simulation of a sunspot with a photospheric size of about\n20 Mm are presented. The simulation has been carried out with the MURaM code,\nwhich includes a realistic equation of state with partial ionization and\nradiative transfer along many ray directions. The largely relaxed state of the\nsunspot shows a division in a central dark umbral region with bright dots and a\npenumbra showing bright filaments of about 2 to 3 Mm length with central dark\nlanes. By a process similar to the formation of umbral dots, the penumbral\nfilaments result from magneto-convection in the form of upflow plumes, which\nbecome elongated by the presence of an inclined magnetic field: the upflow is\ndeflected in the outward direction while the magnetic field is weakened and\nbecomes almost horizontal in the upper part of the plume near the level of\noptical depth unity. A dark lane forms owing to the piling up of matter near\nthe cusp-shaped top of the rising plume that leads to an upward bulging of the\nsurfaces of constant optical depth. The simulated penumbral structure\ncorresponds well to the observationally inferred interlocking-comb structure of\nthe magnetic field with Evershed outflows along dark-laned filaments with\nnearly horizontal magnetic field and overturning perpendicular (`twisting')\nmotion, which are embedded in a background of stronger and less inclined field.\nPhotospheric spectral lines are formed at the very top and somewhat above the\nupflow plumes, so that they do not fully sense the strong flow as well as the\nlarge field inclination and significant field strength reduction in the upper\npart of the plume structures.\n