A Dynamical Model for the Penumbral Fine Structure and the Evershed Effect in Sunspots

Relying on the assumption that the interchange convection of magnetic fluxtubes is the physical cause for the existence of sunspot penumbrae, we proposea model in which the dynamical evolution of a thin magnetic flux tubereproduces the Evershed effect and the penumbral fine structure such as brightand dark filaments and penumbral grains. According to our model, penumbral grains are the manifestation of thefootpoints of magnetic flux tubes, along which hot subphotospheric plasma flowsupwards with a few km/s. Above the photosphere the hot plasma inside the tubeis cooled by radiative losses as it flows horizontally outwards. As long as theflowing plasma is hotter than the surroundings, it constitutes a bright radialfilament. The flow confined to a thin elevated channel reaches the temperatureequilibrium with the surrounding atmosphere and becomes optically thin near theouter edge of the penumbra. Here, the tube has a height of approximately 100 km above the continuum andthe flow velocity reaches up to 14 km/s. Such a flow channel can reproduce theobserved signatures of the Evershed effect.Comment: 5 pages, 2 figures, accepted for publication in ApJ letter