  Relying on the assumption that the interchange convection of magnetic flux
tubes is the physical cause for the existence of sunspot penumbrae, we propose
a model in which the dynamical evolution of a thin magnetic flux tube
reproduces the Evershed effect and the penumbral fine structure such as bright
and dark filaments and penumbral grains.
  According to our model, penumbral grains are the manifestation of the
footpoints of magnetic flux tubes, along which hot subphotospheric plasma flows
upwards with a few km/s. Above the photosphere the hot plasma inside the tube
is cooled by radiative losses as it flows horizontally outwards. As long as the
flowing plasma is hotter than the surroundings, it constitutes a bright radial
filament. The flow confined to a thin elevated channel reaches the temperature
equilibrium with the surrounding atmosphere and becomes optically thin near the
outer edge of the penumbra.
  Here, the tube has a height of approximately 100 km above the continuum and
the flow velocity reaches up to 14 km/s. Such a flow channel can reproduce the
observed signatures of the Evershed effect.
