Fine scale structures of sunspots and their role on global sunspot energetics

The sunspot has highly structured atmosphere of strongly magnetized plasma; In visible light, the umbra contains a number of small bright patches called as umbral dots in its dark background, the penumbra consists of numerous bright and dark filaments with a width of 0.2--0.3 arcsec. The origin of these features has been a topic under debate for long time in the sunspot physics.The 'brightness' of sunspots is another topic under debate on sunspot physics, i.e., how the energy for maintaining the brightness of umbrae (10- 20% of normal photosphere) and penumbrae (70-80% of normal photosphere) are carried to the solar surface. The Solar Optical Telescope (SOT) aboard Hinode revealed the close relationship between the fine scale filamentary structures and the plasma motions (Evershed flow) in sunspot penumbra; The Evershed flow is confined in narrow channels with nearly horizontal magnetic fields embedded in deep layer of penumbral atmosphere. It is a dynamic phenomenon with the flow velocity close to the sound speed in the photosphere, and individual flow channels are associated with tiny upflow (source) at the inner end and down flow (sink) at the outer end. The penumbral bright grains are well correlated with the upflowing gas, thus the Evershed effect can be interpreted as a consequence of the thermal convection under the strong inclined magnetic field of sunspot penumbra. We will present the new picture of the sunspot penumbra, and discuss the role of the fine structures on the global sunspot energetics.