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Astron. Astrophys. 364, 829-834 (2000)

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4. Discussion

The penumbral flow field is predominantly horizontal, but a vertical component is present which decreases with radial distance from spot center. This leads to an upflow component in the inner part of the penumbra, which gradually changes into a downflow component in the outer penumbra. In the previous section we presented significant differences in the material flow geometry for bright and dark penumbral features.

The bright component of the flow is less inclined with respect to the surface normal than the dark component throughout the entire penumbra. In the inner penumbra, bright features correspond to patches of upflows, whereas dark features are in average more horizontal. This indicates that bright penumbral features, i.e. penumbral grains and bright filaments, are consistent with the moving tube model (Schlichenmaier et al. 1998) which suggests that hot sub-photospheric plasma is transported by upflow channels into the penumbral photosphere. We have estimated that these upflows are sufficient to explain the surplus brightness of the penumbra relative to the umbra (Schlichenmaier & Schmidt 1999).

In the outer part of the penumbra only the dark component shows a downflow. The flow speed in the dark component is somewhat higher than in the bright features. Assuming that the flow channels are confined by magnetic flux tubes, they may be advected by the flow field of the surrounding granulation into the cool intergranular lanes.

The spatial resolution of our data is about 500 km, just good enough to distinguish between two, locally defined, intensity ranges and to measure the material flow speed there. We expect that at higher resolution much higher flow speeds and larger velocity fluctuations would result.

The analysis of two-dimensional velocity data, covering the complete spot reveal some of the difficulties that arise when working with one-dimensional spectra taken with a grating spectrograph: the slit (assumed to be straight) always intersects the penumbra at different spot radii, and the data base from a single spectrum is too small to establish a relationship between intensity (or temperature) and other quantities, such as velocity and magnetic field.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001