Magnetic Flux Tubes in Sunspot Penumbrae


Magnetic Flux Tubes in Sunspot Penumbrae

With increasing spatial resolution, observations reveal more and more details of the fine structure of sunspots. What are these remarkable features made of? Numerical MHD-simulations (Schlichenmaier et al., 1998) suggest that the penumbra is heated by magnetic flux tubes in which hot plasma flows up from below and subsequently cools off by radiation when reaching the surface. The picture shows the magnetostatic sunspot model of Jahn & Schmidt (1994), which is used as a background field in which thin magnetic flux tubes evolve with time.
Spectropolarimetry serves as a powerful tool to probe photospheric magnetic fields. We investigate the implications of the moving tube model by calculating synthetic line profiles of the penumbra of an axisymmetric sunspot. Our model reproduces observational results such as the center-to-limb varation of the net circular polarization (NCP) in visible wavelengths and predicts a significantly different shape of the NCP curve in the infrared, an effect which is due to the combination of longer wavelength and higher excitation potential. A recent discovery (Müller et al. 2002) is the fact that the spatial symmetry of the NCP within the penumbra is broken by anomalous dispersion (cf. results).