Tadpoles in Sunspot Penumbrae

Observations of magnetic fields and flows in sunspot penumbrae at a spatial resolution of 0.1"-0.2" revealed many earlier unavailable details (see e.g. K. Langhans et al. 2005, A&A, 436, 1087). We concentrate here on some properties of the fine structure of penumbrae that briefly may be characterized as a dense ensemble of dark-cored and bright magnetic filaments, highly dynamic at short time scales and preserving their general properties for hours. Inclination of dark and bright filaments, their magnetogram signals (having rapid azimuthal variation), and plasma flows associated with them, are significantly different. Topologically, dark-cored filaments surrounded by bright threads, have a peculiar (and ubiquitous) shape of a tadpole with a thick head ``diving'' into the umbra and a long tail reaching sometimes the outer edge of the penumbra. The filaments have their own sub-structure of various forms that continuously evolve from one to another. We use the model of highly inhomogeneous ``magnetic fluid'' with the sheared mass flows to explain the observed regularities. We show that differences in the inclination of magnetic fields and sheared velocities result in the nonlinear instabilities associated with the vortex motion and interaction of the poloidal and toroidal components of magnetic fields. This, in turn, leads to a filamentation process, as well as formation of the structures having the appearance of tadpoles.