  This paper is a ``Habilitationsschrift'', a second thesis required until
recently by universities in Germany and in a few other countries to obtain the
right to lecture. It was accepted by the University of Goettingen in 1990 after
review by a number of german and international experts. Although the
introduction and the references represent the state of research in 1990, most
of the remaining content is still relevant and has never been published
elsewhere. The most important part is the derivation of a linear stability
formalism for thin magnetic flux tubes following an arbitrary path in a
gravitationally stratified medium with a stationary velocity. It was found
later (Ferriz-Mas & Schuessler, Geophys. Astrophys. Fluid Dyn. vol. 72, 209;
1995) that, for consistency, the inertial term in the equation of motion for
the external medium should be included in Eq. (3.24), which leads to an
additional term in the stability equations in the case of a spatially varying
external velocity. This term is missing in the present text, but can be easily
introduced into the formalism. The full abstract can be found at the beginning
of the paper.
