THE GALACTIC DYNAMO EFFECT DUE TO PARKER-SHEARING INSTABILITY OF MAGNETIC-FLUX TUBES .1. GENERAL FORMALISM AND THE LINEAR-APPROXIMATION

In this paper we investigate the idea of Hanasz & Lesch 1993 that the galactic dynamo effect is due to the Parker instability of magnetic fl ux tubes. In addition to the former approach, we take into account mor e general physical conditions in this paper, by incorporating cosmic r ays and differential forces due to the axisymmetric differential rotat ion and the density waves as well. We present the theory of slender ma gnetic flux tube dynamics in the thin flux tube approximation and the Lagrange description. This is the application of the formalism obtaine d for solar magnetic flux tubes by Spruit (1981), to the galactic cond itions. We perform a linear stability analysis for the Parker-shearing instability of magnetic flux tubes in galactic discs and then calcula te the dynamo coefficients. We present a number of new effects which a re very essential for cosmological and contemporary evolution of galac tic magnetic fields. First of all we demonstrate that a very strong dy namo alpha-effect is possible in the limit of weak magnetic fields in presence of cosmic rays. Second, we show that the differential force r esulting from axisymmetric differential rotation and the linear densit y waves causes that the cr-effect is essentially magnified in galactic arms and switched off in the interarm regions. Moreover, we predict a non-uniform magnetic field in spiral arms and well aligned one in int erarm regions. These properties are well confirmed by recent observati onal results by Beck gr Hoernes (1996).