The galactic dynamo effect due to Parker-shearing instability of magnetic flux tubes I. 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 flux tubes. In addition to the former ap- proach,wetakeintoaccountmoregeneralphysicalconditionsin this paper, by incorporating cosmic rays and differential forces due to the axisymmetric differential rotation and the density waves as well. We present the theory of slender magnetic flux tube dy- namics in the thin flux tube approximation and the Lagrange description. This is the application of the formalism obtained for solar magnetic flux tubes by Spruit (1981), to the galactic conditions.WeperformalinearstabilityanalysisfortheParker- shearing instability of magnetic flux tubes in galactic discs and then calculate the dynamo coefcients. We present a number of new effects which are very essen- tial for cosmological and contemporary evolution of galactic magnetic elds. First of all we demonstrate that a very strong dynamo-effect is possible in the limit of weak magnetic elds inpresenceofcosmicrays.Second,weshowthatthedifferential force resulting from axisymmetric differential rotation and the linear density waves causes that the -effect is essentially mag- nied in galactic arms and switched off in the interarm regions. Moreover, we predict a non-uniform magnetic eld in spiral arms and well aligned one in interarm regions. These properties are well conrmed by recent observational results by Beck & Hoernes (1996)