The galactic dynamo effect due to Parker-shearing instability of magnetic flux tubes. III. The fast dynamo model

We present a new fast dynamo model for galactic magnetic fields, which is based on the Parker-shearing instability and magnetic reconnection, in the spirit of the model proposed by Parker (1992). We introduce a new scenario of flux tube interactions and estimate the dynamo transport coefficient basing on simple geometrical arguments. The obtained expressions are equivalent to the formally derived helicity $\alpha_d$ and diffusivity $\eta_d$ in the first paper of this series. The model we propose predicts that the $\alpha$-effect in galactic discs has opposite sign with respect to that resulting directly from the sign of the Coriolis force. We estimate the rate of magnetic heating due to the reconnection of magnetic flux tubes, which plays an important role in our dynamo model. The corresponding luminosities of the diffuse X-ray emission are consistent with the ROSAT observations of nearby galaxies. The present considerations synthesize the ideas of Parker with our own results presented in the preceding papers (Hanasz & Lesch 1993, 1997; Hanasz 1997).