  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).
