## 3. Mean-field electrodynamicsThe evolution of the mean regular magnetic field is governed by the dynamo equation where is the turbulent electromotive force (EMF), with the turbulence velocity and the turbulent magnetic field, and the mean velocity of the differentially rotating interstellar gas (Krause & Rädler 1980). As usual, we assume approximate scale separation and write For the EMF we adopt here the concept used in the more general frame by Rohde & Elstner (1998). The -tensor takes the form with the diagonal terms turbulent diamagnetism and magnetic buoyancy We introduced here the turbulence intensity (rms velocity) The angular velocity is given via Eq. (11); the density distribution is described in Sect. 4.3. The different quenching functions (, , , ) represent the influence of the magnetic field strength onto the turbulence effects. They are discussed in detail by Kitchatinov & Rüdiger (1992), Rüdiger & Kitchatinov (1993) and also by Elstner et al. (1996). The field strength is normalized as with the equipartition field (where is the permeability, in the cgs-system). The scalar field is given as with = 0.3 (cf. Ruzmaikin et al. 1988). The feedback of the magnetic field onto the eddy diffusivity (Kitchatinov et al. 1994), which is not taken into account here, should be investigated in a future work. © European Southern Observatory (ESO) 1999 Online publication: October 4, 1999 |