3. Mean-field electrodynamics
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).
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
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