SpringerLink
Forum Springer Astron. Astrophys.
Forum Whats New Search Orders


Astron. Astrophys. 350, 423-433 (1999)

Previous Section Next Section Title Page Table of Contents

3. Mean-field electrodynamics

The evolution of the mean regular magnetic field [FORMULA] is governed by the dynamo equation

[EQUATION]

where [FORMULA] is the turbulent electromotive force (EMF), [FORMULA] with [FORMULA] the turbulence velocity and [FORMULA] the turbulent magnetic field, and [FORMULA] the mean velocity of the differentially rotating interstellar gas (Krause & Rädler 1980).

As usual, we assume approximate scale separation and write

[EQUATION]

For the EMF we adopt here the concept used in the more general frame by Rohde & Elstner (1998). The [FORMULA]-tensor takes the form

[EQUATION]

with the diagonal terms

[EQUATION]

[EQUATION]

turbulent diamagnetism

[EQUATION]

and magnetic buoyancy

[EQUATION]

We introduced here the turbulence intensity (rms velocity)

[EQUATION]

The angular velocity [FORMULA] is given via Eq. (11); the density distribution [FORMULA] is described in Sect. 4.3. The different quenching functions ([FORMULA], [FORMULA], [FORMULA], [FORMULA]) represent the influence of the magnetic field strength [FORMULA] 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 [FORMULA] is normalized as [FORMULA] with the equipartition field

[EQUATION]

(where [FORMULA] is the permeability, [FORMULA] in the cgs-system).

The scalar field [FORMULA] is given as

[EQUATION]

with [FORMULA] = 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.

Previous Section Next Section Title Page Table of Contents

© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999
helpdesk.link@springer.de