Annu. Rev. Astron. Astrophys. 1996. 34: 155-206 Copyright © 1996 by . All rights reserved

5.3. The Dynamo Origin of Magnetic Field

Any dynamo requires a seed field because Equation (5) is homogeneous in . There are two possibilities for the seed field: It can be essentially of cosmological origin or it can result from processes occurring in the ISM.

The large-scale dynamo timescale in a typical galaxy cannot be shorter than ~ 5 × 10⁸ yr (see Section 4.4). A primordial field on a protogalactic scale then needs to be at least O(10^-18) G in order to be amplified to 10^-6 G in 10¹⁰ yr (when the amplification by protogalaxy contraction is considered). With the estimates (8) and (9), we conclude that a cosmological magnetic field is not viable as a seed field for a galactic dynamo. Moreover, for the Milky Way and M31, the timescale is more like 10⁹ yr, so that for these galaxies a primordial magnetic field needs to be at least about 2 × 10^-14 G, assuming that has not varied significantly during galactic evolution.

A sufficiently strong seed field for the large-scale galactic dynamo can be generated by a small-scale dynamo. The scale height of the disk of a young galaxy is estimated as h 100-500 pc (Briggs et al 1989) and the turbulent velocity as v 10 km s^-1 (Turnshek et al 1989). Assuming that l 100-300 pc and 10^-24 g cm^-3, we conclude that a random magnetic field b (4 v²)^1/2 2-2.5 µG of a scale 100-300 pc is generated by the fluctuation dynamo on a timescale of order ₁ ~ l / v 10⁶-10⁷ yr. Because a galactic disk contains about N₁ = (h / l)(R / l)² turbulent cells, the resulting mean field dynamo seed field is about bN₁^-1/2 10^-8 G. This is much larger than possible cosmological seed fields (8, 9), even if the field compression during galaxy formation is taken into account.

The resulting small-scale field is strong enough to produce, via a mean field galactic dynamo, a large-scale magnetic field of µG-strength in ~ (1-2) × 10⁹ yr (Beck et al 1994a). This means that even the presence of regular magnetic fields in galaxies with redshifts of z 2 or even z 3.4 (Wolfe et al 1992, White et al 1993) does not contradict the picture of generation and maintenance of large-scale fields by a mean-field dynamo mechanism. The possible role of the halo (Chiba & Lesch 1994) and radial motions (Camenzind & Lesch 1994) has also been investigated.

The fluctuation dynamo also needs a seed but, because of the very short fluctuation dynamo timescale, even the magnetic fields generated by the battery effects in stars (Biermann 1950, Mestel & Roxburgh 1962), and subsequently ejected into the ISM, or a cosmological field (Section 5.1) would suffice.

Thus, large-scale dynamo action in a galaxy is preceded by a small-scale dynamo that prepares the seed for the former. These may operate at different epochs. Small-scale dynamo action has been considered by Pudritz & Silk (1989) for the protogalaxy, by Zweibel (1988) during the post-recombination epoch, and before recombination by Tajima et al (1992).

A rather radical view of the role of the Galactic center in the origin of the global galactic magnetic field was proposed by Hoyle (1969), who suggested that the magnetic field observed in the solar vicinity had been ejected from the Galactic center. This idea was rejected because the required magnetic field in the nucleus is 10⁹ G, and its energy exceeds the gravitational energy of a black hole with a mass of 10⁸ M. Nevertheless, Chakrabarti et al (1994) proposed a similar hypothesis, with the azimuthal field being amplified up to _core 3 × 10⁵ G within r₀ 3 × 10¹¹ cm of the center. A galactic wind is then supposed to carry this field to the outer parts of the Galaxy. However, this gives for the solar vicinity an extremely weak field of (r₀ / r) (h₀ / h) _core 6 × 10^-16 G, where h₀ r₀ and r = 8.5 kpc and h = 500 pc are the radius and half-thickness of the magnetoionic disk in the Solar vicinity. Chakrabarti et al obtained for a value about 10¹⁰ times larger by overlooking a factor h₀ / h .