![]() | Annu. Rev. Astron. Astrophys. 1996. 34:
155-206 Copyright © 1996 by Annual Reviews. 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
.