4.1.3. Cosmic-ray Method
A third method to determine the galactic magnetic field
strength is through the
gamma-ray emission (2.5 × 10-5 Å) by collisions of
cosmic-ray protons and cosmic-ray
electrons with the interstellar matter, as well as collisions of
cosmic-ray electrons with
optical interstellar photons (inverse-Compton), or else through
the x-ray emission (12 Å)
by collisions of cosmic-ray electrons with infrared interstellar
photons (inverse-Compton).
The derivation of cosmic-ray particle densities K in starforming
sites of size P is made first,
and the diffusion assumption that these cosmic-ray particles can
expand from size P to
cover the whole galactic disk of size L is then made to explain
the radio synchrotron
emission I from relativistic electrons over the whole galactic
size L, finally enabling the final
derivation of the total galactic magnetic field strength, i.e.,:
Btot ~ Ivs
K-s L-s , where s = 2 /
[ + 1]
and
is the
cosmic ray electron energy index
(
2.5). Here we call
Bcr the results obtained
by this cosmic-ray particle method. Cosmic-ray electrons from a
galactic nucleus (i.e., P) can
find it difficult to diffuse quickly over a whole galactic disk
(i.e., L). More details on the derivation of
Bcr can be found in
Chi & Wolfendale
(1993),
etc. The basic diffusion
assumption of the cosmic ray method, i.e., that cosmic rays, gas,
and magnetic fields are
"well mixed" in the galaxy, has been questioned in the cases of
the galaxies M82 and of the
Large Magellanic Cloud (e.g., Table 2 in
Vallée 1995a).
In the Milky Way, most cosmic rays
seem to come from SNR in the galactic disk (e.g., see
Völk et al. 1984;
Völk et al. 1989),
and the basic diffusion assumption seems to work. Also, the wide use
of a negligible cosmic-ray
e/p flux ratio, in the cosmic-ray method, has been challenged recently
(Pohl, 1993).