3.3. Distribution of Breg, with galactic radius
The use of synchrotron radiation is often employed to get
the total magnetic field
strength. The average 'equipartition'-derived magnetic field
strength follows from the
assumption of energy-density equipartition between cosmic-ray
particles and magnetic
fields, the assumption or knowledge of the ratio between cosmic-ray
electrons and protons
(and its variation with particle energy), the synchrotron
spectral index , the
extent of the radio emission along the line of sight, and the volume filling
factor. Still, large
uncertainties in these factors lead to only moderate errors in
the total magnetic field strengths (e.g.,
Beck 1996;
Heiles 1996b;
Fitt & Alexander
1993).
The use of the Faraday rotation is often employed to get the
uniform component of
the magnetic field strength. The average 'Faraday'- derived
magnetic field component follows
from the assumption or knowledge of the density distribution of
the thermal (non-synchrotron) electrons in the Galaxy. Since pulsars are
often near the galactic plane, their
emitted light often travels through the spiral arms. A
distribution model for thermal
electrons is thus used to analyse pulsar dispersion measures DM
and pulsar rotation measures RM. The recent model of
Taylor and Cordes (1993)
for the distribution of thermal
electrons in the galactic disk, employing 4 spiral arms, has been
often employed, and its accuracy has been confirmed by
Weisberg et al. (1995),
and very slightly modified by
Heiles et al. (1996)
in the innermost areas. The use of this model allows
small distance errors to pulsars, with a relative random error
r / r
= 0.20 (e.g., Fig. 7
in Taylor and Cordes
1993).
In this case, after looking through 4 spiral arms, one has an error
r = 0.2
× 4 arm widths = 0.8 arm, enabling a proper
study to be done all the way to the region of the galactic center (e.g.,
Vallée 1996b).