4.1.2. Equipartition Method
A second method to determine the strength of a galactic
magnetic field is through the
total continuum (Stokes I) observation of the radio synchrotron
flux density Sv from
relativistic electrons over the whole galactic disk of size
gal,
often made near a radio
frequency between 1 and 3 GHz to cover the whole galaxy (i.e.,:
Btot ~ Sv2/7
gal-4/7). The
equipartition assumption postulates equal energy densities in the
magnetic field and in
cosmic-ray particles, very close to that of minimum energy
conditions in a synchrotron
plasma. For an energy index of -1, minimum energy estimates are
equal to equipartition estimates. Here we call
Beq the results obtained by this
equipartition method. More details on the derivation of
Beq can be found in
Fitt & Alexander
(1993).
Models showed that the
equipartition assumption is likely to be fulfilled eventually in
time, because galaxies are
approaching a form of steady-state while remaining a dynamic
system, through normal redistribution of energy by known interactions
among magnetic fields and cosmic-ray
particles. Thus some atypical galaxies may currently have values
of B different to what
equipartition would predict, althought they may later reach the
equipartition value. The equipartition assumption avoids unrealistic
conditions in galaxies (e.g.,
Duric 1990),
and such unrealistic conditions are seldom observed in nearby galaxies.