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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 Thetagal, often made near a radio frequency between 1 and 3 GHz to cover the whole galaxy (i.e.,: Btot ~ Sv2/7 Thetagal-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.