Filling factors and filamentation
Detailed observations of Cygnus A (Perley, Dreher & Cowan 1984)
have shown that the emission is filamentary. Perley et al. gave a
volume filling factor in the range 0.03-0.3. However, the emission from
a volume containing a random magnetic field will have structure (for an
example, see Tribble
1991). Half of the low frequency (unaged) emission from a three
dimensional random magnetic field comes from only 23% of the volume. In
two and one dimensions the fraction of the volume required for half the
emission is 0.2 and 0.13 respectively.
Filamentary structure (Perley et al. 1984; Fomalont et al. 1989; Hines,
Owen & Eilek 1990) is observed at high frequencies (for high
resolution) where aging is important. As the electron population ages
and the radio spectrum steepens, the emissivity contrast between
regions of different field strength increases. The observed emission
variations might simply be due to a random magnetic field combined with
some spectral aging. If this is the case, one should probably take a
value for the filling factor close to unity.
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Peter Tribble, peter.tribble@gmail.com