I have considered the small scale structure of radio halos from
analytic estimates and simulations. Because the magnetic field is not
uniform, the strength of emission varies from one coherent cell of the
field to another. This leads to granularity in the observed emission,
which is reduced if the cell size of the field is smaller. The
smoothness of the central parts of the Coma radio halo, having
fluctuations of approximately 5% or less about the mean level,
indicates that the scale length of the field is less than about 15 kpc.
Further high resolution observations would be valuable in further
studying the structure of radio halos, as substantial emission
structure and possibly polarization will be present in the halo even
for a scale length of 15 kpc that is presently smeared by poor
resolution, and the limits derived here are close to the currently
accepted values for the scale of the field, 10-20 kpc. In combination
with RM data, a lower limit of approximately 1 can be set on
the rms magnetic field strength in the Coma cluster.
Models in which the halo is due to overlapping wakes were considered
and the wakes shown to emit less than 10% of the total flux, showing
that the halo emission and the magnetic field is spread rather
uniformly throughout the entire cluster rather than being localised.
Dynamo models in which the field is concentrated in intense ropes are
also constrained to have an outer turbulent scale less than 15 kpc.
The shortcomings and limitations of the models must be taken into
account. The structure of the field is rather simple, being
characterized by a single scale. A range of scales, as might be
expected in turbulence, might look different. Even here, though, the
field has coherence up to the outer scale on which energy is put into
the gas, so that structure is expected on these scales.
Peter Tribble, email@example.com