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 1microGauss 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.

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Peter Tribble,