I present model calculations of profiles and two-dimensional images of the radio synchrotron emission of young supernova remnants, concentrating on observable effects of relativistic electrons diffusing upstream of the shock wave. If the preshock electron scattering mean free path is sufficiently long, observable synchrotron halos outside the bulk of the radio emission can potentially result; their absence can constrain the mean free path from above. If scattering is primarily due, as expected, to Alfvén waves with amplitude δB, the halo is expected to extend a distance of order rg c(δB/B)−2 /vs beyond the shock, where rg is the gyroradius of the electrons emitting at the observed frequency, B is the upstream magnetic field strength, vs is the shock velocity, and the amplitude δB refers to waves with wavelength comparable to rg , of order 1013 cm for typical supernova-remnant parameters. However, the detailed geometry of the halo varies with the assumptions about particle acceleration in the shock wave. I present an atlas of model profiles and images as a function of preshock diffusion length, of aspect angle between the magnetic field and the line of sight, and of other relevant parameters.

Subject headings: radiation mechanisms: miscellaneous — shock waves — supernova remnants