RADIO SPECTRAL AGING IN A RANDOM MAGNETIC-FIELD

I study the ageing of radio synchrotron emission from relativistic electrons in a random magnetic field. There is a range of break frequencies corresponding to the range in field strength, so that the sharp break in the spectrum given by the Jaffe-Perola model is not always seen, and the spectra can be very similar to that of the Kardashev-Pacholczyk model. This explains the good fit of the Kardashev-Pacholczyk model to observations, even though the Jaffe-Perola model is more physically reasonable. The lack of an exponential break in the spectrum does not mean that pitch angle scattering does not occur. In the presence of inverse Compton losses, the spectral shape is strongly dependent on rms field strength, allowing equipartition estimates to be checked.