the radio spectrum, throughout the whole data set. It is possible that the variation consists of step changes, with the most obvious step times being around day 3000 and day 4700. At early times, the mean spectral index was about $-0.97$, but in the last year (2000) it was $-0.88$.

The radio spectra are remarkably close to power law and show no sign of either positive curvature, as would be expected if either free-free absorption or synchrotron self-absorption were important, or negative curvature as is predicted by diffusive shock acceleration theories (e.g. Reynolds & Ellison 1992). The radio spectral index of about $-0.9$ is considerably steeper than the canonical $-0.5$ expected for synchrotron emission from relativistic electrons accelerated in strong shocks, for which the spectral index $\alpha = (1-s)/2$ and the electron energy index $s = (r+2)/(r-2)$, where $r$ is the compression ratio in the shock (Jones & Ellison 1991). For high Mach-number shocks in a monatomic non-relativistic gas, $r=4.0$, giving $s=2.0$ and $\alpha =
-0.5$.

A steeper radio spectrum implies a steeper electron energy distribution and a smaller compression ratio in the shock; $s \sim 2.8$ and $r \sim 2.7$ for $\alpha = -0.9$. Monte-Carlo modelling of non-linear diffusive 