useful diagnostics of the shock velocity using emission line ratios. To avoid issues of chemical abundance difference, the line ratio used should involve different levels of the same ion, or else different ions of the same element. In @Dopita16, we already noted that the \[\] $\lambda\lambda 4363/5007$ ratio was a useful indicator of shock velocity. This is because the \[\] temperature in the cooling zone of the shock is high at low shock velocities (faster than $\sim85$km/s, when this ion is first produced in the cooling zone of the shock), reaching a maximum at $v_s \sim 140$km/s when starts to become ionised to higher ionisation stages. As the velocity increases further, is ionised to , and the \[\] emission becomes confined to the cooler region nearer the recombination zone of the shock. At still higher velocities, a zone photoionised by EUV photons generated in the cooling zone of the shock develops adjacent to the recombination zone. This has still lower mean temperature; $T_e\lesssim 10^4$K. For these physical reasons, the \[\] temperature is more or less a decreasing function of shock velocity, and is relatively insensitive to $\log D_{\rm Fe}$. This behaviour is shown in Figure \[fig6\].

A second diagnostic is 