Photoionisation fraction of Na I and rate as a function of phase at distances between 10^16.5 and 10^18.5 cm from a SN. The dashed vertical lines associate the curves of the two plots with one another and are labeled by their respective radii at which the gas is situated from the explosion. The shaded bands indicate the ± 1 mag change in ionising UV flux from the SN. The Na I gas is assumed to be optically thin and is completely ionised at all radii considered in this plot.