Top panel: electron temperature (T_e) of the plasma as a function of ionisation parameter (ξ). The labelled circles correspond to the four PION components for each epoch (red for 2000 and blue for 2014) at their respective ionisation parameter values from Table 4. Bottom panel: thermal stability curve of electron temperature (T_e) as a function of the pressure form of the ionisation parameter (Ξ). On the curve, the rate of cooling equals the rate of heating. Left of the curve, cooling dominates over heating and to the right, heating dominates over cooling. The labelled circles correspond to the Ξ values of the four PION components for each epoch: red for 2000 and blue for 2014.