NH₂ OPR as a function of temperature computed: (i) at thermal equilibrium; (ii) with Model 1 at steady state and an earlier time which best fits the observed values; and (iii) with the analytical formulae (3) and (9). The details of Model 1 are described in Sect. 2.3. The observed OPR values from Persson et al. (2016) are represented within their error bars in blue, cyan and pink at the lowest values of the observed temperature ranges (Persson et al. 2016). For the sake of clarity the uncertainties of the observed temperatures are omitted.