Time-dependent HCN modelling with parameters corresponding to the fragment B of 73P/Schwassmann-Wachmann. The upper plot shows the fractional level populations for levels J = 2,3,4 as a function of cometocentric distance. The middle plot shows the relative importance of different downward rates for the HCN J = 3 − 2 transition. As indicated, the included rates are radiative (excluding IR pumping rates), and collisional rates both for electrons and water molecules. The sum of these rates is included as a black line. They are normalized with the time-dependent rate due to destruction and constant expansion; see Eq. (5). The bottom panel depicts the radial density variations. The blue curve is the total HCN density and the green curve represents the water density behaviour. The electron density, ne(r), is drawn in red. Also the HCN/H₂O ratio is shown. It is not entirely flat, because when the photodissociation becomes important in the outer coma, HCN and H2O molecules are destroyed with slightly different rates.