Thanks for the reference. ( I am on a steep learning curve here !) I can understand how the Swings effect could enhance or suppress the strength of the resonance emission due to the velocity relative to the sun (The distributions are normalised relative to the intensity at the coma centre so don’t show the absolute strengths) but I don’t follow how that could alter the shape of the CN distribution. Would this be due to the resulting radiation pressure sweeping the CN away from the coma into the tail, like Sodium in 2020 F3 ?
That reference to parent molecules and the Haser model on your Fernandez page 68 did remind me of some comments from Jeff Morgenthaler of the IoIO team on my F3 cross sections.
“CN is produced from a parent molecule, likely HCN. The combination of that double process ends up extending the lifetime of CN to photodissociation (destruction by sunlight). I have not done the conversion of angular scale to distance from the comet on your night of observation, but the characteristic scale length of CN emission when comets are at 1 AU from the sun tends to be about 20,000 km. C2 is a little weirder in terms of parent molecules. It seems to have two dominant ones, leading to a flat profile in the inner few thousand km of the coma. Then it drops off with a profile slightly shorter than CN.”
So perhaps I need to consider the effect of sunlight on both the rate of production and destruction of CN and C2 and follow up on the Haser model.