Photochemistry, outer region

The chemistry changes drastically when interstellar UV radiation can penetrate the more tenuous outer regions of the CSE, and photochemistry starts to control molecular abundances. Neutral molecule plus radical reactions and neutral molecule plus ion ( ion-molecule ) reactions are important for the... 

Frequently employed tracers for photochemistry in CSE are CO, which photodissociates in the outer regions of the CSE, and CN and OH, which are photodissociation products of HCN and H2O, respectively. Photodissociation of photospheric parent molecules (e.g., H2O + hv— OH + H C2H2 + hv—> C2H + H HCN + hv- CN +H) increases the radical abundances, and abundances of parent molecules start to decline. The distance in the CSE at which photodissociation begins is different for different parent molecules and reflects the distance where the CSE becomes transparent for the UV radiation to drive the photodissociation reaction. 

The atmosphere is quite clearly and distinctly divided into a series of vertical layers of air (see Fig. 5.4), some of which have already been referred to. The layers are labelled (with increasing z) the troposphere, stratosphere, mesosphere and thermosphere, and they are caused by a combination of photochemistry, air temperature, and the effect of gravity on gas mixing due to convection. As discussed above, air pressure decays exponentially with increasing z, finally reaching the pressure of outer space at a distance >10 km however 99.99 % of the air mass is located below z = 100 km. The depth of identified air layers is therefore a small fraction of the diameter of the Earth (d = 12740 km the thickness of the troposphere is <0.12 % of this value). The stratosphere, which is photochemically the most active region, lies approximately in the range 10 < z < 50 km (the actual boundaries vary with location, season and time of day). 

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