The heterosphere is characterized by variable chemical composition with regions that are dominated by only a few atomic or molecular species. Because there is no convective heating at this height, the material found in the heterosphere is layered according to its mass. The lower levels are dominated by nitrogen and oxygen molecules, oxygen atoms are found in the next layer and then helium and free hydrogen atoms near the boundary of the earth's atmosphere.
The lower heterosphere also consists of a small area dominated by ionic species or free radicals. such as O+, NO+, O2+ , N2+ , and free electrons. This region of the atmosphere is called the ionosphere . The chemistry in the ionosphere features numerous photodissociation reactions, light induced breaking of chemical bonds, and photoionization processes, light induced removal of electrons.
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