The various regions of the ionosphere have
higher concentrations of charged particles (ions) than do other parts of the thermosphere,
which consists mostly of electrically neutral atoms and molecules.
These ions are created when energetic photons from
the Sun collide with atoms and molecules in the atmosphere, knocking electrons out
of the atoms and molecules in a process called photoionization. The composition
of the ionosphere, in terms of elements and compounds, reflects the composition
of the neutral atmosphere from which it is generated.
The graph below shows the density of seven important constituents of the ionosphere
(ionized forms of six gases plus free electrons). Click on the check boxes
at the bottom of the graph to hide or show individual curves. (If you don't
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