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 see the graph, you may need to install the latest version of the Flash player onto your computer.)