An ion is an atom or molecule that has lost or gained an electron, compared to its neutral state. The ionosphere in the thermosphere contains a lot of ions, which were produced when the atoms/molecules absorbed high-energy sunlight. These ions are "charged" or "excited", and when they react with each other or some other substance, the excess energy absorbed earlier is released in the form of light.

At night, the ionosphere tends to reflect radio waves originating from the Earth's surface. This increases the broadcasting range of strong AM radio stations.

When the sun is unusually "active", the solar wind emitted from it carries a large number of ions from the sun. These particles are captured by the Earth's magnetic field lines of force. This lines all go to either magnetic pole, thus concentrating these solar wind ions over the polar regions. The solar wind particles then react with the ions in the ionosphere, which produces photons of visible light; the concentration of the reactions there can produce enough light in one place to be visible to the naked eye as the aurora borealis.

After sunset, the D-region layer disappears. During the daytime, this ionospheric layer scatters AM radio waves, so radio stations cannot transmit to locations that are not line-of-sight from the transmitting antenna. The E- and F-regions can reflect the radio waves more directly instead of scattering them, so at night, AM radio stations can sometimes be heard thousands of miles away. In the "Sunrise Effect", one loses the signals from the distant radio stations at the moment the Sun rises over the horizon, which is when the D-region re-forms.