A region called the ionosphere
From about 50 km above the Earth’s surface electrons and ions start to play an important role in the behavior of the upper atmosphere in a region called the ionosphere. It is important to notice that at the altitude where the highest electron density occurs (106 electrons/cm³ at 250 km) about one in every 1,000 air particles is ionized.
Despite the low density ratio between charged and neutral particles the ionosphere dictates its own conditions by initiating an electric field which binds together charged particles of opposite signs. The Earth’s magnetic field also acts on the charged particles of the ionosphere and consequently, their aeronomic behavior strongly differs from that of the neutral particles.
The existence of the Earth’s ionosphere was confirmed in 1901 by Guglielmo Marconi who achieved the first transatlantic radio communication. ()
The main source of this environment is the Sun’s ultraviolet light which ionizes atoms and molecules in the Earth’s upper atmosphere (50-1000 kilometers altitude). This process called “photoionization” refers to that an electron is knocked free from a neutral atmospheric particle (atom or molecule) during a collision, which then becomes an ion.
Not only solar radiation at ultraviolet wavelengths, but also X-ray and gamma-rays are produced when solar events such as solar flares occur. Arriving at the Earth just eight minutes later they increase the density of the ionosphere on the dayside of Earth.
Solar events can also produce high velocity protons and electrons (arriving at Earth hours to days later) that precipitate into the ionosphere in the polar regions producing large increases in the density of the ionosphere at low altitudes. Particles from the solar wind and cosmic radiation can also ionize the upper atmosphere of Earth.
In general, the ionosphere is horizontally divided into three regions called D, E, F, in which the ionization increases with height. These regions stem from the difference in the energy with which the solar rays penetrate the atmosphere.
Photodissociation and Photoionization
Although the integrated energy in UV rays and X rays emitted from the Sun represents less than 10 percent of the total energy that the Sun sends in our direction, its interaction with the Earth’s atmosphere is considerable.
Photodissociation is a process in which a photon from the Sun fragments an atmospheric atom or molecule into new atomic or molecular components. Photodissociation plays a very important role in the chemistry of planetary atmospheres, including the atmospheric chemistry here on Earth.
The Sun’s radiation also causes photoionization in which a photon ejects one or more electrons from a neutral atmospheric particle (atom or molecule) turning it into a charged particle - an ion.