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Ionosphere, upper atmospheric layer, what is it?

Ionosphere altitude

The ionosphere is the upper layer of the Earth’s atmosphere. The atmosphere is partly ionised by solar UV-light, and such ionisation can persist at high altitude: the ionosphere starts at about 80 km altitude and reaches up to more than 1000 km.

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.

Beyond an altitude of between 2000 and 3000 km, the frequency of collisions between charged particles in the ionosphere is so low that these particles can travel great distances without suffering further collisions. The movements of the particles are then determined solely by the Earth's gravitational and magnetic fields and by the presence of electric fields.

Energy transport from magnetosphere to ionosphere

The ionosphere and the magnetosphere are connected to each other by the geomagnetic field lines.

Because these behave as very good electrical conductors, electromagnetic energy can be transported from the magnetosphere down into the ionosphere through electric currents, the so-called Birkeland currents.

The aurora is the most spectacular manifestation of this energy transfer.

Ionosphere sources

De atmosfeer wordt voortdurend blootgesteld aan ultraviolette straling, röntgenstralen, zonnedeeltjes en kosmische straling. Het resultaat is ionisatie.

  • The Sun’s ultraviolet light which ionizes atoms and molecules in the Earth’s upper atmosphere (50-1000 kilometers altitude) is the main source of this environment. This process is called “photoionization”.
  • 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.
  • Protons and electrons with high velocity produced by solar events (arriving at Earth hours to days later) that precipitate into the ionosphere in the polar regions also produce 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.

Three regions in the ionosphere (density and altitude)

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.

D-region between 50 and 90 km mainly contains polyatomic hydrate ions between 102 and 104 ions/cm³
E-region between 90 and 150 km contains the ions, NO+ and O2+, and metal ions with a density between 103 and 105 ions/cm³
F-region from 150 to almost 1,000 km where the region merges into the magnetosphere between 105 and 106 ions/cm³


History, the discovery of the ionosphere

In 1901, Guglielmo Marconi established the first transatlantic radio transmisson. But electromagnetic waves only propagate in a straight line, at least in a homogeneous medium.

To explain how Marconi's radiotelegraphic signals were able to bypass the Earth's roundness, Heaviside in England and Kennelly in America imagined as early as 1902 that there were reflecting layers for radio waves at very high altitudes: the Kennelly-Heaviside layers.

In 1925, the English physicist Appleton demonstrated by experiment the presence of these layers imagined by Heaviside and Kennelly. Shortly after Appleton, the American physicists Breit and Tuve measured the height of the ionospheric layers using a radio pulse transmitter.

It wasn't until 1929 that the word ionosphere finally replaced Appleton's layer.

Aurora, Lofoten, Norway. Image credits: Jeroen van Gent.
The different layers of the ionosphere as a function of altitude. Image credits: Navy)
The existence of the Earth’s ionosphere was confirmed in 1901 by Guglielmo Marconi who achieved the first transatlantic radio communication.