Basic Facts About the Ionosphere

• Invisible layers of ions and electrons are suspended in the Earth's atmosphere above about 60 kilometers in altitude.
• The main source of these layers is the Sun's ultraviolet light which ionizes atoms and molecules in the Earth's upper atmosphere. During this process, called photoionization, an electron is knocked free from a neutral atmospheric particle, which then becomes an ion.
• Because the Sun's light is responsible for most of the ionization, the ionosphere reaches maximum densities just after local noon. In this region at altitudes where the highest densities occur, about one in every 1000 air particles is ionized. Resulting ionospheric densities are ~ a million ions and electrons per cubic centimeter.
• Ion - electron pairs are also created during collisions of high velocity electrons and ions with the neutral atmosphere mostly at polar and auroral latitudes.
• Flares and other energetic events on the Sun produce increased ultraviolet, x-ray and gamma-ray photons that arrive at the Earth just 8 minutes later and dramatically increase the density of the ionosphere on the dayside. These solar events also can 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.
• Ions and electrons in the ionospheric layer are constantly being lost and replenished. The lifetime of a given ion is only a fraction of a day and decreases with decreasing altitude. Lower altitude ions have shorter lifetimes.
• The low-altitude portion of the ionosphere (D and E-layers) rapidly reaches low densities at night because ion and electron lifetimes are very short at these altitudes and the major source of new ion-electron pairs (solar photons) is absent.

The source of this material is Windows to the Universe, at http://www.windows.ucar.edu/ at the University Corporation for Atmospheric Research (UCAR). ©1995-1999, 2000 The Regents of the University of Michigan; ©2000-05 University Corporation