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
- 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
- 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