The ionosphere is a shell of electrons and electrically charged
atoms and
molecules that surrounds the Earth, stretching from a height of about 50 km to more than 1000 km. It owes its existence primarily to
ultraviolet radiation from the
Sun.
The lowest part of the
Earth's atmosphere, the
troposphere extends from the surface to about 10
km (6 miles). Above 10 km is the
stratosphere, followed by the
mesosphere. In the stratosphere incoming solar radiation creates the
ozone layer. At heights of above 80 km (50 miles), in the
thermosphere, the atmosphere is so thin that free
electrons can exist for short periods of time before they are captured by a nearby positive
ion. The number of these free electrons is sufficient to affect
radio propagation. This portion of the atmosphere is
ionized and contains a
plasma which is referred to as the ionosphere. In a plasma, the negative free electrons and the positive ions are attracted to each other by the electromagnetic force, but they are too energetic to stay fixed together in an electrically neutral molecule.
Ultraviolet (UV),
X-Ray and shorter
wavelengths of
solar radiation are
ionizing, since
photons at these frequencies contain sufficient energy to dislodge an
electron from a neutral gas
atom or
molecule upon absorption. In this process the light electron obtains a high velocity so that the
temperature of the created electronic gas is much higher (of the order of thousand K) than the one of ions and neutrals. The reverse process to
Ionization is
recombination, in which a free electron is "captured" by a positive ion, occurs spontaneously. This causes the emission of a photon carrying away the energy produced upon recombination. As gas density increases at lower altitudes, the recombination process prevails, since the gas molecules and ions are closer together. The balance between these two processes determines the quantity of ionization present.
Ionization depends primarily on the
Sun and its
activity. The amount of ionization in the ionosphere varies greatly with the amount of radiation received from the Sun. Thus there is a
diurnal (time of day) effect and a seasonal effect. The local winter
hemisphere is tipped away from the Sun, thus there is less received solar radiation. The activity of the Sun is associated with the
sunspot cycle, with more radiation occurring with more sunspots. Radiation received also varies with geographical location (polar, auroral zones, mid-latitudes, and equatorial regions). There are also mechanisms that disturb the ionosphere and decrease the ionization. There are disturbances such as
solar flares and the associated release of charged particles into the
solar wind which reaches the Earth and interacts with its
geomagnetic field.