About the Ionosphere
What is the Ionosphere?
Earth's atmosphere varies in density and composition as the altitude increases
above the surface. The lowest part of the atmosphere is called the troposphere
(the light blue shaded region in the figure to the left) and it extends from
the surface up to about 10 km (6 miles). The gases in this region are predominantly
molecular Oxygen ( ) and molecular Nitrogen
( ). All weather is confined to this lower region
and it contains 90% of the Earth's atmosphere and 99% of the water vapor.
The highest mountains are still within the troposphere and all of our normal
day-to-day activities occur here. The high altitude jet stream is found near the
tropopause at the the upper end of this region.
The atmosphere above 10 km is called the stratosphere. The gas is still dense
enough that hot air balloons can ascend to altitudes of 15 - 20 km and Helium
balloons to nearly 35 km, but the air thins rapidly and the gas composition
changes slightly as the altitude increases. Within the stratosphere, incoming
solar radiation at wavelengths below 240 nm. is able to break up (or dissociate)
molecular Oxygen () into individual Oxygen atoms, each
of which, in turn, may combine with an Oxygen molecule ( ),
to form ozone, a molecule of Oxygen consisting of three Oxygen atoms
(). This gas reaches a peak density of a few parts per
million at an altitude of about 25 km (16 miles). The ozone layer is shown by the
yellow shaded region in the figure to the left.
The gas becomes increasingly rarefied at higher altitudes. At heights of 80 km
(50 miles), the gas is so thin that free electrons can exist for short
periods of time before they are captured by a nearby positive ion. The
existence of charged particles at this altitude and above, signals the beginning
of the ionosphere a region having the properties of a gas and of a plasma. The
ionosphere is indicated by the light green shading in the figure to the left.
How is the Ionosphere Formed?
At the outer reaches of the Earth's environment, solar radiation strikes the
atmosphere with a power density of 1370 Watts per or
0.137 Watts per , a value known as the "solar
constant." This intense level of radiation is spread over a broad spectrum
ranging from radio frequencies through infrared (IR) radiation and visible light to
X-rays. Solar radiation at ultraviolet (UV) and shorter wavelengths is considered to
be "ionizing" since photons of energy at these frequencies are capable of
dislodging an electron from a neutral gas atom or molecule during a collision.
The conceptual drawing below is a simplified explanation of this process.
Incoming solar radiation is incident on a gas atom (or molecule). In the process,
part of this radiation is absorbed by the atom and a free electron and a
positively charged ion are produced. (Cosmic rays and solar wind particles also
play a role in this process but their effect is minor compared with that due to
the sun's electromagnetic radiation.)
At the highest levels of the Earth's outer atmosphere, solar radiation is very
strong but there are few atoms to interact with, so ionization is small. As the
altitude decreases, more gas atoms are present so the ionization process increases.
At the same time, however, an opposing process called recombination
begins to take place in which a free electron is "captured" by a positive ion if
it moves close enough to it. As the gas density increases at lower altitudes, the
recombination process accelerates since the gas molecules and ions are closer
together. The point of balance between these two processes determines the degree
of "ionization" present at any given time.
At still lower altitudes, the number of gas atoms (and molecules) increases
further and there is more opportunity for absorption of energy from a photon of
UV solar radiation. However, the intensity of this radiation is smaller at these
lower altitudes because some of it was absorbed at the higher levels. A point is
reached, therefore, where lower radiation, greater gas density and greater
recombination rates balance out and the ionization rate begins to decrease with
decreasing altitude. This leads to the formation of ionization peaks or layers
(also called "Heaviside" layers after the scientist who first proposed their
existence).
Because the composition of the atmosphere changes with height, the ion production
rate also changes and this leads to the formation of several distinct ionization
peaks, the "D," "E," "F1," and "F2" layers.
Additional information about the Ozone layer is available from the National Oceanic
and Atmospheric Administration (NOAA).
References:
[1] Kelley, M. C., The Earth's Ionosphere, Academic Press, Inc:San
Diego, 1989.
[2] Davies, Kenneth, Ionospheric Radio, Peter Peregrinus Ltd.:London,
1990.
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