Geoscience Reference
In-Depth Information
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With decreasing altitude and increasing numbers of gas atoms and molecules,
there should be more opportunity for energy absorption. But the energy from the solar
UV radiation is already absorbed at higher altitudes, and the intensity of radiation
is less lower down. At certain points with a balance of lower radiation and greater
gas density, recombination rates balance the ionization rate, leading to formation of
ionization peaks at different layers, known as the Chapman layers.
4.2 Chapman Layer Profile
It is known that the density of the atmosphere decreases exponentially with alti-
tude (neglecting, for the moment, variations caused by the temporal structure or the
diffusive separation of species). Also, monochromatic radiation is attenuated expo-
nentially by an absorbing medium (Aghanajafi
2000
). Using these two facts, we can
conceive how solar radiation produces ionized layers. At the outer fringes of the
atmosphere, the density is low and radiation is absorbed only slightly; but deeper in
the atmosphere, both the density and the absorption of radiation increase exponen-
tially. At a certain region, this process produces very rapid attenuation of radiation
at a particular wavelength, with virtually no penetration below. If the absorption is
caused by ionization processes, an ionized layer will result. This phenomenon is
represented schematically by Fig.
9
(Whitten and Poppoff
1971
).
This layer formation theorywas put in good quantitative formby SydneyChapman
in 1931. The Chapman lawdescribes the direct relation of the density of free electrons
and ions to height and daily solar motion. The production rate of ion pairs is given by
Fig. 9
Schematic represen-
tation of layer formation
(modified from Whitten and
Poppoff
1971
)
Radiation intensity
Atmospheric density
Ion-electron concentration
