Geoscience Reference
In-Depth Information
motions of the air. However, the Earth and atmosphere then radiate out the energy in
the form of longwave radiation in the infrared. The overturning of the atmosphere
relates to the prevailing meridional circulation. The fundamental physical elements
of the atmospheric climate system are thus the momentum, heat and moisture cycles
(see Peixoto and Oort (
1992
)). The differential heating in the troposphere, because
of latitude, cloud effects, and the irregular distribution of land and ocean, and land
surface characteristics, leads to patterns of atmospheric mass, along with the changes
in temperature and circulation structure. The total atmospheric mass patterns have
distinct climatological distribution, such as the strong prevailing high pressures over
Siberia in the cold winter. These patterns, and changes in them on daily to weekly
weather time scales and more seasonal time scales, also is a cause of angular momen-
tum variation, leading in particular to exchanges from the Earth causing changes in
positions of the pole, and to a lesser extent, changes in the speed of the Earth's
rotation.
4 The Ionosphere
The ionosphere is an upper part of the Earth's atmosphere, extending from about
60-2000km, with the main concentration of its particles between 300 and 400km
(Rishbeth and Garriott
1969
; Hargreaves
1995
). Solar Extreme Ultra Violet (EUV)
radiation at wave lengths
<
130nm significantly ionizes the neutral gas. In addi-
tion to photo-ionization by electromagnetic radiation, energetic particles from the
solar wind and cosmic rays contribute to the ionization but to a much lesser extent
(Hunsucker and Hargreaves
2002
). The electron production in the ionosphere is a
direct consequence of the interaction of the solar radiation with atoms and molecules
in the Earth's upper atmosphere. These free electrons and ions affect the propaga-
tion of electromagnetic waves. This effect is called ionosphere refraction and has
to be considered when determining the propagation velocity of signals of all space
geodetic techniques operating in the microwave band.
4.1 Ionization and Recombination
Within the ionosphere, solar radiation hits the atmosphere with a power density of
1370W/m
2
, a value known as the solar constant. The intense level of solar radiation
is spread over a wide spectrum, ranging from radio frequency through infrared (IR)
radiation and visible light to X-ray. Due to the fact that the photons of energy at
ultraviolet (UV) or shorter wavelengths are capable of separating an electron from
a neutral gas atom or molecule during a collision, they are considered as ionizing.
During a strike, the incoming solar radiation hits a gas atom (or molecule); the atom
absorbs part of this radiation and produces a free electron and a positively charged ion.
