Environmental Engineering Reference
In-Depth Information
In reality, the Earth's ionosphere is not a uniform plasma because of different
compositions of the atmosphere and different degrees of ionization for different
altitudes. Hence, the ionosphere is divided into layers, as given in Table 6.8.
The presence of electrons in the ionosphere prevents long electromagnetic waves
from propagating therein if their frequencies are less than the frequency of plas-
ma oscillations. Hence, radio waves reflect from the ionosphere and return to the
Earth's surface [142, 143]. Table 6.9 gives limiting electron number densities at
which the plasma frequency coincides with the frequency of plasma oscillations,
so longer radio waves cannot propagate in such a medium. The maximum elec-
tron number density of the order of 10
6
cm
3
may be reached at an altitude of
about 200 km. Therefore, the best reflection conditions are fulfilled for short waves
within the range of 30-100 m. Because the electron number density at any partic-
ular altitude depends on season, time of day, and other factors, the quality of radio
communications at a given wavelength varies continually. Long waves are reflected
at low altitudes, where the atmospheric density is relatively high, and reflection of
the waves is accompanied by damping.
It has been established that several ionized layers differ in their properties, and
different processes are of importance for each layer [144-147]. The lower D layer
of the atmosphere occupies the altitude region from 50 to 90 km. A typical num-
ber density of charged particles therein is of the order of 10
3
cm
3
.Thenegative
charge of the D layer of the Earth's atmosphere arises primarily from the presence
of negative ions, and a great variety of both negative and positive ions reside there.
In particular, the most widespread positive ion is the H
3
O
C
H
2
Oclusterion.Fig-
ure6.22showstheaveragedistributionoftheelectronandionnumberdensity
with altitude. These number densities vary significantly depending on the condi-
Ta b l e 6 . 8
Layers of the ionosphere.
Layer
Altitude, km
D
60-90
E
90-140
F
1
140-300
F
2
300-1000
Ta b l e 6 . 9
Critical number densities of plasma electrons (
N
cr
) for propagation of radio waves.
N
cr
,cm
3
Type of radio waves
Wavelength, m
10
3
Long
10 000-1 000
10
10
3
10
5
Medium
1 000-100
10
5
10
7
Short
100-10
