Environmental Engineering Reference
In-Depth Information
photorecombination of electrons and oxygen ions, photoattachment of electrons to
oxygen atoms, and by transport of electrons to lower atmospheric layers.
The E and F layers of the ionosphere, as layers with a high number density of
electrons, are responsible for reflection of radio signals, so these layers are the radio
mirror of the atmosphere for radio waves. But their role is broader than this func-
tion. The altitudes at which they occur are the most convenient for operation of
artificial satellites. Also, the interesting physical phenomenon, the aurora, occurs
at these altitudes. An aurora occurs when a flow of solar protons penetrates the
atmosphere. Charge exchange of these protons and their deceleration takes place
in the E and F layers of the atmosphere. The subsequent chain of elementary pro-
cesses leads to formation of excited atoms whose radiation is observed from the
Earth as an aurora.
One of the characteristics of auroras is that their radiation is created by forbid-
den transitions of atoms and ions that cannot be modelled in the laboratory. In
particular, processes 7-10 in Table 6.6 usually give the principal contributions to
the radiation of auroras. Under laboratory conditions, these excited states of atoms
and ions are quenched by collisions with air molecules. The probabilities for an ex-
cited atom to radiate and to be quenched are equal at a number density of quench-
ing particles given by (
k
)
1
,where
is the radiative lifetime of the excited atom,
and
k
is the rate constant for collision quenching. The excited oxygen atom O(
1
S
)
is quenched in the upper atmosphere mainly by atomic oxygen (process 38 in Ta-
ble 6.6), and the threshold number density of oxygen atoms is 2
τ
τ
10
11
cm
3
.In
thecaseofO(
1
D
) the main quenching mechanism in the ionosphere is process
36 in Table 6.6. This gives the threshold number density of nitrogen molecules as
10
8
cm
3
. If the number density of quenching particles is lower than the values
given above, radiation of the corresponding excited atoms may be significant.
6.4.4
Atomic Oxygen in the Upper Atmosphere
Atomic oxygen is one of the basic components of the upper atmosphere, so we shall
examine those processes that determine its relative abundance in the atmosphere.
The formation of atomic oxygen results from photodissociation of molecular oxy-
gen (process 4 in Table 6.6) by solar radiation in the spectral range from 132 to
176 nm (corresponding to the photon energy range from 6 to 10.3 eV). This absorp-
tion range is called the Schumann-Runge continuum and is characterized by cross
sections of 10
19
10
17
cm
2
. The Schumann-Runge continuum is the determin-
ing factor in the generation of atomic oxygen at altitudes higher than 120 km. At
lower altitudes, up to
h
80 km, the generation of atomic oxygen is accomplished
mostly by dissociation of oxygen molecules due to the weak Herzberg continuum
in the wavelengths range from 140 to 175 nm, where the absorption cross section
is less than 2
10
24
cm
2
.
The recombination rate of two oxygen atoms in three body collisions decreases as
the altitude increases, and hence the concentration of atomic oxygen increases. The
number density of atomic oxygen becomes equal to the number density of molecu-
