Environmental Engineering Reference
In-Depth Information
3
10
19
cm
6
. This corresponds to altitudes of approximately 200 km. The maxi-
mum number density of atomic ions
N
i
follows from the balance equation
Z
σ
ion
dI
ion
k
[N
2
]
N
i
.
For photoionization of atomic oxygen, we have
R
σ
10
7
s
1
,sothe
ion
dI
ion
D
2
maximum number density is
10
5
cm
3
[O]
10
6
cm
3
.
N
i
2
[N
2
]
(6.101)
In higher layers of the atmosphere, the number density of atomic ions is deter-
mined by the barometric formula, because it is proportional to the number density
of primary atoms, and declines with increasing altitude.
At altitudes where photoionization occurs, the negative charge of the atmospher-
ic plasma comes from electrons. In the D-layer of the ionosphere, electrons attach
to oxygen molecules in accordance with processes 20-22 in Table 6.6, and it is
negative ions that govern the negative charge of the atmosphere. At the altitudes
where this transition takes place, the balance equation
wN
e
/
L
KN
e
[O
2
]
2
is ap-
propriate, where
K
10
31
cm
6
/s is the rate constant for processes 20 and 21 in
Table 6.6. From this it follows that formation of negative ions occurs at altitudes
where
N
[O
2
]
2
10
13
cm
3
. We account for the coeffi-
cient of ambipolar diffusion of ions being of the order of the diffusion coefficient
for atoms. In the D layer of the ionosphere, recombination proceeds according
to the scheme
A
C
10
39
cm
9
or [O
2
]
3
B
C
!
A
C
B
, and is characterized by the rate constant
10
9
cm
3
/s. This leads to the relation
α
10
17
cm
6
N
i
N
(6.102)
for the number density of ions
N
i
. Thus, the properties of the middle and the upper
atmosphere are established by processes in excited and dissociated air involving
ions, excited atoms, and excited molecules.
6.5
Electric Machine of the Earth's Atmosphere
6.5.1
The Earth as an Electric System
Processes in the lower atmosphere lead to charging and discharging of the Earth,
and we consider below the Earth as an electric system and the processes which gov-
ern its electric properties. The Earth is negatively charged, and the average electric
field strength near the Earth's surface is
E
D
πσ
D
is the average sur-
face charge density) [149, 150]. This gives the total Earth charge
Q
4
130 V/m (
σ
10
5
C.
In addition, the electric potential of the Earth with respect to the environment is
D
5.8
