Geoscience Reference
In-Depth Information
Fig. 3.1
Typical profile of
the electric field in the
atmosphere.
1
—fair weather
conditions over ocean, arctic
region and etc.
2
—over
mainland. Adapted from the
the land, as shown in Fig.
3.1
with line 2. Above the mixing layer, whose depth is
about 0.3-3 km, the field value decreases approximately exponentially. The voltage
drop between the Earth and the ionosphere is about 200-250 kV.
The atmospheric conductivity at the ground level is about
a
D
.2
3/
10
14
S/m which is smaller than that of the ionosphere by several orders of
magnitude. In the mixing layer the atmospheric conductivity
a
increases insignifi-
cantly and then it rises nearly exponentially with altitude, with a characteristic scale
of 3-7 km, the value of which depends on altitude. For example, at the daytime
conditions the approximate law for the conductivity as a function of altitude
z
can
be written as (Chalmers
1967
)
a
D
0
exp .
z
=
z
0
/;0<
z
<3:6km;
a
D
1
exp .
z
=
z
1
/; 3:6 <
z
<17:7km;
a
D
2
exp .
z
=
z
2
/;1 :7<
z
<40km;
(3.1)
where
z
0
D
0:82 km,
z
1
D
4:1km,
z
2
D
7:0 km,
0
D
1:14
10
14
S/m,
1
D
0:38
10
12
S/m, and
2
D
2:29
10
12
S/m. This exponential tendency holds true
for larger altitudes although the atmospheric conductivity depends on local time.
Above the lower edge of the E region at altitudes 75-90 km the ratio of the
electron gyrofrequency to the electron-neutral collision frequency is no longer
negligible, and the conductivity converts from a scalar quantity to a tensor one.
Note that the atmospheric conductivity undergoes diurnal variations depending on
latitude, local meteorological conditions, and so on.
The so-called fair weather current, that is, a weak background current flowing
from the mesosphere to the ground plays an important role in the generation of
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