Environmental Engineering Reference
In-Depth Information
is the only way to create an electric current that charges the Earth's surface. In turn,
the Earth's electric field is of importance for charging of aerosols, and such pro-
cesses will be considered below. Aerosol particles in the Earth's atmosphere may
be formed by natural processes and as a result of human activity [140].
When solid clusters join as a result of contact between them in atmospheric air,
porous structures can be formed in which solid clusters conserve their individuali-
ty. These structures are fractal aggregates if their joining takes place in the absence
of external fields. This name results from the dependence of the matter density in
fractal aggregates on their size. The specifics of growth of fractal aggregates in a
buffer gas are connected to the diffusion character of their motion. This can lead
to a nonlinear dependence of the nucleation rate on the aggregate number den-
sity. Because of the micrometer size of fractal aggregates, they interact effectively
with external fields. Even small electric fields can cause an effective interaction be-
tween fractal aggregates through the interaction of induced charges. As a result of
nucleation in an external electric field, so-called fractal fibers are formed that are
elongated fractal structures.
6.4.3
The Ionosphere as a Mirror for Electromagnetic Waves
The history of exploration of the Earth's ionosphere starts from Marconi's exper-
iment in 1901 when he tried to establish radio contact between two continents.
The transmitter had been set up in Europe on the Cornwall peninsula in England,
and the receiver was located in Canada, on the Newfoundland peninsula. From
the standpoint of wave propagation theory, this experiment seemed to be hopeless.
According to the laws of geometrical optics, radio waves should propagate at such
distances with rectilinear beams, and radio connection for these distances seemed
to be precluded by the spherical formof the Earth's surface. The experiment led to a
surprising result: the signal was detected by the receiver and its intensity exceeded
estimates by many orders of magnitude.
The only explanation for this discrepancy was the existence of a radio mirror in
the Earth's atmosphere that reflects radio waves. The radio mirror model assumes
that waves follow rectilinear paths and repeatedly reflect between the surface of the
Earth and this upper-atmosphere radio mirror. In 1902, O. Heaviside and A. Ken-
nelly assumed that the role of this radio mirror is played by an ionized layer in
the atmosphere. This was confirmed in 1924-1925 by an experiment conducted by
a group of English physicists from Cambridge University ([180, 181]). Placing the
receiver at a distance of 400m from the transmitter, they determined by measure-
ment of the time delay for the reflected signal to arrive that the reflector is at an
altitude of 100-120 km. This reflecting layer, which was referred to earlier as the
Heaviside layer, is now called the E layer (based on the designation of the electric
field vector for radio waves). Subsequently, the existence of ionized gas was discov-
ered at other altitudes. The part of the atmosphere that contains ionized gases is
called the ionosphere [141].
