The ionosphere consists of a rarefied ZONE in the earth's atmosphere =
where most of the particles floating around have an electrical charge. =
These particles may be free electrons or atomic nuclei that have lost =
part or all of their electrons. Because the particles are scattered out =
in three dimensions the opportunities for collisions to take place and =
recombine the electrons with the atomic nuclei are few and far between. =
When high energy rays and particles from the sun hit the ionosphere the =
particles already there get stirred up and broken apart even more. Since =
the ionosphere is several hundred miles thick the upper layers get most =
of the energy transferred to them and the lower layers only get stirred =
up when there is enough energy in the rays and enough mass in the =
particles from the sun to push through the upper layers.
The earth's magnetic and electrostatic fields act on the charged =
particles in the ionosphere, generally moving them outward from the =
earth's surface. Since these fields are essentially static (changes in =
them take place in relative slow motion compared to movement of the =
particles) they tend to stabilize and stratify the ionosphere.=20
When an electromagnetic wave enters the ionosphere it begins to be =
affected by the charged particles. The waves are pushed in curving paths =
by the electrostatic charges on the particles in the ionosphere. Since =
these charged particles are moving at fairly rapid rates they create =
magnetic fields. These magnetic fields also push the electromagnetic =
wave around but their force on it is at right angles to the force =
exerted by the electrostatic fields of the particles. If all the =
particles were moving in random paths at random speeds these forces =
would cancel out and the electromagnetic wave would be able to go along =
on its merry way. However it would be weakened because part of its =
energy would be transferred to the particles as it passed among them. =
Fortunately the earth's electrostatic and magnetic fields and the "wind =
from the sun" force the charged particles in the ionosphere into more or =
less uniform paths so the forces on the electromagnetic waves add into =
vector sums that bend them into a new path. This is such a dynamic =
activity that the vector sum of the forces can change in a fraction of a =
cycle of the electromagnetic wave. As the time period of a particular =
vector sum approaches the frequency of the electromagnetic wave the path =
it is bent into becomes momentarily stable and, if the curvature of the =
path is just right, it is sent back towards the surface of the earth. It =
may go through multiple stages of refraction as it penetrates deeper =
into the ionosphere. There may be several stages where part of the wave =
is refracted sufficiently to return to the earth's surface. This is =
called "multiple bounce skip". (In fact it is anything but bouncy.) When =
outside influences (primarily the effects of charged particles from the =
sun) are of sufficient magnitude, stability and duration this path may =
be maintained for a period of several hours. This we call a band =
opening!=20
As an electromagnetic wave enters the bottom of the ionosphere it is =
immediately assailed by the randomly moving charged particles in the =
lowest layer. The wave begins to lose energy to these particles but, if =
it is strong enough, part of the electromagnetic wave will penetrate to =
the higher layers where the forces on it are additive vectorially and it =
is refracted. Due to the stratification caused by the earth's =
electrostatic and magnetic fields sharp lines of demarcation at times =
will exist between the zones of random movement and zones of coherent =
movement of particles. This means that an electromagnetic wave may have =
to exceed a certain energy level before it can reach the layer where it =
will be refracted back towards earth. Below that energy level it simply =
disappears. Once it gets past these random movement areas it will be =
refracted. The amount of refraction will be independent of the energy in =
the wave once it is through the lossy layers. The deeper into the =
ionosphere the uniform layers are pushed by outside forces the less =
random movement of particles the electromagnetic wave will be exposed to =
and the lower the threshold energy level necessary before refraction can =
take place.
In some instances the vector sum of the forces on the electromagnetic =
wave may be parallel to the earth's surface and incorporate a sideways =
shift from the straight line path the wave was initially sent along. =
This will result in a "skewed path" and cause signals to come in from =
directions other than the great circle path normally expected.
This is a greatly simplified explanation of the refraction theory of =
ionosphere propagation but it explains most of the phenomenon =
experienced by radio amateurs in high frequency operations.
The one thing that it does establish is that the ionosphere is a =
non-linear medium.=20
73
N7DF
Larry
--
CQ-Contest on WWW: http://lists.contesting.com/_cq-contest/
Administrative requests: cq-contest-REQUEST@contesting.com
|