Page 96 - Maxwell House

76 Chapter 2

there are roughly equal numbers of positively and negatively charged particles, produced when
the atoms in a gas become ionized. It is sometimes referred to as the fourth state of matter,
distinct from the solid, liquid, and gaseous states.” It is worthwhile to point out that nearly all
the visible matter in the universe exists in the plasma state. Roughly speaking, our Sun is a
massive plasma fireball, plasma is the part of Earth’s upper (60 km and higher) atmosphere
called ionosphere, our Earth has blown around by plasma solar wind, plasma exists in neon and
fluorescent tubes, welding arcs, etc.
Typical electron density in Earth’s ionosphere is quite low in comparison with metals and
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3
10
fluctuates between 10 and 10 electrons per m depending on altitude, day or night time,
season, Sun activity, geographical region, Earth’s magnetic field, lightning, and some other
factors. That all makes the prediction of the ionosphere behavior a quite difficult task.
Undoubtedly, the leading source of plasma ionization is the solar radiation. Since the
fundamental forces governing the electron movements in plasma and metal are practically the
same, we can apply the equation (2.79) to study the ionospheric plasma performance. Our point
of interest is the relatively low frequencies below 30-50 MHz which is far-far away from any
resonances we have mentioned before. As such, the term ( ) in (2.79) can be ignored. Then
2
0
providing some transformations to separate the real and imaginary parts we obtain
2
� �
′
() = 1 −
() 2 +1 � (2.83)
2
� �
′′
() = (() 2 +1)

According to these equities, the real part () is negative while
′

≤ � − 1 (2.84)
2
⁄
2

The relaxation time of ionospheric plasma is
measured in seconds while = 56√ has the

5 6 2
order of 10 – 10 . Therefore, in (2.83) ≫

1 and
2
⁄
2 2

′
() ≈ 1 − = 1 − (2.85)

2 2
Figure 2.5.3 Long-distance
communication using reflections from Here = ⁄ 2 = 9√ [Hz] is the plasma

ionosphere resonance frequency. It would rather better to
consider this frequency as critical because at all
frequencies f < / cos (while () < 0) the electromagnetic waves incidenting from the
′

ground on the ionosphere layer are back-scattered as shown in Figure 2.5.3 . The “reluctance”
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of the waves to penetrate into a medium with negative permitibility can be explain by the fact
that the wavenumber ~� () defining the wave propagation is pure imaginary while
′

() < 0. It means that the substantial portion of penetratated energy is reactive. As a result,
′

only small portion of incident wave called sky wave can go through and propagate in plasma.
The most part of its active energy should be reflected. More accurate analysis shows that the
13 Public Domain Image, source: http://3.bp.blogspot.com/-
asxz7PHGTuA/UTFTwNGzbMI/AAAAAAAAAS4/fkJDEPbTjRY/s1600/IMG_1541.GIF