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DIGISONDE-4D
SYSTEM MANUAL
VERSION 1.2.11
SECTION 1 - GENERAL SYSTEM DESCRIPTION 1-13
that the wave slows down to zero which is obviously required at some point in the process of reflection since
the propagation velocity reverses.
1:18. The total internal reflection from the ionosphere is similar to reflection of RF energy from a metal sur-
face in that the re-radiation of the incident energy is caused by the free electrons in the medium. In both cases
the wave penetrates to some depth. In a plasma the skin depth (the depth into the medium at which the electric
field is 36.8% of its incident amplitude) is defined by:
where
0
is the free space wavelength.
1:19. The major difference between ionospheric reflection and reflection from a metallic surface is that the
latter has a uniform electron density while the ionospheric density increases roughly parabolic ally with alti-
tude, with densities starting at essentially zero at stratospheric altitudes and rising to a peak at about 200 to 400
km. In the case of a metal there is no region where the wave propagates below the resonance frequency, while
in the ionosphere the refractive index and therefore the wave velocity change with altitude until the plasma res-
onance frequency is reached. Of course if the RF frequency is above the maximum plasma resonance frequen-
cy the wave is never reflected and can penetrate the ionosphere and propagate into outer space. Otherwise what
happens on a microscopic scale at the surface of a metal and on a macroscopic scale at the plasma resonance in
the ionosphere is very similar in that energy is re-radiated by electrons which are responding to the incident
electric field.
MOTIVATION FOR A SMALL FLEXIBLE IONOSPHERIC SOUNDER
1:20. Current applications of ionospheric sounders fall into two categories:
a. Support of operational systems, including shortwave radio communications and Over-The-Horizon
(OTH) radar systems. This support can be in the form of predictions of propagating frequencies at
given times and locations in the future (e.g., over the ensuing month) or the provision of real-time
updates (updated as frequently as every 15 minutes) to detect current conditions such that system
operating parameters can be optimized.
b. Scientific research to enable better prediction of ionospheric conditions and to understand the plas-
ma physics of the solar-terrestrial interaction of the Earth’s atmosphere and magnetic field with the
solar wind.
1:21. There has been considerable effort in producing global models of ionospheric densities, temperature,
chemical constitution, etc, such that a few sounder measurements could calibrate the models and improve the
reliability of global predictions. It has been shown that if measurements are made within a few hundred kilo-
meters of each other, the correlation of the measured parameters is very high [Rush, 1978]. Therefore a net-
work of sounders spaced by less than 500 km can provide reliable estimates of the ionosphere over a 250 km
radius around them.
1:22. The areas of research pursued by users of the more sophisticated features of the Digisonde
®
sounders
include polar cap plasma drift, auroral phenomena, equatorial spread-F and plasma irregularity phenomena, and
sporadic E-layer composition [Buchau et al., 1985; Reinisch 1987; and Buchau and Reinisch 1991]. There may
be some driving technological needs (e.g., commercial or military uses) in some of these efforts, but many are
simply basic research efforts aimed at better understanding the manifestations of plasma physics provided by
nature.
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