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Ionosonde Stations
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Ionograms are recorded tracings of reflected high frequency radio pulses generated
by an ionosonde. Unique relationships exist between the sounding frequency and the
ionization densities which can reflect it. As the sounder sweeps from lower to higher
frequencies, the signal rises above the noise of commercial radio sources and records
the return signal reflected from the different layers of the ionosphere. These echoes
form characteristic patterns of
"traces" that comprise the ionogram. Radio pulses travel more slowly within the ionosphere
than in free space, therefore, the apparent or "virtual" height is recorded instead
of a true height. For frequencies approaching the level of maximum plasma frequency
in a layer, the virtual height tends to infinity, because the pulse must travel a
finite distance at effectively zero speed. The frequencies at which this occurs are
called the critical frequencies. Characteristic values
of virtual heights (designated as h'E, h'F, and h'F2, etc.) and critical frequencies
(designated as foE, foF1, and foF2, etc.) of each layer are scaled, manually or by
computer, from these ionograms. Typically, an ionosonde station obtains one ionogram
recording every 15 minutes. When the scaling is done manually only the hourly recordings
are routinely reduced to numerical data. Modern ionosondes with computer-driven automatic
scaling procedures routinely scale all the
ionograms recorded. The resulting numerical values, along with the original ionograms
and station reports, are archived at five World Data Centers (WDCs) for Ionosphere.
The ionosphere is divided into four broad regions called D,E, F, and topside. These
regions may be further divided into several regularly occurring layers, such as F1
or F2.D Region: The region between about 75 and 95km above the Earth in which the
relatively weak) ionization is mainly responsible for
absorption of high-frequency radio waves. E Region: The region between about 95 and
150km above the Earth that marks the height of the regular daytime E layer. Other
subdivisions isolating separate layers of irregular occurrence within this region
are also labeled with an E prefix, such as the thick layer, E2, and a highly variable
thin layer, Sporadic E. Ions in this region are mainly O2+. F Region: The region above
about 150km in which the important reflecting layer, F2, is
found. Other layers within this region are also described using the prefix F, such
as a temperate-latitude regular stratification, F1, and a low-latitude, semi-regular
stratification, F1.5. Ions in the lower part of the F layer are mainly NO+ and are
predominantly O+ in the upper part. The F layer is the region of primary interest
for radio communications.
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