| 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. |