DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

ionosonde
Each sporadic E layer can be described by three
parameters: hEs, foEs, and fbEs. Finally, two
further important parameters are fmin, the minimum
frequency returned from the ionosphere
and recorded on the ionogram and M(3000)F2.
These parameters are tabulated hourly, together
with other information, and exchanged between
World Data Centers. There is now a serious
attempt to collect these data in real time and
construct global maps of the ionosphere. To obtain
the electron density profile in real height
units, sometimes called true height, it is necessary
to invert the ionogram, and correct for the
time delay caused by the ionosphere. See criticalfrequency,
ionosonde, ionosphericsounding,
plasma frequency.
ionosonde A swept frequency pulsed radio
transmitter on the ground cosited with a receiver
forming a vertically directed radar system.
A single frequency pulse is transmitted upwards
and may be reflected from the ionosphere,
the reflected signal being observed with the receiver.
This process is repeated at a sequence
of frequencies usually starting at a frequency of
1.0 MHz and finishing at around 20 MHz. A picture
of the ionization above the ionosonde is then
built up into an ionogram. This is constructed
by displaying the returned signal as a function
of frequency (on the horizontal axis) and time
delay (on the vertical axis). See ionogram.
ionosphere The layer of the Earth’s atmosphere
extending from the top of the mesosphere
(typically 80 to 90 km above the surface)
to about 500 km. The thermosphere, or
ionosphere, is the uppermost layer of a planetary
atmosphere. It is also often called the
upper atmosphere. It is characterized by low
density ( 3 × 10 −9 gm/cm 3 ; number density
7 × 10 −11 /cm 3 ) and pressure ( 0.1 Pa),
and substantial ionization of the atoms, resulting
in significant free electron density, which
is a function of solar activity. The temperature
increases rapidly with increasing altitude up to
about 200 km, followed by a leveling off in the
300 to 500 km region. This heating is the result
of photoionization and photodissociation of
molecules into atoms and ions by direct absorption
of solar photons.
© 2001 by CRC Press LLC
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ionospheric absorption Radio waves passing
through the ionosphere experience interactions
with the free electrons and may lose energy
by these interactions if the electrons experience
collisions with the neutral atmosphere.
This process is called absorption, or sometimes
non-deviative absorption, because the radiowave
path through the absorbing region is not
affected by the absorption. Collisions between
electrons and the neutral atmosphere increase
with decreasing altitude, so the D region is
particularly important for absorption processes.
Absorption varies as the inverse square of the
frequency, so the losses are larger at low HF
frequencies. In normal circumstances, during
the daytime, ionospheric absorption prevents
radio frequencies greater than about 2 or 3 MHz
propagating via the ionosphere unless very large
powers are transmitted. During nighttime, when
the D-region ionization drops to very low levels,
these same transmissions can propagate by the
ionosphere to large distances. During a solar
flare, X-ray radiation from the sun can increase
significantly, resulting in greater ionization of
the D region and subsequently larger absorption
of radio waves. These increases may be sufficiently
large to prevent all HF signals propagating
by the ionosphere. See D region, short wave
fadeout, solar flare.
ionosphericindex Anindexusuallybasedon
observations of the F region critical frequency
(foF2), which gives a measure of the solar cycle
effects on the ionosphere. There are a variety
of ionospheric indices of which the best known
are IF2 and T. Both these indices are calculated
in similar ways. First, past data are used to estimate
calibration curves then, using current data
with the calibration curves, an index is calculated
that best represents the solar cycle effects
for this epoch. Ionospheric indices are effective
because the F region is strongly dependent on
changes in solar activity. The correlation between
the monthly median hourly foF2 and the
smoothed sunspot number, for a location, can be
higher than 0.95. Ionospheric indices are often
used with HF propagation models for predicting
propagation conditions. See ionospheric radio
propagation path.