DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

trix G so that:
d = Gp
in which case a least-squares solution p ′ can be
obtained via:
p ′ = G T G −1 G T d .
However, it may be that other solutions fit the
data equally well, in which case the inversion
is said to be nonunique. Complications may be
introduced by a nonlinear relationship between
p and d, by constraints on the parameters, or by
biases concerning the model (e.g., a preference
for simple models). See nonuniqueness.
inverted barometer effect (inverse barometer
response) The inverse response of sea level
to changes in atmospheric pressure. A static reduction
of 1.005 mb in atmospheric pressure will
cause a stationary rise of 1 cm in sea level. The
dynamic response, especially at periods shorter
than a few days is not constant.
Io Moon of Jupiter, also designated JI. Discovered
by Galileo in 1610, it is one of the four
Galilean satellites. Its orbit has an eccentricity
of 0.004, an inclination of 0.04 ◦ , a precession
of 48.6 ◦ yr −1 , and a semimajor axis of
4.22 × 10 5 km. Its radius is 1815 km, its mass
8.89 × 10 22 kg, and its density 3.55 g cm −3 .It
has a geometric albedo of 0.61, and orbits Jupiter
once every 1.769 Earth days. Io is tidally heated
by Jupiter, and has active volcanoes as a result.
ion acoustic waves Ion acoustic waves are
among the simplest wave motions supported
by a plasma. They behave in a fashion similar
to normal sound waves in neutral gas, with
one principal difference: the ion acoustic wave
speed is determined by the ion’s inertia and the
combined pressure of both ions and electrons in
the plasma. In this wave, the ions and electrons
oscillate synchronously, coupled by the electric
field generated.
ionization The process by which ions are
produced, typically occurring by collisions with
atoms or electrons (“collisional ionization”), or
by interaction with electromagnetic radiation
(“photoionization”).
© 2001 by CRC Press LLC
ionogram
ionization equilibrium A plasma at a given
temperature is in ionization equilibrium if the
ratio of the number of ions in a given ionization
state for a given element, nion, to the total
number of ions of that element, nel, is governed
by the equations of thermodynamic equilibrium.
Departures from ionization equilibrium imply
changes in the temperature of the peak abundance
of the various ions, so that the corresponding
lines would be formed at temperatures significantly
different from the formation temperature
deduced from equilibrium.
ionizing radiation Electromagnetic radiation
with energy exceeding the typical binding
energy of electrons in molecules; electromagnetic
radiation with wavelength shorter than that
of ultraviolet light; also applied to particulate radiation
of similarly high energy.
ionogram The conventional display obtained
from an ionosonde which contains information
about the ionosphere. An ionogram is constructed
by displaying the returned signal as a function
of frequency (on the horizontal axis) and
time delay (on the vertical axis). The vertical
axis, called the virtual height, is the height that
the returned signal would have reached had it
traveled in free space and been reflected from
a perfect reflector above the ionosonde. The E
and F layers of the ionosphere can be recognized
easily on most middle and low latitude ionograms,
recognition being more complex at high
latitudes due to the presence of particle effects.
The layers of the ionosphere can be parameterized
in terms of their peak electron density
(measured in megahertz, the units used on an
ionogram for measuring electron density) and
base height (measured in virtual height units,
usually kilometers), both of which can be readily
measured from ionograms. Conventionally,
these are only recorded for the ordinary ray polarization,
information from the extraordinary
ray, when present on the ionogram, being used
to improve these estimates. The most familiar
internationally agreed parameters are hE, foE
(the E region), hF, foF1 (the daytime F1 region),
and hF2, foF2 (the daytime F2 region). At night
these become just hF and foF2 (the nighttime F
region). When the F1 region is not visible, the
daytime F region parameters are hF and foF2.
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