DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

iron Kα line
iron Kα line A spectral line at energy of
approximately 6.4 keV from the transition between
the L and K shells (i.e., the second innermost
and the innermost atomic shells, corresponding
to quantum number n = 2 and n = 1,
respectively) of an iron atom. The iron fluorescence
Kα line is a strong emission feature in
the X-ray spectra of active galactic nuclei and
of cataclysmic variables. It can be produced by
recombination following photoionization in gas
irradiated by an intense X-ray source, as in systems
powered by accretion. An electron of the
K-shell may also be removed by Compton scatteringduetoanX-rayphoton.
Thelineisexcited
collisionallyinthehotgasofstellarflares, supernova
remnants and in the intra-cluster medium
in clusters of galaxies. Observation of the line
has been made possible by X-ray space-borne
observatories. The instruments on board the
orbiting Japanese observatory ASCA have revealed,
in the Kα line profile of several Seyfert
galaxies, characteristic effects predicted for radiation
coming from a gaseous, rotating disk at
a few gravitational radii from a black hole. See
accretion disk, Seyfert galaxies.
iron meteorite A meteorite consisting of
solid nickel-iron alloy. Such an object must have
formed in the core of a substantial planet which
was then disrupted, presumably in a collision or
close encounter with another object. Iron (or
iron-nickel) meteorites are classified as Hexahedrites
(less than 6.5% nickel); Octahedrites
(6.5% to 13% nickel); and Ataxites with greater
amounts of nickel.
irradiance The radiant power per unit area
per unit wavelength interval [W m−2 nm−1 ].
1. [downward (upward) plane irradiance]
The downward (upward) directed radiant power
per unit area onto an upward (downward) facing
horizontal surface [W m−2 nm−1 ].
2. [downward (upward) scalar irradiance]
The downward (upward) directed radiant power
per unit area onto a spherical collecting surface
[W m −2 nm −1 ].
3. [net plane irradiance] The downward
plane irradiance minus the upward plane irradiance
[W m −2 nm −1 ].
© 2001 by CRC Press LLC
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4. [irradiance ratio] The ratio of the upward
plane irradiance to the downward plane irradiance.
5. [reflectance] irradiance ratio.
6. [scalar irradiance] The power per unit
area incident from all directions onto a spherical
collecting surface [W m −2 nm −1 ]; it equals
the downward scalar irradiance plus the upward
scalar irradiance.
irregular galaxies Galaxies that lack a central
bulge, azimuthal symmetry, and that most
often show a rather patchy appearance. They
are of blue color, have high neutral gas content,
and show evidence of ongoing star formation.
Irregular galaxies are often dwarf galaxies,
on average smaller and less massive than
spiral galaxies, with typical masses ∼ 10 8 solar
masses. They also have lower rotational velocity,
if any, and lower luminosity than spirals.
Although a small fraction in large catalogs of
galaxies, such as the Revised New General Catalog,
irregular galaxies are thought to account
for 1/2 to 1/3 of all galaxies in the local universe.
They have been further subdivided into
the Magellanic type (from the prototype galaxy,
the Large Magellanic Cloud), and the amorphous
type (or M82-type) galaxies. Magellanic
irregulars have a patchy appearance due to clusters
of hot stars in star-forming regions spread
over the whole galaxy, while amorphous irregulars
are smoother in appearance, with a single
supergiant star-forming region at the center of
the galaxy. This subdivision is now considered
mainly of historical importance.
irregular waves Waves without a single,
clearly defined period. Also referred to as random
waves.
irribarren number A dimensionless parameter
used to predict wave breaker type. Equal
to S/ √ Hb/Lo, where S is beach slope, Hb is
a breaking wave height, and Lo is deepwater
wavelength.
irrotational The property of a vector field
v (e.g., in fluid dynamics a velocity field) that
its curl vanishes: ∇ × v = 0. Hence, v is the
gradient of a scalar function: v = ∇φ.