DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

inshore The portion of a beach profile lying
between the foreshore and offshore.
insolation Shortwave solar radiation (UV,
visible, near infrared) per unit area that is received
in the Earth’s atmosphere or at its surface,
taking into account the angle of incidence
to the horizontal.
Institute of Space and Astronautical Studies
(ISAS) Japanese space agency responsible
for a number of successful scientific spacecraft,
including the solar missions Hinotori and
Yohkoh.
integrated Sachs–Wolfe effect In cosmology,
in the linear regime where fluctuations can
be considered small perturbations on an expanding
homogeneous isotopic cosmology, the contribution
to temperature anisotropies from varying
gravitational potentials along the line of
sight. For a flat universe with no cosmological
constant, the gravitational potentials do not
evolve with time and this effect is zero. In open
and flat models with cosmological constant (see
Friedmann models) it can dominate over the
gravitational potential variations across the last
scattering surface.
intensity The radiant power in a given direction
per unit solid angle per unit wavelength
interval [W sr −1 nm −1 ].
interaction of galaxies The gravitational attraction
between two or more galaxies, which
can induce notable modifications in their morphology,
as well as in their photometric and
spectroscopic properties. Interacting galaxies
are often classified as peculiar, since their morphology
does not fit the criteria of any of the
main classification schemes for galaxies. The
effect of interaction among galaxies depends
strongly on their mutual distance from a companion
galaxy, depending on the inverse cube of
the distance. Extensive observation, as well as
computational simulation, suggests that for disk
galaxies, the effects of interaction on morphology
encompass the formation or enhancement
of a spiral pattern, the formation of a bar, and,
in more extreme cases, the formation of tidal
tails, or of a prominent outer ring, as in ring
© 2001 by CRC Press LLC
intercommutation
galaxies or, ultimately, the production of a remnant
(after a merger) which resembles an elliptical
galaxy. Collisions involving only elliptical
galaxies may lead to the production of ripples,
extended halos, and asymmetries in the photometric
profiles, but they do not produce such
spectacular features as tidal tails. Interaction
of galaxies has been linked to an enhancement
of star formation in the host galaxies and, more
speculatively, to the occurrence of quasar-type
nuclear activity. See elliptical galaxies, galaxy,
spiral galaxy, starburst galaxy.
intercloud medium The warm (≈ 3000 K)
lowdensity(≈ 10 −2 /cm 3 )gasinroughpressure
equilibrium with interstellar clouds.
intercommutation In the physics of cosmic
strings, after a cosmic phase transition with
the generation of cosmic strings, the resulting
network is free to evolve and multiple interactions
between single strings will take place. For
the simplest (Abelian–Higgs) model, numerical
simulations show that two strings will, in
general, interchange extremes when crossing.
This exchange of partners is intercommutation
(also sometimes referred to as reconnection). In
strings that might be generated from more complicated
fields (non-Abelian π1(M)) string entanglement
may take place, due to the fact that
exchange of partners is topologically forbidden.
See cosmic string, cosmic topological defect,
cusp (cosmic string), kink (cosmic string).
(a) (b)
String interactions: (a) intercommutation, with the possible
appearance of kinks and cusps, (b) no intercommutation,
i.e., the strings cross each other and are left
essentially unaffected, and (c) entanglement.
(c)
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