DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

with F being the density of particles in a given
magnetic flux tube, t the time, v the particle
speed, s the distance along the magnetic
field spiral, p the momentum in the solar wind
frame, vsowi the solar wind speed, ψ the garden
hose angle between the magnetic field line and
the radial direction, ζ the focusing length, and
E ′ /E= 1 − µvsowiv secψ/c 2 the ratio of the
total energy in the solar wind frame to that in a
fixed frame.
This transport equation can be solved only
numerically. Different approximations exist:
the focused transport equation can be applied
to MeV particles in the inner heliosphere (see
focused transport equation); the diffusion convection
equation, on the other hand, is applied
to particles with energies in the keV range, to
observations beyond 1 AU, and to the modulation
of galactic cosmic rays (see diffusionconvection
equation).
interplanetary scintillation (IPS) Solar
wind induced scintillation of cosmic radio signals.
interplanetary scintillation (IPS) observations
Remote-sensing observations of the inner
solar wind, based on analysis of how a radio
beam from a distant source (natural or from a
spacecraft) is disturbed as it passes near the sun.
interplanetary sector structure See heliospheric
magnetic field.
interplanetary shock wave See hydromagnetic
shock wave.
interplanetary stream structure See heliospheric
stream structure.
interplate earthquake An earthquake that
occurs on the boundary fault dividing two lithospheric
plates, as opposed to intraplate earthquakes
that occur within a plate.
interstellar clouds Dark material along the
spiral arms of our galaxy, consisting of clouds of
neutral hydrogen, or of interstellar molecules.
interstellar dust Small dust grains of size
5×10 −9 mto2×10 −7 m, apparently consisting
© 2001 by CRC Press LLC
interstellar molecules
of refractory materials (silicates and graphite)
(≈ 10 8 atoms/grain) with a power law distribution
n(r) ∝ r −3.5 where r is the radius of
the particle. Additional components consisting
of polyaromatic hydrocarbons, and of water ice,
are also present. Interstellar dust is responsible
for extinction (especially visible in dark nebulae),
and for reddening (a general phenomenon)
of starlight.
Dust production presumably occurs in cool
red giants, which are old stars with enhanced
metals and substantial stellar winds, and also in
supernova explosions, which can transport large
amounts of metals into the interstellar environment.
interstellar gas The gas lying between stars
in the plane of the galaxy, composed principally
(≈ 90%) of atomic hydrogen, H , approximately
half of which is neutral (HI), observable by its
21 cm radiation, and half is ionized (HII, ionized
by hot O and B stars). The number density
varies from 10 −2 to 10 6 cm 3 , averaging about
1cm 3 . There is a small admixture of “metals”.
interstellar medium The matter composed
of dust and gas in diffuse form that fills the space
between the stars of a galaxy. Interstellar matter
varies widely in temperature, density, and
chemical composition. The interstellar medium
reaches to the outer layers of the atmosphere of
stars where the density and temperature of the
gas increase steeply, but far from the stars its
density drops to less than 1 particle cm −3 .However,
in a galaxy there are many places where the
density of interstellar matter increases significantly.
These concentrations or regions of the
interstellar medium, sometimes called clouds,
or shells because of their appearance, have different
physical and chemical characteristics that
depend on the amount of light and energy that
they receive.
interstellar molecules Inorganic (silicates);
and especially organic molecules identified via
infrared spectroscopy in cool dense dark interstellar
clouds (of typically solar system size) in
the nucleus and along the spiral arms of the
galaxy; about 10 −4 the interstellar mass. At
present (AD 2000) over 100 organic molecules
have been fairly securely identified, involving H,
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