DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

sionally, toward the second or higher diffraction
orders. In this case, a grating is said to be blazed.
diffuse absorption coefficient For downwelling
(upwelling) irradiance: the ratio of the
absorption coefficient to the mean cosine of the
downward (upward) radiance. See absorption
coefficient.
diffuse attenuation coefficient For downwelling
(upwelling) irradiance: the ratio of the
sum of the absorption coefficient and the scattering
coefficient to the mean cosine of the downward
(upward) radiance.
diffuse aurora A weak diffuse glow of the
upper atmosphere in the auroral zone, caused
by collisions with the upper atmosphere of electrons
with energies around 1 keV. It is believed
that these electrons leak out from the plasma
sheet of the magnetosphere, where they are
trapped magnetically. The diffuse aurora is not
conspicuous to the eye, but imagers aboard satellites
in space see it as a “ring of fire” around the
magnetic pole. Discovered by ISIS-1 in 1972,
its size, intensity, and variations — in particular,
its intensifications and motions in substorms —
are important clues to the state of the magnetosphere.
diffuse galactic light The diffuse glow observed
across the Milky Way. A large part of
the brightness of the Milky Way, which is the
disk of our galaxy seen from the inside, can be
resolved into stellar sources. The diffuse galactic
light is a truly diffuse glow which accounts
for the remaining 25% of the luminosity and, by
definition, is unresolved even if observed with
large telescopes. The diffuse galactic light is
due to light emitted within our galaxy and scattered
by dust grains, and it is not to be confused
with light coming from extended sources like
reflection or emission nebulae. The brightness
close to the galactic equator due to diffuse galactic
light is equivalent to 50 stars of 10th magnitude
per square degree; for comparison, the total
star background is 170 10th magnitude stars per
square degree, and the zodiacal light 80.
diffuse interstellar bands (DIBs) A series
of interstellar absorption features recorded on
© 2001 by CRC Press LLC
diffusion-convection equation
photographic plates in the early 1900s. They
were labeled “diffuse” because they arise from
electronic transitions in molecules, so they are
broad in comparison to atomic lines. There are
now well over 100 such bands known in the UV,
visible, and near IR regions of the spectrum arising
in interstellar clouds. DIBs must be molecular,
given the complexity of the absorption lines.
DIBs are easily seen when observing spectra
of hot, fast rotating stars whose spectrum has a
strong continuum. Even with very high resolution
spectroscopy, the diffuse interstellar bands
continue to show blended structures. DIBs
show considerable scatter in strength vs. the
amount of stellar reddening suggesting inhomogeneous
variation of chemistry and dust-to-gas
ratio. This may arise because the molecule(s) in
the volume may be able to add hydrogen to the
molecular structure in certain circumstances, as
is known for some carbon compounds. Identifying
the carriers of these absorptions has become
perhaps the classic astrophysical spectroscopic
problem of the 20th century, and numerous
molecules have been put forth as the
source of these features. Recently most attention
has focused on carbon rich molecules such
as fullerenes and polycyclic aromatic hydrocarbons.
diffuse scattering coefficient for downwelling
(upwelling) irradiance The ratio of the scattering
coefficient to the mean cosine of the
downward (upward) radiance.
diffusion The gradual mixing of a quantity
(commonly a pollutant) into a fluid by random
molecular motions and turbulence.
diffusion-convection equation Transport
equation for energetic charged particles in interplanetary
space considering the effects of spatial
diffusion and convection of particles with the solar
wind. The transport equation can be derived
from the equation of continuity by supplementing
the streaming with the convective streaming
vf , yielding
∂f
∂t +∇·(v sowif ) −∇·(D∇f)= 0
with f being the phase space density, D the (spatial)
diffusion coefficient, and v sowi the solar