DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

length range, normally associated with optical
and/or X-ray flares. Microwave bursts provide
a powerful diagnostic of energetic electrons in
the solar atmosphere.
mid-ocean ridge A location of seafloor
spreading (divergence zone). The more or less
continuous line centered in the oceans where
new material reaches the surface of the Earth,
driven by convection in the Earth’s mantle. The
Mid-Atlantic ridge begins in the North polar region,
wanders south between W15 ◦ and W45 ◦ ,
essentially staying in the middle of the Atlantic
Ocean. It connects to the Southwest Indian
Ridge at about latitude S50 ◦ , longitude 0 ◦ . The
Southwest Indian Ridge connects with the Indian
Ridge at about S30 ◦ , E70 ◦ . From this
junction, the Indian Ridge extends north as the
Central Indian Ridge through the Red Sea; it
extends South and East as the Southeast Indian
Ridge along about S45 ◦ , becoming the Pacific
Antarctic Ridge at S60 ◦ , E150 ◦ . This becomes
the East Pacific Rise at about S60 ◦ , W120 ◦ . The
East Pacific Rise runs north along approximately
W100 ◦ , up the west coast of North America,
ending around N50 ◦ as the Juan de Fuca ridge.
Mie scattering Scattering of light by a spherical
particle. Given the complex index of refraction
of the particle, and the ratio of its radius to
the wavelength of the light, it is possible to exactly
solve Maxwell’s equations to find the fraction
of light absorbed, and the fraction scattered,
as well as the phase function of the scattered
light and its polarization. The detailed solution
of this problem is called Mie scattering.
Mie size parameter For scattering by
spheres, the ratio of a sphere’s circumference
to the wavelength.
migration (seismic) This is a technique
which, when implemented on data from seismic
reflection surveys, can help to elucidate the
structure of the underlying rock. In a reflection
survey, seismic waves are generated at one
point on the Earth’s surface and recorded at geophones
(i.e., seismic wave detectors) distributed
nearby (for example, in a linear fashion behind
a ship). Each time the seismic signal encounters
a “reflector”, i.e., a rock layer that causes part
© 2001 by CRC Press LLC
Milankovich cycle
of the seismic energy to be reflected back to the
surface, a seismic pulse is returned to the geophones.
The time delay between the generation
of the seismic signal and its detection at a geophone
depends on the speed of seismic waves
in the rock, the depth of the reflector, and the
horizontal distance between seismic source and
geophone, but also on the geometry of the reflector.
For example, if the reflector is inclined
upwards toward the geophone from the direction
of the source, then the part of the reflection that
is recorded at the geophone will have been reflected
closer to the geophone, rather than midway
between the geophone and the source if the
reflector had been horizontal. Migration corrects
this effect, and also removes other geometrical
artifacts such as diffractions associated
with scattering centers in the rock.
Milankovich cycle Cyclic variations in climate
driven by periodic changes in orbital and
Earth orientation parameters. Climate in a particular
area depends on the solar flux, and therefore
on both the distance from the Earth to the
sun and the angle between the surface of the area
in question and the sun’s rays over the course
of a day (overhead sunlight leading to a greater
flux than tangential sunlight). These both cause
the seasonal variations in weather. The importance
of the former depends on the eccentricity
of the Earth’s orbit, which varies on a 96,000year
cycle, while the importance of the latter
depends on the obliquity, i.e., the angle between
the planes defined by the Earth’s equator and
by the Earth’s orbit around the sun (the ecliptic
plane), which varies between 21 ◦ and 24 ◦
(it is currently at 23.5 ◦ ) and which varies on a
41,000-year cycle. The effect of the eccentricity
may be either to augment the seasonal variations
in either the northern or southern hemisphere
while reducing the variations in the other hemisphere
(if the closest approach of the Earth to
the sun occurs near a solstice, i.e., northern or
southern winter), or to have a relatively neutral
effect (if it occurs near an equinox, i.e., northern
or southern spring). The orientation of the
orbit is also cyclic, so that these effects vary on
a 22,000-year timescale. As long, hard winters
are thought to be important for growing ice
sheets, these cycles may therefore have significant
impact on global climate, although they
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