DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Pleistocene, about 2 million years ago, ending
only about 10,000 years ago.
glacier A mass of moving ice formed by accumulation
and compactification of snow, and
flowing from an accumulation source to an edge
where it is ablated. Glaciers are found on Earth
and are suspected to have occurred on Mars in
the past. Ground ice is present on both Earth and
Mars. Some of the icy moons of the outer solar
system show evidence of ice moving on top
of liquid water oceans, either presently (such
as Europa) or in the past (such as Ganymede).
Glaciers contained within mountain valleys are
called valley or alpine glaciers, while those
spread out laterally over large areas are called
continental glaciers, found in Greenland and
Antarctica. Glaciers are very efficient at eroding
the underlying material and transporting this
material to other locations where it is deposited
as eskers, moraines, and drumlins. Ground ice
also creates a number of geologic features, primarily
thermokarst features produced by the collapse
of overlying material as subterranean ice
is removed by heating.
global loop oscillation Coronal loops act as
high quality resonance cavities for hydromagnetic
waves and, consequently, display a largescale
frequency response sharply peaked at the
global resonant frequency of the loop, defined
by vAlfvén = 2L/period. This global mode is crucial
to wave heating models of the solar corona
since, in order to be efficient, the resonant absorption
of waves requires a close matching to
the length of the coronal loop through the condition
of global-mode resonance.
globally hyperbolic space-time (Leray,
1952.) A space-time (M,g)is globally hyperbolic
if
1. (M,g)is strongly causal.
2. For any two points p, q ∈ M, the intersection
of the causal sets J + (p)∩J − (q) is compact
where J + (p) is the causal future set of p, i.e.,
those points that can be influenced by p, and
J − (q) is the causal past set of q. In a globally
hyperbolic space-time, the wave equation with
source term δ(p) has a unique solution which
vanishes outside the causal future set J + (p).
© 2001 by CRC Press LLC
global thermohaline circulation
global positioning system (GPS) A U.S.
Department of Defense system of 24 satellites
used as timing standards and for navigation
to monitor positions on the Earth’s surface.
Satellites carrying precise cesium or rubidium
atomic clocks which are synchronized with Coordinated
Universal Time broadcast coded data
streams giving their time and their position. By
observing four satellites, all four parameters (x,
y, z, time) describing the location and time at
a near Earth receiver can be extracted. Precise
mode (a military classified mode) allows locations
to less than 1 m. Commercial devices are
available that will provide a location of the device
with an accuracy of 10s of meters using
signals from these satellites. Differential GPS
and interferometric applications can reduce errors
to millimeters.
global seismology A research field of seismology
aimed at elucidation of the Earth’s
deep structure, dynamics, and its driving forces
through an international exchange of information
and technique in seismology. In the 1980s,
through analyses of digital data of global seismograph
networks, three-dimensional velocity
structural models of the Earth were proposed,
and research on global seismology made remarkable
progress. In order to know more about
the structures and dynamics of the Earth, a seismograph
network with broadband and high dynamic
range was equipped, and many countries
started to participate in a global seismograph
network. As representative networks, there are
GEOSCOPE (France), IRIS (U.S.), CANDIS
(Canada), CDSN (China), FKPE (Germany),
DRFEUS (Europe), and POSEIDON (Japan).
global thermohaline circulation A vertical
overturning circulation in which cold water
sinks in localized polar/subpolar regions,
spreads to and rises to the surface in the rest
of the world ocean. At low temperatures close
to the freezing point, sea water density becomes
less sensitive to temperature than to salinity differences.
So the formation of deep water is
determined by salinity differences among the
oceans. Currently, most of the world ocean’s
deep water sinks in the northern North Atlantic,
and flows southward along the western boundary
into the Southern Ocean. Riding on the
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