DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

sometimes called sand seas since they represent
vast regions of sand. Most ergs are located in
desert basins downwind from terrain which is
experiencing high amounts of erosion. Various
types of dunes occur within the ergs. Ergs are
found on Earth and around the north polar cap
on Mars.
Ernst equation (1967) The complex partial
differential equation
ReEE =∇E·∇E
in a Euclidean 3-space introduced by F.J. Ernst.
The axially symmetric solutionsE=E(x 1 ,x 2 )
represent the metric potentials of stationary, axisymmetric
vacuum gravitational fields. A second
form
(¯ξξ− 1)ξ = 2 ¯ξ∇ξ·∇ξ
is obtained by the substitutionE=(ξ−1)/(ξ+
1). The Ernst equation has been found to
describe additionally various physical systems,
such as colliding plane waves in general relativity,
monopoles in SU(2) gauge theory, and states
of Heisenberg ferromagnets. The symmetries of
the equation and solution generating techniques
have been extensively studied.
erosion Rock is destroyed by weathering.
Mechanical weathering breaks up rock into
small particles which can be transported hydrologically.
In chemical weathering the rock is
dissolved by water. In either case, the removal
of the rock is erosion.
ERP Earth rotation parameters.
Ertel potential vorticity See potential vorticity.
eruption Volcanos are subject to periodic
eruptions during which molten rock (magma)
and/or volcanic ash flows to the Earth’s surface.
Eruptions can occur for months between periods
of quiescence.
eruptiveprominence Solarprominencescan
become activated and exhibit several types of
large-scale motion. The prominence may grow
and become brighter with a corresponding in-
© 2001 by CRC Press LLC
Euler equations
crease in helical motion or flow along the prominence
axis. This type of activation can lead to
an eruption, especially if it exceeds a height of
50,000 km. In this case, it rises as an erupting
prominence and eventually disappears. The
eruption of a quiescent prominence (disparition
brusque) is a slow process lasting several hours.
The eruption of an active region prominence is
much more rapid taking about 30 min.
escape velocity In Newtonian physics, under
gravitational interactions, the gravitational field
contributes negatively to the total energy of a
particle. The escape velocity is that speed giving
a zero total energy (kinetic + gravitational potential).
A particle moving without dissipation
at or exceeding the escape velocity will escape
to infinity.
eskers Long narrow ridges of sand and gravel
deposited in the middle of glaciers. They have
stream-like shapes and are believed to form from
meltwater streams flowing in tunnels beneath
a melting glacier. Sinuous ridges in the high
latitude regions of Mars have been suggested to
be eskers.
estuary A sheltered region that lies inshore
of a sea and is subject to tidal action. Generally
has a reduced salinity, compared to the adjoining
sea, as a result of river and other freshwater
inputs.
ET See ephemeris time (ET).
eternal black hole A black hole that was
not formed in the collapse of matter, but was
present ab initio as a stable topological structure
in spacetime.
Eudoxos of Cnidus Greek geometer and astronomer,
born in Cnidus, Asia Minor approximately
408 B.C., died about 353 B.C. Eudoxos
established the foundation of geometrical principles
developed by Euclid, and applied the subject
to the study of the moon and the planets.
Euler equations The equations of motion
for an inviscid fluid. Equivalent to the Navier–
Stokes equations if viscosity is taken as zero.
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