DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

elevation
particles have been interpreted as an indication
of the primitive, pre-solar nature of comets, or at
least their sub-components. For example, the ratios
of D/H in water from comets Halley, Hyakutake,
and Hale–Bopp were all around 3 × 10 −4 ,
similar to what is seen in certain interstellar environments
and approximately a factor of two
over ocean water, and DCN/HCN ratios up to
2.5 × 10 −2 were measured in Hale–Bopp.
elevation The vertical distance measured
from the geoid, from the earth ellipsoid, or from
the local terrain. When interpreting elevation
data, care is needed to ascertain which of the
three surfaces mentioned is the reference one.
When no specification is given, the context must
be examined. For geophysics, hydrology, and
cartography, the geoid is the usual reference surface
because most elevation data are produced in
surveys based on the local horizontal, as found
by the spirit level or the normal to the plumb
line. For construction or related work, the local
mean terrain (earth surface) is the more likely
reference. The ellipsoid is rarely the reference
surface, but it may be so in space science contexts.
elevation head (sometimes gravitational
head) (z) The potential energy per unit fluid
weight is given by the elevation above an arbitrary
horizontal datum. The elevation head has
units of length and is a component of the hydraulic
head.
Eliassen–Palm (EP) flux A vector in the
meridional (y,z) plane, which has the eddy momentum
flux and eddy heat flux as its horizontal
and vertical components. Its convergence is directly
related to the eddy forcing on the zonal
mean flow.
elliptical galaxies Galaxies of regular, ellipsoidal
appearance, and of rather reddish colors.
The photometric profiles of most elliptical
galaxies are described by empirical
laws in which the surface brightness decreases
smoothly as a function of the distance from the
galaxy center. Elliptical galaxies do not show
features such as bars, spiral arms, or tails. Only
a minority of them show ripples, shells, or asymmetric
radial distribution of surface brightness.
© 2001 by CRC Press LLC
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Elliptical galaxies are characterized by the absence
of significant neutral or molecular gas,
and hence the absence of star formation, and
by a stellar content mostly made of old stars belonging
to stellar population II. They account
for about 1/3 of all observed galaxies, and are
the majority of galaxies in dense cluster environments.
They cover a wide range of masses,
from ∼ 10 6 to ∼ 10 11 solar masses, the most
massive being located at the center of clusters
of galaxies (cD galaxies), the less massive being
dwarf elliptical galaxies. See cD galaxies,
dwarf spheroidal galaxies.
Ellison scale Like the Thorpe scale, the Ellison
scale is a quantity to estimate the overturning
eddy size. The Ellison scaleLE is based on
densityρ instead of temperature T, and the definition
LE =< ρ 2 >/(∂ρ/∂z) deviates slightly
from the procedure for the Thorpe scale LT estimation.
Both scales, LE and LT are considered
adequate measures for the overturning eddy
size and generally agree well with the Ozmidov
scale.
El Niño The warm phase of the Southern
Oscillation beginning at about Christmas time
(hence the name “El Niño”, Spanish for “Christ
child”) and lasting 12 to 18 months. Characterized
by warming of sea surface temperatures in
the central and eastern equatorial Pacific Ocean.
The anomalously warm water causes the sardine
population to die off the Peru coast. A
series of effects arise, including an increased
westerly wind and a shift in Pacific ocean circulation.
This warming occurs in the entire tropics
and causes drought in Indonesia and Australia.
Enhanced North-South temperature differences
transport energy into the atmosphere, modifying
global atmospherics flow, causing warm dry
weather in Northern U.S., and wet cool weather
in the Southern U.S. See Southern Oscillation
Index, La Niña.
elongation The angle between the sun and the
observer, measured at the object being observed.
elongation The apparent angular separation
between the sun and a solar system object
as viewed by a distant observer, i.e., the sunobserver-object
angle.