DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

equatorial surface of the magnetosphere A
surface with respect to which the Earth’s magnetospheric
field and its trapped plasma exhibit
approximate north-south symmetry. It is usually
defined as a collection of the points of weakest
magnetic intensity |B| on “closed” field lines
which start and end on the surface of Earth. In
the plasma sheet the surface is often called the
“neutral sheet” and it undergoes a periodic deformation
(“warping”) due to the daily and annual
variation of the geomagnetic tilt angle ψ.
See minimum-B surface.
equatorial undercurrent A strong narrow
eastward current found in the region of strong
density gradient below the ocean mixed layer
and with its core close to the equator. Its vertical
thickness is around 100 m and its half-width is
a degree of latitude. The maximum current is
typically 1 ms −1 . This eastward current is a
majorfeatureoftheequatorialoceancirculation,
particularly in the Pacific and Atlantic Oceans.
equatorial upwelling The resultant volume
transport in the Ekman layer is at right angles
to and to the right of the wind direction in the
northern hemisphere. If the wind is easterly, water
in the Ekman layer will tend to move away
from the equator on both sides of the equator
and will be replaced by water moving upward
from below the layer. This is called equatorial
upwelling. If the wind is westerly, water in the
Ekman layer will tend to move toward the equator
in both sides of the equator and repel the
water below the layer. This is called equatorial
downwelling.
equatorial waveguide An oceanic or atmospheric
wave that is confined to propagate near
the equator due to the vanishing of the Coriolis
force at the equator. While the conditions
for geostrophic balance theoretically fail at the
equator, in practice any mass crossing the equator
will be influenced by the Coriolis force on
either side. The force turns such motion back
towards the equator, thus creating a trap or a
waveguide.
equatorial waves A class of equatorially
trapped wave solutions first obtained by Taroh
Matsuno in 1966, consisting of the Rossby, in-
© 2001 by CRC Press LLC
equilibrium tide (gravitational tide)
ertial gravity, mixed Rossby-gravity, and Kelvin
waves. The Rossby and inertial gravity waves
are waves of rotating fluid, having their counterparts
in off-equatorial regions. The Kelvin and
mixed Rossby-gravity waves are unique to the
equatorial waveguide arising from the singularity
of the equator where the Coriolis parameter
f = 2 sinθ vanishes. Here is the angular
velocity of the Earth’s rotation around the north
pole and θ is latitude.
equilibrium In mechanics, a configuration
in which the total force on a system vanishes,
so if placed in such a configuration the system
remains in it.
equilibrium beach profile A theoretical
beach profile shape that results if wave conditions
and water level are held constant for an
indefinite period. It has a monotonic, concaveup
shape.
equilibrium range High-wavenumber part
of the turbulent kinetic energy spectrum that includes
the inertial subrange and the dissipation
range. The turbulence at these wavenumbers is
nearly isotropic and the shape of the spectrum
at these wavenumbers does not depend on the
amount of kinetic energy present at larger scales
or the size of these energy containing scales.
Kinetic energy is merely transferred by inertial
forces through the inertial subrange until the energy
is dissipated into heat at the Kolmogorov
microscale.
equilibrium space-times Stationary electrovacuum
space-times describing the external
fields of an arbitrary array of electrically
charged, massive sources. Equilibrium is
achieved by the balance of gravitational and
electromagnetic forces. The static equilibrium
space-times were found by S.D. Majumdar
and A. Papapetrou (1947). See Israel–Wilson–
Perjés space-times.
equilibrium tide (gravitational tide) Ahypothetical
ocean tide that responds instantly to
tide producing forces, forming an equilibrium
surface. The effects of friction, inertia, and the
irregular distribution of land mass are ignored.
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