DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Jerlov water type
one because of gravitational attraction; the fluid
collapses under its own weight. For scales
smaller than this critical length, the fluid can
build enough hydrostatic pressure to halt the collapse
and perturbations oscillate as sound waves.
For a perfect Newtonian fluid the Jeans length
is given by:
λJ =
πv2 1/2
s
Gρ
where vs is the sound speed of the fluid, G the
gravitational constant, and ρ the mean density.
For a perturbation longer than the Jeans length
collapsing in a non-expanding fluid, the density
grows exponentially on a dynamical time scale
τdyn = (4πGρ) −1/2 . If the fluid is expanding,
the background perturbation growth is like
a power law with an exponent close to unity, the
exact value depending on the equation of state
of the fluid. The Jeans length during the conditions
believed to be present at the time after the
Big Bang when decoupling of ordinary matter
and radiation occurred was 6 × 10 17 m, or about
60 light years.
Jerlov water type A water clarity classification
scheme based on the downward diffuse
attenuation coefficient just below the sea surface.
jet In fluid mechanics, a compact, coherent,
collimated stream of relatively fast moving fluid
impinging into still fluid.
In meteorology, a shorthand for jet stream.
In astrophysics, usually bipolar streams of
high velocity gas emitted from an active region.
Examples include jets from star formation regions
(collimated by magnetic fields) and from
black hole accretion in stellar mass or in supermassive
black holes, in which the collimation is
accomplished by the rotation of the black hole.
jet stream Relatively strong west to east
winds concentrated in a narrow stream up to
500 km across, located in a thin vertical range
10 to 15 km in altitude, with winds exceeding
90 km/h. Winds, especially in the winter, can
approach 500 km/h. In North America, major
cold outbreaks are associated with movement of
the jet stream south. In summertime, and during
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260
mild winter weather, the jet stream stays to the
north. Its actual fluctuations range from northern
Canada to over the Gulf of Mexico.
jetty A structure at a tidal inlet. Commonly
built as a rubble mound structure, to keep waves
and sediment out of a harbor entrance.
jog In geology, segments of dislocation line
which have a component of their sense vector
normal to the glide plane. Jogs are abrupt
changes in direction taking the dislocation out
of the glide plane. When the normal component
extends over only a single interplanar spacing
d, as for A and B, the jogs are unit jogs, or for
brevity just jogs. If the normal component extends
over more than one interplanar spacing,
the jog is called superjog.
joints Fractures in the Earth’s crust across
which there has been no relative motion. If relative
motion has occurred, the fracture is a fault.
JONSWAP Acronym for Joint North Sea
Wave Project, an experiment involving measurement
of ocean wave energy spectra yielding an
empirical spectrum of the same name.
Joule (J) The SI unit of energy, equal to Nm.
Joule heating The dissipation of current in
a plasma in the form of heat due to the presence
of a finite electrical conductivity. The efficient
dissipation of currents in the corona requires the
currents to be confined to small volumes because
of the extremely high conductivity. The heating
is given by WJ = j · E = j 2 /σ where E is the
electric field, j is the current density, and σ is
the electrical conductivity of the plasma. Joule
heating is also known as Ohmic heating.
Jovian planet A planet that has characteristics
similar to Jupiter, the largest planet in our
solar system. There are four Jovian planets in
our solar system: Jupiter, Saturn, Uranus, and
Neptune. The primary characteristics of Jovian
planets are large size (>48,000 km in diameter),
large distance from the sun (>5 astronomical
units), low densities (< 1600 kg/m 3 , indicating
gaseous compositions of primarily hydrogen
and helium), many moons, and ring systems.