DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

valley networks
tectonic rifting (by analogy with Earth), with
collapse along the crest of the bulge due to subsurface
withdrawal of material.
valley networks The small dendritic channels
found on Mars. These channels are found
on the flanks of volcanos and in the ancient terrain
of the planet. Although similar in some
respects to terrestrial channels formed by rainfall,
the detailed morphologies of these channels
suggest they were formed by sapping, whereby
groundwater is removed and the overlying surface
collapses along the path of the underground
river, producing the channel shape. Those found
on the flanks of volcanos may have been produced
by hydrothermal activity. Valley networks
are typically about 1 km in width and tens to
hundreds of kilometers in length.
Van Allen Belts Named after their discoverer,
James Van Allen. First detected in 1958
by Explorer 1; James Van Allen correctly interpreted
the results. Two distinct toroidal belts;
the inner one, located between about 1.1 to 3.3
Earth radii, near the equatorial plane, contains
primarily protons with energies exceeding 10
MeV. Flux maximum is at about 2 Earth radii.
This population varies with 11-year solar cycle
and is subject to occasional perturbations
due to geomagnetic storms. The protons in this
region are produced from cosmic rays striking
the atmosphere. The outer belt (between 3 to
9 Earth radii, with a maximum around 4 Earth
radii) contains mainly electrons with energies
up to 10 MeV arising from solar wind electron
injection and acceleration in geomagnetic
storms. The population of this belt thus shows
day/night variation and is sensitive to solar activity.
The charged particles contained in the Van
Allen belts remain trapped along field lines of
the Earth. The particles drift and spiral around
the Earth’s magnetic field lines. As the particles
approach the converging field lines near the
poles, they are reflected back towards the opposite
pole. “Horns” of especially the outer belt
dip sharply in toward the polar caps.
VAN method A method of earthquake prediction
proposed by three Greek researchers,
Varotsos, Alexopouls, and Nomicos in the mid
1980s. According to VAN method, earthquakes
© 2001 by CRC Press LLC
498
could be predicted based on the observed facts
that abnormal signals of Earth current with a
duration time of several minutes to hours (SES)
and continuing more than several days appear
weeks before generations of disastrous earthquakes.
VAN claim that the magnitude of earthquakes,
epicentral distance, and occurrence time
of earthquakes are respectively predicted empirically
from maximum amplitude of SES, station
distribution where SES appears, and kinds
of SES. VAN method was tested throughout
Greece. Some researchers reported that prediction
of disastrous earthquakes succeeded with
substantially high probability, but others are critical
owing to lack of objectivity of the method.
vapor A gas whose temperature is less than
its critical temperature. In such a situation an increase
of pressure at constant temperature will
cause the gas to condense (liquify or solidify);
an example is as water vapor in air at any temperature
below 374 ◦ C. (The corresponding pressure
is 221 bar.) More colloquially, a dispersion
of molecules of a substance that is a liquid or a
solid in its normal state at the given temperature,
through a substrate gas.
vapor concentration The vapor concentration
or absolute humidity is the mass of vapor
per unit of volume of moist air.
vapor pressure The partial pressure of the
water vapor in moist air. It is a measurement of
water vapor content in air. The pressure of moist
air is equal to the partial pressure of the dry air in
the moist air plus the vapor pressure, according
to Dalton’s law (the additivity of partial pressures).
Vapor pressure e is directly proportional
to vapor density ρw and temperature T as
e = R
ρwT
mw
where R is the gas constant, R = 8.314 J/mol ·
K, and mw is the water vapor molecular weight.
For a given temperature, a certain volume of
moist air can only contain a limited content of
water vapor. If vapor content reaches this limit,
the air is said to be saturated, and its vapor pressure
is the saturated vapor pressure or the maximum
vapor pressure. As temperature increases,
saturated vapor pressure increases.