DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

active front
active front An active anafront or an active
katafront. An active anafront is a warm front at
which there is upward movement of the warm
sector air. This is due to the velocity component
crossing the frontal line of the warm air being
larger than the velocity component of the cold
air. This upward movement of the warm air usually
produces clouds and precipitation. In general,
most warm fronts and stationary fronts are
active anafronts. An active katafront is a weak
cold frontal condition, in which the warm sector
air sinks relative to the colder air. The upper
trough of active katafront locates the frontal line
or prefrontal line. An active katafront moves
faster than a general cold front.
active galactic nuclei (AGN) Luminous nuclei
of galaxies in which emission of radiation
ranges from radio frequencies to hard-X or, in
the case of blazars, to γ rays and is most likely
due to non-stellar processes related to accretion
of matter onto a supermassive black hole. Active
galactic nuclei cover a large range in luminosity
(∼ 10 42 −10 47 ergs s −1 ) and include, at the low
luminosity end, LINERs and Seyfert-2 galaxies,
and at the high luminosity end, the most
energetic sources known in the universe, like
quasi-stellar objects and the most powerful radio
galaxies. Nearby AGN can be distinguished
from normal galaxies because of their bright nucleus;
their identification, however, requires the
detection of strong emission lines in the optical
and UV spectrum. Radio-loud AGN, a minority
(10 to 15%) of all AGN, have comparable optical
and radio luminosity; radio quiet AGN are
not radio silent, but the power they emit in the
radio is a tiny fraction of the optical luminosity.
The reason for the existence of such dichotomy
is as yet unclear. Currently debated explanations
involve the spin of the supermassive black
hole (i.e., a rapidly spinning black hole could
help form a relativistic jet) or the morphology
of the active nucleus host galaxy, since in spiral
galaxies the interstellar medium would quench
a relativistic jet. See black hole, QSO, Seyfert
galaxies.
active margins The boundaries between the
oceans and the continents are of two types, active
and passive. Active margins are also plate
boundaries, usually subduction zones. Active
© 2001 by CRC Press LLC
margins have major earthquakes and volcanism;
examples include the “ring of fire” around the
Pacific.
active region A localized volume of the solar
atmosphere in which the magnetic fields are extremely
strong. Active regions are characterized
as bright complexes of loops at ultraviolet and
X-ray wavelengths. The solar gas is confined
by the strong magnetic fields forming loop-like
structures and is heated to millions of degrees
Kelvin, and are typically the locations of several
solar phenomena such as plages, sunspots,
faculae, and flares. The structures evolve and
change during the lifetime of the active region.
Active regions may last for more than one solar
rotation and there is some evidence of them recurring
in common locations on the sun. Active
regions, like sunspots, vary in frequency during
the solar cycle, there being more near solar
maximum and none visible at solar minimum.
The photospheric component of active regions
are more familiar as sunspots, which form at the
center of active regions.
adiabat Temperature vs. pressure in a system
isolated from addition or removal of thermal
energy. The temperature may change, however,
because of compression. The temperature
in the convecting mantle of the Earth is closely
approximated by an adiabat.
adiabatic atmosphere A simplified atmosphere
model with no radiation process, water
phase changing process, or turbulent heat transfer.
All processes in adiabatic atmosphere are
isentropic processes. It is a good approximation
for short-term, large scale atmospheric motions.
In an adiabatic atmosphere, the relation between
temperature and pressure is
T
T0
p
=
p0
AR
Cp
where T is temperature, p is pressure, T0 and
p0 are the original states of T and p before adiabatic
processes, A is the mechanical equivalent
of heat, R is the gas constant, and Cp is the specific
heat at constant pressure.
adiabatic condensation point The height
point at which air becomes saturated when it