DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

elevation. Altitude is normally one coordinate
of the three in the topocentric system of coordinates.
See also azimuth and zenith angle.
Amalthea Moon of Jupiter, also designated
JV. Discovered by E. Barnard in 1892, its orbit
has an eccentricity of 0.003, an inclination
of 0.4 ◦ , a precession of 914.6 ◦ yr −1 , and a
semimajor axis of 1.81 × 10 5 km. Its size is
135 × 83 × 75 km, its mass, 7.18 × 10 18 kg, and
its density 1.8 g cm −3 . It has a geometric albedo
of 0.06 and orbits Jupiter once every 0.498 Earth
days. Its surface seems to be composed of rock
and sulfur.
Amazonian Geophysical epoch on the planet
Mars, 0 to 1.8 Gy BP. Channels on Mars give
evidence of large volumes of water flow at the
end of the Hesperian and the beginning of the
Amazonian epoch.
Ambartsumian, Viktor Amazaspovich
(1908–1996) Soviet and Armenian astrophysicist,
founder and director of Byurakan Astrophysical
Observatory. Ambartsumian was
born in Tbilisi, Georgia, and educated at the
Leningrad State University. His early work
was in theoretical physics, in collaboration with
D.D. Ivanenko. Together they showed that
atomic nuclei cannot consist of protons and electrons,
which became an early indication of the
existence of neutrons. The two physicists also
constructed an early model of discrete spacetime.
Ambartsumian’s achievements in astrophysics
include the discovery and development of
invariance principles in the theory of radiative
transfer, and advancement of the empirical approach
in astrophysics, based on analysis and
interpretation of observational data. Ambartsumian
was the first to argue that T Tauri stars
are very young, and in 1947, he discovered stellar
associations, large groups of hot young stars.
He showed that the stars in associations were
born together, and that the associations themselves
were gravitationally unstable and were
expanding. This established that stars are still
forming in the present epoch.
ambipolar field An electric field amounting
to several volts/meter, maintaining charge neu-
© 2001 by CRC Press LLC
Am star
trality in the ionosphere, in the region above the
E-layer where collisions are rare. If that field did
not exist, ions and electrons would each set their
own scale height — small for the ions (mostly
O + ), large for the fast electrons — and densities
of positive and negative charge would not match.
The ambipolar field pulls electrons down and
ions up, assuring charge neutrality by forcing
both scale heights to be equal.
Amor asteroid One of a family of minor
planets with Mars-crossing orbits, in contrast to
most asteroids which orbit between Mars and
Jupiter. There are 231 known members of the
Amor class.
ampere Unit of electric current which, if
maintained in two straight parallel conductors
of infinite length, of negligible circular crosssection,
and placed 1 m apart in vacuum, produces
between these conductors a force equal to
2 × 10 −7 N/m of length.
Ampere’s law If the electromagnetic fields
are time independent within a given region, then
within the region it holds that the integral of the
magnetic field over a closed path is proportional
to the total current passing through the surface
limited by the closed path. In CGS units the constant
of proportionality is equal to 4 π divided
by the speed of light. Named after A.M. Ampere
(1775–1836).
amphidrome (amphidromic point) A stationary
point around which tides rotate in a counterclockwise
(clockwise) sense in the northern
(southern) hemisphere. The amplitude of a
tide increases with distance away from the amphidrome,
with the amphidrome itself the point
where the tide vanishes nearly to zero.
Am star A star of spectral type A as determined
by its color but with strong heavy metal
lines (copper, zinc, strontium, yttrium, barium,
rare earths [atomic number = 57 to 71]) in its
spectrum. These stars appear to be slow rotators.
Many or most occur in close binaries
which could cause slow rotaton by tidal locking.
This slow rotation suppresses convection and allows
chemical diffusion to be effective, producing
stratification and differentiation in the outer