DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Allan Hills meteorite
Allan Hills meteorite A meteorite found in
Antarctica in 1984. In August of 1996, McKay
et al. published an article in the journal Science,
purporting to have found evidence of ancient
biota within the Martian meteorite ALH
84001. These arguments are based upon chemically
zoned carbonate blebs found on fracture
surfaces within a central brecciated zone. It has
been suggested that abundant magnetite grains
in the carbonate phase of ALH 84001 resemble
those produced by magnetotactic bacteria,
in both size and shape.
allowed orbits See Störmer orbits.
all sky camera A camera (photographic, or
more recently, TV) viewing the reflection of the
night sky in a convex mirror. The image is
severely distorted, but encompasses the entire
sky and is thus very useful for recording the distribution
of auroral arcs in the sky.
alluvial Related to or composed of sediment
deposited by flowing water (alluvium).
alluvial fan When a river emerges from a
mountain range it carries sediments that cover
the adjacent plain. These sediments are deposited
on the plain, creating an alluvial fan.
alongshore sediment transport Transport
of sediment in a direction parallel to a coast.
Generally refers to sediment transported by
waves breaking in a surf zone but could include
other processes such as tidal currents.
Alpha Centauri A double star (α-Centauri
A, B), at RA 6 h 45 m 9 s , declination
−16 ◦ 42 ′ 58 ′′ , with visual magnitude −0.27.
Both stars are of type G2. The distance to α-
Centauri is approximately 1.326 pc. In addition
there is a third, M type, star (Proxima Centauri)
of magnitude 11.7, which is apparently bound
to the system (period approximately 1.5 million
years), whichatpresentisslightlyclosertoEarth
than the other two (distance = 1.307 pc).
α effect A theoretical concept to describe
a mechanism by which fluid flow in a dynamo
such as that in the Earth’s core maintains a magnetic
field. In mean-field dynamo theory, the
© 2001 by CRC Press LLC
magnetic field and fluid velocities are divided
into mean parts which vary slowly if at all and
fluctuating parts which represent rapid variations
due to turbulence or similar effects. The
fluctuating velocities and magnetic fields interact
in a way that may, on average, contribute to
the mean magnetic field, offsetting dissipation
of the mean field by effects such as diffusion.
This is parameterized as a relationship between
a mean electromotive force ɛ due to this effect
and an expansion of the spatial derivatives of the
mean magnetic field B0:
∂B0j
ɛi =αijB0j +βijk
∂xk
+···
with the first term on the right-hand side, usually
assumed to predominate, termed the “alpha effect”,
and the second term sometimes neglected.
∇×ɛ is then inserted into the induction equation
for the mean field. For simplicity,α is often
assumed to be a scalar rather than a tensor in
mean-field dynamo simulations (i.e., ɛ =αB0).
For α to be non-zero, the fluctuating velocity
field must, when averaged over time, lack certain
symmetries, in particular implying that the
time-averaged helicity (u ·∇×u) is non-zero.
Physically, helical fluid motion can twist loops
into the magnetic field, which in the geodynamo
is thought to allow a poloidal magnetic field to
be created from a toroidal magnetic field (the opposite
primarily occurring through the ω effect).
See magnetohydrodynamics.
alpha particle The nucleus of a 4 He atom,
composed of two neutrons and two protons.
Altair Magnitude 0.76 class A7 star at RA
19h50.7m, dec +8 ◦ 51 ′ .
alternate depths Two water depths, one subcritical
and one supercritical, that have the same
specific energy for a given flow rate per unit
width.
altitude The altitude of a point (such as a
star) is the angle from a horizontal plane to that
point, measured positive upwards. Altitude 90 ◦
is called the zenith (q.v.), 0 ◦ the horizontal, and
−90 ◦ the nadir. The word “altitude” can also
be used to refer to a height, or distance above
or below the Earth’s surface. For this usage, see