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Algol system

region in the nightside magnetosphere where region 2 Birkeland currents apparently originate. Magnetospheric plasma must be (to a high degree of approximation) charge neutral, with equal densities of positive ion charge and negative electron charge. If such plasma convects earthward under the influence of an electric field, as long as the magnetic field stays constant (a fair approximation in the distant tail) charge neutrality is preserved.

Near Earth, however, the magnetic field begins to be dominated by the dipole-like form of the main field generated in the Earth’s core, and the combined drift due to both electric and magnetic fields tends to separate ions from electrons, steering the former to the dusk side of Earth and the latter to the dawn side. This creates Alfvén layers, regions where those motions fail to satisfy charge neutrality. Charge neutrality is then restored by electrons drawn upwards as the downward region 2 current, and electrons dumped into the ionosphere (plus some ions drawn up) to create the corresponding upward currents.

Alfvén shock See intermediate shock.

Alfvén speed In magnetohydrodynamics, the speed of propogation of transverse waves in a

direction parallel to the magnetic field B. In SI

units, vA = B/ (µρ) where B is the magnitude of the magnetic field [tesla], ρ is the fluid density [kg/meter3], and µ is the magnetic permeability [Hz/meter].

Alfvén’s theorem See “frozen-in” magnetic field.

Alfvén wave A hydromagnetic wave mode in which the direction (but not the magnitude) of the magnetic field varies, the density and pressure are constant, and the velocity fluctuations are perfectly aligned with the magnetic-field fluctuations. In the rest frame of the plasma, energy transport by an Alfvén wave is directed along the mean magnetic field, regardless of the direction of phase propagation. Largeamplitude Alfvén waves are predicted both by the equations of magnetohydrodynamics and the Vlasov–Maxwell collisionless kinetic the-

ory, without requiring linearization of the theory.

In magnetohydrodynamics, the characteris-

tic propagation speed is the Alfvén speed C = √ A

B/ 4πρ (cgs units), where B is the mean magnetic field and ρ is the gas density. The ve-

locity and magnetic fluctuations are related by

δV = δB/ 4πρ; the upper (lower) sign applies to energy propagation parallel (antiparallel) to the mean magnetic field. In collisionless kinetic theory, the equation for the characteristic propagation speed is generalized to

V 2 = C2 1 + 4π P P E ,

A A B2

where P and P are, respectively, the pressures transverse and parallel to the mean magnetic field,

E = 1 ρα (*Vα)2 . ρ α

ρα is the mass density of charge species α, and *Vα is its relative velocity of streaming relative to the plasma. Alfvén waves propagating through a plasma exert a force on it, analogous to radiation pressure. In magnetohydrodynamics the force per unit volume is δB 2 /8π, where δB 2 is the mean-square magnetic fluctuation amplitude. It has been suggested that Alfvén wave radiation pressure may be important in the acceleration of the solar wind, as well as in processes related to star formation, and in other astrophysical situations.

In the literature, one occasionally finds the term “Alfvén wave” used in a looser sense, referring to any mode of hydromagnetic wave. See hydromagnetic wave, magnetoacoustic wave.

Algol system A binary star in which mass transfer has turned the originally more massive component into one less massive than its accreting companion. Because the time scale of stellar evolution scales as M2, these systems, where the less massive star is the more evolved, were originally seen as a challenge to the theory. Mass transfer resolves the discrepancy. Many Algol systems are also eclipsing binaries, including Algol itself, which is, however, complicated by the presence of a third star in orbit around the eclipsing pair. Mass transfer is proceeding on the slow or nuclear time scale.

© 2001 by CRC Press LLC

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 6h45m9s, declination 1642 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), which at present is slightly closer to Earth 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

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 G due to this effect and an expansion of the spatial derivatives of the mean magnetic field B0:

Gi = αij B0j + βijk ∂B0j + · · ·


with the first term on the right-hand side, usually assumed to predominate, termed the “alpha effect”, and the second term sometimes neglected.× 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., = 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 4He atom, composed of two neutrons and two protons.

Altair Magnitude 0.76 class A7 star at RA 19h50.7m, dec +851 .

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 90is called the zenith (q.v.), 0the horizontal, and 90the 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

© 2001 by CRC Press LLC

Am star

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 914.6yr1, and a

semimajor axis of 1.81 × 105 km. Its size is 135 ×83 ×75 km, its mass, 7.18 ×1018 kg, and

its density 1.8 g cm3. 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-

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 × 107 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

© 2001 by CRC Press LLC

anabatic wind

layers of the star, the currently accepted explanation for their strange appearance.

anabatic wind A wind that is created by air flowing uphill, caused by the day heating of the mountain tops or of a valley slope. The opposite of a katabatic wind.

analemma The pattern traced out by the position of the sun on successive days at the same local time each day. Because the sun is more northerly in the Northern summer than in Northern winter, the pattern is elongated North-South. It is also elongated East-West by the fact that civil time is based on the mean solar day. However, because the Earth’s orbit is elliptical, the true position of the sun advances or lags behind the expected (mean) position. Hence, the pattern made in the sky resembles a figure “8”, with the crossing point of the “8” occurring near, but not at, the equinoxes. The sun’s position is “early” in November and May, “late” in January and August. The relation of the true to mean motion of the sun is called the equation of time. See equation of time, mean solar day.

Ananke Moon of Jupiter, also designated JXII. Discovered by S. Nicholson in 1951, its orbit has an eccentricity of 0.169, an inclination of 147, and a semimajor axis of 2.12 ×107 km. Its radius is approximately 15 km, its mass, 3.8 × 1016 kg, and its density 2.7 g cm3. Its geometric albedo is not well determined, and it orbits Jupiter (retrograde) once every 631 Earth days.

Andromeda galaxy Spiral galaxy (Messier object M31), the nearest large neighbor galaxy, approximately 750 kpc distant, centered at RA 00h42.7m, dec +4116 , Visual magnitude 3.4 , angular size approximately 3by 1.

anelastic deformation Solids creep when a sufficiently high stress is applied, and the strain is a function of time. Generally, the response of a solid to a stress can be split into two parts: elastic part or instantaneous part, and anelastic part or time-dependent part. The strain contributed by the anelastic part is called anelastic deformation. Part of the anelastic deformation can be recovered with time after the stress is removed

(retardation strain), and part of it becomes permanent strain (inelastic strain). Anelastic deformation is usually controlled by stress, pressure, temperature, and the defect nature of solids. Two examples of anelastic deformation are the attenuation of seismic waves with distance and the post-glacial rebound.

anemometer An instrument that measures windspeed and direction. Rotation anemometers use rotating cups, or occasionally propellers, and indicate wind speed by measuring rotation rate. Pressure-type anemometers include devices in which the angle to the vertical made by a suspended plane in the windstream is an indication of the velocity. Hot wire anemometers use the efficiency of convective cooling to measure wind speed by detecting temperature differences between wires placed in the wind and shielded from the wind. Ultrasonic anemometers detect the phase shifting of sound reflected from moving air molecules, and a similar principle applies to laser anemometers which measure infrared light reemitted from moving air molecules.

angle of repose The maximum angle at which a pile of a given sediment can rest. Typically denoted by φ in geotechnical and sediment transport studies.

angle-redshift test A procedure to determine the curvature of the universe by measuring the angle subtended by galaxies of approximately equal size as a function of redshift. A galaxy of size D, placed at redshift z will subtend an angle



DIo2(1 + z)2





Ioz + (Io 2) (Ioz + 1)1/2 1 ,

in a universe with mean density Io and no cosmological constant. In models with cosmological constant, the angle also varies in a defined manner but cannot be expressed in a closed form. However, since galaxies are not “standard rods” and evolve with redshift, this test has not been successful in determining cosmological parameters.

© 2001 by CRC Press LLC

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