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arcade

one instant. The crater produced would be about 10 km across. This event would raise matter into the atmosphere that would cause dramatic surface cooling by blocking sunlight for at least several years.

There are 240 known Apollos. See Amor asteroid, Aten asteroid.

apparent horizon A spacelike topological 2-sphere from which the outgoing null rays all have zero expansion. In gravitational theories, especially in general relativity, a horizon is a boundary between events visible from infinity and those that are not. The surface of a black hole, for instance, consists of those marginally trapped rays (which just fail escape to infinity); these constitute the event horizon. Generators of the event horizon are not truly identified until the evolution of the spacetime is complete into the future. A more local definition is the apparent horizon, the outermost surface defined by the null rays which instantaneously are not expanding. See event horizon, trapped surface.

apparent magnitude See magnitude.

apparent optical property (AOP) A ratio of radiometric quantities that depends both on the inherent optical properties and on the directional nature of the ambient light field and which is spatially and temporally stable. Applied in oceanography to describe a water body; examples include the average cosine of the light field, the irradiance reflectance, the remote sensing reflectance, and the diffuse attenuation coefficients.

apparent solar time Time based on the diurnal motion of the true (observed) sun, as opposed to mean solar time, to which it is related by the equation of time. The rate of diurnal motion undergoes seasonal variations because of the obliquity of the ecliptic, the eccentricity of the Earth’s orbit, and irregularities in the Earth’s orbit.

apse Line connecting the pericenter to the apocenter of an orbit, the longest axis of the orbit.

Ap star A chemically peculiar star of temperature classification A, which is a slow rotator and has a strong gravitational field. Ap stars have a pattern of overabundance including silicon, chromium, strontium, and europium and other rare earths. Their magentic fields are measured by the polarization induced in their spectral lines by the Zeeman effect; the fields have been measured up to 34000 Gauss (compared to 1G for the sun). Present understanding is that the slow rotation and the magnetic field together suppress convection to allow chemical segregation and enhancement in the surface layers of the stars.

aquifer A highly pervious geological formation, empirically defined as a geologic formation saturated with water and sufficiently permeable to transmit “significant” quantities of water under normal field conditions. On land, water enters an aquifer through precipitation or influent streams and leaves an aquifer through springs or effluent streams. An unconfined aquifer is a geologic formation in which the upper boundary of the saturated zone is the water table. A confined aquifer is an aquifer that is overlain by a confining bed with significantly lower hydraulic conductivity (an aquitard); water in a well or piezometer within a confined aquifer will rise above the top of the confined aquifer to the potentiometric surface. A perched aquifer is a region in the unsaturated zone that may be temporarily saturated because it overlies an area with lower hydraulic conductivity such as an aquitard or aquiclude.

aquitard A semipervious geological formation that transmits water very slowly as compared to an aquifer.

Arago point One of three points on the sky in a vertical line through the sun at which the polarization of skylight vanishes. Usually located at about 20above the antisolar point (the point opposite the sun on the sky). See Babinet point, Brewster point.

arcade A configuration of coronal loops spanning a magnetic neutral line. The loops are often perpendicular to the neutral line but can be sheared due to the forces of differential rotation.

© 2001 by CRC Press LLC

archaeoastronomy

A coronal arcade is frequently associated with a filament channel.

archaeoastronomy The study of the astronomical knowledge and techniques of prehistorical societies by studies of archaeological structures.

archaeomagnetism The study of the Earth’s magnetic field using archaeological artifacts. Historical magnetism uses explicit historical measurements of the Earth’s magnetic field, which (despite claims that the compass was invented as far back as the second century BC) are only useful back to around 1600 AD. Paleomagnetism relies on measurements of the magnetization of geological materials, such as lava flows and lake bed sediments, and tends to have coarser resolution in time. Archaeomagnetism attempts to bridge the gap between the two by providing measurements of field older than historical but with better resolution than paleomagnetism. A magnetic measurement may be obtained from an excavation from, for example, a kiln whose last firing may be determined using radiocarbon dating. The kiln may record the magnetic field of that time through thermoremanent magnetization.

Archean The period in the Earth’s evolution prior to 2.5 billion years ago.

Archimedes’ principle An object partially or totally submerged in a liquid is buoyed up by a force equal to the weight of the displaced liquid.

Archimedian spiral Shape of the interplanetary magnetic field line. Physically, the solar magnetic field is frozen into the radially streaming solar wind (see frozen-in flux theorem). Because the footpoint of the field line is fixed on the sun, the sun’s rotation winds up the field to a spiral with constant distances between neighboring windings.

Mathematically, such a spiral is called an

Archimedian spiral. In polar coordinates (r, ϕ) it is described as

r = v sowi · ϕ ϕo + ro

ω

with v sowi being the solar wind speed, ω the angular speed of the sun in the equatorial plane, r o the source height of the plasma parcel, and ϕ o its source longitude. With ψ = ω r/v sowi the path length s along the spiral is

s =

2 ·

ω

ψ · ψ2 + 1+

 

1

 

v sowi

 

 

 

+ ln ψ + ψ2 + 1 .

For sufficiently large distances, the garden hose angle δ, that is the angle between the magnetic field line and a radius vector from the sun, can be written as

tan δ = ω r .

v sowi

At Earth’s orbit, the garden hose angle is about 45for the average solar wind speed of 400 km/s, and the distance s to the sun along the Archimedian magnetic field spiral is about 1.15 AU.

arc minute A measure of angular size, abbreviated arcmin or . There are 60 arc minutes in 1 arc degree. On the surface of the Earth 1 arc minute of latitude corresponds very closely to a north-south distance of 1 nautical mile (1852 m).

arc second A measure of angular size in the plane of the sky, abbreviated arcsec or . There are 60 arc seconds in 1 arc minute and, therefore, 3600 arc seconds in 1 arc degree. One arc second corresponds to about 725 km on the surface of the sun, as viewed from the Earth.

Arctic circle The latitude 6632 N. North of this line the sun does not rise on the northern winter solstice and does not set on the day of the northern summer solstice.

arctic oscillation (AO) Dominant mode of atmospheric sea level pressure (SLP) variability in the Northern Hemisphere, most pronounced in winter. At its positive phase, the AO features a deepened Icelandic low and Azores high in the North Atlantic but a weakened Aleutian low in the North Pacific. Surface air temperature rises over northern Eurasia but falls over high-latitude North America. The AO involves changes in

© 2001 by CRC Press LLC

arrow of time

the latitude and strength of westerly jet in the troposphere and in the intensity of polar vortex in the lower stratosphere.

Arcturus 0.2 magnitude star, of spectral type K2, at RA 14h 15m 39.6s,dec +1910 57 .

argon Inert (noble) gas which is a minor (0.94%) constituent of the Earth’s atmosphere. Atomic number 18, naturally occurring atomic mass 39.95, composed of three naturally occurring isotopes A36 (0.34%), A38 (0.06%), and A40 (99.60%). A40 is produced by decay of K40, and potassium-argon dating is used to date the solidification of rocks, since the gas escapes from the melt, but is then regenerated by the decaying potassium.

argument of periapse The angle from the ascending node of an orbit to the periapse.

Ariel Moon of Uranus, also designated UI. It was discovered by Lassell in 1851. Its orbit has an eccentricity of 0.0034, an inclination of 0.3, a semimajor axis of 1.91 × 105 km, and a precession of 6.8yr1. Its radius is 576 km, its mass is 1.27 × 1021 kg, and its density is 1.59 g cm3. Its geometric albedo is 0.34, and it orbits Uranus once every 2.520 Earth days.

Arnowitt–Deser–Misner (ADM) decomposition of the metric In a four-dimensional space-time I, with Lorentzian metric tensor g, consider any one-parameter (t) family of spacelike hypersurfaces 4t with internal coordinates x = (xi , i = 1, 2, 3) and such that, by continuously varying t, 4t covers a domain D I of non-zero four-dimensional volume. Inside D, on using (t, x) as space-time coordinates, the proper distance between a point Ax on 4t and a point Bx+dx on 4t+dt can be written according to the Pythagorean theorem

 

2

 

 

 

ds2 = γij dxi

+ βi dt

 

dxj + βj dt

(α dt)

 

,

 

 

where γ is the metric tensor (pull back of g) on 4t , α (lapse function) gives the lapse of proper time between the two hypersurfaces 4t and 4t+dt , βi (shift vector) gives the proper displacement tangential to 4t between Ax and the

point Ax, which has the same spatial coordinates as Ax but lies on 4t+dt .

In matrix notation one has for the covariant metric tensor

g

=

 

α2 + βk βk

βi

,

 

βj

γij

 

where latin indices are lowered by contraction with γij . The contravariant metric (the matrix inverse of g) is given by

 

 

 

 

1

 

 

βi

 

,

g

1

=

β

 

 

γ ij

α2

βi βj

j

 

 

 

 

α2

 

 

 

 

 

 

 

α2

 

α2

 

where γ ik γkj

= δji .

Any geometrical quan-

tity can then be decomposed in an analogous way. For instance, the determinant of g becomes g = α2γ , where γ is the determinant of the 3-dimensional metric γ .

The above forms (in four dimensions) are also called 3+1 splitting of space-time and can be generalized easily to any dimension greater than one. There is a large amount of freedom in the choice of this splitting which reflects the absence of a unique time in general relativity (multifingered time). See ADM form of the Einstein–Hilbert action.

array seismic observation A seismic observation system improving S/N (signal to noise) ratio of seismic waves by deploying many seismometers in an area and stacking their records, giving appropriate time differences. It is also possible to identify a location of a hypocenter of an earthquake by obtaining direction of arrived seismic waves and apparent velocity. As a large-scale array system, there is the LASA (Large Aperture Seismic Array) in Montana, where more than 500 seismometers were deployed in an area about 200 km in diameter.

arrow of time A physical process that distinguishes between the two possible directions of flow of time. Most of the equations that describe physical processes do not change their form when the direction of flow of time is reversed (i.e., if time t is replaced by the parameter τ = −t, then the equations with respect to τ are identical to those with respect to t). Hence, for every solution f (t) of such equations (f (t) represents here a function or a set

© 2001 by CRC Press LLC

ascending node

of functions), f (t) is also a solution and describes a process that is, in principle, also possible. Example: For a planet orbiting a star, the time-reversed motion is a planet tracing the same orbit in the opposite sense. However, for most complex macroscopic processes this symmetry is absent; nature exhibits histories of directed events in only one direction of time, never the reverse. This is known as the arrow of time. The arrow of time is provided by the expansion of the universe, the thermodynamics of the physical system, or the psychological process. The most famous example is the entropy in thermodynamics: All physical objects evolve so that their entropy either increases or remains constant. The question of whether an arrow of time exists in cosmology is a theoretical problem that has not been solved thus far. Observations show that the universe is expanding at present, but the Einstein equations allow a time-reversed solution (a contracting universe) as well. Note also that at a microscopic level certain quantum particle interactions and decays are not time reversal invariant, and thus define a direction of time. However, no completely convincing connection has yet been made to the large-scale or cosmological arrow of time.

ascending node For solar system objects, the right ascension of the point where the orbit crosses the ecliptic travelling to the North; in other systems, the equivalent definition.

aseismic front An ocean-side front line of an aseismic wedge-shaped region located between a continental plate and an oceanic plate subducting beneath an island arc such as the Japanese islands. An aseismic front is almost parallel to a trench axis and a volcanic front. Very few earthquakes whose hypocentral depths range from 40 to 60 km between the oceanic and the continental plates occur on the continental side of the aseismic front. This is thought to be because temperature is high and interplate coupling is weak on the continental side of the aseismic front. These are closely related to slow velocity structure of the uppermost mantle beneath the island arc, detected from an analysis of observed Pn waves.

aseismic region A region with very few earthquakes.

asperity Earthquakes occur on faults. Faults are approximately rough planar surfaces. This roughness results in asperities that impede displacements (earthquakes) on the fault. An extreme example of an asperity would be a bend in a fault.

association An obvious collection of stars on the sky that are part of, or contained within, a constellation.

A star Star of spectral type A. Vega and Sirius are examples of A stars. A0 stars have color index = 0.

asterism A small collection of stars (part of a constellation) that appear to be connected in the sky but form an association too small to be called a constellation.

asteroid Small solid body in orbit around the sun, sometimes called minor planet. Asteroids are divided into a number of groups depending on their reflection spectrum. The major classes are C-type, characterized by low albedo (0.02 to 0.06) and a chemical composition similar to carbonaceous chondrites; S-type, which are brighter (albedo between 0.07 and 0.23) and show metallic nickel-iron mixed with iron and magnesium silicates; and M-type with albedos of 0.07 to 0.2 which are nearly pure nickeliron. C-type asteroids comprise about 75% of all main belt asteroids, while S-type comprise about 17%. Additional rare classes are E (enstatite), R (iron oxide?), P (metal?), D (organic?), and U (unclassifiable). Asteroids are also classified according to location. Main belt asteroids lie in roughly circular orbits between Mars and Jupiter (2 to 4 AU from the sun). The Aten family has semimajor axes less than 1.0 AU and aphelion distances larger than 0.983 AU. These form a potential hazard of collision with Earth. The Apollo family has semimajor axes greater than 1.0 AU and perihelion distances less than 1.017 AU. Amor asteroids have perihelia between 1.017 and 1.3 AU. Trojan asteroids lie at the L4 and L5 Lagrange points of Jupiter’s orbit around the sun. Centaurs have orbits that bring them into the outer solar system. Obervationally, the distinction between asteroids and comets is that comets display a coma and tail.

© 2001 by CRC Press LLC

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