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African waves

quence, luminosity of the advection dominated disk can be much lower than that of a standard thin accretion disk. Advection dominated disks are expected to form if the accretion rate is above the Eddington limit, or on the other end, if the accretion rate is very low. Low accretion rate, advection dominated disks have been used to model the lowest luminosity active galactic nuclei, the galactic center, and quiescent binary systems with a black hole candidate. See active galactic nuclei, black hole, Eddington limit.

advective heat transfer (or advective heat transport) Transfer of heat by mass movement. Use of the term does not imply a particular driving mechanism for the mass movement such as thermal buoyancy. Relative to a reference temperature T0, the heat ﬂux due to material of temperature T moving at speed v is q = v ρc(T − T0), where ρ and c are density and speciﬁc heat, respectively.

aeolian

See eolian.

aerosol Small size (0.01 to 10 µm), relatively stable suspended, colloidal material, either natural or man-made, formed of solid particles or liquid droplets, organic and inorganic, and the gases of the atmosphere in which these particles ﬂoat and disperse. Haze, most smokes, and some types of fog and clouds are aerosols. Aerosols in the troposphere are usually removed by precipitation. Their residence time order is from days to weeks. Tropospheric aerosols can affect radiation processes by absorbing, reﬂecting, and scattering effects, and may act as Aitken nuclei. About 30% of tropospheric aerosols are created by human activities. In the stratosphere, aerosols are mainly sulfate particles resulting from volcanic eruptions and usually remain there much longer. Aerosols in the stratosphere may reduce insolation signiﬁcantly, which is the main physics factor involved in climatic cooling associated with volcanic eruptions.

aesthenosphere Partially melted layer of the Earth lying below the lithosphere at a depth of 80 to 100 km, and extending to approximately 200 km depth.

afﬁne connection A non-tensor object which has to be introduced in order to construct the covariant derivatives of a tensor. Symbol: :βγα . Under the general coordinate transformation xµ −→ x µ = xµ +ξµ(x) the afﬁne connection possesses the following transformation rule:

∂xα ∂xν ∂xλ

∂xα ∂2xτ

:β

γ =

:νλ

∂xµ

∂xβ

∂xγ

∂xτ

∂xβ ∂xγ

while for an arbitrary tensor

µ ...µ

l one

T A = Tν1...1 νk

has

α1 ...αl

∂xα1

. . .

∂xαl ∂xν1

Tβ1 ...βk

∂xµ1

∂xµl

∂xβ1

. . .

∂xνk

T µ1...µl

∂xβk

ν1...νk

The non-tensor form of the transformation of afﬁne connection guarantees that for an arbitrary tensor Tρναβ......αγ its covariant derivative

αβ	...γ	=	αβ	...γ	α σβ	...γ	+
µTρν...	α	=	Tρν...	α,µ	+ :σµTρν...	α	+

− :ρµσ Tσναβ......αγ − . . .

is also a tensor. (Here the subscript “µ” means ∂/∂Xµ.) Geometrically the afﬁne connection and the covariant derivative deﬁne the parallel displacement of the tensor along the given smooth path. The above transformation rule leaves a great freedom in the deﬁnition of afﬁne connection because one can safely add to :βγα any tensor. In particular, one can provide the symmetry of the afﬁne connection :βγα = :γβα (which requires torsion tensor = 0) and also metricity of the covariant derivative µgαβ = 0. In this case, the afﬁne connection is called the Cristoffel symbol and can be expressed in terms of the sole metric of the manifold as

:α = 1 gαλ ∂β gγ λ + ∂γ gβλ − ∂λgβγ βγ 2

See covariant derivative, metricity of covariant derivative, torsion.

African waves During the northern hemisphere summer intense surface heating over the Sahara generates a strong positive temperature gradient in the lower troposphere between the equator and about 25◦ N. The resulting easterly thermal wind creates a strong easterly jet core near 650 mb centered near 16◦ N. African waves

afternoon cloud (Mars)

are the synoptic scale disturbances that are observed to form and propagate westward in the cyclonic shear zone to the south of this jet core. Occasionally African waves are progenitors of tropical storms and hurricanes in the western Atlantic. The average wavelength of observed African wave disturbance is about 2500 km and the westward propagation speed is about 8 m/s.

afternoon cloud (Mars) Afternoon clouds appear at huge volcanos such as Elysium Mons, Olympus Mons, and Tharsis Montes in spring to summer of the northern hemisphere. Afternoon clouds are bright, but their dimension is small compared to morning and evening clouds. In their most active period from late spring to early summer of the northern hemisphere, they appear around 10h of Martian local time (MLT), and their normal optical depths reach maximum in 14h to 15h MLT. Their brightness seen from Earth increases as they approach the evening limb. Afternoon clouds show a diurnal variation. Sometimes afternoon clouds at Olympus Mons and Tharsis Montes form a W-shaped cloud together with evening clouds, in which the afternoon clouds are identiﬁed as bright spots. The altitude of afternoon clouds is higher than the volcanos on which they appear. See evening cloud, morning cloud.

aftershocks Essentially all earthquakes are followed by a sequence of “aftershocks”. In some cases aftershocks can approach the main shock in strength. The decay in the number of aftershocks with time has a power-law dependence; this is known as Omori’s law.

ageostrophic	ﬂow	The ﬂow that is not
geostrophic. See geostrophic approximation.
agonic line	A line of zero declination. See
declination.

air The mixture of gases near the Earth’s surface, composed of approximately 78% nitrogen, 21% oxygen, 1% argon, 0.035% carbon dioxide, variable amounts of water vapor, and traces of other noble gases, and of hydrogen, methane, nitrous oxide, ozone, and other compounds.

airfoil probe A sensor to measure oceanic turbulence in the dissipation range. The probe is an axi-symmetric airfoil of revolution that senses cross-stream velocity ﬂuctuations u = |u| of the free stream velocity vector W (see ﬁg- ure). Airfoil probes are often mounted on vertically moving dissipation proﬁlers. The probe’s output is differentiated by analog electronic circuits to produce voltage ﬂuctuations that are proportional to the time rate of change of u, namely ∂u(z)/∂t, where z is the vertical position. If the proﬁler descends steadily, then by the Tayler transformation this time derivative equals velocity shear ∂u/∂z = V −1 ∂u(z)/∂t. This microstructure velocity shear is used to estimate the dissipation rate of turbulent kinetic energy.

airglow Widely distributed ﬂux predominately from OH, oxygen, and neon at an altitude of 85 to 95 km. Airglow has a brightness of order 14 magnitudes per square arcsec.

air gun An artiﬁcial vibration source used for submarine seismic exploration and sonic prospecting. The device emits high-pressured air in the oceanic water under electric control from an exploratory ship. The compressed air is conveyed from a compressor on the ship to a chamber which is dragged from the stern. A shock produced by expansion and contraction of the air in the water becomes a seismic source. The source with its large capacity and low-frequency signals is appropriate for investigation of the deeper submarine structure. An air gun is most widely used as an acoustic source for multi-channel sonic wave prospecting.

Airy compensation The mass of an elevated mountain range is “compensated” by a low density crustal root. See Airy isostasy.

Airy isostasy An idealized mechanism of isostatic equilibrium proposed by G.B. Airy in 1855, in which the crust consists of vertical rigid rock columns of identical uniform density ρc independently ﬂoating on a ﬂuid mantle of a higher density ρm. If the reference crustal thickness is H , represented by a column of height H , the extra mass of a “mountain” of height h must be compensated by a low-density “mountain root” of length b. The total height of the

Alba Patera

Geometry of the airfoil probe, α is the angle of attack

of the oncoming ﬂow.

rock column representing the mountain area is then h + H + b. Hydrostatic equilibrium below the mountain root requires (ρm − ρc)b = ρch.

Airy phase When a dispersive seismic wave propagates, the decrease of amplitude with increasing propagation distance for a period whose group velocity has a local minimum is smaller than that for other periods. The wave corresponding to the local minimum is referred to as an Airy phase and has large amplitude on a

record of surface waves. An Airy phase appears at a transition between normal dispersion and reverse dispersion. For continental paths an Airy phase with about a 20-sec period often occurs, while for oceanic paths an Airy phase with 10to 15-sec period occurs, reﬂecting the thickness of the crust.

Airy wave theory First-order wave theory for water waves. Also known as linear or ﬁrstorder theory. Assumes gravity is the dominant restoring force (as opposed to surface tension). Named after Sir George Biddell Airy (1801– 1892).

Aitken, John (1839–1919) Scottish physicist. In addition to his pioneering work on atmospheric aerosol, he investigated cyclones, color, and color sensations.

Aitken nucleus count One of the oldest and most convenient techniques for measuring the concentrations of atmospheric aerosol. Saturated air is expanded rapidly so that it becomes supersaturated by several hundred percent with respect to water. At these high supersaturations water condenses onto virtually all of the aerosol to form a cloud of small water droplets. The concentration of droplets in the cloud can be determined by allowing the droplets to settle out onto a substrate, where they can be counted either under a microscope, or automatically by optical techniques. The aerosol measured with an Aitken nucleus counter is often referred to as the Aitken nucleus count. Generally, Aitken nucleus counts near the Earth’s surface range from average values on the order of 103 cm−3 over the oceans, to 104 cm−3 over rural land areas, to 105 cm−3 or higher in polluted air over cities.

Alba Patera A unique volcanic landform on Mars that exists north of the Tharsis Province. It is less than 3 km high above the surrounding plains, the slopes of its ﬂanks are less than a quarter of a degree, it has a diameter of ≈ 1600 km, and it is surrounded by an additional 500 km diameter annulus of grabens. Its size makes it questionable that it can properly be called a volcano, a name that conjures up an image of a distinct conical structure. Indeed from the ground on Mars it would not be discernible

albedo

because the horizontal dimensions are so large. Nevertheless, it is interpreted as a volcanic structure on the basis that it possesses two very large summit craters from which huge volumes of lava have erupted from the late Noachian until the early Amazonian epoch; hence, it might be the largest volcanic feature on the entire planet. The exact origin is unclear. Possible explanations include deep seated crustal fractures produced at the antipodes of the Hellas Basin might have subsequently provided a conduit for magma to reach the surface; or it formed in multiple stages of volcanic activity, beginning with the emplacement of a volatile rich ash layer, followed by more basaltic lava ﬂows, related to hotspot volcanism.

albedo Reﬂectivity of a surface, given by I/F , where I is the reﬂected intensity, and πF is the incident ﬂux. The Bond albedo is the fraction of light reﬂected by a body in all directions. The bolometric Bond albedo is the reﬂectivity integrated over all wavelengths. The geometric albedo is the ratio of the light reﬂected by a body (at a particular wavelength) at zero phase angle to that reﬂected by a perfectly diffusing disk with the same radius as the body. Albedo ranges between 0 (for a completely black body which absorbs all the radiation falling on it) to 1 (for a perfectly reﬂecting body).

The Earth’s albedo varies widely based on the status and colors of earth surface, plant covers, soil types, and the angle and wavelength of the incident radiation. Albedo of the earth atmosphere system, averaging about 30%, is the combination of reﬂectivity of earth surface, cloud, and each component of atmosphere. The value for green grass and forest is 8 to 27%; over 30% for yellowing deciduous forest in autumn; 12 to 18% for cities and rock surfaces; over 40% for light colored rock and buildings; 40% for sand; up to 90% for fresh ﬂat snow surface; for calm ocean, only 2% in the case of vertically incident radiation but can be up to 78% for lower incident angle radiation; 55% average for cloud layers except for thick stratocumulus, which can be up to 80%.

albedo neutrons Secondary neutrons ejected (along with other particles) in the collision of

cosmic ray ions with particles of the upper atmosphere. See neutron albedo.

albedo of a surface For a body of water, the ratio of the plane irradiance leaving a water body to the plane irradiance incident on it; it is the ratio of upward irradiance to the downward irradiance just above the surface.

albedo of single scattering The probability of a photon surviving an interaction equals the ratio of the scattering coefﬁcient to the beam attenuation coefﬁcient.

Alcyone Magnitude 3 type B7 star at RA 03h47m, dec +24◦06 ; one of the “seven sisters” of the Pleiades.


Aldebaran	Magnitude 1.1 star at RA
04h25m, dec +16◦31 .
Alfvénic ﬂuctuation		Large amplitude ﬂuc-

tuations in the solar wind are termed Alfvénic ﬂuctuations if their properties resemble those of Alfvén waves (constant density and pressure, alignment of velocity ﬂuctuations with the magnetic-ﬁeld ﬂuctuations; see Alfvén wave). In particular, the ﬂuctuations δv sowi in the solar wind velocity and δB in magnetic ﬁeld obey the relation

δv sowi = ±√δB

4πC

with C being the solar wind density. Note that in the deﬁnition of Alfvénic ﬂuctuations or Alfvénicity, the changes in magnetic ﬁeld and solar wind speeds are vector quantities and not the scalar quantities used in the deﬁnition of the Alfvén speed.

Obviously, in a real measurement it will be impossible to ﬁnd ﬂuctuations that exactly fulﬁll the above relation. Thus ﬂuctuations are classiﬁed as Alfvénic if the correlation coefﬁcient between δvsowi and δB is larger than 0.6. The magnetic ﬁeld and velocity are nearly always observed to be aligned in a sense corresponding to outward propagation from the sun.

Alfvénicity See Alfvénic ﬂuctuation.

Alfvén layer Term introduced in 1969 by Schield, Dessler, and Freeman to describe the