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Dictionary of Geophysics, Astrophysic, and Astronomy.pdf

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overwash

release will make the loop shrink, and therefore its current will augment in a way inversely proportional to its size. By the end of the loop life, the energy output due to this increased electromagnetic power may supersede that in gravity waves. In such a case, plasma in the vicinity of the string loop will be subject to a strong radiation pressure, and thus thin expanding plasma shells will be generated. These dense gas shells would then eventually be the loci for the formation of galaxies and other structures.

While in standard scenarios cosmic strings attract surrounding matter via their gravitational ﬁelds, here instead string loops repel shells of matter via electromagnetic pressure, forming large-scale structures on the shell boundaries, and voids in the shell interiors. This model is importantly constrained by present measurements of distortions in the 3K cosmic microwave background radiation, notably by the detectors in the COBE satellite. See conducting string, cosmic string, Witten conducting string.

Ottawa 10.7 cm ﬂux The solar radio ﬂux observed at a wavelength of 10.7 cm (or 2800 MHz) by the Ottawa Radio Observatory at 1700 UT (local noon) has been accepted as a standard measurement of solar ﬂuxes. Nor-

mally, the ﬂux is reported in solar ﬂux units (1 sfu = 1 × 10−22 webers m−2 Hz−1) The

variations of the 10.7 cm ﬂux, of 2800 MHz ﬂux as it is sometimes called, is closely associated with enhanced thermal radiation from solar active regions, and thus to the overall level of solar activity.

outer core In geophysics, the Earth’s outer core has an inner radius of 1215 km and an outer radius of 3480 km. It is primarily composed of molten iron with other components dissolved in it. Convection in the outer core produces the geodynamo that generates the Earth’s magnetic ﬁeld.

outfall A pipeline placed along the seaﬂoor to disperse wastewater into the sea.

outﬂow channel Very large dry river channels seen on Mars. They typically begin in a region of jumbled terrain (called chaotic terrain) and cross terrain of various ages, often termi-

nating in the lower-lying northern plains. The channels have widths between 10 and 100 km and are 400 to over 2000 km in length. Features associated with outﬂow channels suggest formation by catastrophic ﬂoods, probably caused by groundwater breaking out of the source region. Outﬂow channels are young, typically with formation ages less than 3 × 109 years.

outgoing longwave radiation (OLR) Infrared radiation emitted by the Earth toward the space. The OLR displays strong temporal and spatial variations, affected by surface temperature, moisture, and clouds. The maximum annual-mean OLR occurs over the Saharan desert where the surface temperature is high and the atmosphere is dry. Satellite-measured OLR is used to determine the temperature and hence the approximate height of clouds. Over the tropics where heavy precipitation is caused mostly by deep convection, OLR can be used as a proxy of convective activity and precipitation rate.

oval equation

The partial differential equa-

tion

∂u2

∂σ 2

χ(u)

∂2

σ 2)α

1 + u2

λ(u)

∂2α

for the unknown functions α = α(σ, u), χ = χ(u) and λ = λ(σ). The ﬁeld equation describing stationary and axially symmetric spacetimes in general relativity containing only electric ﬁelds. Additionally, the cylindrical radius function ρ given by

ρ2 = (1 + σ 2)α − 1 + u2 3 αλχ

is required to be a harmonic function in these solutions.

overtopping A term used to denote the process of waves pushing water over the top of a structure such as a breakwater or jetty.

overturning scale See Ozmidov scale.

overwash A term used to describe the ﬂow of water across a barrier island and into the bay behind. An overwash fan of deposited sediments generally results.

OVV quasar

OVV quasar Optically violently variable quasar. Their optical spectrum closely resembles that of BL Lac objects, with a “blue bump” in some cases. They exhibit strong variability high polarization, and a ﬂat radio spectrum.

oxygen Colorless, odorless gas, O, atomic number 8, appears in the form of molecules O2 and the unstable O3, called ozone. O2 is a strongly reactive gas, comprising about 21% of the sea level atmosphere. Naturally occurring atomic weight 15.9994. Naturally occurring isotopes: 16O (99.76%), 17O, 18O melting point 54.8 K, boiling point 90.2 K. Constituent of water (H2O). Produced by photosynthesis from CO2 by plants, essential for respiration of most animals.

Ozmidov scale The vertical scale, at which the buoyancy force is of the same magnitude as the inertial forces, is deﬁned as LO = (ε/N3)1/2, where ε is the dissipation rate of turbulent kinetic energy and N is the stability frequency. The Ozmidov scale quantiﬁes the maximum size of overturning eddies for a given level of turbulence (characterized by ε) and stratiﬁcation (characterized by N). Experimental evidence indicates that the Ozmidov scale (LO) and the Thorpe scale (LT) are strongly related and about equal (Dillon, 1982). See buoyancy scale.

ozone A form of molecular oxygen, each molecule consisting of three oxygen atoms. It is colorless but has a characteristic odor. Its molecular formula is O3, and molecular weight is 47.9982. Ozone is a common atmospheric constituent when oxygen is abundant, as is the case for the Earth. Ozone is important for the development and survival of life on Earth since it absorbs the deadly ultraviolet radiation produced by the sun. Most of the Earth’s ozone is found within the stratosphere, although ozone is also a primary component of the smog produced by photochemical reactions with industry and car exhaust in cities. Within the stratosphere, the ozone layer continuously absorbs the incoming ultraviolet radiation, which causes the ozone molecule to split apart into an O2 molecule and oxygen (O) atom. Normally the O2 molecule and oxygen atom combine to reform ozone and the process repeats. However, the introduction

of chloroﬂuorocarbons (CFCs) into the atmosphere by humans alters this ozone production process since chlorine atoms will combine with the oxygen atoms. The result is a thinning of the ozone layer (and production of the Antarctic ozone hole), which allows increased amounts of ultraviolet radiation to reach the surface.

ozone layer The region in the stratosphere between altitudes of approximately 15 and 50 km (approximately coinciding with the stratosphere) where most of the atmosphere’s ozone (O3) is found, with the maximum concentrated ozone at about 20 to 25 km. At the maximum concentrated ozone layer, ozone is 1 to 10 parts per million. Ozone is an important constituent of the atmosphere as it acts to absorb most of the ultraviolet radiation from the sun in the wavelength range 230 to 320 nm, which is damaging to the complex molecules found in the cells of living organisms. In addition, heating by ozone absorption plays a profound role in determining the structure of the atmosphere. The heat absorption affects the lapse rate of the stratosphere in a way to make it much more stable, inhibiting vertical air movements, and stratospheric circulation. Ozone is formed by the breakdown of molecular oxygen (O2) caused by short-wavelength solar radiation to form atoms of oxygen (O). These atoms collide with molecular oxygen (O2) to form ozone (O3), which in turn absorbs solar radiation for future dissociation to O and O2. The distribution of atmospheric ozone layer varies at the different latitudes and seasons. The minimum concentration of ozone appears near the equator and the maximum locates at 60◦N and 60◦S. Ozone has its maximum concentration in spring and minimum in fall. Additionally, the ozone concentration has clear diurnal variation with higher concentration during night. This is because the dissociated oxygen atoms can continuously combine with oxygen molecules at night, yet the ozone decomposing process does not exist without solar radiation. The vertical distribution of ozone concentration has discontinuous or jump phenomena. The distribution of ozone is additionally inﬂuenced by a complex set of chemical destruction mechanisms involving atmospheric trace gases, as well as by transport by stratospheric winds.

ozonosphere

Although signiﬁcant decreases in ozone have been observed over much of the globe since the 1970s, the mechanisms are presently only partly understood. The appearance of the Antarctic ozone hole in the early 1980s demonstrated that polar ozone can be drastically reduced by manmade substances, namely chloroﬂuorocarbons (CFCs). These CFCs provide a source of chlorine in the atmosphere. Atomic chlorine is produced from Cl2 or from CFCs by photodissociation in sunlight. Hydrogen chloride and chlorine nitrate are the most important two stratospheric chlorine reservoirs. Especially in polar regions, stratospheric clouds can serve as the reaction sites to produce molecular chlorine and nitric acid from the reservoirs. Chlorine is extremely damaging to ozone, because it acts catalytically:

Cl + O3	→	ClO + O2
O3 + hν	→	O + O2
ClO + O	→	Cl + O2 .

The net result is to reform the chlorine radical, and to convert two ozone molecules to three O2 molecules. The Montreal Protocol, signed in 1987, and its subsequent amendments, provide an international framework for the phasing-out of CFCs, although the extent and timescale of the recovery in the ozone layer are yet to be determined.

ozonosphere The atmospheric layer with higher concentrated ozone, which lies about 10 to 50 km altitude and thus is essentially synonymous with the stratosphere. The maximum concentration of ozone occurs at about 20 to 25 km.

pancake dome

package effect In oceanography and other areas, discrepancy between the spectral absorption coefﬁcient of a particle suspension, consisting of a spatially nonuniform distribution of pigment molecules in cells, and the corresponding coefﬁcient of a homogeneous solution containing the same amount of pigment.

Palatini (ﬁrst order) formalism In general relativity, in the framework of the ﬁrst order formalism the metric gµν and the afﬁne connection βγα are considered as independent variables for the purpose of obtaining the Einstein equations from a variational principle. Taking variational derivatives of the otherwise standard Hilbert form of the variational action for general relativity with respect to those variables, one arrives at the dynamical Einstein equations in ﬁrst order formalism. In the action

SEH = dnx√−ggµνRµν( )

where Rµν( ) depends on the connection only, but not on the metric, the equation of motion forβγα gives the same result as the metricity condition, that is, it ﬁxes connection βγα to be the

Cristoffel symbol αβγ . Taking this constraint into account, the equations for the metric become usual Einstein equations. One may apply similar concepts in gauge theories or in other variations of gravity theories. See christoffel symbol, curvature tensor, metricity of covariant derivative.

paleoclimate The science that treats the global climates throughout the geological ages. Its data are the distribution of glacier deposits, nature of plant and animal fossils, topography and geography of former periods, and character of sedimentary rocks. Based on the information extracted from these data, the ice period, the distribution of temperature and moisture can be identiﬁed.

palimpsest In planetary physics, circular features found on icy moons (particularly Ganymede) that appear to be the imprints of impact craters without any topographic relief. They typically display a bright interior zone, thought to represent the excavated crater, surrounded by a rough region representing part of the ejecta blanket. Although it may be argued that these are ancient craters which have relaxed over time due to the low strength of ice, palimpsests are more likely pristine forms that resulted from impact into ice which then melted and deformed as part of the cratering process. They probably represent features formed during the time when Ganymede’s ice crust was solidifying.

Pallas One of the larger main belt asteroids. Second asteroid to be discovered, in 1802. Its diameter is estimated at 540 km, and a crude mass estimate, based on close encounters with Vesta and Ceres, gives M = 2.16(±0.44) × 1024 g. The best estimate of its density (good to about 35%) is 2.6 g cm−3. It has a visual albedo of 0.66 and the spectral reﬂection characteristics of an S-type asteroid. Its composition has not been well determined. Orbit: semimajor axis 2.772 AU, eccentricity 0.22965, inclination to the ecliptic 34◦.84603, period 4.62 years.

Pan The innermost of Saturn’s known moons, also designated SXVIII. It was discovered by M. Showalter in 1990 in Voyager photos, and orbits within the Encke Gap in Saturn’s A ring. It has a radius of approximately 10 km. Its orbit has a semimajor axis of 1.34 ×105 km. Its mass has not been measured.

pancake distribution In plasma physics, a pitch angle distribution in which the highest intensities are orthogonal to the magnetic ﬁeld so that a contour of the ﬂux density against polar angle θ is an oblate spheriod (pancake). See cigar distribution.

pancake dome A volcanic feature on the surface of Venus composed from viscous lava. The lava was probably quartz-rich, granitic, or a frothy, gas-rich basaltic lava. Pancake domes are widely scattered, often forming small groups or clusters. Dome clusters comprise 5 or 6

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