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Dictionary of Geophysics, Astrophysic, and Astronomy.pdf

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elemental abundances: of comet(s)

electromagnetism The study of the relationships between electric and magnetic ﬁelds, their causes, and their dynamical effects.

electromagnetism, Lorentz transformation and An important property of Lorentz transformations is that the equations that describe the relationship between electric and magnetic ﬁeld, their causes and effects, are Lorentz invariant. It then follows that special relativity presents a consistent theoretical groundwork for both mechanics and electromagnetism.

electron An elementary particle in the family of leptons, with mass of 9.1093897 × 10−31kg, negative charge and spin of 1/2.

electron precipitation In solar physics, the transport of an accelerated population of electrons from the corona to the chromosphere where they deposit the bulk of their energy. The electrons can precipitate directly, immediately following their production, or after a delay during which they are trapped in the corona by converging magnetic ﬁeld. The pitch-angle of the electrons, the nature of the ﬁeld convergence, and the collision rate in the corona are all important in deciding which electrons precipitate and when.

electron temperature The kinetic temperature of electrons in an ionized gas.

electron volt The energy an electron (or proton) gains when accelerated through an elec-

tric potential difference of 1 volt. Equal to 1.602177 × 10−19 J.

electrostatic unit A unit of charge deﬁned so that two equal such charges exert a mutual electrostatic force of 1 dyne, when separated by 1 cm.

electrovacuum A space-time in general relativity containing a source-free electromagnetic ﬁeld interacting with the gravitational ﬁeld.

element A group of atoms, each with the same number of protons, deﬁnes an element. The number of protons in a given atom is called the atomic number. There are 92 naturally oc-

curring elements. Elements with over 100 protons can be synthesized. Although an atom consists of protons and neutrons in the nucleus surrounded by clouds of electrons, it is only the number of protons that deﬁnes the atom as belonging to a particular element. Atoms of the same element that have different numbers of neutrons are known as isotopes of the element. Atoms of the same element that have different numbers of electrons are called ions.

elemental abundances: general The bulk elemental composition of most solar system objects are roughly the solar system average, but contain distinct sub-components that are presolar. Isotopic anomalies represent the only unequivocal signatures of the survival of circumstellar and interstellar materials within extraterrestrial objects such as meteorites and interplanetary dust particles (IDPs).

elemental abundances: in minerals Isotopic anomalies in a variety of elements have been used to identify a number of circumstellar mineralogical species in meteorites. These isotopic anomalies are thought to have a nucleosynthetic origin, i.e., their carrier grains were formed in the circumstellar environments of various kinds of stars, survived transport through the interstellar medium, and incorporation into our solar system.

elemental abundances: in organics Significant deuterium enrichments have been seen in the organic components of both meteorites and interplanetary dust particles. Unlike the isotopic signatures in the circumstellar materials, denterium anomalies do not have a nucleosynthetic origin. Instead, it has been proposed that they result from reactions at low temperatures in the interstellar medium. In a few cases, the D/H ratios of speciﬁc classes of organic compounds have been measured, for example in PAHs in meteorites and IDPs.

elemental abundances: of comet(s) The overall elemental composition of Comet Halley resembled the solar system average, but abundances and deuterium enrichments of molecules in cometary tails and comae, and the types of solid materials observed in interplanetary dust

elevation

particles have been interpreted as an indication of the primitive, pre-solar nature of comets, or at least their sub-components. For example, the ratios of D/H in water from comets Halley, Hyakutake, and Hale–Bopp were all around 3 × 10−4, similar to what is seen in certain interstellar environments and approximately a factor of two over ocean water, and DCN/HCN ratios up to 2.5 × 10−2 were measured in Hale–Bopp.

elevation The vertical distance measured from the geoid, from the earth ellipsoid, or from the local terrain. When interpreting elevation data, care is needed to ascertain which of the three surfaces mentioned is the reference one. When no speciﬁcation is given, the context must be examined. For geophysics, hydrology, and cartography, the geoid is the usual reference surface because most elevation data are produced in surveys based on the local horizontal, as found by the spirit level or the normal to the plumb line. For construction or related work, the local mean terrain (earth surface) is the more likely reference. The ellipsoid is rarely the reference surface, but it may be so in space science contexts.

elevation head (sometimes gravitational head) (z) The potential energy per unit ﬂuid weight is given by the elevation above an arbitrary horizontal datum. The elevation head has units of length and is a component of the hydraulic head.

Eliassen–Palm (EP) ﬂux A vector in the meridional (y,z) plane, which has the eddy momentum ﬂux and eddy heat ﬂux as its horizontal and vertical components. Its convergence is directly related to the eddy forcing on the zonal mean ﬂow.

elliptical galaxies Galaxies of regular, ellipsoidal appearance, and of rather reddish colors. The photometric proﬁles of most elliptical galaxies are described by empirical laws in which the surface brightness decreases smoothly as a function of the distance from the galaxy center. Elliptical galaxies do not show features such as bars, spiral arms, or tails. Only a minority of them show ripples, shells, or asymmetric radial distribution of surface brightness.

Elliptical galaxies are characterized by the absence of signiﬁcant neutral or molecular gas, and hence the absence of star formation, and by a stellar content mostly made of old stars belonging to stellar population II. They account for about 1/3 of all observed galaxies, and are the majority of galaxies in dense cluster environments. They cover a wide range of masses, from 106 to 1011 solar masses, the most massive being located at the center of clusters of galaxies (cD galaxies), the less massive being dwarf elliptical galaxies. See cD galaxies, dwarf spheroidal galaxies.

Ellison scale Like the Thorpe scale, the Ellison scale is a quantity to estimate the overturning eddy size. The Ellison scale LE is based on density ρ instead of temperature T, and the deﬁnition LE =< ρ2 > /(∂ρ/∂z) deviates slightly from the procedure for the Thorpe scale LT estimation. Both scales, LE and LT are considered adequate measures for the overturning eddy size and generally agree well with the Ozmidov scale.

El Niño The warm phase of the Southern Oscillation beginning at about Christmas time (hence the name “El Niño”, Spanish for “Christ child”) and lasting 12 to 18 months. Characterized by warming of sea surface temperatures in the central and eastern equatorial Paciﬁc Ocean. The anomalously warm water causes the sardine population to die off the Peru coast. A series of effects arise, including an increased westerly wind and a shift in Paciﬁc ocean circulation. This warming occurs in the entire tropics and causes drought in Indonesia and Australia. Enhanced North-South temperature differences transport energy into the atmosphere, modifying global atmospherics ﬂow, causing warm dry weather in Northern U.S., and wet cool weather in the Southern U.S. See Southern Oscillation Index, La Niña.

elongation The angle between the sun and the observer, measured at the object being observed.

elongation The apparent angular separation between the sun and a solar system object as viewed by a distant observer, i.e., the sun- observer-object angle.

emission lines: interstellar and cometary

elves Transient air glow events observed near 90 km, nearly simultaneously with a strong cloud-to-ground lightning stroke. They often precede sprites, which may occur at lower altitudes a few milliseconds later. It is currently believed that elves are the result of wave heating by very low frequency (VLF) radio pulses emitted by the lightning discharge current. See sprites.

Elysium Province The second most pronounced region of central vent volcanism on Mars. It is 5 km high, ≈ 2000 km in diameter, and is centered on latitude 25◦ N and longitude 210◦ W. It is considerably smaller than Tharsis and has only three shield volcanoes of appreciable size (namely, Albor Tholus, Hecates Tholus, and Elysium Mons). However, the volcanoes are still large compared to those on Earth, with Hecates Tholus standing ≈ 6 km above the plains and Elysium Mons standing ≈ 9 km above the plains. All of them are greater than 150 km in diameter. The volcanoes are considered to be older than those at Tharsis formed during the late Noachian to early Hesperian. They all present a diverse range of volcanic morphologies, all of which indicate more pyroclastic activity compared to the Tharsis volcanoes. Conﬁned outﬂow channels exist NW of Elysium Mons and Hecates Tholus. They drain north-west and extend ≈ 1000 km into the plains, and may be of volcanic origin.

Similarities of the Elysium Province to Tharsis include the volcanic and tectonic history, a broad free air gravity anomaly, and Phlegra Montes which is assumed to be an island of old cratered terrain on the northern ﬂank of Elysium Mons. If the favored proposal for subsurface thermal activity at Tharsis is applicable to Elysium, then it appears that there is a bimodal distribution of hotspots on Mars.

embedded defect Cosmic topological defects are predicted to form at phase transitions when the symmetry G of a system is broken down to a smaller symmetry group H and there exists a nontrivial homotopy group of the vacuum manifold M G/H . In the case where all the homotopy groups of M are trivial, no topological defect can form but there might still be defect-like solutions. These arise from the

existence of subgroups g G and h H that are also broken through the scheme g → h during the breaking G → H . For the same reasons as with ordinary topological defects, if there is a non-trivial structure of the vacuum submanifold g/h, then defects might form. However, here stability is not ensured by topology, but by dynamical arguments.

Even these unstable defects could have a role in cosmology, as for instance the so-called Z−strings, associated with the total breaking of a U(1) subgroup of SU(2) in the scheme SU(2) × U(1) → U(1) of the electroweak model (the actual group that is broken is essentially SU(2), which contains a U(1) subgroup, and it is the breaking of this subgroup that gives rise to the defect-like solutions), and that could be responsible for the primordial baryogenesis. Other embedded defects are observed in other branches of physics such as condensed matter experiments. See cosmic phase transition, cos- mic topological defect, homotopy group, vac- uum manifold.

emerging ﬂux region New bipolar active regions emerge from below the solar photosphere in a characteristic pattern known as an emerging ﬂux region. A ﬂux loop brought to the surface by magnetic buoyancy intersects the surface to form a bipole. As the loop emerges, the opposite poles move apart, the preceding spot moving 1 km s−1 and the following spot less than 0.3 kms−1. In Hα the phenomenon is characterized by arch ﬁlaments which appear to trace the rising ﬂux loops.

emission line A feature in the spectrum of the light produced by a medium that emits light by a quantum transition, thus increasing the intensity of the light at certain wavelengths. See spectrum. Compare with absorption line.

emission lines: interstellar and cometary

Extraterrestrial molecules, fragments, and atoms can be detected or characterized based on emission, the emanation of radiation as a result of excitation such as resulting from collisions or the absorption of photons. Interstellar examples include Lyman and Balmer series of molecular and atomic hydrogen, respectively, in the UV/Vis, and in the infrared, the ﬂuores-

emissivity

cence of aromatic hydrocarbons pumped by UV photons. Similarly, emission from both atoms (i.e., sodium D lines) and ﬂuorescence from molecules (i.e., 3.4 µm, methanol) and fragments (i.e., Swan bands of C2 ) excited by sunlight have been observed from cometary comae.

emissivity An indirect measure of reﬂectivity, where emissivity = 1 − reﬂectivity and reﬂectivity is the reﬂected energy measured from a surface by a single energy bounce. Mirrored surfaces reﬂect large amounts of energy (around 98%) but absorb very little (around 2%). A blackbody surface, on the other hand, reverses the ratio, absorbing 98% of the energy and reﬂecting only 2%. Emissivity is affected by the geometric shape of the object, the electrical properties of the radiating surface, and the measuring wavelength.

A Magellan radiometer experiment observed the 12.6-cm-wavelength radiothermal emissivity on Venus. The nominal pattern of radiothermal emissivity shows high mountain summits display abnormally low emissivity and plains regions high emissivity. One explanation for the low emissivity of mountaintops is the presence of electrically conductive minerals, produced by weathering, embedded in the surface rocks.

empirical model of the magnetosphere A mathematical representation of the global magnetic ﬁeld of the magnetosphere, whose coefﬁcients are ﬁtted to data. It is a convenient tool for predicting the magnetic ﬁeld vector or ﬁeld line linkage to the ionosphere which a satellite at some point P in space is most likely to ﬁnd. The construction of empirical ﬁelds is also the best way of extracting global information from magnetic ﬁeld data.

The parameters of modern empirical models are generally derived from ﬂexible representations of the ﬁelds of the different magnetospheric current systems, e.g., those of the magnetopause, tail, ring current, and the Birkeland current circuit. The ﬁeld predicted at P depends not only on the location but also on the tilt angle ψ, on the dynamic pressure of the solar wind, on the interplanetary magnetic ﬁeld and on geomagnetic activity indices such as Dst and AE.

Enceladus Moon of Saturn, also designated SII. Discovered by Herschel in 1789 its surface has the highest albedo of any solar system body. It also displays evidence of resurfacing, possibly as a result of water volcanism. Since it is too small to retain radioactive heat, tidal heating aided by a 1:2 resonance with Dione may provide the required energy. Its orbit has an eccentricity of 0.0045, an inclination of 0.00◦, a precession of 156.2◦ yr−1, and a semimajor axis

of 2.38 ×105 km. Its radius is 250 km, its mass, 8.40 × 1019 kg, and its density 1.28 g cm−3. It

has a geometric albedo of 1.0, and orbits Saturn once every 1.370 Earth days.

Encke’s comet Comet with the shortest known orbital period: 3.30 years. Its orbit has semimajor axis 2.21 AU and perihelion distance 0,338 AU.

endothermic A process that absorbs heat as it proceeds. Opposite of exothermic.

energetic particles Supra-thermal particles; that is all particles with speeds large compared to the thermal plasma speed.

energetic particles in interplanetary space

Energetic particles in interplanetary space can be observed with energies ranging from the supra-thermal up to 1020 eV. The main constituents are protons, α-particles, and electrons; heavier particles up to iron can be found in substantially smaller numbers. The particle populations originate in different sources, all having their typical spectrum, temporal development, and spatial extent.

1.Galactic cosmic rays (GCR) are the high-

energy background with energies extending up to 1020 eV. They are incident upon the heliosphere uniformly and isotropically. In the inner heliosphere, the galactic cosmic radiation is modulated by solar activity: the intensity of GCRs is highest during solar minimum and reduced during solar maximum conditions. See modulation of galactic cosmic rays.

2.Anomalous galactic cosmic rays (AGCR), also called anomalous component, energetically connect to the lower end of the galactic cosmic rays but differ from them with respect to composition, charge states, spectrum, and variation

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