dynamic pressure

two after their outbreaks, are white visually and bright in blue photographically, but not in red.

dwarf galaxy A galaxy having a small mass and low luminosity. They are categorized into: D Ir dwarf irregular; D Sp dwarf spiral; D El dwarf elliptical; and D Sph dwarf spheroidal (and nucleated dwarf spheroidal).

dwarf nova The subset of cataclysmic variables in which hydrogen accreted on the white dwarf has ignited while degenerate and so burned explosively. The system survives and the explosions repeat in periods from about 30 years (recurrent novae) up to 104−5 years (classical novae). Most or all of the accreted hydrogen and its burning products are expelled at speeds of about 1000 km/sec, leading to a visible nebula called a nova remnant. These remain detectable for decades until the gas (only about 10−5 solar masses) dissipates. A few novae are found in the Milky Way each year, and the total number is probably 20 to 50, many of which are hidden behind galactic dust. The ejecta make some contribution to the galactic inventory of carbon and of rare isotopes like nitrogen-15 and (probably) aluminum-26.

dwarf spheroidal galaxies Low mass ellipsoidal galaxies, which differ from other dwarf galaxies because of their lower surface brightness and lower luminosity. More quantitatively, a dwarf spheroidal (dSph) galaxy can be defined as a galaxy with absolute blue magnitude fainter than −14 (−8.5 for the faintest dwarf spheroidal known), surface brightness lower than 22 visual magnitudes per square arcsec (a surface brightness comparable to that of the night sky at new moon), and no nucleus. dSph galaxies have been discovered in the local group, including the first ever, the sculptor system, discovered in 1937 by H. Shapley. The local group dSph galaxies appear as a collection of faint stars, with no diffuse light coming from a background of unresolved, less luminous stars. In addition, dSph galaxies are extremely poor in atomic gas.

dynamical friction The retarding effect on a massive body as it moves through a cloud of lighter bodies, arising from the fact that momentum is transferred to the lighter bodies by their

gravitational interaction with the massive body. Hence, heavier objects tend to sink to the center of clusters (and some of the lighter objects are expelled in the interactions).

dynamical time (DT) A family of atomic times introduced in 1984 to replace ephemeris time. DT incorporates a relativistic correction for the change in the frequency of a clock due to the effect of its different, or changing, gravitational potential. The heavier the clock, the slower it runs. The second of DT is scaled from the second of International Atomic Time. “Terrestrial Dynamical Time” (TDT) is measured on the Earth’s geoid and is meant to approximate Terrestrial Time used in solar motion theories. “Barycentric Dynamical Time” (TDB) is referred to as the solar system barycenter. TDB differs from TDT only by periodic variations. Planetary ephemerides are computed using TDB, but published referred to TDT. See Barycentric Dynamical Time, Terrestrial Dy- namical Time.

dynamic height Height measured in units of dynamical meters.

dynamic meter A unit of gravity potential used to represent the amount of work performed in lifting a unit mass vertically 1 m and defined as 105 dyn-cm/gm or 10 J/kg. Dynamic meters can be related to a depth, D, in the ocean as D = gh/10, where g is the gravitational acceleration and h is the geometric depth. One dynamic meter corresponds roughly to 1.02 ge-

ometric meters at the surface of the Earth where g = 9.8m/sec2.

dynamic pressure The inferred momentum flux ρv2 of the solar wind (ρ density, v velocity) before it strikes the magnetosphere. It is widely used as a parameter in theoretical or empirical models of the magnetopause and bow shock, and equals the pressure on a plate perpendicular to the flow of the solar wind, far ahead of Earth, assuming it absorbs all solar wind particles that hit it. A typical value is 2 nP (nanopascal). The size of the magnetosphere shrinks (expands) with growing (diminishing) dynamic pressure.

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dynamic recrystallization

dynamic recrystallization Deformationinduced reworking of grain size, shape, or orientation in a crystal with little or no chemical change. An important mechanism of ridding the crystal of tangled dislocations, in which the new dislocation-free crystals nucleate and grow through the deformed or partially recovered structure. Dynamic recrystallization will result in a new undeformed polycrystalline state with high-angle grain boundary (i.e., no particular orientation relationship between grains).

dynamics The study of motion arising from interaction between bodies, and of bodies under the influence of external fields (gravitational, electric, and magnetic).

dynamic viscosity The coefficient of viscosity, usually denoted µ equal to the ratio of the shearing stress to the shear of the motion. See kinematic viscosity, eddy viscosity, abso- lute viscosity.

dynamo The mechanism whereby strong magnetic fields are produced from small field fluctuations through distortion of field lines by motion of conducting fluids. A dynamo effect is responsible for the Earth’s magnetic field. Numerical simulations of the Earth’s internal dynamo can, in fact, reproduce the field reversals that have taken place in the Earth’s field.

© 2001 by CRC Press LLC

earthquake

E

Eady, E.T. (1915–1966) English mathematician and meteorologist. Worked virtually alone in developing the theory of baroclinic instability during World War II.

Eady model A baroclinic geophysical fluid dynamic model that clearly illustrates the baroclinic instability process.

EAL See International Atomic Time.

Earth Third planet in the solar system from the sun, orbital semimajor axis 1.496 × 108 km, eccentricity 0.0167, polar inclination to the ecliptic 23◦.45; Mass 5.9736 × 1024 kg, radius 6126 km. Home of the human race. A planet with active plate tectonics, and frequently (by geologic standards) reversing magnetic field. The Earth has a dense but transparent atmosphere (approximately 1.013 bar at the surface). The atmosphere consists of 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide, and water and produces a greenhouse effect contributing about 40◦C to its average temperature of approximately 14◦C. Oceans cover 71% of the Earth’s surface, which acts as a thermal reservoir, and CO2 buffer. Earth is the only planet with a known active biosphere, extending from as much as 1 km into the Earth to 30 km above it. Photosynthetic organisms produce copious amounts of oxygen (O2) and remove CO2 from the atmosphere, using energy from the sun (G2 dwarf).

earth ellipsoid and flattening factor The figure of the Earth is approximately an oblate spheroid, symmetrical about its North-South axis. A spheroid is a special case of an ellipsoid, with two of its three principal axes (A, B, C) equal, viz: A = B > C. The flattening factor f = (A − C)/A, now approximately 1/298, slowly decreases, as tidal dissipation slows the Earth’s rotation. The “weighted mean axis” is (2A+C)/3. The “GCT Sphere” is used in map-

ping transformations that do not accommodate oblateness. Most models were determined by fitting a spheroid to surveys. Ground-based surveying methods depend on the plumb bob or its equivalent to find the local vertical, yielding not the spheroid, but the geoid. Some models were determined dynamically, by measuring the Earth’s gravity field through the analysis of spacecraft orbits. Among these, cases such as GEM-9 and GEM-10, with identical axes, were derived from different parent models, whose gravity fields differ in detail.

Earth Orbiter (EO-1) A spacecraft belonging to the New Millennium Program, intended to validate revolutionary technologies for future land imaging missions. Launched November 21, 2000, with three advanced land imaging instruments that collect multispectral and hyperspectral scenes in coordination with the Enhanced Thematic Mapper (ETM+) on Landsat-7. EO-1 flies in a 705-km circular, sunsynchronous orbit at a 98.7◦ inclination allowing it to match within 1 min the Landsat-7 orbit and collect identical images for later comparison.

earth orientation parameters (EOP) Coordinates of the Earth’s rotational pole as measured along the Greenwich meridian and the meridian 90◦ west. They are determined geodetically by very long baseline radio interferometry or satellite or lunar laser ranging.

earthquake The sudden movement of the ground caused by the release of stress along a fracture (or fault) within the lithosphere. Earthquakes usually occur along plate boundaries, although intraplate earthquakes do occur and probably represent the release of excess stress. When too much stress builds up within the planet’s lithosphere, the material will fracture and elastic waves carry the resulting energy throughout the planet. The actual location of the initial slip along the fault is called the focus of the earthquake; it is usually located at some depth below the surface. The location on the surface directly above the focus is called the epicenter of the earthquake. Energy released by an earthquake is measured in terms of magnitude on the Richter scale — an increase of one magni-

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earthquake intensity

tude represents a 32-fold increase in the energy released by the earthquake. The study of earthquakes is called seismology. Similar releases of energy are seen on other bodies, resulting in moonquakes, Marsquakes, etc.

earthquake intensity A measure of the local perceived strength of an earthquake based on the local damage that is done.

earthquake magnitude The Richter magnitude is a measure of the intensity of an earthquake. It is a logarithmic scale with an increase of one in magnitude corresponding to about a factor of 32 increase in energy (a factor of 10 in amplitude recorded on a seismogram). Earthquake magnitudes are instrumental in that they are obtained from earthquake seismographs. The table on page 141 relates the earthquake magnitude to earthquake intensity and the associated damage.

earthquake moment The earthquake moment M is defined by the relation

M = mdA

where m is the shear modulus in the rock where the earthquake occurs, d is the mean displacement across the fault during an earthquake, and A is the rupture area. The earthquake moment is empirically related to the earthquake magnitude.

earthquake precursor Any phenomenon that occurs prior to an earthquake that warns of its future occurrence. Examples are foreshocks, ground motion (uplift, tilt), electromagnetic signals, emission of gases (for example, radon), changes in the water table, and animal behavior. Although earthquake precursors have been documented in some cases, no reliable precursors to earthquakes have been found.

earthquake prediction	Predictions of earth-
quakes can be divided	into two classes:

(1) earthquake hazard assessment and (2) prediction of a particular earthquake. Hazard assessment provides an estimate of probability that an earthquake of a specified magnitude will occur in a specified region in a specified time interval. Hazard assessments are based on a number of observations including the number of smaller

earthquakes that occur in a region. Reasonably accurate hazard assessments are available. No reliable short-term predictions of actual earthquakes have been documented.

earthquake swarm A swarm of earthquakes that takes place, concentrating spatially and temporally. The relation among foreshocks, mainshock, and aftershocks is obscure, and an earthquake swarm does not include a remarkably large event (mainshock). Earthquake swarms tend to occur in markedly heterogeneous crustal structures such as a volcanic region and crush zone. Possible generation mechanisms are

(1) region of accumulated elastic strain is divided into blocks, and earthquakes take place in the respective subregions, or (2) earthquakes occur at the same place repeatedly during short period due to fast supply of energy. A remarkable example of earthquake swarm is the Matsushiro earthquake swarm in Japan (Maximum magnitude M 5.4), for which more than 60,000 felt earthquakes were observed for several years from 1965.

earth radius (RE) A widely used unit in measuring distances in the Earth’s magnetosphere, usually measured from the center of the Earth. (In other planetary magnetospheres, the planet’s radius is often used in a similar way.) The mean earth radius is 6,371,315 m. The average distance to the subsolar point of the magnetopause is about 11 RE, to that of the bow shock 14 RE, to the synchronous orbit 6.6 RE, to the moon about 60 RE and to the L1 and L2 Lagrangian points 236 RE.

earth rotation parameters (ERP) Earth orientation parameters and Universal Time.

earthshine Reflected light from the dayside of the Earth that illuminates the part of the moon that is not directly lit by sunlight.

East Australian current An ocean current flowing southward along the east coast of Australia.

East Greenland current An ocean current flowing southward along the east coast of Greenland.

© 2001 by CRC Press LLC