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Great Red Spot

picture in the weak-field limit in which the graviton is the quantum mode of a polarized weak gravitational plane wave. This particle then is a spin-2 and zero-mass quantum, bearing a close kinship with the photon. However, the perturbational approach to quantum gravitation is nonrenormalizable, and approaches to quantization from a nonlinear viewpoint, or by embedding relativity in a larger theory (e.g., string theory), are being sought.

gravity According to Newton’s law of gravity, any mass m exerts a gravitational attraction g on any other mass which is given by

g = Gmm

r2

where r is the distance between the masses and the universal constant of gravity G is a fundamental constant in physics. The gravitational attraction of the mass of the Earth is responsible for the acceleration of gravity at its surface. In general relativity, gravity is the expression of the intrinsic curvature of the 4-dimensional spacetime.

gravity anomaly To a first approximation the earth is a sphere. However, the rotation of the earth causes it to deform into an oblate spheroid. The result is polar flattening and an equatorial bulge. The equatorial radius is larger than the polar radius by about 3 parts in 1000. Thus, the reference gravitational field of the Earth is taken to be that of an oblate spheroid. Gravity anomalies are the differences between the actual gravity field and the reference field.

gravity assist A way of changing the energy of a spacecraft by an encounter with a planet. When distant from the planet, the spacecraft is essentially in orbit around the sun. As it approaches the planet, it is best to consider the interaction in the planet’s frame of reference. In this case the spacecraft is approaching the planet at some velocity, and leaves in a different direction at the same speed. However, from the point of view of the solar system, the energy of the spacecraft has changed by the addition of the planet’s velocity. For instance, a satellite in an extreme elliptical orbit (essentially only radial motion) around the sun has the minimum en-

ergy possible to achieve a particular aphelion, and the negative binding energy is twice that of a circular orbit at that radius. If the satellite encounters a planet at the radius, it is essentially overtaken by that planet, and a parabolic orbit reverses the satellite’s motion with respect to the planet. Hence, after encounter, the satellite speed is twice that of the√planet in its orbit, and the satellite velocity is 2Vescape. Thus, after the one encounter the satellite can escape the solar system.

gravity wave A water wave that has gravity as its primary restoring force (as opposed to surface tension); waves with wavelength >1.7 cm are considered gravity waves.

grazing incidence optics An important means by which to image radiation at X-ray wavelengths. X-ray radiation entering a telescope is reflected at a very shallow angle (grazing incidence), by a special combination of hyperboloid and paraboloid mirrors, before being brought to a focus.

Great Attractor An as yet unidentified and somewhat hypothetical entity, probably a massive super-cluster of galaxies, whose existence is suggested by a large flow of galaxies toward an apex at galactic longitude ≈ 307◦, galactic latitude ≈ 9◦, and recessional velocity 4500 km s−1. The Local Cluster, as well as the Virgo Cluster and the Hydra-Centaurus cluster, are thought to be falling toward the putative Great Attractor, located at a distance from the galaxy which should be 3 to 4 times the distance to the Virgo cluster. The structure associated to the Great Attractor has yet to be identified, not least because the center of the Great Attractor probably lies close to the galactic plane, where gas and dust heavily obscure any extragalactic object. The very existence of the Great Attractor is debatable, since there is no consensus of the observation of infall motions toward the Great Attractor of galaxies located beyond it. See galactic coordinates.

Great Red Spot A long-lived atmospheric phenomenon in the southern hemisphere of the planet Jupiter with a distinctive orange-red color. It is an approximate ellipse approximately

© 2001 by CRC Press LLC

great salinity anomaly (GSA)

14,000 × 40,000 km across. The Great Red Spot has been in existence for at least 300 years; it was probably first observed by Jean Dominique Cossinin (1625–1712). The Great Red Spot is an anticyclone with counter-clockwise winds of up to 400 km/h.

great salinity anomaly (GSA) A low salinity event that first appeared in the late 1960s northeast of Iceland, then propagated counterclockwise in the northern North Atlantic and returned to the Norwegian, Greenland, and Iceland Seas in the late 1970s. The low salinity cap reduced the formation of various deep water masses in the North Atlantic. There is evidence that similar low salinity events — of smaller magnitude though — occurred both before and after the one in the 1970s.

Greenhouse Effect The enhanced warming of a planet’s surface temperature caused by the trapping of heat in the atmosphere by certain types of gases (called greenhouse gases; primarily carbon dioxide, water vapor, methane, and chlorofluorocarbons). Visible light from the sun passes through most atmospheres and is absorbed by the body’s surface. The surface reradiates this energy as longer-wavelength infrared radiation (heat). If any of the greenhouse gases are present in the body’s troposphere, the atmosphere is transparent to the visible but opaque to the infrared, and the infrared radiation will be trapped close to the surface and will cause the temperature close to the surface to be warmer than it would be from solar heating alone. In the case of Venus, the extensive amount of atmospheric carbon dioxide has created a runaway Greenhouse Effect with a surface temperature of 740 K. For the Earth, the net warming is about 17 K above the expected radiative equilibrium temperature of 271 K. Most climate models suggest that as more carbon dioxide is introduced into the atmosphere from the burning of fossil fuels, there is an enhanced Greenhouse Effect leading to global warming. This strongly suggests that there will be anthropogenically driven global warming becoming apparent in the next few decades, which would affect the Earth’s climate, and agricultural practices throughout the world. Mechanisms that take up carbon dioxide, by the oceans (where it is transformed into

carbonate rocks) and by plants (particularly the tropical rain forests), apparently have longer timescales than the current timescale for adding greenhouse gases to the atmosphere.

greenhouse gases The gases that can cause the greenhouse effect. Water vapor, carbon dioxide, ozone, methane, chlorofluorocarbons, and some oxides of nitrogen are greenhouse gases.

green line A coronal line observed at 5303 Å resulting from a forbidden transition in highly ionized iron atoms (Fe XIV). Important for the study of coronal structures at temperatures of order 2 MK.

Green’s theorem An integral of the divergence theorem. For two scalar fields, φ and ψ,

· (φ ψ) = φ 2ψ + φ · ψ ,

and

φ ψ · n = φ∂ψ/∂n .

Inserting this into the divergence theorem, constructing the similar expression with ψ and φ exchanged, and subtracting the two yields

φ 2ψ − ψ 2φ d3x

V

=[φ∂ψ/∂n − ψ∂φ/∂n] da ,

S

where S is the closed surface bounding the volume V , and a is the area element on S.

Greenwich mean (solar) time (GMT) A time system based on the angle from the Greenwich meridian to the “mean sun”, an artificial construct moving on the celestial equator at a constant rate with one revolution per year. The sun actually traverses the ecliptic, not the celestial equator, during the year, and does that at a nonconstant rate, as well, because of the eccentricity of the Earth’s orbit, and because the inclination of the ecliptic causes further variation in the motion of the projection of the sun’s position on the celestial equator. Greenwich mean solar noon is when this “mean sun” is on the meridian at Greenwich, England. Other times of the day were derived from noon using clocks or astronomical observations. GMT was split into Coordinated Universal Time (UTC) and Universal

© 2001 by CRC Press LLC

ground stroke

Time (UT1) on January 1, 1972. In the United Kingdom, GMT is sometimes used to refer to Coordinated Universal Time (UTC).

Greenwich sidereal date (GSD) The number of days of sidereal time elapsed at Greenwich, England since the beginning of the Greenwich sidereal day that was in progress at Julian date 0.0.

Gregorian The type of reflecting telescope invented by James Gregory, with a small concave secondary mirror mounted beyond focus in front of the primary mirror, to reflect rays back through the primary mirror, where they are viewed using a magnifying lens (eyepiece) from behind the telescope.

Gregorian calendar The calendar introduced by Pope Gregory XIII in 1582 to replace the Julian calendar and currently in civil use in most countries. Every year therein consists of 365 days, except that every year exactly divisible by 4 is a leap year aside from centurial years, which are leap years only if they are also exactly divisible by 400.

Greisen–Kuzmin–Zatsepin cutoff An energy limit at about 5 × 1019 eV per nucleon, beyond which, theoretically, cosmic rays have great difficulty in penetrating the 2.75◦ cosmic microwave background radiation (q.v.). In 1966, Kenneth Greisen (U.S.) and, independently, Vadem Kuzmin and Georgi Zatsepin (U.S.S.R.), pointed out that cosmic rays of very high energy would interact with the photons of the microwave background. This happens because, at their very high velocity, the cosmic rays find the photons colliding from in front (the most likely direction, of course) blue-shifted by the Doppler effect up to gamma ray energies. As a result, cosmic ray protons undergo pi-meson production, while other cosmic ray nuclides are photo-disintegrated. The largest distance from which cosmic rays of this high energy could presumably originate is 50 Mpc, quite local on the cosmic scale. Recent observations with the “Fly’s Eye” (Utah, U.S.), AGASA (Japan), and Haverah Park (England) detectors have, puzzlingly, established that the cosmic ray spectrum extends, basically unchanged, to energies

higher than 1020 eV. There are very few plausible sources within 50 Mpc, and the arrival directions seem isotropic, as well, deepening the puzzle.

grism (After the contraction of grating and prism.) A diffraction grating coated onto a prism. Grisms are instrumental to the design of highly efficient spectrographs devoted to the observations of faint objects. For example, in the Faint Object Spectrograph at the 3.6 meter William Herschel telescope on La Palma (Canary Islands), there is no collimator, and light allowed into the spectrograph by the slit directly illuminates a grism which is mounted on the corrector plate of a Schmidt camera to minimize optical elements, and consequently, light losses. The prism acts as a cross disperser, separating the first and second order spectrum produced by the diffraction grating, in a similar way as obtained with a second diffraction grating employed as a cross-disperser in an Echelle spectrograph. See diffraction grating, echelle spec- trograph.

groin (also groyne) A structure built perpendicular to the trend of the coastline, lying partially in and partially out of the water. May be built of stone or other materials (wood, steel sheet piles, sand bags). Generally placed in order to impede longshore transport of sediment.

gross photosynthetic rate The total rate of carbon dioxide fixation with no allowance for the CO2 simultaneously lost in respiration. [µmoles CO2 (or O2) (mg chl)−1 h−1] or [mg C (carbon) (mg chl)−1 h−1].

ground level event (GLE) In sufficiently strong solar flares, particles can be accelerated up to GeV energies. Given a suitable magnetic connection (see connection longitude) these particles can be detected on Earth as an increase in neutron monitor counting rates a few hours after the flare. Ground level events are rare and even during solar maximum only a few GLEs can be detected each year.

ground stroke A lightning stroke that flashes between a cloud and the ground, usually from

© 2001 by CRC Press LLC

groundwater

negative charges at the bottom of the cloud to positive charges on the ground.

groundwater Water within the pores of a soil. May be still or moving, and have a pressure corresponding to hydrostatic conditions, pressure flow (as in an artesian acquifer), or below atmospheric pressure, due to the effects of surface tension.

group velocity The speed at which the profile of a wave form (and its energy) propagates. Specifically, for waves in water, the group velocity reduces to one-half the wave phase speed

in deep water; wave phase speed and group ve-

√

locity are the same in shallow water ( gh per linear water wave theory, where g is acceleration of gravity and h is water depth).

growing season Generally, and vaguely, the interval between the last killing frost in Spring and the first killing frost in Autumn; or some suitable historical average of this period.

GSD See Greenwich sidereal date.

G star Star of spectral type G. Capella and our sun are G stars.

Guiana current An ocean current flowing northwestward along the northeast coast of South America.

guiding center The concept of the guiding center was introduced in the 1940s by H. Alfvén to describe the motion of particles in electromagnetic fields. It separates the motion v of a particle into motions v parallel and v perpendicular to the field. The latter can consist of a drift vD and a gyration ω around the magnetic field line:

v = v + v = v + vD + ω = vgc + ω .

If we follow the particle for a longer time period, for instance from the sun to the orbit of Earth, the gyration itself is of minor importance. In some sense the gyration can be averaged out. Then we can describe the particle motion by the motion vgc of its guiding center, consisting of a fieldparallel motion and a drift. The particle then is always within a gyro-radius of this position.

guiding center approximation (GCA) A regime of plasma behavior which applies to most plasmas in the Earth’s magnetosphere and in interplanetary space. In this regime, the motion of a particle can be separated (to a good approximation) into a rotation around a guiding field line of the magnetic field (“gyration”) and a sliding motion (like a bead on a wire) along that line. Modifications of the motion are introduced as added corrections, e.g., as guiding center drifts such as the gradient drift and the curvature drift. The instantaneous center of rotation is known as the guiding center and the instantaneous radius as the gyroradius.

Alfvén’s criterion states that the GCA is applicable if the gyroradius is much smaller than the scale distance over which the magnetic field B varies. The GCA does not apply to cosmic rays in the Earth’s field because their gyration radius is too big, nor to dense plasmas dominated by particle collisions, where collisions usually intervene before a particle manages to complete even one circle.

Guiding center motion is also known as adiabatic motion, because particles undergoing it in the Earth’s magnetic field conserve one or more adiabatic invariants, each associated with a nearly periodic component of the motion. The magnetic moment is associated with gyration around the guiding field line, the second invariant or longitudinal invariant is associated with the bounce motion between mirror point, and the third invariant is associated with drift around the Earth. The term “nonadiabatic motion” is frequently applied to motion near neutral points, lines, or sheets, where for at least part of the orbit the magnetic field is so weak (and the gyration radius so big) that Alfvén’s criterion no longer holds.

gulf stream Warm ocean current that flows up the eastern coast of North America. It turns eastward at about 40◦N and flows toward Europe.

Gunn–Peterson bound Limit on the abundance of neutral hydrogen in the intergalactic medium. Photons of wavelength 1216 Å present in the radiation emitted by a quasar at redshift z would be absorbed if neutral hydrogen is present in the interstellar medium. The quasar spec-

© 2001 by CRC Press LLC

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