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Dictionary of Geophysics, Astrophysic, and Astronomy.pdf

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extratropical storm

for the expansion of the universe is the redshift of light received from distant sources, ﬁrst discovered by E.P. Hubble in 1929 (see Hubble parameter). Expansion, in turn, is the evidence that the universe is evolving (see steady-state models) from a state of higher density toward lower density. Extrapolation of this phenomenon backward in time leads to the notion of the Big Bang. The rate of expansion depends on the value of the cosmological constant ;. With ; = 0, the rate of expansion is constantly being decreased by gravitational attraction between any two massive objects. The Friedmann–Lemaître cosmological models predict the following possible scenarios for the future fate of the universe when ; = 0. If the mean matter-density in the universe, ρ¯, is at present lower than the critical density ρ0 ≈ 10−29g/cm3, then the decelerating force is small and the universe will go on expanding forever. If ρ¯ > ρ0, then the expansion will be completely halted at a certain moment in the future and followed by collapse toward the ﬁnal singularity; this collapse is the timereverse of the Big Bang. The determination of the present value of ρ¯ is thus of crucial importance and is currently one of the main (unsolved) problems of cosmology. Certain vigorously advertised theories are critically dependent on ρ¯ being equal to ρ0, but unquestionable observations imply only that ρ¯ ≥ 0.2ρ0.

experimental craters A means of observing craters as they form, during which the experimental conditions are changed in order to determine the effects of different parameters. Results show that the impact process consists of three formation stages: compression, excavation, and modiﬁcation, and that the strength of the impact target inﬂuences crater shape and diameter. For example, a low target strength reduces the threshold diameter for the formation of complex craters.

Experimental studies are usually of two types. In the ﬁrst, a small projectile is ﬁred (at several kilometers per second) from a light gas gun toward a target in a vacuum chamber. Very high-speed cameras photograph the craters as they form. Scaling over several orders of magnitude is then applied to extrapolate the results (this introduces some error since crater formation is dependent on size). In the second method,

explosive charges (such as nuclear or chemical explosives) are used. This method reduces the scaling problem but introduces a gas acceleration stage that does not exist in natural impacts and causes a slightly different ﬁnal impact morphology.

extended object Astronomical source that has an angular diameter larger than the resolving power of the instrument used to observe it.

extensions of space-times The omission of some region from a space-time may be detected by the appearance of incomplete geodesics. These are geodesics for which the afﬁne parameter does not assume all real values. Thus, an observer moving along an incomplete timelike geodesic will reach the boundary in a ﬁ- nite time. The extension of a space-time will produce a spacetime that contains the original spacetime (is isometric to it where they overlap). In the presence of singularities, inextendable geodesics will occur. See maximal extension of a space-time.

extinction and reddening Extinction of light equally at all visible wavelengths can be caused by dust in interstellar clouds between the observer and the continuum source. Grains of all sizes contribute to absorption and scattering of light, resulting in a general obscuration of the continuum light. However, most interstellar dust grains are slightly smaller than visible wavelengths. Reddening is a selective extinction of bluer wavelengths due to scattering off of these small dust grains. The extinction of the bluer light causes the object to appear redder than it should compared with the predicted temperature based on its spectral type. Reddening increases the color index of a star (see color index, color excess).

extratropical storm A storm that occurs when cold, denser air moving toward the equator meets the warmer, more humid air moving away from the equator. A circulation results that can cause high winds over a large area. It is more common in winter when temperature gradients between the equatorial regions and the higher latitudes are stronger.

extreme ultraviolet (EUV)

The portion of

the electromagnetic spectrum in the wavelength range from approximately 100 to 1000 Å.

extreme ultraviolet imaging telescope (EIT)

Telescope aboard the SOHO spacecraft which obtains images of the sun at extreme-ultraviolet wavelengths. The EIT is able to image the solar transition region and inner corona in four selected bandpasses in the EUV, namely, Fe IX/X at 171 Å, Fe XII at 195 Å, Fe XV at 284 Å, and He II at 304 Å. The EIT is used to image active regions, ﬁlaments and prominences,

coronal holes, coronal “bright points”, polar plumes, and a variety of other solar features using either full-disk or sub-ﬁeld images.

eye (of a storm) In strong tropical cyclones, the central region of the storm is often relatively calm, sometimes with a clear cylindrical appearance with blue sky overhead and storm clouds rising all around it. Winds are generally 10 knots or less, and no rain occurs. Sizes may range from 6 to 60 km in diameter, with most frequent size of order 20 to 40 km.

fallout

F1-layer Relatively unstable ionospheric layer at a height of between about 140 and 200 km which ﬁlls the gap between the E-layer and the F2-layer. Ionization is due to EUV radiation. Formally, the F1-layer can be described by a Chapman proﬁle.

F2-layer Uppermost ionospheric layer at heights of between 200 and about 1000 km. In addition to local ionization and recombination, large-scale transport processes inﬂuence the chemistry and charge density. In addition, vertical transport leads to a separation of different particle species and charge states. Despite already reduced particle densities, electron densities are higher than in the other ionospheric layers. The dominant particle species is ionized atomic oxygen O+. N+ and N+2 are dominant in the lower parts, while in the upper part of the F2layer ionized helium He+ and atomic hydrogen H+ become dominant.

Faber–Jackson law An empirical relationship between the total luminosity and the central velocity dispersion of elliptical galaxies: L σ 4, i.e., the galaxy luminosity is proportional to the fourth power of the velocity dispersion σ . This law was ﬁrst discussed by S.M. Faber and R.E. Jackson in 1976. Since the velocity dispersion can be measured from the broadening of absorption lines in the galaxy spectrum, the Faber–Jackson law can, in principle, be used to determine the luminosity and, once the apparent magnitude of the galaxy is measured, to derive the distance of the galaxy. The Faber– Jackson law is analogous to the Tully–Fisher law for spiral galaxies: The stellar velocity dispersion substitutes the HI rotational width, since there is little atomic gas in early-type galaxies. See Tully–Fisher law, velocity dispersion.

facula A bright region of the photosphere seen in white light at the solar limb. Faculae are

the manifestations of the supergranule boundaries, seen at the limb.

fading Fading is a common characteristic of radio propagation and is typiﬁed by aperiodic changes in the received signal amplitude and other signal characteristics. The depth of fading can be described by an amplitude probability distribution and the rapidity of the fading may be described by autocorrelation functions. These statistics are required to make allowances for fading in system design. Understanding the properties of the fading may also be important in constructing systems to capitalize or minimize the effects. Fading on ionospheric propagation paths can arise from sources that alter the properties of a single path (e.g., absorption, ionization changes resulting in pathlength changes or skip distance changes, ionization irregularities near the reﬂection point) and more commonly from multiple propagation paths (e.g., interference fading, polarization fading). Understanding the source of the fading may allow some control over its effects (e.g., skip fading may mean using a lower operating frequency; absorption fading may mean a higher frequency is required). See ﬂutter fading, ionospheric radio propagation path, multipath fading.

failed arm See aulacogen.

faint young sun paradox As the sun ages, the fusion processes at its core gradually intensify, and models predict that the sun has increased brightness by about 35% over the last 4 billion years. The paradox is that one expects the Earth to have been completely icebound, but geological evidence shows liquid water at least that far into the past. A resolution has been proposed in terms of intense greenhouse effect, driven by ammonia and hydrocarbon haze, which could have maintained the surface above freezing.

Falkland current An ocean current ﬂowing northward along the coast of Argentina, between Argentina and the Falkland Islands.

fallout Term describing airborne dangerous particles that deposit from the atmosphere after days, weeks, or longer in suspension in the air. Usually referring to radioactive particles which

fall speed (velocity)

can be lofted into the air by nuclear explosions, or via leaks from nuclear power plants, or by ﬁre associated with catastrophic failure in a nuclear installation.

fall speed (velocity) The equilibrium speed reached by a sediment particle falling in a column of still water. Governed by the size, shape, and density of the particle, as well as any cohesive forces between particles, and the density and viscosity (and thus temperature) of the ﬂuid. Used for classiﬁcation of particle size and assessment of the mobility of the sediment.

Fanaroff–Riley (FR) class I and II radio galaxies Lobe-dominated radio galaxies

whose luminosity at 178 MHz is below (class I) or above (class II) 5 × 1032 ergs s−1Hz−1.

In 1974, B.L. Fanaroff and J.M. Riley noted a dichotomy in the radio morphology of radio galaxies with a sharp threshold luminosity: FR I type galaxies, of lower luminosity, show smooth two-sided and poorly collimated jets and edgedarkened lobe structures. FR II type galaxies, of higher luminosity, show edge-brightened lobes, often with prominent hot spots at the inner end of the lobes, connected to the nucleus by pencillike jets that are usually very faint, and in several sources not visible at all.

Faraday Cup A detector for low-energy plasma, absorbing incident ions or electrons and measuring the rate at which their electric charge is deposited.

Faraday effect The rotation of the plane of plane-polarized light by a medium placed in a magnetic ﬁeld parallel to the direction of the light beam. The effect can be observed in solids, liquids, and gasses.

Faraday rotation When propagating through the ionosphere, a linearly polarized radio wave will suffer a gradual rotation of its plane of polarization due to its interaction with the ionization in the ionosphere in the presence of the Earth’s magnetic ﬁeld. The magnitude of rotation depends on the electron density and the inverse square of the frequency. Typically, for a total electron column of 1016 electrons m−2 and a frequency of 1 GHz the Faraday rotation is

0.01 radian or 0.57◦ and for a total electron column of 1018 electrons m−2 the Faraday rotation is 1 radian or 57◦.

fast magnetohydrodynamic shock A fast magnetohydrodynamic shock forms when a fast magnetohydrodynamic wave steepens. The magnetic ﬁeld increases from the upstream to the downstream medium and is bent away from the shock because the normal component of the ﬁeld is constant. In contrast, in a slow magnetohydrodynamic shock, the magnetic ﬁeld is bent toward the shock normal. The normal component of the upstream (downstream) ﬂow speed is larger (smaller) than the propagation speed of fast magnetohydrodynamic waves and both upstream and downstream ﬂow speeds exceed the Alfvén speed. Traveling interplanetary shocks in general and planetary bow shocks always are fast magnetohydrodynamic shocks.

Change in magnetic ﬁeld direction across a fast and a

slow shock.

fast shock wave See hydromagnetic shock wave.

fast solar wind Solar wind escaping from the open magnetic ﬁeld regions of the coronal holes. Plasma speeds range from 400 km/s to 800 km/s, densities are low with about 3 ions/cm3 at Earth’s orbit. Particles are mostly hydrogen; helium amounts to about 4%. Electron and proton temperatures are 1 × 105 K and 2 × 105 K, respectively. Fast solar wind streams are remarkably stable and variations from stream to stream are small. Despite the differences in individual parameters, the average momentum and total energy ﬂux are similar in fast and slow solar wind streams.

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