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Dictionary of Geophysics, Astrophysic, and Astronomy.pdf

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ﬁrst-order wave theory

the ﬁelds are correlated, whereas no correlation exists between ﬁelds belonging to different domains. After its creation, a bubble will expand at the speed of light surrounded by a “sea” of false vacuum domains. As opposed to secondorder phase transitions, here the nucleation process is extremely inhomogeneous and φ(x) is an abruptly changing function of time. See also Ginzburg temperature, GeV, Kibble mechanism, QCD, spontaneous symmetry breaking.

ﬁrst-order wave theory Also known as linear wave theory or Airy wave theory. The derivation involves assumption that terms of second and higher order are negligible.

fjord (also ﬁord). A long, narrow, typically deep inlet, connected to a sea. Generally found in mountainous regions at high latitudes (Norway, Alaska, New Zealand).

FK Comae stars A rapidly rotating, red giant. Because conservation of angular momentum during the expansion of a star from the main sequence to red giant stage should slow its surface rotation, the phenomenon must arise from the interaction between binary stars, possibly from the mergers of W Ursa Majoris binaries.

ﬂare A pronounced, transient increase in the sun’s global output related to sunspots and solar activity. Flares are generally manifested by a rapid (≈ 1 min) brightening in the red hydrogen line, emitted in the sun’s chromosphere, and a very sudden increase (a few seconds) in X-ray emission from that region. Energies up to 1025 J can be released over a time period of some minutes though much shorter events are known. White-light ﬂares, that is a brightening of the sun in visible light, are rare because even for the largest ﬂares the brightness is less than 1% of the total luminosity of the photosphere. In certain frequency ranges, e.g., at the wings of the black body, the intensity of the electromagnetic radiation can increase by orders of magnitude. Although the ﬂare is deﬁned as an outburst in electromagnetic radiation, large ﬂares are associated with large ﬂuxes of energetic ions from the sun, at times at energies of 1 GeV and more (see energetic particles) and the eruption of ﬁlaments, coronal mass ejections (see coronal mass ejec-

tions). Depending on the time-scales and the occurrence of coronal mass ejections, ﬂares can be classiﬁed as impulsive or gradual. The energy for these rapid events is taken from reconnection in the magnetic ﬁelds of the sunspot region near which the ﬂare occurs. Particle events in interplanetary space caused by these different kinds of ﬂares also show distinct properties. See gradual ﬂare, impulsive ﬂare.

Flares can be classiﬁed according to the size of the Hα emitting region (cf. table), the peak intensity in X-ray emission (SXR class), or the radio ﬂux at 5000 MHz (given in solar ﬂux units sfu).

	Flare Classiﬁcation
Class	Area	SXR	Radio
	(Hα)	Class	Flux
S	≤ 200	C2	5
1	200–500	M3	30
2	500–1200	X1	300
3	1200–2400	X5	3000
3+	>2400	X9	30000

In millionth of sun’s area with 1 millionth equal to 6.08 · 106 km2.

ﬂare electromagnetic radiation The electromagnetic radiation released in a solar ﬂare shows typical time proﬁles in different frequency ranges. In a large ﬂare, the ﬂare emission can be divided into three phases: (a) a precursor (also called preﬂare phase) lasting for some minutes to some 10 minutes, visible as a weak brightening of the ﬂare region in Hα and soft X-rays, indicating the heating of the ﬂare site.

(b) The impulsive or ﬂash phase in which electromagnetic radiation is emitted over the entire frequency range from γ -rays over X-rays, UV, the visible, IR up to radio waves. In this phase, the hardest part of the electromagnetic emission is most abundant, which indicates the acceleration of particles in addition to just a heating of the ﬂare site. The impulsive phase can be observed in most ﬂares, lasting for some minutes. In larger ﬂares it can be followed by (c) a gradual phase lasting for some 10 minutes to some hours during which the emission mainly occurs in Hα and soft X-rays. This emission

ﬂattening of the Earth

stems from the heated plasma at the ﬂare site, and does not indicate the presence of accelerated particles. Microwave and radio emission can also continue, evidence for energetic electrons trapped in closed magnetic loops.

1.Soft X-rays and Hα originate as thermal

emission in a plasma with temperatures of about 107 K. Most of the emission is continuum, lines of highly ionized O, Ca, and Fe are observed as well.

2.Hard X-rays are photons with energies between a few tens of keV and a few hundred keV generated as bremsstrahlung of electrons with slightly higher energies. Only a very small

amount of the total electron energy, about 1 out of 105, is converted into hard X-rays.

3.Microwaves are generated by the same electron population that also generates the hard X-rays as can be deduced from the similarities, in particular multiple spikes, in both intensity time proﬁles. Microwave emission is gyrosynchrotron emission of accelerated electrons.

4.γ -rays are the best indicators for the presence of energetic particles. The spectrum can be divided into three parts: (a) bremsstrahlung of electrons and, to a lesser extent, the Doppler broadening of closely neighbored γ -ray lines leads to a γ -ray continuum. (b) Nuclear radiation of excited CNO-nuclei leads to a γ -ray line spectrum in the MeV range. The most important lines are the 2.23 MeV line due to neutron cap-

ture in the photosphere and the 4.43 MeV line from the relaxation of excited 12C nuclei. These reactions require particle energies of some 10 MeV/nucl, thus the γ -ray line emission indicates the presence of very energetic nuclei at the ﬂare site. (c) Decaying pions lead to γ -ray continuum emission above 25 MeV.

5.Radio emission results from electron streams in the corona, exciting Langmuir oscillations. Frequencies are in the meter range (see metric radio emission). According to their frequency drift, radio bursts are classiﬁed as type I to type V metric bursts. See type i radio burst, i

=I . . . V.

ﬂare star A star whose brightness increases detectably for a few hours at irregular intervals due to ﬂare analogous to those in the sun (solar ﬂares), but much brighter. They are a signature of a star that is rotating rapidly and has a strong

magnetic ﬁeld, either because it is young or because it is in a close binary system. The most conspicuous ﬂare stars are of spectral type MV, both because the stars are intrinsically faint and because the convection zones are deep, producing stronger ﬁelds and brighter ﬂares.

ﬂaring angle On the magnetopause at a given point P on it — the angle χ (sometimes also denoted α) between the magnetopause surface and the ﬂow direction of the distant solar wind (or else, 90◦ − χ is the angle between ﬂow of the distant solar wind and the normal to the magnetopause at P). By the Newtonian approximation, the perpendicular pressure of the solar wind on the magnetopause at P equals p sin χ, where p is the dynamic pressure of the solar wind.

ﬂat ﬁeld, sky ﬂat, dome ﬂat A CCD’s pixel- to-pixel variation in sensitivity is called the ﬂat ﬁeld function. To make accurate measurements of the number of counts from objects on an image, this variation (which is a multiplicative factor) must be removed. This is done by making a ﬂat ﬁeld image, and dividing this image into all the data images.

There are a number of methods for obtaining a ﬂat ﬁeld image, and they are very dependent on the instrument, type of data, and ﬁlters used. Different ﬂat ﬁeld images must be obtained for each ﬁlter, as ﬁlters can modify the illumination across a CCD, and narrowband ﬁlters can produce interference fringes. A common method is to get a number of exposures of a blank screen in the telescope dome and average them together; these are called dome ﬂats. One may also get images of the twilight sky, or combine many data images. In these latter cases, one must be sure that no actual objects (stars, etc.) appear on the ﬂats. If these sky ﬂats are not of the same point in the sky, it is possible to combine them with a median ﬁlter and obtain a good ﬂat ﬁeld frame. Each data image is divided by an average ﬂat ﬁeld image of the same ﬁlter, etc.

ﬂattening of the Earth Because of its rotation the polar radius of the Earth is smaller than the equatorial radius by about 21 km. This leads to polar ﬂattening and an equatorial bulge.

ﬂat universe

ﬂat universe A model of the universe in which the average density is exactly equal to the critical density at which it neither expands freely nor collapses again. It is called ﬂat because at any instant the global geometry of space is Euclidean; space is ﬂat rather than curved.

ﬂexure Bending of a plate. In tectonics, the Earth’s lithosphere is subjected to ﬂexure in a variety of geological settings. Examples include the bending of the lithosphere prior to subduction at an ocean trench, bending under the load of a volcanic island, for example the Hawaiian Islands, and the bending associated with sedimentary basins.

ﬂoating breakwater A breakwater which does not rest on the seaﬂoor, but is instead anchored in place and has a buoyant force equal to its weight.

ﬂocculation The process in which suspended soil forms lumps during the mixing of freshwater with seawater, especially in estuaries. Flocculation occurs as the result of molecular attractive forces known as the van der Waals forces. These forces are weak, and are only signiﬁcant as clay particles are brought close together, such as during mixing turbulence. Normally, in fresh water clay minerals carry a negative charge which repels particles from each other. In saline waters, these charges are neutralized, and the attractive van der Waals forces dominate.

ﬂood current The tidal current that results when the water in bays and estuaries is lower than that in the adjoining sea. The opposite is referred to as an ebb current.

ﬂood delta A deposit of sediments immediately inshore of a tidal inlet, deposited by ﬂood tidal currents.

ﬂood shoal See ﬂood delta.

ﬂood tide See ﬂood current.

Flora Eighth asteroid to be discovered, in 1847. Orbit: semimajor axis 2.2015AU, eccentricity 0.1562, inclination to the ecliptic 5◦.8858, period 3.27 years.

Florida current Ocean current ﬂowing northward along the south-east coast of the U.S. from the Florida Straits to Cape Hatteras, feeding into the Gulf Stream. This current transports approximately 26 million cubic meters per second through the Florida Straits.

ﬂow regime A categorization of open channel ﬂow based on ﬂow speed and bed form. The lower ﬂow regime corresponds to lower ﬂow speeds and ripples or dunes on the bed. A transitional ﬂow is somewhat faster and will yield dunes, a planar bed, or antidunes. The upper ﬂow regime is faster and yields a planar bed, antidunes, or more complicated ﬂow and bed patterns.

ﬂuence The time integrated ﬂux.

ﬂuorescence The inelastic scattering process in which a photon is absorbed by a molecule and shortly thereafter (10−11 to 10−8 s) another photon of greater wavelengh is emitted; the emitted radiance is unpolarized.

ﬂutter fading Very rapid fading of radio signals in an ionospheric circuit (5 to 10 fades per second, or 5 to 10 Hz fades, and faster). Called ﬂutter fading because of the audible effect it has on signals. Fades can be deep, dropping below the local noise levels and resulting in the signal being drowned in the noise. It is a common problem at low latitudes where it is associated with equatorial spread F. Flutter fading is also observed at high latitudes on non-great circle paths, suggesting large ionization gradients, and near the auroral oval where it has been linked with slant-type sporadic E. See fading.

ﬂuvial Refers to processes or features produced by the ﬂow of some liquid, usually water. River channels are among the most common features produced by ﬂuvial processes. These features can be produced by surface ﬂow or by groundwater ﬂow. On Mars, large outﬂow channels have been produced by catastrophic surface ﬂoods while the smaller valley networks have been created by the sapping process (removal of groundwater, causing the overlying terrain to collapse).

focal length

ﬂux The rate of ﬂow of ﬂuid, particles, or energy through a given surface.

ﬂux density In radiation, a measure of ﬂux (power per unit area) per wavelength or fre-

quency interval. The unit of ﬂux density is called a Jansky and is deﬁned as 10−23 Joule s−1 m−2 Hz−1 .

ﬂuxgate A type of magnetometer widely used both for geophysical studies and aboard spacecraft. It utilizes the property of some magnetic materials to saturate abruptly at a welldeﬁned value of the magnetic intensity.

ﬂux, magnetic The magnetic ﬂux crossing a small area dA equals BndA, where Bn is the ﬁeld component perpendicular to dA. The ﬂux crossing a ﬁnite area is accordingly the integral B · dA over the area, and ﬂux density is magnetic ﬂux per unit area. The magnetic ﬂux through a surface is sometimes referred to informally as “the number of magnetic ﬁeld lines crossing it”.

ﬂux, particle Total ﬂux is the number of particles per unit area crossing a given surface each second. Differential ﬂux is the ﬂow of particles per steradian crossing a given surface from a particular direction.

ﬂux Richardson number The non-dimen- sional ratio Rf = Jb/JR expresses the rate of storage of potential energy in the stratiﬁcation of the water column (Jb, buoyancy ﬂux) relative to the rate of production JR of turbulent kinetic energy by Reynolds stress. The ﬂux Richardson number and the mixing efﬁciency γmix are related by Rf = γmix(1 + γmix).

ﬂuxrope A cylindrical body of magnetized plasma with twisted ﬁelds. Fluxrope topologies observed in interplanetary space have come to be known as magnetic clouds and are thought to be the interplanetary counterparts to coronal mass ejections.

ﬂuxtube The volume enclosed by a set of magnetic ﬁeld lines which intersect a simple closed curve. A way of visualizing magnetic ﬁelds, a tube whose surface is formed by ﬁeld

lines. The strength of a ﬂuxtube, F , is often deﬁned as the amount of ﬂux crossing an areal section, S, via F = S B · dS where dS is taken in the same sense as B to yield F ≥ 0. Fluxtube properties include: (i) the strength of a ﬂuxtube remains constant along its length, (ii) the mean ﬁeld strength of a ﬂuxtube varies inversely with its cross-sectional area, (iii) a compression of a ﬂuxtube increases the ﬁeld and gas density in the same proportion, (iv) an extension of a ﬂuxtube without compression increases the ﬁeld strength. Examples of ﬂuxtubes in the solar atmosphere are sunspots, erupting prominences and coronal loops, the Io/Jupiter system where Io injects ions into the Jupiter magnetosphere. Because of the localized source, these ions ﬁll up tubes following the Jovian ﬁeld lines to the poles, where they induce bright isolated auroral displays.

ﬂux tube See ﬂuxtube.

ﬂux tube volume See speciﬁc volume.

ﬂy’s eye A telescope adapted to wide angle sky searches, consisting of a set of ﬁxed mirrors with detectors at the focus of each mirror. Directional information is obtained from the ﬁeld of view of each reﬂector, but typically no attempt is made to obtain an image from each reﬂector.

ﬂywheel effect An exchange of momentum between the neutral and charged components of the ionosphere. If some magnetospheric source creates a strong electric ﬁeld E in a region in the ionosphere, the local plasma ﬂows with the electric drift velocity v, and collisions with neutral gas also impel that gas to move with some fraction of v. Like a ﬂywheel, the neutral gas stores some of its momentum and later, when E is decaying, its collisions help the ions maintain some of their ﬂow, creating in the process a secondary electric ﬁeld.

focal length The distance behind a converging lens (or the distance in front of a converging mirror) that rays arriving parallel from inﬁnity come together in a close approximation to a point (positive focal length); or the distance of the point in front of a diverging lens (or the distance behind a diverging mirror) from which

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