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multiple ring basins

all be possible). This can lead to severe amplitude fading, and hence signal distortion, because of the small time delays (up to 1 msec and more for several modes) in radio signals traveling by different modes. The effects of this form of multipath may be minimized by choosing a working frequency that is supported on fewer modes, and by selecting antennas that favor one mode over others. A single ionospheric mode (e.g., 1-F mode) may encompass four separate paths: the ordinary and extraordinary paths and a high and low ray for each of these polarizations. This situation is common near the maximum useable frequency for the circuit and time delays of the order of 0.5 msec are possible. See fading.

multiple ring basins Very large impact basins often display a pattern of three or more complete or partial rings of mountainous materials. These multiple ring basins provide information on the thickness of the crust at the time the basin formed. Multiple ring basins are created by impact into a rigid surface layer (lithosphere) which overlies a more ﬂuid layer (asthenosphere). If the crater resulting from the impact is smaller than the thickness of the lithosphere, a normal impact crater is produced. However, if the impact crater is greater than the thickness of the lithosphere, the asthenosphere can ﬂow inward beneath the portion of the lithosphere which is pushed down by the impact. This ﬂow exerts a drag force on the descending segment of the lithosphere, which can cause fracturing in the lithosphere surrounding the crater. The number of rings in a multi-ring basin is related to the thickness of the crust at the time of basin formation — fewer rings are produced in a thicker crust, while many rings indicate a relatively thin crust.

multiple shock An earthquake composed of several spatially and temporally discontinuous

subevents. Most large earthquakes are multiple shocks. From detailed analysis of seismograms, we can ﬁnd that an earthquake source fault does not spread continuously and smoothly, but expands intermittently and heterogeneously. This kind of rupture pattern is considered to reﬂect the inhomogeneous structure of a source region.

mushy zone As a melt containing more than one component (e.g., a rock magma with diverse chemistry, or the ﬂuid of the Earth’s outer core, which is thought to be composed of iron, nickel, and some lighter elements) is cooled, the composition of the ﬁrst crystals to solidify are generally different from the bulk composition of the melt as a whole. As cooling continues, the chemical makeup of the material being solidiﬁed evolves, both because temperature of the melt is changing and because the chemical makeup of the remaining melt is itself changing (becoming more enriched in the material excluded from the ﬁrst crystals). These factors also come into play at the boundary between the Earth’s inner core and outer core: Between the depths where a parcel of outer core ﬂuid would start to crystallize and ﬁnish crystallizing, the boundary region may be a mixture of liquid and solid material, commonly referred to as a mushy zone. As neither the composition of the core nor the thermodynamic conditions at the inner core boundary are particularly well constrained, the existence and depth of the mushy zone are not known, but it may be a factor both for the seismic properties of the top of the inner core and the dynamic boundary conditions for the geodynamo at the base of the outer core. The crystallization of aqueous analogs reveals some of the nature of mushy zones: liquid circulates through the zone, creating organized structures such as channels called “chimneys”.

nanotesla (nT)

Nabarro–Herring creep When material is subjected to differential stress, the differential stress sets up a vacancy concentration gradient at the scale of grain-size, which causes spatial distribution of vacancy. Therefore, vacancy ﬂux will occur, producing creep. Creep caused by motions of vacancies through a self-diffusion mechanism is called Nabarro–Herring creep, which was ﬁrst proposed by Nabarro in 1948 and improved by Herring in 1950. Nabarro– Herring creep is described in the form of

ε = α DSD σ

d2 kT

where T is temperature, DSD is the coefﬁcient of self-diffusion, σ is the differential stress, d is the grain size, is the atomic volume, k is Boltzmann’s constant, and α is a numerical coefﬁcient depending on the grain shape and the boundary conditions for σ . It is a linear rheology (n = 1, n is the stress sensitivity of creep rate at steady-state stage) with a high sensitivity to grain-size (the strain rate depends on the grain size as ε 1/d2).

nadir The direction directly downward from the observer, as deﬁned by the local horizontal. A stone dropped from a bridge, for example, falls toward the nadir point. An Earth-orbiting spacecraft has two commonly used nadir directions: geocentric nadir, the direction towards the Earth center, and geodetic nadir, the direction of the normal dropped to the Earth ellipsoid. See also altitude, topocentric system, zenith.

nadir angle The angle between a given direction and the downward vertical direction.

Naiad Moon of Neptune also designated NVIII. Discovered by Voyager 2 in 1989, it is a small, irregularly shaped body approximately 29 km in radius. Its orbit has an eccentricity of 0.00033, an inclination of 4.74◦, a precession of 626◦ yr−1, and a semimajor axis of

4.82 ×104 km. Its mass has not been measured. It orbits Neptune once every 0.294 Earth days.

naked singularity In general relativity, a singularity that is not hidden inside a black hole. A singularity is a place where currently known laws of physics break down. Hence, it can, in principle, emit matter and radiation at unpredictable rates and in unpredictable amounts. Einstein’s equations do not automatically forbid the creation of any naked singularities (see cosmic censorship); examples of exact solutions of Einstein’s equations are known in which naked singularities exist. Whether naked singularities can be created during collapse of massive bodies is a question of active theoretical interest. There have been no observations suggesting astrophysical naked singularities. The only singularity of relevance to observational astronomy has thus far been the one that created our universe, i.e., the Big Bang. See Cauchy singularity, future/past event horizon, real singularity.

nakhlite One of a small class (only three are known) of meteorites, composed principally of pyroxene, a silicate containing calcium, iron, and magnesium. Nakhlites appear to be of Martian origin. Named for Nakhla, Egypt, the location of the ﬁrst reported fall in 1911.

nano-ﬂares Proposed small-scale impulsive energy releases in the solar atmosphere due to reconnection: the photosphere, as the top of the solar convection zone, is in continuous motion with bubbles rising and falling and plasma ﬂowing in and out. Frozen-in into this plasma, the magnetic ﬁeld is shufﬂed around as well. Thus, on small scales, magnetic ﬁeld conﬁgurations suitable for reconnection are likely to form, thereby converting magnetic ﬁeld energy into thermal energy. Observational evidence for such small-scale impulsive energy releases is found in the so-called bright X-ray points and in small-scale exploding EUV events. Nanoﬂares are suggested to play an important role in coronal heating.

nanotesla (nT) A unit of magnetism often used in geomagnetism. 1nT = 1.0×10−9 tesla, and is equivalent to 1.0 ×10−5 gauss. The older

narrow emission line galaxies

name for this magnetic ﬁeld strength was the gamma, where 1γ = 1 nanotesla = 10−5 gauss.

narrow emission line galaxies A broad, heterogeneous class of galaxies showing permitted and forbidden emission lines in their optical nuclear spectra. The term “narrow” is used in juxtaposition to Seyfert-1 galaxies, which show much broader permitted lines. The line proﬁles of narrow emission line galaxies, whose width is several hundred kilometers and may reach 1000 km, are broad for normal galaxies. The term narrow is used to distinguish these lines and lines whose full width at half maximum is several thousands kilometers, emitted in a region of active nuclei (the Broad Line Region) distinct from the Narrow Line Region. The strongest lines in the optical spectrum of the Narrow Line Region are the Balmer lines of hydrogen, the forbidden lines of oxygen twice ionized at 500.7 nm and 495.9 nm, and the lines from ionized nitrogen at 654.8 and 658.3 nm. From the presence of strong forbidden lines, it is inferred that the density of the Narrow Line Region gas must be 104 to 105 particles per cubic centimeter. The absence of variability and the observation of nearby active galaxies, where the Narrow Line Region is partly resolved, suggest that the Narrow Line Region spans distances in the range from a few parsecs to a few hundred parsecs from the galactic center. The excitation mechanism of the line emitting gas is probably photoionization by the radiation from the active nucleus; it has been suggested that mechanical heating might also play a role. In several nearby AGN, emission lines whose intensity ratios are similar to those observed for the Narrow Line Region extend far out from the nucleus, to a distance that can be a signiﬁcant fraction of the size of a galaxy, typically several kiloparsecs, in an exceptional case even 20 kpc. Since these regions appear resolved in long slit spectra, they are often referred to as the Extended Narrow Line Region.

Narrow emission line galaxies are also collectively referred to as type-2 active galactic nuclei, again in juxtaposition to Seyfert-1 galaxies. Narrow emission line galaxies include Seyfert- 2 galaxies or LINERs, and all other type-2 active galactic nuclei; among narrow emission line galaxies several authors also include galaxies

whose nuclei show spectra of HI regions, which are not type-2 AGN.

Narrow Line Region (NLR) A region of active galaxies, where narrow permitted and forbidden emission lines are produced. The term narrow is used to distinguish lines whose width is typically > 300 km s−1, hence already unusually broad for non-active galaxies, and lines whose full width at half maximum is several thousand km s−1, emitted in a region of active nuclei (the Broad Line Region) distinct from the NLR. The excitation mechanism of the line emitting gas is probably photoionization by the radiation from the active nucleus; it has been suggested that mechanical heating might also play a role. Images of several Seyfert-2 galaxies obtained with HST suggest that the morphology of the line emitting gas is closely related to radio plasma ejected from the nucleus. See narrow emission line galaxies.

Nasmyth universal spectrum Empirical energy spectrum of ocean turbulence in the inertial subrange. The spectrum is based on ocean turbulence data measured by Nasmyth in an energetic tidal ﬂow. This spectral form is considered characteristic for oceanic turbulence in various ﬂow regimes, and most oceanic turbulence measurements are routinely compared to Nasmyth’s universal form.

According to Kolmogorov’s hypothesis, the spectrum, , of turbulent kinetic energy in the inertial subrange depends only on the wavenumber k, viscosity ν, and the rate of dissipation of turbulent kinetic energy . Dimensional arguments then require that

(k) = 2/3k−5/3F k/kη

where F (k/kη) is a universal function. The function F (k/kη) was experimentally determined by Nasmyth and it is shown in the ﬁgure on page 329.

natural line broadening Minimum width

λ of a spectral line proﬁle which arises from the ﬁnite t lifetime of the state via Heisenberg’s uncertainty principle:

λ = 2π c t −1

λλ

nearly diurnal free wobble

	106
)	4
η10
F(k/k
spectrum	102
spectrum
Nasmyth	100
Nasmyth
	102
	104	103	102	101	100
	104	103	102	101	100
		normalized wavenumber k/kη

Universal spectrum of oceanic turbulence according to Nasmyth. The circles are the measured points and they are plotted against the cyclic wavenumber normalized by the Kolmogorov wavenumber kη.

and has a Lorentzian shape given by:

γ /4π2

φ( ν) =

γ /4π2 + ( ν)2

where γ is the radiation damping constant. In a description in terms of classical radiation theory,

γ = 8π2e2 , 3mecλ2

where e and me are the charge and mass of the electron, and c is the speed of light.

natural remanent magnetism Many volcanic rocks contain magnetic minerals. When these rocks cool from the molten state, they are permanently magnetized by the Earth’s magnetic ﬁeld at that time. This natural remanent magnetism provides information on the time evolution of the Earth’s magnetic ﬁeld and continental drift.

nautical mile	A unit of length equal to ex-
actly 1852 m.
nautical twilight	See twilight.
Navier–Stokes equations		The complicated

set of partial differential equations for the motion of a viscous ﬂuid subject to external forces.

neap tide The tide produced when the gravitational pull of the sun is in quadrature, i.e., at

right angles, to that of the moon. They occur twice a month at about the times of the lunar ﬁrst and last quarters. In these situations the difference between high and low tides is unusually small, with both the high tide lower and the low tide higher than usual.

NEAR A Near Earth Asteroid Rendezvous (NEAR) spacecraft that was the ﬁrst Discovery mission launched on February 17, 1996. NEAR took photographs of Comet Hyakutake in March 1996, ﬂew by C-asteroid 253 Mathilde on June 27, 1997, and S-asteroid 433 Eros on December 23, 1998, and also past the Main Belt asteroid Illya. It began orbiting S-asteroid 433 Eros in February 2000.

Its primary mission was to explore 433 Eros in detail for approximately 1 year. The mission will help answer basic questions about the nature and origin of near-Earth objects, as well as provide clues about how the Earth itself was formed. Studies were made of the asteroid’s size, shape, mass, magnetic ﬁeld, composition, and surface and internal structure. It achieved this using the spacecraft’s X-ray/gamma ray spectrometer, near infrared imaging spectrograph, multispectral camera ﬁtted with a CCD imaging detector, laser altimeter, and magnetometer. A radio science experiment was performed using the NEAR tracking system to estimate the gravity ﬁeld of the asteroid. Periapsis of the orbit was as low as 24 km above the surface of the asteroid. Although NEAR has no landing legs, on February 12, 2001 it was landed on the surface of Eros at a speed of about 1.9 m/s. It photographed on the way down, the last image from 120 m altitude. Remarkably it continued to function, after impact sending back γ -ray data.

nearly diurnal free wobble The retrograde motion of the Free Core Nutation (FCN) as viewed from points ﬁxed on the Earth. The wobble appears from the Earth to have frequencies of ωwobble = ωnutation − , where is the frequency of the Earth’s rotation. Very long baseline interferometry observations, which measure the nutation in space relative to ﬁxed stars, place the FCN frequency ωnutation ≈ 433.2 cycles/sidereal day.