cyclone

Generic shape of a string segment at the location of a

cusp.

the sunward boundary. In simple idealized models, the cusps contain neutral points, located on the magnetopause. The models predict that at those points field lines intersect, and the field intensity drops to zero. In the actual magnetosphere, the cusps are observed as regions of weak varying field.

The term polar cusp is also applied to the funnel-shaped regions extending from the above weak-field regions near the magnetopause to their footprints on the ionosphere (those regions are also sometimes referred to as the cusps). The polar cleft is the name applied to the same region by researchers who propose its shape to be slit-like rather than funnel-like.

cutoff energy Of cosmic ray protons at a given point P on Earth, the energy below which such protons can no longer reach P. For heavier particles, the cutoff energy can be derived from that of protons.

Proton orbits arriving at P can be trapped in the Earth’s magnetic field (as those of radiation belt protons are), or they can extend beyond the Earth’s magnetic field. All low energy proton orbits at P are trapped, while at sufficiently high energy, none are. The cutoff energy at P may be viewed as the energy below which all orbits at P are trapped.

Actually at any point P, the transition between trapped and non-trapped orbits is somewhat irregular and depends on direction. At high energy E, protons can arrive from any direction. At lower energies, some orbits arriving from the east are trapped and, therefore, empty of cosmic ray protons. As E decreases, the trapped/free boundary (its structure is com-

plex) expands across zenith until it reaches the western horizon, at the value of E below which all access is cut off.

cut-off rigidity Cosmic rays below the cutoff rigidity cannot penetrate down to the Earth’s surface but are reflected back towards space (see Störmer orbits). Since this shielding is due to the geomagnetic field, the cut-off rigidity depends on the geomagnetic latitude and the altitude of the observer: for normally incident protons the cut-off rigidity at the geomagnetic equator is about 15 GeV, that is all particles below 15 GeV are unable to reach the Earth, while the cut-off rigidity is only about 1.4 GeV at 58◦ geomagnetic latitude at sea level. In general, the cut-off rigidity Pcutoff is related to the geomagnetic latitude R c by

P cutoff = 14.9 GeV cos4 R c .

(1)

Thus, the magnetosphere acts as a giant mass spectrometer.

Historically, the term cut-off rigidity refers to the rigidity at which the lower part of the spectrum of galactic cosmic rays is cut off. The cut-off rigidity is also used to characterize the energy threshold of a neutron monitor.

cyclic coordinate In classical mechanics, a coordinate that does not explicitly appear in the Lagrangian (or in the Hamiltonian) for a system, though its velocity may be present. Via Lagrange equations, if pa = ∂L/∂q˙a (where the coordinate qa does not appear in the Lagrangian though q˙a does), then

d		∂L
	pa =		= 0 .
dt		∂qa

Hence, momenta conjugate to cyclic coordinates are constants and provide first integrals for the system. See first integral.

cyclone In meteorology, a 3-dimensional depression vortex system with closed cells and low central pressure. Its horizontal scale is from 200 to 3000 km. A cyclone has a characteristic pattern of wind circulation (counterclockwise in the northern hemisphere, clockwise in the southern). Mid-latitude cyclones are associated with the convergence of polar and tropical

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cyclongenesis

air masses along fronts. According to the structure of temperature and pressure, a cyclone can be classified as a barotropic cyclone (cold cyclone and warm cyclone), a baroclinic cyclone, or a neutral cyclone. Based on the distribution of geography, it can be classified as an extratropical cyclone, subtropic cyclone, or tropical cyclone. Since its lower level convergence can cause ascending air motion, cyclones are often accompanied by clouds and rain.

cyclongenesis A development of synopticscale weather disturbances.

cyclotron damping and instability In a collisionless plasma, damping or instability associated with the n = ±1 resonance, of importance in space physics and astrophysics as a mechanism of pitch-angle scattering of charged particles. See resonant damping and instability.

cyclotron frequency	See Larmor frequency.
cyclotron radius	See Larmor radius.
Cygnus A Nearby (z = .056, about 200Mpc

distant) active galaxy (3C 405) at RA19h59.4m, dec+40◦43 which is a strong radio source, with radio jets that extend for about 50kpc in either direction. The closest and second strongest radio galaxy. Recently detected to have a small quasar-like core.

Cygnus Loop Supernova remnant at RA20h 49m00s dec+30◦30 , 230 × 160 in extent. In the optical, a large filamentary loop, also strong in radio, visible in X-ray. Age of remnant is estimated at 20,000 years.

Cygnus X1 (Cyg-X1) A binary system at RA 19h56m22s.0, dec +35◦03 36 , at distance 2.5 kpc, consisting of the O9.7 supergiant HDE 226868 (Gies and Bolton 1986) and a compact object with an orbital period of 5.6 days. The mass of the unseen companion is significantly larger than 5M , probably as large as 12M . Cygnus X1 is the second brightest X-ray source in the sky, with X-ray emission exhibiting strong variability at time scales from milliseconds to years. It also radiates in γ -radiation. Because no theoretical models exist of compact degenerate (neutron) stars of mass exceeding 5M , the system is taken as the prototype of a black hole binary. In this model, the compact object of mass ≈ 12M is the putative black hole, accreting mass from the hot supergiant companion star which has overflowed its Roche lobe through an accretion disk which thermally emits X-radiation. Cygnus X1 is also a radio source, with radio flux correlated to X-ray output. Also called 4U 1956+35.

cynthion Of or pertaining to the moon.

Cytherean Venusian, referring to the planet Venus.

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dark matter

D

D layer The bottom layer of the Earth’s mantle, approximately 150 km thick. The name derives from K.E. Bullen’s assignment of letters to various layers in the Earth and the subdivision of these layers. D has a distinct seismic signature that distinguishes it from the overlying mantle. Since the mantle convects, and at least enough heat is extracted from the core to power the geodynamo, it seems reasonable to suppose that D is a thermal boundary layer between the bulk of the mantle and the core. However, it is also possible that it is chemically distinct from the bulk mantle, either through differentiation allowed by the elevated temperatures and perhaps partial melting at the bottom of the layer, chemical core-mantle coupling, or perhaps because all or part of D is composed of the dregs of dense plate that was once subducted from the Earth’s surface. There is seismic evidence of lateral variations in the properties of D , and consequently there has been speculation that it is a significantly heterogeneous layer perhaps composed of “crypto-continents” involved in some inverted form of tectonics at the base of the mantle. It is often suggested as the place of origin for some or all of the plumes which rise to the Earth’s surface and are manifested as hot spots such as Hawaii.

Dalton’s Law The additivity of partial pressures. Accurate for ideal gases, approximate law for mixtures of real gases. The total pressure is pi, where pi = kNiT /V and Ni is the number of molecules of type i, k is Boltzmann’s constant, T is the temperature, and V is the volume of the container of gas mixtures. Discovered by John Dalton, 1766–1844.

Darcy’s law An empirical law that governs the macroscopic behavior of fluid flow in porous media. It is credited to H. Darcy, who conducted experiments on the flow of water through sands in 1856. The law ignores the details of tortuous paths of individual fluid particles and defines

average flow rate per unit area (Darcy velocity) q as being related to fluid pressure p in the form of mass diffusion,

q = − κ · p

µ

where κ is the permeability (tensor) of the porous medium, and µ is the viscosity of the fluid. In application to groundwater hydrology, Darcy’s law is modified as follows to include the effect of Earth’s gravity:

q = − κ · ( p − ρg)

µ

where g is the gravitational acceleration vector, and ρ is fluid density. Darcy’s law applies only when flow is laminar within a porous medium and flow velocities are low enough that inertial forces are negligible and breaks down at very high flow velocities and possibly very low permeabilities. See Darcy velocity.

Darcy velocity (or specific discharge) The volume of fluid flow per unit time through a unit area of a porous medium. Darcy velocity q equals the average flow velocity v of fluid particles times porosity n. See Darcy’s law.

Darcy–Weisbach friction factor A dimensionless friction factor intended primarily for determination of head loss (energy loss per unit weight) of a flow in a full conduit, such as a pipe. The friction factor is a function of the Reynolds number for the flow and the relative roughness of the conduit.

dark cloud A part of the interstellar medium that emits little or no light at visible wavelengths and is composed of dust and gas that strongly absorb the light of stars. Most of the gas is in molecular form and the densities are of the order of 103 to 104 particles cm−3 with masses of 102 to 104 solar masses and sizes of a few parsecs. Dark cloud temperatures range from 10 to 20 K. See interstellar medium.

dark matter Matter component that does not radiate in the electromagnetic spectrum and, therefore, is not detected by means of telescopes. The first evidence of existence of large fractions of non-luminous matter came from the

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dark matter, cold

study of clusters of galaxies by Zwicky in the early 1930s. If clusters of galaxies form bound systems, the velocities of the member galaxies within the clusters are characteristic of cluster mass. This turns out to be about an order of magnitude larger than the sum of the luminous masses observed within the galaxies themselves. Since the 1970s it was known that there is a similar situation in the outer parts of spiral galaxies and in some elliptical galaxies. At that time it was assumed that the dark mass was ordinary (baryonic) matter in some not readily detectable form such as gas, low mass stars, planets, or stellar remnants (white dwarfs, neutron stars, and black holes). However, the nucleosynthesis bound limits B ≤ 0.1, while dynamical measurements suggest matter ≤ 0.3. To explain the discrepancy, the existence of a more exotic and yet undetected form of matter has been postulated. A wide class of dark matter candidates fall into two categories depending on their mean kinetic energy at high redshifts: cold and hot. The main difference lays in the behavior of the post-recombination power spectrum at galactic scales. See matter density perturbations, dark matter, cold and dark matter, hot.

dark matter, cold Dark matter made of particles with negligible random velocities. The standard cold dark matter model of structure formation assumes the dark matter particle contribution makes the universe flat. The primordial Gaussian density field is characterized by a power spectrum with a spectral index n = 1. Density perturbations that come within the horizon before matter-radiation equality (see thermal history of the universe) are frozen. As a result, the post-recombination power spectrum is modified and bends gently from n = −3 on subgalactic scales to the initial n = 1. The model had severe observational difficulties and several variants have been proposed: spectral index n ≤ 1, spectral index n ≥ 1, a 20% fraction of hot dark matter, and a 70% contribution of a cosmological constant, etc. Figure on page 113.

dark matter, hot Dark matter made of particles that are highly relativistic at early times. As the universe cools, their momentum is redshifted away and becomes non-relativistic. The standard example would be massive neutrinos.

Several extensions of the standard model of particle interactions suggest the existence of neutrinos with masses up to mν 90 eV, enough to equate the mean density to the critical density. The standard hot dark matter model, like its cold counterpart, assumes the primordial Gaussian density field is characterized by a power spectrum with a spectral index n = 1. The postrecombination spectrum is rather different. Perturbations that came within the horizon while the neutrinos were relativistic (of wavelength 30h−1 Mpc and smaller) are erased. Due to their large velocity, neutrinos diffuse out of the perturbation in a Hubble time. The final result is that while on large scales the power spectrum retains its original n = 1 spectral index, on galaxy and cluster scales it is exponentially damped.

The standard hot dark matter model is ruled out by observations. However, neutrinos could still have a smaller mass and give a contribution of 20 to 30%, the rest being made of cold dark matter and baryons. This model, termed mixed dark matter, seems to fit observations of large scale structure acceptably well. Figure on page 113.

dark nebula A nebula that can be seen because the dust within it obscures the light coming from stars or bright nebulae behind it.

dart leader In a lightning flash, a second flow of current from the cloud along the channel already opened by the stepped leader and the return stroke. The dart leader does not step; it rapidly and smoothly flows along the channel about 50 ms after the first flash.

data assimulation A process in which the observational data are modified in a dynamically consistent fashion in order to obtain a suitable data set for numerical model initialization in the weather predication and climate modeling.

Davidson current An ocean countercurrent flowing northward during the winter months between the California Current and the coasts of northern California, Oregon, and Washington.

day An interval of 86,400 seconds approximating one rotation of the Earth relative to the

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