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quay A wharf used for docking, loading, and unloading ships.

quenching See current saturation (cosmic string).

quicksand See liquefaction.

quiet sun The state of the sun when no sunspots, solar flares, or solar prominences are

taking place. Also refers to the regions of the sun outside the active regions containing no filaments or prominences. The quiet sun typically consists of regions of mixed-polarity magnetic field (“salt-and-pepper”), which are continuously being reprocessed in a series of flux cancellations and flux emergence. Quiet sun coincides with minimal magnetospheric storms and minimal solar wind flux on the Earth. The last quiet sun minimum occurred in 1989.

© 2001 by CRC Press LLC

radiation belts


R 136 See 30 Doradus.


Raadu–Kuperus configuration

See fila-



rad In describing the interaction of nuclear particles with matter, an amount of radiation equivalent to the deposition of an absorbed energy of 100 ergs per gram of the absorbing material.

radar An acronym for RAdio Detection And Ranging. Although early radar operated at a frequency of tens of Megahertz, current radars operate in the microwave portion of the electromagnetic spectrum and work by “shining” a pulsed or continuous beam of microwave energy at an object and measuring the time delay for energy reflected back. Radar is often referred to as L-Band (1–2 GHz), S-Band (2–4 GHz), C- Band (4–8 GHz), X-Band (8–12 GHz), and K- band (12–18, 27–40 GHz). These designations were assigned by the military during the early development of radar in WWII. Each designation has a specific application. Imaging radars are generally considered to include wavelengths from 1 mm to 1 m. Navigation radars make use of the longer wavelengths (lower frequencies).

radian A (dimensionless) measure of angle defined by the ratio of the length of the arc s on a circle contained within the angle θ, to the radius r of the circle:

θ = s/r .

Because a full circle has circumference of 2πr, the radian measure of a circle (360) is 2π. Hence,

1= 2π/360 radians 0.01745 radians ,


1 radian = 360/(2π)57.296.

radiance The radiant power in a beam per unit solid angle per unit area perpendicular to

the beam per unit wavelength interval [W m2 sr1 nm1].

radiation Any sort of propagation of particles or energy from a source. Examples include electromagnetic radiation (visible light, X-rays, gamma rays, radio, infrared, ultraviolet), neutrino radiation, gravitational radiation, and particle streaming (alpha radiation, beta radiation, neutron flux, heavy ion radiation).

radiation belts Regions containing substantial densities of energetic particles, typically electrons or protons, confined to the near magnetic equatorial regions of a planet, due to the planet’s magnetic field. Particle injection into the radiation belts is from solar wind and solarmagnetosphere interactions.

The Van Allen belts are the terrestrial example. Trapping occurs as a result of the Earth’s near-dipolar shape of the magnetic field characterized by magnetic field lines that converge at high latitudes resulting in an increased field strength compared with the equatorial region. The radiation belts were one of the first major discoveries made by satellites and are often called the “Van Allen radiation belts” after J. A. Van Allen. The Van Allen belts are divided into two regions, an inner region centered at 1.5 Earth radii at the equator, and an outer region centered at 3.5 Earth radii. Particle populations in the inner belt are very stable and long-lived, while the outer zone particle populations vary with solar and geomagnetic activity. Inner belt particles mostly have high energies (MeV range) and originate from the decay of secondary neutrons created during collisions between cosmic rays and upper atmospheric particles. Outer belt protons are lower in energy (about 200 eV to 1 MeV) and come from solar wind interactions. The outer belt is also characterized by highly variable fluxes of energetic electrons sensitive to the geomagnetic storms. During storm periods the particles may undergo radial and pitch angle diffusion propelling them toward Earth where they collide with exospheric or geocorona particles. The disturbed electrons can be precipitated into the middle ionosphere enhancing ionization and thus affecting radio propagation. The giant

© 2001 by CRC Press LLC

radiation cooling

planets (Jupiter, Saturn, Uranus, and Neptune) also carry such radiation belts, as does Mercury, but Venus and Mars have very weak magnetic fields and do not.

radiation cooling The transfer of heat energy away from an object by the net outward transport of radiation. In geophysics, at night, there is no incoming solar radiation, and the outgoing radiation from the Earth’s surface exceeds the incoming radiation. The loss of radiant energy lowers the temperature of the surface, and the radiational cooling in surface layers of the atmosphere often causes a strong inversion of lapse rate. In atmosphere radiational processes, the vertical variations are much larger than their horizontal variations. Thus, the radiation cooling rate CR can be expressed approximately by its vertical component. That is

CR = 1 ∂FN ρCp ∂h

where ρ is air density, Cp is specific heat at constant pressure, FN is vertical component of net radiation flux, and h is height.

radiation gauge In linearized (weak field) descriptions of gravity, one may use the re-

stricted coordinate transformations to achieve hαα = 0, h0µ = 0, and hαµ= 0 (where µ = 0, 1, 2 or 3) for the linearized gravitational potential hab in a source-free region. (The comma denotes partial derivative.) Gravitational potentials meeting these requirements are said to be in the radiation gauge. See linearized gravitation, summation convention.

radiation pressure The transfer of momentum to an object by radiation through the scattering, absorption, and emission of the radiation. The momentum transported by a single photon is given by p = hν/c where h is Planck’s constant, ν is the frequency of the photon, and c is the speed of light. The change in the momentum of a flux of photons as it is reflected (or absorbed) results in a net pressure being applied to the object. If the radiation is reflected, then the momentum transferred by the photon is twice the normal incident momentum.

radiation tide Periodic variations in sea level primarily related to meteorological changes, such as the twice daily cycle in barometric pressure, daily land and sea breezes, and seasonal changes in temperature. Other changes in sea level due to meteorological changes that are random in phase are not considered to be part of the radiation tides.

radiation zone An interior layer of the sun, lying between the core and the convection zone, where energy transport is governed by radiation. In the radiation zone of the sun, the temperature is a little cooler than the core, and as a result some atoms are able to remain intact. Their opacity influences the flow of radiation through this zone.

radiative-convective equilibrium An equilibrium state where the outgoing radiation would be equal to the absorbed radiation at all latitudes.

radiative transfer equation The linear integrodifferential equation that describes the rate of change with distance of the radiance in a collimated beam at a specified location, direction, and wavelength; the equation accounts for all losses (e.g., due to absorption and scattering out of the beam) and gains (e.g., by emission or scattering into the beam).

radio absorption Absorption occurring to radio waves as a result of interaction between an electromagnetic wave and free electrons in the ionosphere. See ionospheric absorption.

radioactive Containing isotopes that undergo radioactive decay to another isotope, or another element, by emitting a subatomic particle: typically electron (beta particle) or helium nucleus (alpha particle). In some cases, the radioactive nucleus undergoes an internal rearrangement and emits an energetic gamma ray (a photon), without changing its atomic number or atomic weight.

radioactive decay The process by which unstable atomic nuclei (radioisotopes) spontaneously break down into one or more nuclei of other elements. During this process, energy and subatomic particles are released. The radioac-

© 2001 by CRC Press LLC

radio galaxy

tive element is called the parent element, while the element produced by the decay is called the daughter element. There are three major processes by which radioactive decay occurs: alpha decay (loss of a helium nucleus, hence the atomic number decreases by two and the atomic weight decreases by four), beta decay (a neutron decays to a proton and electron, leading to an increase in the atomic number by one), and electron capture (where an electron is captured and combines with a proton to form a neutron, thus decreasing the atomic number by one). The half-life is the amount of time it takes for onehalf of the parent to decay into the daughter. The radioactive decay of isotopes of uranium, thorium, and potassium is responsible for the heating of the Earth’s interior.

radio burst Short, pulse-like radio emission.

1.Radio bursts or pulses can be emitted from astrophysical objects (radio pulsars) where the radiation is created as synchrotron radiation of streaming electrons in the pulsar’s magnetic field.

2.In the heliosphere, radio bursts can be observed on the sun, on Jupiter, and in interplanetary space. On Jupiter the source of the radio emission is the gyro-synchrotron radiation of electrons in the Jovian magnetosphere, and on the sun and in interplanetary space radio bursts result from the excitation of Langmuir waves

by streams of electrons. See metric radio burst, type i radio burst, i = I . . . V .

radio core Compact radio emitting region at the center of radio galaxies and quasars. Radio cores are usually unresolved at angular resolution > 0.1 sec of arc. At resolution of the order of 1 milliarcsecond, achieved with very long baseline interferometers, cores are resolved into one-sided jets, whose size is typically of the order of a parsec in nearby radio-galaxies. There is always a continuity with larger scale jets connecting the core to the radio lobes, although the jets often bend significantly passing to larger scale. The spectral energy distribution of a core is nearly constant over frequency in the radio domain, so that radio cores are said to be flat spectrum sources, with spectral index 0. Core-dominated (or equivalently, flat spec-

trum) radio sources do not show evident radio jets extended on scales of kiloparsecs, nor lobes.

radio emission: types I-IV Emissions of the sun in radio wavelengths from centimeters to dekameters. Radio emission is classified by four types:

1.Type I: A noise storm composed of many short, narrow-band bursts in the range of 50 to 300 MHz.

2.Type II: Narrow-band emission that begins in the meter range (300 MHz) and sweeps slowly toward dekameter wavelengths (10 MHz). Type II emissions occur in loose association with major flares and are indicative of a shock wave moving through the solar atmosphere.

3.Type III: Narrow-band bursts that sweep rapidly from decimeter to dekameter wavelengths (500 to 0.5 MHz).

4.Type IV: A smooth continuum of broadband bursts primarily in the meter range (30 to 300 MHz). These bursts are associated with some major flare events beginning 10 to 20 min after the flare maximum and can last for hours.

radio frequency interference (RFI) The degradation of a wanted radio signal due to the effect of an unwanted or interfering signal. Interference, unlike natural noise sources, is assumed to be manmade in origin and may be minimized by frequency management or, if the source can be identified, by negotiation. At night, RFI rather than natural noise is often the limiting factor for HF propagation.

radio frequency spectrum The electromagnetic spectrum extends from DC to gamma rays. The radio frequency spectrum is part of this spectrum. It is further split into a series of frequency bands. A frequency band is a continuous set of frequencies lying between two specified limiting frequencies and generally contains many channels. The radio frequency bands are listed in the table on page 388.

radio galaxy An active galaxy that radiates strongly in the radio. Instead of a point source, the emission comes from large radio lobes extending from jets that are located on either side of the parent galaxy, up to megaparsecs in scale.

© 2001 by CRC Press LLC

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