DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

adioisotopes
Radio Frequency Bands
Designation Band Frequency Wavelength
Extremely Low Frequency ELF 0.003 – 3 kHz 100,000 – 100 km
Very Low Frequency VLF 3 – 30 kHz 100 – 10 km
Low Frequency LF 30 – 300 kHz 10 –1km
Medium Frequency MF 300 kHz –3MHz 1km –100 m
High Frequency HF 3 – 30 MHz 100 – 10 m
Very High Frequency VHF 30 – 300 MHz 10 –1m
Ultra High Frequency UHF 300 MHz – 3GHz 1m–10 cm
Super High Frequency SHF 3 – 30 GHz 10 –1cm
Extra High Frequency EHF 30 – 300 GHz 1 cm –1mm
radioisotopes The isotopes of elements
which are unstable and change into other elements
via radioactive decay.
radio lobe Extended, often irregular and filamentary
region of radio emission observed at
the end of radio jets on opposite sides of the nucleus,
in powerful radio galaxies and quasars.
Radio lobes often extend beyond the optical image
of a galaxy and typically reach end-to-end
sizes of 50 kpc to 1 Mpc (the largest radio lobes
known span 9 Mpc). The radio spectral energy
distribution of lobes decreases sharply with increasing
frequency; lobes are said to be steep
spectrum sources in opposition to radio cores
which show a flat spectral specific flux distribution.
Emission is due to synchrotron processes,
as in radio cores, but produced by electrons with
lower energy. A prototypical lobe-dominated
radio source is the elliptical galaxy Centaurus A
(the brightest radio source in the Centaurus constellation),
with two lobes extending far beyond
the optical image of the galaxy, each ∼ 300 kpc
in projected linear size.
radiometer An instrument used to measure
radiant energy, as opposed to the number of photons.
radiometric dating A method of obtaining
absolute ages of geological specimens by comparing
relative concentrations of parents and
daughter elements for a particular radioactive
decay.
radiometry The science of the measurement
of radiant energy.
© 2001 by CRC Press LLC
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radio stars Stars whose radio emission is
strong enough to be detected from Earth. This
is rather rare and generally associated with extreme
youth (see stellar activity) or presence of a
companion (see RS Canum Venaticorum stars).
Radio stars are particularly important because
they permit coordinate systems on the sky determined
separately from radio and optical data
to be combined.
radius of deformation A horizontal length
scale defined as r = c/f, where c is the phase
speed of long gravity waves in the ocean or atmosphere,
and f = 2 sin θ is the Coriolis parameter
with being the angular velocity of
Earth’s rotation and θ latitude. For motion of
horizontal scales larger than the deformation radius,
rotation effects are dominant, whereas for
motion of smaller horizontal scales, rotation effects
are less important. On the equator, f vanishes
and the equatorial radius of deformation is
defined as rE = √ c/β where β = 2 cos θ is
the meridional gradient of the Coriolis parameter.
radon A radioactive colorless noble gas, Rn.
Atomic number 86, natural atomic number 222.
Melting temperature 202 K; boiling temperature
211.3K.Radon exhibits an orange to yellow fluorescense
in the solid state. Radon is produced
as a daughter element in α− decay of radium.
The longest-lived isotope is 222 Rn which has a
half-life of 3.825 days, much longer than that of
the other known isotopes ( 210 Rn to 221 Rn). All
isotopes decay by α emission; 210 Rn and 211 Rn
also have an electron capture branch, and 221 Rn
also has a β− decay branch.