DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

heard as “whistles” with steadily falling pitch
that lasts for a few tenths of a second (short
whistlers) to several seconds (long whistlers).
Whistlers are generated by lightning sources or
atmospherics. The electromagnetic energy produced
in the lightning sources travels along the
Earth’s magnetic lines in the ionosphere almost
without any attenuation to the other hemisphere.
A part of this energy in the other hemisphere
is reflected back along the same magnetic lines
producing whistlers in a radio receiver in the
hemisphere of origin, and another part enters
the ionosphere and propagates in the Earth –
ionosphere waveguide mode to the hemisphere
of origin producing short whistlers. Since the
losses in the ionosphere are small, many echoes
may occur in what is known as an “echo” train
beforetheyarelostinthebackgroundnoise. The
changing pitch indicates that the path time “t”
along the magnetic line of force is related to the
frequency “f” by the dispersion relation
D = t √ f
where D is the dispersion and may range from
about 10 to 200 √ sec. Since t will depend on
the line of force along which the energy travels
and the electron density which decreases with
height, D is found to increase with increase in
electron density. The whistler properties can be
explainedbytheapplicationofthemagnetoionic
theory to low frequency wave propagation in the
ionosphere. Whistlers are used as a research
tool to study the electron density and associated
properties in the Earth’s magnetosphere.
whistler waves A low-frequency plasma
wave which propagates parallel to the magnetic
field at a frequency less than the electroncyclotron
frequency. Whistler waves are circularly
polarized, rotating in the same sense as the
electrons in the plasma. Whistlers are so-named
because of their characteristic audio frequency
tone.
white dwarf A star supported entirely by
electron degeneracy pressure, formed by contraction
when internal fusion energy sources are
exhausted.
white hole The time-reverse of a black hole,
i.e., a region of the spacetime that can only eject
© 2001 by CRC Press LLC
Wien’s displacement law
matter and radiation but can accrete none of it.
A white hole would be a naked singularity, with
a past event horizon, which does not shield it
from our view. Light from a white hole would
be blue-shifted. A white hole can be imagined
as a Big Bang taking place in a limited volume
of space, while the remaining part of the space
already exists. White holes are natural in inhomogeneous
models of the universe — they are
places where the Big Bang occurs later and was
still going on while it had already been completed
elsewhere. Since literally everything can
come out of these holes, thus strongly violating
causality, a cosmic censorship principle has
been invoked to avoid their existence in our universe.
There is currently no evidence that they
exist. See black hole, future/past event horizon,
naked singularity.
white light Solar radiation integrated over the
visible portion of the spectrum (4000 to 7000 Å)
to produce a broad-band (white light) signal.
Best used to observe the photosphere and also in
eclipse or coronagraph observations of the solar
corona.
white light flare A major flare in which compact
regions become visible in white light. Such
flares are usually strong X-ray, radio, and particle
emitters.
Widmanstä tten pattern A distinctive pattern
of crystal faces found in nickel-rich iron meteorites.
When an iron-nickel mixture is heated
to near melting and slowly cooled, two types of
crystals are formed: a nickel-poor phase (kamacite)
and a nickel-rich phase (taenite). The
pattern formed by the growth of these two crystal
phases can be seen when the meteorite is polished
and etched with acid.
Wien’s displacement law (Wien law) The
wavelength λm at which the maximum radiation
intensity (ergs/sec/cm 2 /steradian/wavelength
interval) of a black body radiator occurs is inversely
proportional to the absolute temperature
T , so:
Tλm = C,
where C is a constant, equal to 0.28978 cm-K.
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