DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Electra
Electra Magnitude 3.8 type B5 star at RA
03 h 44 m , dec +24.06’; one of the “seven sisters”
of the Pleiades.
electric drift See drift, electric.
electric field, parallel See parallel electric
field.
electric regime (cosmic string) It has been
known since the seminal work of E. Witten in
1985 that a current can build up in a cosmic
string. As current generation proceeds via random
choices of the phase of the current carrier,
the resulting current can be of two distinct kinds,
timelike or spacelike, depending on whether its
time component is greater or smaller than its
space component (there is also the possibility
that they are equal, leading to a so-called lightlike
current, but this is very rare at the time of
current formation and would only occur through
string intercommutation). Explicitly, setting the
phase of the current carrier as a function of the
time t and the string coordinate z in the form
ϕ=ωt −kz ,
withω andk arbitraryparameters, onecandefine
a state parameter through
w=k 2 −ω 2 ,
the case w 0) corresponding
to a timelike (respectively, spacelike)
current.
In the timelike case, the configuration is said
to be in the electric regime, whereas for spacelike
currents it is in the magnetic regime. The
reason for these particular names stems from the
possibility that the current is electromagnetic in
nature, which means coupled with electric and
magnetic fields. Then, for the electric regime,
one can always find a way to locally remove the
magnetic field and thus one is led to describe
solely an electric field surrounding a cosmic
string. The same is true in the magnetic regime,
where this time it is the electric field that can be
removed and only a magnetic field remains. See
current carrier (cosmic string), current generation
(cosmic string), intercommutation (cosmic
string).
© 2001 by CRC Press LLC
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electroglow A light emitting process in the
upper atmospheres of Jupiter, Saturn, Uranus,
and Saturn’s satellite Titan. Sunlight dissociates
some H2 and ionizes the hydrogen; the electrons
are accelerated and interact with H2, producing
the glow.
electromagnetic current meter A device
that uses Faraday’s Law of magnetic induction
to measure flow velocities. The current meter
head establishes a magnetic field. A moving
conductor (water) creates an electrical potential
that is measured by the instrument. The electrical
potential is proportional to the speed of the
current. Often used for one- or two-dimensional
velocity measurements.
electromagnetic induction This is the generation
of currents in a conductor by a change in
magnetic flux linkage, which produces a magnetic
field that opposes the change in the inducing
magnetic field, as described by Faraday’s
laws and Lenz’s law. In geophysics, this can
be used to study the conductivity of the mantle
through measurement of its response to magnetic
fluctuations originating in the ionosphere
and/or magnetosphere. Maxwell’s equations
can be used to show that in the appropriate limit,
a magnetic field obeys a diffusion equation:
∂B
∂t =−∇×
1
µ0σ ∇×B
where σ is the local conductivity. Fluctuating
external fields of frequency ω diffuse into
a layer at the top of the mantle with a skin depth
√ 2/µ0σω. The induced field is then essentially
a reflection of this external field from the skindepth
layer, and it has a phase relative to the external
field that relates the conductivity structure
of the layer to the frequency of the signal (essentially,
long period signals penetrate deeper and
therefore depend more on deeper conductivity
than do short period signals).
electromagnetic radiation Radiation arising
from the motion of electric charges, consisting
of variations in the electric and magnetic
fields, and propagating at the speed of light. Depending
on the wavelength, observable as radio,
infrared, visible light, ultraviolet, X-rays,
or gamma rays.