DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

electromagnetism The study of the relationships
between electric and magnetic fields, their
causes, and their dynamical effects.
electromagnetism, Lorentz transformation
and An important property of Lorentz transformations
is that the equations that describe the
relationship between electric and magnetic field,
their causes and effects, are Lorentz invariant.
It then follows that special relativity presents a
consistent theoretical groundwork for both mechanics
and electromagnetism.
electron An elementary particle in the family
of leptons, with mass of 9.1093897 × 10 −31 kg,
negative charge and spin of 1/2.
electron precipitation In solar physics, the
transport of an accelerated population of electrons
from the corona to the chromosphere
where they deposit the bulk of their energy. The
electrons can precipitate directly, immediately
following their production, or after a delay during
which they are trapped in the corona by converging
magnetic field. The pitch-angle of the
electrons, the nature of the field convergence,
and the collision rate in the corona are all important
in deciding which electrons precipitate
and when.
electron temperature The kinetic temperature
of electrons in an ionized gas.
electron volt The energy an electron (or proton)
gains when accelerated through an electric
potential difference of 1 volt. Equal to
1.602177 × 10 −19 J.
electrostatic unit A unit of charge defined
so that two equal such charges exert a mutual
electrostatic force of 1 dyne, when separated by
1 cm.
electrovacuum A space-time in general relativity
containing a source-free electromagnetic
field interacting with the gravitational field.
element A group of atoms, each with the
same number of protons, defines an element.
The number of protons in a given atom is called
the atomic number. There are 92 naturally oc-
© 2001 by CRC Press LLC
elemental abundances: of comet(s)
curring elements. Elements with over 100 protons
can be synthesized. Although an atom consists
of protons and neutrons in the nucleus surrounded
by clouds of electrons, it is only the
number of protons that defines the atom as belonging
to a particular element. Atoms of the
same element that have different numbers of
neutrons are known as isotopes of the element.
Atoms of the same element that have different
numbers of electrons are called ions.
elemental abundances: general The bulk
elemental composition of most solar system objects
are roughly the solar system average, but
contain distinct sub-components that are presolar.
Isotopic anomalies represent the only unequivocal
signatures of the survival of circumstellar
and interstellar materials within extraterrestrial
objects such as meteorites and interplanetary
dust particles (IDPs).
elemental abundances: in minerals Isotopic
anomalies in a variety of elements have
been used to identify a number of circumstellar
mineralogical species in meteorites. These
isotopic anomalies are thought to have a nucleosynthetic
origin, i.e., their carrier grains were
formed in the circumstellar environments of various
kinds of stars, survived transport through
the interstellar medium, and incorporation into
our solar system.
elemental abundances: in organics Significant
deuterium enrichments have been seen in
the organic components of both meteorites and
interplanetary dust particles. Unlike the isotopic
signatures in the circumstellar materials, denterium
anomalies do not have a nucleosynthetic
origin. Instead, it has been proposed that they
result from reactions at low temperatures in the
interstellar medium. In a few cases, the D/H
ratios of specific classes of organic compounds
have been measured, for example in PAHs in
meteorites and IDPs.
elemental abundances: of comet(s) The
overall elemental composition of Comet Halley
resembled the solar system average, but abundances
and deuterium enrichments of molecules
in cometary tails and comae, and the types of
solid materials observed in interplanetary dust
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