DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

emissivity
cence of aromatic hydrocarbons pumped by UV
photons. Similarly, emission from both atoms
(i.e., sodium D lines) and fluorescence from
molecules (i.e., 3.4 µm, methanol) and fragments
(i.e., Swan bands ofC2 ∗) excited by sunlight
have been observed from cometary comae.
emissivity An indirect measure of reflectivity,
where emissivity = 1 − reflectivity and reflectivity
is the reflected energy measured from
a surface by a single energy bounce. Mirrored
surfaces reflect large amounts of energy (around
98%) but absorb very little (around 2%). A
blackbody surface, on the other hand, reverses
the ratio, absorbing 98% of the energy and reflecting
only 2%. Emissivity is affected by
the geometric shape of the object, the electrical
properties of the radiating surface, and the
measuring wavelength.
A Magellan radiometer experiment observed
the 12.6-cm-wavelength radiothermal emissivity
on Venus. The nominal pattern of radiothermal
emissivity shows high mountain summits
display abnormally low emissivity and plains regions
high emissivity. One explanation for the
low emissivity of mountaintops is the presence
of electrically conductive minerals, produced by
weathering, embedded in the surface rocks.
empirical model of the magnetosphere A
mathematical representation of the global magnetic
field of the magnetosphere, whose coefficients
are fitted to data. It is a convenient tool
for predicting the magnetic field vector or field
line linkage to the ionosphere which a satellite
at some point P in space is most likely to find.
The construction of empirical fields is also the
best way of extracting global information from
magnetic field data.
The parameters of modern empirical models
are generally derived from flexible representations
of the fields of the different magnetospheric
current systems, e.g., those of the magnetopause,
tail, ring current, and the Birkeland
current circuit. The field predicted at P depends
not only on the location but also on the tilt angle
ψ, on the dynamic pressure of the solar wind,
on the interplanetary magnetic field and on geomagnetic
activity indices such as Dst and AE.
© 2001 by CRC Press LLC
152
Enceladus Moon of Saturn, also designated
SII. Discovered by Herschel in 1789 its surface
has the highest albedo of any solar system body.
It also displays evidence of resurfacing, possibly
as a result of water volcanism. Since it is
too small to retain radioactive heat, tidal heating
aided by a 1:2 resonance with Dione may
provide the required energy. Its orbit has an eccentricity
of 0.0045, an inclination of 0.00 ◦ ,a
precession of 156.2 ◦ yr −1 , and a semimajor axis
of 2.38 ×10 5 km. Its radius is 250 km, its mass,
8.40 × 10 19 kg, and its density 1.28 g cm −3 .It
has a geometric albedo of 1.0, and orbits Saturn
once every 1.370 Earth days.
Encke’s comet Comet with the shortest
known orbital period: 3.30 years. Its orbit has
semimajor axis 2.21 AU and perihelion distance
0,338 AU.
endothermic A process that absorbs heat as
it proceeds. Opposite of exothermic.
energetic particles Supra-thermal particles;
that is all particles with speeds large compared
to the thermal plasma speed.
energetic particles in interplanetary space
Energetic particles in interplanetary space can
be observed with energies ranging from the
supra-thermal up to 10 20 eV. The main constituents
are protons,α-particles, and electrons;
heavier particles up to iron can be found in substantially
smaller numbers. The particle populations
originate in different sources, all having
their typical spectrum, temporal development,
and spatial extent.
1. Galactic cosmic rays (GCR) are the highenergy
background with energies extending up
to 10 20 eV. They are incident upon the heliosphere
uniformly and isotropically. In the inner
heliosphere, the galactic cosmic radiation
is modulated by solar activity: the intensity of
GCRs is highest during solar minimum and reduced
during solar maximum conditions. See
modulation of galactic cosmic rays.
2. Anomalous galactic cosmic rays (AGCR),
also called anomalous component, energetically
connect to the lower end of the galactic cosmic
rays but differ from them with respect to composition,
charge states, spectrum, and variation