DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Pandora
domes. Most occur in the lowland plains far
from the largest shields, and often near the lowland
coronas. “Tick-like” features also show
a pattern similar to the pancake domes, both in
their spatial distribution and morphological similarities.
Thus, it is thought that many ticks are
modified dome features.
Pandora Moon of Saturn, also designated
SXVII. It was discovered on Voyager photos in
1980. Its orbit has an eccentricity of 0.004, an
inclination of essentially 0, and a semimajor axis
of 1.42 × 10 5 km. Its size is 57 × 42 × 31
km, its mass is 2.2 × 10 17 kg, and its density
0.71 g cm −3 . It has a geometric albedo of 0.9,
and orbits Saturn once every 0.629 Earth days.
Pandora is the outer shepherd of Saturn’s F ring.
Pangaea In geophysics, the single supercontinent
in which all the continental areas were
connected, and which broke up under continental
drift about 180 million years ago.
Papapetrou spacetimes (1953) Stationary
empty space-times for which the norm and curl
scalars of the Killing vector are in a functional
relationship. Their asymptotic expansion lacks
the mass monopole term, so they are in a sense
zero-mass gravitational fields.
PAR See photosynthetically available radiation.
paraboloid A figure of revolution based on
a parabola. The preferred shape for reflecting
telescope primaries because it brings all rays
parallel to the axis to a focus at the same unique
point.
parallax Motion of nearby stars against the
more distant background stars, as a result of the
different vantage points from the Earth’s orbit at
different times of the year. Essentially a baseline
of 2 AU (astronomical units) is used to give
a perspective on the distance to the star. See
astronomical unit, parsec.
parallel electric field The component E|| of
the electric field in a plasma parallel to the magnetic
field vector. Because charges can easily
flow along magnetic field lines, in many plasma
© 2001 by CRC Press LLC
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configurations E|| is expected to be virtually
zero. However, observations on auroral field
lines (e.g., that of beams of ions) suggest that a
non-zero E|| may exist there, balanced either by
the mirror force (see quasi neutral equilibrium)
or by turbulent plasma processes. The ambipolar
electric field, in which a relatively small E||
is balanced by gravity, is another example.
Parker limit A limit on the cosmological
flux of magnetic monopoles:
FM ≤ 10 −15 cm −2 · sec −1 .
Acceleration of magnetic monopoles in the presence
of a magnetic fieldB (like that of a galaxy,
for instance), makes the magnetic field energy
density decrease with time. The characteristic
decay time is
eB
τd = ,
4πnM vM
where e is the electric charge, nM the magnetic
monopole number density, and 〈vM 〉 their averagevelocity.
Asthetimenecessarytoregenerate
the magnetic field in a galaxy is comparable to
the galactic rotation period τr ∼ 108 years, the
limit results from the requirement that the decay
time τd be larger than τr (so that magnetic
monopoles do not affect galactic physics noticeably).
The maximum allowed flux is much less
than what is expected for monopoles formed at
the grand unified energy scale. See monopole,
monopole excess problem.
parsec The distance corresponding to a
parallax of one second of arc. A unit of distance
defined as the distance at which 1 astronomical
unit (AU) subtends an angle of 1 sec of arc. It is
equal to 206264.806 AU = 3.08568×10 16 m =
3.26166 light years.
partial derivative For a function f of multiple
independent variables xa , the derivative
with respect to one variable, say xi , computed
by treating the other independent variables xa ,
a = i constant. Notation:
∂f
∂xi partial pressure In any mixture of gases, of
each component gas, the pressure that the gas