DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

polycyclic aromatic hydrocarbons (PAHs)
entirely on spherical surfaces, while the poloidal
potentialP can only be zero on a spherical surface
if the radial component of v is also zero on
that surface. It can further be shown that the curl
of the toroidal term is poloidal, and the curl of
the poloidal term is toroidal.
polycyclic aromatic hydrocarbons (PAHs)
Flat molecules made primarily of carbon atoms
arranged in the graphitic form; in hexagons so
that their skeleton looks like chicken wire. Pollutants
on Earth, they are commonly observed in
meteorites, and are widely distributed throughout
the galaxy, accounting for 15 to 20% of the
galactic carbon. The UV absorption of these stable
molecules may be the source of the diffuse
interstellar bands, and their emission in the IR
is seen towards H II and star forming regions.
polynya An area of the ocean that persists
in being either partially or totally free of sea ice
under surface conditions where the sea would be
expected to be ice covered. Polynyas appear in
winter when air temperatures are well below the
freezing point of sea water and are bordered by
water that is covered with ice. They are typically
rectangular or elliptical in shape and tend to recur
in the same regions of the Arctic and Southern
Ocean. The size of polynyas can range from
a few hundred meters to hundreds of kilometers.
Polynyas form via two mechanisms, which
often operate simultaneously. In the first, ice
is continually formed and removed by winds,
ocean currents, or both. The latent heat of fusion
of the ice provides the energy necessary to
maintain open water. The second mechanism
requires oceanic heat to enter a region in quantities
sufficient to prevent local ice formation.
Polynyas are sites for active brine formation,
which may affect the local water density structure
and current field, which in turn modifies
large-scale water masses. They are also an interface
for gas exchange between the ocean and
atmosphere in polar regions. The large sensible
heat fluxes (along with fluxes due to evaporation
and longwave radiation) tend to dominate regional
heat budgets. They are also of biological
interest because their regular occurrence makes
them important habitats.
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polytropic process In thermodynamics, any
process that can be described by the statement
pV n = constant (for some fixed n). Here p
is the pressure in the experiment, and V is the
volume of the sample. Thus n= 0 yields p=
constant (constant pressure process), n = ∞
yieldsV= constant (constant volume process).
For a process at constant temperature in an ideal
gas, n = 1, and for a constant entropy process
in an ideal gas, n is the ratio of specific heat at
constant pressure to that at constant volume.
poorly graded sediment A sediment with
grain sizes that have a small standard deviation.
Most grains are very close to the median grain
size.
Pop I See Population I.
Pop II See Population II.
Population I Stars having a composition similar
to the sun. See also main sequence star,
metallicity.
Population II Stars significantly deficient,
relative to the sun, in elements beyond helium.
See also metallicity, main sequence star.
pore fluid pressure ratio The ratio of pore
fluid pressure to lithostatic pressure, defined as
λ = pf − ps
pl − ps
where pf is the pore fluid pressure, ps is pressure
at the surface, and pl is the lithostatic pressure,
the weight of the rock column above containing
the pore fluid.
poroelastic medium A porous medium that
deforms elastically under loading. A poroelastic
medium can be envisaged to consist of an
elastic matrix frame hosting an interstitial fluid.
The elastic moduli of the matrix frame are defined
under the drained condition, a situation in
which the pore fluid freely enters and exits the
medium such that the pore fluid pressure does
not affect matrix deformation. In a poroelastic
medium, strain and effective stress of the matrix
frame are governed by its constitutive law
(such as Hooke’s law for infinitesimal strain),