DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

point object
point object Astronomical source that has
an angular diameter smaller than the resolving
power of the instrument used to observe it.
point spread function (PSF) In an optical
system, the apparent radiance due to an
unresolved Lambertian (cosine-emitting) point
source, as normalized to the source intensity
in the direction of maximum emission [m −2 ];
numerically equal to the beam spread function.
The PSF is the intensity distribution of a point
source as it appears on the detector. The PSF
is the result of any distorting factors induced
by the full optical path including that of the instrument
and environmental effects (e.g., atmospheric
seeing).
Poiseuille’s law The mean velocity (U) for
steady laminar flow through a uniform pipe
can be described as U=−(dp/dx)(D 2 /32µ),
where D is the diameter of the pipe, µ is the
viscosity of the fluid, anddp/dx is the pressure
decrease in the direction of flow. Poiseuille’s
law is derived from the Bernoulli equation, assuming
that elevation along the pipe is constant.
We can generalize the equation by allowing the
pipe to be inclined at an angle and adding a term
related to the elevation difference, giving
U=− d
p
ds ρg +z
D2ρg 32µ
whereds is the length of a short section of pipe,
ρg is the fluid weight, andz is the elevation difference.
Note that the term in brackets is the
hydraulic head h, a term used frequently in hydrology.
Poisson’s ratio When an isotropic elastic
object is subject to elongation or compressive
stressTz a strainεz results in the direction of the
stress. At the same time a strain arises in the
transverse directions,εx ,εy.Ifεz is an elongation,
thenεx =εy are compressive. The ratio
ν=− εx
εz
is called Poisson’s ratio.
=− εy
εz
polar cap The region surrounding one of the
poles of the Earth. In magnetospheric physics,
© 2001 by CRC Press LLC
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the region surrounding the magnetic pole, in
general the one inside the auroral oval. That
region is believed to be connected to “open”
field lines which extend to great distances from
Earth and which to all intents and purposes can
be considered to be linked to Earth at one end
only. The existence of polar cap precipitation
(infalling charged particles) suggests the other
end is linked to the interplanetary magnetic field.
polar cap absorption (PCA) At the polar
cusps, even solar energetic particles with relativelylowenergies(ingeneralprotonswithenergies
between about 1 MeV and 100 MeV) from
a solar flare or coronal mass ejection can follow
magnetic field lines to penetrate down to
the ionosphere and stratosphere into heights between
30 and 90 km. These particles lead to
an increased ionization, which in turn leads to
the absorption of radio waves that may last up
to days. Since this absorption is limited to the
polar regions, this phenomenon is called polar
cap absorption (PCA). Owing to the propagation
time of particles between the sun and the
Earth, a polar cap absorption generally starts a
few hours after a flare. In contrast, a similar effect
on radio waves on the dayside atmosphere,
the sudden ionospheric disturbance (see sudden
ionospheric disturbance), starts immediately after
the flare because it is caused by hard electromagnetic
radiation. Polar cap absorptions are
limited to a small latitudinal ring around the geomagnetic
pole. Part of the particles’ energy is
transferred to electromagnetic radiation in the
visible range, the polar glow aurora, a diffuse
reddish glow of the entire sky. Since the incoming
protons also influence the upper atmospheric
chemistry, in particular the NO production,
strong polar cap absorptions lead to a decrease
in the total ozone column at high latitudes.
polar cap arc (or sunward arc) A type of
aurora most often observed by satellite-borne
imagers. Polar cap arcs extend into the polar
cap, usually starting near midnight and stretching
sunward. Polar cap arcs can extend many
hundreds of kilometers or even all the way across
the polar cap, in which case they are known as
the theta aurora because the auroral configuration
seen by orbiting imagers — the auroral oval