DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

displacement of air and water masses cause an
approximately circular motion about the mean
rotation pole. The mean pole itself has an irregular
drift in a direction ∼ 80 ◦ west. Thus,
the wobble is centered at about 3.4” west of the
CTRS pole as of this writing. Worldwide observations
of polar motion of the CTRS pole are
reduced and provided to the public by the International
Earth Rotation Service.
polar plume Bright ray-like solar structure
of out-flowing gas which occurs along magnetic
field lines in coronal holes. Plumes deviate noticeably
from the radial direction and tend to angle
towards lower latitudes as expected if they
follow the global solar magnetic field. Most
prominent at times near solar minimum.
polar wander See polar motion.
polar wind In the polar regions of the Earth
where the geomagnetic lines of force are open,
the continuous expansion of the ionospheric
plasma consisting of O + ,H + ,He + leads to supersonic
flow into the magnetospheric tail. This
is known as polar wind and is analogous to the
solar wind from the sun. This supersonic flow is
reached beyond about 1500 km for lighter ions
depending upon the plasma temperature.
pole-on magnetosphere When the solar
wind directly hits one pole of a planet’s magnetic
field, a pole-on magnetosphere results.
Such a magnetosphere is cylinder-symmetric
with an axial-symmetric neutral and plasma
sheet around the axis of the magneto-tail. Neptune’s
magnetosphere oscillates between such a
pole-on magnetosphere and an earth-like magnetosphere:
its magnetic field axis is tilted by
47 ◦ with respect to its axis of rotation, which
itself is inclined by 28.8 ◦ with respect to the
plane of ecliptic. Thus during one rotation of the
planet (0.67 days), the inclination of the magnetic
dipole axis with respect to the plane of
the ecliptic, and thus the solar wind flow, varies
between 90 ◦ − 28.8 ◦ − 47 ◦ = 14.2 ◦ , which
leads to a nearly pole-on magnetosphere, and
90 ◦ + 28.8 ◦ − 47 ◦ = 71.8 ◦ , which gives an
earth-like magnetosphere.
© 2001 by CRC Press LLC
poloidal/toroidal decomposition
poles of Mars Regions with distinctly different
physiography from the rest of Mars. First,
layered deposits are unique to the polar regions
and extend outwards for a little over 10 ◦ . They
are arranged in broad swirls such that individual
layers can be traced for hundreds of kilometers.
The layer deposits are almost entirely devoid of
craters, indicating they are among the youngest
features of the planet. In the north the deposits
lie on plains and are surrounded by a vast array
of sand dunes, which form an almost complete
collar around the polar region. Dune fields of
comparable magnitude do not occur in the south.
Elevations from the Mars Orbiter Laser Altimeter
show the northern ice cap has a maximum elevation
of 3 km above its surroundings, but lies
within a 5-km deep hemispheric depression that
adjoins the area into which the outflow channels
emptied.
The polar caps are at their minimum size at
the start of fall, and are at their maximum size
in the spring. Clouds form a polar hood over
the north polar cap in the winter months. In
the south, on the other hand, the Viking Orbiter
showed only discrete clouds existed in the polar
regions during the fall and winter.
Pole star North pole star, Polaris. There is
no bright star near the southern celestial pole.
Pollux 1.14 magnitude star of spectral type
K0 at RA07 h 45 m 18.9 s , dec +28 ◦ 01 ′ 34 ′′ .
poloidal/toroidal decomposition It can be
shown that a sufficiently differentiable vector
field v that vanishes at infinity may be written in
the form:
v =∇φ +∇×(T r) +∇×∇×(P r)
where r is the radius vector from the origin
of the coordinate system. The three potentials
φ, T , and P are, respectively, the scaloidal,
toroidal, and poloidal potentials. If v represents
a solenoidal vector such as magnetic field
or incompressible flow, then the term in φ does
not contribute (∇φ = 0), leaving the other two
terms as the poloidal/toroidal decomposition of
v. This is a convenient representation in many
geophysical systems because of the near spherical
symmetry of the Earth. The toroidal portion
of v has no radial component, i.e., ∇×(T r) lies
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