DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

plus the arc stretching across it — resembles the
Greek letter theta (). Polar cap arcs are associated
with northward interplanetary magnetic
fields and their origin is not well understood.
polar cap (Mars) Snow fields covering polar
regions. Their main composition is CO2 ice
(dry ice). In winter the atmospheric temperature
of the polar region is below the freezing
point of the CO2 gas which accounts for 95%
of the Martian atmosphere. The polar region
in winter does not receive sunlight (the inclination
of Mars to its orbit is quite similar to
that of Earth). Moreover, the Martian polar regions
are covered by the polar hood cloud. Becauseitgrowsindarkness,
observationshavenot
been made of a growing polar cap. In late winter
or early spring, the polar cap appears to us.
The north polar cap extends from around 60 ◦ N.
It recedes slowly during early spring and then
quickly until a permanent cap is exposed in late
spring. The north permanent cap extends from
about 75 ◦ N. The south polar cap extends from
55 ◦ S in early spring and recedes with constant
speed. Mars is near aphelion when the southern
hemisphere is in winter, so that the south
seasonal cap grows larger than the north seasonal
cap. In the south, the permanent polar cap
extends from about 85 ◦ S in summer. According
to observations of the surface temperature
by Viking spacecraft, the north permanent cap
consists of H2O ice and the south cap consists of
CO2 ice. There is no current explanation of the
fact that the south permanent cap consists of dry
ice. Martian atmospheric pressure varies with
waxing and waning of seasonal polar caps. It
reaches primary minimum in late winter of the
southern hemisphere. The amplitude of the atmospheric
pressure suggests 8.5 × 10 15 kg for
the mass of the south seasonal cap in late winter.
See polar hood.
polar cap precipitation The polar cap region
is roughly the position of the so-called “inner”
auroral zone and is a circular region around the
geomagnetic pole of radius of about 10 ◦ . There
are three types of low energy electron precipitation
in the polar cap ionosphere: polar rain,
polar showers, and polar squalls.
The polar rain particles seem to be of magnetosheath
origin and fill the entire cap with ther-
© 2001 by CRC Press LLC
polar glow
mal electrons of about 100 ev and a flux of about
10 2 ergs/cm 2 /s.
The polar showers are embedded in the polar
rain and consist of enhanced fluxes of precipitating
electrons of mean energy around 1 kev.
The showers are probably responsible for “sun
aligned” arcs. Polar squalls are also localized
intense fluxes of electrons of several kev during
geomagnetic storms, which occur as a result of
field aligned accelerations.
polar crown Region around poles of sun at
about latitude 70 ◦ of filaments oriented nearly
parallel to the equator. The polar crown is occupied
by prominences and large arcades which
can erupt and result in CMEs.
polar cusp See cusp, polar.
polar dunes Dune-like features evident on
Mars. The similarity in size and form between
dunes on Earth and Mars indicates that surface
materials have responded to wind action
in the same way on both planets, despite differences
in atmospheric density, and wind speeds.
However, the global distribution of dunes differs
greatly on the two planets. On Earth the most extensive
sand dunes are in the mid- to low-latitude
deserts, whereas on Mars most dunes are in high
latitudes. In the Martian North polar region, an
almost continuous expanse of dunes forms a collar,
in places 500 km across, around the layered
terrain, while in the south dune fields form discrete
deposits within craters. The dunes imaged
by Mars Global Surveyor Orbiter Camera are
classic forms known as barchan and transverse
dunes. These two varieties form from winds that
persistently come from a single direction (in this
case, from the southwest).
The source of material involved in the formation
of the Martian dunes is unclear. As an
alternative to quartz (silicic rocks are thought to
be lacking on Mars) garnet has been proposed.
(It is sufficiently hard to withstand the erosive
action of wind.) Alternatively, the sand-sized
particles could be produced by electrostatic aggregation,
or frost cementation in the polar regions,
of smaller particles.
polar glow Reddish proton aurora at heights
between 300 and 500 km. The polar glow covers
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