DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

solar year
mic rays into the outer heliosphere, and can induce
magnetic fields in objects containing conductive
material. As the solar wind interacts
with the comae of comets, it pushes the material
back into the comet tails.
As the solar wind flows outward, its ram pressure
declines as r −2 , and eventually becomes
comparable to the local interstellar pressure due
to interstellar matter or the galactic magnetic
field or cosmic rays.
At such distances the interstellar medium is
an effective obstacle to the solar wind, which
must adjust by passing through a shock to become
subsonic; the shocked wind is swept away
by and eventually merged with the interstellar
gas. Thus, the termination of the solar wind is
thought to be characterized by two boundaries,
the termination shock where the flow becomes
subsonic, and the “heliopause,” the boundary
between the (shocked) solar material and exterior
interstellar material. Spacecraft have not yet
(June 1998) reached either of these boundaries,
which are expected to be at distances of order
100 to 200 AU from the sun.
solar year The amount of time between successive
returns to the Vernal equinox. For precision
we refer to tropical year 1900 as the standard
because the solar year is lengthening by
about 1 millisecond per century.
solar zenith angle The zenith angle is the angle
between the overhead point for an observer
and an object such as the sun. The solar zenith
angle is zero if the sun is directly overhead and
90 ◦ when the sun is on the horizon.
solid Earth tides Astronomical bodies such
as the sun and moon deform the heights of gravitational
equipotentials around the Earth. The
disturbance in the height of an equipotential due
to an external body such as the moon takes the
formofasecondorderzonalsphericalharmonic,
which rotates around the Earth with a period
slightly greater than one day (because as the
Earth rotates, the moon itself moves in orbit).
If the Earth’s surface were inviscid and fluid on
a diurnal timescale, then its surface would move
so as to coincide with an equipotential (although
the redistribution of mass at the surface would
itself adjust the equipotentials). This is what
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happens with the world’s oceans, leading to high
and low tides approximately twice per day (as
there are two tidal bulges rotating around the
Earth). For the viscoelastic solid Earth, the relaxation
to an equipotential is incomplete: the
proportion of actual readjustment to theoretical
readjustment can be found from the appropriate
Love number h2, which (using the PREM Earth
model) is 0.612. Due to irregularities such as the
fact that the plane of the Earth’s orbit around the
sun, that of the moon’s orbit around the Earth
and the Earth’s equatorial plane are not all parallel,
and that the orbits are elliptical and have
different periods, there are in fact many different
frequencies in the solid Earth tides. See Love
numbers.
solitary wave A wave with a single crest;
wavelength is thus undefined.
soliton A spatially localized wave in a
medium that can interact strongly with other
solitons but will afterwards regain its original
form. In hydrodynamical systems whose description
involves non-linear equations, one can
find solitary waves that are non-vanishing only
in small regions of space, stable, and which can
travel with constant velocity. They were first
experimentally evidenced in 1842 by J. Scott
Russel.
In non-linear field theories, equivalent stable
bound state solutions called solitons can also
exist both at the classical and quantum level.
Their most remarkable property is that they do
not disperse and thus conserve their form during
propagation and collision.
Topological solitons, such as topological defects,
require non-trivial boundary conditions
and are produced by spontaneous symmetry
breaking; non-topological solitons require only
the existence of an additive conservation law.
See cosmic topological defect, non-topological
soliton, sine-Gordon soliton.
solstice Dates on which the day in one hemisphere
(Northern or Southern) is of greatest
length. Dates on which the sun is at one of
two locations on the ecliptic which are most
distant from the celestial equator. Because the
Earth poles are inclined by 23 ◦ 27 ′ to its orbital
plane, northern and southern hemispheres typi-