DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

layer, which can be seen as a small reddish ring
during a total eclipse. Its name, chromosphere,
means colored sphere and stems from this reddish
color, which results from emission in the
Hα line of hydrogen. The chromosphere shows
highly variable structures, giving it the nickname
“burning prairie”. In the chromosphere,
the density decreases by about three orders of
magnitude while the temperature stays roughly
constant. The next layer is the transition region,
where the temperature increases by a factor of
about 200, the density decreases by one order of
magnitude and the collision time increases by
more than 4 orders of magnitude — over a thin
layer which is only a few hundred kilometers
thick. The outer solar atmosphere, the corona,
fills the entire heliosphere as solar wind. Aside
from just above the transition region, temperature
and density are roughly constant. The
corona can be seen as a broad structured ring
around the occulting disk of the moon during
a total eclipse. These structures, in particular
arcs, rays, or helmet streamers, reflect the magnetic
field pattern in the corona, allowing the
distinction between open and closed magnetic
fields. Despite its high temperature, the corona
does not radiate like a black body because its
density is too low. Instead, the radiation is photospheric
light scattered by the electrons. Thus,
the light intensity also reflects the electron density
in the corona. See chromosphere, corona,
photosphere, transition region.
solar B-angle The angle, relative to the center
of the solar disk, between the north pole of
the sun and the zenith, measured from north to
west in degrees.
solar constant The rate at which solar radiant
energy at all wavelengths is received outside
the atmosphere on a surface normal to the incident
radiation, at the Earth’s mean distance from
the sun. The amount of variability is still a subject
of debate, but is certainly very small (less
than 1%) apart from the long-term development
in the history of the sun. The symbol of solar
constant is S. Unit of S is Wm −2 or cal · cm −2 ·
min −1 . Since 99.9% of the solar radiation energy
is within the wave band 0.2 to 10.0 µm,
the measurment of solar constant does not need
to involve a wide wave band. In 1981, World
© 2001 by CRC Press LLC
solar dynamo
Meteorological Organization suggested the use
of S = 1367 ± 7 W/m 2 as the solar constant
value, equivalent to 1.945 cal · cm −2 · min −1 .
solar cycle The solar cycle, the 11-year periodicity
in the occurrence of sunspots, first was
recognized in 1843 by H. Schwabe. Its features
can best be seen in a butterfly diagram: at the
beginning of a solar cycle a few sunspots start to
appear at latitudes around 30 ◦ . These spots are
relatively stable and can often be observed over
a couple of solar rotations. The spots move toward
the equator while at their original latitudes
new spots appear. The number of sunspots increases
until solar maximum. Afterwards, only
a few new sunspots emerge while the spots at
lower latitude dissolve. The total number of
sunspots decreases until just after solar minimum
when new sunspots begin to emerge at
higher latitudes. The average duration of such a
cycle is 11 years with variations between 7 and
15 years. Successive cycles have opposite magnetic
polarity, so that the full cycle is actually
22 years. The maximum number of sunspots
also differs from cycle to cycle by up to a factor
of 4. Occasionally, the solar cycle, and with it
the sunspots, can disappear nearly completely
over time scales of some 10 years. The best
documented case of such “missing sunspots” is
the Maunder Minimum. Solar Cycle #23 began
in October 1996. See butterfly diagram, Hale’s
polarity law, Maunder Minimum, Spoerer’s law,
sunspot, sunspot cycle.
solar day The length of time from noon to
noon; It varies from the mean solar day (24 h)
as described by the equation of time.
solar disk The visible surface of the sun projected
against the sky.
solar dynamo The process by which the
interaction between magnetic field and plasma
deep in the solar interior is thought to occur. The
dynamo process results in the intensification of
magnetic fields via the induction of plasmas trying
to cross field lines. The action of the solar
dynamo is thought to explain the existence of
the solar cycle, the butterfly diagram, Spörer’s
Law, and the reversal of the sun’s polar fields
near sunspot maximum.
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