DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

the radial component of the magnetic field is either
toward or away from the sun. The region in
which such an orientation is maintained is called
a sector. There are either away or toward sectors
depending on the direction of the radial component
of the magnetic field. At the transition from
away to toward or vice versa, the magnitude of
the magnetic field is fairly constant. Normally
two or four sectors are observed during a solar
rotation. The sectors are dependent on the solar
latitude and disappear at higher latitudes where
the Interplanetary Magnetic Field (IMF) has the
same sign as the appropriate solar pole.
solar maximum The time at which the solar
cycle reaches its highest level as defined by the
12-month smoothed value of the sunspot number.
During solar maximum there are more active
regions and sunspots on the sun, as well as
more solar flares leading to greater numbers of
geomagnetic storms at the Earth. The most recent
solar maximum occurred in July 1989. The
next is expected to occur sometime in the year
2000. See solar cycle.
solar maximum mission (SMM) The SMM
spacecraft was launched on February 14, 1980
near the height of the solar cycle, to examine
solar flares in more physically meaningful detail
than ever before. SMM recorded its final data
in November 1989.
solar minimum The time at which the solar
cycle reaches its lowest point as described by the
12-month smoothed value of the sunspot number.
During solar minimum, there may be no
sunspots or solar flares. The most recent minimum
occurred around October 1996. See solar
cycle.
solar nebula The cloud of gas and dust out of
which our solar system formed. About 4.6×10 9
years ago, a slowly rotating cloud of gas and dust
began to collapse under its own gravitational influence.
As the cloud collapsed, it began to form
a flattened disk with a central bulge. The central
bulge collapsed down to eventually form the
sun. In the outer part of the disk, at least two
other blobs of gas and dust collapsed down to
form the planets Jupiter and Saturn. Elsewhere,
small material condensed out of the cloud and
© 2001 by CRC Press LLC
solar neutrinos
began to accrete to form the other planets, most
of the moons, and the small icy and rocky debris
that became the comets and asteroids. This
scenario helps to explain the counterclockwise
orbital motion of all the planets, the counterclockwise
orbital motion of most of the larger
moons, and the counterclockwise rotation direction
of most of the planets by proposing that the
solar nebula was rotating in a counterclockwise
direction.
Alternately, the disk of gas and dust that surrounded
the newly formed sun at the time of solar
system formation. The planets were formed
from the material in the nebula. If we take the
current masses of all the planets, and add enough
hydrogen and helium to make the composition
identical to that of the sun, the resulting mass is
the mass of a minimum mass solar nebula.
solar neutrinos Low-energy electron neutrinos
released in nuclear reactions in the sun
and detectable from Earth. Fewer are seen than
predicted by standard physics and astrophysics.
The reactions of the proton-proton chain produce
neutrinos of energy less than 1 MeV at
the deuterium-production stage and more energetic
ones in connection with the reactions involving
beryllium and boron. The first experiment,
using Cl 37 as a detector, was constructed
by Raymond Davis in the Homestake Gold Mine
in Lead, South Dakota beginning in 1968. By
1971 it was clear that only about a third of the
expected neutrinos were being captured, but the
experiment was sensitive only to the Be and B
products, thus explanations focused on mechanisms
that might cool the center of the sun and
reduce production of these high energy particles
without affecting the main reaction chain.
A second experiment, at Kamioka, Japan, confirmed
the deficiency of B 8 neutrinos starting
in 1989, but also showed that the ones being
seen were definitely coming from the direction
of the sun. Two experiments of the 1990 (SAGE
in the Caucasus Mountains and GALLEX under
the Alps) look at the lower energy neutrinos
from the main reaction chain. They are also deficient
by a factor of about two. The full pattern
is best accounted for if the solar model is the
standard one, but electron neutrinos can change
into mu or tau neutrinos en route to us. A possible
mechanism, called MSW (for its inven-
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