The Geometry The Nucleus

ASTRONOMY
Inspecting Our Closest Star: The Sun
by David Cherry
The entire disk of the Sun as seen in
"hydrogen light, "a narrow wavelength
range emitted by excited hydrogen atoms,
known to astronomers as the Ha
spectral line. Hydrogen light discloses
the chromosphere, normally invisible
to us because of the brightness of the
photosphere beneath it. All the photographs
here were taken in hydrogen
light.
The Sun's magnetic fields control the
chromosphere, but have little effect on
the photosphere because it is denser.
Both layers are partially ionized gasplasma.
The bright area just left of center in
this Aug. 26, 1979 photo is an enormous
solar flare. The less bright patches
in its immediate neighborhood are
the terminations of magnetic lines of
force that emerge from the flare. The
dark curving lines are solar prominences,
bright when seen against the
sky, dark against the disk.
All photos courtesy of Big Bear Solar Observatory, California Institute of Technology.
One of the most exciting frontiers in
solar astronomy is helioseismology, the
study of the Sun's pulsations or oscillations.
While some stars observed at
a distance are seen to pulsate in a simple
mode—expanding and contracting
in a steady rhythm, the Sun's pulsations
are a different phenomenon.
In fact, the oscillations observed at
close range in the Sun are a complex
of thousands of individual modes of
oscillation. In the quadrupole mode
(which has two axes), the ends of one
axis travel outward while the ends of
another axis at right-angles to it are
moving inward. This oscillation is
superimposed on an octopole mode
(four axes), and so on.
Prof. Ken Libbrecht at the California
Institute of Technology, together with
T.L. Duvall and other astronomers of
the National Solar Observatory at Kitt
Peak, Ariz., have just published exhaustive
lists of solar oscillation frequencies
in the Jan. 15 issue of the Astrophysical
Journal. These oscillations
are the only means of directly detecting
what is going on deep within the
Sun.
Libbrecht's more recent data have
produced measurements of the differential
rates of rotation of the Sun's gases
to a depth halfway to its center—by
tracking the oscillations. Images taken
once a minute are processed at the National
Science Foundation supercomputing
center in San Diego, where
three-dimensional Fourier transforms
Big Bear Solar Observatory is built in
the middle of a lake. Since water
changes its temperature more slowly
than land, the air in the path of the
telescope suffers less local convection
as the Sun warms the Earth. The observatory
issues "BearAlerts"reporting
solar phenomena likely to affect space
flight and electromagnetic transmissions
on Earth.
56 May-June 1988 21st CENTURY ASTRONOMY