A Tale of Two Mysteries on shuttle Discovery
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A Tale of Two Mysteries
Solar physics experiments on STS-95 will address
two of the biggest puzzles of science
November 5, 1998: Shuttle Discovery
is carrying two telescopes, SOLCON and SPARTAN, designed to unravel
two of the most perplexing mysteries in science. One mystery
concerns the temperature of the Earth: Did the Sun cause a "Little
Ice Age" two hundred years ago? A second conundrum involves
the temperature of the Sun: Why is the solar corona, the outermost
layer of the sun's atmosphere, 2,000 times hotter than the sun's
surface? The STS-95 mission may not solve these longstanding
puzzles, but scientists hope to uncover some important clues.
Mystery the First: the Solar Constant
In the late 17th century there was a 70 year period called
the Maunder Minimum when no sunspots were observed on the Sun.
The normal 11-yr sunspot cycle essentially stopped, and solar
activity was abnormally low. At the same time Northern Europe
experienced the "Little Ice Age", a series of bitter
winters lasting 50 years. There was another decrease in sunspot
activity between 1800 and 1830. It wasn't as severe as the Maunder
Minimum, but temperatures in Europe and America took another
dip. The year 1816 is sometimes referred to as "The Year
without a Summer" because of unusually cold weather. Many
of the novels of Charles Dickens, which depict harsh winters
in London, were set in this period.
Today, scientists are wondering if there is a connection. Does
solar activity influence Earth's climate, and just how constant
is the sun, anyway? Today the sun deposits 1370 Watts of power
on every square meter of the Earth's upper atmosphere. That number
is called the solar constant. Since the early 1980's orbiting
spacecraft have been keeping an eye on the Sun to monitor possible
changes the solar constant. The Solar Maximum Mission, which
flew from 1980 until 1989 established that the sun's radiance
does fluctuate by a small amount. "It [the solar radiance]
is 0.1% less during the sunspot minimum than during solar maximum",
according to Dr. David Hathaway, a solar physicist at the NASA
MSFC Space Sciences Lab. "These small changes are probably
not enough to affect climate here on Earth, but what we really
want to know is this: are there larger changes that take place
on time scales of 100 years or more? If there are, then there
could be a connection between the Sun and the Little Ice Age.
Right now we just don't know."
Our Sun has spots!
These spots appear dark in photographs like the one above, but
in fact sunspots are quite bright - they are just dark compared
to the rest of the Sun. Although sunspots are cooler than the
rest of the sun, the sun is generally hotter when there are many
sunspots. The Solar Maximum Mission found that solar radiance
increased by 0.1% at the maximum of the sunspot cycle. Scientists
are predicting that the next
solar maximum will occur in the year 2000.
Since
SOLCON returns to Earth in the shuttle for recalibration, its
measurements are not subjected to possible degradation from solar
radiation like other orbiting instruments. It provides a "standard
candle" for comparison with all other measurements. Measuring
the variability of the sun on 100-yr time scales takes time,
and SOLCON is a crucial component of the process. With enough
measurements by SOLCON and other satellites scientists are hopeful
that they can answer some of the outstanding questions about
climate change here on Earth.
Mystery the Second: the Solar Corona
The SPARTAN
solar observatory was captured and returned to its berth yesterday
after successfully completing a two-day solar science mission.
About 30 percent of the science data has already been transmitted
to the ground and the remainder will be off-loaded at landing.
SPARTAN Scientist Dr. Richard Fisher noted that investigators
were pleased to have the satellite in orbit near a solar maximum
cycle and that its instruments had captured sought-after data
on a solar mass ejection event.
Right, above: The free-flying SPARTAN satellite trailing
Discovery earlier this week.
 A solar mass ejection occurred while
SPARTAN was taking data. Click here
for a JavaScript movie of the event recorded by the coronagraph
on SOHO. |
"It should be possible to heat the corona with waves,"
says Dr. John Davis at the NASA/MSFC Space Sciences Lab. "All
kinds of waves are generated in the photosphere - such as acoustic
waves, from mechanical motions, or Alfven waves, from shaking
magnetic fields. These waves spread upward into the corona which
absorbs energy from the waves.
"The trouble with this idea is that none of the waves likes
to be absorbed by the corona. They either go right through, or
are reflected back to their starting point. This is a long-standing
problem and nobody has a good solution for it.
Alternatively, scientists from Marshall have suggested that energy
is pumped into the corona through a series of little explosive
effects - microflares - that occur all over the place."
Left: A
composite of images captured by SPARTAN from Nov. 1 to Nov. 2,
1998. It shows coronal streamers in white polarized light.
There are lots of theories, but no one knows the answer. The
corona is hard to study from Earth because its light is relatively
dim compared to the blindingly bright disk of the sun. The white
light corona can be viewed from Earth only during a solar eclipse
or with a special instrument called a coronagraph. Ground-based
astronomers are never able to see the corona's ultraviolet radiation
because Earth's atmosphere blocks UV rays.
SPARTAN is equipped with two telescopes that can measure both
white light and UV emissions from the sun's corona.
The white light coronagraph, developed by the High-Altitude Observatory
in Boulder, Colo., will measure the density of the electrons
in the coronal white light. The ultraviolet coronal spectrometer
from the Smithsonian Astrophysical Observatory at Harvard will
measure the velocities, temperatures, and densities of the coronal
gases.
By comparing the data collected by the two telescopes and
combining the observations of the SPARTAN 201 missions and Ulysses
and observations made by ground-based instruments, scientists
expect to gain a much more complete picture of the solar corona
and some insight into what might be heating it.
SPARTAN 201-05 observations this week were coordinated with observations
made from the Solar and Heliospheric Observatory (SOHO) satellite.
The second and third missions were coordinated with the passage
of the Ulysses spacecraft over the sun's south and north poles.
Sunspots and the Solar Cycle from Science@NASA
SpaceWeather.com Current conditions and forecasts of solar storms, plus news about space weather.
NASA Space Shuttle home page
ShuttlePressKit.com
-- background information about STS-95
SPARTAN Home Page
-- at GSFC
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Author: Dr.
Tony Phillips
Production Editor: Dr.
Tony Phillips
Curator: Bryan Walls
Responsible NASA official: Ron
Koczor