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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."

The Solar Cycle

Close-up of a sunspot and solar granulation.

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.

A plot of sunspot number from 1975 through October 1998. The number of sunspots changes with an 11 year cycle.

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An instrument called SOLCON has been sent aloft on STS-95 to help answer this question. SOLCON is a radiometer, a device used to measure the total power radiated by the sun at all wavelengths. Its measurements will be used to calibrate instruments on satellites that are continuously monitoring the sun's output. "One problem with instruments in orbit is that they tend to be good at measuring small changes in the solar radiance, but not so good at measuring its absolute value," continued Hathaway. "Calibration is very important." Below: SOLCON in the cargo bay of Shuttle Discovery prior to launch last Thursday. SOLCON is mounted on the International Extreme Ultraviolet Hitchhiker platform.

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.
The main target of SPARTAN's observations was the solar corona. Astronomers believe something odd is happening there. Here on Earth as you move from sea level to higher altitude the air generally becomes colder, but the Sun works in reverse. Its outermost layer, the corona, is hotter than 1,000,000 degrees C while the visible surface, or photosphere, has a temperature of only about 6,000 degrees C. How the corona is heated is one of the great mysteries of solar physics.

"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.

Web Links

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