The Solar and Heliospheric Observatory (SOHO) spacecraft will assist eclipse expeditions from around the world in making key solar physics observations during the Feb. 26 total solar eclipse over the Caribbean.

The path of the Moon's umbral shadow begins at sunrise in the Pacific, continues through northern South America and the Caribbean Sea, and ends at sunset off the Atlantic coast of Africa. A partial eclipse will be seen within the much broader path of the Moon's penumbral shadow, which includes parts of the United States and eastern Canada, Mexico, Central America and the northern half of South America. The SOHO spacecraft is a joint European Space Agency (ESA)/NASA project and is controlled from NASA’s Goddard Space Flight Center, Greenbelt, Md. The observations are being coordinated at Goddard by the SOHO Science Operations Coordinator, Dr. Petrus Martens of ESA. The simultaneous observations are of great scientific value because they reveal a more complete picture of what is occurring on the Sun than separate measurements.

"Each method of observation has its strengths," said Martens. "By combining the two, we can learn more than by operating separately. Ground observers can get more detailed views of the corona, because they take advantage of new technology and the absence of weight, power and size restrictions imposed on spacecraft. On the other hand, SOHO creates an artificial eclipse continuously."

There are six international teams participating in the coordinated observations. The expeditions will be conducted from the Caribbean islands of Aruba and Guadeloupe, with support, in some cases, from C-130 aircraft flying in the eclipse path. Spacecraft participating in the coordinated observations include SOHO, YOHKOH, a Japanese solar observing spacecraft, and Galileo, NASA’s probe to Jupiter.

SOHO can create an artificial eclipse of any duration with coronagraphs installed in its Large Angle Spectrometric Coronagraph and Ultraviolet Coronagraph Spectrometer instruments. An occulting disk in each coronagraph blocks direct light from the Sun so that the much dimmer corona can be seen. The corona, made up of hot, electrically charged gas streaming from the surface of the Sun, is seen as a shimmering, white veil around the moon during a total solar eclipse.

"By working together, we can determine how the detailed snapshot from the ground fits in our SOHO observations, and perhaps learn more about how coronal structure changes with time," said Martens.

"You can think of the moon as a natural occulter," said Dr. Madhulika Guhathakurta of the Catholic University of America, Washington, D.C., a member of the Harvard-Smithsonian eclipse expedition. "Light from the Sun scatters off the occulting disk in an artificial coronagraph, making it difficult to measure the density of the innermost corona near the edge of the disk. Because the moon is so far away, we do not experience that problem during a natural eclipse. The moon is the original and still the best."

"The position of SOHO in space one million miles from Earth puts it far above the Earth’s atmosphere," said Dr. Arthur Poland, SOHO NASA project scientist. "This permits SOHO to observe the Sun directly, without interference from the atmosphere. SOHO can then observe the Sun using colors (wavelengths) of light that are blocked by the atmosphere. Looking at the Sun in these other wavelengths is like looking inside a car and under its hood to find out how it works. Because you can see more of the parts, you can start to see what makes it work.

"Also, SOHO can make measurements without the distortion produced by turbulence in the atmosphere, which is especially important when looking at the solar surface. We need to look at the surface because the ever-changing structure of the corona is controlled by magnetic fields, which are generated inside the Sun. By looking at the surface of the Sun, we can infer what is going on in the interior and correlate that activity with changes in coronal structure. Measuring the magnetic field of the Sun and its corona will help us understand solar storms, because the Sun’s magnetic field is a major driver in solar storms that hit the Earth," added Poland.

Solar storms occur when clouds of hot, electrically charged gas are ejected from the surface of the Sun and impact Earth’s magnetic field. The storms can cause communication and power disruptions.

"Since the corona can only be seen on Earth during a total solar eclipse, many of the expeditions are using the eclipse as an opportunity to test new equipment and techniques that astronomers hope to later use in space," said Martens.

"The observations offer benefits to SOHO as well. SOHO has a chance to calibrate its instruments using the ground based observations. Instruments on the ground are used as a reference because they can be tested in a laboratory setting," said Martens.

The expeditions are being led by Dr. Philip Judge of the High Altitude Observatory at the National Center for Atmospheric Research, Boulder, Colo., Dr. Frederic Clette of the Royal Observatory of Belgium, Brussels, Belgium, Dr. Bernard Foing of the European Space Agency’s Solar System Division at the European Space Research and Technology Center, Noordwijk, Netherlands, Dr. Jay Pasachoff of Williams College, Williamstown, Mass., Dr. Shadia Habbal of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., and Dr. Serge Koutchmy of the Institut d’Astrophysique of the Centre National de la Recherche Scientifique in Paris, France.

Images and eclipse information may be found at the following sites: