Don Savage Headquarters, Washington, DC May 2, 1996 (Phone: 202/358-1547) Embargoed until 1 p.m. EDT Jim Sahli Goddard Space Flight Center, Greenbelt, MD (Phone: 301/286-0697) RELEASE: 96-87 UNEXPECTEDLY ACTIVE SUN AND NEW INSIGHTS INTO SOLAR PLUMES FOUND Initial observations by the recently launched Solar and Heliospheric Observatory (SOHO) spacecraft reveal unexpected activity on the Sun and the best views yet of the sources of strange, chaotic "plume" structures that extend from the solar poles to high altitudes within its outer atmosphere, or corona. SOHO was specifically designed to observe the Sun during a supposedly "quiet" period near the bottom of its 11-year sunspot cycle, when solar disturbances are at a minimum and the undisturbed solar atmosphere and interior could best be studied. To the surprise and excitement of SOHO's scientific investigators, "movies made from SOHO ultraviolet data show that there is continuous motion and action everywhere on the Sun," said Dr. Joseph Gurman of NASA's Goddard Space Flight Center, Greenbelt, MD. Disturbances are occurring even within so-called "coronal holes," areas of particularly low density, low temperature and open magnetic field lines in the corona, where such disturbances were least expected. An open magnetic field is one in which the lines of magnetic force stretch out indefinitely into interplanetary space, rather than looping back downward onto the Sun. The dramatic new ultraviolet movies from SOHO also have revealed the source areas of the long, feathery plumes that extend from regions near the poles of the Sun to more than 13 million miles into interplanetary space. The sequences of ultraviolet images, combined into movies by SOHO researchers, show the polar plumes standing in the solar wind, an outward streaming of electrified gas from the corona. The movies clearly reveal the bases of the plumes, never characterized before, to be seething regions of wildly gyrating magnetic fields and turbulent solar gases. "Polar plumes are a natural laboratory to explore two of the main objectives of the SOHO mission," explained Dr. Craig DeForest of Stanford University, Palo Alto, CA. These goals are to learn how the Sun's corona is heated to about 3.6 million degrees Fahrenheit and why the solar wind in some places reaches speeds of almost two million miles per hour. The plume data may even shed light on the third main goal of SOHO, to determine what occurs below the solar surface to generate the strong flows and intense magnetism that produce solar disturbances such as sunspots and solar flare explosions. The chaotic conditions discovered by SOHO at the base of the polar plumes also are under analysis. Early data show that often the base of a plume is a bright point in the solar atmosphere, as seen in ultraviolet light. These bright points fluctuate rapidly in intensity and are at locations where magnetic fields are rapidly changing. "These magnetic changes may represent the release of significant amounts of energy on the Sun, and they contribute to the heating of the corona," said Gurman. A chief aim of the SOHO polar plume research is to see if plumes can be positively identified as the sources of high-speed streams of solar wind that were directly sampled by the European Space Agency's Ulysses spacecraft when it passed over the poles of the Sun in 1994 and 1995. In earlier work with sounding rocket experiments, DeForest concluded that the plumes may contain high-speed outward gas flows. "Now, we are trying to determine whether the plumes are in fact the fundamental source of the high speed streams," he said. The dramatic changes found at the foot of the polar plumes "appear to include the breaking open of small magnetic arches to form 'jets' that expel mass at coronal temperatures," Gurman added. Such jets have been observed in the X-ray portion of the spectrum by a telescope on Japan's Yohkoh spacecraft, "but this is the first time that small-scale jets have been observed outside large active regions on the Sun," he said. "In fact, we are seeing them in a coronal hole at the south pole, along with the plumes." Much of the new information on polar plumes and on the unexpected disturbances on the "quiet Sun" comes from the SOHO Extreme-ultraviolet Imaging Telescope (EIT). Dr. Jean- Pierre Delaboudiniere, of the Institut d'Astrophysique Spatiale, Orsay, France, is principal investigator for EIT. The EIT images show the portion of the plumes from their bases in the solar surface layer out to heights of about 93,000 miles. At its base, a plume is about 1.5 times as wide as the diameter of the Earth--which is 7,928 miles. Other important information comes from SOHO's Michelson Doppler Imager (MDI), which measures underlying magnetic fields and gas flow patterns on the solar surface, and from the Large-Angle Spectroscopic Coronagraph (LASCO), which images the plumes as far as 30 solar radii from the surface of the Sun. (The radius of the Sun is about 440,000 miles.) Professor Philip Scherrer of Stanford University is the principal investigator for MDI and Dr. Guenther Brueckner of the Naval Research Laboratory, Washington, DC, is the principal investigator for LASCO. SOHO is on location in space near the L-1 Lagrangian point, where the Earth's and Sun's gravitational forces balance, some one million miles sunward from the Earth. This vantage point enables solar astronomers to observe the Sun continuously, with no intervening "night." SOHO is a project of international cooperation between NASA and the European Space Agency. It was launched on Dec. 2, 1995 from the Cape Canaveral Air Force Station, FL. The Extreme-ultraviolet Imaging Telescope is the product of laboratories in France, Belgium, and the U.S. The Large- Angle Spectroscopic Coronagraph was fabricated at laboratories in the U.S., Germany, France, and the United Kingdom. The Michelson-Doppler Imager was produced at laboratories in the United States. -end- NASA press releases and other information are available automatically by sending an Internet electronic mail message to domo@hq.nasa.gov. In the body of the message (not the subject line) users should type the words "subscribe press-release" (no quotes). The system will reply with a confirmation via E-mail of each subscription. 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