RELEASE NO: 98-071

SOLAR "CAT SCAN" PROVIDES MOST DETAILED LOOK TO DATE AT CORONA

An international team of researchers using a constellation of spacecraft and ground based observatories recently compiled the most detailed view to date of the Sun’s outer atmosphere, called the corona. The project, designated the Whole Sun Month, includes a complete three dimensional tomograph of the corona from about 600,000 miles to 1.2 million miles above the Sun’s surface.

Whole Sun Month began as a series of coordinated observations by solar researchers Dr. Sarah Gibson of the Catholic University, Washington, D.C., and Dr. Douglas Biesecker of Space Applications Corporation, Vienna, Va., both located at NASA’s Goddard Space Flight Center, Greenbelt, Md., and rapidly grew into an international cooperative project as scientists recognized the benefits of simultaneous observations. The research is scheduled to be published this fall in the Journal of Geophysical Research.

"The corona is partially transparent, like thin clouds. When you look at features in the Sun’s atmosphere, especially on the limb (edge) of the Sun, sometimes you can’t tell if the feature is in front or behind another structure. A three dimensional view lets you move around in the image so you can correlate the atmospheric structure both with the surface features below it, and structures in the solar wind above it. This will help us better understand how coronal structures are ultimately connected to the Earth," said Gibson.

"Building this view of the corona was similar to taking a CAT scan of a person. We took one picture per day over the entire twenty eight day rotation period of the Sun. We then used a computer to assemble the images into a three dimensional view," said Biesecker.

The tomograph images were developed using the Ultraviolet Coronograph Spectrometer onboard the European Space Agency/NASA Solar and Heliospheric Observatory (SOHO) spacecraft. The corona is made up of hot, electrically charged gas called plasma. It is only visible from Earth’s surface during a total solar eclipse, when it appears as a shimmering, white veil around the moon. This plasma becomes the solar wind as it streams away from the Sun at speeds of one million to one and a half million miles per hour. Periodically, energetic events on the Sun’s surface send massive amounts of plasma hurtling toward the Earth. Because the plasma is electrically charged and contains magnetic fields, its impact compresses the Earth’s magnetic field. A severe impact can cause geomagnetic storms capable of disrupting communication and power systems.

Whole Sun month occurred in two phases. Phase one, the data collection period, began on Aug. 10, 1996 and ended Sept. 8, 1996. Phase two was the data analysis and theoretical modeling period. The more difficult of the two periods, it continues to the present day.

"We had an ideal situation in 1996 when we collected our data. That was during a relatively calm period in the 11 year cycle of solar activity known as solar minimum. There were enough coronal structures to make things interesting, but not so many that their connections to solar wind behavior at the Earth could not be distinguished. We were able to learn new things without being overwhelmed by complexity," said Gibson.

"Whole Sun Month was so successful that we are planning to repeat it later this year in August. With the next maximum in solar activity predicted sometime around 2000, we are now in a relatively active solar period. The payoff from the first Whole Sun Month has been the ability to refine our models of the corona. This will really help us understand what is happening in the corona during this more active period," said Biesecker.

Whole Sun Month includes measurements of the corona out to approximately four times the distance between the Earth and the Sun. Participating spacecraft include the Solar and Heliospheric Observatory (SOHO) spacecraft, a joint European Space Agency (ESA) and NASA project; YOHKOH, a Japanese X-ray observatory; Ulysses, a joint ESA/NASA interplanetary probe to explore the solar poles; and Wind, a NASA spacecraft designed to study the solar wind near Earth.

Participating ground based observatories include the Mauna Loa Solar Observatory, Hawaii, with the National Center for Atmospheric Research, the National Solar Observatory at Sacramento Peak, Sunspot, N.M., and the National Solar Observatory at Kitt Peak, Tucson, Ariz., both part of the National Optical Astronomy Observatories sponsored by the National Science Foundation; the Nobeyama Radio Observatory, Japan, part of the Japanese National Astronomical Observatory (NAO), and the European Incoherent Scatter Scientific Association (EISCAT), a scientific association between Germany, France, Great Britain, Finland, Norway and Sweden and Japan.

NOTE TO EDITORS: Video to support this story will be broadcast on NASA TV Thursday, May 28 at Noon, 3, 6, and 9 p.m. EDT. NASA Television is located on GE-2, Transponder 9C located at 85 degrees West longitude.

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