The intrepid Yohkoh spacecraft has been taking X-ray pictures of the Sun for more than 10 years, and is still going strong. More than six million Yohkoh “X-rays” of the Sun are helping astronomers better understand the nearest star.

The Japanese-led international mission was launched August 30, 1991 from Kagoshima Space Center, Japan. Astronomers are celebrating Yohkoh’s 10th anniversary with a scientific conference September 17 – 20 in Kona, Hawaii to discuss its latest discoveries.

“Yohkoh was designed and flown to address the how, the why, and the where of high energy processes on the Sun,” said Dr. Loren W. Acton of Montana State University-Bozeman, the US Principal Investigator for the Yohkoh Soft X-ray Telescope (SXT) instrument. “It has succeeded beyond our original hopes and dreams in all of these areas. The quality, coverage, and duration of Yohkoh data are unprecedented in a solar space mission.”

Among the many results from Yohkoh are discoveries about:

– The Sun’s corona, including information about how and where this multi-million degree outer layer of the Sun’s atmosphere is heated to temperatures up to hundreds of times greater than the solar surface. Yohkoh also tracked the dramatic year-to-year evolution of the corona.

– The physics of solar flares, titanic explosions in the atmosphere of the Sun caused by the violent release of magnetic energy. A typical solar flare can release in less than one hour as much as 10,000 times the annual energy consumption of the US. Yohkoh observations have helped astronomers understand better than ever before how the Sun’s magnetic fields are deformed and twisted, broken and reconnected during flares; and how the electrified gas (plasma) of the Sun’s corona is heated to millions of degrees during flares.

– The structures that produce ejections of material from the Sun, helping astronomers to understand and begin to predict “space weather.” Although the prediction tools are still rudimentary, two items of note along this line are the discoveries that certain structures on the Sun, namely sigmoids and trans-equatorial interconnecting loops (TILs), are more likely to be the sites of solar eruptions. The sigmoids — S-shaped regions seen in coronal imagery — have been found to be more likely to erupt than non-S-shaped regions. The TILs have recently received attention as another possible source of mass ejections.

Yohkoh is the first spacecraft to continuously observe the Sun in X-rays over an entire sunspot cycle, the roughly 11-year cycle in which the Sun goes from a period of numerous intense storms and sunspots to a period of relative calm and then back again. “The value of the Yohkoh observations increases as the mission continues because they better reveal the many faces of our variable Sun,” said Acton. Additionally, the Yohkoh SXT carries the longest-operating Charge Coupled Device (CCD) camera in space. After 10 years, the CCD camera — similar in operation to digital cameras now popular worldwide — is still taking beautiful X-ray pictures after collecting more than 6 million images.

Yohkoh is a mission of Japan’s Institute of Space and Astronautical Sciences with the cooperation of the United States and the United Kingdom. The US part is funded by NASA; it comprises the building of the SXT by Lockheed-Martin Solar & Astrophysics Laboratory (LMSAL), under the leadership of Dr. Acton. A consortium of organizations is responsible for the science operations of SXT and the Yohkoh data analysis, including LMSAL, Montana State University, Stanford University, and the University of Hawaii.

The collaboration has been extremely fruitful, with more than 900 peer-reviewed publications and 100 master’s and doctoral theses to date. Yohkoh data are freely available on-line for interested scientists worldwide, and are being analyzed in many countries, including China, Saudi Arabia, India, Argentina, Brazil, Russia, Australia, most European countries, and Canada.

According to the latest projections, Yohkoh will stay in orbit until the next solar maximum, around 2010. In the coming years, Yohkoh will closely collaborate with the High Energy Solar Spectroscopic Imager (HESSI), an upcoming NASA mission, providing crucial calibration data for its high-resolution hard X-ray images. Solar-B is the Japanese follow-up mission, again with involvement from the US and the UK. It will look at the Sun in soft X-rays, as Yohkoh before, but it will also make very high resolution images in visible light.

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