N° 01-99 - Paris, 3 February 1999

Orbiting observatory SOHO finds source of high-speed "wind" blowing from the Sun

Like water gushing through cracks in a dam, "fountains" of electrified gas, called the solar wind, has been observed by scientists flowing around magnetic regions on the Sun to begin their 3 million kilometres per hour rush into space. Scientists have identified regions on the Sun where the high speed solar wind - a stream of electrified gas affecting Earth's space environment - originates. Using ESA's Solar and Heliospheric Observatory (SOHO) spacecraft, American and European scientists, have observed solar wind flows coming from the edges of honeycomb-shaped patterns of magnetic fields at the surface of the Sun. These observations are presented in the 5 February issue of Science magazine. The research will lead to better understanding of the high-speed solar wind, a stream of electrified gas that affects the Earth's space environment.

"The search for the source of the solar wind has been like the hunt for the source of the Nile," said Dr. Don Hassler of the Southwest Research Institute, Boulder, Colorado, lead author of the paper in Science. "For 30 years, scientists have observed high-speed solar wind coming from regions in the solar atmosphere with open magnetic field lines, called coronal holes. However, only recently, with the observations from SOHO, have we been able to measure the detailed structure of this source region".

The solar wind comes in two varieties : high-speed and low-speed. The low-speed solar wind moves at "only" 1.5 million kilometres per hour, while the high-speed wind is even faster, moving at speeds as high as 3 million kilometres per hour.

As it flows past Earth, the solar wind changes the shape and structure of the Earth's magnetic field. In the past, the solar wind didn't affect us directly, but as we become increasingly dependent on advanced technology, we become more susceptible to its effects. Researchers are learning that variations in the solar wind flow can cause dramatic changes in the shape of the Earth's magnetic field, which can damage satellites and disrupt communications and electrical power systems.

The nature and origin of the solar wind is one of the main mysteries ESA's solar observatory SOHO was designed to solve. It has long been thought that the solar wind flows from coronal holes; what is new is the discovery that these outflows are concentrated in specific patches at the edges of the honeycomb-shaped magnetic fields. Just below the surface of the Sun there are large convection cells, and each cell has a magnetic field associated with it. "If one thinks of these cells as paving stones in a patio, then the solar wind is breaking through like grass around the edges, concentrated in the corners where the paving stones meet", said Dr. Helen Mason, University of Cambridge, England, and co-author of the paper to appear in Science. "However, at speeds ranging from 30,000 km/h at the surface to over 3 million km/h, the solar wind "grows" much faster than grass". "Looking at the spot where the solar wind actually appears is extremely important", says co-author Dr. Philippe Lemaire of the Institut d'Astrophysique Spatiale in Orsay, France.

The Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on SOHO detected the solar wind by observing the ultraviolet spectrum over a large area of the solar north polar region. The SUMER instrument was built under the leadership of Dr. Klaus Wilhelm at the Max-Planck-Institut für Aeronomie in Lindau, Germany, with key contributions from the Institut d'Astrophysique Spatiale in Orsay, France, the NASA Goddard Space Flight Center in Greenbelt, Maryland, and the University of California at Berkeley, with financial support from German, French, US and Swiss national agencies. "Identification of the detailed structure of the source region of the fast solar wind is an important step in solving the solar wind acceleration problem. We can now focus our attention on the plasma conditions and the dynamic processes seen in the corners of the magnetic field structures", says Dr. Wilhelm, also co-author of the Science paper.

A spectrum results from the separation of light into its component colours, which correspond to different wavelengths. Blue light has a shorter wavelength and is more energetic than red.

A spectrum is similar to what is seen when a prism separates white light into a rainbow of distinct colours. By analysing light this way, astronomers learn a great deal about the object emitting the light, such as its temperature, chemical composition, and motion. The ultraviolet light observed by SUMER is actually invisible to the human eye and cannot penetrate the Earth's atmosphere.

The hot gas in the solar wind source region emits light at certain ultraviolet wavelengths. When the hot gas flows towards Earth, as it does in the solar wind, the wavelengths of the ultraviolet light emitted become shorter, a phenomenon called Doppler shift. This is similar to the way an ambulance siren appears to change tone as it speeds by. When the ambulance moves towards us, its sound is compressed to a shorter wavelength, resulting in a higher tone. As it moves away, its sound is stretched to a longer wavelength, resulting in a lower tone. Motion towards us, away from the solar surface, was detected as blueshifts and identified as the beginning of the solar wind.

SOHO operates at a special vantage point 1.5 million kilometres out in space, on the sunward side of the Earth. The project is an international collaboration between ESA and NASA. SOHO was launched on an Atlas rocket from Cape Canaveral Air Station, Florida, in December 1995 and is operated from the Goddard Space Flight Center in Greenbelt, Maryland.

Note to editors : A high-resolution image to support this story is available on the internet at : http://www.estec.esa.nl/spdwww/cw/soho/4feb.html and a web-quality picture on the internet at : http://sci.esa.int/soho

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