Getting The Wind Up In SOHO: ESA/NASA Spacecraft Detects Source Of High Speed "Wind" From The Sun

February 04, 1999

An international team of scientists have identified regions on the Sun where the high speed solar wind originates. Using the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on the space-based Solar and Heliospheric Observatory (SOHO) of ESA and NASA, they observed solar wind flows coming from the edges of honey-comb shaped patterns of magnetic fields at the surface of the Sun. These observations are presented in the Feb. 5 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 inside coronal holes".

The 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 Institute of Aeronomy (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, the University of California in Berkeley, with financial support from German, French, US and Swiss national agencies. "The 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 dynamical processes seen in the corners of the magnetic field structures", says Klaus Wilhelm, also co-author of the SCIENCE paper.

A spectrum results from the separation of light into its component colors, which correspond to different wavelengths. Blue light has a shorter wavelength and is more energetic than red light. A spectrum is similar to what is seen when a prism separates white light into a rainbow of distinct colors. By analyzing 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 used by SUMER is actually invisible to the human eye and cannot penetrate the Earth's atmosphere.

The solar wind is a high speed stream of electrically charged gas flowing from the surface of the Sun. It comes in two varieties: high speed and low speed. The low speed solar wind moves at "only" 1.5 million kilometers per hour (roughly 400 kilometers per second), while the high speed wind is even faster, moving at speeds as high as 3 million kilometers per hour (800 km/s). 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 effect. 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 blueshift in the spectrum and was identified as the beginning of the solar wind.

The nature and origin of the solar wind is one of the main mysteries which the Solar and Heliospheric 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 honey-comb 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 8 km/s at the surface to over 800 km/s, the solar wind 'grows' much faster than grass". "Looking in the spot where the solar wind actually appears is an important step", says co-author Dr. Philippe Lemaire of the Institut d'Astrophysique Spatiale in Orsay, France.

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 did not directly affect us, 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 power systems.

SOHO operates at a special vantage point 1.5 million kilometers out in space, on the sunward side of the Earth. SOHO is a project of international collaboration between the European Space Agency and NASA. SOHO was launched on an Atlas II-AS rocket from NASA's Kennedy Space Center, Florida, in December, 1995 and is operated from the Goddard Space Flight Center.
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Max-Planck-Gesellschaft

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