SOHO Spacecraft Detects Source of High Speed "Wind" From The Sun
Like water gushing through cracks in a dam, scientists observed "fountains" of electrified gas, called the solar wind, flowing around magnetic regions on the Sun to begin their two million mile per hour rush into space.Using the Solar and Heliospheric Observatory (SOHO) spacecraft, American and European scientists, including Dr Helen Mason of the University of Cambridge, 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 5th 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 inside coronal holes."
The solar wind comes in two varieties: high speed and low speed. The low speed solar wind moves at roughly a million miles per hour, while the high speed wind is even faster, moving at speeds as high as two million miles 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 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.
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 of the University of Cambridge, UK, co-author of the paper to appear in SCIENCE. "However, at speeds ranging from 20,000 mph at the surface to over two million mph, 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.
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 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," said Dr. Klaus Wilhelm at the Max-Planck-Institut fuer Aeronomie in Lindau, Germany, 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 light. 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 used 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 about one million miles 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 in Greenbelt, Maryland.
For the lead author of the paper in SCIENCE
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Extreme ultraviolet image of the Sun taken with the EAS/NASA Solar and Heliospheric Observatory (SOHO) Spacecraft revealing gas at 1.5 million degrees shaped by magnetic fields. Bright regions indicate hot, dense plasma loops with strong magnetic fields, while dark regions imply an open magnetic field geometry, and are the source of the high speed solar wind.
The "zoomed-in" or "close-up" region shows a Doppler velocity map of million degree gas at the base of the solar atmosphere, where the solar wind originates. Blue represents blue shifts or outflows and red represents red shifts or downflows. The atmospheric motion toward us, away from the solar surface, is seen as a blue shift, and is the beginning of the solar wind. The blue regions are inside a coronal hole, or open magnetic field region, where the high speed solar wind is accelerated. Superposed are the edges of "honey-comb" shaped patterns of magnetic fields at the surface of the Sun, where the strongest flows (dark blue) occur. These flows begin at 20,000 mph at the surface, and accelerate to over a million mph as they stream toward the Earth.
These observations were taken on 22nd September 1996 with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on SOHO. (See February 5 issue of SCIENCE magazine.) The SUMER spectrometer analyses ultraviolet light which is given off by the hot gases in the Sun's atmosphere, and is ideal for studying atmospheric motions and turbulence.
The full Sun image was taken by the Extreme Ultraviolet Imaging Telescope (EIT) on SOHO (courtesy of the SOHO/EIT Consortium). The SUMER project is financially supported by German, French, American and Swiss national agencies. SOHO is a project of international co-operation between the European Space Agency (ESA) and NASA.
Photo Credit: ESA/NASA
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