Oct. 13, 2008 Complicated networks of magnetic fields power the sun's atmosphere and create both the beautiful structures and violent explosions that scientists study. Active regions, anchored in sunspots, are areas of the sun where the concentrated magnetic fields that give rise to these phenomena are extremely strong. Measured in ultraviolet or X-ray wavelengths these regions appear bright against the cooler surface.
Using new information from Hinode, an international mission to study the sun, scientists have found that the dim areas at the edges of active regions may hold the key to understanding how the sun converts vast amounts of energy from its surface into the solar wind. The solar wind is a stream of electrically conductive gas, flowing out from the sun that defines the Earth's space environment. Previously, solar researchers believed that solar wind originated in areas of the sun away from active regions. This new discovery indicates that solar wind may have a second origin.
Using data from one of three telescopes on the Japanese Hinode spacecraft, a team of researchers at the Naval Research Laboratory in Washington, has explored that hidden world at the edges of active regions. Hinode’s Extreme-ultraviolet Imaging Spectrometer (EIS) measures temperatures, densities, speeds and directions of the flows of gas in the part of the sun's atmosphere called the corona, the glowing halo around the sun seen during a total solar eclipse. Understanding the origin of solar wind will improve our forecast of space weather and help astrophysicists to understand the nature of stellar wind blowing from the surfaces of many other stars.
Because Hinode’s Extreme-ultraviolet Imaging Spectrometer (EIS) can make precise measurements of solar wind speed, Dr. George Doschek, a solar physicist at the Naval Research Laboratory and the EIS principal investigator, scrutinized large areas near active regions on the sun where the million-degree solar atmosphere accelerates away from the sun at speeds approaching 90,000 mph. The hot gas is channeled along magnetic field lines that open into interplanetary space. In addition to flowing outward, the gas shows violent internal turbulent motions, creating winds approaching 150,000 mph. This may be caused by different outflow speeds, which, like hurricane winds on Earth, create rotating patterns in the solar wind powering an energetic storm.
These massive outflows arise from previously ignored edges around active regions may prove be a significant contributor to the solar wind. These results will be presented this week at the second Hinode science meeting in Boulder, Colo.
"The fringes of active regions are far more interesting than we thought," said Doschek. "Outflow areas from sunspots are faint in extreme-ultraviolet radiation, and it was a surprise to measure such huge amounts of plasma streaming out of these dim boundary regions."
Moreover, the outflowing gas is not all at one temperature, but is at temperatures ranging between 1 million and 3 million degrees. It is exceedingly rarified; an average-size living room would contain only about 0.1 billionths of a pound of gas. While this may seem small, the outflows cover roughly 1 billion square miles, pushing billions of tons of gas out from the sun into interplanetary space every day. In Earth terms, over a year’s time, this flow of solar plasma is equivalent to expelling the total mass of water from Lake Erie into space. And this is only one active region. At the peak of the solar cycle, several active regions may be present on the solar surface at any particular time.
Doschek notes that studies of the moving gas are just beginning. "We need to know much more about these remarkable flows before we can understand their cause. If we can figure this out, we may learn much about the origin of at least part of the solar wind. We need to look at the sun’s magnetic fields where the flows are occurring, and relate the flows to cooler gas near the sun's surface. This is just the beginning of an exciting new area of solar research."
Hinode's Extreme-ultraviolet Imaging Spectrometer was built by a team of scientists from the United Kingdom, the United States, Japan, and Norway. The telescope contains state-of-the-art technology that breaks down images of the sun into a color spectrum of extreme-ultraviolet radiation, allowing the study of individual atomic transitions in atoms of different elements in the sun’s atmosphere. Hinode is revolutionizing our understanding of the solar atmosphere.
Launched from southern Japan in September 2006, it provides vital information about the sun's magnetic field and how its explosive energy propagates through the different layers of the solar atmosphere. It is a Japanese-led mission in collaboration with NASA and the space agencies of the United Kingdom, Norway and Europe and Japan's National Astronomical Observatory. NASA’s Marshall Space Flight Center in Huntsville, Ala. manages its science operations and managed the development of the scientific instrumentation provided for the mission by NASA, industry and other federal agencies.
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