Hinode mission delves into solar mysteries

December 06, 2007

New, peer-reviewed results from the Hinode space mission ("Sunrise" in English) should help explain some long-standing mysteries of the Sun, such as the huge temperature difference between its relatively cool surface and its white-hot atmosphere, and the origins of the solar wind that blasts through the solar system and buffets planetary atmospheres.

These results appear in a special collection of 10 articles, by scientists in Japan, Europe and the United States, in the 7 December issue of the journal Science. Science is published by AAAS, the nonprofit science society.

Many of Hinode's key goals involve understanding the basic physics that operate on the Sun, providing Earth with the heat and energy to sustain life.

The discoveries may also have a practical edge, since eruptions of magnetic energy from the Sun are responsible for "space weather" events that can threaten telecommunications, navigation systems and electric power grids on Earth. A better understanding of these eruptions and of the solar wind, the huge volume of ionized material that the Sun spews into interplanetary space, may help people predict or plan for space weather events.

"Some of the first known scientific observations were of the Sun, but many of the processes that take place on our nearest star remain a mystery," said Brook Hanson, Science's Deputy Editor, Physical Sciences.

"The papers in this special issue present some of the first peer-reviewed results from Hinode, and though there is still much to learn, the findings show that the mission is well on its way toward providing a new view of the Sun."

The Hinode spacecraft was launched in September 2006 and has been orbiting Earth along a path that keeps it constantly in view of the Sun. The mission is led by the Japan Aerospace Exploration Agency (JAXA), with collaboration from the National Astronomical Observatory of Japan (NAOJ), the National Aeronautics and Space Administration (NASA) in the United States, the Science and Technology Facilities Council (STFC) in the United Kingdom, and the European Space Agency (ESA).

The spacecraft has spectrometers that can view the Sun in optical, x-ray, and extreme ultraviolet wavelengths. These devices have allowed researchers to capture images and video, with particularly high resolution in time and space, showing structures and magnetic fields within the Sun's high-energy plasma.

One of the key results reported in the special Science issue is the discovery of a type of magnetic wave, known as an Alfvén wave, which ripples through the plasma of the Sun's atmosphere, or "corona." Swedish physicist Hannes Alfvén predicted these waves theoretically, which won him a Nobel Prize, but they have not been detected definitively until now.

Several research teams report evidence of Alfvén waves, which could potentially heat the corona to extreme temperatures by releasing energy as they travel outward from the Sun along magnetic field lines. These findings may help solve the so-called "corona problem," which refers to the fact that the sun's surface, the photosphere, is only about 6,000 Kelvin, while the corona is at least 1 million Kelvin.

The Alfvén wave discoveries appear in articles by Jonathan Cirtain and colleagues, Takenori J. Okamoto and colleagues, and Bart De Pontieu and colleagues. De Pontieu's team also shows that the energy associated with the waves is sufficient to heat the corona and accelerate the solar wind.

Another possible method for heating the corona is the release of energy that occurs when magnetic field lines cross and reconnect. Reconnection events are also primarily responsible for the violent explosions known as solar flares. Hinode observed a variety of high-speed jets of material that were ejected from these reconnection sites.

Studying the Hinode data, Kazunari Shibata and colleagues report a higher-than-expected number of "anemone" jets (shaped like an upside-down Y) in active sunspot regions, which are relatively cool areas with intense magnetic activity. Yukio Katsukawa and colleagues also detected many small-scale, short-lived jets associated with sunspots, while De Pontieu's group found jets throughout the chromosphere. Cirtain and colleagues also identified much larger jets, up to 20,000 kilometers wide and 100,000 kilometers long. These jets may also contribute to the solar wind.

Another possible source of the solar wind has been detected by Taro Sakao and colleagues, who identified a region where x-ray-emitting plasma is continuously flowing into the upper corona. They estimated the temperature and density for the outflowing plasma and report that it could be supplying the solar wind with up to one-fourth of its mass.

A Perspective article by Robertus Erdelyi and V. Ferdun discusses these findings and others in the special Science issue, concluding that Hinode has "opened new avenues for solar observation and theory."

The titles of the Science articles are listed below. The articles will be available to subscribers at www.sciencemag.org on 7 December.
The American Association for the Advancement of Science (AAAS) is the world's largest general scientific society, and publisher of the journal Science (www.sciencemag.org). AAAS was founded in 1848, and serves 262 affiliated societies and academies of science, reaching 10 million individuals. Science has the largest paid circulation of any peer-reviewed general science journal in the world, with an estimated total readership of 1 million. The nonprofit AAAS (www.aaas.org) is open to all and fulfills its mission to "advance science and serve society" through initiatives in science policy; international programs; science education; and more. For the latest research news, log onto EurekAlert!, www.eurekalert.org, the premier science-news Web site, a service of AAAS.

American Association for the Advancement of Science

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