Introduction to special issue


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Science  07 Dec 2007:
Vol. 318, Issue 5856, pp. 1571
DOI: 10.1126/science.318.5856.1571

Given the importance of the Sun to sustaining life on Earth from season to season, it is not surprising that some of the first scientific observations were of the Sun or that the sunspot record is one of our longest scientific records. Nor is it surprising that exploration of the Sun has continued vigorously, more recently through a wide range of approaches using computer simulations, neutrino detectors, and spacecraft sampling of the solar wind, and of course a wide variety of telescope observations. Yet many of the Sun's mechanisms remain a mystery. We know that nuclear reactions in its core fuel its energy and magnetism and the emission of most of its light from its nearly 6000-kelvin surface, though many questions remain about the details of all of these processes. Moving out from the Sun's surface, things get particularly interesting. The solar corona, which is orders of magnitude less dense than the surface region, is heated to near 3,000,000 kelvin. The mechanism that transfers energy from the solar surface to heat the corona has remained a mystery. This heating, as well as the corona's intense magnetic fields and loops, stretch the ionized gas of the corona out to millions of kilometers and produce solar flares and the solar wind.

Hinode (or “sunrise”) is a solar space telescope mission spearheaded by the Japan Aerospace Exploration Agency, in collaboration with partners in the United States and United Kingdom, aimed at addressing many of the questions associated with the Sun's corona. It was launched in September 2006 and began observing the Sun about 1 month later from its orbit around Earth. The papers in this special issue, as highlighted in a Perspective by Erdélyi and Fedun (p. 1572), present some of the first results from Hinode, including important clues to the mechanisms heating the corona and the acceleration of the solar wind. Hinode has spectrometers that can view the Sun in optical wavelengths as well as in extreme-ultraviolet and x-ray wavelengths that can image structures and magnetic fields within the high-energy solar plasma. It provides particularly high resolution in time and space, allowing the dynamics of small wavelike structures to be captured as movies, and revealing features, such as fine-scale jets related to magnetic fields in the corona, that have not been seen previously. New information is also provided about how magnetic lines may cross and energetically reconnect, causing solar flares. Observations of one type of wave, known as an Alfvén wave and involving a traveling oscillation of the magnetic field and plasma along a magnetic loop, may resolve the corona heating problem and explain the acceleration of the solar wind. Much more is expected to come from Hinode, yet it has already provided a new view of the Sun well-fitting its name.

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