New eye on the sun

Why is the sun’s outer atmosphere so much hotter than the star’s roiling interior? Astronomers hope that the recently launched Hinode spacecraft, which has begun staring at the sun with a trio of telescopes, will help solve that puzzle.



Hinode’s visible-light telescope shows details of the sun’s turbulent surface, where great plumes of hot gas rise and fall in convection cells to give the surface a mottled, granular appearance. JAXA

Images taken during the testing phase of the Hinode mission, a Japanese-British–U.S. collaboration, have examined the solar surface as well as the sun’s outer atmosphere, or corona. Focusing on the sun’s surface, the craft’s half-meter-wide visible-light telescope is the largest solar telescope ever flown in space.

Hinode’s low-energy X-ray telescope records emissions of hot gases from the corona. The device registers an unusually wide range of temperatures, from about 1 million to 4 million kelvins, enabling Hinode to track both cooler, quiescent features and hotter, explosive events.

The X-ray telescope’s flexibility will enable astronomers “to study coronal dynamics better than ever before,” says Ed DeLuca, whose team at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., collaborated with Japanese researchers to design and build the telescope.

The whole-sun X-ray portrait above, taken Oct. 28, reveals features of the corona known as X-ray bright points (two are in the box marked on the image). Close-ups not shown here indicate for the first time that the points are simple magnetic loops entraining hot gas. Using Hinode to monitor bright points across the sun provides astronomers with new clues about why the sun’s corona becomes so hot and how changes in magnetic fields alter other aspects of the corona, DeLuca says.

The simplicity of the X-ray bright points makes them much easier to analyze than the larger, more complex, and explosive events known as solar flares and coronal mass ejections. In the latter events, the sun hurls billion-ton clouds of hot, electrified gas into space. Some of them head toward our planet.

The X-ray image above, recorded Nov. 4 and never before released, zooms in on a patch of the corona above two so-called active regions of the sun’s surface. These are places that have high magnetic field strength and are often the sites of explosions. The image shows several thin magnetic loops never before imaged by a low-energy X-ray telescope. Although the two active regions below these coronal features cover just a fraction of the sun, many Earths would easily fit into the area covered by the image.

In addition to the X-ray telescope and visible-light telescope to study the sun’s surface, Hinode carries a magnetograph to monitor magnetic fields associated with sunspots. The craft also has an extreme-ultraviolet imaging spectrometer to track hot gas in the corona (SN: 8/19/06, p. 120: Available to subscribers at The Sun’s Halo in 3-D).

After additional testing, the craft, known before launch as Solar-B, will begin its 3-year mission in December. “Hinode” means sunrise in Japanese.

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