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Solar-B gets an A

Hinode gathers round-the-clock data on solar magnetism

by Bob Henson


Three postdoctoral researchers at NCAR are working with scientist Bruce Lites (far right) on analysis of solar magnetism data from Hinode. From left: Alfred de Wijn, Rebecca Elliott, and Masuhito Kubo. Behind the group is a Hinode/NASA image of the vertical magnetic field emanating from a sunspot. (Photo by Carlye Calvin.)

It pays to keep an eye on the Sun's magnetic fields. Churning, twisting, and more than a trillion square kilometers in size, the fields shape sunspots and the solar storms that can toss radio signals and power grids into disarray. NCAR's High Altitude Observatory (HAO) has been observing solar magnetic fields for more than 15 years, but its observations have been limited to an earthbound vantage point, with the view restricted to daylight hours and sometimes obscured by clouds or haze.

Now, there's a satellite-borne instrument keeping tabs on the Sun's magnetic contortions 24 hours a day. Solar physicists at NCAR and elsewhere are elated by the new data.

"This is extremely significant for understanding the Sun's activity in a global and comprehensive way," say HAO director Michael Knölker.

The satellite Hinode (meaning "sunrise" in Japanese) was launched on 22 September from the Uchinoura Space Center in Kyoshu. The Japanese mission includes contributions from space and science agencies in the United States and Europe. Hinode is a sequel of sorts to Yohkoh, another Japanese mission with international involvement, which flew from 1991 to 2005. Saku Tsuneta, of Japan's National Optical Observatory, lauds the teamwork behind the latest satellite. "We were able to build Hinode by combining the best scientists and technology, funding from several space agencies, and the strengths of several different cultures," says Tsuneta.

Yohkoh and Hinode were commonly referred to as Solar-A and Solar-B during their developmental stages. Hinode has also been called a Hubble Space Telescope for the Sun. The nickname arose in large part from the stunning clarity and sheer volume of imagery gathered by the satellite's Solar Optical Telescope (SOT) as Hinode circles Earth in a Sun-synchronous orbit. The two other major instruments on board are an X-ray telescope and a spectrograph in the extreme ultraviolet range.

With a main aperture 50 centimeters (18 inches) wide, the SOT can provide high-resolution broadband data over an area spanning roughly 150,000 kilometers (93,000 miles) in its long dimension. The SOT's resolution is 0.00006 degrees. That's comparable to discerning a single strand of hair on the head of someone half a block away. What's more, the images are gathered as often as several times a minute, giving scientists unprecedented animations of solar flares and other key phenomena as they unfold.

There's another payoff that's less spectacular to the untrained eye but perhaps even more pathbreaking. It's the result of a spectropolarimeter attached to the back of the SOT. About the size of a suitcase, this instrument measures the polarization of light in a narrow range around 630.2 nanometers. The data are converted into color-coded images (see graphic) that reveal the three-dimensional orientation of the magnetic field vectors across the Sun's photosphere, its visible surface.

Bruce Lites has been working for more than 20 years toward images like these. The NCAR solar physicist led the development of the Advanced Stokes Polarimeter (ASP), housed at the National Solar Observatory since 1991. ASP was the first instrument to resolve solar magnetic fields in detail, and it inspired a number of ground-based successors. Together, these instruments provided critical glimpses of magnetism on the Sun's surface. But because they were limited by atmospheric conditions and the length of the day itself, the glimpses left solar physicists hungry for a more complete picture.

When magnetic fields twist and reconnect, they spawn gigantic mass ejections that can fling energized particles into Earth's atmosphere, triggering auroras and solar storms. However, most of the Sun's magnetism leads a less glamorous life. Some of it gets neutralized near the surface; some gets redirected downward. What's not clear is how these and other processes are apportioned across the Sun from hour to hour, day to day, and month to month.

"You really need long time sequences to follow the evolution of the magnetic fields responsible for heating the upper layers of the solar atmosphere and for the variables that affect our climate," explains Lites.

One surprise already revealed by Hinode is the sheer liveliness of the magnetic fields across the Sun's surface, not only in and near sunspots but across quiet regions as well, according to Lites. "We're seeing these fields all over the place. We'd gotten hints of these with ASP, but we had no idea they were so widespread or so strong." The new data may compel physicists to revise their understanding of how the Sun's corona, or uppermost atmosphere, is heated from below.

During its stay in space, the Yohkoh satellite gathered a smorgasbord of solar data across a variety of wavelengths, but polarization wasn't measured. When Japan's solar physicists began discussing Hinode in the mid-1990s, they invited Lites to participate. With NASA support, Lockheed Martin built a focal plane package for Hinode that included a spectropolarimeter. Working with NCAR engineers David Elmore and Kim Streander, Lites collaborated with Lockheed principal investigator Alan Title (succeeded by Theodore Tarbell), NASA program manager Lawrence Hill, and others to develop and build the instrument. "In this kind of alliance, we can focus on what we do best," says Knölker. Thanks to advances in optics and miniaturization, the group managed to create a sensor for Hinode that was smaller yet more precise than ASP.

The first few months of magnetism data from Hinode debuted at the annual meeting of the American Association of Astronometers in May. "There was a lot of excitement," says Lites. "People are clamoring for these data. The instrument is absolutely unique, and it's telling us an awful lot about solar magnetic activity." According to Lockheed scientist Title, "Hinode images are revealing irrefutable evidence for the presence of turbulence-driving processes that are bringing magnetic fields, on all scales, to the Sun's surface."

To help keep up with Hinode's data gusher, Lites has brought three new postdoctoral researchers on board (see photo). Alfred de Wijn, a recent Ph.D. from the University of Utrecht, plans to carry out detailed analyses of particular solar events. In May, Hinode tracked a sunspot along its 12-day, rotation-induced path from one side of the Sun to the other.

"With ground-based imagers, you'd be lucky to get a couple of good hours of observations in a day and a few good days in a year," says de Wijn. "Now we're getting good data 24 hours a day—maybe 100 or 200 times more than we've ever had before." The only substantial breaks in the Hinode data stream occur from about May to August of each year, when Hinode's view of the Sun is eclipsed by Earth for up to 15 minutes during each 100-minute orbit.

With its round-the-clock coverage and its anticipated lifetime of at least three years, Hinode will not only track particular solar features but also help foster a holistic understanding of the entire Sun's magnetic budget—much as the advent of global radiosonde launches in the mid-20th century transformed our picture of weather circulation.

These images from Hinode's Solar Optical Telescope show the intensity of light (top) and the magnetic field (bottom) around a sunspot on 12 December 2006. Hues indicate the angle of the horizontal magnetic field as shown by the color wheel at top right. The field extends beyond the granular borders of the sunspot itself. (Images courtesy Bruce Lites/Hinode.)



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