Mar 3, 1999

Future telescope could shatter solar high-resolution barrier

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Future telescope could shatter
solar high-resolution barrier

Study looks to launch in 2012

March 3, 1999: Discovery is always just beyond the limit of resolution, scientists like to say. To reach beyond limits that are about to be stretched by the Solar-B satellite, scientists at NASA's Marshall Space Flight Center and the University of Alabama propose to design a telescope that would virtually put solar flares under a microscope.

Right: A tornado's power pales compared to this twister. A computer model depicts the likely cross-section of a sunspot where magnetic flux tubes rise through the visible surface and stir flares and other active events. (NASA)

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"What we'd like to do is shatter the resolution barrier and routinely observe magnetic fields with near kilometer-scale resolution over active-region fields of view," said Dr. John Davis, deputy chief of the Physics and Astronomy Division in the Space Sciences Laboratory at NASA/Marshall.

Davis will present a proposal for a "Next Generation of Solar High-Resolution Imaging Instrumentation" at a meeting today of the Mechanisms of Solar Variability Science Working Group in Washington, D.C. NASA charters such working groups to look ahead to the kinds of advanced instruments that will be needed to answer questions that will be raised by new instruments that are still being built.

"The Transition Region and Coronal Explorer, launched in 1998, demonstrates that improvements in resolution reveal new and unexpected phenomena," Davis explained. This was true of each new solar telescope that preceded TRACE, all the way back to Galileo's discovery of sunspots in 1610. TRACE is studying the mysterious transition region where the solar atmosphere's temperature soars to millions of degrees even as it is thinning out.

Left: A composite image of the sun built up from multiple exposures taken by TRACE. (NASA)

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The next advance will be the Japanese Solar-B satellite, now starting development. Davis and his team are looking a step or two beyond that and anticipating that Solar-B will reveal tougher questions that will require the ability to see even finer details on the visible surface of the sun.

"We want to understand the dynamics and internal structure of magnetic flux tubes down between the granules that we can see now," Davis explained. The flux tubes are involved in the formation and growth of sunspots and solar flares.

"We also want to understand the changes in magnetic energy, structure, and helicity in active region magnetic fields. What are the relationships between fine-scale magnetic activity and overlying coronal structures? How do the structure and evolution of magnetic fields connect with convection below surface?"

Right: What's going on in there? Spicules - narrow, short-lived jets of gas - are part of the dynamic visible surface of the sun. (NASA)

The questions are more than academic. Understanding what controls solar flares and sunspots ­ and the solar cycle - can help in understanding what drives space weather effects around the Earth. The sun is also the best plasma physics laboratory for understanding other stars and the inner workings of controlled fusion experiments on Earth.

One of the competing concepts for the Next Generation Space Telescope is a deployable reflector that would unfold like a flower. TRW has demonstrated large reflectors (right) using this technology
Web Links

TRACE home page (Lockheed-Martin)

The Solar-B mission fact sheet

Next Generation Space Telescope home page at Goddard Space flight Center

To get these answers will require seeing the sun in exquisite detail. The angular resolution will have to be 20 to 40 milli-arcseconds. That's about 20,000 times more detail than the human eye can see and 10 times better than the best current solar telescope. That translates to seeing about 1/5 the diameter of a flux tube between grains on the visible surface of the sun. It would also see events as brief as 1 to 2 seconds since magnetic structures can rearrange themselves almost that fast. The field of view will be generous, 3x3 arc-minutes, about 1/10th the apparent diameter of the sun. To achieve this, the study team proposes to borrow from the best technologies being developed for two new missions that will look in the other direction altogether.
Another concept for the Next Generation Space Telescope will use a large, thin-shell mirror backed with actuators that adjust its shape and focus. The University of Arizona is testing scale mirrors (right).

The Next Generation Space Telescope will succeed the Hubble Space Telescope early in the 21st Century. To take the NGST's resolution a step beyond the phenomenal views that Hubble provides, NASA is studying several large mirror concepts, including a 6-meter (19.7-ft) thin-shell mirror, and a deployable 8-meter (26-ft) mirror. Both would have special systems to fine-tune their shape, thus avoiding the expense of polishing a perfect mirror on Earth.

The Space Interferometry Mission will use four or six smaller mirrors separated by 10 meters (33 feet). The multiple mirrors will focus their images together to give the same sharpness as a 10-meter mirror, although the image is not as bright.

Right: The Space Interferometry Mission, under study by the Jet Propulsion Laboratory, will employ 4 or 6 mirrors that combine an image to give the same sharpness as a single, larger mirror.

One of these concepts, or the best elements of both, will probably find its way into the design of the next-generation solar telescope. Davis says that final design decisions and instrument selections will be made after experience is gained from the NGST, Space Interferometry Mission, and Solar B. If all goes well, the next-generation solar instrument would become a "new start" in 2008, and be launched in 2012.

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Author: Dave Dooling
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