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June 22, 1998: Understanding what's going on with the Sun may require a second look - from a different perspective. Most of the images scientists now have of the Sun are based on a limited view from Earth. Even telescopes in space are so near the Earth that they see the same face as we do.
In order to get a better look, Dr. Allen Gary, Dr. John Davis
and Dr. Ron Moore of NASA's Marshall Space Flight Center have
proposed using two separate spacecraft - one in Earth's orbit
and one in deep space - to simultaneously observe the Sun's activity
in true stereo (stereo is Greek for solid).
In earlier work, Gary, a solar physicist at Marshall's Space Sciences Laboratory, was able to create simulations of three-dimensional loop-like structures in the Sun's outer atmosphere, or corona. The loops resemble bunches of bananas. Rather than being full of vitamins, though, these tubes are made of hot plasma surrounding magnetic field lines.
At right, magnetic field lines loop through the solar atmosphere and interior to form a complicated web of magnetic structures linking sunspots. Click on image for larger version. Links to 495x393-pixel, 34K JPG. Credit: NASA/Marshall Space Flight Center. More details are available at the 3-D solar magnetic fields site in related links below.
Gary produced these images by combining observations from different types of telescopes including the solar vector magnetograph at NASA/Marshall and an X-ray telescope aboard Japan's Yohkoh satellite. These images have been used to analyze the three-dimensional coronal loops and the effects of magnetic fields on the Sun's coronal mass.
A New Perspective
In an article for an upcoming issue of Solar Physics, Gary and his colleagues have proposed taking this research a step further to try to predict the explosive effects of the coronal loops. They plan to improve the imaging techniques for the 3-D structures by using two identical X-ray telescopes, one planned for Earth, the other in deep space.
The Earth-orbiting telescope is being developed for launch on a future Geostationary Operational Environmental Satellite (GOES) mission. The GOES spacecraft are a continuing series of NOAA weather satellites aimed primarily at forecasting terrestrial weather. However, they have also carried solar X-ray monitors to watch for flares. Starting with the GOES M spacecraft, expected to be launched in 2000, all will carry instruments to observe solar events. The proposed Stereo X-Ray Coronal Imager (SXCI) mission will send the second, identical telescope out expressly to record stereoscopic images of the solar corona.
"To
determine how far away from a telephone pole you are," said
Gary; "you use both your eyes to get a sense of depth and
distance." The same process of triangulation applies to his
plan for viewing the coronal loops. The low-cost SXCI spacecraft,
designed specifically for this mission, will lead the Earth in
an orbit of the Sun at about one astronomical unit (AU) - the
average distance between the Earth and the Sun. This way the two
telescopes will be operating at equal distances from the sun,
creating a viewing angle ranging from 0 degrees at the beginning
of the mission to 25 degrees at nine months to 50 degrees at the
end of 18 months.
The image at left depicts the earth like orbit of the SXCI spacecraft and the Earth, which will be orbited by a GOES weather satellite carrying an identical telescope. Links to 600x600-pixel, 10K GIF. Credit: NASA/Marshall Space Flight Center.
"The simultaneous imaging will not only tell us how far away these things are, but also will help us understand the distribution of matter within the corona," said Gary, who plans to use tomographic techniques to enhance the images. Tomography is a process of creating models of three-dimensional objects by imaging one slice at a time. A CAT scan, for example, uses X-ray imaging to gather representations of multiple slices of an object and combines these images to make a composite picture. The images would be viewed on a computer screen in a manner similar to how scientists viewed Mars Pathfinder images in 3-D.
Using
this technique to look at the structures in the Sun's atmosphere
requires at least two different viewpoints. Using only one telescope
would be like looking at a curved palm tree from a single perspective
and believing it has a straight trunk.
The same tube structure appears to have a different shape when the perspective changes by 20 degrees. The lines link the same points in the flare. (Links to 850x1050-pixel, 207K JPG with graphs depicting the view.)
The second telescope gives scientists an additional perspective, like being able to walk around the palm tree a bit to see how the tree trunk bends directly towards, or away from, the initial point of observation. So, while the loop models Gary has built are astounding, they can't show the whole picture because they are based on flat images rather than stereoscopic images.
Solar Forecast: Stormy and Hot
But unlike stationary palm trees gently swaying on the beach, the Sun's corona is a dynamic million-degree Kelvin (1.8 million degree F) plasma extending outward from the 6,000-degree Kelvin (11,291 degree F) solar surface, the photosphere.
"The energy for heating the corona and solar wind is supplied by magnetic fields which are generated deep within the Sun and emerge through the photosphere," said Gary. "The buildup and release of magnetic energy in the corona is accompanied by changes in the 3-D structure of the corona and its magnetic fields which cannot be determined from a single viewpoint."
By comparing images taken simultaneously from both telescopes a short time later, scientists will be able to see new structures develop.
"Getting
images at two times from two spacecraft will allow us to see loops
being heated and popping out," Gary said. This type of observation
will help scientists make predictions about events on the Sun
that affect space weather.
Two different perspectives of heated coronal tube structures (in a computer simulation) illustrate how a tube's apparent shape varies with the viewpoint. Links to 600x756-pixel, 81K JPG with detailed views from all three axes. Credit: NASA/Marshall Space Flight Center.
Among the explosive solar events Gary and his colleagues are working to predict are coronal mass ejection's (CMEs), prominence eruptions and solar flares. These huge energy releases can cause geomagnetic storms and surges of energetic particles at Earth with harmful effects on spacecraft, humans in space, communication and navigation systems, and electrical power grids on Earth.
"The main goal is to get a whole 3-D model of the structure and dynamics of the corona," said Gary; "This will help us predict and explain CMEs and other solar eruptions which ultimately have an impact on Earth."
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The image at left (309x394-pixel, 39K JPG)is an anagram in which the left and right views are presented in red and blue, respectively. You need glasses with a red and blue filter to get the 3-D effect. The image at right (800x800-pixel, 69K JPG) comprises true left and right images that must be viewed up close and then shifting your eyes to fuse the two images into one. |
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NASA research
Papers
External link (implies no endorsement by NASA)
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ON ANALYSIS OF DUAL SPACECRAFT STEREOSCOPIC OBSERVATIONS TO DETERMINE THE THREE-DIMENSIONAL MORPHOLOGY AND PLASMA PROPERTIES OF SOLAR CORONAL FLUX TUBES
G. Allen Gary, John M. Davis, and Ronald Moore, Space Sciences Laboratory/ES82 NASA/Marshall Space Flight Center, Space Sciences Laboratory/ES82, George C. Marshall Space Flight Center/NASA, Huntsville, AL 35812, U.S.A.
Abstract. By using two spacecraft equipped with multi-bandpass X-ray telescopes, it is possible to obtain direct 3-dimensional morphology of coronal structures which is essential for understanding the energetics and dynamics of the solar atmosphere. X-ray observations taken only in orbit about the Earth are inadequate to fully resolve the 3-dimensional nature of the solar corona. These Earth-orbit observations produce 2-dimensional images and an appropriate model must be included to derive the 3-dimensional structures from the line-of-sight information. Stereoscopic observations from space will remove this limitation and are needed if we are to improve our knowledge of the 3-dimensional morphology of the corona. Several important points regarding a stereoscopic mission are investigated and illustrated using model coronal flux tubes and image-rendering techniques. Synthesized images are formed by integrating the emission from volume elements along the line-of-sight path through a 3- dimensional volume in which a set of model flux tubes are located. The flux tubes are defined by (1) a plasma model defining the emissivity for a specific density, temperature, and pressure distribution, and (2) a magnetic field model from which a set of field lines are selected to define the geometry of the flux tubes. The field lines are used to define the flux-tube volume by assuming an initial base radius and conservation of flux. An effective instrumental spectral-response function is folded into the integration. Analysis of pairs of these synthesized images with various angular perspectives are used to investigate the effect of angular separation on mission objectives. The resulting images and analysis provide guidelines for developing a stereoscopic mission.
Our study produced four important results, namely: (1) An angular separation of ~30 degrees maximizes the scientific return by direct triangulation analysis because of the tradeoff between increased line-of-sight resolution of position and decreased recognition of individual loop structures arising from the overlapping of multiple loops with increasing angular separation. (2) The analysis benefits from the use of time differential images to select flux tubes from the collection of numerous overlapping systems by selecting only recently heated or cooled flux tubes. (3) An analysis needs to be developed for algebraic reconstruction techniques applying a priori information, specific to the solar coronal structures, i.e., flux-tube continuity, maximum emission strength, non-negative emission, previous history, and maximum gradients of emission. (4) An analysis strategy combining triangulation, modeling techniques, and algebraic restoration is necessary to derive a complete understanding of the 3-dimensional morphology of the magnetic field. In the same way that helioseismology is classical viewing of the Sun with a tailored set of analysis tools for probing the interior of the Sun, heliostereoscopy is classical viewing of the X-ray emitting corona and requires a tailored set of analysis tools to deduce the true 3-dimensional structure of the corona.
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Author: Tom
Kelleher
Curator: Linda Porter
NASA Official: Gregory
S. Wilson