Getting A Solid View Of The Suns's Corona

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.

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 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 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.

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."

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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