A group of new orbiting instruments is sending back surprising images of our best-known star, the Sun. The composite images shown here represent four days (June 16-19) of recent observations in the extreme ultraviolet by Solar and Heliospheric Observatory (SOHO) These and images in other wavelengths are posted to the Internet on a daily basis by researchers from the National Aeronautics and Space Administration and the European Space Agency, who are collaborating on the project.
SOHO was launched by NASA Dec. 2, 1995 from Cape Canaveral Air Station in Florida. It was designed and built in Europe; NASA provided the launch and is operating the satellite from its Goddard Space Flight Center in Maryland. European scientists provided eight of the observatory's instruments and US scientists a further three. In late April, SOHO reached its final destination near the L-1 Lagrangian point, where the Earth's and Sun's gravitational forces balance, some one million miles sunward from the Earth. This vantage point enables solar astronomers to observe the Sun continuously, with no intervening "night."
The spacecraft's mission is to study the internal structure of the Sun, its extensive outer atmosphere and the origin of the solar wind, the stream of highly ionized gas that blows continuously outward through the Solar System. It was specifically designed to observe the Sun during a supposedly "quiet" period near the bottom of its 11-year sunspot cycle, when solar disturbances are at a minimum and the undisturbed solar atmosphere and interior could best be studied.
But initial observations from SOHO revealed that the "quiet" sun is far from quiescent. The spacecraft's Extreme-ultraviolet Imaging Telescope revealed unexpected activity on the Sun and the best views yet of the sources of strange, chaotic "plume" structures that extend from the solar poles to high altitudes within its outer atmosphere, or corona.
Dramatic movies from SOHO have revealed the source areas of the long, feathery plumes that extend from regions near the poles of the Sun to more than 13 million miles into interplanetary space. The sequences of ultraviolet images show the polar plumes standing in the solar wind, an outward streaming of electrified gas from the corona. The movies clearly reveal the bases of the plumes, never characterized before, to be seething regions of wildly gyrating magnetic fields and turbulent solar gases.
A chief aim of the SOHO polar plume research is to see if plumes can be positively identified as the sources of high-speed streams of solar wind that were directly sampled by the European Space Agency's Ulysses spacecraft when it passed over the poles of the Sun in 1994 and 1995. Another instrument on SOHO, the Large-Angle Spectroscopic Coronagraph (LASCO) images the plumes as far as 30 solar radii from the surface of the Sun. (The radius of the Sun is about 440,000 miles.)
SOHO is also probing the sun's interior structure with instruments that detect variations in the intensity of light or its wavelength --SOHO's Michelson Doppler Imager (MDI) measures underlying magnetic fields and gas flow patterns on the solar surface. The Solar Oscillations Investigation (SOI) uses the Michelson Doppler Imager (MDI) instrument to probe the interior of the Sun.
The data sent back by MDI will be analyzed along with that collected by other groups peering beneath the solar surface. For example, a Japanese satellite called Yohkoh is returning spectacular images of the sun taken in x-rays that can be viewed as movies. Its Soft X-ray Telescope (SXT) was developed by the Lockheed Palo Alto Research Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo.
Similarly, a collaboration of earth-bound researchers known as GONG, are applying the techniques of seismology to building models of the Sun's interior. GONG stands for Global Oscillation Network Group, an array of six identical instruments spread around the globe that provide essentially continuous observations of the sun. Like others, these researchers have also displayed their findings as movies.
The GONG researchers are using a technique called helioseismology. Like terrestrial seismology, it utilizes the low-frequency sound waves that propagate throughout the Sun to measure, for the first time, the invisible internal structure and dynamics of a star. By analyzing the myriad waves that appear and dissipate across the sun, researchers can gather information about virtually any point of the sun's interior and can, for the first time, confirm the details of theoretical models about the sun - and raise some interesting problems for further consideration. A group of papers by the GONG researchers were published in the May 31 issue of Science.
As our nearest star, the Sun has always been in the forefront of astrophysics. These new images provide a clearer picture and will surely lead to theoretical insights.