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Spewing out of the sun at a million miles an hour, the great gushing geyser of broiling gas and supercharged particles known as the solar wind blasts through space, enveloping all the planets, moons and small bodies of the solar system in a hot, seething soup that affects everything in this corner of the universe, including human life itself.

As this cosmic jet stream rams into the Earth's magnetic field, most of it is thankfully deflected into interstellar space. But billions of wind-driven particles still manage to break through the planet's warped and twisting protective shield. They spiral down into the atmosphere over the poles to produce aurorae, geomagnetic storms, disruptions in radio communications and power surges among transmission lines.The existence of this powerful solar wind had been known since the early days of space exploration, but its origin inside the sun - like much else about our nearest star - remains a mystery.

Sometime in the next decade, however, a satellite experiment called the Ultraviolet Coronagraph-Spectrometer, working in concert with several other space instruments, may at last reveal the secrets of the solar wind. The project is part of a massive international campaign to study the complex relationship among the sun, Earth and human activities.

"An ultraviolet coronograph-spectrometer is our name for an artificial eclipse machine," says Dr. John Kohl of the Smithsonian Astrophysical Observatory in Cambridge, Mass., and principal investigator for the project now being developed with researchers in Italy and Switzerland as well as at several U.S. universities.

The Smithsonian's solar wind experiment is one of several aboard a satellite called the Solar and Heliospheric Observatory (SOHO) to be built by the European Space Agency in cooperation with the National Aeronautics and Space Administration.

"The solar wind seems to originate in the sun's corona, or outer atmosphere," Kohl explains. "This pearly white ring of hot gas is visible form Earth only during a total solar eclipse. To see what's happening in this region on a regular basis, we have to block out the sun's bright disk, which otherwise simply overwhelms the corona."

In fact, most aspects of the hot gas and charged particles in the wind - detectable as high-energy ultraviolet light - are invisible to ground-based optical telescopes. But observations of this light to determine the solar wind's velocity, temperature and composition can be made from space.

"Our chronograph-spectrometer is a device that creates a permanent eclipse," says Kohl. "Thus we can observe the corona - and the outpouring of the wind - continuously.

Continuous observance is absolutely necessary because the flow of the solar wind is not constant. It changes dramatically and frequently, often over periods of a few hours. Since the sun makes one full rotation every 27 days, streams of hot, high-speed gas spiral out like the spray of water from a spinning lawn sprinkler. The wind also displays other long-term variations linked, perhaps, with the same 11-year cycle that brings sunspots and flares to the sun's surface.

The solar wind was first detected by equipment aboard the Mariner 2 spacecraft in 1961. Since then, numerous spacecraft in orbit around both the Earth and the sun have measured the wind at distances as close as 30 million miles from the solar surface.

In the mid-1970s, for example, X-ray observations from the manned Skylab space station revealed that the sun's outer atmosphere was laced by great dark regions and that some of the powerful solar wind was pouring out of those "coronal holes."

"Except for the coronal holes," says Dr. George Withbroe, an associate director at the Harvard-Smithsonian Center for Astrophysics, also in Cambridge, "we have so far been unable to determine where the wind originates - or what mechanism drives it away from the sun. Some of the energy required to accelerate the wind and allow it to escape the sun's great gravitational pull must be associated with the same unknown mechanism that heats the coronal gas to 1 million degrees Celsius."

The coronal holes, some large enough to cover 20 percent of the sun's visible surface, change shape, size and location with time. At periods of maximum solar flare and sunspot activity, they may disappear completely and, with them, the high-speed solar wind streams.

The variability of the solar wind, combined with the dynamic and complicated system of magnetic fields and electrical currents in the vast domain of "geospace" surrounding Earth, add up to a complex array of physical effects.

The largest region of geospace, for example, is the "magnetosphere" where all phenomena are dominated by the Earth's own magnetism. The gusty solar wind compresses the magnetosphere into a giant, bowlike shock wave on the day side of Earth and pulls it out like a string of taffy on the opposite, or night side.

The second region is the "ionosphere" - the broad band of atmosphere some 40 to 300 miles above the earth. Here, solar interactions, including the charged particles driven inward by the wind, give rise to the great magnetic storms that produce spectacular Northern and Southern lights displays as well as disrupt communications.

In the Earth's lower atmosphere, chemical reactions and wind patterns, triggered by solar wind interactions hundreds of miles above, may affect local weather conditions and, perhaps, even global climatic patterns. The mechanism linking these various phenomena, however, is still poorly understood.

More alarming, increasing evidence suggests that many human activities - fossil fuel burning and unrestrained release of chlorofluorcarbons in aerosols among them - may be affecting not only the lower atmosphere, but the entire geospace. The outstanding example of this is the "ozone hole" detected in recent years over Antarctica.

Understanding the solar wind is essential to unraveling this intertwining of many related phenomena, and a diverse battery of experiments planned in the next decade under the broad umbrella of the International Solar Physics Program will study the solar wind both at its source and as it enters the Earth's environs.

Now planned for launch in 1995 - aboard either the space shuttle or an expendable rocket - the SOHO satellite will observe the sun continuously without interference from the Earth's atmosphere or its day-night cycle.

Among the SOHO experiments are several concerned with "helioseismology," that is, measuring oscillations in the sun's surface as a means of probing its interior, just as geologists have probed the Earth's core by studying earthquakes. Such studies of solar dynamics may hold the key to understanding similar physical processes in other stars.

Since its position some 1 million miles from Earth will place SOHO directly in the path of the outrushing solar wind, instruments on board will be able to make direct measurements of the wind's chemical components. These measurements, combined with data from the Smithsonian's coronagraph-spectrometer and other SOHO optical experiments, will offer the first opportunity to chart the evolution of the solar wind - from the time it leaves the sun until it arrives at the Earth.

The sun-watching SOHO will be joined in space by a small armada of four other scientific satellites collectively called "Cluster." Placed in near-Earth orbit by European Ariane rockets, also in 1995, the quartet will fly in formation, separated from each other by distances ranging from a few hundred to a few thousand miles.

In addition, as part of its own Global Geospace Science program, NASA will launch two spacecraft in the mid-1990s. The first, called "Wind," will be positioned to remain permanently on Earth's daylight side so it can monitor shock reactions generated by the incoming solar wind. The second, "Polar," will travel in a north-south orbit taking it directly over the Earth's poles so it can watch auroral phenomena from space.

A joint U.S.-Japanese satellite called "Geotail" will be launched into a long, looping orbit to study that part of the Earth's magnetosphere stretched by the solar wind into a cometlike "tail" millions of miles long.

Experiments such as these have the potential not only for determining the origin of the solar wind and the engine that drives it, but also for defining exactly how this powerful force affects the Earth's environment - and our lives.