Satellites yield hypothesis-busting data on Earth's magnetosphere

Remember AMPTE, the satellite that loosed a cloud of barium to make the Christmas ``comet''? It released another, even more spectacular display a few weeks ago to end nearly a year of experimentation with Earth's magnetosphere. That's the invisible sheath of magnetic fields and charged particles surrounding our planet, that the space shuttle Challenger has also been investigating. It was too early at this writing to know what the shuttle-borne experiments were finding. But data from the three-nation AMPTE project have shown scientists they don't know as much as they thought they did about Earth's magnetic envelope. As Stamatos Krimigis of Johns Hopkins University has observed, the results make the experts wonder: ``Are all the things we've done for 20 years wrong?''

The AMPTE adventure illustrates the discipline which the ability to experiment with nature imposes on the formation of scientific ideas. Satellite measurements of Earth's space environment have been worked into admittedly simple models of the magnetosphere. These have led scientists to believe they understand, at least roughly, what is going on there. Yet a series of well-defined experiments has refused to conform to that ``understanding.''

AMPTE stands for Active Magnetospheric Particle Tracer Explorers, with emphasis on the word ``active.'' It is a project involving three satellites which has allowed scientists to experiment on a planetary scale. It has also been an exercise in effective international space science.

West Germany supplied a 1,554-pound satellite called an Ion Release Module, or IRM. This carried 16 ejectable aluminum canisters of lithium and barium -- the experimental agents. Britain's 170-pound UKS (United Kingdom Subsatellite) dogged the German module to measure the magnetic fields and charged particles. These satellites, whose orbits took them as far as 70,000 miles from Earth, stayed just outside or just within the outer edge of the magnetosphere. Meanwhile, the US-supplied Charged Composi tion Explorer (CCE) monitored the experiments from inside the magnetosphere. Its orbital high point is 30,000 miles.

The AMPTE team has been especially interested in the interaction of the magnetosphere with the solar wind, a stream of electrically charged particles expanding outward from the sun at speeds around a million miles an hour. Physicists call such a gas with equal numbers of positively and negatively charged particles a plasma.

Earth's magnetosphere sits in the solar wind like an obstacle in a river. The solar plasma streams around it, and is drawn out into a long tail that extends beyond the orbit of the moon. This is Earth's so-called magnetotail. Scientists believe that the solar wind sometimes penetrates the magnetosphere, circulates through the system, and becomes energized as its particles are accelerated to high speeds. But they don't know how this happens. The AMPTE experiments were planned to try to answer this fundam ental question of space physics.

Just as hydrologists drop marker dyes to trace water flow, so too have AMPTE experimenters painted the sky with lithium and barium to make visible the solar plasma streams. They began with lithium releases last Sept. 11 and 20. The Christmas ``comet'' followed when barium was released Dec. 27 -- a display hidden by clouds from ground observers but tracked by aircraft. More lithium and barium releases followed in the spring. Then on July 17 -- nearly a year after AMPTE was launched last Aug. 16 -- the fi nal barium ``comet'' put on a spectacular and well-observed display. Although the British satellite was lost when its power supply failed in January, AMPTE has been largely successful.

Herein lie the surprises. Instead of penetrating the magnetosphere as expected, the marker ``dyes'' disappeared with scarcely a trace. The effect of the solar wind as it swept the barium out into cometlike tails was sometimes startling. These tails did not always move with the wind, sometimes going at right angles to it. Also, particles in those tails may be energized to millions of degrees C. instead of less than a thousand degrees as expected.

The opportunity to experiment with their subject is forcing space physicists to reassess preconceived notions, as so often happens in the development of a science.

A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.

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