A NASA satellite has located a third radiation belt surrounding the Earth, one that traps flotsam and jetsam from exploded stars, galactic dust clouds and the Big Bang itself, scientists said yesterday.
The space agency's SAMPEX satellite, launched in June 1992, has mapped a portion of the new belt -- where particles snared in the Earth's magnetic field bounce back and forth between the north and south magnetic poles -- only a few hundred miles above the Earth's surface.
In 1958, James A. Van Allen, now of the University of Iowa, discovered two radiation belts, one of which bottles up neutrons and the other protons, using data from NASA's Explorer I satellite.
The new belt, which dips closest to Earth over the South Atlantic, is embedded in the lower of the two Van Allen belts, scientists said.
"It's sort of a melting pot for everything that's been happening in the galaxy over the last 10-to-15 billion years," said Richard A. Mewaldt of the California Institute of Technology, speaking at the American Geophysical Union meeting in Baltimore. "It's interstellar material right in our own backyard."
The SAMPEX satellite pinpointed the new belt, the existence of which was first predicted 15 years ago. In 1991, a team of Russian and United States scientists detected a cloud of the nuclei of gas atoms trapped in the Earth's magnetic field, but they were unable to fix its location.
The satellite's name stands for Solar, Anomalous and Magnetospheric Particle Explorer.
The new belt traps heavier materials, including the nuclei of atoms of helium, nitrogen, oxygen and neon.
These gases help make up the thin mist -- of about one atom per cubic inch -- that fills the void between stars in our Milky Way galaxy. This material, called the interstellar medium by astronomers, represents the debris from the birth and death of stars, as well as particles that have been drifting around since the creation of the universe about 15 billion years ago.
Some of these gases penetrate the heliosphere -- the vast cloud of particles trapped in the Sun's magnetic field and stirred by solar winds. Then, they can be accelerated by energy from the Sun and wind up trapped in the Earth's magnetic field.
Once inside the new belt, these atoms may bounce back and forth between the Earth's magnetic poles for weeks before leaking out into space or the atmosphere. As a result, the amount of matter inside the new belt waxes and wanes. It doubled, for example, from August to November 1992.
Another NASA scientist at the Baltimore meeting said SAMPEX data supports the theory that a significant fraction of global ozone depletion is due to high-energy electrons bombarding the atmosphere -- and not to chemicals produced by agriculture and industry.
Linwood B. Callis, an atmospheric scientist at NASA's Langley Research Center in Hampton, Va., said that between 1980 and 1985, when global ozone levels dropped 4.5 percent, "Our calculations suggest that about 78 percent of that was natural."
He said that many other natural conditions, such as global temperatures and volcanic eruptions, influence ozone destruction and regeneration.
Even if the Earth's ozone layer fluctuates naturally due to natural forces, Dr. Callis cautioned, man-made chemicals can still harm the ozone layer.
Ozone, made up of three atoms of oxygen, helps protect life on Earth by shielding the planet's surface from destructive ultraviolet radiation from the Sun.
Dr. Callis said that SAMPEX has measured levels of high-energy electrons striking the atmosphere sufficient to account for a significant amount of the ozone loss before 1985. After the mid-1980s, ozone levels rose again.