Solar Probe Catches New Waves From Sun's Core
Joint European-American space probe Ulysses feels its way around the sun to discover the boundaries of the star's influence. The probe's mission involves both the wonder of scientific discovery and the search to better understand practical effects of solar activity
ULYSSES - the probe that's gone where no spacecraft has been before - is giving scientists a new perspective on our solar system. It may also give them a glimpse of what's going on in the sun's energy-producing core.
That's not bad for a space probe that can't even see what it's doing.
Ulysses has no imaging equipment, explains Edward Smith, Ulysses project scientist at the California Institute of Technology's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. But, he adds, ''we've learned a lot of really interesting things'' as the spacecraft feels, listens, and tastes its way through unexplored regions high above the sun's poles.
Cameras wouldn't do scientists any good as they try to understand what's going on in the bulk of our solar system. Most of it is invisible. Ulysses' detectors are just what's needed to chart the tangle of magnetic fields and outward rushing solar winds of electrically charged particles that fill a seemingly empty void. Its sensors trace out the magnetic force patterns the probe encounters. They listen for the ''ping'' of small dust particles. They discriminate among different chemical elements to list the ingredients in the solar wind.
It is the outward push of that wind and the deflecting force of the magnetic fields that set the boundaries of the solar system - not the planets, moons, and other solid bodies. This seemingly ethereal interplanetary medium determines where the sun's sphere of influence ends and interstellar space begins.
Before Ulysses, space probes had sampled only a small part of that medium in regions near the plane of the sun's equator where the planets orbit. Now Ulysses is enabling scientists to study it in a way that was impossible before, by giving them a three-dimensional view.
To reach stations over the solar poles, Ulysses had to turn out of the plane of the sun's equator. No available rocket was powerful enough to launch it on such a course. Instead, Ulysses headed for Jupiter, arriving Feb. 11, 1992. The giant planet's gravity flung it into its final orbit.
The main exploration began last fall as the spacecraft moved toward the region over the sun's south pole. On Sept. 13, it reached 80.22 degrees south solar latitude. Then it swung back up, crossing the solar equator again March 12. And yesterday, it reached its highest point at north latitude 80.2 degrees.
In late September, Ulysses will head back out to Jupiter's orbit. It will return to the sun in the year 2000, just in time to make a second set of observations as the sun, which is now quiescent, enters a new active phase as part of the normal sun-spot cycle.
Looking inside the sun
Ulysses never came closer to the sun than 1.34 times Earth's average distance away. Indeed, it wasn't designed to study the sun at all. That's why Dr. Smith calls it ''truly astounding'' that the most recently reported findings seem to reveal processes deep within the solar interior.
Last month, statisticians Louis Lanzerotti, Carol Maclennan, and David Thomson reported in the journal Nature that they have found signs of waves from within the sun's core impressed on magnetic field lines and on the solar wind spewing outward far beyond the sun's surface. Solar physicists have been looking unsuccessfully for signs of such waves on the sun itself.
They have seen plenty of oscillations made by sound waves. But these deep solar waves are gravity waves, like waves on water. The Bell Labs team has finally found them in Ulysses data and in data gathered by the Voyager 2 spacecraft in 1985.
If this discovery is confirmed, it will be what Dr. Lanzerotti calls ''a breakthrough for studies of the sun.'' By reflecting conditions at the sun's core, the gravity waves would help solar physicists test their understanding of what goes on as hydrogen fusion reactions generate the sun's energy.
This is more than an arcane phenomenon to delight astrophysicists. The Ulysses mission involves both the wonder of scientific discovery and the search to better understand practical effects of solar wind activity on Earth.
The interaction of solar wind particles and magnetic fields sometimes forms shock waves. When these hit Earth's magnetic field, they can cause magnetic storms that send charged particles into the outer atmosphere and cause auroras. They stir up electric currents that can disrupt communications and electrical transmission.
The better scientists understand how the sun wields its interplanetary influence, the better they can understand and perhaps predict what that influence can do on Earth. That's why Lanzerotti says the discovery he and his colleagues have made is also ''a breakthrough for studies of ... the interplanetary medium and the detrimental effects of energetic particles on terrestrial systems.''
Scientists have thought of the sun's general magnetism, which internal electric currents generate, as being like a bar magnet. They expected the force lines to loop out of the north and south poles to great distances. Smith, the JPL scientist, notes that this has not turned out to be the case. Instead, Ulysses has found a more or less uniform field all around the sun.
This disappoints scientists who had hoped that the field lines from the sun's poles would open a channel to interstellar space for cosmic rays - electrically charged atoms of various elements coming from distant space. It now seems clear that the sun's magnetism acts as a deflector rather than a conduit for interstellar cosmic rays.
Also, Ulysses has confirmed that waves of many frequencies - besides the newly found gravity waves - move through the interplanetary medium. These also deflect cosmic rays. All in all, ''it's a very bumpy road for the cosmic rays'' trying to enter the solar system, says cosmic ray physicist John Simpson of the University of Chicago. He compares the situation to standing at the top of an upward moving escalator and trying to toss tennis balls to the bottom. Most of the balls will come right back and never make it to the bottom. The upshot of Ulysses discovery is that ''we're enclosed like a cocoon,'' cut off from interplanetary space, Dr. Simpson says.
Two political orbits
Ulysses is a joint European-American project managed by the National Aeronautics and Space Administration (NASA). The European Space Agency (ESA) supplied the spacecraft. NASA's shuttle Discovery launched it on Oct. 6, 1990. American and European scientists each have half the experiments on board. JPL's deep-space-tracking network maintains communication.
The project has been a successful, but troubled, partnership. Originally designated the Solar Polar Mission, two spacecraft were to be launched to study regions above the sun's poles simultaneously.
That symmetry was lost in 1981 when the Reagan White House forced NASA to cancel the American spacecraft unilaterally. As Dr. Simpson notes, ''If we had not been on the European spacecraft, we would have been dead.'' He adds, ''European scientists who were on the American spacecraft were dead.''
Now American and European scientists hope their governments will continue to fund Ulysses during its cruise to Jupiter's orbit and back. They need data on the active sun to round out discoveries already made.