Solar magnetism study advanced

Press, 9 December 1976, Page 9

Solar magnetism study advanced

By JOHN NOBLE WILFORD, of the “New York Times,” through NZPA New York The structure of the Sun’s magnetic field has been determined for the first time from data returned by the Pioneer II spacecraft, American scientists have reported. The magnetic field envelops and pervades the entire solar system, according to the data. It is roughly spherical, extending several billion miles above the Sun’s north and south poles and probably as far out as the orbit of Pluto, the outermost planet. It is split into northern and southern hemispheres by a thin sheet of electric current like a warped disk, the spacecraft data have revealed.

Physicists describe the discovery as a major advance in knowledge of the Sun and space, and it may well apply to the magnetic fields of other Sun-like stars in the universe. It could also be important, they say, in understanding how solar magnetic storms possibly affect weather on Earth. Pioneer II made its discoveries by exploring a region of space, high above the plane of Earth’s orbit, that had never been traversed by a craft from Earth before. Spacecraft ordinarily remain in the Earth’s orbital plane. But Pioneer 11, on its way to Saturn, was thrown 100 million miles above the plane by the force of Jupiter’s gravity on a reconnaisance fly-by in 1974. Dr Edward J. Smith, of the Jet Propulsion Laboratory at Pasadena, California, the Pioneer II magnetometer experimenter, reported the findings at the autumn meeting of the American Geophysical Union in San Francisco. Dr Smith said that the solar magnetic structure, as observed by the spacecraft, appeared to be in close agreement with a number of

theories advanced in recent years.

The Sun’s magnetic field, Dr Smith said, also seemed to have many characteristics in common with the smaller field of Jupiter, the largest of the planets, which is often likened to a stillborn star.

Using the new information, primarily from the Pioneer II data, the nature and structure of the solar magnetic field are described as follows:

The field is generated by electric currents in the Sun and has a simple north polesouth pole structure, the magnetic axis being tilted 15 degrees off the Sun’s rotation axis. This is about the same tilt as is found on Earth and Jupiter. The field stretches outwards in every direction from the northern hemisphere of the Sun. It is carried by the million-mile-an-hour “solar wind” of charged particles, which flows constantly out from the Sun. It is known to reach as far as Saturn, and thought to extend to Pluto, about four billion miles from the Sun. Then, somewhere out on the fringes between the solar system and interstellar space, the field reverses direction and comes back in toward the Sun’s southern hemisphere.

This is the way it is now, but the Sun’s field has been found to reverse itself every 11 years near the time of maximum activity of the sunspot cycle. So, in another five years and a half, the magnetic field should move outwards from the Sun’s southern hemisphere and return to the northern hemisphere. A scource of puzzlement had been the apparently erratic nature of the solar magnetic field—at least as it was observed by earlier spacecraft coursing through the plane of planetary orbits. Pioneer II had suggested a solution to the puzzle, Dr Smith said.

Near the Sun’s magnetic

equator the northern and southern fields are separated by a warped sheet of electric current. Pioneers 10 and 11 observed a similar phenomoneon at Jupiter. These electric currents tend to circle the Sun in the inner solar system, but gradually turn and finally flow outwards in the outer part of the solar system, Dr Smith explained. As the Sun rotates the warped sheet appears to move up and down relative to the Earth’s orbital plane, he said.

This is-what deceived earlier probes. The spacecraft remained on a plane, but the warped current sheet bobbed up and down. Sometimes the spacecraft instruments, detected a magnetic field going away from the Sun, sometimes going towards the Sun. The warped current sheet had been postulated by Dr Leif Svalgaard and Dr John M. Wilcox, of Stanford University. Dr Wilcox said that the Pioneer II discovery was “very interesting and very important." Dr Hannes Alfven, a Nobel prize-winner at the University of California at San Diego who is another Pioneer theoretician in the field of solar magnetism, said that the discovery was “essential” to understanding Sun-Earth interactions and possibly in the quest to learn how the solar system originated. Pioneer 11, the second spacecraft to make a close reconnaisance of Jupiter, is operated by the Ames Research Centre of the National Aeronautics and Space Administration.