Tantalizing Taste of Things to Come : Voyager Discovers Magnetic Field on Neptune

For the first time, the spacecraft Voyager has detected intense radio signals coming from Neptune, revealing that the planet has a magnetic field and indicating that next week’s close encounter will be as intriguing as scientists had hoped.

The discovery suggests that the spacecraft will find a wide range of interesting phenomena as it passes close to the distant planet Thursday night, including the kind of auroral activity that produces the northern and southern lights on Earth.

The signals were first detected last Monday, but scientists at the Jet Propulsion Laboratory were not sure they were coming from Neptune, according to James Warwick, principal investigator on the planetary radio astronomy experiment.

Not What Was Expected

“Their character was different than what we were expecting,” Warwick said.

“The radio emissions are very intense,” Warwick said, adding that they came in short, powerful bursts over a limited frequency range. Scientists had expected radio signals from Neptune, but they were looking for something less intense and less limited in frequencies.

Warwick and fellow scientists working on the project decided to announce the discovery Friday after they grew more confident of what they were seeing.

The discovery means Neptune has a strong magnetic field because radio waves are generated by charged particles that become trapped in the magnetic field and spiral into the planet’s atmosphere.

It is the same process that produces the northern lights on Earth. Charged particles from space flow along the Earth’s magnetic field lines and into the atmosphere at either the North or South Pole, producing the celestial fireworks over the higher latitudes. Now, Voyager is expected to see the same thing at Neptune.

Within 3,000 Miles

The Voyager spacecraft, which will make its fourth and final planetary flyby next week when it passes within about 3,000 miles of Neptune’s cloud tops, used a radio receiver and a giant pair of “rabbit ears” to detect the radio emissions--just as it did at Jupiter, Saturn and Uranus. Each 33-foot-long ear scans a series of frequencies and, since they work separately, the “polarization,” or vibration, of the signals can be determined, thus distinguishing between radio signals generated by lightning and those created by a magnetic field.

“We know we are dealing with a magnetic field,” Warwick said.

The discovery of a magnetic field also tells scientists something about the interior of Neptune. The Earth’s magnetic field is generated by the movement of its molten iron core, and Neptune’s strong magnetic field suggests that Neptune also has a region of fluid in its interior.

Venus, Mars and probably Pluto do not have magnetic fields.

Early analysis of the data from Voyager indicates that Neptune’s magnetic field is similar in intensity to Earth’s.

As more and more data is collected on the magnetic field, scientists will be able to determine more precisely the length of a Neptunian day--the time it takes for one rotation. That is believed to be about 18 hours, but by the time Voyager is through that figure should be known exactly.

Until now, scientists have known little about Neptune other than such things as its atmospheric composition, which includes the methane that makes it appear blue. But Voyager will pass right over the planet’s north pole, and that is fortuitous for scientists who are interested in its magnetic field. That trajectory will carry it through the intense field where charged particles are being funneled into the polar atmosphere, and that should reveal much about the field.

In fact, by the time Voyager zips past Neptune it will have collected enough data for scientists to learn more about its magnetic field than any other planet except Earth.