Clouds above Neptune (Click on the image for a larger view)

The character of Neptune's magnetic field is important because it helps scientists understand what goes on deep in the planet's interior.

To have a magnetic field, scientists believe, a planet must fulfill these conditions:

• There must be a region within the planet that is liquid;
• The region must also be electrically conducting;
• There must be an energy source that sets the region in motion and then keeps it moving.

Neptune's magnetic field is tilted 47 degrees from the planet's rotation axis, and is offset at least 0.55 radii (about 13,500 kilometers or 8,500 miles) from the physical center. The dynamo electric currents produced within the planet, therefore, must be relatively closer to the surface than for Earth, Jupiter or Saturn. The field strength at the surface varies, depending on which hemisphere is being measured, from a maximum of more than 1 gauss in the southern hemisphere to a minimum of less than 0.1 gauss in the northern. (Earth's equatorial magnetic field at the surface is 0.32 gauss.) Because of its unusual orientation and the tilt of the planet's rotation axis, Neptune's magnetic field goes through dramatic changes as the planet rotates in the solar wind.

Voyager's first indication of the Neptunian magnetic field was the detection of periodic radio emissions from the planet. The spacecraft crossed the bow shock, the outer edge of the field that stands ahead of the planet like a shield in the solar wind, as a shock wave stands out before a supersonic airplane, at 7:38 a.m. August 24, and shortly thereafter entered the planet's magnetosphere. Voyager 2 remained within the magnetosphere for about 38 hours, or slightly more than two planetary rotations, before passing once again into the solarwind.

Because Neptune's magnetic field is so highly tilted, and the timing of the encounter was such that the south magnetic pole was very nearly pointed at the Sun, Voyager 2 flew into the southern cusp of the magnetosphere, providing scientists a unique opportunity to observe this region of a gigantic magnetic field.

Magnetospheric scientists compared Neptune's field with that of Uranus, which is tilted 59 degrees from the rotation axis, with a center that is offset by 0.3 Uranus radii. After Voyager 2 passed Uranus in January 1986, some scientists thought they might have seen the planet as its magnetic field was reversing direction. Others found it difficult to believe such a coincidence just happened as Voyager passed through the neighborhood. Scientists have no definite answers yet, but think that the tilt may be characteristic of flows in the interiors of both Uranus and Neptune and unrelated to either the high tilt of Uranus' rotation axis or possible field reversals at either planet.

Neptune's magnetic field polarity is the same as those of Jupiter and Saturn, and opposite to that of Earth.

Neptune's magnetic field provided another clue to the planet's structure and behavior. Observers on Earth hadn't been able to determine the length of a Neptunian day. Cloud motions are a poor indicator of the rotation of the bulk of the planet, since they are affected by strong winds and vary substantially with latitude. The best telescopic estimate was a rotation period of approximately 18 hours. The best indicator of the internal rotation period of the planet is periodic radio waves generated by the magnetic field. Voyager's planetary radio astronomy instrument measured these periodic radio waves, and determined that the rotation rate of the interior of Neptune is 16 hours, 7 minutes.

Voyager detected auroras, similar to the northern and southern lights on Earth, in Neptune's atmosphere. The auroras on Earth occur when energetic particles strike the atmosphere as they spiral down the magnetic field lines. But because of Neptune's complex magnetic field, the auroras are extremely complicated processes that occur over wide regions of the planet, not just near the planet's magnetic poles. The auroral power on Neptune is weak, estimated at about 50 million watts, compared to 100 billion watts on Earth.