Neptune's magnetic field

Neptune is the eighth planet outward from the Sun. Although classified as a Jovian or gas giant planet, Neptune shares more similarities with Uranus than with Jupiter or Saturn. Neptune’s magnetic field is inferred to be generated like that which produces Uranus’s magnetic field.

Overview

The first planet discovered during the telescope age was Uranus, the seventh planet from the Sun. The planets from Mercury (closest to the Sun) through Saturn (sixth from the Sun) were known to the ancients. No records pinpoint a discovery event in each of these planets’ cases. After Sir William Herschel announced the discovery of Uranus in 1781, observers began making precise determinations of Uranus’s orbit around the Sun.

Unexpected motions in the orbit of Uranus led many to suspect that a planet exists farther out in the solar system, one that exerts gravitational influences on Uranus that perturb its orbit. A search followed for a new world to join the family of seven planets in the contemporary solar system model. This search was extensively guided by mathematical analysis using the young celestial mechanics field based on an understanding of Newtonian gravitation. The basis of that analysis involved calculating where a body would have to be located beyond Uranus to account for measured variations in the orbit of Uranus.

Who discovered Neptune has been a matter of debate. Calculations in 1843 by John Couch Adams were dispatched to Astronomer Royal Sir George Airy, but the latter needed to be convinced. He even asked Adams to send proof of the validity of his work. Adams appeared disappointed or insulted by Airy’s dismissal, and he never responded. Two years later, Urbain Le Verrier published an independent calculation that generated some interest in Airy's work. Efforts at Cambridge Observatory failed to find the eighth planet. Still, when Le Verrier asked the Berlin Observatory to aim a telescope toward a particular region of the sky, Johann Gottfried Galle reacted immediately to the request. Neptune was discovered on September 23, 1846. Its position was just one degree off from Le Verrier’s calculation and twelve from Adams’s. In the aftermath, the British and French argued over who had discovered the new planet. Adding to the controversy were Cambridge records indicating that Neptune was charted months earlier than Galle’s observation. However, those records did not recognize Neptune as a planet. Even in the last years of the twentieth century, some argued against giving Le Verrier and Adams joint credit as Neptune’s discoverers.

In the decades following its discovery, information about Neptune had been garnered only by using ever-increasingly large Earth-based telescopes. Then, in 1977, an opportunity arose to send a single spacecraft to explore the outer solar system. Using gravity assists in turn from Jupiter to Saturn, Uranus to Neptune. The National Aeronautics and Space Administration (NASA) was able to launch Voyager 2 on such a “grand tour.” Voyager 2 encountered Jupiter in 1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989.

Just as it had at Uranus, Voyager 2 would answer whether Neptune has a magnetic field. The answer, be it in the affirmative or the negative, would provide considerable insight into the interior structure of Neptune. Voyager 2 indeed found Neptune to be a dynamic world that generates more heat energy than the radiation it receives from the Sun. It generates a complex and dynamic magnetic field that has left scientists with much to consider concerning the implications of that magnetic field for the structure and evolution of the planet.

Knowledge Gained

There are many means by which a magnetic field can be detected. Most of them use electromagnetic induction in that a voltage is induced in a coil by intercepting a time-dependent magnetic flux. That is the basis of sophisticated magnetometers, such as that flown on the Voyager 2 spacecraft. Scientists had to be careful in interpreting readings from the spacecraft’s magnetometer, in that it picked up magnetic effects created by the solar wind even at the distant position of Neptune. Solar wind particles become trapped within a planet’s magnetosphere, and that generates its magnetic field.

The Voyager 2 Neptune encounter began on June 5, 1989, with the spacecraft still 117 million kilometers from the planet. Voyager first detected radio waves, indicating it had crossed into Neptune’s magnetosphere, on August 24. The spacecraft came within 4,400 kilometers of Neptune’s upper atmosphere on August 25. The spacecraft remained inside Neptune’s magnetosphere for thirty-eight hours and noted that Neptune’s magnetic “bubble” was affected by both the planet’s ring system and its satellites. The planet’s magnetosphere exists for thirty-five Neptune radii on the sunward side and out to seventy-two radii behind Earth. That varies with the strength of the solar wind. Several days later, the Neptune near-encounter phase ended after Voyager passed close to Neptune’s large satellite Triton. Triton significantly alters the outermost portion of Neptune’s magnetosphere. Triton revolves around Neptune in a retrograde fashion. Charged particles within the magnetosphere near Triton include nitrogen ions from this satellite's cryovolcano eruptions.

Voyager 2 also found radiation belts trapped in the planet’s expansive magnetosphere, although charged particle density in Neptune’s magnetosphere was found to be less than that at Uranus. The composition of the trapped particles primarily includes protons, electrons, and ionized molecular hydrogen and helium.

Analysis of Voyager 2 data from the spacecraft’s magnetometer and radio astronomy experiment indicated that Neptune’s magnetic field varies considerably as the planet rotates in the presence of the solar wind. Neptune’s magnetic field is dominated by its dipole character, but it also has a vital quadrupole component. A quadrupole can be understood as two bar magnets oriented at right angles to each other. The dipole field strength near the equatorial region is 1.42 microteslas. Neptune’s dipole strength is 2.2x10¹⁷ tesla-meters cubed. Uranus’s dipole moment is nearly double that value. The planet’s quadrupole moment is mainly due to an offset of the field center from the planet’s center. Octupole and higher moments could not be determined accurately.

The center of Neptune’s field is displaced from the planet’s center by approximately 0.55 planetary radii, which corresponds to 55 percent of the planet’s radius. The planet’s magnetic field is also tilted significantly, 46.9 degrees relative to the planet’s rotational axis.

On Earth, trapped particles spiral around magnetic field lines and dip into the atmosphere over polar regions, creating auroras by exciting atmospheric gases. Voyager 2 detected auroral activity in Neptune’s atmosphere. Still, largely because of the complexity of Neptune’s magnetic field structure, the observed auroral activity was not confined to areas near the planet's magnetic poles.

Observations of Neptune’s magnetic field provided insight into Neptune’s interior. Like Uranus, Neptune is believed to have a liquid layer of high electrical conductivity in motion powered by internal heat flow from the core beneath. This results in a dynamo effect from this fluid mantle region, composed of water, ammonia, methane, and lesser amounts of volatile substances. Planetary rotation results in complex variations of Neptune’s magnetic field.

Neptune is a radio source because of its rotation and the fact that it possesses a reasonably strong magnetic field. The rotation of the magnetic field is determined by the observed periodicity in the planet’s radio emissions. That has provided a better means of deciding Neptune’s rotation rate than attempting to monitor the time taken for atmospheric features, such as streamers or large storms, to make a complete rotation. A gas giant’s atmosphere rotates differentially. The planetary core produces the magnetic field, and radio emissions led scientists to determine that Neptune’s core rotates once every 16.11667 hours.

Context

After Uranus, Neptune was the second planet in the solar system to be discovered by telescopic observations. Just as the search for Neptune originated based on irregularities noticed in the orbit of Uranus, perturbations of Neptune’s orbit led to a search for yet another planet in the outer solar system. In 1930, Pluto was discovered by Clyde Tombaugh, but ironically, Pluto did not explain Neptune’s orbital irregularities. Also, in 2006, the International Astronomical Union no longer classified Pluto as a planet; Pluto is considered to be either a dwarf planet or the first representative of a class of objects called plutoids.

For more than one hundred years since the planet’s discovery, using ever-larger Earth-based optical telescopes was the only means to investigate Neptune. Then, with the advent of the space age and the confluence of a particular configuration of planets in the outer solar system that arises only once every 176 years, it became possible to send a spacecraft from Earth: Voyager 2. It traveled, in turn, to Jupiter, Saturn, Uranus, and Neptune. The Voyager 2 flyby of Neptune generated the best available data and images about this mysterious blue world at that time.

After that Voyager flyby, the Hubble Space Telescope was used in the 1990s and 2000s to make continual observations of Neptune. However, only spacecraft missions can advance our knowledge of Neptune’s magnetic field. A proposal offered in 2002 for a Neptune orbiter probe was investigated in 2003 as part of NASA’s response to the Bush administration’s Vision for Space Exploration. Still, it was not funded at that time. Planetary scientists have continued to indicate strong support for a mission in the class of the Cassini orbiter to investigate Uranus and Neptune.

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