How are planetary magnetic fields produced?

In basic electrodynamics, all magnetic fields are produced by currents of charge in motion, so ultimately within each planet there must be a rotating current of conducting, charged material. There is a reasonably good theory of solar and planetary magneto-dynamics worked out in the 1960's and 1970's by Eugene Parker that relates the rotation period and conductivity of the Sun and each planet to the geometry and strength of their magnetic fields. It even predicts that periodically, these magnetic fields should change their polarity. The Sun does this every 22 years ( twice the sunspot cycle); the Earth every 250,000 years or so. The periods can actually be computed just by knowing the conductivity and rotation period of the Sun and Earth.

One feature of planetary magnetic fields is that they are almost always NOT aligned with the rotation axis of the planet. This indicates that the core of a planet is subject to its own physics, and that the flows of conducting material deep inside do not slavishly follow the rotation axis of the planet. In fact, the magnetic field of Neptune is located 1/3 of the way to the surface from its center!

 

There is still a lot that astronomers do not understand about how planetary and stellar magnetic fields are generated, maintained, and undergo periodic changes, but empirically there is quite a bit we have learned about them in the last 50-100 years!

Here is an excerpt from the C.T. Russel's review paper on planetary magnetospheres published in Science Progress, 75, 93-105, 1991....

Mercury The magnetic moment of Mercury is about one 1/3000th of the terrestrial magnetic moment. The equatorial surface magnetic field strength is about 250 nT. Mercury has been explored by only one spacecraft Mariner 10 which passed by Mercury 3 times in 1974 and 1975. On two of these passes the spacecraft passed through the wake of the planet encountering a mini-magnetosphere much like that of the Earth. These two passes gave us only a brief glimpse of the nature of the Mercury magnetosphere. This glimpse was not enough to precisely determine the strength of the magnetic moment of the planet. It did however suggest that the magnetosphere more efficiently extracts energy from the solar wind than does the Earth's magnetosphere. Scientists hope to revisit Mercury in the future with one or more orbiting spacecraft, but presently it is expected that this will not happen until early in the 21st century.

Earth The equatorial surface field of the Earth is about 31,000 nT. It is strong enough to activate rudimentary magnetic compasses and has been used as a navigational aid for at least 1000 years. The investigation of the earth's magnetic field began in about the 16th century but reached its zenith in the space age when it could be more fully explored with spacecraft. The spacecraft which have examined the Earth's magnetosphere are too numerous to name and have been launched by all the spacefaring nations. At present the most active area of research in magnetospheric physics is energy transfer from the solar wind to the magnetosphere. In the mid-1990's a consortium of space agencies (ESA, Intercosmos and NASA) are going to launch a flotilla of spacecraft into the magnetosphere to study this problem. This program is called the International Solar Terrestrial Program and will consist of over 15 different spacecraft. Jupiter The magnetic moment of Jupiter, as befitting the largest planet in the solar system, is also the largest of the planetary system over 10,000 times that of the earth. Its equatorial surface field is over 10 times that of the Earth. The strength of its magnetic field combined with the weakness of the solar wind at Jupiter produces a magnetosphere that is enormous. The sun could easily fit inside the magnetosphere. Its tail is thought to extend past Saturn, over 5 AU away. If Jupiter's magnetosphere could be seen from Earth it would appear to be larger than the Earth's moon. Deep inside the jovian magnetosphere orbit the Galilean satellites. One of these, Io, has a volcanically produced atmosphere that is constantly being bombarded by the intense radiation belts of jupiter. This bombardment knocks atoms out of the atmosphere of Io into the magnetosphere of Jupiter where they become ionized. This process produces a torus, or doughnut, of hot ions circling Jupiter near Io's orbit. This torus together with the enormous electrical and magnetic forces in the Jovian magnetosphere leads to intense radiation belts and radio emissions. These emissions can be detected from Earth and were the first indication of Jupiter's enormous magnetic field well before the first interplanetary spacecraft were launched.

Jupiter has been visited four times by spacecraft: Pioneer 10 in 1973; Pioneer 11 in 1974; and Voyager 1 and 2 in 1979. Each of these spacecraft were on flyby trajectories. At this writing the Galileo spacecraft is on its way to Jupiter when it will be injected into an elliptic near equatorial orbit in 1995.

Saturn The magnetosphere of Saturn is quite benign compared to that of Jupiter. Since Saturn is a smaller planet, its conducting core in which the planetary magnetic field is generated is smaller, and so is the planetary magnetic field. The magnetic moment of Saturn is 580 times that of the Earth but its equatorial surface magnetic field strength is about equal that of the Earth. In stark contrast to the magnetic fields of all the other planets, the Saturnian dipole moment is not tilted with respect to the rotation axis of the planet. This observation was a great surprise to those studying planetary magnetic dynamos. Saturn's ring system absorbs radiation belt particles so that the radiation belts are weaker than at Jupiter and none of Saturn's moons exhibits volcanic activity similar to that of Io. As a consequence Saturn's radiation belt resemble more those of the Earth than those of Jupiter and few radio emissions are produced.

Saturn has been visited by 3 spacecraft Pioneer 11 in 1979, Voyager 1 in 1980 and Voyager 2 in 1981. Each of these were on flyby trajectories. Currently, NASA and ESA are working on an orbiter/probe mission called Cassini/Huygens which is scheduled to arrive at Saturn early in the 21st century.

Uranus and Neptune The magnetic fields of Uranus and Neptune are quite unlike those of the other planets. The magnetic fields are quite irregular and cannot be well represented by a simple dipole field. When a dipole moment is fit to the flyby data available from Voyager 2 which flew by these planets in 1986 and 1989 respectively, a very large tilt angle between the rotation axis and the dipole axis is found, about 50 degrees. The magnetic fields are also much weaker than those found at Jupiter and Saturn. The magnetic moments are about 40 times that of Earth and their surface magnetic fields slightly less than the terrestrial field. The reason for this weakness and the irregularity may be that the magnetic field is generated, not in a deep molten core like the Earth's, but in salty ice/water oceans closer to the surface. The radiation both of Uranus and Neptune are quite weak. There are no present plans to explore these planets further.

 


Copyright 1997 Dr. Sten Odenwald

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