Magnetospheres: Plasma of the Solar System
We often hear about magnetic fields on Earth as well as surrounding other planets. Also known as magnetospheres, what exactly are they and how do they affect our universe?
Magnetospheres are composed of plasma -- highly energetic ions and electrons that behave as a fluid. Studying the properties of these fluids in magnetospheres of other planets provides information that will help us better understand our own magnetosphere here on Earth. The Earth's own magnetic field is probably created in its core, believed to contain molten iron, by a "fluid dynamo." These are rather special conditions and scientists who studied the Earth's dynamo in the 1950's and 1960's must have wondered whether the Earth's planetary magnetism was unique.
We now know better -- space probes have found that Jupiter, Saturn, Uranus and Neptune all have magnetic fields, as does tiny Mercury. The Moon has patches of magnetized rocks and might have had a field when those rocks formed long ago. Up until 1997 it was unknown whether Mars had a magnetic field, but the Mars Global Surveyor mission discovered it had patches of magnetic rocks as well.
The sources of magnetic fields on other planets are quite different from Earth's and their origins have perplexed scientists for decades. For example, where do the magnetic fields of Jupiter and Saturn originate -- in their cores, made up of highly pressurized hydrogen? And what about Uranus and Neptune?
Where do their internal currents come from? Water ice? Methane?
According to R.M. Nelson in "Mercury: The Forgotten Planet," the magnetic fields of Mercury, Mars and Earth's Moon are of a different origin -- magnetized rocks, from which lava poured out and became magnetized themselves. The Messenger spacecraft, set to reach Mercury in 2008, may shed more light about the source of these magnetic rocks.
As for the rest of the solar system: Venus does not appear to have a magnetic field, yet Jupiter, the largest planet in our solar system also has the largest and most powerful magnetic field. Space probes Pioneer 10 and 11, Voyager 1 and 2, Ulysses and Galileo, first explored its immense magnetosphere.
Differences in Scale
The magnetospheres of the giant planets differ from the Earth's in at least four ways. First, they are much bigger, not only because the planetary magnets are stronger but also because the solar wind weakens as it moves away from the sun and spreads out. Both of these factors cause the solar wind to be stopped further away from the planet than is the case with Earth.
The speed of the solar wind however, remains the same, about 400 km/sec. As a result, the wind needs a much longer time to traverse the length of the magnetosphere.
With the Earth's magnetosphere, it takes the solar wind about one hour to advance from the "nose" to the distant tail regions where ISEE-3 and Geotail have probed it, some 200 Re downstream. During that one hour the Earth rotates by a rather small angle, 15 degrees, and if "open" field lines in the lobes connect it to the solar wind, those lines might become twisted by about 15 degrees.
If Jupiter's magnetosphere has the same proportions, the solar wind would need 2-3 days to cover the corresponding distance (equal to about half the Earth-Sun distance!), during which the planet might have rotated 5-7 times around its axis. One might therefore expect the lobes of Jupiter's magnetotail (and Saturn's too) to be severely twisted, and the Galileo mission might be the first opportunity to examine this point. All other probes sent to Jupiter used the planet to gain extra speed; the way "Wind" used the moon and the orbits required for this maneuver kept them out of the lobes.
Secondly, all of these planets possess satellites and rings within their radiation belts (all four have rings, but only Saturn's rings are big enough to easily be seen from Earth). These absorb some of the trapped ions and electrons and produce dips in the profiles of the belts.
A third difference is the role of planetary rotation. The Earth is surrounded by a cloud of cool plasma-essentially, the upward continuation of the ionosphere-which extends to about 5 Earth radii and which rotates the Earth.
Tilt
Finally, there exist differences in the tilt of the magnetic axis. Earth has a magnetic axis inclined by 11.2 degrees to its rotation axis, which itself is inclined by 23.5 degrees to the direction perpendicular to the plan of the Earth's orbit; that plane also contains the direction from which the solar wind arrives.
Jupiter's magnetic axis is inclined to its rotation axis by about the same amount as Earth's. Its magnetic north-south polarity is the opposite of the Earth's-but incidentally, there is evidence that Earth's polarity has reversed several times in the distant past. Saturn's magnetic axis seems exactly aligned with its rotation axis.
Uranus's rotation axis is nearly parallel to its orbital plane. At the time of Voyager 2's 1986 flyby, the axis had pointed almost exactly at the sun. But as Voyager 2 found, the magnetic axis of Uranus was actually steeply inclined to its rotation axis, at nearly 60 degrees, causing it to spin around like the axis of a top that is about to fall over. Neptune's magnetic axis was similar-about 47 degrees.
All of this suggests that not only isn't the Earth's magnetosphere unique, but different kinds of magnetospheres are possible and some of them can be found throughout our solar system. The study of magnetospheres is of great scientific interest and practical importance because it provides a natural laboratory for studying cosmic plasmas, but different examples of such plasmas are also accessible and waiting to be studied in more detail.
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