Magnetism is a force in nature that is produced by electric fields in motion. This movement can involve electrons 'spinning' around atomic nuclei, flowing through a conducting wire or ions moving through space in an organized stream.
Earth's magnetic field is familiar to us through its effects: our compasses point to the magnetic poles (north and south); it protects our atmosphere from the blast of the solar wind; and particles interact with it to produce the auroras, or northern and southern lights. Similarly, the magnetic fields of Mercury, Jupiter, Saturn, Uranus, and Neptune are detectable with compasses, and we have seen beautiful auroras on Jupiter and Saturn!
Planetary magnetic fields originate from processes deep in each planet's interior. Earth's is generated from the electric current caused by the flow of molten metallic material within its outer core. Mercury's may be generated from its liquid core. Jupiter and Saturn are composed of gases crushed to such incredible pressures that they are forced beyond the common states of liquid, solid, or gas that we find on Earth. One such a layer inside Jupiter and Saturn is metallic hydrogen, and the electric current caused by swirling movements in this substance produces a magnetic field so large that the tail of Jupiter's magnetic field reaches the edge of Saturn's orbit!
Scientists map planetary magnetic fields with a more sophisticated version of a compass, called a magnetometer. They also "listen" for the radio signals given off by charged particles as they move through the magnetic field, and measure the properties of ions and electrons directly with particle detectors.
The Sun has a very large and complex magnetic field. It actually extends far out into space, beyond the furthest planet. The solar wind, the stream of charged particles that flows outward from the Sun, carries the Sun's magnetic field to the planets and beyond. While the basic shape of the Sun's magnetic field is like the shape of Earth's field, with a north and south pole, superimposed on this basic field is a much more complex series of local fields that vary over time. Places where the Sun's magnetic field is especially strong are called active regions, and often produce sunspots. Disruptions in magnetic fields near active regions can create energetic explosions on the Sun such as solar flares and coronal mass ejections. The exact nature and source of the Sun's magnetic field are areas of ongoing research. Turbulent motions of charged plasmas in the Sun's convective zone clearly play a role.
In spite of the low density, the solar wind and its accompanying magnetic fields are strong enough to interact with the planets and their magnetic fields to shape magnetospheres. A magnetosphere is the region surrounding a planet where the planet's magnetic field dominates. Because the ions in the solar plasma are charged, they interact with these magnetic fields, and solar wind particles are swept around planetary magnetospheres, as are particles from the planet's atmosphere. At Jupiter and Saturn, the plasma inside the magnetosphere is almost entirely from their moons. Robotic missions investigating these worlds are challenged by the energetic charged particles that are trapped in these planets' magnetic fields as radiation belts.
The shape of the Earth's magnetosphere is the direct result of being blasted by solar wind. Solar wind compresses its sunward side to a distance of only 6 to 10 times the radius of the Earth. Solar wind drags out the night-side magnetosphere to possibly 1,000 times Earth's radius; this extension of the magnetosphere is known as the magnetotail. Many other planets in our solar system have magnetospheres of similar, solar wind-influenced shapes.
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| Tracking a Solar Storm: Adventures in Geospace || These five short articles provide an excellent background to understand Earth's magnetosphere. The articles; Adventures in Geospace, Magnetism, A Magnet in Space, Electricity, Electromagnetism, and The Earth's Magnetosphere; explain the basic of magnetism, electricity, electromagnetism and the structure and origin of Earth's magnetic field. |
| Magnetism from A to B || This article by Sun-Earth Day has a variety of details about magnetism. |