Photoglossary

Planetary Magnetospheres   Interplanetary space was once believed to be empty, but that is not the case. It has two primary constituents - the solar wind and planetary magnetospheres. The solar wind is an extension of the hot, million degree, outer atmosphere of the Sun (corona). It is an electrified gas, or plasma, which consists of free electrons and free atomic nuclei. A candle flame is an example of plasma. Planetary magnetospheres are the other important constituents of space. A magnetosphere is the region of space surrounding a planet which is dominated its magnetic field rather than the solar wind. When the solar wind encounters a planetary magnetic field, the magnetic field deflects the solar wind, and forms a protective cocoon around the planet called the magnetosphere. The solar wind shapes the magnetosphere, compressing it on the sunward side and drawing it out to a long tail on the nightward side. Earth's magnetosphere is especially important because it protects the planet from the potentially lethal particles of the solar wind which could be damaging to many forms of life. Two ingredients are necessary in order to generate a planetary magnetic field--an electrically conductive liquid in the interior of the planetary body, and sufficiently rapid rotation to create circulation of the metallic liquid. On Earth, the outer core is composed of a highly conductive liquid alloy of iron and nickel. Earth's rapid rotation, once every 24 hours, creates convection in this electrically conductive liquid, which thereby generates the magnetic field. Not all planets have magnetic fields. Venus does not have one. This may be due to its extremely slow rotation rate. It takes 243 Earth days for Venus to spin once on its axis. Mars also has no detectable global magnetic field. Although its rotation is rapid enough, it may no longer have an electrically conductive liquid in its interior. Although most particles from the solar wind are deflected, sometimes they do penetrate the Earth's magnetosphere at the north and south magnetic poles. Here they strike molecules of hydrogen, nitrogen, and oxygen causing them to emit red, green, and blue light. The shimmering glow that is produced are the aurora borealis and aurora australis, or the northern and southern lights.   Illustration Credit: NASA      Io Home Page Photoglossary Home Page