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The Magnetosphere is the immense magnetic field that surrounds those celestial objects that have a ferrous liquid outer-core. One such example is the planet Earth, whose magnetic field we are all familiar with.

The field is thought be be caused by the movement of the iron-rich liquid of the outer core. The movement of this liquid forms a magnetic field stretching out up to many hundreds of times the radius of the earth. Because it is the liquid core that produces this, celestial objects such as the Moon do not have a magnetosphere. The Sun also has a magnetosphere. The cause of this is a subject of debate, some claim it has a liquid iron core, others a charged hydrogen/helium core. An argument for the iron model is the fact that the sun is powered on fusion and that Iron is the most stable element.

On Earth, the magnetic field is currently orientated so the magnetic poles are approximately 10° from their geographic counterparts. The field reverses its polarity every few hundred thousand years, something which could prove devestating to our modern, technological world. One theory is that the poles are currently in the process of reversing, which is supported by the fact that their magnetic flux density has been decreasing over the past few thousand years (currently 6×10-5 tesla). Given the long timescale of this process (in human terms, not geological) it is not a problem humans in the near future need to worry about.

The field is partly responsible for the magnificent aurorae (Australis and Borealis) that are visible at the Earth's poles. The other requirement is the solar wind, particles that are ejected from the Sun. As these particles hit the magnetosphere they cause it to deform. Most of these particles are deflected, but some are pulled into the regions around the poles, where they produce a physical reaction with the atmosphere, and give off photons. The reason that the polar lights are around the poles is due to the fact that this is where the deformation of the field creates an area called the magnetopause, where the solar wind can slip through.

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