Since the time of the ancient greeks and the ancient chinese it was known thatlodestone was magnetized, and that under proper conditions it tended to prefera particular orientation. Primitive compasses were used for navigation as long ago as 2000 B.C by some historical accounts. By the 17th century the Earth'smagnetic field was pretty well established, and there were speculations that it was shaped like the field of a bar magnet. During the 18th and 19th centuries, the shape of this field near the Earth's surface came under increasing study, and during the dawn of the Space Age, the first magnetometers were launched into space to measure the complete 3-dimensional shape of the field. The following sketch gives you the essential ingredients of this field, called the magnetosphere:

The magnetosphere is very complex. The Sun emits a solar wind of charged electrons and protons traveling at over 400 km/sec which flows along the open magnetic field lines from giant solar coronal holes. This magnetized plasmaimpacts the Earth's 'bar magnet' shaped magnetosphere at the magnetopause and sweeps it back into a tear-drop shape that extends over 1000 times the radius of the Earth in the direction opposite the Sun. Some of these particles get trapped in the so-called Van Allen radiation belts, a major hazard to astronauts, but physicists still do not know how this actually happens.

Why do physicists study the magnetosphere? Because there are many importantphenomena that seem to have their origins in this invisible cloak that the Earth covers itself in. Solar activity can cause interference in long distance communication by affecting the properties of the Earth's ionospherewhich is partially shielded by the magnetosphere. Aurora Borealis are produced when charged particles trapped in the magnetosphere get 'dumped' into the atmosphere causing some gases to fluoresce in 'curtains' and 'ribbons' of light. Physicists think they understand how magnetospheres are generated by a planet, but there are many details that remain poorly understood. The magnetosphere protects life on Earth from potentially mutagenic cosmic radiation, however, the geologic record and direct measurements show that theEarth's magnetic field reverses its polarity every 250,000 years or so. Over a dozen of these polarity reversals have been found in magnetized rock, but what is most disturbing is that the Earth's field is CURRENTLY declining in strength. It is predicted that in 2 - 5,000 years the Earth's field will temporarily vanish, before reinstating itself with a reversed polarity. Manybiologists have wondered what will happen to life on Earth when the magnetic field vanishes for several centuries.

All of these things make it very important for physicists to learn as much as they can about Earth's magnetosphere. If you want to learn more about the magnetosphere, visit the NASA, IMAGE satellite education area which I have created for this space mission. Click on the magnetosphere icon in the upper left corner, or on the 'resources' link.

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