The magnetic field near the Earth is from a combination of three sources:

97 - 99 % Main Field (From electric currents in the Outer Core)

1 - 2 % Crustal Field (From magnetized rock in the Crust)

1 - 2 % External Field (From ionized particles above the Earth)

The Main Field is near dipolar and varies in strength from approximately 30,000 nT near the equator to 60,000 nT at the poles. Its secular variation or amount of change is about 1% per year. The north and south pole undergo a reversal (change in direction) every 500,000 years, on average.

The External Field varies on time scales of seconds to days, primarily due to solar interactions. These fields result from current systems and range in intensity from fractions of a nT to thousands of nT.

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Responsible NASA official:Dr. Herbert Frey
Web Curator: Katia Nazarova (Raytheon)
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Last modified on May 15, 2001