Earth's magnetic field varies on timescales from
milliseconds to millions of years. Below are a couple of images of
various timescales indicating how Earth's magnetic field has varied
in the past. Click on the map menu below of Earth's magnetic field intensity
to see how it has changed over the past century. Note the growth of
the magnetic low over Brazil and the weakening of the magnetic high
in northern Canada. The map is generated from a mathematical model
of the geomagnetic field generated from observations.
The figure above shows the intensity of Earth's magnetic
field at Woods Hole, Massachusetts back to 1900. These are values estimated
from a mathematical model of Earth's magnetic field: the International Geomagnetic Reference Field (IGRF) based on satellite and in situ measurements. The IGRF is a spherical harmonic expansion of Earth's magnetic field that is recomputed every 5 years.
This picture above shows Earth magnetic dipole moment (a measure of how strong Earth's field is) plotted as a function of year since 1600. The data for this plot come from IGRF coefficients back to 1945 and then from a compilation by A.C. Fraser-Smith, Rev. of Geophysics, 25, 1-16, 1987.
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