Geomagnetism

Geomagnetism includes all the phenomena related to the earth's magnetic field.

The Earth has a complex magnetic field that varies in space and time and that protects the atmosphere and life on Earth from the harmful effects of solar wind and cosmic rays. Without it the Earth's atmosphere would be blown away just as the solar wind produces comets' tails. The charged particles emitted by the Sun and cosmic rays would prevent complex life from developing. The most spectacular phenomenon associated with geomagnetism is the polar aurora.

Aurora Borealis (Credit: Gilles Boutin).

The magnetic field produced by the planet makes up what we call the magnetosphere. In addition to the dipolar field, which is relatively stable but which inverts periodically, there is a more complex fluctuating component that depends on the Sun's activity and the convection currents inside the Earth's core. We have good reason to suppose that the origin of the Earth's magnetic field lies in the fluid part of the core composed of an iron-nickel alloy. A process called the dynamo effect is said to be at work.

Inverting periodically, the fossilizing of the magnetic field in lava and sediments containing iron particles has been, and still is, a valuable indicator of the movements of tectonic plates in the course of the geological history of the planet. The discovery and above all the establishing of the tectonic plate theory owe a lot to the study of geomagnetism and the associated magnetic inversions.

The nature of the materials making up the Earth's crust influences the Earth’s local magnetic field. We can use this effect for geophysical prospecting and to study the processes and structures related to volcanism.

To find out more:

• http://gsc.nrcan.gc.ca/geomag/index_f.php ;
• http://geomag.usgs.gov/intro.php ;
• Articles by Philippe Cardin.