Earth's magnetic field is produced by a number of current systems. By far the most important from the standpoint of trapped radiation is the interior current system of the Earth's dynamo. Deep in the Earth the convection of hot, conducting material forms a system of moving conductors. The motion of these conductors across the geomagnetic field induces electric currents, which in turn reinforce the magnetic field. Thus, convection driven by heat acts as a self-exciting dynamo to produce the main part of the geomagnetic field. While this field is steady on a time scale of less than a year, secular changes do occur and have been measured directly for several centuries. Systematic variations in the shape of the field are taking place and the overall geomagnetic field is becoming weaker at a rate which, if continued, will cause the Earth's field to disappear in about 2000 years. However, the present downward trend may only be a temporary fluctuation and could change at any time.

There is clear geological evidence that the polarity of the geomagnetic field has reversed at irregular intervals of about one million years. The reason for these reversals is not known, but explanations proposed include internally driven oscillations similar to those causing the solar sunspot cycles.