It is theorised that the source of the magnetic field is a result of many geophysical processes including a geomagnetic dynamo and several smaller currents and ‘electrojets’. The geomagnetic dynamo is seen as the most important source because it creates a base field upon which other currents can form.
Dynamo Theory:
In which motion of the fluid material in the outer core of the Earth moves conducting material (such as liquid iron) across the existing weak magnetic field and generates an electric current. It is hypothesised that the heat from the radioactive decay in the core of the Earth induces convective movement. The electric current then interacts with the fluid motion to create a second stronger magnetic field and both lie along the axis of the Earth.
Variation of the Earth’s Magnetic Field:
Magnetic fields can also be affected by seasonal, solar flares and lunar changes because of their impact on the ‘ionospheric dynamo’ (the movement of charged particles of the ionosphere across Earth’s main magnetic field).
Geomagnetic Reversal:
One theory suggests that direction of the dipoles reverse approximately every 300,000 to 1,000,000 years. On a geological scale this reversal is very rapid - taking place over 5000 years. However, the time between reversals follows no discovered pattern therefore there could be many years of one polarity and then only short interval at the opposite polarity.
Diagram: Magnetic Fields of Earth
"...dipoles reverse approximately every 300,000 to 1,000,000 years." "