This cutaway shows the magnetic field lines due the Earth's magnetic field generated in the molten outer core. The positive and negative polarity portions of the field are shown as either orange or blue. Half of the core shows the strength of the radial field (blue and yellow being opposite polarities), the other half shows the tangential component of the fluid velocities which are responsible for maintaining the field and for generating the secular variation (the change in the magnetic field with time). The "geodynamo" process has maintained the field for almost the entire life of the Earth, four and a half billion years.
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