Why Does the Earth Have a Magnetic Field?

Earth outer core is a 2,200-km-thick layer of liquid iron-nickel alloy at temperatures of 4,000-6,000 K. Heat flowing from the solidifying inner core drives vigorous convection currents. Because the liquid is electrically conductive and the planet rotates, these currents generate and sustain a self-exciting magnetic dynamo (the Coriolis effect organises the flow). The resulting dipole field extends tens of thousands of kilometres into space, forming the magnetosphere that deflects solar wind particles and cosmic radiation. Without it, Earth atmosphere would be gradually stripped away, as may have happened to Mars after its dynamo shut down roughly 4 billion years ago. The field periodically reverses polarity; the last reversal (Brunhes-Matuyama) occurred about 780,000 years ago.