My girlfriend Hannah asked me the other day how the Earth’s magnetic field was generated. I knew a little about the rotation of the liquid core of the Earth having to do with producing its protective magnetic field. However, I couldn’t full satisfy her question to her nor to myself. So I thought I’d do a little investigating.
Geographic north pole is the point on the north tip of the Earth's rotational axis. Magnetic North Pole is the north tip of the magnetic dipole which is tilted at 11 degrees with respect to the rotational axis
The Earth’s core drives the magnetic field of the Earth. The inner core is believed to be primarily a solid ball of iron, which is surrounded by a liquid outer core made up of effectively an ocean sphere of molten iron and nickel. These oceans of molten metal are in perpetual motion within the Earth.
When a celestial body, such as the Earth or the Sun, has a magnetic field being generated around it this is caused by a phenomenon named geodynamo.
In geophysics, dynamo theory proposes a mechanism by which a celestial body such as the Earth or a star generates a magnetic field. The theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time scales.
So three things are clearly required:
1. a rotating core
2. a liquid metal
3. the presence of convection
The inner core, outer core and mantle of the Earth were formed early on in the planet’s life when it was still incredibly hot. When you heat a mixture of solid material made up of rock and iron to very high temperatures the iron will begin to separate because it is heavy. And so it will start to sink under gravity to the centre of the earth, and thus the core will be formed.
So as a result of this separation of molten rock and metals we get these different layers of the Earth, and as the heat from the centre of the Earth started escaping it caused the liquid metal within the core to move. This perpetual motion within the depths of the Earth is what generates its magnetic field.
If you want to generate a magnetic field the way the earth does all you need is a metal, like iron in our case, it needs to be liquid, which means it has to be very hot, but you also need a temperature difference. Where the heat flows from the hot inner core to the cooler mantle of the planet it causes convection currents to form in the molten metal of the outer core. Those motions, through the process of electromagnetic induction, is the way in which the magnetic field of the Earth is produced.
Without the Earth’s magnetic field we’d have a lot more to worry about besides out compasses not working. Its most important role is protecting the biosphere on the surface of the planet from being hit by the infinite number of charged particles thrown at us by the Sun (solar wind). If you want to know what happens to a planet when its core ceases to rotate, and it loses its magnetic field, take a look at the Moon.
Earth's magnetic field deflecting solar wind
For more information I highly recommend watching this documentary on the core of the Earth.
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