When conducting metals such as iron and nickel move about, they generate magnetic fields. Because the entire iron-nickel outer core of Earth is constantly in motion, circulating around the inner core because of a process called convection, it produces an enormous magnetic field that extends many thousands of miles out into space.
The significance of this field to life on Earth becomes obvious when one considers the hazards posed by the Sun, a runaway nuclear reactor located just ninety-three million miles away. The Sun's enormous gravity ensures that most of its exploding matter never gets too far away, but some particles do escape. Collectively, they are known as the solar wind. In order to break free of the Sun's gravity, they have to be moving very fast, about a million miles an hour. This high rate of speed makes them dangerous to anything in their path, including life on Earth. Fortunately, the magnetic field generated by the circulating metals in Earth's outer core shields us from these particles.
The process is easily observed in the nighttime sky above Earth's two magnetic poles. While most of the charged particles are deflected into space, a few do collect at the poles, where they release energy in the form of light. (Think of a truck plowing snow with its windows open. Although most of the snow is pushed aside, a little does get in the windows.) Around the northern magnetic pole, this release of energy is called the aurora borealis, or northern lights. Around the southern pole, it is known as the aurora australis, or southern lights.
If not for the shielding effect of Earth's magnetic field, the planet would be subject to a constant bombardment of highly charged particles akin to solar sandblasting. Some scientists believe the effect would be sufficient to strip Earth of its atmosphere and its oceans, leaving the planet as sterile and lifeless as Mars, which has no molten outer core and thus no magnetic shield. (‘The Bedside Baccalaureate’, edited by David Rubel)