The aurora borealis, seen here over northern Iceland, happens when electrically charged particles accelerate along the lines of the Earth's magnetic field into the upper atmosphere, where they collide with gas atoms, causing the atoms to give off light. (Mariana Suarez/AFP via Getty Images) At a Glance Earth's original magnetic field formed 4.2 billion years ago.Zircon crystals indicate the field was much stronger than previously thought.The field shielded the planet from cosmic rays and solar winds. Earth's first magnetic field...