How Auroras Work

When electricity runs through a neon sign, fast-moving electrons excite the gas inside the glass tube, and it glows in that gas's own color. The aurora that lights up the night sky shines in a very similar way.

The Sun doesn't just sit there giving off light. It also blasts out huge numbers of charged particles in every direction. This is called the "solar wind." These particles travel toward Earth without a break, hundreds of times faster than a bullet.

When the fast solar particles slam into the air around Earth, the air particles take in energy and become excited. Then, when they settle back to their calm state, they release the energy they were holding as light. Countless tiny flashes of light pile up and form a giant curtain of light across the sky.

The solar wind streams toward the whole Earth evenly. So why can we see auroras only near the North and South Poles, and not at the equator or in the middle latitudes?

The secret is that Earth itself is a giant magnet. It wears an invisible, huge "magnetic shield" that pushes away dangerous particles from space. Thanks to this shield, living things can stay safe.

Solar particles rushing toward Earth get blocked by the magnetic field. They can't fall straight to the ground. Instead, they slide down toward the two ends of the magnet, the North and South Poles. It's like sprinkling iron filings around a magnet: they line up tightly and gather at both poles. Because particles gathered in a dense stream above the polar regions pour into the atmosphere and collide violently with the air, auroras mostly form in ring-shaped zones around the poles. More precisely, it's not that the solar wind slams down directly. Instead, electrons that had built up inside Earth's magnetic field get accelerated by the solar wind's energy, rush down, and cause the glow.

The different colors of an aurora, such as green, red, and purple, depend on "what height and which gas" the solar particles collide with.

At about 100 to 200 kilometers above the ground, the particles hit oxygen atoms and make the mysterious green light you often see in photos. Higher up, above 200 kilometers, where the air is thin, oxygen gives off red light. Below 100 kilometers, at lower altitudes, the particles hit nitrogen molecules and make bright pink or purple light.

When an explosion such as a coronal mass ejection happens near a sunspot and the Sun becomes more active, many more particles than usual arrive all at once. At times like this, auroras can be seen well beyond the polar regions, in the night sky of the middle latitudes much farther from the poles. Other planets, such as Jupiter and Saturn, also have magnetic fields and atmospheres, so they have auroras too.