What Causes the Northern Lights
On a dark, clear night in places such as northern Canada, Alaska, or Norway, the sky sometimes fills with curtains of green light that ripple and drift. This glowing display is called the aurora borealis, or the northern lights. It looks like magic, but it has a scientific explanation that begins ninety-three million miles away, at the sun.
The sun constantly sends out a stream of tiny charged particles called the solar wind. Most of these particles would sweep right past Earth, but our planet is surrounded by a magnetic field, an invisible shield that bends their paths. Earth's magnetic field is weakest near the north and south poles, so the particles are guided down toward those regions.
High above the ground, at a height of roughly sixty miles or more, the particles crash into atoms of gas in the upper atmosphere. Each collision gives an atom extra energy, and the atom gets rid of that energy by giving off a tiny flash of light. Millions of these flashes at once make the glow we see.
The color depends on the gas and the height. Oxygen atoms usually glow green, which is the most common color of the aurora, and sometimes red at great heights. Nitrogen can add blue or purple edges.
The same thing happens at the opposite end of the planet, where it is called the aurora australis, or southern lights. Auroras are strongest when the sun is especially active. Skywatchers who want to see them try to get far from city lights and wait for a dark, cloudless night.