At certain times of year in the northern latitudes of Canada, a strange and colorful display of light can be seen in the sky. Composed of red, blue, and green, the display can be compared to a sunset—except it happens at night! This is the aurora borealis, and it has captured the imagination of humankind for centuries.
What causes the aurora borealis? Benjamin Franklin, who is considered by many to be the father of electricity, thought that the mysterious and colorful lights were caused by a concentration of electrical charges in the polar regions. Though he was right about the location of the aurora borealis (which happens near the North Pole), today our understanding of the phenomenon is far more sophisticated.
The aurora borealis differs from other displays of light in the sky. Most of the light we see in the sky is due to reflected or scattered sunlight. Other times, the light comes from internal energy sources, such as when lightning strikes and we see a flash in the sky.
The aurora borealis, by contrast, is caused by particles that enter Earth’s atmosphere from above. The process begins with solar winds flowing past Earth. These solar winds come into contact with Earth’s magnetic field, creating the magnetosphere. The magnetosphere contains an abundance of electrons.
What happens next is that the electrons from the magnetosphere are drawn to Earth’s North Pole. As these electrons descend lower into our atmosphere, they collide with atoms. The atoms then become “excited.” In other words, the atoms absorb and store some of the energy from the passing electrons. This energy is then released as photons, i.e., light.
The color of the aurora borealis depends on the types of atoms that collide with the electrons. Because we know that the atmosphere is composed mostly of nitrogen and oxygen, we can guess that the electrons from the magnetosphere normally collide with atoms of nitrogen and oxygen. Oxygen atoms are known to emit photons of green and red. Nitrogen atoms, meanwhile, emit blue photons.