What Causes Auroras? Learn the Physics of These Awesome Light Shows

The Northern Lights appear above Canada. Photo Credit: NASA

By Amber Bennett

Few people have the opportunity to view one of Earth's natural wonders: the awesome light shows known as auroras. This is because the northern and southern lights light up the sky primarily in the polar regions. So, what causes the ribbons of light and glowing skies?

The cause is the solar wind that flows from the sun to the Earth. The majority of the charged particles are deflected by the magnetic field. However, some of them move along the magnetic field to the north or south pole. Once there, they enter the atmosphere, bumping into nitrogen and oxygen atoms.

This excites the atoms, which makes them unstable. The atoms then relax and enter a ground (unexcited) state, releasing the photons (light particles) that create auroras.

This photo of an aurora was taken
on the International Space Station.
Photo Credit: ESA/NASA 

Where Can You See the Auroras?

You can watch the breathtaking light show known as the aurora borealis (northern lights) in Canada and Alaska, as well as sometimes in Norway, Iceland, and Greenland. In the Southern Hemisphere, you can watch the shimmering lights of the aurora australis (southern lights) in Antarctica, Tasmania, and New Zealand.

Typically, the auroras are only visible at latitudes north of 70 degrees N and south of 70 degrees S. However, you sometimes can see them in other latitudes during a coronal mass ejection, an intense burst of plasma. It can lead to severe distortions in the Earth's magnetic field, causing auroras to stretch farther toward the equator.

What Causes Different Colors in Auroras?

How much energy the incoming charged particles are carrying determines the color of the aurora. For example, particles with less energy can't go as deep into the thermosphere. As a result, the oxygen atoms are excited at altitudes higher than 150 miles (240 kilometers), causing a red glow.

Particles with slightly more energy reach lower altitudes of about 60-150 miles (100-240 kilometers) above the Earth. When they collide with oxygen atoms at those altitudes, they create a green glow.

The charged particles with the most energy reach altitudes below 60 miles (100 kilometers). At these altitudes, nitrogen atoms are more common than oxygen atoms. The collisions cause nitrogen to glow purple or blue.

A coronal mass ejection caused the northern lights to be seen closer to the equator in 2015. Photo Credit: NASA/Christian Begeman

What Kinds of Auroras Are There?

Auroras can take many shapes depending on atmospheric conditions and Earth's magnetic field. (Click the links in the list below to see a video of each type):

• Curtain: One common aurora is a curtain of light caused by Alfvén waves. These magnetic waves draw down particles into Earth's atmosphere. 
• Dunes: The dunes are less common than curtain auroras. They look like green stripes running parallel with the ground and appear in dense pockets of oxygen atoms.
• Pulsating: A pulsating aurora appears when ripples in Earth's magnetic field knock charged particles into the atmosphere. These large auroras appear to get brighter and dimmer as charged particles collide with gases in the atmosphere. 
• STEVE: Strong Thermal Emission Velocity Enhancement (STEVE) isn't technically an aurora, but it is beautiful just the same. These shimmering lights appear in lower latitudes than auroras and are not caused by charged particles entering the atmosphere. Instead, the friction from particles running into a river of plasma in the atmosphere causes purple smears of light to appear.

NASA's Juno Spacecraft took this infrared picture of Jupiter's southern aurora. Photo credit: NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM

Where Else Can You See Auroras?

These beautiful lights shows can be seen in several other places in our solar system. For example, gas giants Saturn and Jupiter both sport auroras. In fact, scientists believe the powerful auroras may be responsible for heating these planets' atmospheres, making them warmer than expected considering their distance from the sun. You can see a video of Saturn's northern aurora here and Jupiter's auroras here.

Auroras aren't confined to planets, either. Two of Jupiter's moons (Ganymede and Europa) and Comet 67P have auroras, as well.

If you'd like to learn more about auroras, check out NASA's collection of articles about space weather. You can also check out these videos about electromagnetism on our YouTube channel.