The aurora borealis (Northern Lights) and the aurora australis (Southern Lights) are natural light displays that appear as shifting curtains, rays, or arcs of light, predominantly in shades of green, across the polar night skies. This phenomenon results from energetic interactions between solar activity and Earth’s atmosphere. Understanding the exact atmospheric layer where this light show takes place provides insight into the physics that creates the aurora.
Pinpointing the Aurora’s Location
Auroras span a significant vertical distance in the upper atmosphere, primarily situated in the thermosphere. They extend slightly downward into the upper mesosphere and upward toward the exosphere. The light typically begins around 80 kilometers (50 miles) above the Earth’s surface, marking the lower boundary of the thermosphere.
The most intense and visible part of the display occurs between 100 and 300 kilometers (62 to 186 miles) in altitude. Although the light can sometimes reach as high as 600 kilometers (370 miles), most photon emission happens in this central zone. This broad range exists where atmospheric density is extremely low, near the edge of space. The region where the aurora occurs also corresponds to the ionosphere, an electrically charged layer that overlaps with the thermosphere.
Characteristics of the Thermosphere
The thermosphere is the fourth major layer of Earth’s atmosphere, beginning above the mesosphere at approximately 80 to 90 kilometers. Temperatures in this layer dramatically increase with altitude, potentially soaring to over 2,000 degrees Celsius (3,600 degrees Fahrenheit) at its upper limits. This heat results from the absorption of intense solar X-ray and ultraviolet radiation.
Despite the high temperatures, the air density in the thermosphere is incredibly low. Molecules travel great distances before colliding, unlike the frequent collisions closer to the ground. This low density is a prerequisite for the aurora, allowing energetic solar particles to penetrate deep into the atmosphere before interacting. The International Space Station also orbits within the thermosphere.
The Mechanism of Light Production
The light of the aurora begins with the Sun, which constantly releases a stream of charged particles known as the solar wind. During periods of heightened activity, such as coronal mass ejections (CMEs), the Sun ejects massive clouds of energized electrons and protons into space.
When this stream of charged particles reaches Earth, most are deflected by the planet’s powerful magnetic field, or magnetosphere. However, some particles become trapped and are funneled along the magnetic field lines toward the North and South magnetic poles.
These high-speed particles then plunge into the atmosphere, slamming into the atoms and molecules of the thermosphere. The collision transfers energy to the atmospheric gases, primarily oxygen and nitrogen, causing them to become “excited.” The atoms quickly stabilize by releasing this excess energy as photons (light particles). The process is similar to how a neon sign works. The resulting cascade of light emission is what the eye perceives as the glowing curtains of the aurora. The specific color depends on the type of gas atom struck.
Why Aurora Colors Change with Altitude
The variety of auroral colors is directly linked to the altitude where particle collisions occur, because the density and composition of atmospheric gases change significantly with height. The most commonly observed color, bright green, is produced when incoming particles excite oxygen atoms between 100 and 300 kilometers. This range offers the optimal balance of oxygen concentration and atmospheric density for this specific energy transition to emit green light.
Red auroras are also caused by oxygen but are generated at higher altitudes, typically above 300 kilometers. At these greater heights, the much lower atmospheric density changes the time the excited oxygen atom takes to release its energy. This longer relaxation time results in the emission of a different wavelength of light, appearing as a deep, crimson red. The red color is less frequently seen because it requires a more intense solar event to excite the fewer atoms present at these extreme heights.
Blue, violet, and pink hues are produced by collisions with nitrogen molecules, which concentrate at lower altitudes, often below 100 kilometers. Nitrogen emissions appear toward the bottom edge of the display, where incoming particles have penetrated the deepest. The mixing of red oxygen and blue nitrogen emissions can create the pink or magenta colors occasionally seen near the lower border of the auroral curtains.