The Northern Lights, or Aurora Borealis, are a spectacular natural light display caused by energetic particles from the sun colliding with Earth’s atmosphere. These collisions create a phenomenon that is scientifically vibrant, composed of reds, greens, blues, and purples. However, those observing the aurora often see a ghostly, colorless glow instead of the vivid hues captured in photographs. The display frequently appears to the casual observer as a shimmering white or grey band, even though it is fundamentally a multi-colored event.
The Physics Behind Aurora Colors
The light show begins with the solar wind, a stream of charged particles ejected from the sun’s outer atmosphere. These particles are channeled toward the polar regions by Earth’s magnetic field, where they plunge into the upper atmosphere. The resulting light is generated when these solar particles strike and “excite” atoms and molecules of atmospheric gases, causing them to release photons of light.
The specific color emitted depends on the type of gas molecule being struck and the altitude of the collision. Oxygen atoms are responsible for the two most common auroral colors. When excited oxygen is struck at an altitude between 100 and 150 kilometers (60 and 90 miles), it emits a bright, yellowish-green light.
The deep red color is also produced by atomic oxygen, but it occurs at much higher altitudes, typically above 200 kilometers (120 miles). Collisions with nitrogen molecules create the blue and purplish-red hues, primarily appearing at lower atmospheric levels, below 100 kilometers.
Why the Aurora Appears White to the Naked Eye
The primary reason the aurora often looks white or colorless is a limitation of human vision in low-light conditions, known as scotopic vision. The retina contains two types of light-sensitive cells: rods and cones. Rod cells are extremely sensitive to light intensity and handle vision in the dark, but they cannot distinguish color.
Cone cells are responsible for color perception, but they require a much higher level of incoming light to be activated. When the aurora is relatively dim, the light intensity falls below the threshold needed to stimulate the cone cells. The display is then processed almost entirely by the rod cells, which perceive the light as a colorless, black-and-white image, resulting in a shimmering white or grey band.
Only during an exceptionally bright display, one that provides enough light to activate the cone cells, will the human eye clearly perceive the green and red colors. Cameras easily overcome this biological limitation by using long-exposure settings. This process allows the sensor to collect photons over time, revealing the true colors that were too faint for direct human perception.
Influence of Solar Activity on Visibility
The intensity of the solar event directly dictates the brightness of the aurora and, consequently, its perceived color. Displays resulting from weak solar activity are inherently dimmer, making them far more likely to appear white or grey. Geomagnetic activity is measured using the Kp index, which ranges from 0 to 9, with higher numbers indicating stronger disturbances.
When the Kp index is low (0 to 2), the aurora is typically faint and confined to high-latitude regions. Under these conditions, the light is insufficient to activate the cone cells, and the white glow dominates the view.
Conversely, a strong geomagnetic storm (Kp index of 5 or more) pushes the auroral oval to lower latitudes and produces significantly brighter light. These intense events generate enough energy to stimulate the cone cells, allowing observers to see the true green and red colors. Visibility is also influenced by secondary factors, such as the speed and density of the solar wind particles, and local conditions like light pollution.