The aurora borealis, often called the Northern Lights, is a natural light display occurring primarily in the high-latitude regions of the Northern Hemisphere. It is the result of charged particles from the sun interacting with gases in the Earth’s upper atmosphere.
The Reality of the Visual Experience
The visual reality of the aurora often differs significantly from what is captured in photographs, primarily due to the limitations of human night vision. When the display is faint, the human eye relies on rod cells and perceives the light as a colorless, grayish-white glow. Cone cells, which detect color, require more light than a typical aurora provides. Therefore, vibrant hues are often not visible to the naked eye unless the display is exceptionally bright.
Only during intense geomagnetic events, comparable in brightness to a full moon, does the display become vivid enough for the eye to clearly perceive the green color. The camera, however, uses long exposure times to gather photons, which intensifies the light and reveals the full spectrum of color the human eye misses. This explains why photographs often show deep reds and purples that an observer only saw as a faint white arc.
The speed of the aurora’s movement also varies, influencing the overall experience. At its mildest, the aurora appears as a diffuse, static glow or a slowly shifting, smooth arc low on the horizon. During strong solar events, the light becomes dynamic, transforming into rapidly “dancing” curtains that flicker and ripple across the entire night sky in a matter of seconds. This rapid, undulating movement offers a more immediate visual spectacle than the slower, cloud-like forms.
The Chemistry of Color
The colors of the aurora are determined by the specific type of atmospheric gas particle struck by solar particles, as well as the altitude of the collision. The most frequently observed color, a bright yellowish-green, is produced by oxygen atoms located between 100 and 300 kilometers. This is the most common emission because it occurs within the densest part of the atmosphere where the solar particles deposit most of their energy.
A deep red color is also emitted by oxygen atoms, but this occurs at higher altitudes, typically above 200 or 300 kilometers. The red emission requires a longer time for the excited oxygen atom to release its energy, a process only possible in the thin air of the upper atmosphere. Nitrogen molecules contribute blue and purple hues when ionized, and sometimes a pinkish-red fringe at the bottom edge of the auroral curtains, usually below 100 kilometers.
The Geometry of the Forms
The visible structure of the aurora is shaped by the Earth’s magnetic field lines, causing the charged particles to descend in patterns. The simplest and most stable form is the homogeneous arc, which appears as a smooth, long band of light stretching across the sky. These arcs typically lack internal structure and are characteristic of periods with lower solar activity.
With increasing intensity, the arc develops vertical streaks, transforming into a rayed curtain or band, which is the drapery-like appearance. These curtains consist of many parallel, vertical light rays that give the aurora a three-dimensional, folding look as they ripple and sway. When the display occurs directly overhead, the perspective changes, and the rays appear to converge at a single point in the sky, creating a form known as a corona. This crown-like structure is a sign of an intense geomagnetic event.
The Solar Engine: How the Lights Are Created
The auroral process begins at the sun, which constantly emits a stream of charged particles known as the solar wind. This wind, composed primarily of electrons and protons, streams through space, carrying an embedded magnetic field. When this stream reaches Earth, most of the particles are deflected by the planet’s magnetic field, the magnetosphere.
The magnetosphere funnels some of these charged particles toward the magnetic poles. The particles are channeled down the magnetic field lines, eventually precipitating into the upper atmosphere. These high-speed electrons and protons collide with the oxygen and nitrogen atoms in the atmosphere. The energy transfer excites the atmospheric atoms, which then release the excess energy as photons, creating the visible light we observe as the aurora.