The Aurora Borealis, commonly known as the Northern Lights, is a natural light display visible in high-latitude regions near the Arctic. This phenomenon occurs when energetic particles from the sun interact with Earth’s upper atmosphere, creating dynamic patterns of light. The aurora primarily takes place within the thermosphere, sometimes extending into the exosphere. This location is a direct consequence of the physics governing the interaction between solar energy and the layers of gases surrounding our planet.
The Definitive Answer: Location in the Atmosphere
The thermosphere is the atmospheric layer situated above the mesosphere, beginning at approximately 80 kilometers (50 miles) above Earth’s surface. The majority of visible auroral activity occurs within this layer, specifically between 80 kilometers and about 600 kilometers in altitude. The thermosphere’s upper boundary, the thermopause, varies due to solar activity, and the fainter portions of the aurora can sometimes extend into the outermost layer, the exosphere.
The altitude where the aurora begins, around 80 kilometers, is determined by atmospheric density. Below this point, the air is too dense. Charged particles lose their energy too quickly through frequent collisions to produce sustained light. The lower edge of the auroral curtain marks the altitude where the atmosphere becomes sufficiently tenuous for energetic particles to penetrate and excite the gases. This region, which includes the thermosphere and part of the exosphere, is also known as the ionosphere due to the high concentration of charged ions and electrons.
The visible display is concentrated in a ring-shaped zone around the geomagnetic poles, called the auroral oval. The brightest and most common auroral emissions occur between 100 and 300 kilometers. Although the thermosphere is part of the “upper atmosphere,” the highest red auroral emissions can sometimes be observed from the International Space Station, which orbits around 400 kilometers above Earth.
The Mechanism of Light Production
The aurora begins at the sun, which constantly emits a stream of charged particles, predominantly electrons and protons, known as the solar wind. This solar wind travels through space and encounters Earth’s magnetic field, or magnetosphere. Most of the solar wind is deflected, but some particles become trapped and funneled by the magnetic field lines toward the North and South magnetic poles.
These charged particles are highly energetic and are accelerated as they travel along the magnetic field lines. When guided into the polar regions, they plunge into the upper atmosphere, which is composed primarily of atomic oxygen and molecular nitrogen. The light display is produced when these high-speed particles collide with the atmospheric gas atoms and molecules.
The collision transfers energy from the solar particles to the atmospheric gases, causing the atoms and molecules to jump to an unstable, higher-energy state. The atoms immediately release this excess energy as a photon, a particle of light. The color of the emitted light is determined by the type of gas atom involved in the collision and the energy level it was excited to. This process of excitation and photon emission creates the dynamic curtains and rays of the aurora.
Altitude and Color Variations
The color of the aurora is a direct result of which atmospheric gas is struck and the altitude of the collision. The chemical composition and density of the atmosphere change throughout the thermosphere, dictating the resulting colors. The most commonly seen auroral color is green, produced when charged particles collide with atomic oxygen.
Green light is generated between approximately 100 and 300 kilometers. At these intermediate heights, oxygen concentration is sufficient, and the atmospheric density allows for the energy transition that produces the green glow. Red light is also caused by collisions with atomic oxygen but occurs at much higher altitudes, typically above 300 kilometers.
At these higher altitudes, the lower atmospheric density allows excited oxygen atoms more time before colliding, enabling the slower energy transition that releases red light. Below 100 kilometers, where the atmosphere is denser, collisions with molecular nitrogen produce blue and purple hues. These nitrogen-induced colors are often seen as a pink or dark red fringe along the lower border of the aurora.