An aurora is a natural light display observed in the high-latitude regions near the Earth’s poles, known as the aurora borealis in the North and the aurora australis in the South. This dynamic light show is the result of an interaction between charged particles ejected from the Sun and the gases present in our planet’s upper atmosphere. The appearance of the lights is highly variable, changing in color, shape, and movement based on the intensity of solar activity and atmospheric conditions.
The Visual Spectacle: Shapes, Forms, and Movement
The most commonly observed color during an auroral display is green, which often dominates the lower edge of the visible phenomenon. The lights rarely hold a single form; instead, they are characterized by dynamic patterns that seem to writhe and flow across the night sky.
A frequent structure is the auroral arc, a smooth, steady band of light stretching across the horizon, sometimes remaining static for extended periods. As solar activity intensifies, these arcs can evolve into pleated curtains or draperies. These curtains are composed of numerous vertical rays or streaks, resembling searchlight beams that follow the Earth’s magnetic field lines.
The movement of the aurora is often described as shimmering or dancing, with the lights flickering and pulsating. During a highly energetic event, the rays may converge directly overhead, creating a corona, where the light appears to radiate outwards from a single point in the zenith. The brightness of the display can range drastically, from a faint, milky-white glow barely perceptible to the naked eye to a display so vibrant it can illuminate the landscape below.
The Physics of Aurora Colors
The varied colors seen in an aurora are determined by which atmospheric gas is struck by the incoming charged particles and the altitude at which the collision occurs. When electrons from the solar wind slam into atoms and molecules in the upper atmosphere, they excite the gas particles, causing them to emit light as they return to a stable state.
The most familiar green hue is produced when oxygen atoms are excited at lower altitudes, typically between 60 and 120 miles (100–200 kilometers) above the surface. At much higher altitudes, usually above 120 miles (200 kilometers), collisions with oxygen atoms yield the deep red light.
The other colors, including blue, violet, and pink, are primarily produced by nitrogen molecules. Excited nitrogen emits blue light at lower altitudes, while a mixture of red oxygen and blue nitrogen emissions creates the pink or magenta colors often seen along the active lower border of the auroral curtains. The color spectrum indicates both the gas composition and the energy level of the particles impacting different layers of the atmosphere.
Viewing an Aurora: Location and Timing
Auroras are concentrated within the auroral oval, a region circling the magnetic poles that generally falls between 60 and 75 degrees latitude. Observers must position themselves within this zone, which includes territories like Alaska, Northern Canada, Iceland, and Scandinavia.
The lights occur year-round, but visibility requires darkness, making the winter months between late August and mid-April the optimal viewing season. The best time of night to witness the strongest displays is typically around local midnight, between 9:00 PM and 2:00 AM.
Successful viewing requires clear, cloudless skies and a location with minimal light pollution, as the aurora’s brightness can be easily washed out by city lights. While solar activity is the primary driver of intensity, the equinox months of September and March often see slightly increased activity due to a favorable alignment of the Earth’s magnetic field with the solar wind.