The aurora borealis, a natural light display most often seen in high-latitude regions, paints the night sky with vibrant colors. A common question arises: can the aurora be heard? This article explores the science behind whether the northern lights produce any audible sound.
How Aurora Produces Light
The aurora forms when energetic charged particles, primarily electrons and protons, from the Sun interact with Earth’s magnetic field and upper atmosphere. These particles, carried by the solar wind, are channeled towards the magnetic poles. Collisions between these solar particles and atmospheric gases, like oxygen and nitrogen, cause the atmospheric gases to become excited. As these excited atoms and molecules return to their normal state, they release energy as photons, which are tiny packets of light. This process, similar to how a neon lamp works, primarily converts energy into light, not sound.
The Silent Sky
The aurora typically occurs at altitudes ranging from 80 to several hundred kilometers above Earth’s surface, in a very thin part of the atmosphere. Sound, unlike light, requires a medium through which to travel, and the extremely low density of air at these high altitudes makes it nearly impossible for sound waves to be produced or to propagate effectively to the ground. Even if a sound were generated at auroral altitudes, light travels far faster than sound. The visual display would be seen minutes before any theoretical sound could reach an observer on the ground, making simultaneous perception impossible. Therefore, the aurora itself, as a light-emitting phenomenon high in the atmosphere, does not directly create sounds audible at ground level.
Unraveling Auroral Sound Phenomena
Despite the scientific understanding that the aurora high in the atmosphere does not produce audible sound, historical accounts and modern reports describe faint sounds associated with intense auroral displays. For centuries, observers have reported sounds such as crackling, hissing, popping, or swishing noises. Early scientific skepticism often attributed these reports to psychological phenomena or auditory illusions, suggesting that the awe-inspiring visual display might trick the brain into “hearing” sounds.
Recent research, particularly from studies conducted by Professor Unto Laine in Finland, has provided evidence that sounds can indeed be associated with the aurora, though not directly from the auroral lights themselves. These recorded sounds are generated much closer to the ground, typically at an altitude of about 70 to 100 meters (230-330 feet). The current hypothesis suggests these sounds arise from electrical discharges within a temperature inversion layer in the lower atmosphere.
During cold, clear nights, a warmer air layer can trap colder air near the ground, leading to a buildup of electrical charges. When strong auroral activity causes geomagnetic disturbances, these disturbances can trigger the discharge of these accumulated charges, creating audible “clapping” or “crackling” sounds near the observer. This phenomenon is distinct from the light-producing processes occurring far above, though both are linked to solar activity.