The Unheard Symphony
The aurora borealis fills the Alaskan sky with silent, shimmering light, but the spectacle is not quiet. During intense displays, the aurora generates powerful low-frequency sound waves, known as infrasound. These sound waves fall below the 20 Hz threshold of human hearing and travel vast distances with little loss of energy. Researchers at the Geophysical Institute of the University of Alaska Fairbanks use arrays of sensitive microbarometers to detect these atmospheric pressure waves. The data shows that auroral infrasound often has frequencies below 1 Hz, with some signals having periods between 0.1 and 100 seconds.
Two primary mechanisms are thought to produce these inaudible sounds. One hypothesis suggests that the supersonic motion of auroral arcs, which contain strong electrical currents called electrojets, creates shock waves that can be observed on the ground. Another explanation involves the rapid heating of the lower ionosphere by precipitating electrons from pulsating auroral patches, which generates pressure disturbances. Scientists at facilities like the Poker Flat Research Range, operated by the University of Alaska Fairbanks, launch sounding rockets to study the atmospheric and electrical phenomena that cause both the visible light and the inaudible sound.
A Sound Source Closer to Home
Auroral infrasound originates high in the atmosphere, historical accounts have long told of audible crackling or swishing sounds during intense auroral displays. For years, these reports were dismissed because the aurora itself is typically 100 kilometers or higher, and the thin atmosphere at that altitude cannot transmit audible sound to the ground effectively. However, research from Aalto University in Finland has provided a compelling explanation.
This research, confirmed by recordings, places the source of these audible sounds much lower, at an altitude of about 70 to 100 meters. The mechanism involves a temperature inversion layer, which occurs on cold, calm nights when a layer of warmer air sits above colder air near the surface. Electrical charges build up in these layers, and during a geomagnetic storm, these charges can discharge, creating sparks that produce a crackling sound. This discharge happens almost simultaneously with the visual display, explaining why observers hear the sounds in sync with the auroral movements. The sound is generated just 0.2 seconds before it reaches the ear. Because this phenomenon requires specific atmospheric conditions, audible auroral sounds are reported in only about 5% of intense displays.