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.