An auroral chorus is a series of
electromagnetic wave
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...
s at frequencies which resemble chirps, whistles, and quasi-musical sounds in predominantly rising tones when played as pressure waves (sound), which are created by
geomagnetic storm
A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave and/or cloud of magnetic field that interacts with the Earth's magnetic field.
The disturbance that d ...
s also responsible for the
aurora
An aurora (plural: auroras or aurorae), also commonly known as the polar lights, is a natural light display in Earth's sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). Auroras display dynamic patterns of bri ...
s. The sounds last approximately 0.1 to 1.0 seconds. Other auroral sounds includes hissing, swishing, rustling and cracking.
The electromagnetic waves are a type of
natural radio waves, vibrations of electric and magnetic energy occurring at the same frequency as sound.
Detection
Auroral chorus can be detected primarily around the magnetic equator, specifically in two distinct frequency bands, one above the equatorial half gyro-frequency and one below it. The gyro-frequency ranges from 0.6 kHz to about 1.6 kHz. Distinguishable on high resolution wideband spectrographs, the wave amplitude grows linearly then switches to non-linear. Demonstrating a peak distribution near dawn, the auroral chorus is most detectable via
ELF
An elf () is a type of humanoid supernatural being in Germanic mythology and folklore. Elves appear especially in North Germanic mythology. They are subsequently mentioned in Snorri Sturluson's Icelandic Prose Edda. He distinguishes "ligh ...
/
VLF
Very low frequency or VLF is the ITU designation for radio frequencies (RF) in the range of 3–30 kHz, corresponding to wavelengths from 100 to 10 km, respectively. The band is also known as the myriameter band or myriameter wave a ...
Radio receivers in the middle latitude around 30-60 degrees N. The most numerous recordings of the auroral chorus has been by the Iowa Plasma Wave Group. They have released many audio interpretations of chorus recordings online along with spectrograph measurements.
Historically, the sounds have been associated with spiritual events by
Inuit
Inuit (; iu, ᐃᓄᐃᑦ 'the people', singular: Inuk, , dual: Inuuk, ) are a group of culturally similar indigenous peoples inhabiting the Arctic and subarctic regions of Greenland, Labrador, Quebec, Nunavut, the Northwest Territories ...
in Canada who regularly experienced auroral chorus on cold, windless nights.
Explanation
The specific nature and source of the auroral chorus is a continuing question in space and atmospheric research. Cluster satellite observations suggest that the sounds are seemingly generated by numerous sources in rapid motion.
Studies have shown a definite correlation on/off with fluctuations of solar wind and southward turning of IMF (which is correlated with aurora). The strength of noise correlates with strength of geomagnetic activity in the Earth's
ionosphere. Other studies show a clear correlation peak at upper infrasound range (less than 20 Hz). A delay between the peak of auroral sounds and the peak of electromagnetic activity, corresponds to the
speed of sound as if it were traveling from the auroral heights (80–100 km) to the Earth's surface. However, local
electric field signals do not correlate well with sound signals.
Some believe that it is probable that the auroral chorus doesn't originate at the point of aurorae but rather is transformed from slight wave ripples in the air into audible sound waves by objects closer to the observer. There remains a question of whether real sound waves exist or if somehow
electromagnetic waves affects the human ear.
See also
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Dawn chorus (electromagnetic)
The electromagnetic dawn chorus is a phenomenon that occurs most often at or shortly after dawn local time. With the proper radio equipment, dawn chorus can be converted to sounds that resemble birds' dawn chorus (by coincidence).
The electroma ...
*
Aurora
An aurora (plural: auroras or aurorae), also commonly known as the polar lights, is a natural light display in Earth's sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). Auroras display dynamic patterns of bri ...
*
Geomagnetic storm
A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave and/or cloud of magnetic field that interacts with the Earth's magnetic field.
The disturbance that d ...
*
Radio waves in the Ionosphere
References
External links
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*{{Cite web, url=http://www.auroralchorus.com/natradio.htm, title=Natural VLF Radio - Sounds of Space Weather - Stephen P. McGreevy, website=Auroralchorus.com, access-date=8 August 2022
*N. G. Kleimenova and O. V. Kozyreva. "ELF Polar Chorus and Magnetic Storms",Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Russia, Geomagnetism and Aeronomy, 2007, Vol. 47, No. 3, pp. 336–342. © Pleiades Publishing, Ltd., 2007.
*Yang JunYing, CAO JinBin, Yan ChunZiao, Li LiuYuan, Ma YuDuan "The mid-high latitude whistler mode chorus waves observed around substorm onsets", Science in China Series E: Technological Sciences, October 2008, Vol.51, No 10, Springerlink
Earth phenomena
Electrical phenomena