Auroral chorus
Encyclopedia
An auroral chorus is a series of chirps, whistles, and quasi-musical sounds in predominantly rising tones created by geomagnetic storm
s also responsible for the aurora
s. The sounds last approximately 0.1-1.0 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.
/VLF 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. http://www-pw.physics.uiowa.edu/plasma-wave/istp/polar/home.html They have released many recordings of auroral sounds available online along with spectrograph measurements. Historically, the sounds have been associated with spiritual events by Inuit
in Canada who regularly experienced auroral chorus on cold, windless nights.
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 just play tricks on our ears. http://docs.lib.noaa.gov/rescue/whytheweather/1933/19330411.pdf
Geomagnetic storm
A geomagnetic storm is a temporary disturbance of the Earth's magnetosphere caused by a disturbance in the interplanetary medium. A geomagnetic storm is a major component of space weather and provides the input for many other components of space weather...
s also responsible for the aurora
Aurora (astronomy)
An aurora is a natural light display in the sky particularly in the high latitude regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere...
s. The sounds last approximately 0.1-1.0 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.
Experiencing/Detecting
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 to about 1.6 MHz. 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 ELFELF
ELF may be:Science* Electron localization function, a concept in quantum mechanics* Extremely low frequency, the band of radio frequencies from 3 to 30 HertzLinguistics...
/VLF 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. http://www-pw.physics.uiowa.edu/plasma-wave/istp/polar/home.html They have released many recordings of auroral sounds available online along with spectrograph measurements. Historically, the sounds have been associated with spiritual events by Inuit
Inuit
The Inuit are a group of culturally similar indigenous peoples inhabiting the Arctic regions of Canada , Denmark , Russia and the United States . Inuit means “the people” in the Inuktitut language...
in Canada who regularly experienced auroral chorus on cold, windless nights.
Explaining/Understanding
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
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
. 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
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....
as if it were traveling from the auroral heights (80–100 km) to the Earth's surface. However, local electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
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 just play tricks on our ears. http://docs.lib.noaa.gov/rescue/whytheweather/1933/19330411.pdf
See also
- Dawn chorus (electromagnetic)Dawn chorus (electromagnetic)The electromagnetic dawn chorus is an unexplained phenomenon that occurs most often at sunrise or shortly after, that resembles the sound of the birds' dawn chorus. Chorus is believed to be generated by a Doppler-shifted cyclotron interaction between anisotropic distributions of energetic ...
- AuroraAurora (astronomy)An aurora is a natural light display in the sky particularly in the high latitude regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere...
- Geomagnetic stormGeomagnetic stormA geomagnetic storm is a temporary disturbance of the Earth's magnetosphere caused by a disturbance in the interplanetary medium. A geomagnetic storm is a major component of space weather and provides the input for many other components of space weather...
- SoundSoundSound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
- Radio waves in the Ionosphere
External links
- http://www-pw.physics.uiowa.edu/plasma-wave/istp/polar/home.html
- http://www.auroralchorus.com/
- http://www.acoustics.hut.fi/projects/aurora/index.html
- http://lib.tkk.fi/Dipl/2005/urn007898.pdf
- http://members.tripod.com/~auroralsounds/
- http://home.pacific.net.au/~ddcsk1/aurora1.htm
- http://docs.lib.noaa.gov/rescue/whytheweather/1933/19330411.pdf
- 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
- http://www.auroralchorus.com/natradio.htm