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An auroral chorus is a series of electromagnetic waves 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 storms also responsible for the auroras. The electromagnetic waves are a type of natural radio waves, vibrations of electric and magnetic energy occurring at the same frequency as sound.
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/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.  They have released many audio interpretations of chorus recordings online along with spectrograph measurements.
- 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