Polar Audio Tape: VLF Plasma Waves

Chorus Emissions

Chorus emissions are electromagnetic emissions propagating in the right-hand polarized whistler mode. They are among the most intense plasma waves in the outer magnetosphere. Chorus emissions are observed at intermediate invariant latitudes, between L=4 and L=10, and over a wide range of local times with a peak in the distribution near local dawn. Typical wave spectra of chorus emissions show a characteristic frequency that varies inversely with invariant latitude. The chorus occurs primarily in two distinct frequency bands, one above and the other below the equatorial half-gyrofrequency. A characteristic null in the distribution of these emissions at the half-gyrofrequency is clearly visible in the low-resolution frequency-time spectrograms.

The spectral characteristic which gives these emissions their name is the succession of predominantly rising tones which sound like a chorus of chirping birds. These rising tones are very short in duration, typically only 0.1-1.0 seconds. Because of their short duration, these tones can only be distinguished on high-resolution wideband spectrograms.

The wideband spectrogram for May 31, 1996 was taken from a dayside pass at latitudes just below the dayside auroral zone. The receiver was connected to the magnetic loop antenna. The discrete tones characteristic of chorus can be seen as a dense population of short, very intense rising tones between 500 Hz and 1.2 kHz.