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Polar Sound Descriptions
VLF Plasma Waves

Whistlers Saucers Chorus Emissions
Auroral Hiss Auroral Kilometric Radiation (AKR)

Whistlers Whistlers were first detected during World War I. They are audio frequency electromagnetic waves produced by lightning. Once produced, these waves travel along closed magnetic field lines from one hemisphere to the other in the right-hand polarized, whistler mode of propagation. The duration of the whistling tone is related to the length of the propagation path. Because of anisotropies in the index of refraction, the wave energy is confined within a cone that makes an angle of 19° 28' with respect to the local magnetic field.

On a high-resolution wideband spectrogram, the whistler's characteristic spectral feature is a clearly defined tone descending rapidly in frequency over several seconds. The name "whistler" refers to this characteristic whistling sound in the audio frequency range.

The first spectrogram is a 48-second wideband spectrogram taken from a nightside plasmaspheric pass on March 26, 1996. Initially the wideband receiver was connected to the electric Eu antenna, but was switched to the Bu magnetic search coil antenna at 07:59:06 UT. A series of brief whistlers is evident throughout this interval below 1.5 kHz.

The second spectrogram is a 48-second wideband spectrogram taken from a dayside plasmaspheric pass on May 10, 1996. The wideband receiver was connected to the magnetic loop antenna throughout this interval. Two clusters of whistlers of varying duration are seen below 8 kHz at 00:16:25 UT and 00:16:44 UT.

The third spectrogram is a 48-second wideband spectrogram taken from a nightside plasmaspheric pass on June 12, 1996. The wideband receiver was again connected to the magnetic loop antenna. Some whistlers can be seen up to 9 kHz (13:58:24 UT and 13:58:29 UT) and several more below 4 kHz (13:58:32 UT and 13:58:44 UT).

Saucers Saucer emissions are found near the low-latitude boundary of the auroral precipitation region. Saucers are electromagnetic whistler-mode emissions characterized by a V-shaped or saucer-shaped signature on high resolution frequency-time spectrograms.

Saucers are upward-propagating emissions that usually last only seconds. The short time duration of the saucers is the most significant spectral difference between these emissions and the broadband auroral hiss found in the same region. On the audio tape, the saucers have distinct falling and rising tones.

In a wideband spectrogram from March 27, 1996, two distinct saucers can be seen over-lapping each other. The V-shaped saucer is centered on 20:05:42 UT and extends in frequency up to 5 kHz. The dish-shaped saucer is centered on 20:05:47 UT and extends in frequency up to 2.5 kHz. Both saucers are found on dayside auroral field lines near the poleward edge of the auroral zone. For this pass, the wideband receiver was connected to the electric Eu antenna.

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.

Auroral Hiss Auroral hiss emissions are broad, intense electromagnetic emissions which occur over a wide frequency range from a few hundred Hz to several tens of kHz. At low frequencies, auroral hiss occurs in a narrow latitudinal band, typically only 5-10 degrees wide, centered on the auroral zone. At high frequencies, the emission spreads out over a broad region, both toward the polar cap, and to a lesser extent toward the equator. This spreading at high frequencies is caused by the anisotropic character of whistler mode propagation.

Auroral hiss is emitted in a beam around an auroral magnetic field line at altitudes of 2-4 RE. The beam width increases with increasing frequency. As the spacecraft approaches the source field line, the higher frequencies are detected first, thereby producing the "funnel-shaped" frequency-time signature that is the characteristic feature of the auroral hiss spectrogram. At high altitudes, the auroral hiss often has a sharp high frequency cutoff. This cutoff is a propagation effect that occurs because the whistler mode has an upper frequency limit of either the electron plasma frequency or the electron cyclotron frequency, whichever is smaller.

Poynting flux measurements have shown that auroral hiss propagates both upward and downward along auroral field lines. Typically above 10,000 km, the emissions are propagating upward and at low altitudes. Below 1000 km, the radiation is usually propagating downward. The source of the auroral hiss emissions is in the intermediate region, between 2 and 4 RE. Downward propagating auroral hiss emissions are closely correlated with intense, downgoing 100 eV to 1 keV inverted-V electron beams. Upward propagating auroral hiss is correlated with upgoing ~50 eV electron beams.

Because the auroral hiss emissions appear as a uni-directional signal to the spacecraft antennas, the continuous, featureless spectrum of the hiss emissions is strongly spin-modulated when observed on high- resolution wideband spectrograms. Well-defined nulls in the signal occur every half-spin when the electric antennas are aligned perpendicular to wave propagation direction. The resulting tones on the audio tape are strongly modulated hiss-like tones.

The first wideband spectrogram is taken from a nightside auroral zone pass in the northern hemisphere on May 28, 1996. The wideband receiver is connected to the electric Eu antenna during this pass. The strongly spin-modulated hiss signal is found below 3 kHz.

The second wideband spectrogram is also taken from a nightside auroral zone pass in the northern hemisphere. For this pass, on June 11, 1996, the wideband receiver is again connected to the electric Eu antenna. The strongly spin-modulated hiss signal is found below 1 kHz.

Auroral Kilometric Radiation (AKR) Auroral Kilometric Radiation (AKR) is an intense radio emission escaping outward from the earth's auroral regions at frequencies above the local electron plasma frequency. AKR usually consists of a very intense band of emission in the frequency range of about 50-500 kHz. The AKR intensity is usually highly variable, often changing by as much as 60-80 dB on time scales of ten minutes or less. The periods of high intensity tend to occur in storms lasting from a fraction of an hour to days and are closely correlated with global auroral displays, particularly with discrete auroral arcs in the evening sector. The occurrence of intense bursts of AKR is closely associated with the occurrence of inverted-V electron precipitation events. Direction-finding measurements have shown that the most intense bursts of AKR come from a source region on the nightside auroral field lines at radial distances ranging from 2-4 RE. Dayside sources are also observed and are associated with the dayside cusp region. These dayside sources are typically less intense than the nightside sources. AKR is observed in both hemispheres and has been found to propagate in both the right- and left-hand polarization R-X and L-O free space modes.

High-resolution wideband AKR spectrograms consist of many narrowband emissions with rapidly varying center frequencies. This spectral structure is responsible for the rapid combinations of rising and falling tones that are heard on the audio tape.

The wideband spectrogram for May 10, 1996 is taken from a pass through the nightside auroral zone in the southern hemisphere. The wideband receiver is configured to obtain data in the frequency range of 250-340 kHz and is connected to the electric Eu antenna. The multiple discrete spectral features are predominantly rising tones of varying frequency dispersions between 270 kHz and 340 kHz.

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