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 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.

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 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.

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.