Polar Audio Tape: VLF Plasma Waves

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.