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Whistler mode auroral hiss emissions observed near Jupiter's moon Io

[1] On 16 October 2001, the Galileo spacecraft made a close flyby of Jupiter's moon Io. During the flyby the plasma wave instrument detected an electric field emission with spectral characteristics very similar to a type of whistler-mode noise called “auroral hiss” that is commonly observed in Earth's auroral region. This paper gives a detailed analysis of the “auroral hiss” observed near Io. The frequency-time spectrum of the emission has a sharp high-frequency cutoff near the electron cyclotron frequency and a V-shaped low-frequency cutoff. On a frequency-time spectrogram these cutoffs give the emission a characteristic funnel shape that is very similar to the spectrum of terrestrial auroral hiss. Strong magnetic field perturbations occurred near the vertex of the funnel indicating the presence of a field-aligned current. To explain the origin of the emission, a brief review is given of whistler-mode wave propagation and the unipolar inductor model of Io's interaction with the magnetosphere of Jupiter. Assuming propagation near the whistler-mode resonance cone, ray-tracing analyses show that the radiation originates from a source very close to the surface of Io. The source is located in the same region where field-aligned currents are believed to originate in the ionosphere of Io. Since terrestrial auroral hiss is known to be produced by beams of low-energy auroral electrons, these observations suggest that the auroral hiss at Io is generated by an electron beam that is part of the field-aligned current system induced by the interaction of Io with the rapidly rotating magnetosphere of Jupiter.