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

Abstract

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.