Detection of Jovian whistler mode chorus; Implications for the Io torus aurora

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Abstract
Near the Io torus outer boundary (L ≃ 8), the Voyager 1 plasma wave instrument detected high frequency (f) waves near one‐half the electron cyclotron frequency fc. High resolution waveform measurements demonstrate that these f ≃ fc/2 signals are banded whistler mode chorus at f ≲ fc/2 and half‐cyclotron frequency emissions with f slightly above fc/2. The high resolution spectral information, and the theory of whistler mode waves, permit us to estimate the density (≃2.5 cm−3), energy (few keV), and omnidirectional energy flux (10² ergs/cm²‐sec) of the electrons resonant with the chorus. Chorus precipitates about 6 ergs/cm²‐sec of few keV electron energy to the Jovian ionosphere at L = 8. Electrostatic emissions, probably electron cyclotron half‐harmonic modes, have also been detected near the magnetic equator in the Io torus region. At L = 8, the multimode pitch‐angle diffusion associated with the detected waves should produce a precipitation flux about a factor two below the 50 ergs/cm²‐sec required to generate the observed auroral emissions; however, the flux could well be larger deeper within the torus.

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