Resonant instability near the two‐ion crossover frequency in the Io plasma torus

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Abstract
The propagation characteristics of electromagnetic waves below the proton gyrofrequency are strongly influenced by the changing ion composition in the Io plasma torus. Obliquely propagating waves experience a natural reversal of their dominant sense of polarization when their frequency becomes equal to the crossover frequency. This dramatically modifies both the wave growth characteristics and their effect on scattering resonant particles. A numerical simulation of the path‐integrated gain of unducted waves in the torus has confirmed the importance of L mode ion cyclotron instability as the most likely mechanism for wave excitation. Whistler (R mode) instability is insignificant and it should be totally quenched by strong ion cyclotron damping. The favored region for wave excitation is at intermediate latitudes (λ ≥ 12°) on field lines passing through the outer torus (L ≥ 6). Unstable waves can subsequently propagate toward lower latitude following the natural polarization reversal (to R mode waves) at the wave crossover frequency. But the wave amplitudes observed in the low‐latitude region sampled by Voyager 1 are expected to be significantly lower those that in the source region due to strong ion cyclotron damping. This is consistent with recent observational evidence on the power spectral intensity of low‐frequency waves in the torus.