Ion‐cyclotron waves at Jupiter: Possibility of detection by Ulysses

Abstract

We evaluate the convective growth of ion‐cyclotron waves in the Io plasma torus using realistic plasma parameters. Significant wave amplification is restricted to two dominant frequency bands. As originally proposed by Thorne and Moses [1983,1985], waves between the O+ and H+ gyro‐frequencies may be excited at higher latitudes (λ >15°) in the region where H+ becomes the dominant ion. Strong cyclotron resonant damping should prevent wave propagation to lower latitude. Even under optimum conditions the path integrated gain of such waves is modest and extremely sensitive to the properties of both the thermal plasma and the cyclotron resonant energetic ions. Consequently, these waves should be confined to a limited region of the torus (6<L<9). The equatorial region of the torus can be unstable to L‐mode waves below the O+ gyro‐frequency. Rapid amplification should drive waves to non linear amplitudes for any reasonable choice of the plasma properties. While the orbit of Ulysses passes through the Io torus close to the optimum location (near L=8) to detect both the high frequency (10Hz) waves at λ >15° and the low frequency waves (<0.5Hz) near the equator, the sensitivity of the magnetometer and the lower frequency threshold of the plasma wave detector offer only a marginal possibility to clearly identify either of these important waves.