Ion cyclotron waves in the Io torus: Wave dispersion, free energy analysis, and SO2+ source rate estimates


Institute of Geophysics and Planetary Physics, University of California, Los Angeles
1Also at Department of Earth and Space Sciences

Abstract.   As the Galileo spacecraft passed through the Io torus, ion cyclotron waves were observed near the sulfur dioxide ion gyrofrequency. The torus plasma is continually replenished by the ionization of neutral particles from Io. It is well known that sulfur dioxide dissociates rapidly, so that the corotating torus plasma consists of predominantly sulfur and oxygen ions. However, for the small fraction of molecules that become ionized before dissociation, the appearance of SO2+ gyroresonant waves near Io indicates that the wave growth timescale (or wave-particle scattering time) is short compared with the lifetime of these SO2+ ions. Newly created ions initially form "ring"-type ion distributions which are highly unstable and generate the observed ion cyclotron waves. A warm plasma dispersion analysis finds that growth at the SO2+ gyrofrequency dominates over that at the O+ and S+ gyrofrequencies, partly because the ring energy scales with ion mass, but mainly due to the absence of a thermalized "background" component of SO2+ which would otherwise damp these waves. At the growth rate peak, the wave frequency is just below the SO2+ gyrofrequency (0.4 Hz) and the phase velocity is ~55 km/s. A free energy analysis for wave-particle scattering of ions toward a "bispherical" shell-type distribution suggests that the SO2+ density in the torus falls off steeply with distance from Io wake values. These density estimates obtained from the observed wave power do not rely on assumptions of the exact plasma composition, but require that the SO2+ wave dominates the spectrum and is not strongly damped (as verified by the dispersion analysis). From our density estimates, we infer an ion production rate for SO2+ of ~8 x 1026 /s, representing the small fraction of sulfur dioxide that survives in molecular form long enough to be ionized and generate waves before dissociation occurs. This is consistent with 5% of the total ion source at the time of the Galileo flyby but is less than 3% of the widely accepted total torus supply rate of ~3 x 1028 particles/s from Io.

        There are two major differences which distinguish the Io pickup process from many solar wind pickup situations.   (FIGURE 1).   These are, first, that the vast majority of the Iogenic ions are picked up into a "same-species" background plasma and, second, that the plasma characteristics (including high B and a corotating flow) dictate a high ratio R ~1 of wave phase velocity to ion injection speed. Both of these factors have important consequences as discussed herein.
        A warm plasma dispersion analysis for conditions appropriate to the Io plasma torus has been presented.   (FIGURE 6).   Results show that the SO2+ ion cyclotron wave mode is dominant, partly because the pickup ring distribution energy scales with ion mass, but mainly because of the absence of thermalized SO2+ torus plasma due to rapid dissociation, while the S+ and O+ waves may be gyro-resonantly damped by the torus background ions of the same species. Probable sulfur dioxide dissociation timescales are of the order of 0.5 to 5 hours [e.g., Smyth and Marconi, 1998; Warnecke et al., 1997], which is much shorter than the ~13 hours required for ions to corotate back to Io as part of the thermalized background torus population. The ionization rate of the average SO2 molecule is slower than the dissociation rate and sufficient only to allow a small fraction (as observed) of the molecular particles to become ionized [e.g., Smyth and Marconi, 1998], whence they rapidly pitch angle scatter. The presence of the observed SO2+ gyrofrequency waves thus indicates that the ion pitch angle scattering to a shell-type distribution must occur on timescales at least comparable with the subsequent SO2+ dissociation lifetime. Note that a pitch angle scattered velocity-space shell distribution, while more isotropic than the ring, is far from thermalized. Timescales for total thermalization of the pickup distributions in the torus (filling in of the shell and development of a Maxwellian) are much longer.
        For newly ionized particles, waves are generated just below the pickup ion gyrofrequency for a perpendicular pickup geometry (=90o case), and both above and below the gyrofrequency for the drifting ring beam (= 80o case), consistent with the appearance of double peaks in some observed spectra in Figure 3 [see also Warnecke et al., 1997].   (FIGURE 7).   The peak SO2+ branch growth rate in the dispersion analysis corresponds to waves of frequency ~0.4 Hz (as observed) propagating along the magnetic field lines with a phase velocity Vph ~ 55 km/s.
        In the R~1 parameter regime, the shape and average v// of the resulting pitch angle scattered distribution is highly dependent.   (FIGURE 8).     (FIGURE 9).   For the nearly perpendicular geometry as appropriate for the torus, there is substantial free energy for the ion cyclotron interaction to generate waves efficiently, and the scattering timescales are expected to be fast. For low values of , there would be little free energy for the ion cyclotron instability on scattering to a conventional bispherical shell, whence perhaps energy diffusion (on the Vph scattering centers) might occur or other instability modes might take over. Low values of are not encountered in the torus, except (we speculate) perhaps very close to Io on the high-latitude edges of the wake or on the upstream side, if the plasma is deflected north-south, the field is distorted, and convected B fluctuation levels are high. Future planned Galileo-Io flybys may help to resolve this and other issues in the extended mission phase.
        The free-energy analysis enables us to relate the density of ions to the observed wave amplitudes and indicates that SO2+ densities in the torus are considerably lower than the wake values and fall off with distance from Io as expected.   (FIGURE 10).   Flux calculations based on our inferred density profile give an approximate estimate of the SO2+ ion production rate of 8 x 1026 ions/s, for the sulfur dioxide that becomes ionized before dissociation.   (FIGURE 11).     (TABLE 2).   This is an "effective" production rate, less the dissociation losses of SO2 molecules; it is ~5% of the total ion source rate inferred by Bagenal [1997] from the observed total densities at the Galileo encounter, and is consistent with the PLS wake composition observation [Frank et al., 1996]. Our SO2+ source ionization rate accounts for less than 3% of the total 3 x 1028 particles/s torus material supply rate [e.g., Hill et al., 1983] that is widely used in the literature.


Copyright 1998 by the American Geophysical Union.
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