One-dimensional hybrid simulations of planetary ion pickup: Techniques and verification

Journal of Geophysical Research Atmospheres (Impact Factor: 3.43). 12/2006; 111(A12). DOI: 10.1029/2006JA011996

ABSTRACT 1] Previously, hybrid simulation techniques using massless fluid electrons and kinetic ions have been successfully applied to study the electromagnetic plasma waves generated by ion pickup in the solar wind, where instability is driven by the large drift velocities of newborn ion populations. For ion pickup at Jupiter and Saturn's magnetospheres where instability is driven by heavy ions with a ring velocity distribution, we show that the one-dimensional hybrid simulation technique can successfully reproduce the behavior of this instability as predicted by linear dispersion theory as well as the important nonlinear wave-particle interactions. The simulated ion cyclotron waves have frequencies near the ion gyrofrequency and are generated as the anisotropic newborn ion ring distribution scatters to a more isotropic configuration. Simulated maximum wave amplitudes and instability growth rates increase with newborn ion density and pickup velocity. For appropriate heavy pickup ion densities and velocities the simulated wave amplitudes are within the range observed by spacecraft.

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    ABSTRACT: Ion pickup in planetary environments results in unstable newborn ion populations with sufficient free energy to generate electromagnetic plasma waves. Estimates of the SO2+ pickup rates near the Jovian moon, Io, based on ion cyclotron wave observations assumed that the newborn pickup ions lose 50% of their energy to wave growth. Using one-dimensional initial-value hybrid simulation, we test this assumption and predict that at most ∼25% of the energy of the newborn ion population is lost to wave growth for conditions at Io. This energy is lost over >1400 SO2+ gyroperiods (∼1 hour), with the majority lost after saturation of the instability. Thus the saturation wave energy is low, at
    Geophysical Research Letters 01/2007; 34(2). DOI:10.1029/2006GL028285 · 4.20 Impact Factor
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    ABSTRACT: 1] In Jupiter and Saturn's magnetospheres, ionization of neutrals produced by the satellites and rings results in populations of newborn pickup ions with T ? > T k , which are unstable to the generation of electromagnetic ion cyclotron waves. Linear dispersion analysis of this anisotropy instability finds maximum growth at parallel propagation, with decreasing growth rates at wave vector angles, q, oblique to the ambient magnetic field, B 0 . Observed S + and SO + ion cyclotron waves near the Jovian moon, Io, propagate at a variety of angles within 60° of B 0 . Using one-dimensional hybrid simulation, we study the properties of the obliquely propagating waves in the Io plasma torus environment for q 60°. We find that the maximum growth rate decreases with increasing angle from B 0 , as predicted by linear theory, with the growth rate at q = 60° equal to $70% the growth rate at parallel propagation. At the oblique angles, cyclotron harmonics of S + and SO + are excited and grow larger with q. At q = 60°, the fundamental mode saturation amplitude is roughly four times that of the first harmonic. However, we do not expect to see these harmonics in the Io torus because thermalized O + and S + damps their growth. We also find that SO + cyclotron waves may dominate over S + if n SO +/n SO 2 + > 2.5 – 3.
    Journal of Geophysical Research Atmospheres 06/2007; 112(A6). DOI:10.1029/2006JA012230 · 3.43 Impact Factor
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    ABSTRACT: [1] This study reports direct detection by the Cassini plasma spectrometer of freshly-produced water-group pick-up ions within the proposed Enceladus torus, a radially narrow toroidal region surrounding Saturn that contains a high density of water-group neutrals. This torus is produced by the icy plumes observed near the south pole of Enceladus. The ions are created by charge exchange collisions between water-group neutrals in the Enceladus torus and thermal ions corotating with Saturn. They are identified in the Cassini data via their characteristic ring-like signatures in ion velocity distributions. In the radial distance range of 4.0 to 4.5 RS, the density of these non-thermalized ions is estimated to be at least 5.2 cm−3, about 8% of the total ion density. The estimated density together with ionization, charge exchange, and loss times, yield an ion thermalization time of at least 3150 s, in reasonable agreement with hybrid particle simulations.
    07/2008; 35(14). DOI:10.1029/2008GL034749
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