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

Journal of Geophysical Research (Impact Factor: 3.17). 01/2006; 111. 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: [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.
    Geophysical Research Letters - GEOPHYS RES LETT. 01/2008; 35(14).
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    ABSTRACT: 1] Recent Cassini observations of active venting of water molecules from Enceladus indicate that the moon is the primary source of Saturn's extended neutral cloud. Ionization of the neutrals through charge exchange creates a population of newborn ions with a velocity space distribution, which is highly unstable to the generation of electromagnetic ion cyclotron waves. Cassini observed such ion cyclotron waves, finding spatial and temporal variability in the wave amplitudes throughout the extended neutral cloud region. Since the amount of energy in the ion cyclotron waves is proportional to the number of newborn ions generating them, it is possible to infer the ion production rate in the region. To do so, we use two-dimensional electromagnetic hybrid (kinetic ions, fluid electrons) simulations to investigate the growth and nonlinear evolution of ion cyclotron waves. We focus on conditions near Enceladus' L shell and compare the simulated and observed ion cyclotron wave amplitudes to estimate the neutral densities and ion production rates. Our simulation results find a relatively linear relation between ion production rate and quasisteady wave energy level (dB 2). For conditions near Enceladus' L shell, we find that water group ion production rates of 0.007–0.014/cc/s (which yield wave amplitudes of $0.1–0.3 nT) are appropriate. For ion production within an annulus volume from 3.9 to 4 R S , we obtain ion production rates of 3.8 Â 10 26 to 7.6 Â 10 26 ions/s or 10.2–20.4 kg/s.
    Journal of Geophysical Research 01/2009; 114. · 3.17 Impact Factor
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    ABSTRACT: A positive slope in a velocity distribution function perpendicular to the ambient magnetic field, such as due to a loss cone or ring velocity distribution, can become a free energy source for the excitation of various plasma waves. Since there exists no analytic expression for integrals of Maxwellian ring velocity distribution functions, their linear properties have previously been studied using several approximations or modeled distributions. In this paper, a numerical method for analyzing the linear dispersion relation for Maxwellian ring-beam velocity distributions is developed. The obtained linear properties are confirmed by direct comparison with full particle simulation results.
    Physics of Plasmas 07/2012; 19(7). · 2.38 Impact Factor


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