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

Journal of Geophysical Research Atmospheres (Impact Factor: 3.44). 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: Titan's dense atmosphere is thought to be an important source of pickup ions in the Saturnian magnetosphere. Cassini spacecraft measurements of the plasmas near Titan show evidence of pickup ions, yet electromagnetic ion cyclotron waves, which are direct indicators of ion pickup, have not yet been detected. Pickup ions and their associated ion cyclotron waves have been observed at Saturn's Extended Neutral Cloud (˜4-10 Rs), as well as at Jupiter's moon, Io, suggesting that the plasma and pickup conditions near Titan may be fundamentally different than these other environments. Using 1-D hybrid simulations, we investigate ion cyclotron wave growth for variable conditions in the Titan environment to test which conditions could yield waves with sufficient amplitude to be clearly detected by spacecraft. Results suggest that ion cyclotron waves have not been observed at Titan because their growth time is too long compared to the convection time of background plasma through the interaction region and therefore will not be of sufficient amplitude to be observed by Cassini. Because Titan can be located in either the magnetically noisy Saturnian current sheet or in the magnetically quiet lobe region, we consider both of these environments in this study.
    Journal of Geophysical Research Atmospheres 01/2010; 115. DOI:10.1029/2010JA015769 · 3.44 Impact Factor
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    ABSTRACT: Enhanced electromagnetic plasma waves generated by unstable pickup ion distributions have been detected by spacecraft in several planetary environments including those of Venus, Earth, Mars, comets, Jupiter and Saturn. These waves are important because they can be used as a diagnostic of the local plasma and the ion pickup conditions. Interpretation of these waves relies on understanding the source of free energy and how that free energy is reduced through instability growth. In this manuscript, we focus on the regime where the pickup velocity is sub-Alfvénic and systematically illustrate some basic differences in instability behavior by carrying out one-dimensional hybrid simulations of pickup ion-generated waves for varying pickup angle, α. Although several pickup ion-driven instabilities are possible in this regime, the dominant electromagnetic instability that is observed is the ion cyclotron ring instability, which generates waves in the spacecraft frame with frequencies near the pickup ion cyclotron frequency. To better understand the instability behavior as well as what wave properties a spacecraft would potentially observe, we carry out the simulations in the bulk plasma frame and in the planetary rest frame (i.e. spacecraft frame). The simulations presented here are not meant to reproduce any particular planetary environment but are intended to illustrate the general behavior in the sub-Alfvénic pickup velocity regime. We also review the previous observations and dispersion analyses of pickup ion-driven waves in the relevant planetary environments, and discuss them in terms of our simulation results.
    Journal of Geophysical Research Atmospheres 06/2012; 117(A6):6215-. DOI:10.1029/2012JA017568 · 3.44 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). DOI:10.1063/1.4736848 · 2.25 Impact Factor


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