Electromagnetic ion cyclotron wave generation by planetary pickup ions: One‐dimensional hybrid simulations at sub‐Alfvénic pickup velocities

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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.