Abstract
Abstract The flow direction of low-energy ions around comet 67P/Churyumov–Gerasimenko has previously been difficult to constrain due to the influence of the spacecraft potential. The Ion Composition Analyzer of the Rosetta Plasma Consortium (RPC-ICA) on Rosetta measured the distribution function of positive ions with energies down to just a few eV/q throughout the escort phase of the mission. Unfortunately, the substantial negative spacecraft potential distorted the directional information of the low-energy data. In this work, we present the flow directions of low-energy ions around comet 67P, corrected for the spacecraft potential using Particle-In-Cell simulation results. We focus on the region in and around the diamagnetic cavity, where low-energy ions are especially important for the dynamics. We separate between slightly accelerated ‘burst’ features and a more constant ‘band’ of low-energy ions visible in the data. The ‘bursts’ are flowing radially outwards from the nucleus with an antisunward component while the ‘band’ is predominantly streaming back towards the comet. This provides evidence of counter-streaming ions, which has implications for the overall expansion velocity of the ions. The backstreaming ions are present also at times when the diamagnetic cavity was not detected, indicating that the process accelerating the ions back towards the comet is not connected to the cavity boundary.
Highlights
Low-energy ions play important roles in cometary environments
We present the first results from this correction, focusing on the flow directions in the region inside and in the vicinity of the diamagnetic cavity
The XCSEQ axis points towards the Sun along the Sun-comet line, the ZCSEQ axis is orthogonal to XCSEQ and directed along the solar rotation axis, and the YCSEQ axis completes the right-handed system
Summary
Low-energy ions play important roles in cometary environments. When the neutral particles in the coma get ionized, primarily through photoionization and through electron-impact ionization and charge exchange with the solar wind (Galand et al 2016; Simon Wedlund et al 2017), the newly created ions are initially cold and moving with the same velocity as the neutral gas (0.5–1 km s−1; Gulkis et al 2015). A radial ambipolar electric field, created due to the charge separation arising from warm electrons in a steep density gradient, accelerates the ions outwards from the comet nucleus The different gyro radii of ions and electrons give rise to a tailward polarization electric field (Nilsson et al 2018), resulting in a flow direction of new born ions radially outwards from the nucleus with a clear antisunward component When the solar wind encounters the plasma cloud around the comet, the cometary particles are ‘picked up’ and accelerated along the convective electric field of the solar wind, forcing the solar wind The different gyro radii of ions and electrons give rise to a tailward polarization electric field (Nilsson et al 2018), resulting in a flow direction of new born ions radially outwards from the nucleus with a clear antisunward component (Bercicet al. 2018).
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