Effective ion speeds at ~200–250 km from comet 67P/Churyumov–Gerasimenko near perihelion

In 2015 August, comet 67P/Churyumov–Gerasimenko, the target comet of the ESA Rosetta
mission, reached its perihelion at ~1.24 au. Here, we estimate for a three-day period near perihelion, effective ion speeds at distances ~200–250 km from the nucleus. We utilize two different methods combining measurements from the Rosetta Plasma Consortium (RPC)/Mutual Impedance Probe with measurements either from the RPC/Langmuir Probe or from the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA)/Comet Pressure Sensor (COPS) (the latter method can only be applied to estimate the effective ion drift speed). The obtained ion speeds, typically in the range 2–8 km s⁻¹, are markedly higher than the expected neutral outflow velocity of ~1 km s⁻¹. This indicates that the ions were de-coupled from the neutrals before reaching the spacecraft location and that they had undergone acceleration along electric fields, not necessarily limited to acceleration along ambipolar electric fields in the radial direction. For the limited time period studied, we see indications that at increasing distances from the nucleus, the fraction of the ions’ kinetic energy associated with radial drift motion is decreasing.