tokens: here but contains also a 1/ r dependence. Recently, Vigren Eriksson ( 2017 ) developed a 1D model to test the ability of H 2 O molecules to interrupt ion acceleration along weak electric fields in the radial direction. From the model results, they raised concerns about the correctness in assuming u I, flow = u for r r IN as given by Equation ( 1 ) or ( 2 ). As an example, for perihelion conditions, r IN from equation (1 ) is at ∼1000 km, while the estimated effective ion flow speed at 200 km was calculated as several km s −1 , assuming a rather weak electric field strength of the order of ∼0.03–0.1 mV m -1 , which may commence due to the electron pressure gradient force. Their finding would support the observations of H 3 O + /H 2 O + number density ratios observed at times to be lower than predicted by models running with the assumption of collisionally coupled ions (see fig. 7 of Fuselier et al. 2016 ). The observed ratios were observed to be variable and at times also at the level predicted by collisionally coupled models (see in particular, Fig. 6 of Fuselier et al. 2016 ), particularly so when the RPC-Ion Electron Sensor (RPC-IES, see Mandt et al. 2016 ) did not see clear signs of ion acceleration. Vigren Eriksson ( 2017 ) referred also to preliminary values of effective ion speeds (not restricted to ion flow speeds) of several km s −1 as calculated from combined measurements by the Mutual Impedance Probe (MIP, Trotignon et al. 2007 ) and the Langmuir Probe (LAP, Eriksson et al. 2007 ), both being subsystems of the Rosetta Plasma Consortium (RPC, Carr et al. 2007 ). In this work, we will first (Section 2 ) discuss the MIP and LAP measurements and how they can serve to estimate the mean (or rather the effective) ion speed. The selected time interval for the study, 2015 August 2–4, near perihelion, is discussed in Sec-tion 3 . Results are presented and discussed in Section 4 . The derived effective ion speeds are markedly higher than the neutral outflow velocity and also shown to at least periodically be in good agreement with effective ion drift speeds derived from an independent method; a simple flux conservation model relying on MIP and COPS measurements. The work is briefly summarized in Section 5 . 2 METHOD Our method utilized to derive effective ion speeds is based on combining electron number densities measured by the MIP with voltage–current characteristics from the ion side (negative voltages) of Langmuir probe bias voltage sweeps (we used the LAP1 sensor, see Eriksson et al. 2007 for details). The MIP measurements consist of determining the mutual impedance between two electric antennas – a transmitting and a receiving one. The transmitting electrode injects an oscillating current in the plasma, at a given frequency, while the receiving one measures the potential difference induced in the plasma at the same frequency. The frequency is increased step by step, in a frequency range that ideally contains the plasma frequency. The
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