Low-energy (order 10 eV) ion flow in the magnetotail lobes inferred from spacecraft wake observations

Cold ionospheric ions with eV energies are common in the magnetosphere and can travel far out in the magnetotail. However, they are difficult to measure with conventional ion spectrometers mounted on spacecraft, since the potential of a sunlit spacecraft often reaches several tens of volts. In this paper we present two alternative methods of measuring the cold-ion flow with the Cluster spacecraft and apply them on one case in the magnetotail at 18 R-E: 1. Ion spectrometer in combination with artificial spacecraft potential control; 2. Deriving ion flow velocity (both perpendicular and parallel) from electric field instruments. The second method takes advantage of the effect on the double-probe instrument of the wake formed behind a spacecraft in a plasma flow. The results from the two methods show good agreement and are also consistent with polar wind models and previous measurements at lower altitudes, confirming the continuation of low-energy ion outflows.