Spatial distribution of low‐energy plasma around comet 67P/CG from Rosetta measurements

Abstract We use measurements from the Rosetta plasma consortium Langmuir probe and mutual impedance probe to study the spatial distribution of low‐energy plasma in the near‐nucleus coma of comet 67P/Churyumov‐Gerasimenko. The spatial distribution is highly structured with the highest density in the summer hemisphere and above the region connecting the two main lobes of the comet, i.e., the neck region. There is a clear correlation with the neutral density and the plasma to neutral density ratio is found to be ∼1–2·10 −6 , at a cometocentric distance of 10 km and at 3.1 AU from the Sun. A clear 6.2 h modulation of the plasma is seen as the neck is exposed twice per rotation. The electron density of the collisionless plasma within 260 km from the nucleus falls off with radial distance as ∼1/ r . The spatial structure indicates that local ionization of neutral gas is the dominant source of low‐energy plasma around the comet.