Radial distribution of plasma at comet 67P and implications for cometary flyby missions

The Rosetta spacecraft followed comet 67P/Churyumov-Gerasimenko (67P) for more than two years, at slow walking pace (about 1 m/s) within 1500 km from the nucleus. During one of the radial movements of the S/C in the early phase of the mission the radial distribution of the plasma density could be estimated, and the ionospheric density was found to be inversely proportional to the cometocentric distance r from the nucleus (a 1/r-distribution). In this study we characterise the radial distribution of plasma around 67P throughout the mission further, and expanding on the initial results. We also investigate how a 1/r-distribution can be perceived during a flyby with a fast (10's km/s) spacecraft, such as the upcoming Comet Interceptor mission, when there is also an asymmetry introduced to the outgassing over the comet surface. We use data from the Rosetta Plasma Consortium (RPC) Langmuir probe (LAP) and Mutual Impedance (MIP) instruments during six  intervals throughout the mission, when Rosetta moved radially with respect to the comet, to determine the radial distribution of the plasma. We then simulate what radial distribution a fast flyby mission would actually observe during its passage through a coma when there is a 1/r plasma density distribution but also an asymmetric outgassing introduced. The plasma density around comet 67P is found to roughly follow a 1/r dependence, although significant deviations occur in some intervals. If normalizing all data to a common outgassing rate (or heliocentric distance) and combining the intervals to cover a the full radial range of 10-1500 km, a 1/r^1.19 average distribution is found. The simulated observed density from a fast S/C flying through a coma with a 1/r-distribution but with an asymmetric outgassing can in fact appear as both a 1/r-distribution, a 1/r^2-distribution, or an even steeper distribution, due to the combined effect of the radial variation in plasma with the variation in density arising from the spacecraft moving over an asymmetrically outgassing body.