Radial distribution of plasma at comet 67P : Implications for cometary flyby missions

Context. The Rosetta spacecraft followed comet 67P/Churyumov-Gerasimenko (67P) for more than two years at a slow walking pace (similar to 1 m s(-1)) within 1500 km from the nucleus. During one of the radial movements of the spacecraft 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). Aims. This study aims to further characterise the radial distribution of plasma around 67P throughout the mission and to expand on the initial results. We also aim to investigate how a 1/r distribution would be observed during a flyby with a fast (similar to 10's km s(-1)) spacecraft, such as the upcoming Comet Interceptor mission, when there is also an asymmetry introduced to the outgassing over the comet surface. Methods. To determine the radial distribution of the plasma, we used data from the Langmuir probe and Mutual Impedance instruments from the Rosetta Plasma Consortium during six intervals throughout the mission, for which the motion of Rosetta was approximately radial with respect to the comet. We then simulated what distribution a fast flyby mission would actually observe during its passage through a coma when there is a 1/r plasma density distribution as well as a sinusoidal variation with a phase angle (and then a sawtooth variation) multiplied to the outgassing rate. Results. The plasma density around comet 67P is found to roughly follow a 1/r dependence, although significant deviations occur in some intervals. If we normalise all data to a common outgassing rate (or heliocentric distance) and combine the intervals to a radial range of 10-1500 km, we find a 1/r(1.19) average distribution. The simulated observed density from a fast spacecraft flying through a coma with a 1/r distribution and an asymmetric outgassing can, in fact, appear anywhere in the range from a 1/r distribution to a 1