Radial distribution of plasma at comet 67P

Context. The Rosetta spacecraft followed comet 67P/Churyumov-Gerasimenko (67P) for more than two years at a slow walking pace (~1 m s ?1 ) within 1500 km from the nucleus. During one of radial movements in early phase mission, distribution plasma density could be estimated, and ionospheric was found to inversely proportional cometocentric distance r nucleus (a 1/ distribution). Aims. This study aims further characterise around 67P throughout mission expand on initial results. We also aim investigate how would observed during aflyby with fast (~10’s spacecraft, such as upcoming Comet Interceptor when there is an asymmetry introduced outgassing over surface. Methods. To determine plasma, we used data Langmuir probe Mutual Impedance instruments Plasma Consortium six intervals which motion approximately respect comet. then simulated what flyby actually observe its passage through coma well sinusoidal variation angle (and sawtooth variation) multiplied rate. Results. roughly follow dependence, although significant deviations occur some intervals. If normalise all common rate (or heliocentric distance) combine range 10–1500 km, find 1.19 average distribution. flying asymmetric can, fact, appear anywhere 2 distribution, or even slightly outside this range. Conclusions. distributed way that it decreases manner distance. expected photoionisation collision-less, expanding neutral gas constant ionisation expansion speed. deviation pure many cases caused by surface, additional sources being present, electric fields accelerating changing upstream solar wind conditions. A can deviates significantly trend if not symmetric altitude profile will depends very much level asymmetry, velocity, rotation rate, phase. It therefore essential include both inbound outbound legs, compare get complete understanding plasma.