We present and compare measurements of the spacecraft potential (Vs/c) of the Rosetta space- craft throughout its stay in the inner coma of comet 67P/Churyumov-Gerasimenko, by the Langmuir probe ...(RPC-LAP) and Ion Composition Analyzer (RPC-ICA) instruments. Vs/c has mainly been negative, driven by the high temperature (~5-10 eV) of the coma photoelectrons. The magnitude of the negative Vs/c traces heliocentric, cometocentric, seasonal and diurnal variations in cometary outgassing, consistent with production at or inside the cometocentric distance of the spacecraft being the dominant source of the observed plasma. LAP only picks up a portion of the full Vs/c since the two probes, mounted on booms of 2.2 and 1.6 m length, respectively, are generally inside the potential field of the spacecraft. Comparing with the minimum energy of positive ions collected by ICA, we find numerous cases with strong cor- relation between the two instruments, from which the fraction of Vs/c picked up by LAP is found to vary between about 0.7 and 1. We also find an ICA energy offset of 13.7 eV (95% CI: 12.5, 15.0). Many cases of poor correlation between the instruments are also observed, predominantly when local ion production is weak and accelerated ions dominate the flux, or during quiet periods with low dynamic range in Vs/c and consequently low signal-to-noise ratios.
A major point of interest in cometary plasma physics has been the diamagnetic cavity, an unmagnetized region in the inner-most part of the coma. Here, we combine Langmuir and Mutual Impedance Probe ...measurements to investigate ion velocities and electron temperatures in the diamagnetic cavity of comet 67P, probed by the Rosetta spacecraft. We find ion velocities generally in the range 2-4 km/s, significantly above the expected neutral velocity \(\lesssim\)1~km/s, showing that the ions are (partially) decoupled from the neutrals, indicating that ion-neutral drag was not responsible for balancing the outside magnetic pressure. Observations of clear wake effects on one of the Langmuir probes showed that the ion flow was close to radial and supersonic, at least w.r.t. the perpendicular temperature, inside the cavity and possibly in the surrounding region as well. We observed spacecraft potentials \(\lesssim\)-5~V throughout the cavity, showing that a population of warm (\(\sim\)5~eV) electrons was present throughout the parts of the cavity reached by Rosetta. Also, a population of cold (\(\lesssim0.1\)~eV) electrons was consistently observed throughout the cavity, but less consistently in the surrounding region, suggesting that while Rosetta never entered a region of collisionally coupled electrons, such a region was possibly not far away during the cavity crossings.
Comets hold the key to the understanding of our solar system, its formation and its evolution, and to the fundamental plasma processes at work both in it and beyond it. A comet nucleus emits gas as ...it is heated by the sunlight. The gas forms the coma, where it is ionised, becomes a plasma and eventually interacts with the solar wind. Besides these neutral and ionised gases, the coma also contains dust grains, released from the comet nucleus. As a cometary atmosphere develops when the comet travels through the solar system, large-scale structures, such as the plasma boundaries, develop and disappear, while at planets such large-scale structures are only accessible in their fully grown, quasi-steady state. In situ measurements at comets enable us to learn both how such large-scale structures are formed or reformed and how small-scale processes in the plasma affect the formation and properties of these large scale structures. Furthermore, a comet goes through a wide range of parameter regimes during its life cycle, where either collisional processes, involving neutrals and charged particles, or collisionless processes are at play, and might even compete in complicated transitional regimes. Thus a comet presents a unique opportunity to study this parameter space, from an asteroid-like to a Mars- and Venus-like interaction. Fast flybys of comets have made many new discoveries, setting the stage for a multi-spacecraft mission to accompany a comet on its journey through the solar system. This white paper reviews the present-day knowledge of cometary plasmas, discusses the many questions that remain unanswered, and outlines a multi-spacecraft ESA mission to accompany a comet that will answer these questions by combining both multi-spacecraft observations and a rendezvous mission, and at the same time advance our understanding of fundamental plasma physics and its role in planetary systems.
