Ion Velocity and Electron Temperature Inside and Around the Diamagnetic Cavity of Comet 67P

Odelstad, E.; Eriksson, A. I.; Johansson, F. L.; Vigren, E.; Henri, P.; Gilet, N.; Heritier, K. L.; Vallières, X.; Rubin, M.; André, M. (2018). Ion Velocity and Electron Temperature Inside and Around the Diamagnetic Cavity of Comet 67P. Journal of Geophysical Research: Space Physics, 123(7), pp. 5870-5893. AGU Publications 10.1029/2018ja025542

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A major point of interest in cometary plasma physics has been the diamagnetic cavity,
an unmagnetized region in the innermost 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 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 with respect to the perpendicular temperature, inside
the cavit y and possibly in the surrounding region as well. We observed spacecraft potentials  −5 V
throughout the cavity, showing that a population of warm (∼5 eV) electrons was present throughout
the parts of the cavity reached by Rosetta. Also, a population of cold (0.1 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.

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