The Evolution of the Electron Number Density in the Coma of Comet 67P at the Location of Rosetta from 2015 November through 2016 March

Vigren, E.; Edberg, N. J. T.; Eriksson, A. I.; Galand, M.; Henri, P.; Johansson, F. L.; Odelstad, E.; Rubin, Martin; Valliéres, X. (2019). The Evolution of the Electron Number Density in the Coma of Comet 67P at the Location of Rosetta from 2015 November through 2016 March. Astrophysical journal, 881(1), p. 6. Institute of Physics Publishing IOP 10.3847/1538-4357/ab29f7

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A comet ionospheric model assuming the plasma moves radially outward with the same bulk speed as the neutral
gas and not being subject to severe reduction through dissociative recombination has previously been tested in a
series of case studies associated with the Rosetta mission at comet 67P/Churyumov–Gerasimenko. It has been
found that at low activity and within several tens of kilometers from the nucleus such models (which originally
were developed for such conditions) generally work well in reproducing observed electron number densities, in
particular when plasma production through both photoionization and electron-impact ionization is taken into
account. Near perihelion, case studies have, on the contrary, shown that applying similar assumptions
overestimates the observed electron number densities at the location of Rosetta. Here we compare Rosetta Orbiter
Spectrometer for Ion and Neutral Analysis/Comet Pressure sensor-driven model results with Rosetta Plasma
Consortium/Mutual Impedance Probe-derived electron number densities for an extended time period (2015
November through 2016 March) during the postperihelion phase with southern summer/spring. We observe a
gradual transition from a state when the model grossly overestimates (by more than a factor of 10) the observations
to being in reasonable agreement during 2016 March.

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