Abstract
The highly varying plasma environment around comet 67P/Churyumov–Gerasimenko inspired an upgrade of the ion mass spectrometer (Rosetta Plasma Consortium Ion Composition Analyzer) with new operation modes, to enable high time resolution measurements of cometary ions. Two modes were implemented, one having a 4 s time resolution in the energy range 0.3–82 eV/q and the other featuring a 1 s time resolution in the energy range 13–50 eV/q. Comparing measurements made with the two modes, it was concluded that 4 s time resolution is enough to capture most of the fast changes of the cometary ion environment. The 1462 h of observations done with the 4 s mode were divided into hour-long sequences. It is possible to sort 84 per cent of these sequences into one of five categories, depending on their appearance in an energy–time spectrogram. The ion environment is generally highly dynamic, and variations in ion fluxes and energies are seen on time-scales of 10 s to several minutes.
Highlights
Comets in the inner Solar system are highly variable and dynamic objects
The highly dynamic plasma processes suggested by the observed fine-structured cometary tails and disconnecting tail events (Mendis & Ip 1977) could be investigated with the plasma instruments in situ by the Rosetta Plasma Consortium (RPC; Carr et al 2007)
This paper focuses on rapid changes in the ion environment around comet 67P as detected by the Rosetta Plasma Consortium Ion Composition Analyzer (RPC-ICA) (Nilsson et al 2007) on board the Rosetta spacecraft
Summary
Comets in the inner Solar system are highly variable and dynamic objects. As they approach the Sun, sublimation of the surface and subsurface material creates expanding atmospheres and long cometary tails. As long as the neutral gas is thin enough, the solar wind permeates the comet’s atmosphere, but when the outgassing increases as the comet moves closer to the Sun, more or less stable plasma boundaries form: a cometary magnetosphere, similar to what is observed around. On shorter time-scales, the comet’s rotation period of 12.4 h (Sierks et al 2015) manifests itself as a variation of 6.2 h in the observed neutral density (Hassig et al 2015) as well as in the ion and electron density (Edberg et al 2015). We will first describe how the time resolution was increased and two new observation modes introduced These modes enabled measurements of cometary ions with a temporal resolution of 4 or 1 s in limited energy ranges and were not planned prior to the arrival at the comet in 2014 August. We will analyse the recorded high time resolution data with the focus on identifying typical time-scales
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