Abstract
Rosetta's unique trajectory is allowing exciting measurements of the development of cometary activity between ∼3.6 and 1.2 AU for the first time. For a few months following Rosetta's arrival at comet 67P in August 2014, data from the Rosetta Plasma Consortium (RPC) particle instruments (the Ion and Electron Spectrometer (IES) and the Ion Composition Analyser (ICA)), have shown that the low activity cometary environment was initially dominated by the solar wind. This was expected in the early stages of the mission. In addition to the solar wind and related He+ populations, a low energy pickup ion population is seen intermittently in the early phase of the mission near the comet. The population is very time dependent, but at times reaches higher energy approaching the solar wind energy. During these intervals, ICA data indicate that the composition is mainly water group ions. The rising energy signatures of these ions observed at times indicate that they are in the early phases of the pickup process, initially accelerated by the electric field (‘early phase pickup’). Here, we compare these exciting pickup ion measurements with Giotto measurements at the relatively weak (compared to Halley) comet Grigg-Skjellerup, where early phase pickup was seen including non-gyrotropic cometary ions, and with the AMPTE lithium and barium releases. Our results reveal some striking similarities with the AMPTE releases, particularly the rising energy signature related to early pickup, and a momentum balance between the pickup ions and the deflected solar wind. There is also evidence for momentum transfer between the pickup ions and the solar wind, with less velocity change seen in the solar wind alpha particles compared to the protons; this was also observed in an AMPTE lithium release. We discuss the effects of increasing activity observed between 3.6 to 1.8 AU, including the increasing dominance and energisation of pickup ions, increasing ionospheric effects and the decreasing effect of the solar wind.
Highlights
As Rosetta accompanies its comet from ~3.6 AU to perihelion (~1.2 AU), it provides an unique opportunity to follow the solar wind-comet interaction from relatively dormant to well-developed (e.g. [4])
The solar wind interaction with a comet has been observed at comets Giacobini-Zinner (GZ, e.g. [9]), Halley (e.g. [10]), Grigg-Skjellerup (GS, e,g.[11]), Borrelly (e.g.12) and ChuryumovGerasimenko (CG, 67P, e.g. [1,2])
Before the ‘Halley Armada’ in the mid-1980s, relevant processes were studied by the US-German-UK AMPTE mission [13,14] that released neutral lithium and barium atoms in the solar wind in the vicinity of the Earth, giving an early chance to study cometary plasma physics and processes [6,15,16,17]
Summary
As Rosetta accompanies its comet from ~3.6 AU to perihelion (~1.2 AU), it provides an unique opportunity to follow the solar wind-comet interaction from relatively dormant to well-developed (e.g. [4]). Initial pickup ion measurements from Rosetta at comet 67P indicate the importance of the solar wind in the early measurements [1,2], as had been expected before the mission [3,4]. We will compare the early Rosetta results with pickup ion measurements from earlier cometary missions, pointing out the likely importance of non-gyrotropic ion distributions [5] and comparing with the AMPTE lithium [6,8] and Barium [7] releases. Before the ‘Halley Armada’ in the mid-1980s, relevant processes were studied by the US-German-UK AMPTE mission [13,14] that released neutral lithium and barium atoms in the solar wind in the vicinity of the Earth, giving an early chance to study cometary plasma physics and processes [6,15,16,17]
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