Abstract
Aims.Cometary ions are constantly produced in the coma, and once produced they are accelerated and eventually escape the coma. We describe and interpret the dynamics of the cometary ion flow, of an intermediate active comet, very close to the nucleus and in the terminator plane.Methods.We analysed in situ ion and magnetic field measurements, and characterise the velocity distribution functions (mostly using plasma moments). We propose a statistical approach over a period of one month.Results.On average, two populations were observed, separated in phase space. The motion of the first is governed by its interaction with the solar wind farther upstream, while the second one is accelerated in the inner coma and displays characteristics compatible with an ambipolar electric field. Both populations display a consistent anti-sunward velocity component.Conclusions.Cometary ions born in different regions of the coma are seen close to the nucleus of comet 67P/Churyumov–Gerasimenko with distinct motions governed in one case by the solar wind electric field and in the other case by the position relative to the nucleus. A consistent anti-sunward component is observed for all cometary ions. An asymmetry is found in the average cometary ion density in a solar wind electric field reference frame, with higher density in the negative (south) electric field hemisphere. There is no corresponding signature in the average magnetic field strength.
Highlights
Comets are, thanks to their enormous tails, the smallest bodies in the solar system observable from Earth with the naked eye (Yeomans et al 1986)
We study the dynamics of the ionised coma at low and intermediate activity
In this article we provide observations of the cometary ion dynamics and their interpretation for a medium activity comet and in the terminator plane, at a distance of ∼28 km from the comet
Summary
Thanks to their enormous tails, the smallest bodies in the solar system observable from Earth with the naked eye (Yeomans et al 1986). At some heliocentric distance the sublimated cometary gas together with dust starts to form a thin and gravitationally unbound atmosphere, which gets ionised mostly by photoionisation, charge exchange, and electron impact ionisation. In contrast with most bodies in the solar system, this partially ionised atmosphere is continuously escaping. The dust and ions form two distinct tails behind the comet. The dust tail generally points away from the Sun, but is slightly curved following the direction of the cometary orbit, while the ion tail is formed closer to the anti-sunward direction through a complex and highly variable interaction with the solar wind (Alfvén 1957)
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