 mass ejections or co-rotating interacting regions (McKenna-Lawlor et al. 2016; Edberg et al. 2016a,b), the net effects of which are mainly to compress the comet’s plasma environment and increase the ionisation due to particle impact. Solar wind variations may also include those of the solar wind dynamic pressure. Using RPC instruments, Mandt et al. (2016) report the presence of plasma boundaries between April 2015 and February 2016. The authors in turn link these observations to the predominance of ion-neutral collisions and the so-called “collisionopause” deep in the comet’s atmosphere, modulated by the neutrals and the solar wind dynamic pressure.
Because the comet’s atmosphere typically extends in space on scales of hundreds of thousands of kilometres, mass loading of the solar wind is expected to play a major role in the formation of boundaries and plasma regions (Biermann et al. 1967; Szegö et al. 2000). Its effects, such as deflection and deceleration of the solar wind flow, are investigated with RPC-ICA by Behar et al. (2016b) and later by Behar et al. (2016a): they find that the observed solar wind deflection was the clearest indication of ongoing mass loading, with only little slowing down of the flow. Deflection of the solar wind from the Sun-comet line occurs because solar wind ions undergo a Lorentz force in an opposite direction to pickup ions. The pickup ions are accelerated mostly in the direction of the solar wind convection electric field Esw, which is necessary to conserve energy and momentum between fast-moving solar wind ions and slow-moving cometary ions. As a result, a weak pile-up of the plasma may occur, which can be seen in the magnetic field data. This effect is reported at 67P by Koenders et al. (2016a), with strong signatures observed in RPC-MAG data at 2.0 AU that arise from the B-field piling up and draping. Using a hybrid model, these authors argue for a strongly deflected plasma flow in the first few tens of kilometres from the nucleus, in agreement with the particle instruments onboard Rosetta.
Ions that are newly picked up by the solar wind, originally at rest, will accelerate along ESW to a maximum speed of 2 × USW in the situation where the Interplanetary Magnetic Field (IMF) is perpendicular to the flow of the solar wind, where USW is the solar wind speed; they will also gyrate about the magnetic field BSW with a typical radius rL = miUSW/e | BSW |, where mi is the mass of the ion i and e is the elementary charge. A typical value of rL at 1.3 AU is about 1.5 − 2 × 104 km, which is of the order of magnitude of the macroscopic plasma boundaries found at weakly outgassing comets such as the bow shock and cometopause. Pickup ions adopt ring or ring-beam velocity distribution functions (Coates & Jones 2009), giving rise to instabilities which are in turn dissipated by the generation of ion cyclotron or mirror-mode waves (Volwerk et al. 2016). Though no ring-like distributions have so far been detected at 67P (Goldstein et al. 2015; Koenders et al. 2016b), Volwerk et al. (2016) report mirror-mode waves in the RPC-MAG data. Recently, ion acoustic waves were reported by Gunell et al. (2017) using RPC-LAP, RPC-ICA and RPC-MIP, calculating wave dispersion relations based on the ion distribution functions. Their excitation mechanism is still under debate, but such wave studies may contribute to our understanding of how phenomena on different scales, from the ion acoustic wavelength of ~ 10 m, via mean free paths for ion-neutral collisions of 1 − 10 km, to characteristic length scales for electric fields of a few hundred kilometres, interact to shape the plasma environment of a weakly active comet.
Due to the mass loading upstream of the nucleus, the super-magnetosonic solar wind may become sub-magnetosonic close to perihelion when the neutral outgassing is larger than about 1027 s-1 and a weak subcritical shock at low Mach numbers may form as a result, situated at a few thousand kilometres from the nucleus at the subsolar point (Coates & Jones 2009). This shock is distinct from other planetary bow shocks while most resembling that of Venus (Wallis 1973; Slavin et al. 1984; Russell 1985; Mendis et al. 1986; Coates & Jones 2009; Balogh & Treumann 2013) because the flow deceleration and energy dissipation may take place over an extended region of space. Since the first possible identification of a cometary bow shock at a comet (Russell et al. 1984), many observations of cometary bow shocks have been performed: for pre-Rosetta missions, the reader is referred to Coates (1995) and Coates et al. (1997). Cometary bow shock boundaries have also been modelled extensively (Biermann et al. 1967; Galeev & Khabibrakhmanov 1990) and more recently by Koenders et al. (2013, 2015). Despite Rosetta’s excursion on the dayside to nearly 1500 km, attempts to detect a bow shock forming upstream of the nucleus or indications of a change in cometary ion density were unsuccessful; this was in contrast to hybrid model expectations predicting such a boundary at about that cometocentric distance (Koenders et al. 2013). However, the absence of detection does not necessarily imply the absence of any bow shock, in particular as Rosetta appeared to be still inside a cometopause-like boundary (Edberg et al. 2016a). For comparison, Neubauer et al. (1993) report a standoff bow shock distance of ~ 1.7 × 104 km at 1.0 AU at comet 26P/Grigg-Skjellerup, which has a similar outgassing rate at perihelion as 67P.
It is therefore more likely that Rosetta crossed another plasma boundary, such as the so-called cometopause, due to the spacecraft-nucleus distance of a few 100 km adopted throughout the mission. One definition of the cometopause is (i) the region where water-group cometary ions start to dominate the plasma over solar wind ions (Galeev et al. 1988). Gombosi (1987) and Cravens (1989) also defined the cometopause as (ii) the solar wind charge-exchange collisionopause. The nature of the cometopause has been vigorously analysed for comet 1P/Halley (see Gringauz et al. 1986; Gombosi 1987; Ip 1989; Reme et al. 1994; Tátrallyay et al. 1995; Coates & Jones 2009); at comet 26P, the inner boundary of the cometopause was found at about 1.8 × 103 km (Johnstone et al. 1993). At 67P, Mandt et al. (2016) remarked that all of the Rosetta observations of collision-related boundaries occurred within the cometary ion-dominated cometopause shell, defined as in (ii), that is the collisionopause for charge exchange of solar wind protons. If defined as in (i), one may argue that the disappearance of the solar wind signature from the field of view of the RPC ion spectrometers in April 2015 (Nilsson et al. 2015b; Simon Wedlund et al. 2016) and its later reappearance in December 2015 (Nilsson et al., in prep.) may be the signature of the spacecraft gradually transiting in and out of a cometary ion-dominated region, and hence the cometopause. Although predicted by numerical simulations, a more refined analysis of the archived RPC data will likely decide this problem.
For a discussion of the plasma boundaries found so far at comet 67P by Rosetta and how they relate to earlier missions’ findings, the reader is referred to Mandt et al. (2016). For a discussion of more transient plasma boundaries as seen by the Rosetta mission, such as the diamagnetic cavity, the reader is referred to Goetz et al. (2016b), Goetz et al. (2016a) and Nemeth et al. (2016).
One key aspect involved in the formation of these transitions and boundaries is cometary ion production and how it relates to the solar wind plasma and the neutral atmosphere. Three main physico-chemistry processes are responsible for the creation of heavy cometary ions: photoionisation, charge exchange and electron ionisation, sometimes referred to as “electron impact ionisation” in older literature. They take the following form: Ambient neutral species X, mostly H2O molecules at small velocities, become ionised in processes 1-3 (X → X+), after photons hν, solar wind ions Yn+ of charge n +, or suprathermal electrons e− collide with them. Cometary ions X+ are produced.
Loss processes of these cometary ions need also to be taken into account, namely, electron recombination with thermalised electrons: (4)This process results in a range of neutral atoms and molecules, through dissociative recombination.
Processes 1−3and their known impact on the cometary environment are described in more detail below. Because they change the overall ion composition and are efficient at different cometocentric distances, they constitute the backbone of any attempt to model the formation of the macroscopic plasma boundaries of a comet.
Photoionisation due to solar extreme ultraviolet (EUV) radiation, process 1, is one of the main contributors of water-group ions in the close vicinity of the nucleus because the atmosphere is nearly always optically thin for medium activity comets (Huebner 1985; Cravens 1987). In the process, the produced