The Plasma Environment of Comet 67P/Churyumov-Gerasimenko

Abstract

The environment of a comet is a fascinating and unique laboratory to study plasma processes and the formation of structures such as shocks and discontinuities from electron scales to ion scales and above. The European Space Agency's Rosetta mission collected data for more than two years, from the rendezvous with comet 67P/Churyumov-Gerasimenko in August 2014 until the final touch-down of the spacecraft end of September 2016. This escort phase spanned a large arc of the comet's orbit around the Sun, including its perihelion and corresponding to heliocentric distances between 3.8 AU and 1.24 AU. The length of the active mission together with this span in heliocentric and cometocentric distances make the Rosetta data set unique and much richer than sets obtained with previous cometary probes. Here, we review the results from the Rosetta mission that pertain to the plasma environment. We detail all known sources and losses of the plasma and typical processes within it. The findings from in-situ plasma measurements are complemented by remote observations of emissions from the plasma. Overviews of the methods and instruments used in the study are given as well as a short review of the Rosetta mission. The long duration of the Rosetta mission provides the opportunity to better understand how the importance of these processes changes depending on parameters like the outgassing rate and the solar wind conditions. We discuss how the shape and existence of large scale structures depend on these parameters and how the plasma within different regions of the plasma environment can be characterised. We end with a non-exhaustive list of still open questions, as well as suggestions on how to answer them in the future.

Fig. 1

Closest approach distance vs gas production rate for the spacecraft that have visited comets. The gas production rate was derived from in-situ observations. The shaded, red area indicates the approximate range of distances and gas production rates covered by Rosetta during its two year mission. Abbreviations: GZ: 21P/Giacobini-Zinner, ICE: International Cometary Explorer, DS1: Deep Space 1, GS: 26P/Grigg-Skjellerup. Values are from Gringauz et al. (1986b), Johnstone et al. (1993), Richter et al. (2011), Goetz (2019), Cowley (1987)

Fig. 2

View of the interaction pre-Rosetta, for a highly active comet such as 1P/Halley. The Sun is on the right, $v_{\text{sw}}$ and $B$ denote the solar wind flow velocity and the interplanetary magnetic field. The comet is outgassing water that is quickly turned into ${\text{H}}_2{\text{O}}^{+}$. The water ions expand from the comet and create an inner shock where the expansion speed exceeds the acoustic speed. A diamagnetic cavity is formed and the interplanetary magnetic field drapes around the inner coma. The solar wind flow is deflected. For more details see Coates and Jones (2009)

Fig. 3

Water production rate as a function of heliocentric distance reproduced from Hansen et al. (2016), Fig. 9. The results from several Rosetta instruments have been added and include ROSINA (blue diamonds; Hansen et al. 2016), VIRTIS (green triangles; Bockelée-Morvan et al. ; Fink et al. ; Fougere et al. 2016a), RPC/ICA (red triangles; Simon Wedlund et al. 2016), MIRO (yellow circles; Biver et al. ; Lee et al. 2015), and the scaled dust brightness from ground-based observations for comparison (tan crosses; Snodgrass et al. 2016)

Fig. 4

Estimate of the solar wind parameters at 67P during the Rosetta mission. From top to bottom: magnetic field magnitude, Parker angle, density, speed. We show the results of three different propagation models, description see text. From Goetz (2019)

Fig. 5

Separation angle between Earth and comet 67P and Mars and 67P for the duration of the comet phase of the Rosetta mission

Fig. 6

(Top) Ionisation frequencies of water from electron impact (EI, blue), photo-ionisation (PI, red), Solar wind ionisation (SWI, yellow) and solar wind charge exchange (SWCX, purple) at the Rosetta spacecraft (see also Simon Wedlund et al. 2019b, 2020). The ionisation frequency of PI and EI may not represent the ionisation rate throughout the coma, especially near perihelion when electron degradation is significant and the optical depth becomes large. SWCX and SWI are calculated only when Rosetta was outside of the solar wind ion cavity as defined by Behar et al. (2017). (Bottom) Heliocentric distance of 67P throughout the Rosetta mission

Fig. 7

Overview of spectra at $16\text{u}{\text{q}}^{-1}$ over the whole mission in high resolution mode. Colours depend on the period during the escort phase. The detected ion species are ${\text{O}}^{+}$, ${\text{NH}}_2^{+}$, and ${\text{CH}}_4^{+}$. From Beth et al. (2020), reproduced with permission ©ESO

Fig. 8

Schematic of the role of species with high proton affinity molecules on the ion composition in the coma. Arrows represent ion-neutral reactions, from the reactant to the product. Framed ions have been detected by ROSINA/DFMS (Beth et al. 2020). Only H₂O does not result from protonation (of HO)

Fig. 9

Total ion density as a function of the cometocentric distance computed with Eq. 10 for different neutral outgassing rates, outflow bulk velocities (ion and neutral) and (total) ionization frequencies (from Heritier 2018)

Fig. 10

Upper row: illustration of three different regimes for the ion dynamics in physical space (from Behar 2018). Lower row: 3-dimensional evolution of two ion beams given by the generalised gyromotion in velocity space (from Behar et al. , Fig. 1)

Fig. 11

First and second rows: Observations of the deflection angle and bulk speed of the solar wind protons (adapted from Behar ; Behar et al. , Fig. 1). Third row: Observations of bulk velocities of two cometary ion populations. Last row, schematics of the acceleration of cometary ions by an already deflected solar wind ion flow. Third and last taken form Berčič et al. (2018), reproduced with permission ©ESO

Fig. 12

Left: Observed plasma density close to the nucleus during two flybys in February 2015 (Edberg et al. 2015). Right: The effects of an ambipolar electric field and of plasma compression on electron spectra in a kinetic model (Madanian et al. 2016)

Fig. 13

The electron environment close to the nucleus in a particle in cell simulation. For description see text. From Sishtla et al. (2019)

Fig. 14

Left: Observed IES electron spectra (daily averages) for selected intervals. The grey spectrum (Aug 1, 2014) is typical for the solar wind, the other represent various activity stages and cometocentric distances (Madanian et al. 2016). Right: An example IES electron spectrum with fit to two kappa distributions. The high kappa value ${\kappa }_w\approx 18$ of the warm population ($T_w\approx 7\text{eV}$) means it is close to Maxwellian, while the lower ${\kappa }_h\approx 6$ implies a significantly stronger high energy tail in the hot ($T_w\approx 34\text{eV}$) population (Broiles et al. 2016b)

Fig. 15

Presence of cold electrons during the Rosetta mission according to RPC-LAP is colour coded in the large panel, the red line indicates the electron exobase distance. The first panel shows the latitude (blue) and solar zenith angle (orange) of the spacecraft. The two bottom panels show the final energies when reaching Rosetta position of two electrons starting near the nucleus at initial energy 10 eV and 1 eV, respectively, using a simple cooling model and the neutral gas density observed by ROSINA-COPS. Grey overlays indicate times of excursions to larger distance from the nucleus. From Engelhardt et al. (2018a), reproduced with permission ©ESO

Fig. 16

Warm (top) and cold (bottom) electron temperatures derived from RPC-MIP spectra. The colour bar shows the normalised occurrence, calculated for 6 h intervals, so each column can be seen as a colour coded histogram. From Wattieaux et al. (2020), reproduced with permission ©ESO

Fig. 17

Observation of oppositely charged energetic nanograins by the Rosetta Electron and Ion Sensor, RPC-IES. The energy flux of negative (left, denoted “Electrons”) and positive (right, denoted “Ions”) dust grains is colour coded in this energy-azimuth polar plot from which low energy particles have been excluded. Detections of particles of both charge signs over a wide energy range are indicated by the orange ellipses, with their velocity directions compared to the solar direction shown in the insert. This is one of the few (as yet) published pieces of direct Rosetta evidence of dust-plasma interactions. Figure from LLera et al. (2020)

Fig. 18

The wave environment of comets on different scales. (a) Ion pickup-related waves at distances 1–2 orders of magnitude larger than the bow shock for a comet at high activity. The bow shock is shown in orange in the centre of the panel, and the blue, fading into grey, cloud represents the coma. On this scale, the nucleus is too small to be seen. (b) Waves in the foreshock and at the bow shock for a high activity comet. The orange region represents the bow shock. (c) Waves in the inner coma, inside of the infant bow shock, at intermediate activity. The infant bow shock is here illustrated through the corresponding proton density structure in a hybrid simulation (see Sect. 4.2 and Gunell et al. 2018). The red parts correspond to the highest density. (d) The multitude of waves seen in and around the diamagnetic cavity: ion acoustic waves near the boundary but on the inside; and on the outside lower hybrid waves, steepened waves, and ion Bernstein waves. In addition, the boundary itself may exhibit surface waves. (e) Singing comet waves in the coma of a weak activity comet, where the wavelength is on the same order of magnitude as typical length scales of the ionised coma itself, the latter being represented by the green to grey region. From Gunell (2020)

Fig. 19

Time series of waves observed by the Rosetta spacecraft during its mission. (a) Ion acoustic waves 28 km from the centre of the comet, which was at 2.5 AU heliocentric distance (power spectral densities were published by Gunell et al. 2017b). (b) Low frequency ion acoustic waves in the diamagnetic cavity at the same time as waves in the same frequency range were seen outside the cavity (both reported by Madsen et al. 2018). (c) Electric field observations of lower hybrid waves during a part of the interval shown in panel (d) (interval selected from André et al. 2017). (d) Steepened waves seen outside the diamagnetic cavity in the magnetic field components as well as the magnitude of the magnetic field. (e) Langmuir probe current showing waves interpreted as ion Bernstein waves (Odelstad et al. 2020). (f) Singing comet waves shortly after the spacecraft arrived at comet 67P (originally reported by Richter et al. 2015)

Fig. 20

Wave observations when Rosetta was in the diamagnetic cavity. The power spectral density of the waves was measured using the Langmuir probe instrument, RPC-LAP, on 3 August 2015. From (Gunell 2020)

Fig. 21

A–B: The spectral index $\alpha$ as a function of radial distance from the comet for the singing dominated intervals and the no singing intervals. C–D: The spectral index $\alpha$ as a function of $log(P_0)$, from the power law fittings. In all panels the median (red), mean (green), and standard deviation (blue) of the $\alpha$-distributions are plotted. From (Volwerk et al. 2018), reproduced with permission ©ESO

Fig. 22

Example of the cometary plasma response to a CIR as measured by the RPC instruments. a) Proton velocity in km/s, b) proton density in ${\text{cm}}^{-3}$, c) proton temperature in eV, d) magnetic field in nT, e) and f) ion and electron energy spectra with energy in eV, g) electron density in ${\text{cm}}^{-3}$, h) latitude of the spacecraft in degrees, i) cometocentric (blue) and heliocentric (red) distance of the spacecraftin km. From Hajra et al. (2018a), Fig. 3

Fig. 23

Measurements during the October 2015 ICME impact. From top to bottom: a) magnetic field components in CSEQ and magnitude, b) solar wind ion flux, c) electron flux, d) Langmuir probe current-voltage sweep, e) spacecraft potential, f) plasma density. The start of the ICME impact is marked as a vertical dashed line. From Edberg et al. (2016a), Fig. 4

Fig. 24

NAVCAM context images (left) with the Alice slit superimposed and near simultaneous spectral images (right) for 2014 November 29 (top) and 2015 January 30 (bottom). The white horizontal lines outline the 4 rows used in the spectral extraction. In all of the images the Sun is towards the top. From Feldman et al. (2018), ©AAS. Reproduced with permission

Fig. 25

Coma spectra corresponding to the spectral images in Fig. 24. All of the spectra are summations over 4 rows (${0.05}^{\circ }\times {1.2}^{\circ }$) in the narrow center of the slit. The blue line is a synthetic spectrum of electron impact on H₂O. The green line is the same for CO₂. Both are adjusted to compensate for uncertainties in the energy distribution of both the cross section and electron flux. The positions of the ($2,0$) and ($1,0$) CO Cameron bands are also indicated. Note the scale change at 1750 Å by a factor of 2.5. From Feldman et al. (2018), ©AAS. Reproduced with permission

Fig. 26

Top: Cometocentric distance $r$ (red) of Rosetta and heliocentric distance $R$ (blue) of the comet during the main phase of the Rosetta mission. Major milestones are marked by arrows. Middle: Phase angle between the spacecraft and the Sun in a cometocentric system. Bottom: Longitude of the Sun and spacecraft in a cometocentric system. The gray shaded boxes show northern summer, while the white background denotes southern summer. The latitude was not included as it changes periodically with the comet’s rotation rate ($\sim 12\text{hours}$) and would not be visible at this scale

Fig. 27

Left: Dayside excursion, Right: Nightside excursion in a CSEQ system. Color code is blue to yellow increasing in time

Fig. 28

Image of 67P/C-G obtained with the 2.5 m Isaac Newton Telescope on La Palma on the morning of 19 January 2016. The picture was taken through a red filter; the apparent colour has been added to help pick out faint structures by eye. The tail extends 0.5 degrees from the nucleus (the apparent size of the full moon) before reaching the edge of the image, corresponding to a minimum length of 2.2 million km. Note that the thick black lines are gaps between CCDs in the array (the camera has 4 CCDs to cover half a degree). Credit: Alan Fitzsimmons/Isaac Newton Telescope

Fig. 29

Multi-instrument approach applied to the analysis of the plasma density. Coincident neutral gas, particle, and plasma observations are linked though the solution of the continuity equation in order to assess the plasma balance. The ion bulk velocity is derived from observations from MIRO. The solar flux observed by TIMED-SEE at Earth is extrapolated in phase and distance to comet 67P. From Heritier et al. (2018), reproduced with permission ©ESO

Fig. 30

Top: ROSINA-COPS total neutral number density (solid line) measured at Rosetta and the sub-spacecraft latitude (dashed line) as a function of time. Bottom: Ionospheric density as a function of time. The period shown is 2014 October 17–18. The blue (red) curves correspond to the calculated plasma density assuming photoionization alone (photoionization and electron-impact ionization). The vertical spread of these curves corresponds to the range of ion outflow velocity considered, spreading from 400 m/s (top boundary) to 700 m/s (bottom boundary). The RPC-MIP electron density is shown with large, violet dots. The RPC-LAP electron density is shown with small green dots, assuming an electron temperature of 7.5 eV (light green). Due to a different operation mode, there is no RPC-LAP electron density available between 07:00 and 08:30 UT on 2014 October 18. The subsolar latitude was ${40}^{\circ }$ and $\mathit{\text{R}}osetta$ was in the solar terminator plane. From Galand et al. (2016), Fig. 15

Fig. 31

Top: Time series of the ROSINA-COPS measured neutral number density (full line) and cometocentric distance of the spacecraft (dashed line). Middle: Time series of the photo-ionisation frequency ${\nu }^{h\nu }$ (blue curve) and total ionisation frequencies (${\nu }^{h\nu }+{\nu }^{e-}$) (red dots). Seasonal variations are colour coded in the top panel with pink for spring (northern hemisphere) and yellow for autumn (southern hemisphere). Bottom: Time series of the RPC-MIP measured electron number density (pink dots), smoothed using a 5-minute average (purple) and RPC-LAP derived total ion densities (green). Simplified modelled ionospheric densities (Eq. (9)) using photo-ionisation only (blue) and both photo-ionisation and electron-impact ionisation (red), assuming outflow velocity from 400 m/s (upper bound) to 700 m/s (lower bound). The solar wind dynamic pressure as predicted by the Tao et al. (2005) model is plotted in green (middle panel) to illustrate the effect of the CIR impact. From Heritier et al. (2018), reproduced with permission ©ESO

Fig. 32

Schematic of the multi-instrument analysis applied to FUV emissions. From Stephenson et al. (2021), reproduced with permission ©ESO

Fig. 33

(a) Comparison of modelled (black) and observed (magenta) atomic oxygen emissions for seven cases in the northern hemisphere of 67P with a nadir viewing. Measured and modelled points for a given date and time are offset for visibility. From Galand et al. (2020). (b) and (c) Comparison of modelled (solid lines) and observed (crosses) FUV emissions during a corotating interaction region at comet 67P on 4th Aug 2016. Each time interval of consecutive measurements is plotted in its own color to better distinguish them. Both from Stephenson et al. (2021), reproduced with permission ©ESO. Further atomic emission lines to those shown have been modelled in both of the above studies

Fig. 34

Illustration of the boundaries formed by the interaction of the solar wind with the coma that have been observed during spacecraft flybys of comets. All of these boundaries are thought to be permanent features of the interaction when the outgassing rate is high enough. The grey shaded region is the portion of the comet solar wind interaction explored by Rosetta between 2015 May and December

Fig. 35

Illustration of the electric fields in the inner part of the coma of a comet that has not developed a diamagnetic cavity. From (Gunell 2020)

Fig. 36

The infant bow shock at comet 67P. In the left-hand panel the red circles mark the position of comet 67P on each of the 152 days on which shocked solar wind plasma was observed (Goetz et al. 2020a). The second panel shows the magnetic flux density magnitude in a hybrid simulation of the infant bow shock. The light blue circle marks the position of the nucleus. The three panels on the right-hand side show from the top: the energy spectrum of the light ions, the electron energy spectrum, and the components and magnitude of $\overrightarrow{B}$ as observed by the Rosetta spacecraft (Gunell et al. 2018). The vertical light blue and purple lines mark when the spacecraft entered and exited the region with shocked plasma in an infant bow shock encounter. From (Gunell 2020)

Fig. 37

Analytical trajectories of solar wind protons, Sun to the right and upstream magnetic field directed upward, in the same plane. Around the nucleus at the centre, a region is created in which solar wind ions cannot penetrate. According to the model, these trajectories simply scale with the activity and the heliocentric distance. From Behar et al. (2018a), reproduced with permission ©ESO

Fig. 38

Radial magnetic field profile for the two models as applied to comet 67P/Churyumov-Gerasimenko. From Goetz et al. (2017), Fig. 1

Fig. 39

Two-dimensional histograms of the cone angle of the magnetic field along the Rosetta orbit for the outbound (left) and inbound (right) leg. The green lines show the aberrated Parker spiral angles. From Volwerk et al. (2019), reproduced with permission ©ESO

Fig. 40

Observations of crossings of the ion-neutral collisionopause during the excursion to 1500 km in September and October 2015. The bar at the top of the figure indicates whether the spacecraft was in the inner (black) or the outer (red) region. In the outer region ions with energies greater than 50 eV are observed (panel a). Ion energies in the inner region are observed at the spacecraft potential (panel c), which varied between 0 and $-20\text{V}$. The boundary between the two regions appears in some crossings to be sharp, such as on 15 October, while at other points to be broad. The sharp boundary crossings likely appear to be sharp because the solar wind dynamic pressure increased due to an event such as a CIR or CME, causing the boundary to move across the spacecraft (see Mandt et al. , for details). From Mandt et al. (2019), reproduced with permission ©ESO

Fig. 41

Comparison of plasma observations as a function of the distance from the nucleus scaled by the production rate $\times {10}^{-27}$ as observed by Rosetta during the dayside excursion, Giotto during flybys of 1P/Halley (Altwegg et al. 1993) and 26P/Grigg-Skjellerup (Goldstein et al. 1994) and Deep Space 1 during the flyby of 19P/Borrelly (Young et al. 2004). The top panel shows the total ion number density for the flybys and the Rosetta electron density. The centre panel illustrates the ion bulk velocity, and the bottom panel shows the ratio of ions with mass 19 (${\text{H}}_3{\text{O}}^{+}$) to ions with mass 18 (mostly ${\text{H}}_2{\text{O}}^{+}$ with some uncertain contribution from ${\text{NH}}_4^{+}$). The horizontal dashed lines in the centre panel represent the estimated bulk velocities for RPC-IES observations of collisionopause crossings after correcting for spacecraft charging effects. The shaded regions indicate where Rosetta, DS1, and Giotto at 1P/Halley crossed into a similar range of velocities indicating crossing of the cometary ion collisionopause. The ion velocities at 26P/GS did not show an indication of crossing of this boundary. From Mandt et al. (2019), reproduced with permission ©ESO

Fig. 42

RPC observations of the plasma from 30 July 2015. From top to bottom: ICA heavy ions (high time resolution mode and negative spacecraft potential in black), IES ions, IES electrons, MIP spectra, plasma density from LAP and MIP, magnetic field, cone and clock angle of the magnetic field. The white line on the IES electrons indicates the summed flux over the $80\text{–}120\text{eV}$ interval. The horizontal magenta bars mark the time that Rosetta was measuring inside the diamagnetic cavity. e1 and e2 mark the times of two density enhancements in the diamagnetic cavity

Fig. 43

Left: Occurrence of cavity measurements over the spacecraft-comet distance in electron collision lengths. Right: spacecraft-comet distance over gas production rate (derived from in-situ measurements and a Haser model). The colour scale gives the dwell time of the spacecraft over the entire Rosetta comet phase, and the blue points mark the points at which a cavity was detected

Fig. 44

Sketch of the diamagnetic cavity at comet 67P. The magnetic field (green) is draped around the cavity and solar wind electrons (sw ${\text{e}}^{-}$) are moving along the field lines. Rosetta is located at the terminator, the most common configuration during the mission. Ion velocities are indicated in dark purple, with the accelerated ions outside the cavity moving anti-sunward and the newborn ions moving radially outward. The ambipolar electric field (white) is confined to the cavity. The cold (blue) and warm (red) ion populations are indicated as well. The light red background indicates the presence of warm electrons in the entire inner coma. The ion-neutral collisionopause is shown in black dashed lines

Fig. 45

Sketch of the plasma environment at 67P at three different activity stages as defined in Sect. 1. Note that the sketch is in a CSE system, aligned with the magnetic and convective electric fields and that the cross-sections are different for the three panels. While the structures are approximately to scale with respect to each other for the low and medium activity cases. The high activity case is not to scale with regards to the other stages and the scale is approximately logarithmic with respect to the radial distance to the nucleus