| 1. |
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| 2. |
- Breuillard, H., et al.
(author)
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Properties of the singing comet waves in the 67P/Churyumov-Gerasimenko plasma environment as observed by the Rosetta mission
- 2019
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In: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 630
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Journal article (peer-reviewed)abstract
- Using in situ measurements from different instruments on board the Rosetta spacecraft, we investigate the properties of the newly discovered low-frequency oscillations, known as singing comet waves, that sometimes dominate the close plasma environment of comet 67P/Churyumov-Gerasimenko. These waves are thought to be generated by a modified ion-Weibel instability that grows due to a beam of water ions created by water molecules that outgass from the comet. We take advantage of a cometary outburst event that occurred on 2016 February 19 to probe this generation mechanism. We analyze the 3D magnetic field waveforms to infer the properties of the magnetic oscillations of the cometary ion waves. They are observed in the typical frequency range (similar to 50 mHz) before the cometary outburst, but at similar to 20 mHz during the outburst. They are also observed to be elliptically right-hand polarized and to propagate rather closely (similar to 0-50 degrees) to the background magnetic field. We also construct a density dataset with a high enough time resolution that allows us to study the plasma contribution to the ion cometary waves. The correlation between plasma and magnetic field variations associated with the waves indicates that they are mostly in phase before and during the outburst, which means that they are compressional waves. We therefore show that the measurements from multiple instruments are consistent with the modified ion-Weibel instability as the source of the singing comet wave activity. We also argue that the observed frequency of the singing comet waves could be a way to indirectly probe the strength of neutral plasma coupling in the 67P environment.
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| 3. |
- Bucciantini, L., et al.
(author)
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Instrumentation for Ionized Space Environments : New High Time Resolution Instrumental Modes of Mutual Impedance Experiments
- 2023
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In: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 128:2
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Journal article (peer-reviewed)abstract
- Mutual impedance experiments are in situ plasma diagnostic techniques for the identification of the plasma density and the electron temperature. Different versions of mutual impedance instruments were included in past and present space missions (e.g., Rosetta, BepiColombo, JUICE and Comet Interceptor). New versions are currently being devised to fit the strong mass, volume and power constraints on nanosatellite platforms for future multi-point space missions. In this study, our goal is to define and validate two new instrumental modes (i.e., chirp and multi-spectral modes) to improve the time resolution of the experiment with respect to typical mutual impedance instrumental modes (i.e., frequency sweep). Higher time resolution measurements are expected to simplify the integration of mutual impedance experiments onboard nanosatellite platforms by facilitating antenna sharing between different experiments. The investigation is performed both (a) numerically, using a 1D-1V electrostatic full kinetic Vlasov-Poisson model and, (b) experimentally, with laboratory tests using a vacuum chamber and a plasma source. From a plasma diagnostic point of view, we find that both the chirp and multi-spectral modes provide measurements identical to the (reference) frequency sweep mode. From an instrumental point of view, multi-spectral measurements are faster than frequency sweep measurements but they require larger amounts of onboard computing resources (i.e., larger power consumption). Chirp measurements, instead, outperform frequency sweep measurements both in terms of measurement duration (20 times faster) and onboard processor usage (20% less).
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| 4. |
- Engelhardt, Ilka. A. D., et al.
(author)
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Cold electrons at comet 67P/Churyumov-Gerasimenko
- 2018
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 616
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Journal article (peer-reviewed)abstract
- Context. The electron temperature of the plasma is one important aspect of the environment. Electrons created by photoionization or impact ionization of atmospheric gas have energies ∼10 eV. In an active comet coma the gas density is high enough for rapid cooling of the electron gas to the neutral gas temperature (few hundred kelvin). How cooling evolves in less active comets has not been studied before.Aims. To investigate how electron cooling varied as comet 67P/Churyumov-Gerasimenko changed its activity by three orders of magnitude during the Rosetta mission.Methods. We use in-situ data from Rosetta plasma and neutral gas sensors. By combining Langmuir probe bias voltage sweeps and Mutual Impedance Probe measurements we determine when cold electrons form at least 25% of the total electron density. We compare the results to what is expected from simple models of electron cooling, using the observed neutral gas density as input.Results. We demonstrate that the slope of the Langmuir probe sweep can be used as a proxy for cold electron presence. We show statistics of cold electron observations over the 2 year mission period. We find cold electrons at lower activity than expected by a simple model based on free radial expansion and continuous loss of electron energy. Cold electrons are seen mainly when the gas density indicates an exobase may have formed.Conclusions. Collisional cooling of electrons following a radial outward path is not sufficient for explaining the observations. We suggest the ambipolar electric field is important for the observed cooling. This field keeps electrons in the inner coma for much longer time, giving them time to dissipate energy by collisions with the neutrals. We conclude there is need of better models to describe the plasma environment of comets, including at least two populations of electrons and the ambipolar field.
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| 5. |
- Engelhardt, Ilka. A. D., et al.
(author)
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Plasma Density Structures at Comet 67P/Churyumov-Gerasimenko
- 2018
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 477:1, s. 1296-1307
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Journal article (peer-reviewed)abstract
- We present Rosetta RPC case study from four events at various radial distance, phase angle and local time from autumn 2015, just after perihelion of comet 67P/Churyumov-Gerasimenko. Pulse like (high amplitude, up to minutes in time) signatures are seen with several RPC instruments in the plasma density (LAP, MIP), ion energy and flux (ICA) as well as magnetic field intensity (MAG). Furthermore the cometocentric distance relative to the electron exobase is seen to be a good organizing parameter for the measured plasma variations. The closer Rosetta is to this boundary, the more pulses are measured. This is consistent with the pulses being filaments of plasma originating from the diamagnetic cavity boundary as predicted by simulations.
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| 6. |
- Galand, M., et al.
(author)
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Ionospheric plasma of comet 67P probed by Rosetta at 3 au from the Sun
- 2016
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 462, s. S331-S351
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Journal article (peer-reviewed)abstract
- We propose to identify the main sources of ionization of the plasma in the coma of comet 67P/Churyumov-Gerasimenko at different locations in the coma and to quantify their relative importance, for the first time, for close cometocentric distances (< 20 km) and large heliocentric distances (> 3 au). The ionospheric model proposed is used as an organizing element of a multi-instrument data set from the Rosetta Plasma Consortium (RPC) plasma and particle sensors, from the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis and from the Microwave Instrument on the Rosetta Orbiter, all on board the ESA/Rosetta spacecraft. The calculated ionospheric density driven by Rosetta observations is compared to the RPC-Langmuir Probe and RPC-Mutual Impedance Probe electron density. The main cometary plasma sources identified are photoionization of solar extreme ultraviolet (EUV) radiation and energetic electron-impact ionization. Over the northern, summer hemisphere, the solar EUV radiation is found to drive the electron density - with occasional periods when energetic electrons are also significant. Over the southern, winter hemisphere, photoionization alone cannot explain the observed electron density, which reaches sometimes higher values than over the summer hemisphere; electron-impact ionization has to be taken into account. The bulk of the electron population is warm with temperature of the order of 7-10 eV. For increased neutral densities, we show evidence of partial energy degradation of the hot electron energy tail and cooling of the full electron population.
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| 7. |
- Gilet, N., et al.
(author)
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Observations of a mix of cold and warm electrons by RPC-MIP at 67P/Churyumov-Gerasimenko
- 2020
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 640
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Journal article (peer-reviewed)abstract
- Context. The Mutual Impedance Probe (MIP) of the Rosetta Plasma Consortium (RPC) onboard the Rosetta orbiter which was in operation for more than two years, between August 2014 and September 2016 to monitor the electron density in the cometary ionosphere of 67P/Churyumov-Gerasimenko. Based on the resonance principle of the plasma eigenmodes, recent models of the mutual impedance experiment have shown that in a two-electron temperature plasma, such an instrument is able to separate the two isotropic electron populations and retrieve their properties.Aims. The goal of this paper is to identify and characterize regions of the cometary ionized environment filled with a mix of cold and warm electron populations, which was observed by Rosetta during the cometary operation phase.Methods. To reach this goal, this study identifies and investigates the in situ mutual impedance spectra dataset of the RPC-MIP instrument that contains the characteristics of a mix of cold and warm electrons, with a special focus on instrumental signatures typical of large cold-to-total electron density ratio (from 60 to 90%), that is, regions strongly dominated by the cold electron component.Results. We show from the observational signatures that the mix of cold and warm cometary electrons strongly depends on the cometary latitude. Indeed, in the southern hemisphere of 67P, where the neutral outgassing activity was higher than in northern hemisphere during post-perihelion, the cold electrons were more abundant, confirming the role of electron-neutral collisions in the cooling of cometary electrons. We also show that the cold electrons are mainly observed outside the nominal electron-neutral collision-dominated region (exobase), where electrons are expected to have cooled down. This which indicates that the cold electrons have been transported outward. Finally, RPC-MIP detected cold electrons far from the perihelion, where the neutral outgassing activity is lower, in regions where no electron exobase was expected to have formed. This suggests that the cometary neutrals provide a more frequent or efficient cooling of the electrons than expected for a radially expanding ionosphere.
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| 8. |
- Gunell, H., et al.
(author)
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Ion acoustic waves at comet 67P/Churyumov-Gerasimenko : Observations and computations
- 2017
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In: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 600
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Journal article (peer-reviewed)abstract
- Context. On 20 January 2015 the Rosetta spacecraft was at a heliocentric distance of 2.5 AU, accompanying comet 67P/Churyumov-Gerasimenko on its journey toward the Sun. The Ion Composition Analyser (RPC-ICA), other instruments of the Rosetta Plasma Consortium, and the ROSINA instrument made observations relevant to the generation of plasma waves in the cometary environment.Aims. Observations of plasma waves by the Rosetta Plasma Consortium Langmuir probe (RPC-LAP) can be explained by dispersion relations calculated based on measurements of ions by the Rosetta Plasma Consortium Ion Composition Analyser (RPC-ICA), and this gives insight into the relationship between plasma phenomena and the neutral coma, which is observed by the Comet Pressure Sensor of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis instrument (ROSINA-COPS).Methods. We use the simple pole expansion technique to compute dispersion relations for waves on ion timescales based on the observed ion distribution functions. These dispersion relations are then compared to the waves that are observed. Data from the instruments RPC-LAP, RPC-ICA and the mutual impedance probe (RPC-MIP) are compared to find the best estimate of the plasma density.Results. We find that ion acoustic waves are present in the plasma at comet 67P/Churyumov-Gerasimenko, where the major ion species is H2O+. The bulk of the ion distribution is cold, k(B)T(i) = 0.01 eV when the ion acoustic waves are observed. At times when the neutral density is high, ions are heated through acceleration by the solar wind electric field and scattered in collisions with the neutrals. This process heats the ions to about 1 eV, which leads to significant damping of the ion acoustic waves.Conclusions. In conclusion, we show that ion acoustic waves appear in the H2O+ plasmas at comet 67P/Churyumov-Gerasimenko and how the interaction between the neutral and ion populations affects the wave properties.
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| 9. |
- Hajra, R., et al.
(author)
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Impact of a cometary outburst on its ionosphere Rosetta Plasma Consortium observations of the outburst exhibited by comet 67P/Churyumov-Gerasimenko on 19 February 2016
- 2017
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 607
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Journal article (peer-reviewed)abstract
- We present a detailed study of the cometary ionospheric response to a cometary brightness outburst using in situ measurements for the first time. The comet 67P/Churyumov-Gerasimenko (67P) at a heliocentric distance of 2.4 AU from the Sun, exhibited an outburst at similar to 1000 UT on 19 February 2016, characterized by an increase in the coma surface brightness of two orders of magnitude. The Rosetta spacecraft monitored the plasma environment of 67P from a distance of 30 km, orbiting with a relative speed of similar to 0.2 m s(-1). The onset of the outburst was preceded by pre-outburst decreases in neutral gas density at Rosetta, in local plasma density, and in negative spacecraft potential at similar to 0950 UT. In response to the outburst, the neutral density increased by a factor of similar to 1.8 and the local plasma density increased by a factor of similar to 3, driving the spacecraft potential more negative. The energetic electrons (tens of eV) exhibited decreases in the flux of factors of similar to 2 to 9, depending on the energy of the electrons. The local magnetic field exhibited a slight increase in amplitude (similar to 5 nT) and an abrupt rotation (similar to 36.4 degrees) in response to the outburst. A weakening of 10-100 mHz magnetic field fluctuations was also noted during the outburst, suggesting alteration of the origin of the wave activity by the outburst. The plasma and magnetic field effects lasted for about 4 h, from similar to 1000 UT to 1400 UT. The plasma densities are compared with an ionospheric model. This shows that while photoionization is the main source of electrons, electron-impact ionization and a reduction in the ion outflow velocity need to be accounted for in order to explain the plasma density enhancement near the outburst peak.
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| 10. |
- Henri, P., et al.
(author)
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Diamagnetic region(s) : structure of the unmagnetized plasma around Comet 67P/CG
- 2017
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In: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 469, s. S372-S379
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Journal article (peer-reviewed)abstract
- The ESA's comet chaser Rosetta has monitored the evolution of the ionized atmosphere of comet 67P/Churyumov-Gerasimenko (67P/CG) and its interaction with the solar wind, during more than 2 yr. Around perihelion, while the cometary outgassing rate was highest, Rosetta crossed hundreds of unmagnetized regions, but did not seem to have crossed a large-scale diamagnetic cavity as anticipated. Using in situ Rosetta observations, we characterize the structure of the unmagnetized plasma found around comet 67P/CG. Plasma density measurements from RPC-MIP are analysed in the unmagnetized regions identified with RPC-MAG. The plasma observations are discussed in the context of the cometary escaping neutral atmosphere, observed by ROSINA/COPS. The plasma density in the different diamagnetic regions crossed by Rosetta ranges from similar to 100 to similar to 1500 cm(-3). They exhibit a remarkably systematic behaviour that essentially depends on the comet activity and the cometary ionosphere expansion. An effective total ionization frequency is obtained from in situ observations during the high outgassing activity phase of comet 67P/CG. Although several diamagnetic regions have been crossed over a large range of distances to the comet nucleus (from 50 to 400 km) and to the Sun (1.25-2.4 au), in situ observations give strong evidence for a single diamagnetic region, located close to the electron exobase. Moreover, the observations are consistent with an unstable contact surface that can locally extend up to about 10 times the electron exobase.
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| 11. |
- Heritier, K. L., et al.
(author)
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Plasma source and loss at comet 67P during the Rosetta mission
- 2018
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In: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 618
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Journal article (peer-reviewed)abstract
- Context. The Rosetta spacecraft provided us with a unique opportunity to study comet 67P/Churyumov-Gerasimenko (67P) from a close perspective and over a 2-yr time period. Comet 67P is a weakly active comet. It was therefore unexpected to find an active and dynamic ionosphere where the cometary ions were largely dominant over the solar wind ions, even at large heliocentric distances. Aims. Our goal is to understand the different drivers of the cometary ionosphere and assess their variability over time and over the different conditions encountered by the comet during the Rosetta mission. Methods. We used a multi-instrument data-based ionospheric model to compute the total ion number density at the position of Rosetta. In-situ measurements from the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) and the Rosetta Plasma Consortium (RPC)-Ion and Electron Sensor (IES), together with the RPC-LAngmuir Probe instrument (LAP) were used to compute the local ion total number density. The results are compared to the electron densities measured by RPC-Mutual Impedance Probe (MIP) and RPC-LAP. Results. We were able to disentangle the physical processes responsible for the formation of the cometary ions throughout the 2-yr escort phase and we evaluated their respective magnitudes. The main processes are photo-ionization and electron-impact ionization. The latter is a significant source of ionization at large heliocentric distance (>2 au) and was predominant during the last 4 months of the mission. The ionosphere was occasionally subject to singular solar events, temporarily increasing the ambient energetic electron population. Solar photons were the main ionizer near perihelion at 1.3 au from the Sun, during summer 2015.
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| 12. |
- Heritier, K. L., et al.
(author)
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Vertical structure of the near-surface expanding ionosphere of comet 67P probed by Rosetta
- 2017
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 469, s. S118-S129
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Journal article (peer-reviewed)abstract
- The plasma environment has been measured for the first time near the surface of a comet. This unique data set has been acquired at 67P/Churyumov-Gerasimenko during ESA/Rosetta spacecraft's final descent on 2016 September 30. The heliocentric distance was 3.8 au and the comet was weakly outgassing. Electron density was continuously measured with Rosetta Plasma Consortium (RPC)-Mutual Impedance Probe (MIP) and RPC-LAngmuir Probe (LAP) during the descent from a cometocentric distance of 20 km down to the surface. Data set from both instruments have been cross-calibrated for redundancy and accuracy. To analyse this data set, we have developed a model driven by Rosetta Orbiter Spectrometer for Ion and Neutral Analysis-COmetary Pressure Sensor total neutral density. The two ionization sources considered are solar extreme ultraviolet radiation and energetic electrons. The latter are estimated from the RPC-Ion and Electron Sensor (IES) and corrected for the spacecraft potential probed by RPC-LAP. We have compared the results of the model to the electron densities measured by RPC-MIP and RPC-LAP at the location of the spacecraft. We find good agreement between observed and modelled electron densities. The energetic electrons have access to the surface of the nucleus and contribute as the main ionization source. As predicted, the measurements exhibit a peak in the ionospheric density close to the surface. The location and magnitude of the peak are estimated analytically. The measured ionospheric densities cannot be explained with a constant outflow velocity model. The use of a neutral model with an expanding outflow is critical to explain the plasma observations.
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| 13. |
- Karlsson, Tomas, et al.
(author)
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Localized density enhancements in the magnetosheath : Three-dimensional morphology and possible importance for impulsive penetration
- 2012
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117, s. A03227-
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Journal article (peer-reviewed)abstract
- We use Cluster multipoint density measurements, using the spacecraft potential, to identify localized density enhancements (>50%) in the magnetosheath, and estimate their three-dimensional morphology and orientation. Typically one dimension of the density enhancements is shorter than others, is directed perpendicular to the background magnetic field, and varies from similar to 0.1 R-E to 10 R-E, with the other two dimensions a factor 3-10 greater. The density structures are oriented with the longest sides in the general direction of the bow shock and magnetopause. Examples of density structures both convecting with the same velocity as the background magnetosheath flow ("embedded plasmoids"), and convecting with an excess x(GSE) velocity component ("fast plasmoids") are found. Possible importance for the impulsive penetration mechanism for plasma entry in the magnetosphere is analyzed by comparing the results to laboratory results, via a parameter scaling. The estimation of the three-dimensional topology of the density enhancements will enable a comparison with localized magnetosheath populations inside the magnetosphere, observed earlier, to determine if these originate from penetrated magnetosheath density enhancements.
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| 14. |
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| 15. |
- Madsen, B., et al.
(author)
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Extremely Low-Frequency Waves Inside the Diamagnetic Cavity of Comet 67P/Churyumov-Gerasimenko
- 2018
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In: Geophysical Research Letters. - : AMER GEOPHYSICAL UNION. - 0094-8276 .- 1944-8007. ; 45:9, s. 3854-3864
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Journal article (peer-reviewed)abstract
- The European Space Agency/Rosetta mission to comet 67P/Churyumov-Gerasimenko has provided several hundred observations of the cometary diamagnetic cavity induced by the interaction between outgassed cometary particles, cometary ions, and the solar wind magnetic field. Here we present the first electric field measurements of four preperihelion and postperihelion cavity crossings on 28 May 2015 and 17 February 2016, using the dual-probe electric field mode of the Langmuir probe (LAP) instrument of the Rosetta Plasma Consortium. We find that on large scales, variations in the electric field fluctuations capture the cavity and boundary regions observed in the already well-studied magnetic field, suggesting the electric field mode of the LAP instrument as a reliable tool to image cavity crossings. In addition, the LAP electric field mode unravels for the first time extremely low-frequency waves within two cavities. These low-frequency electrostatic waves are likely triggered by lower-hybrid waves observed in the surrounding magnetized plasma. Plain Language Summary As sunlight heats a comet nucleus, frozen volatile gases sublimate are ionized and interact with the solar wind and its embedded magnetic field, inducing a dynamical plasma environment around the comet. With the cornerstone European mission Rosetta and its 2years of near-continuous orbiting of comet 67P/Churyumov-Gerasimenko, the origin, structure, and evolution of this environment are only starting to be unveiled. Exciting are the numerous crossings of the diamagnetic cavity, the innermost plasma region from which the solar wind magnetic field is excluded. Whilst the magnetic field structure of the cavity crossings is well studied, the related electric field activity remains until now unexplored. Studying the electric field with the Langmuir probes onboard Rosetta, we find that whereas the large-scale electric field structure agrees well with the observed magnetic field behavior during cavity crossings, unexpected short-lived low-frequency electric field signals manifest themselves within the cavity. We interpret these as electrostatic waves triggered by a modulating of the cavity boundary caused by observed electrostatic waves at the same frequency in the surrounding magnetized plasma. This unravels a new aspect of the electromagnetic activity in the inner cometary environment, which is crucial for our understanding of the comet-solar wind-induced plasma environment.
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| 16. |
- Nilsson, Hans, et al.
(author)
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Size of a plasma cloud matters The polarisation electric field of a small-scale comet ionosphere
- 2018
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In: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 616
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Journal article (peer-reviewed)abstract
- Context. The cometary ionosphere is immersed in fast flowing solar wind. A polarisation electric field may arise for comets much smaller than the gyroradius of pickup ions because ions and electrons respond differently to the solar wind electric field. Aims. A situation similar to that found at a low activity comet has been modelled for barium releases in the Earth's ionosphere. We aim to use such a model and apply it to the case of comet 67P Churyumov-Gerasimenko, the target of the Rosetta mission. We aim to explain the significant tailward acceleration of cometary ions through the modelled electric field. Methods. We obtained analytical solutions for the polarisation electric field of the comet ionosphere using a simplified geometry. This geometry is applicable to the comet in the inner part of the coma as the plasma density integrated along the magnetic field line remains rather constant. We studied the range of parameters for which a significant tailward electric field is obtained and compare this with the parameter range observed. Results. Observations of the local plasma density and magnetic field strength show that the parameter range of the observations agree very well with a significant polarisation electric field shielding the inner part of the coma from the solar wind electric field. Conclusions. The same process gives rise to a tailward directed electric field with a strength of the order of 10% of the solar wind electric field. Using a simple cloud model we have shown that the polarisation electric field, which arises because of the small size of the comet ionosphere as compared to the pick up ion gyroradius, can explain the observed significant tailward acceleration of cometary ions and is consistent with the observed lack of influence of the solar wind electric field in the inner coma.
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| 17. |
- Vigren, Erik, et al.
(author)
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Effective ion speeds at similar to 200-250 km from comet 67P/Churyumov-Gerasimenko near perihelion
- 2017
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 469, s. S142-S148
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Journal article (peer-reviewed)abstract
- In 2015 August, comet 67P/Churyumov-Gerasimenko, the target comet of the ESA Rosetta mission, reached its perihelion at similar to 1.24 au. Here, we estimate for a three-day period near perihelion, effective ion speeds at distances similar to 200-250 km from the nucleus. We utilize two different methods combining measurements from the Rosetta Plasma Consortium (RPC)/Mutual Impedance Probe with measurements either from the RPC/Langmuir Probe or from the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA)/Comet Pressure Sensor (COPS) (the latter method can only be applied to estimate the effective ion drift speed). The obtained ion speeds, typically in the range 2-8 km s(-1), are markedly higher than the expected neutral outflow velocity of similar to 1 km s(-1). This indicates that the ions were de-coupled from the neutrals before reaching the spacecraft location and that they had undergone acceleration along electric fields, not necessarily limited to acceleration along ambipolar electric fields in the radial direction. For the limited time period studied, we see indications that at increasing distances from the nucleus, the fraction of the ions' kinetic energy associated with radial drift motion is decreasing.
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| 18. |
- Vigren, Erik, et al.
(author)
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Model-Observation Comparisons Of Electron Number Densities In The Coma Of 67P/Churyumov-Gerasimenko During 2015 January
- 2016
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In: Astronomical Journal. - : American Astronomical Society. - 0004-6256 .- 1538-3881. ; 152:3
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Journal article (peer-reviewed)abstract
- During 2015 January 9-11, at a heliocentric distance of similar to 2.58-2.57 au, the ESA Rosetta spacecraft resided at a cometocentric distance of similar to 28 km from the nucleus of comet 67P/Churyumov-Gerasimenko, sweeping the terminator at northern latitudes of 43 degrees N-58 degrees N. Measurements by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Comet Pressure Sensor (ROSINA/COPS) provided neutral number densities. We have computed modeled electron number densities using the neutral number densities as input into a Field Free Chemistry Free model, assuming H2O dominance and ion-electron pair formation by photoionization only. A good agreement (typically within 25%) is found between the modeled electron number densities and those observed from measurements by the Mutual Impedance Probe (RPC/MIP) and the Langmuir Probe (RPC/LAP), both being subsystems of the Rosetta Plasma Consortium. This indicates that ions along the nucleus-spacecraft line were strongly coupled to the neutrals, moving radially outward with about the same speed. Such a statement, we propose, can be further tested by observations of H3O+/H2O+ number density ratios and associated comparisons with model results.
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| 19. |
- Vigren, Erik, et al.
(author)
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The Evolution of the Electron Number Density in the Coma of Comet 67P at the Location of Rosetta from 2015 November through 2016 March
- 2019
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In: Astrophysical Journal. - : IOP PUBLISHING LTD. - 0004-637X .- 1538-4357. ; 881:1
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Journal article (peer-reviewed)abstract
- A comet ionospheric model assuming the plasma moves radially outward with the same bulk speed as the neutral gas and not being subject to severe reduction through dissociative recombination has previously been tested in a series of case studies associated with the Rosetta mission at comet 67P/Churyumov-Gerasimenko. It has been found that at low activity and within several tens of kilometers from the nucleus such models (which originally were developed for such conditions) generally work well in reproducing observed electron number densities, in particular when plasma production through both photoionization and electron-impact ionization is taken into account. Near perihelion, case studies have, on the contrary, shown that applying similar assumptions overestimates the observed electron number densities at the location of Rosetta. Here we compare Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Comet Pressure sensor-driven model results with Rosetta Plasma Consortium/Mutual Impedance Probe-derived electron number densities for an extended time period (2015 November through 2016 March) during the postperihelion phase with southern summer/spring. We observe a gradual transition from a state when the model grossly overestimates (by more than a factor of 10) the observations to being in reasonable agreement during 2016 March.
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| 20. |
- Volwerk, M., et al.
(author)
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Mass-loading, pile-up, and mirror-mode waves at comet 67P/Churyumov-Gerasimenko
- 2016
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In: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 34:1, s. 1-15
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Journal article (peer-reviewed)abstract
- The data from all Rosetta plasma consortium instruments and from the ROSINA COPS instrument are used to study the interaction of the solar wind with the outgassing cometary nucleus of 67P/Churyumov-Gerasimenko. During 6 and 7 June 2015, the interaction was first dominated by an increase in the solar wind dynamic pressure, caused by a higher solar wind ion density. This pressure compressed the draped magnetic field around the comet, and the increase in solar wind electrons enhanced the ionization of the outflow gas through collisional ionization. The new ions are picked up by the solar wind magnetic field, and create a ring/ring-beam distribution, which, in a high-beta plasma, is unstable for mirror mode wave generation. Two different kinds of mirror modes are observed: one of small size generated by locally ionized water and one of large size generated by ionization and pick-up farther away from the comet.
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