Rosetta Spacecraft Research Articles

Charging effects on Rosetta dust measurements

ABSTRACT Dust particles released from comet 67P/Churyumov-Gerasimenko collect electrostatic charges. Their motion is influenced by the electric fields induced by the flow of the solar wind and by the charging of the Rosetta spacecraft itself. Dust grains with sufficiently low tensile strength might even be destroyed en route from the nucleus to Rosetta. A simple model of the plasma environment is discussed here to enable simultaneously following the charging and the dynamics of dust particles as a function of the heliocentric distance of the comet, the distance between Rosetta and the nucleus, the asymmetry in gas production between the northern and southern hemispheres of the nucleus, the amplitude and timing of ultraviolet flares, and the possible outbursts intermittently increasing the production rate of the comet. The electrostatic disruption, and the combination of attractive and repulsive forces between the dust grains and Rosetta might significantly alter the conclusions about the size and spatial distributions of dust grains released from 67P/Churyumov-Gerasimenko. These calculations are presented to help assess the effects of dust and spacecraft charging in the analysis and interpretation of dust measurements by Rosetta.

Cometary ion drift energy and temperature at comet 67P/Churyumov–Gerasimeko

ABSTRACT The Ion Composition Analyzer (ICA) on the Rosetta spacecraft observed both the solar wind and the cometary ionosphere around comet 67P/Churyumov–Gerasimenko for nearly two years. However, observations of low energy cometary ions were affected by a highly negative spacecraft potential, and the ICA ion density estimates were often much lower than plasma densities found by other instruments. Since the low energy cometary ions are often the highest density population in the plasma environment, it is nonetheless desirable to understand their properties. To do so, we select ICA data with densities comparable to those of Rosetta’s Langmuir Probe (LAP)/Mutual Impedance Probe (MIP) throughout the mission. We then correct the cometary ion energy distribution of each energy-angle scan for spacecraft potential and fit a drifting Maxwell–Boltzmann distribution, which gives an estimate of the drift energy and temperature for 3521 scans. The resulting drift energy is generally between 11–18 eV and the temperature between 0.5–1 eV. The drift energy shows good agreement with published ion flow speeds from LAP/MIP during the same time period and is much higher than the cometary neutral speed. We see additional higher energy cometary ions in the spectra closest to perihelion that would be well described by a second Maxwellian-like distribution. The energy and temperature are negatively correlated with heliocentric distance, with a stronger dependence on heliocentric distance for temperature. It cannot be quantitatively determined whether this trend is primarily due to heliocentric distance or spacecraft distance to the comet, which increased with decreasing heliocentric distance.

Modeling the plasma composition of 67P/C-G at different heliocentric distances

The Rosetta spacecraft accompanied the comet 67P/C-G for nearly 2 years, collecting valuable data on the neutral and ion composition of the coma. The Rosetta Plasma Consortium (RPC) provided continuous measurements of the in situ plasma density while ROSINA-COPS monitored the neutral composition. In this work, we aim to estimate the composition of the cometary ionosphere at different heliocentric distances of the comet. Läuter et al. (2020) derived the temporal evolution of the volatile sublimation rates for 50 separated time intervals on the orbit of 67P/C-G using the COPS and DFMS data. We use these sublimation rates as inputs in a multifluid chemical-hydrodynamical model for 36 of the time intervals for heliocentric distances <3 au. We compare the total ion densities obtained from our models with the local plasma density measured by the RPC instruments. We find that at the location of the spacecraft, our modeled ion densities match with the in situ measured plasma density within factors of 1−3 for many of the time intervals. We obtain the cometocentric distance variation of the ions H2O+ and H3O+ and the ion groups created respectively by the ionization and protonation of neutral species. We see that H3O+ is dominant at the spacecraft location for nearly all the time intervals while ions created due to protonation are dominant at low cometocentric distances for the intervals near perihelion. We also discuss our ion densities in the context of their detection by DFMS.

Mass calibration of Rosetta’s ROSINA/DFMS mass spectrometer

The Double Focusing Mass Spectrometer (DFMS) onboard the Rosetta spacecraft employs an electrostatic and a magnet sector for energy and mass discrimination, resulting in a high mass resolution. A built-in feedback loop uses the measured magnet temperature to compensate for the temperature dependence of the magnet’s field strength. Still, large onboard temperature variations and other effects cause any given mass peak to move over a range of 30 pixels or more on the detector during the mission. The present paper discusses the various factors that contribute to the time variations in the mass calibration relation. A technique is developed to evaluate and correct for these factors. A mass calibration relation that is valid for the DFMS neutral high mass resolution mode measurements throughout the entire mission for the mass range m/z=13–69 is established and its accuracy is evaluated. The 1σ precision turns out to be less than a single pixel, which is excellent as full peak width at half height is about 12 pixels. The proposed approach provides an a posteriori mass calibration and is useful for all magnet-based mass spectrometers where experimental mass calibration by comparison to reference species, temperature stabilization, and/or electrostatic compensation, are not possible or fail to deliver a mass scale precision that is comparable to the mass resolution of the instrument.

Sublimation of volatiles from H2O:CO2 bulk ices in the context of comet 67P/Churyumov–Gerasimenko

Context. The ROSINA instrument on board the Rosetta spacecraft measured, among others, the outgassing of noble gases from comet 67P/Churyumov–Gerasimenko. The interpretation of this dataset and unravelling underlying desorption mechanisms requires detailed laboratory studies. Aims. We aim to improve our understanding of the desorption patterns, trapping, and fractionation of noble gases released from the H2O:CO2-dominated ice of comet 67P. Methods. In the laboratory, ice films of neon, argon, krypton, or xenon (Ne, Ar, Kr, and Xe) mixed in CO2:H2O were prepared at 15 K. Temperature-programmed desorption mass spectrometry is employed to analyse the desorption behaviour of the noble gases. Mass spectrometric ROSINA data of 67P were analysed to determine the fraction of argon associated with CO2 and H2O, respectively. Results. CO2 has a significant effect on noble gas desorption behaviour, resulting in the co-release of noble gases with CO2, decreasing the amount of noble gas trapped within water, shifting the pure phase noble gas peak desorption temperature to lower temperatures, and prolonging the trapping of neon. These effects are linked to competition for binding sites in the water ice and the formation of crystalline CO2. Desorption energies of the pure phase noble gas release were determined and found to be higher than those previously reported in the literature. Enhancement of the Ar/Kr and Ar/Xe ratios are at best 40% and not significantly influenced by the addition of CO2. Analysis of ROSINA mass spectrometric data shows that the fraction of argon associated with H2O is 0.53 ± 0.30, which cannot be explained by our laboratory results. Conclusions. Multicomponent ice mixtures affect the desorption behaviour of volatiles compared to simple binary mixtures and experiments on realistic cometary ice analogues are vital to understanding comet outgassing.

The response of the cometary ionosphere to space weather forcing

ABSTRACT The Rosetta spacecraft observed the temporal evolution of the ion populations within the ionosphere of comet 67P/Churyumov–Gerasimenko. A striking feature of the ion spectrum is represented by the so-called medium-energy ion peaks, which recurrently emerge from the low-energy ion background with their energy levels typically reaching 50 to 1000 eV before their energy gradually decreases, and they disappear from the measurements. These peaks are believed to be caused by space weather forcing, but there was no conclusive evidence until now. We investigated the characteristics of these ions, paying special attention to the connection between the solar wind dynamic pressure and the amount and energy of the medium-energy ions. Our findings reveal a strikingly accurate direct correlation between the dynamic pressure of the solar wind at the position of the comet and the amount of medium-energy ions measured by Rosetta. The ion energy also unquestionably reacts to the effects of solar wind pressure variation, but this parameter is strongly affected by the production rate and the distance from the nucleus as well. We explain these close correlations between cometary ion and solar wind characteristics using the well-founded assumption that certain boundary layers of the magnetosphere can move in and out under the influence of higher and lower solar wind pressure.

Ejection and dynamics of aggregates in the coma of comet 67P/Churyumov-Gerasimenko

The process of gas-driven ejection of refractory materials from cometary surfaces continues to pose a challenging question in cometary science. The activity modeling of comet 67P/Churyumov-Gerasimenko, based on data from the Rosetta mission, has significantly enhanced our comprehension of cometary activity. But thermophysical models have difficulties in simultaneously explaining the production rates of various gas species and dust. It has been suggested that different gas species might be responsible for the ejection of refractory material in distinct size ranges. This work focuses on investigating the abundance and the ejection mechanisms of large aggregates (gtrsim 1 cm) from the comet nucleus. We aim to determine their properties and map the distribution of their source regions across the comet surface. This can place constraints on activity models for comets. We examined 189 images acquired at five epochs by the OSIRIS/NAC instrument on board the Rosetta spacecraft. Our goal was to identify bright tracks produced by individual aggregates as they traversed the camera field of view. In parallel, we generated synthetic images based on the output of dynamical simulations involving various types of aggregates. By comparing these synthetic images with the observations, we determined the characteristics of the simulated aggregates that most closely resemble the observations. We have identified over 30000 tracks present in the OSIRIS images, derived constraints on the characteristics of the aggregates, and mapped their origins on the nucleus surface. The aggregates have an average radius of $ cm and a bulk density consistent with that of the comet's nucleus. Due to their size, gas drag exerts only a minor influence on their dynamical behavior, so an initial velocity is needed to bring them into the camera field of view. The source regions of these aggregates are predominantly located near the boundaries of distinct terrains on the surface.

Detection and characterisation of icy cavities on the nucleus of comet 67P/Churyumov-Gerasimenko

Abstract We report on the detection of three icy cavities on the nucleus of comet 67P/Churyumov-Gerasimenko. They were identified on high-resolution anaglyphs built from images acquired by the OSIRIS instrument aboard the Rosetta spacecraft on 2016 April 9-10. Visually, they appear as bright patches of typically 15 to 30 m across whose large reflectances and spectral slopes in the visible substantiate the presence of sub-surface water ice. Using a new high-resolution photogrammetric shape model, we determined the three-dimensional shape of these cavities whose depth ranges from 20 to 47 m. Spectral slopes were interpreted with models combining water ice and refractory dark material and the water ice abundances in the cavities were found to amount to a few per cent. The determination of the lifetime of the icy cavities was strongly biased by the availability of appropriate and favourable observations, but we found evidences of values of up to two years. The icy cavities were found to be connected to jets well documented in past studies. A thermal model allowed us to track their solar insolation over a large part of the orbit of the comet and a transitory bright jet on 2015 July 18 was unambiguously linked to the brief illumination of the icy bottom of one of the cavities. These cavities are likely to be the first potential subsurface access points detected on a cometary nucleus and their lifetimes suggest that they reveal pristine sub-surface icy layers or pockets rather than recently recondensed water vapor.

Upper limit of the solar wind protons backscattering efficiency from Comet 67P/Churyumov-Gerasimenko

Context. Solar wind ions backscattering is a fundamental plasma-surface interaction process that may occur on all celestial bodies exposed to the solar wind and lacking a significant atmosphere or magnetosphere. Yet, observations have been limited to the regolith-covered Moon and Phobos, one of the Martian moons. Aims. We aim to expand our knowledge of the process to include comets by investigating the backscattering of solar wind protons from the surface of comet 67P/Churyumov-Gerasimenko. Methods. We used one of the ion spectrometers on board ESA’s Rosetta spacecraft to search for evidence of backscattered solar wind protons from the cometary surface. The signal of interest was expected to be very weak and several statistical treatments of the data were essential to eliminate any influence from background noise and instrumental effects. Due to limited knowledge of the signal location within the observed parameter space, we conducted a statistical analysis to identify the most probable conditions for detecting the signal. Results. No significant solar wind backscattered protons were ever observed by the instrument. The statement applies to the large spectrum of observation conditions. An upper limit of the backscattered proton flux is given, as well as an upper limit of the backscattering efficiency of 9 × 10−4. Conclusions. The surface of comet 67P/Churyumov-Gerasimenko distinguishes itself as a notably weak reflector of solar wind protons, with its backscattering efficiency, at most, as large as the lowest observed backscattering efficiency from the lunar regolith.

Volatiles in the H2O and CO2 ices of comet 67P/Churyumov–Gerasimenko

ABSTRACT European Space Agency’s Rosetta spacecraft at comet 67P/Churyumov–Gerasimenko (67P) was the first mission that accompanied a comet over a substantial fraction of its orbit. On board was the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis mass spectrometer suite to measure the local densities of the volatile species sublimating from the ices inside the comet’s nucleus. Understanding the nature of these ices was a key goal of Rosetta. We analysed the primary cometary molecules at 67P, namely H2O and CO2, together with a suite of minor species for almost the entire mission. Our investigation reveals that the local abundances of highly volatile species, such as CH4 and CO, are reproduced by a linear combination of both H2O and CO2 densities. These findings bear similarities to laboratory-based temperature-programmed desorption experiments of amorphous ices and imply that highly volatile species are trapped in H2O and CO2 ices. Our results do not show the presence of ices dominated by these highly volatile molecules. Most likely, they were lost due to thermal processing of 67P’s interior prior to its deflection to the inner solar system. Deviations in the proportions co-released with H2O and CO2 can only be observed before the inbound equinox, when the comet was still far from the sun and the abundance of highly volatile molecules associated with CO2 outgassing were lower. The corresponding CO2 is likely seasonal frost, which sublimated and lost its trapped highly volatile species before re-freezing during the previous apparition. CO, on the other hand, was elevated during the same time and requires further investigation.

Oxygen-bearing organic molecules in comet 67P’s dusty coma: First evidence for abundant heterocycles

The puzzling complexity of terrestrial biomolecules is driving the search for complex organic molecules in the interstellar medium (ISM) and serves as a motivation for many in situ studies of reservoirs of extraterrestrial organics, from meteorites and interplanetary dust particles to comets and asteroids. Comet 67P/Churyumov-Gerasimenko (67P), the best-studied comet to date, has been visited and accompanied for 2 yr by the European Space Agency’s Rosetta spacecraft. Around 67P’s perihelion and under dusty conditions, the high-resolution mass spectrometer on board Rosetta has provided a spectacular glimpse into this comet’s chemical complexity. For this work, we analyzed the O-bearing organic volatiles in unprecedented detail. Through a comparison of 67P’s inventory with molecules detected in the ISM, in other comets, and in soluble organic matter extracted from the Murchison meteorite, we also highlight the (pre)biotic relevance of different chemical groups of species. We report first evidence for abundant extraterrestrial O-bearing heterocycles (with abundances relative to methanol often on the order of 10% and a relative error margin of 30–50%) and various representatives of other molecule classes, such as carboxylic acids and esters, aldehydes, ketones, and alcohols. As with the pure hydrocarbons, some hydrogenated forms seem to be dominant over their dehydrogenated counterparts. An interesting example is tetrahydrofuran, as it might be a more promising candidate for searches in the ISM than the long-sought furan. Our findings not only support and guide future efforts to investigate the origins of chemical complexity in space, but they also strongly encourage the study, in the laboratory as well as by modeling, of such topics as the ratios of unbranched versus branched species and hydrogenated versus dehydrogenated species in astrophysical ice analogs.

On the similarity of rotational motion of dust particles in the inner atmosphere of comets

ABSTRACT In situ measurements of individual dust grain parameters in the immediate vicinity of a cometary nucleus are being carried by the Rosetta spacecraft at comet 67P/Churyumov–Gerasimenko. For interpretation of these observational data, a model of dust grain motion as realistic as possible is requested. In particular, the results of Stardust mission and analysis of samples of interplanetary dust have shown that these particles are highly non-spherical. In many cases precise simulations of non-spherical grain’s dynamics is either impossible or computationally too expensive. In such situation it is proposed to use available experimental or numerical data obtained for other conditions and rescale them considering similarity of the physical process. In the present paper we focus on the derivation of scaling laws of rotational motion applicable for any shape of particles. We use a set of universal, dimensionless parameters characterizing the dust motion in the inner cometary coma. The scaling relations for translational and rotational motion of dust grains in a cometary environment are proposed. The scaled values are compared with numerically computed ones in our previous works.

Indirect Observations of Electric Fields at Comet 67P

AbstractNo spacecraft visiting a comet has been equipped with instruments to directly measure the static electric field. However, the electric field can occasionally be estimated indirectly by observing its effects on the ion velocity distribution. We present such observations made by the Rosetta spacecraft on 19 April 2016, 35 km from the nucleus. At this time comet 67P was at a low outgassing rate and the plasma environment was relatively stable. The ion velocity distributions show the cometary ions on the first half of their gyration. We estimate the bulk drift velocity and the gyration speed from the distributions. By using the local measured magnetic field and assuming an E × B drift of the gyrocentre, we get an estimate for the average electric field driving this ion motion. We analyze a period of 13 hr, during which the plasma environment does not change drastically. We find that the average strength of the perpendicular electric field component is 0.21 mV/m. The direction of the electric field is mostly anti‐sunward. This is in agreement with previous results based on different methods.

The Evolution of Activity and Chemical Composition in Rosetta’s Comet Targets across Multiple Apparitions: Complications for CS2 as the CS Parent in Comet Nuclei

Jupiter-family comets are ephemeral small bodies injected into the inner solar system from the Kuiper Belt, doomed to either sublimate all their volatiles and become inert or violently shatter from the activity. We investigate two target candidates of the ESA Rosetta mission, comets 46P/Wirtanen and 67P/Churyumov-Gerasimenko, which had favorable apparitions for Earth-based observations in 2018–2019 and 2021, respectively. Using the Hubble Space Telescope STIS and COS instruments, we observed OH and CS emissions to characterize production rates of H2O and CS, established Af ρ values, and placed upper limits on the production rate of C2 and its parent. We find CS/H2O relative abundances that are significantly (5σ–7σ) larger than previous remote near-UV (NUV) measurements of 46P and 67P at similar heliocentric distances and CS/H2O values larger than those obtained via contemporaneous submillimeter observations for the same apparitions. We also find that for 67P the remote derivations of CS2/H2O ratios are substantially (∼50×) higher than the values measured by the ROSINA mass spectrometer on board the Rosetta spacecraft for all NUV-derived CS2 production rates. The discrepancy points toward an unidentified CS parent or parents with contributing factors from uncertainties with the fluorescence efficiencies of the CS (0,0) band of the A 1Π–X 1Σ+ system around 2580 Å. Given the significance of understanding the chemistry and dissociation physics of sulfur-bearing molecules in comets for tracing planetesimal formation environments, as well as the limited studies in this area, we propose several hypotheses to explain this discrepancy and outline future studies to address these issues.

Solar Wind Protons Forming Partial Ring Distributions at Comet 67P

AbstractWe present partial ring distributions of solar wind protons observed by the Rosetta spacecraft at comet 67P/Churyumov‐Gerasimenko. The formation of ring distributions is usually associated with high activity comets, where the spatial scales are larger than multiple ion gyroradii. Our observations are made at a low‐activity comet at a heliocentric distance of 2.8 AU on 19 April 2016, and the partial rings occur at a spatial scale comparable to the ion gyroradius. We use a new visualization method to simultaneously show the angular distribution of median energy and differential flux. A fitting procedure extracts the bulk speed of the solar wind protons, separated into components parallel and perpendicular to the gyration plane, as well as the gyration velocity. The results are compared with models and put into context of the global comet environment. We find that the formation mechanism of these partial rings of solar wind protons is entirely different from the well‐known partial rings of cometary pickup ions at high‐activity comets. A density enhancement layer of solar wind protons around the comet is a focal point for proton trajectories originating from different regions of the upstream solar wind. If the spacecraft location coincides with this density enhancement layer, the different trajectories are observed as an energy‐angle dispersion and manifest as partial rings in velocity space.

Gas Dynamic Models of the Interaction between the Solar Wind

Gas dynamic models of the interaction between the solar wind and cometary atmospheresare considered. Interest in the development of such models arose sharply after the beginning of the investigations of comets with the use of spacecraft launched at distances close to their surfaces. The instruments on this spacecraft gave the possibility to investigate experimentally the parameters of gas flow out from the cometary surfaces when the comets approach the Sun and its interaction with the solar wind plasma flow, which could not be made using only the spectral photometry. The beginning of such studies was started by almost instantaneous approaching of several space probes to Halley’s Comet on March 1986. Only after 28 years, the Rosetta spacecraft launched by the European Space Agency (ESA) along a complex trajectory have approached comet Churyumov–Gerasimenko and, maneuvering in the neighborhood of this comet during more than two years, it, in particular, have investigated the interaction of the cometary atmosphere and the solar wind

Observations of a Solar Energetic Particle Event From Inside and Outside the Coma of Comet 67P.

We analyze observations of a solar energetic particle (SEP) event at Rosetta's target comet 67P/Churyumov-Gerasimenko during 6-10 March 2015. The comet was 2.15AU from the Sun, with the Rosetta spacecraft approximately 70km from the nucleus placing it deep inside the comet's coma and allowing us to study its response. The Eastern flank of an interplanetary coronal mass ejection (ICME) also encountered Rosetta on 6 and 7 March. Rosetta Plasma Consortium data indicate increases in ionization rates, and cometary water group pickup ions exceeding 1keV. Increased charge exchange reactions between solar wind ions and cometary neutrals also indicate increased upstream neutral populations consistent with enhanced SEP induced surface activity. In addition, the most intense parts of the event coincide with observations interpreted as an infant cometary bow shock, indicating that the SEPs may have enhanced the formation and/or intensified the observations. These solar transient events may also have pushed the cometopause closer to the nucleus. We track and discuss characteristics of the SEP event using remote observations by SOHO, WIND, and GOES at the Sun, in situ measurements at Solar Terrestrial Relations Observatory Ahead, Mars and Rosetta, and ENLIL modeling. Based on its relatively prolonged duration, gradual and anisotropic nature, and broad angular spread in the heliosphere, we determine the main particle acceleration source to be a distant ICME which emerged from the Sun on 6 March 2015 and was detected locally in the Martian ionosphere but was never encountered by 67P directly. The ICME's shock produced SEPs for several days which traveled to the in situ observation sites via magnetic field line connections.

First investigation of the diamagnetic cavity boundary layer with a 1D3V PIC simulation

Context. Amongst the different features and boundaries encountered around comets, one remains of particular interest to the plasma community: the diamagnetic cavity. Crossed for the first time at 1P/Halley during the Giotto flyby in 1986 and later met more than 700 times by the ESA Rosetta spacecraft around Comet 67P/Churyumov-Gerasimenko, this region, almost free of any magnetic field, surrounds nuclei of active comets. However, previous observations and modelling of this part of the coma have not yet provided a definitive answer as to the origin of such a cavity and on its border, the diamagnetic cavity boundary layer. Aims. We investigate which forces and equilibrium might be at play and balance the magnetic pressure at this boundary down to the spatial and temporal scales of the electrons in the 1D collisionless case. In addition, we scrutinise assumptions made in magneto-hydrodynamic and hybrid simulations of this environment and check for their validity. Methods. We simulated this region at the electron scale by means of 1D3V particle-in-cell simulations and SMILEI code. Results. Across this layer, depending on the magnetic field strength, the electric field is governed by different equilibria, with a thin double-layer forming ahead. In addition, we show that the electron distribution function departs from Maxwellian and/or gyrotropic distributions and that electrons do not behave adiabatically. We demonstrate the need to investigate this region at the electron scale in depth with fully kinetic simulations.

Radial distribution of plasma at comet 67P

Context. The Rosetta spacecraft followed comet 67P/Churyumov-Gerasimenko (67P) for more than two years at a slow walking pace (~1 m s−1) within 1500 km from the nucleus. During one of the radial movements of the spacecraft in the early phase of the mission, the radial distribution of the plasma density could be estimated, and the ionospheric density was found to be inversely proportional to the cometocentric distance r from the nucleus (a 1/r distribution). Aims. This study aims to further characterise the radial distribution of plasma around 67P throughout the mission and to expand on the initial results. We also aim to investigate how a 1/r distribution would be observed during aflyby with a fast (~10’s km s−1) spacecraft, such as the upcoming Comet Interceptor mission, when there is also an asymmetry introduced to the outgassing over the comet surface. Methods. To determine the radial distribution of the plasma, we used data from the Langmuir probe and Mutual Impedance instruments from the Rosetta Plasma Consortium during six intervals throughout the mission, for which the motion of Rosetta was approximately radial with respect to the comet. We then simulated what distribution a fast flyby mission would actually observe during its passage through a coma when there is a 1/r plasma density distribution as well as a sinusoidal variation with a phase angle (and then a sawtooth variation) multiplied to the outgassing rate. Results. The plasma density around comet 67P is found to roughly follow a 1/r dependence, although significant deviations occur in some intervals. If we normalise all data to a common outgassing rate (or heliocentric distance) and combine the intervals to a radial range of 10–1500 km, we find a 1/r1.19 average distribution. The simulated observed density from a fast spacecraft flying through a coma with a 1/r distribution and an asymmetric outgassing can, in fact, appear anywhere in the range from a 1/r distribution to a 1/r2 distribution, or even slightly outside of this range. Conclusions. The plasma density is distributed in such a way that it approximately decreases in a manner that is inversely proportional to the cometocentric distance. This is to be expected from the photoionisation of a collision-less, expanding neutral gas at a constant ionisation rate and expansion speed. The deviation from a pure 1/r distribution is in many cases caused by asymmetric outgassing over the surface, additional ionisation sources being present, electric fields accelerating plasma, and changing upstream solar wind conditions. A fast flyby mission can observe a radial distribution that deviates significantly from a 1/r trend if the outgassing is not symmetric over the surface. The altitude profile that will be observed depends very much on the level of outgassing asymmetry, the flyby velocity, the comet rotation rate, and the rotation phase. It is therefore essential to include data from both the inbound and outbound legs, as well as to compare plasma density to neutral density to get a more complete understanding of the radial distribution of the plasma.

Outbursts of Comet 67P/Churyumov-Gerasimenko

Abstract We monitored the comet 67P/Churyumov-Gerasimenko close to its perihelion in November 2021 with the GROWTH-India Telescope. We observed two outbursts of this comet on 2021 October 29.940 and November 17.864 UTC, −3.12 days and +15.81 days respectively from the perihelion date. The brightening in the first outburst appears as a compact source, with a radial extent up to 8.″5. The comet brightened by 0.26 ± 0.03 mag in the outburst, with a 27% increase in the effective geometric cross-section and total outburst dust mass of ∼5.3 × 105 kg. The second outburst caused a brightening of 0.49 ± 0.08 mag with effective geometric cross-section and total outburst dust mass 2.5 times larger than the first event. These outbursts are up to an order of magnitude larger than the strongest outburst event observed in situ by the Rosetta spacecraft in 2015.