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  • 1. Behar, E.
    et al.
    Tabone, B.
    Saillenfest, M.
    Henri, P.
    Deca, J.
    Lindkvist, Jesper
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Holmstrom, M.
    Nilsson, H.
    Solar wind dynamics around a comet: A 2D semi-analytical kinetic model2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 620, artikel-id A35Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aims. We aim at analytically modelling the solar wind proton trajectories during their interaction with a partially ionised cometary atmosphere, not in terms of bulk properties of the flow but in terms of single particle dynamics.

    Methods. We first derive a generalised gyromotion, in which the electric field is reduced to its motional component. Steady-state is assumed, and simplified models of the cometary density and of the electron fluid are used to express the force experienced by individual solar wind protons during the interaction.

    Results. A three-dimensional (3D) analytical expression of the gyration of two interacting plasma beams is obtained. Applying it to a comet case, the force on protons is always perpendicular to their velocity and has an amplitude proportional to 1/r2. The solar wind deflection is obtained at any point in space. The resulting picture presents a caustic of intersecting trajectories, and a circular region is found that is completely free of particles. The particles do not lose any kinetic energy and this absence of deceleration, together with the solar wind deflection pattern and the presence of a solar wind ion cavity, is in good agreement with the general results of the Rosetta mission.

    Conclusions. The qualitative match between the model and the in situ data highlights how dominant the motional electric field is throughout most of the interaction region for the solar wind proton dynamics. The model provides a simple general kinetic description of how momentum is transferred between these two collisionless plasmas. It also shows the potential of this semi-analytical model for a systematic quantitative comparison to the data.

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  • 2.
    Behar, Etienne
    et al.
    Swedish Institute of Space Physics, Kiruna.
    Lindkvist, Jesper
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna.
    Nilsson, Hans
    Swedish Institute of Space Physics, Kiruna.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna.
    Stenberg-Wieser, Gabriella
    Swedish Institute of Space Physics, Kiruna.
    Ramstad, Robin
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna.
    Götz, Charlotte
    Technicsche Universität Braunschweig, Institute for Geophysics an Extraterrestrial Physics, Braunschweig.
    Mass-loading of the solar wind at 67P/Churyumov-Gerasimenko: Observations and modelling2016Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 596, artikel-id A42Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. The first long-term in-situ observation of the plasma environment in the vicinity of a comet, as provided by the European Rosetta spacecraft.

    Aims. Here we offer characterisation of the solar wind flow near 67P/Churyumov-Gerasimenko (67P) and its long term evolution during low nucleus activity. We also aim to quantify and interpret the deflection and deceleration of the flow expected from ionization of neutral cometary particles within the undisturbed solar wind.

    Methods. We have analysed in situ ion and magnetic field data and combined this with hybrid modeling of the interaction between the solar wind and the comet atmosphere.

    Results. The solar wind deflection is increasing with decreasing heliocentric distances, and exhibits very little deceleration. This is seen both in observations and in modeled solar wind protons. According to our model, energy and momentum are transferred from the solar wind to the coma in a single region, centered on the nucleus, with a size in the order of 1000 km. This interaction affects, over larger scales, the downstream modeled solar wind flow. The energy gained by the cometary ions is a small fraction of the energy available in the solar wind.

    Conclusions. The deflection of the solar wind is the strongest and clearest signature of the mass-loading for a small, low-activity comet, whereas there is little deceleration of the solar wind. 

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  • 3.
    Benseguane, Selma
    et al.
    LGL-TPE, Cnrs, Université Lyon, Ucbl, Ensl, Villeurbanne, France.
    Guilbert-Lepoutre, Aurélie
    LGL-TPE, Cnrs, Université Lyon, Ucbl, Ensl, Villeurbanne, France.
    Lasue, Jérémie
    Irap, Université de Toulouse, Cnrs, Cnes, Ups, Toulouse, France.
    Besse, Sébastien
    Aurora Technology B.V. for the European Space Agency, Esac, Villanueva de la Canada, Madrid, Spain.
    Leyrat, Cédric
    Lesia, Observatoire de Paris, Cnrs, Sorbonne Univ., Univ. Paris-Diderot, Meudon, France.
    Beth, Arnaud
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Costa Sitjà, Marc
    Rhea System for the European Space Agency, Esac, Villanueva de la Canada, Madrid, Spain.
    Grieger, Björn
    Aurora Technology B.V. for the European Space Agency, Esac, Villanueva de la Canada, Madrid, Spain.
    Capria, Maria Teresa
    Istituto di Astrofísica e Planetologia Spaziali (IAPS), Inaf, Roma, Italy.
    Evolution of pits at the surface of 67P/Churyumov-Gerasimenko2022Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 668, artikel-id A132Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. The observation of pits at the surface of comets offers the opportunity to take a glimpse into the properties and the mechanisms that shape a nucleus through cometary activity. If the origin of these pits is still a matter of debate, multiple studies have recently suggested that known phase transitions (such as volatile sublimation or amorphous water ice crystallization) alone could not have carved these morphological features on the surface of 67P/Churyumov-Gerasimenko (hereafter 67P).

    Aims. We want to understand how the progressive modification of 67P' s surface due to cometary activity might have affected the characteristics of pits and alcoves. In particular, we aim to understand whether signatures of the formation mechanism of these surface morphological features can still be identified.

    Methods. To quantify the amount of erosion sustained at the surface of 67P since it arrived on its currently observed orbit, we selected 380 facets of a medium-resolution shape model of the nucleus, sampling 30 pits and alcoves across the surface. We computed the surface energy balance with a high temporal resolution, including shadowing and self-heating contributions. We then applied a thermal evolution model to assess the amount of erosion sustained after ten orbital revolutions under current illumination conditions.

    Results. We find that the maximum erosion sustained after ten orbital revolutions is on the order of 80 m, for facets located in the southern hemisphere. We thus confirm that progressive erosion cannot form pits and alcoves, as local erosion is much lower than their observed depth and diameter. We find that plateaus tend to erode more than bottoms, especially for the deepest depressions, and that some differential erosion can affect their morphology. As a general rule, our results suggest that sharp morphological features tend to be erased by progressive erosion.

    Conclusions. This study supports the assumption that deep circular pits, such as Seth_01, are the least processed morphological features at the surface of 67P, or the best preserved since their formation.

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  • 4.
    Beth, Arnaud
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Department of Physics, Imperial College London, London, UK.
    Altwegg, K.
    Balsiger, H.
    Berthelier, J. -J.
    Combi, M. R.
    De Keyser, J.
    Fiethe, B.
    Fuselier, S. A.
    Galand, M.
    Gombosi, T. I.
    Rubin, M.
    Semon, T.
    ROSINA ion zoo at Comet 67P2020Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 642, artikel-id A27Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context: The Rosetta spacecraft escorted Comet 67P/Churyumov-Gerasimenko for 2 yr along its journey through the Solar System between 3.8 and 1.24 au. Thanks to the high resolution mass spectrometer on board Rosetta, the detailed ion composition within a coma has been accurately assessed in situ for the very first time.

    Aims: Previous cometary missions, such as Giotto, did not have the instrumental capabilities to identify the exact nature of the plasma in a coma because the mass resolution of the spectrometers onboard was too low to separate ion species with similar masses. In contrast, the Double Focusing Mass Spectrometer (DFMS), part of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis on board Rosetta (ROSINA), with its high mass resolution mode, outperformed all of them, revealing the diversity of cometary ions.

    Methods: We calibrated and analysed the set of spectra acquired by DFMS in ion mode from October 2014 to April 2016. In particular, we focused on the range from 13–39 u q−1. The high mass resolution of DFMS allows for accurate identifications of ions with quasi-similar masses, separating 13C+ from CH+, for instance.

    Results: We confirm the presence in situ of predicted cations at comets, such as CHm+ (m = 1−4), HnO+ (n = 1−3), O+, Na+, and several ionised and protonated molecules. Prior to Rosetta, only a fraction of them had been confirmed from Earth-based observations. In addition, we report for the first time the unambiguous presence of a molecular dication in the gas envelope of a Solar System body, namely CO2++.

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  • 5.
    Beth, Arnaud
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Simon Wedlund, C.
    Goetz, C.
    Nilsson, H.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    First investigation of the diamagnetic cavity boundary layer with a 1D3V PIC simulation2022Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 667, artikel-id A143Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 6.
    Canu Blot, Romain
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Wieser, Martin
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Wieser, Gabriella Stenberg
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Upper limit of the solar wind protons backscattering efficiency from Comet 67P/Churyumov-Gerasimenko2024Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 683, artikel-id A245Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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 A 104.

    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.

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  • 7.
    Edberg, N.J.T.
    et al.
    Swedish Institute of Space Physics (IRF), Uppsala, Sweden.
    Eriksson, A.I.
    Swedish Institute of Space Physics (IRF), Uppsala, Sweden.
    Vigren, E.
    Swedish Institute of Space Physics (IRF), Uppsala, Sweden.
    Nilsson, H.
    Swedish Institute of Space Physics (IRF), Kiruna, Sweden.
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Götz, C.
    Department of Mathematics, Physics and Electrical Engineering, Northumbria University, Newcastle-upon-Tyne, United Kingdom.
    Richter, I.
    Institut fur Geophysik und Extraterrestrische Physik, Technische Universität Braunschweig, Braunschweig, Germany.
    Henri, P.
    Laboratoire de Physique et Chimie de l'Environnement et de l'Espace, Cnrs, Orléans, France; Laboratoire Lagrange, Oca, Cnrs, Uca, Nice, France.
    De Keyser, J.
    Royal Belgian Institute for Space Aeronomy, BIRA-IASB, Brussels, Belgium.
    Scale size of cometary bow shocks2024Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 682, artikel-id A51Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. In past decades, several spacecraft have visited comets to investigate their plasma environments. In the coming years, Comet Interceptor will make yet another attempt. This time, the target comet and its outgassing activity are unknown and may not be known before the spacecraft has been launched into its parking orbit, where it will await a possible interception. If the approximate outgassing rate can be estimated remotely when a target has been identified, it is desirable to also be able to estimate the scale size of the plasma environment, defined here as the region bound by the bow shock.

    Aims. This study aims to combine previous measurements and simulations of cometary bow shock locations to gain a better understanding of how the scale size of cometary plasma environments varies. We compare these data with models of the bow shock size, and we furthermore provide an outgassing rate-dependent shape model of the bow shock. We then use this to predict a range of times and cometocentric distances for the crossing of the bow shock by Comet Interceptor, together with expected plasma density measurements along the spacecraft track.

    Methods. We used data of the location of cometary bow shocks from previous spacecraft missions, together with simulation results from previously published studies. We compared these results with an existing model of the bow shock stand-off distance and expand on this to provide a shape model of cometary bow shocks. The model in particular includes the cometary outgassing rate, but also upstream solar wind conditions, ionisation rates, and the neutral flow velocity.

    Results. The agreement between the gas-dynamic model and the data and simulation results is good in terms of the stand-off distance of the bow shock as a function of the outgassing rate. For outgassing rates in the range of 1027-1031-s-1, the scale size of cometary bow shocks can vary by four orders of magnitude, from about 102 km to 106 km, for an ionisation rate, flow velocity, and upstream solar wind conditions typical of those at 1 AU. The proposed bow shock shape model shows that a comet plasma environment can range in scale size from the plasma environment of Mars to about half of that of Saturn.

    Conclusions. The model-data agreement allows for the planning of upcoming spacecraft comet encounters, such as that of Comet Interceptor, when a target has been identified and its outgassing rate is determined. We conclude that the time a spacecraft can spend within the plasma environment during a flyby can range from minutes to days, depending on the comet that is visited and on the flyby speed. However, to capture most of the comet plasma environment, including pick-up ions and upstream plasma waves, and to ensure the highest possible scientific return, measurements should still start well upstream of the expected bow shock location. From the plasma perspective, the selected target should preferably be an active comet with the lowest possible flyby velocity.

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  • 8.
    Fatemi, Shahab
    et al.
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Poirier, Nicolas
    École nationale supérieure de mécanique et d’aérotechnique, Chasseneuil-du-Poitou, France .
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Lindkvist, Jesper
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Wieser, Martin
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Barabash, Stas
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    A modelling approach to infer the solar wind dynamic pressure from magnetic field observations inside Mercury's magnetosphere2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 614, artikel-id A132Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aims: The lack of an upstream solar wind plasma monitor when a spacecraft is inside the highly dynamic magnetosphere of Mercury limits interpretations of observed magnetospheric phenomena and their correlations with upstream solar wind variations.

    Methods: We used AMITIS, a three-dimensional GPU-based hybrid model of plasma (particle ions and fluid electrons) to infer the solar wind dynamic pressure and Alfvén Mach number upstream of Mercury by comparing our simulation results with MESSENGER magnetic field observations inside the magnetosphere of Mercury. We selected a few orbits of MESSENGER that have been analysed and compared with hybrid simulations before. Then we ran a number of simulations for each orbit (~30–50 runs) and examined the effects of the upstream solar wind plasma variations on the magnetic fields observed along the trajectory of MESSENGER to find the best agreement between our simulations and observations.

    Results: We show that, on average, the solar wind dynamic pressure for the selected orbits is slightly lower than the typical estimated dynamic pressure near the orbit of Mercury. However, we show that there is a good agreement between our hybrid simulation results and MESSENGER observations for our estimated solar wind parameters. We also compare the solar wind dynamic pressure inferred from our model with those predicted previously by the WSA-ENLIL model upstream of Mercury, and discuss the agreements and disagreements between the two model predictions. We show that the magnetosphere of Mercury is highly dynamic and controlled by the solar wind plasma and interplanetary magnetic field. In addition, in agreement with previous observations, our simulations show that there are quasi-trapped particles and a partial ring current-like structure in the nightside magnetosphere of Mercury, more evident during a northward interplanetary magnetic field (IMF). We also use our simulations to examine the correlation between the solar wind dynamic pressure and stand-off distance of the magnetopause and compare it with MESSENGER observations. We show that our model results are in good agreement with the response of the magnetopause to the solar wind dynamic pressure, even during extreme solar events. We also show that our model can be used as a virtual solar wind monitor near the orbit of Mercury and this has important implications for interpretation of observations by MESSENGER and the future ESA/JAXA mission to Mercury, BepiColombo.

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  • 9. Gunell, H.
    et al.
    Nilsson, H.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Eriksson, A.
    Odelstad, E.
    Maggiolo, R.
    Henri, P.
    Vallieres, X.
    Altwegg, K.
    Tzou, C. -Y
    Rubin, M.
    Glassmeier, K. -H
    Wieser, G. Stenberg
    Wedlund, C. Simon
    De Keyser, J.
    Dhooghe, F.
    Cessateur, G.
    Gibbons, A.
    Ion acoustic waves at comet 67P/Churyumov-Gerasimenko: Observations and computations2017Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 600, artikel-id A3Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 10.
    Gunell, Herbert
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Goetz, Charlotte
    Department of Mathematics, Physics and Electrical Engineering, Northumbria University, Newcastle upon Tyne, United Kingdom; Space Research and Technology Centre, European Space Agency, Noordwijk, Netherlands.
    Particle-in-cell modelling of comet 67P/Churyumov-Gerasimenko: spatial structures of densities and electric fields2023Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 674, artikel-id A65Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context: Sufficiently far from the Sun, all comets go through a phase of low activity. Rosetta observations at large heliocentric distances of approximately 3 au showed that the plasma at a low-activity comet is affected by both steady state electric fields and low-frequency waves.

    Aims: Our goal is to provide a model for the electric fields in the inner coma at a low-activity comet and to simulate waves and field structures farther away from the nucleus.

    Methods: We compare analytical models for the convective, ambipolar, and polarisation electric fields to the results of an electrostatic particle-in-cell simulation of a scaled-down low-activity comet.

    Results: We find good agreement between the steady state field model and the simulation results close to the nucleus. At larger cometocentric distances, waves dominate the electric field. These waves are interpreted as the scaled-down electrostatic limit of the previously observed singing comet waves. The comet ion density is not spherically symmetric.

    Conclusions: Low-activity comets can be modelled using electrostatic particle-in-cell simulations of a scaled-down system. Outside the innermost part of the coma (r ≥ 40 km), the plasma is not spherically symmetric and the electric field is dominated by waves.

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  • 11.
    Gunell, Herbert
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Goetz, Charlotte
    Department of Mathematics, Physics and Electrical Engineering, Northumbria University, Newcastle-upon-Tyne, United Kingdom.
    Fatemi, Shahab
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Impact of radial interplanetary magnetic fields on the inner coma of comet 67P/Churyumov-Gerasimenko: Hybrid simulations of the plasma environment2024Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 682, artikel-id A62Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. The direction of the interplanetary magnetic field determines the nature of the interaction between a Solar System object and the solar wind. For comets, it affects the formation of both a bow shock and other plasma boundaries, as well as mass-loading. Around the nucleus of a comet, there is a diamagnetic cavity, where the magnetic field is negligible. Observations by the Rosetta spacecraft have shown that, most of the time, the diamagnetic cavity is located within a solar-wind ion cavity, which is devoid of solar wind ions. However, solar wind ions have been observed inside the diamagnetic cavity on several occasions. Understanding what determines whether or not the solar wind can reach the diamagnetic cavity also advances our understanding of cometsolar wind interaction in general.

    Aims. We aim to determine the influence of an interplanetary magnetic field directed radially out from the Sun that is, parallel to the solar wind velocity on the cometsolar wind interaction. In particular, we explore the possibility of solar wind protons entering the diamagnetic cavity under radial field conditions.

    Methods. We performed global hybrid simulations of comet 67P/Churyumov-Gerasimenko using the simulation code Amitis for two different interplanetary magnetic field configurations and compared the results to observations made by the Rosetta spacecraft.

    Results. We find that, when the magnetic field is parallel to the solar wind velocity, no bow shock forms and the solar wind ions are able to enter the diamagnetic cavity. A solar wind ion wake still forms further downstream in this case.

    Conclusions. The solar wind can enter the diamagnetic cavity if the interplanetary magnetic field is directed radially from the Sun, and this is in agreement with observations made by instruments on board the Rosetta spacecraft.

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  • 12.
    Gunell, Herbert
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Avenue Circulaire 3, 1180 Brussels, Belgium.
    Goetz, Charlotte
    Wedlund, Cyril Simon
    Lindkvist, Jesper
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Hamrin, Maria
    Nilsson, Hans
    LLera, Kristie
    Eriksson, Anders
    Holmström, Mats
    The infant bow shock: a new frontier at a weak activity comet2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 619, artikel-id L2Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The bow shock is the first boundary the solar wind encounters as it approaches planets or comets. The Rosetta spacecraft was able to observe the formation of a bow shock by following comet 67P/Churyumov-Gerasimenko toward the Sun, through perihelion, and back outward again. The spacecraft crossed the newly formed bow shock several times during two periods a few months before and after perihelion; it observed an increase in magnetic field magnitude and oscillation amplitude, electron and proton heating at the shock, and the diminution of the solar wind further downstream. Rosetta observed a cometary bow shock in its infancy, a stage in its development not previously accessible to in situ measurements at comets and planets.

  • 13.
    Gunell, Herbert
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Avenue Circulaire 3, 1180 Brussels, Belgium .
    Lindkvist, Jesper
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Goetz, Charlotte
    Institut für Geophysik und extraterrestrische Physik, TU Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany.
    Nilsson, Hans
    Swedish Institute of Space Physics, Box 812, 981 28 Kiruna, Sweden .
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Polarisation of a small-scale cometary plasma environment: Particle-in-cell modelling of comet 67P/Churyumov-Gerasimenko2019Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 631, artikel-id A174Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context: The plasma near the nucleus of a comet is subjected to an electric field to which a few different sources contribute: the convective electric field of the solar wind, the ambipolar electric field due to higher electron than ion speeds, and a polarisation field arising from the vastly different ion and electron trajectories.

    Aims: Our aim is to show how the ambipolar and polarisation electric fields arise and develop under the influence of space charge effects, and in doing so we paint a qualitative picture of the electric fields in the inner coma of a comet.

    Methods. We use an electrostatic particle-in-cell model to simulate a scaled-down comet, representing comet 67P/Churyumov-Gerasimenko with parameters corresponding to a 3.0 AU heliocentric distance.

    Results: We find that an ambipolar electric field develops early in the simulation and that this is soon followed by the emergence of a polarisation electric field, manifesting itself as an anti-sunward component prevalent in the region surrounding the centre of the comet. As plasma is removed from the inner coma in the direction of the convectional electric field of the solar wind, a density maximum develops on the opposite side of the centre of the comet.

    Conclusions: The ambipolar and polarisation electric fields both have a significant influence on the motion of cometary ions. This demonstrates the importance of space charge effects in comet plasma physics.

  • 14.
    Gunell, Herbert
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Belgium.
    Maggiolo, Romain
    Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Belgium.
    Nilsson, Hans
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Stenberg Wieser, Gabriella
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Slapak, Rikard
    EISCAT Scientific Association, Kiruna, Sweden.
    Lindkvist, Jesper
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    De Keyser, Johan
    Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Belgium.
    Why an intrinsic magnetic field does not protect a planet against atmospheric escape2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 614, artikel-id L3Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The presence or absence of a magnetic field determines the nature of how a planet interacts with the solar wind and what paths are available for atmospheric escape. Magnetospheres form both around magnetised planets, such as Earth, and unmagnetised planets, like Mars and Venus, but it has been suggested that magnetised planets are better protected against atmospheric loss. However, the observed mass escape rates from these three planets are similar (in the approximate (0.5–2) kg s−1 range), putting this latter hypothesis into question. Modelling the effects of a planetary magnetic field on the major atmospheric escape processes, we show that the escape rate can be higher for magnetised planets over a wide range of magnetisations due to escape of ions through the polar caps and cusps. Therefore, contrary to what has previously been believed, magnetisation is not a sufficient condition for protecting a planet from atmospheric loss. Estimates of the atmospheric escape rates from exoplanets must therefore address all escape processes and their dependence on the planet’s magnetisation.

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  • 15.
    Johansson, F.L.
    et al.
    Swedish Institute of Space Physics, Uppsala, Sweden; Uppsala University, Department of Astronomy and Space Physics, Uppsala, Sweden.
    Eriksson, A.I.
    Swedish Institute of Space Physics, Uppsala, Sweden.
    Vigren, E.
    Swedish Institute of Space Physics, Uppsala, Sweden.
    Bucciantini, L.
    Laboratoire de Physique et Chimie de l'Environnement et de l'Espace, Cnrs, Orléans, France.
    Henri, P.
    Laboratoire de Physique et Chimie de l'Environnement et de l'Espace, Cnrs, Orléans, France; Laboratoire Lagrange, Oca, Cnrs, Uca, Nice, France.
    Nilsson, H.
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Bergman, Sofia
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Edberg, N.J.T.
    Swedish Institute of Space Physics, Uppsala, Sweden.
    Stenberg Wieser, G.
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Odelstad, E.
    Swedish Institute of Space Physics, Uppsala, Sweden.
    Plasma densities, flow, and solar EUV flux at comet 67P: A cross-calibration approach2021Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 653, artikel-id A128Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. During its two-year mission at comet 67P, Rosetta nearly continuously monitored the inner coma plasma environment for gas production rates varying over three orders of magnitude, at distances to the nucleus ranging from a few to a few hundred kilometres. To achieve the best possible measurements, cross-calibration of the plasma instruments is needed. Aims. Our goal is to provide a consistent plasma density dataset for the full mission, while in the process providing a statistical characterisation of the plasma in the inner coma and its evolution.

    Methods. We constructed physical models for two different methods to cross-calibrate the spacecraft potential and the ion current as measured by the Rosetta Langmuir probes (LAP) to the electron density as measured by the Mutual Impedance Probe (MIP). We also described the methods used to estimate spacecraft potential, and validated the results with the Ion Composition Analyser (ICA).

    Results. We retrieve a continuous plasma density dataset for the entire cometary mission with a much improved dynamical range compared to any plasma instrument alone and, at times, improve the temporal resolution from 0.24-0.74 Hz to 57.8 Hz. The physical model also yields, at a three-hour time resolution, ion flow speeds and a proxy for the solar EUV flux from the photoemission from the Langmuir probes.

    Conclusions. We report on two independent mission-wide estimates of the ion flow speed that are consistent with the bulk H2O+ ion velocities as measured by the ICA. We find the ion flow to consistently be much faster than the neutral gas over the entire mission, lending further evidence that the ions are collisionally decoupled from the neutrals in the coma. Measurements of ion speeds from Rosetta are therefore not consistent with the assumptions made in previously published plasma density models of the comet 67P's ionosphere at the start and end of the mission. Also, the measured EUV flux is perfectly consistent with independently derived values previously published from LAP and lends support for the conclusions drawn regarding an attenuation of solar EUV from a distant nanograin dust population, when the comet activity was high. The new density dataset is consistent with the existing MIP density dataset, but it facilitates plasma analysis on much shorter timescales, and it also covers long time periods where densities were too low to be measured by MIP.

  • 16. Kun, E.
    et al.
    Keresztes, Z.
    Simkó, A.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Szucs, G.
    Gergely, L. A.
    Comparative testing of dark matter models with 15 HSB and 15 LSB galaxies2017Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 608, artikel-id A42Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. We assemble a database of 15 high surface brightness (HSB) and 15 low surface brightness (LSB) galaxies, for which surface brightness density and spectroscopic rotation curve data are both available and representative for various morphologies. We use this dataset to test the Navarro-Frenk-White, the Einasto, and the pseudo-isothermal sphere dark matter models.

    Aims. We investigate the compatibility of the pure baryonic model and baryonic plus one of the three dark matter models with observations on the assembled galaxy database. When a dark matter component improves the fit with the spectroscopic rotational curve, we rank the models according to the goodness of fit to the datasets.

    Methods. We constructed the spatial luminosity density of the baryonic component based on the surface brightness profile of the galaxies. We estimated the mass-to-light (M/L) ratio of the stellar component through a previously proposed color-mass-to-light ratio relation (CMLR), which yields stellar masses independent of the photometric band. We assumed an axissymetric baryonic mass model with variable axis ratios together with one of the three dark matter models to provide the theoretical rotational velocity curves, and we compared them with the dataset. In a second attempt, we addressed the question whether the dark component could be replaced by a pure baryonic model with fitted M/L ratios, varied over ranges consistent with CMLR relations derived from the available stellar population models. We employed the Akaike information criterion to establish the performance of the best-fit models.

    Results. For 7 galaxies (2 HSB and 5 LSB), neither model fits the dataset within the 1 sigma confidence level. For the other 23 cases, one of the models with dark matter explains the rotation curve data best. According to the Akaike information criterion, the pseudoisothermal sphere emerges as most favored in 14 cases, followed by the Navarro-Frenk-White (6 cases) and the Einasto (3 cases) dark matter models. We find that the pure baryonic model with fitted M/L ratios falls within the 1 sigma confidence level for 10 HSB and 2 LSB galaxies, at the price of growing the M/Ls on average by a factor of two, but the fits are inferior compared to the best-fitting dark matter model.

  • 17.
    Lindkvist, Jesper
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium.
    Nilsson, Hans
    Swedish Institute of Space Physics.
    Simon Wedlund, Cyril
    University of Oslo, Department of Physics, Oslo, Norway.
    Kallio, Esa
    Aalto University, Department of Electronics and Nanoengineering, Espoo, Finland.
    Mann, Ingrid
    University of Tromsø, Department of Physics and Technology, Tromsø, Norway.
    Pitkänen, Timo
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Karlsson, Tomas
    KTH Royal Institute of Technology, School of Electrical Engineering, Stockholm, Sweden.
    Energy conversion in cometary atmospheres: Hybrid modeling of 67P/Churyumov-Gerasimenko2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 616, artikel-id A81Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aims. We wish to investigate the energy conversion between particles and electromagnetic fields and determine the location where it occurs in the plasma environment of comets.

    Methods. We used a hybrid plasma model that included photoionization, and we considered two cases of the solar extreme ultraviolet flux. Other parameters corresponded to the conditions of comet 67P/Churyumov-Gerasimenko at a heliocentric distance of 1.5 AU.

    Results. We find that a shock-like structure is formed upstream of the comet and acts as an electromagnetic generator, similar to the bow shock at Earth that slows down the solar wind. The Poynting flux transports electromagnetic energy toward the inner coma, where newly born cometary ions are accelerated. Upstream of the shock-like structure, we find local energy transfer from solar wind ions to cometary ions. We show that mass loading can be a local process with a direct transfer of energy, but also part of a dynamo system with electromagnetic generators and loads.

    Conclusions. The energization of cometary ions is governed by a dynamo system for weak ionization, but changes into a large conversion region with local transfer of energy directly from solar wind protons for high ionization.

  • 18. Nilsson, H.
    et al.
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Belgian Institute for Space Aeronomy, Avenue Circulaire 3, 1180 Brussels, Belgium.
    Karlsson, T.
    Brenning, N.
    Henri, P.
    Goetz, C.
    Eriksson, A. I.
    Behar, E.
    Wieser, G. Stenberg
    Vallieres, X.
    Size of a plasma cloud matters: The polarisation electric field of a small-scale comet ionosphere2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 616, artikel-id A50Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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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  • 19.
    Nilsson, Hans
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Möslinger, Anja
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Williamson, H. N.
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Bergman, Sofia
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Stenberg Wieser, Gabriella
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Futaana, Yoshifumi
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Karlsson, T.
    Department of Space and Plasma Physics, School of Electrical Engineering and Computer Science, KTH Royal Institute of Technology, Stockholm, Sweden.
    Behar, E.
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Holmström, Mats
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Upstream solar wind speed at comet 67P: reconstruction method, model comparison, and results2022Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 659, artikel-id A18Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context: Rosetta followed comet 67P at heliocentric distances from 1.25 to 3.6 au. The solar wind was observed for much of this time, but was significantly deflected and to some extent slowed down by the interaction with the coma.

    Aims: We use the different changes in the speed of H+ and He2+ when they interact with the coma to estimate the upstream speed of the solar wind. The different changes in the speed are due to the different mass per charge of the particles, while the electric force per charge due to the interaction is the same. A major assumption is that the speeds of H+ and He2+ were the same in the upstream region. This is investigated.

    Methods: We derived a method for reconstructing the upstream solar wind from H+ and He2+ observations. The method is based on the assumption that the interaction of the comet with the solar wind can be described by an electric potential that is the same for both H+ and He2+. This is compared to estimates from the Tao model and to OMNI and Mars Express data that we propagated to the observation point.

    Results: The reconstruction agrees well with the Tao model for most of the observations, in particular for the statistical distribution of the solar wind speed. The electrostatic potential relative to the upstream solar wind is derived and shows values from a few dozen volts at large heliocentric distances to about 1 kV during solar events and close to perihelion. The reconstructed values of the solar wind for periods of high electrostatic potential also agree well with propagated observations and model results.

    Conclusions: The reconstructed upstream solar wind speed during the Rosetta mission agrees well with the Tao model. The Tao model captures some slowing down of high-speed streams as compared to observations at Earth or Mars. At low solar wind speeds, below 400 km s-1, the agreement is better between our reconstruction and Mars observations than with the Tao model. The magnitude of the reconstructed electrostatic potential is a good measure of the slowing-down of the solar wind at the observation point.

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  • 20. Perjés, Zoltán
    et al.
    Vasúth, Mátyás
    Czinner, Viktor
    Eriksson, Daniel
    Umeå universitet, Teknisk-naturvetenskaplig fakultet, Fysik.
    C-infinity perturbations of FRW models with a cosmological constant2005Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 431, nr 2, s. 415-421Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Spatially homogeneous and isotropic cosmological models, with a perfect fluid matter source and non-vanishing cosmological constant, are studied. The equations governing linear perturbations of the space-time and the variation of energy density are given. The complete solution of the problem is obtained for C∞ perturbations, using a comoving time. The Sachs-Wolfe fluctuations of the temperature of the cosmic background radiation are obtained for the relatively growing density perturbations. It is found that the observable celestial microwave fluctuation pattern underwent a reversal approximately two billion years ago. What is observed today is a negative image of the last scattering surface with an attenuation of the fluctuations, due to the presence of the cosmological constant.

  • 21. Simon Wedlund, Cyril
    et al.
    Alho, Markku
    Gronoff, Guillaume
    Kallio, Esa
    Gunell, Herbert
    Nilsson, Hans
    Swedish Institute of Space Physics, Kiruna.
    Lindkvist, Jesper
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Behar, Etienne
    Swedish Institute of Space Physics, Kiruna.
    Stenberg Wieser, Gabriella
    Swedish Institute of Space Physics, Kiruna.
    Miloch, Wojciech Jacek
    Hybrid modelling of cometary plasma environments: I. Impact of photoionisation, charge-exchange and electron ionisation on bow shock and cometopause at 67P/Churyumov-Gerasimenko2017Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 604, artikel-id A73Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. The ESA/Rosetta mission made it possible to monitor the plasma environment of a comet, from near aphelion to perihelion conditions. To understand the complex dynamics and plasma structures found at the comet, a modelling effort must be carried out in parallel. Aims. Firstly, we present a 3D hybrid model of the cometary plasma environment including photoionisation, solar wind charge exchange, and electron ionisation reactions; this model is used in stationary and dynamic conditions (mimicking the solar wind variations), and is thus especially adapted to a weakly outgassing comet such as 67P/Churyumov-Gerasimenko, the target of the ESA/Rosetta mission. Secondly, we use the model to study the respective effects of ionisation processes on the formation of the dayside macroscopic magnetic and density boundaries upstream of comet 67P in perihelion conditions at 1.3 AU. Thirdly, we explore and discuss the effects of these processes on the magnetic field line draping, ionisation rates, and composition in the context of the Rosetta mission. Methods. We used a new quasi-neutral hybrid model, originally designed for weakly magnetised planetary bodies, such as Venus, Mars, and Titan, and adapted here to comets. Ionisation processes were monitored individually and together following a probabilistic interaction scheme. Three-dimensional paraboloid fits of the bow shock surface, identified for a magnetosonic Mach number equal to 2, and of the cometopause surface, were performed for a more quantitative analysis. Results. We show that charge exchange and electron ionisation play a major role in the formation of a bow shock-like structure far upstream, while photoionisation is the main driver at and below the cometopause boundary, within 1000 km cometocentric distance. Charge exchange contributes to 42% of the total production rate in the simulation box, whereas production rates from electron ionisation and photoionisation reach 33% and 25%, respectively. We also discuss implications for Rosetta's observations, regarding the detection of the bow shock and the cometopause.

  • 22.
    Stephenson, P.
    et al.
    Department of Physics, Imperial College London, London, United Kingdom.
    Galand, M.
    Department of Physics, Imperial College London, London, United Kingdom.
    Feldman, P.D.
    Physics and Astronomy, The Johns Hopkins University, 3400 N. Charles Street, MD, Baltimore, United States.
    Beth, Arnaud
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Rubin, M.
    Physikalisches Institut, University of Bern, Sidlerstrasse 5, Bern, Switzerland.
    Bockelée-Morvan, D.
    Lesia, Observatoire de Paris, Université Psl, Cnrs, Sorbonne Université, Université de Paris, 5 place Jules Janssen, Meudon, France.
    Biver, N.
    Lesia, Observatoire de Paris, Université Psl, Cnrs, Sorbonne Université, Université de Paris, 5 place Jules Janssen, Meudon, France.
    Cheng, Y.-C.
    Lesia, Observatoire de Paris, Université Psl, Cnrs, Sorbonne Université, Université de Paris, 5 place Jules Janssen, Meudon, France.
    Parker, J.
    Southwest Research Institute, Department of Space Studies, 1050 Walnut Street, CO, Boulder, United States.
    Burch, J.
    SouthWest Research Institute, PO Drawer 28510, TX, San Antonio, United States.
    Johansson, F.L.
    Swedish Institute of Space Physics, Ångström Laboratory, Lägerhyddsvägen1, Uppsala, Sweden.
    Eriksson, A.
    Swedish Institute of Space Physics, Ångström Laboratory, Lägerhyddsvägen1, Uppsala, Sweden.
    Multi-instrument analysis of far-ultraviolet aurora in the southern hemisphere of comet 67P/Churyumov-Gerasimenko2021Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 647, artikel-id A119Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aims. We aim to determine whether dissociative excitation of cometary neutrals by electron impact is the major source of far-ultraviolet (FUV) emissions at comet 67P/Churyumov-Gerasimenko in the southern hemisphere at large heliocentric distances, both during quiet conditions and impacts of corotating interaction regions observed in the summer of 2016.

    Methods. We combined multiple datasets from the Rosetta mission through a multi-instrument analysis to complete the first forward modelling of FUV emissions in the southern hemisphere of comet 67P and compared modelled brightnesses to observations with the Alice FUV imaging spectrograph. We modelled the brightness of OI1356, OI1304, Lyman-β, CI1657, and CII1335 emissions, which are associated with the dissociation products of the four major neutral species in the coma: CO2, H2O, CO, and O2. The suprathermal electron population was probed by the Ion and Electron Sensor of the Rosetta Plasma Consortium and the neutral column density was constrained by several instruments: the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA), the Microwave Instrument for the Rosetta Orbiter and the Visual InfraRed Thermal Imaging Spectrometer.

    Results. The modelled and observed brightnesses of the FUV emission lines agree closely when viewing nadir and dissociative excitation by electron impact is shown to be the dominant source of emissions away from perihelion. The CII1335 emissions are shown to be consistent with the volume mixing ratio of CO derived from ROSINA. When viewing the limb during the impacts of corotating interaction regions, the model reproduces brightnesses of OI1356 and CI1657 well, but resonance scattering in the extended coma may contribute significantly to the observed Lyman-β and OI1304 emissions. The correlation between variations in the suprathermal electron flux and the observed FUV line brightnesses when viewing the comet's limb suggests electrons are accelerated on large scales and that they originate in the solar wind. This means that the FUV emissions are auroral in nature.

  • 23. Wedlund, Cyril Simon
    et al.
    Behar, Etienne
    Kallio, Esa
    Nilsson, Hans
    Alho, Markku
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Royal Belgian Institute for Space Aeronomy, Avenue Circulaire 3, 1180 Brussels, Belgium.
    Bodewits, Dennis
    Beth, Arnaud
    Gronoft, Guillaume
    Hoekstra, Ronnie
    Solar wind charge exchange in cometary atmospheres II. Analytical model2019Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 630, artikel-id A36Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. Solar wind charge-changing reactions are of paramount importance to the physico-chemistry of the atmosphere of a comet because they mass-load the solar wind through an effective conversion of fast, light solar wind ions into slow, heavy cometary ions. The ESA/Rosetta mission to comet 67P/Churyumov-Gerasimenko (67P) provided a unique opportunity to study charge-changing processes in situ. Aims. To understand the role of charge-changing reactions in the evolution of the solar wind plasma and to interpret the complex in situ measurements made by Rosetta, numerical or analytical models are necessary. Methods. An extended analytical formalism describing solar wind charge-changing processes at comets along solar wind streamlines is presented. It is based on a thorough book-keeping of available charge-changing cross sections of hydrogen and helium particles in a water gas. Results. After presenting a general 1D solution of charge exchange at comets, we study the theoretical dependence of charge-state distributions of (He2+, He+, He-0) and (H+, H-0, H-) on solar wind parameters at comet 67P. We show that double charge exchange for the He2+-H2O system plays an important role below a solar wind bulk speed of 200 km s-1, resulting in the production of He energetic neutral atoms, whereas stripping reactions can in general be neglected. Retrievals of outgassing rates and solar wind upstream fluxes from local Rosetta measurements deep in the coma are discussed. Solar wind ion temperature effects at 400 km s-1 solar wind speed are well contained during the Rosetta mission. Conclusions. As the comet approaches perihelion, the model predicts a sharp decrease of solar wind ion fluxes by almost one order of magnitude at the location of Rosetta, forming in effect a solar wind ion cavity. This study is the second part of a series of three on solar wind charge-exchange and ionization processes at comets, with a specific application to comet 67P and the Rosetta mission.

  • 24. Wedlund, Cyril Simon
    et al.
    Behar, Etienne
    Nilsson, Hans
    Alho, Markku
    Kallio, Esa
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Royal Belgian Institute for Space Aeronomy, Avenue Circulaire 3, 1180 Brussels, Belgium.
    Bodewits, Dennis
    Heritier, Kevin
    Galand, Marina
    Beth, Arnaud
    Rubin, Martin
    Altwegg, Kathrin
    Volwerk, Martin
    Gronoff, Guillaume
    Hoekstra, Ronnie
    Solar wind charge exchange in cometary atmospheres III. Results from the Rosetta mission to comet 67P/Churyumov-Gerasimenko2019Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 630, artikel-id A37Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context: Solar wind charge-changing reactions are of paramount importance to the physico-chemistry of the atmosphere of a comet. The ESA/Rosetta mission to comet 67P/Churyumov-Gerasimenko (67P) provides a unique opportunity to study charge-changing processes in situ.

    Aims: To understand the role of these reactions in the evolution of the solar wind plasma and interpret the complex in situ measurements made by Rosetta, numerical or analytical models are necessary.

    Methods: We used an extended analytical formalism describing solar wind charge-changing processes at comets along solar wind streamlines. The model is driven by solar wind ion measurements from the Rosetta Plasma Consortium-Ion Composition Analyser (RPC-ICA) and neutral density observations from the Rosetta Spectrometer for Ion and Neutral Analysis-Comet Pressure Sensor (ROSINA-COPS), as well as by charge-changing cross sections of hydrogen and helium particles in a water gas.

    Results: A mission-wide overview of charge-changing efficiencies at comet 67P is presented. Electron capture cross sections dominate and favor the production of He and H energetic neutral atoms (ENAs), with fluxes expected to rival those of H+ and He2+ ions.

    Conclusions: Neutral outgassing rates are retrieved from local RPC-ICA flux measurements and match ROSINA estimates very well throughout the mission. From the model, we find that solar wind charge exchange is unable to fully explain the magnitude of the sharp drop in solar wind ion fluxes observed by Rosetta for heliocentric distances below 2.5 AU. This is likely because the model does not take the relative ion dynamics into account and to a lesser extent because it ignores the formation of bow-shock-like structures upstream of the nucleus. This work also shows that the ionization by solar extreme-ultraviolet radiation and energetic electrons dominates the source of cometary ions, although solar wind contributions may be significant during isolated events.

  • 25. Wedlund, Cyril Simon
    et al.
    Bodewits, Dennis
    Alho, Markku
    Hoekstra, Ronnie
    Behar, Etienne
    Gronoff, Guillaume
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Royal Belgian Institute for Space Aeronomy, Avenue Circulaire 3, 1180 Brussels, Belgium.
    Nilsson, Hans
    Kallio, Esa
    Beth, Arnaud
    Solar wind charge exchange in cometary atmospheres I. Charge-changing and ionization cross sections for He and H particles in H2O2019Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 630, artikel-id A35Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. Solar wind charge-changing reactions are of paramount importance to the physico-chemistry of the atmosphere of a comet, mass-loading the solar wind through an effective conversion of fast light solar wind ions into slow heavy cometary ions. Aims. To understand these processes and place them in the context of a solar wind plasma interacting with a neutral atmosphere, numerical or analytical models are necessary. Inputs of these models, such as collision cross sections and chemistry, are crucial. Methods. Book-keeping and fitting of experimentally measured charge-changing and ionization cross sections of hydrogen and helium particles in a water gas are discussed, with emphasis on the low-energy/low-velocity range that is characteristic of solar wind bulk speeds (<20 keV u-1/2000 km s-1). Results. We provide polynomial fits for cross sections of charge-changing and ionization reactions, and list the experimental needs for future studies. To take into account the energy distribution of the solar wind, we calculated Maxwellian-averaged cross sections and fitted them with bivariate polynomials for solar wind temperatures ranging from 105 to 106 K (12-130 eV). Conclusions. Single- and double-electron captures by He2+ dominate at typical solar wind speeds. Correspondingly, single-electron capture by H+ and single-electron loss by H- dominate at these speeds, resulting in the production of energetic neutral atoms (ENAs). Ionization cross sections all peak at energies above 20 keV and are expected to play a moderate role in the total ion production. However, the effect of solar wind Maxwellian temperatures is found to be maximum for cross sections peaking at higher energies, suggesting that local heating at shock structures in cometary and planetary environments may favor processes previously thought to be negligible. This study is the first part in a series of three on charge exchange and ionization processes at comets, with a specific application to comet 67P/Churyumov-Gerasimenko and the Rosetta mission.

  • 26.
    Williamson, H.N.
    et al.
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Nilsson, H.
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Stenberg Wieser, G.
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Moeslinger, Anja
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Goetz, C.
    European Space Research and Technology Centre, European Space Agency, Noordwijk, Netherlands.
    Development of a cometosheath at comet 67P/Churyumov-Gerasimenko: a case study comparison of Rosetta observations2022Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 660, artikel-id A103Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context: The ionosphere of a comet is known to deflect the solar wind through mass loading, but the interaction is dependent on cometary activity. We investigate the details of this process at comet 67P using the Rosetta Ion Composition Analyzer.

    Aims: This study aims to compare the interaction of the solar wind and cometary ions during two different time periods in the Rosetta mission.

    Methods: We compared both the integrated ion moments (density, velocity, and momentum flux) and the velocity distribution functions for two days, four months apart. The velocity distribution functions were projected into a coordinate system dependent on the magnetic field direction and averaged over three hours.

    Results: The first case shows highly scattered H+ in both ion moments and velocity distribution function. The He2+ ions are somewhat scattered, but less so, and appear more like those of H2O+ pickup ions. The second case shows characteristic evidence of mass-loading, where the solar wind species are deflected, but the velocity distribution function is not significantly changed.

    Conclusions. The distributions of H+ in the first case, when compared to He2+ and H2O+ pickup ions, are indicative of a narrow cometosheath on the scale of the H+ gyroradius. Thus, He2+ and H2O+, with larger gyroradii, are largely able to pass through this cometosheath. An examination of the momentum flux tensor suggests that all species in the first case have a significant non-gyrotropic momentum flux component that is higher than that of the second mass-loaded case. Mass loading is not a sufficient explanation for the distribution functions and momentum flux tensor in the first case, and so we assume this is evidence of bow shock formation.

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