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  • 1.
    Bergman, Sofia
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Stenberg Wieser, Gabriella
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Wieser, Martin
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Nilsson, Hans
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Vigren, Erik
    Swedish Institute of Space Physics, Uppsala, Sweden.
    Beth, Arnaud
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Masunaga, Kei
    Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Yoshinodai 3-1-1, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan.
    Eriksson, Anders
    Swedish Institute of Space Physics, Uppsala, Sweden.
    Flow directions of low-energy ions in and around the diamagnetic cavity of comet 67P2021In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 507, no 4, p. 4900-4913Article in journal (Refereed)
    Abstract [en]

    The flow direction of low-energy ions around comet 67P/Churyumov–Gerasimenko has previously been difficult to constrain due to the influence of the spacecraft potential. The Ion Composition Analyzer of the Rosetta Plasma Consortium (RPC-ICA) on Rosetta measured the distribution function of positive ions with energies down to just a few eV/q throughout the escort phase ofthe mission. Unfortunately, the substantial negative spacecraft potential distorted the directional information of the low-energy data. In this work, we present the flow directions of low-energy ions around comet 67P, corrected for the spacecraft potential using Particle-In-Cell simulation results. We focus on the region in and around the diamagnetic cavity, where low-energy ions are especially important for the dynamics. We separate between slightly accelerated ‘burst’ features and a more constant ‘band’ of low-energy ions visible in the data. The ‘bursts’ are flowing radially outwards from the nucleus with an antisunward component while the ‘band’ is predominantly streaming back towards the comet. This provides evidence of counter-streaming ions, which has implications for the overall expansion velocity of the ions. The backstreaming ions are present also at times when the diamagnetic cavity was not detected, indicating that the process accelerating the ions back towards the comet is not connected to the cavity boundary.

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  • 2.
    Bergman, Sofia
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. 1Swedish Institute of Space Physics, Kiruna, Sweden.
    Wieser, Gabriella Stenberg
    Wieser, Martin
    Johansson, Fredrik Leffe
    Vigren, Erik
    Nilsson, Hans
    Nemeth, Zoltan
    Eriksson, Anders
    Williamson, Hayley
    Ion bulk speeds and temperatures in the diamagnetic cavity of comet 67P from RPC-ICA measurements2021In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 503, no 2, p. 2733-2745Article in journal (Refereed)
    Abstract [en]

    Comets are constantly interacting with the solar wind. When the comet activity is high enough, this leads to the creation of a magnetic field free region around the nucleus known as the diamagnetic cavity. It has been suggested that the ion-neutral drag force is balancing the magnetic pressure at the cavity boundary, but after the visit of Rosetta to comet 67P/Churyumov–Gerasimenko the coupling between ions and neutrals inside the cavity has been debated, at least for moderately active comets. In this study, we use data from the ion composition analyser to determine the bulk speeds and temperatures of the low-energy ions in the diamagnetic cavity of comet 67P. The low-energy ions are affected by the negative spacecraft potential, and we use the Spacecraft Plasma Interaction Software to model the resulting influence on the detected energy spectra. We find bulk speeds of 5–10 km s−1 with a most probable speed of 7 km s−1, significantly above the velocity of the neutral particles. This indicates that the collisional coupling between ions and neutrals is not strong enough to keep the ions at the same speed as the neutrals inside the cavity. The temperatures are in the range 0.7–1.6 eV, with a peak probability at 1.0 eV. We attribute the major part of the temperature to the fact that ions are born at different locations in the coma, and hence are accelerated over different distances before reaching the spacecraft.

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  • 3.
    Beth, Arnaud
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gunell, Herbert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Simon Wedlund, C.
    Goetz, C.
    Nilsson, H.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    First investigation of the diamagnetic cavity boundary layer with a 1D3V PIC simulation2022In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 667, article id A143Article in journal (Refereed)
    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.

  • 4.
    Giang, Tony
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Yamauchi, Masatoshi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lundin, Rickard
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nilsson, Hans
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ebihara, Yusuke
    Institute for Advanced Research, Nagoya University, Japan.
    Rème, Henri
    Centre d'Etude Spatiale des Rayonnements, Toulouse, France.
    Dandouras, Iannis
    Centre d'Etude Spatiale des Rayonnements, Toulouse, France.
    Vallat, C.
    VEGA contracted to Solar System Science Operations Division, ESA/ESAC, Madrid, Spain.
    Bavassano-Cattaneo, M. B.
    L'Istituto di Fisica dello Spazio Interplanetario, Roma, Italy.
    Klecker, B.
    Max Planck Institute for Extraterrestrial Physics, Garching, Germany.
    Korth, A.
    Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany.
    Kistler, L. M.
    University of New Hampshire, Durham, New Hampshire, USA.
    McCarthy, M.
    University of Washington, Seattle, USA.
    Outflowing protons and heavy ions as a source for the sub-keV ringcurrent2009In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 27, no 2, p. 839-849Article in journal (Refereed)
    Abstract [en]

    Data from the Cluster CIS instrument have been used for studying proton and heavy ion (O+ and He+ ) char- acteristics of the sub-keV ring current. Thirteen events with dispersed heavy ions (O+ and He+ ) were identified out of two years (2001 and 2002) of Cluster data. Allevents took place during rather geomagnetically quiet periods. Three of those events have been investigated in detail: 21 August 2001, 26 November 2001 and 20 February 2002. These events were chosen from varying magnetic local times (MLT), and they showed different characteristics. In this article, we discuss the potential source for sub-keV ring current ions. We show that: (1) outflows of terrestrialsub-keV ions are supplied to the ring current also during quiet geomagnetic conditions; (2) the composition of the out-flow implies an origin that covers an altitude interval from the low-altitude ionosphere to the plasmasphere, and (3) terrestrial ions are moving upward along magnetic field lines, at times forming narrow collimated beams, but  frequently also as broad beams. Over time, the ion beams are expected to gradually become isotropised as a result of wave-particleinteraction, eventually taking the form of isotropic drifting sub-keV ion signatures. We argue that the sub-keV energy-time dispersed signatures originate from field-aligned terrestrial ion energising and outflow, which may occur at all local times and persist also during quiet times.

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  • 5.
    Gunell, Herbert
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Avenue Circulaire 3, 1180 Brussels, Belgium.
    Goetz, Charlotte
    Wedlund, Cyril Simon
    Lindkvist, Jesper
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Nilsson, Hans
    LLera, Kristie
    Eriksson, Anders
    Holmström, Mats
    The infant bow shock: a new frontier at a weak activity comet2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 619, article id L2Article in journal (Refereed)
    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.

  • 6.
    Gunell, Herbert
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. 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 University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    De Keyser, Johan
    Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Belgium.
    Why an intrinsic magnetic field does not protect a planet against atmospheric escape2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 614, article id L3Article in journal (Refereed)
    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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  • 7.
    Moeslinger, Anja
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Nilsson, Hans
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Stenberg Wieser, G.
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Gunell, Herbert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Goetz, C.
    Department of Mathematics, Physics and Electrical Engineering, Northumbria University, Newcastle-upon-Tyne, United Kingdom.
    Indirect observations of electric fields at comet 67P2023In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 128, no 9, article id e2023JA031746Article in journal (Refereed)
    Abstract [en]

    No 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.

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  • 8.
    Nilsson, Hans
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Möslinger, Anja
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Williamson, H. N.
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Bergman, Sofia
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Gunell, Herbert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Stenberg Wieser, Gabriella
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Futaana, Yoshifumi
    Umeå University, Faculty of Science and Technology, Department of Physics. 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 University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Upstream solar wind speed at comet 67P: reconstruction method, model comparison, and results2022In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 659, article id A18Article in journal (Refereed)
    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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  • 9. Nilsson, Hans
    et al.
    Wieser, Gabriella Stenberg
    Behar, Etienne
    Gunell, Herbert
    Umeå University, Faculty of Science and Technology, Department of Physics. Royal Belgian Institute for Space Aeronomy, Avenue Circulaire 3, B-1180 Brussels, Belgium.
    Wieser, Martin
    Galand, Marina
    Wedlund, Cyril Simon
    Alho, Markku
    Goetz, Charlotte
    Yamauchi, Masatoshi
    Henri, Pierre
    Odelstad, Elias
    Vigren, Erik
    Evolution of the ion environment of comet 67P during the Rosetta mission as seen by RPC-ICA2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 469, p. S252-S261Article in journal (Refereed)
    Abstract [en]

    Rosetta has followed comet 67P from low activity at more than 3.6 au heliocentric distance to high activity at perihelion (1.24 au) and then out again. We provide a general overview of the evolution of the dynamic ion environment using data from the RPC-ICA ion spectrometer. We discuss where Rosetta was located within the evolving comet magnetosphere. For the initial observations, the solar wind permeated all of the coma. In 2015 mid-April, the solar wind started to disappear from the observation region, to re-appear again in 2015 December. Low-energy cometary ions were seen at first when Rosetta was about 100 km from the nucleus at 3.6 au, and soon after consistently throughout the mission except during the excursions to farther distances from the comet. The observed flux of low-energy ions was relatively constant due to Rosetta's orbit changing with comet activity. Accelerated cometary ions, moving mainly in the antisunward direction gradually became more common as comet activity increased. These accelerated cometary ions kept being observed also after the solar wind disappeared from the location of Rosetta, with somewhat higher fluxes further away from the nucleus. Around perihelion, when Rosetta was relatively deep within the comet magnetosphere, the fluxes of accelerated cometary ions decreased, as did their maximum energy. The disappearance of more energetic cometary ions at close distance during high activity is suggested to be due to a flow pattern where these ions flow around the obstacle of the denser coma or due to charge exchange losses.

  • 10.
    Nilsson, Hans
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Zhang, Qi
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Stenberg Wieser, Gabriella
    Umeå University. Swedish Institute of Space Physics, Kiruna, Sweden.
    Holmström, Mats
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Futaana, Yoshifumi
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Fedorov, Andrey
    Institut de Recherche en Astrophysique et Planétologie, Toulouse, France.
    Persson, Moa
    Swedish Institute of Space Physics, Kiruna, Sweden; Institut de Recherche en Astrophysique et Planétologie, 9 avenue du Colonel Roche BP 44346 31028 Toulouse Cedex 4, France.
    Wieser, Martin
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Solar cycle variation of ion escape from Mars2023In: Icarus, ISSN 0019-1035, E-ISSN 1090-2643, Vol. 393, article id 114610Article in journal (Refereed)
    Abstract [en]

    Using Mars Express data from 2007 until 2020 we show how ion outflow from Mars varied over more than a solar cycle, from one solar minimum to another. The data was divided into intervals with a length of one Martian year, starting from 30 April 2007 and ending 13 July 2020. The net escape rate was about 5×1024s−1 in the first covered minimum, and 2−3×1024s−1 in the most recent minimum. Ion escape peaked at 1×1025s−1 during the intervening solar maximum. The outflow is a clear function of the solar cycle, in agreement with previous studies which found a clear relationship between solar EUV flux and ion escape at Mars. The outflow during solar maximum is 2.5 to 3 times higher than in the surrounding solar minima. The average solar wind dynamic pressure over a Martian year was investigated, but does not vary much with the solar cycle. The escape rate at solar maximum is in good agreement with some recent MAVEN studies, and dominated by low energy ions at most sampled locations. A simple linear fit to the data gives a prediction of the escape rate for the much stronger solar maximum during the Phobos mission in 1989 that is consistent with observations. The fit also implies a non-linear response of ion escape for low solar EUV, with a lower initial escape response for lower solar EUV levels than those of the studied data set.

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  • 11.
    Persson, Moa
    et al.
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Futaana, Yoshifumi
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Nilsson, Hans
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Stenberg Wieser, Gabriella
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Fedorov, Andrei
    IRAP, CNRS, Toulouse, France.
    Zhang, T. L.
    Space Research Institute, Austrian Academy of Sciences, Graz, Austria.
    Barabash, Stas
    Space Research Institute, Austrian Academy of Sciences, Graz, Austria.
    Heavy Ion Flows in the Upper Ionosphere of the Venusian North Pole2019In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, no 6, p. 4597-4607Article in journal (Refereed)
    Abstract [en]

    We investigate the heavy ion density and velocity in the Venusian upper ionosphere near the North Pole, using the Ion Mass Analyzer, a part of the Analyzer of Space Plasmas and Energetic Atoms 4, together with the magnetic field instruments on Venus Express. The measurements were made during June-July 2014, covering the aerobraking campaign with lowered altitude measurements (similar to 130 km). The plasma scale heights are similar to 15 km below 150-km altitude and similar to 200 km at 150-400-km altitude. A clear trend of dusk-to-dawn heavy ion flow across the polar ionosphere was found, with speeds of similar to 2-10 km/s. In addition, the flow has a significant downward radial velocity component. The flow pattern does not depend on the interplanetary magnetic field directions nor the ionospheric magnetization states. Instead, we suggest a thermal pressure gradient between the equatorial and polar terminator regions, induced by the decrease in density between the regions, as the dominant mechanism driving the ion flow. Plain Language Summary We have calculated the ion density and velocities in the Venusian polar ionosphere using measurements from the Ion Mass Analyzer on board the Venus Express spacecraft. During June-July 2014 the periapsis was lowered to similar to 130 km, which allowed for measurements down to low altitudes of the ionosphere near the North Pole. The plasma scale heights are similar to 15 km below 150-km altitude and similar to 200 km at 150-400 km, which is similar to what was found near the equatorial region by the Pioneer Venus mission. In addition, there is a clear trend of dusk-to-dawn flow, along the terminator, for the heavy ions. This is surprising, as a general flow from day-to-night is expected for the Venusian ionosphere due to the long nights and significant heating of the dayside upper atmosphere. The interplanetary magnetic field direction does not appear to affect the ion flow pattern. Instead, we propose a thermal pressure gradient as the dominant accelerating mechanism, induced by the decrease in density from the equator toward the pole.

  • 12.
    Persson, Moa
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Institutet för Rymdfysik, Kiruna, Sverige.
    Futaana, Yoshifumi
    Institutet för Rymdfysik, Kiruna, Sverige.
    Ramstad, Robin
    Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, USA.
    Schillings, Audrey
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Masunaga, Kei
    Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, USA.
    Nilsson, Hans
    Institutet för Rymdfysik, Kiruna, Sverige.
    Fedorov, Andrei
    IRAP, CNRS, Toulouse, France.
    Barabash, Stas
    Institutet för Rymdfysik, Kiruna, Sverige.
    Global Venus-Solar wind coupling and oxygen ion escape2021In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 48, no 4, article id e2020GL091213Article in journal (Refereed)
    Abstract [en]

    The present‐day Venusian atmosphere is dry, yet, in its earlier history a significant amount of water evidently existed. One important water loss process comes from the energy and momentum transfer from the solar wind to the atmospheric particles. Here, we used measurements from the Ion Mass Analyzer onboard Venus Express to derive a relation between the power in the upstream solar wind and the power leaving the atmosphere through oxygen ion escape in the Venusian magnetotail. We find that on average 0.01% of the available power is transferred, and that the percentage decreases as the available energy increases. For Mars the trend is similar, but the efficiency is higher. At Earth, the ion escape does not behave similarly, as the ion escape only increases after a threshold in the available energy is reached. These results indicate that the Venusian induced magnetosphere efficiently screens the atmosphere from the solar wind.

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  • 13.
    Pitkänen, Timo
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Karlsson, Tomas
    Space and Plasma Physics, EES, KTH, Stockholm, Sweden.
    Nilsson, Hans
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Kullen, Anita
    Space and Plasma Physics, EES, KTH, Stockholm, Sweden.
    On IMF By-induced dawn-dusk asymmetries in earthward convective fast flows2017In: Dawn-dusk asymmetries in planetary plasma environments / [ed] Stein Haaland, Andrei Runov, Colin Forsyth, American Geophysical Union (AGU), 2017, 1, p. 95-106Chapter in book (Refereed)
    Abstract [en]

    Studies of earthward plasma and magnetic field transport in the Earth’s magnetotail plasma sheet have shown that, on the average, Earthward ion flows in the premidnight and midnight sectors exhibit a duskward component while flows in the postmidnight sector are dawnward. The flow pattern is more pronounced for slower flows (<100 km/s) and alters gradually to a more symmetric one with respect to midnight for increasing flow speeds. However, recent ionospheric and magnetospheric studies have suggested that a nonzero By component in the interplanetary magnetic field (IMF) may significantly influence the earthward transport, creating previously unnoticed dawn‐dusk asymmetries between the hemispheres. In this article, we give a short overview of the present understanding of the topic, present new results, and briefly discuss the importance of the IMF By component for the Earthward transport processes in the magnetotail plasma sheet.

  • 14.
    Ramstad, Robin
    et al.
    Swedish Institute of Space Physics, Kiruna.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna.
    Futaana, Yoshifumi
    Swedish Institute of Space Physics, Kiruna.
    Nilsson, Hans
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna.
    Effects of the crustal magnetic fields on the Martian atmospheric ion escape rate2016In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 43, no 20, p. 10574-10579Article in journal (Refereed)
    Abstract [en]

    Eight years (2007–2015) of ion flux measurements from Mars Express are used to statisticallyinvestigate the influence of the Martian magnetic crustal fields on the atmospheric ion escape rate.We combine all Analyzer of Space Plasmas and Energetic Atoms/Ion Mass Analyzer (ASPERA-3/IMA)measurements taken during nominal upstream solar wind and solar extreme ultraviolet conditions tocompute global average ion distribution functions, individually for the north/south hemispheres and forvarying solar zenith angles (SZAs) of the strongest crustal magnetic field. Escape rates are subsequentlycalculated from each of the average distribution functions. The maximum escape rate (4.2 ± 1.2) × 1024 s−1 is found for SZA = 60–80, while the minimum escape rate (1.7±0.6)×1024 s−1 is found for SZA = 28–60,showing that the dayside orientation of the crustal fields significantly affects the global escape rate (p=97%). However, averaged over time, independent of SZA, we find no statistically significant difference inthe escape rates from the two hemispheres (escape from southern hemisphere 46% ± 18% of global rate).

  • 15.
    Ramstad, Robin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna.
    Barabash, Stas
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna.
    Futaana, Yoshifumi
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna.
    Nilsson, Hans
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna.
    Holmström, Mats
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna.
    Global Mars-solar wind coupling and ion escape2017In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, no 8, p. 8051-8062Article in journal (Refereed)
    Abstract [en]

    Loss of the early Martian atmosphere is often thought to have occurred due to an effective transfer of the solar wind energy through the Martian induced magnetic barrier to the ionosphere. We have quantified the coupling efficiency by comparing the power of the heavy ion outflow with the available power supplied by the upstream solar wind. Constraining upstream solar wind density nsw, velocity vsw, and EUV intensity IEUV/photoionizing flux FXUV in varying intervals reveals a decrease in coupling efficiency, k,with solar wind dynamic pressure as ∝ pdyn−0.74±0.13 and with FXUV as k ∝ FXUV−2.28±0.30. Despite the decreasein coupling efficiency, higher FXUV enhances the cold ion outflow, increasing the total ion escape rate as Q(FXUV) = 1010(0.82 ± 0.05)FXUV. The discrepancy between coupling and escape suggests that ion escapefrom Mars is primarily production limited in the modern era, though decreased coupling may lead to an energy-limited solar wind interaction under early Sun conditions.

  • 16.
    Ramstad, Robin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna.
    Futaana, Yoshifumi
    Swedish Institute of Space Physics, Kiruna.
    Nilsson, Hans
    Swedish Institute of Space Physics, Kiruna.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna.
    Ion escape from Mars through time: An extrapolation of atmospheric loss based on 10 years of Mars Express measurementsManuscript (preprint) (Other academic)
    Abstract [en]

    Solar wind driven atmospheric ion escape has long been hypothesized as a major influence on the evolution of the Martian atmosphere due to the lack of a Martian global dipole magnetic field. We use 10 years (2007-2017) of Mars Express data to quantify the ion escape rate over the full sampled upstream solar wind dynamic pressure, pdyn, and solar photoionizing flux, FXUV, parameter space. The modeled dependence on the upstream parameters indicates a near-linear dependence on FXUV and weak negative correlation with pdyn. Integrating total heavy ion escape back through time, considering the evolution of the upstream parameters and the modeled trends, can only account for an estimated 4.8 ± 1.1 mbar of atmosphere lost as ions since the mid-late Hesperian (3.3 Ga ago). Accounting for the recently reported stability of ion escape through the energetic oxygen ion plume provides an upper estimate of 6 mbar lost. Extending the extrapolation to the late Noachian (3.9 Ga ago) accounts for 6.3 ± 1.9 mbar, and analogously up to 9 mbar, lost through ion escape since. Thus the ion escape trends observed by Mars Express indicate that atmospheric ion escape contributed only a minor role in the evolution of the Martian atmosphere. We also report solar wind control of the cold ion outflow channel, providing a tentative explanation for the low response of the ion escape rate to upstream solar wind.

  • 17.
    Ramstad, Robin
    et al.
    Swedish Institute of Space Physics, Kiruna.
    Barabash, Stas
    Futaana, Yoshifumi
    Swedish Institute of Space Physics, Kiruna.
    Nilsson, Hans
    Swedish Institute of Space Physics, Kiruna.
    Wang, Xiao-Dong
    Swedish Institute of Space Physics, Kiruna.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna.
    The Martian atmospheric ion escape rate dependence on solar wind and solar EUV conditions: 1. Seven years of Mars Express observations2015In: Journal of Geophysical Research - Planets, ISSN 2169-9097, E-ISSN 2169-9100, Vol. 120, no 7, p. 1298-1309Article in journal (Refereed)
    Abstract [en]

    More than 7 years of ion flux measurements in the energy range 10 eV–15 keV have allowed the ASPERA-3/IMA (Analyzer of Space Plasmas and Energetic Ions/Ion Mass Analyzer) instrument on Mars Express to collect a large database of ion measurements in the Mars environment, over a wide range of upstream solar wind (density and velocity) and radiation (solar EUV intensity) conditions. We investigate the influence of these parameters on the Martian atmospheric ion escape rate by integrating IMA heavy ionflux measurements taken in the Martian tail at similar (binned) solar wind density (nsw), velocity (vsw), and solar EUV intensity (IEUV) conditions. For the same solar wind velocity and EUV intensity ranges (vsw and IEUV constrained), we find a statistically significant decrease of up to a factor of 3 in the atmospheric ion escape rate with increased average solar wind density (5.6 × 1024 s−1 to 1.9 × 1024 s−1 for 0.4 cm−3 and 1.4 cm−3, respectively). For low solar wind density (0.1–0.5 cm−3) and low EUV intensity, the escape rate increaseswith increasing solar wind velocity from 2.4 × 1024 s−1 to 5.6 × 1024 s−1. During high solar EUV intensities the escape fluxes are highly variable, leading to large uncertainties in the estimated escape rates; however, a statistically significant increase in the escape rate is found between low/high EUV for similar solar wind conditions. Empirical-analytical models for atmospheric escape are developed by fitting calculated escape rates to all sufficiently sampled upstream conditions.

  • 18.
    Ramstad, Robin
    et al.
    Swedish Institute of Space Physics, Kiruna.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna.
    Futaana, Yoshifumi
    Swedish Institute of Space Physics, Kiruna.
    Yamauchi, Masatoshi
    Swedish Institute of Space Physics, Kiruna.
    Nilsson, Hans
    Swedish Institute of Space Physics, Kiruna.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna.
    Mars Under Primordial Solar Wind Conditions: Mars Express Observations of the Strongest CME Detected at Mars Under Solar Cycle #24 and its Impact on Atmospheric Ion Escape2017In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007Article in journal (Refereed)
    Abstract [en]

    An extremely strong Coronal Mass Ejection (CME) impacted Mars on 12 July 2011, while theMars Express spacecraft was present inside the nightside ionosphere. Estimated solar wind density andspeed during the event are 39 particles cm−3 and 730 km/s, corresponding to nominal solar wind fluxat Mars when the solar system was ∼1.1 Ga old. Comparing with expected average atmospheric heavy ionfluxes under similar XUV conditions, the CME impact is found to have no significant effect on the escaperate 3.3 × 1024 s−1, with an upper limit at 1025 s−1 if the observed tail contraction is not taken into account.On the subsequent orbit, 7 h later after magnetosphere response, fluxes were only 2.4% of average. As such,even under primordial solar wind conditions we are unable to find support for a strong solar wind-driven ion escape, rather the main effect appears to be acceleration of the escaping ions by ×10–×20 typicalcharacteristic energy.

  • 19. Schillings, Audrey
    et al.
    Gunell, Herbert
    Umeå University, Faculty of Science and Technology, Department of Physics. Belgian Institute for Space Aeronomy, Brussels, Belgium.
    Nilsson, Hans
    De Spiegeleer, Alexandre
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ebihara, Yusuke
    Westerberg, Lars G.
    Yamauchi, Masatoshi
    Slapak, Rikard
    The fate of O+ ions observed in the plasma mantle: particle tracing modelling and cluster observations2020In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 38, no 3, p. 645-656Article in journal (Refereed)
    Abstract [en]

    Ion escape is of particular interest for studying the evolution of the atmosphere on geological timescales. Previously, using Cluster-CODIF data, we investigated the oxygen ion outflow from the plasma mantle for different solar wind conditions and geomagnetic activity. We found significant correlations between solar wind parameters, geomagnetic activity (K-p index), and the O+ outflow. From these studies, we suggested that O+ ions observed in the plasma mantle and cusp have enough energy and velocity to escape the magnetosphere and be lost into the solar wind or in the distant magnetotail. Thus, this study aims to investigate where the ions observed in the plasma mantle end up. In order to answer this question, we numerically calculate the trajectories of O+ ions using a tracing code to further test this assumption and determine the fate of the observed ions. Our code consists of a magnetic field model (Tsyganenko T96) and an ionospheric potential model (Weimer 2001) in which particles initiated in the plasma mantle region are launched and traced forward in time. We analysed 131 observations of plasma mantle events in Cluster data between 2001 and 2007, and for each event 200 O+ particles were launched with an initial thermal and parallel bulk velocity corresponding to the velocities observed by Cluster. After the tracing, we found that 98% of the particles are lost into the solar wind or in the distant tail. Out of these 98 %, 20% escape via the dayside magnetosphere.

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  • 20. Wedlund, Cyril Simon
    et al.
    Behar, Etienne
    Kallio, Esa
    Nilsson, Hans
    Alho, Markku
    Gunell, Herbert
    Umeå University, Faculty of Science and Technology, Department of Physics. 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 model2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 630, article id A36Article in journal (Refereed)
    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.

  • 21. Wieser, Gabriella Stenberg
    et al.
    Odelstad, Elias
    Wieser, Martin
    Nilsson, Hans
    Goetz, Charlotte
    Karlsson, Tomas
    Andre, Mats
    Kalla, Leif
    Eriksson, Anders I.
    Nicolaou, Georgios
    Wedlund, Cyril Simon
    Richter, Ingo
    Gunell, Herbert
    Umeå University, Faculty of Science and Technology, Department of Physics. Royal Belgian Institute for Space Aeronomy, Avenue Circulaire 3, B-1180 Brussels, Belgium.
    Investigating short-time-scale variations in cometary ions around comet 67P2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 469, p. S522-S534Article in journal (Refereed)
    Abstract [en]

    The highly varying plasma environment around comet 67P/Churyumov-Gerasimenko inspired an upgrade of the ion mass spectrometer (Rosetta Plasma Consortium Ion Composition Analyzer) with new operation modes, to enable high time resolution measurements of cometary ions. Two modes were implemented, one having a 4 s time resolution in the energy range 0.3-82 eV/q and the other featuring a 1 s time resolution in the energy range 13-50 eV/q. Comparing measurements made with the two modes, it was concluded that 4 s time resolution is enough to capture most of the fast changes of the cometary ion environment. The 1462 h of observations done with the 4 s mode were divided into hour-long sequences. It is possible to sort 84 per cent of these sequences into one of five categories, depending on their appearance in an energy-time spectrogram. The ion environment is generally highly dynamic, and variations in ion fluxes and energies are seen on time-scales of 10 s to several minutes.

  • 22.
    Zhang, Qi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Wang, Xiao-Dong
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Nilsson, Hans
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna, Sweden.
    The influence of solar irradiation and solar wind conditions on heavy ion escape at MarsManuscript (preprint) (Other academic)
  • 23.
    Zhang, Qi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Wang, Xiao-Dong
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Nilsson, Hans
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna, Sweden.
    The influence of solar irradiation and solar wind conditions on heavy ion escape from Mars2023In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 128, no 10, article id e2023JA031828Article in journal (Refereed)
    Abstract [en]

    We apply a recently proposed method to estimate heavy ion escape from Mars. The method combines in situ observations with a hybrid plasma model, which treats ions as particles and electrons as a fluid. With this method, we investigate how solar upstream conditions, including solar extreme ultraviolet (EUV) radiation, solar wind dynamic pressure, and interplanetary magnetic field (IMF) strength and cone angle, affect the heavy ion loss. The results indicate that the heavy ion escape rate is greater in high EUV conditions. The escape rate increases with increasing solar wind dynamic pressure, and decreases as the IMF strength increases. The ion escape rate is highest when the solar wind is parallel to the IMF and lowest when they are perpendicular. The plume escape rate decreases when the solar wind convective electric field increases.

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