| 1. |
- Bergman, Sofia, 1991-, et al.
(författare)
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Ion bulk speeds and temperatures in the diamagnetic cavity of comet 67P from RPC-ICA measurements
- 2021
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 503:2, s. 2733-2745
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Tidskriftsartikel (refereegranskat)abstract
- Y 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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| 2. |
- Bergman, Sofia, 1991-
(författare)
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Low-energy ions around comet 67P/Churyumov-Gerasimenko
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Low-energy ions play important roles in the formation of the plasma environment around a comet. Reliable ways of measuring these ions are therefore of high importance to fully understand the processes and dynamics of this environment. Unfortunately, low-energy ions are infamously difficult to detect. A spacecraft interacts with the surrounding environment, which leads to an accumulation of charge on the spacecraft surface. As a result, the surface acquires an electrostatic potential with respect to the surrounding plasma, which can be either positive or negative. Low-energy ions are then attracted to or repelled from the charged surface before being detected by the instrument on board, resulting in an energy shift and change of travel direction of the ions. The Rosetta mission studied comet 67P/Churyumov-Gerasimenko during the years 2014-2016, and provided the most detailed observations of a comet and its environment to date. The Ion Composition Analyzer of the Rosetta Plasma Consortium (RPC-ICA) measured positive ions in the cometary environment with energies down to just a few eV. The low-energy part of the data is, however, difficult to interpret due to the distortions caused by the spacecraft potential. In this thesis, the Spacecraft Plasma Interaction Software (SPIS) is used to correct the low-energy ion measurements made by RPC-ICA for the effects introduced by the spacecraft potential. The distortion of the effective field of view is modelled for different ion energies and plasma environments, and the results are used to correct the flow direction of low-energy ions around the comet. The FOV distortion can be considered insignificant when the energy of the ions (in eV) is twice the value of the spacecraft potential (in volts). The FOV distortion at lower energies is geometry dependent, and varies substantially between different pixels of the instrument. The FOV distortion is furthermore dependent on the Debye length of the surrounding plasma. The knowledge obtained from the simulations is subsequently used to study the flow direction of low-energy ions in and around the diamagnetic cavity, a region where the magnetic field is essentially zero and low-energy ions are important for the dynamics. Evidence of counter-streaming ions are found, with ions flowing both radially away from and back towards the nucleus. SPIS is also used to model the influence of the spacecraft potential on the energy spectrum of the ions, and from this the bulk speed and temperature of the low-energy ions in the diamagnetic cavity were determined to 5-10 km/s and 0.7-1.6 eV, respectively. The bulk speed is significantly above the speed of the neutral particles, indicating a weak coupling between ions and neutrals in the diamagnetic cavity.
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| 3. |
- Bergman, Sofia, 1991-
(författare)
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The effect of spacecraft charging on low-energy ion measurements around comet 67P/Churyumov-Gerasimenko
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A spacecraft in space interacts with the surrounding environment and aqcuires an electrostatic potential. Charged particles are constantly bombarding the surface of the spacecraft, and at the same time solar EUV radiation induces photoemission, causing electrons to be emitted from the surface. The result is a transfer of charge between the environment and the spacecraft surface, and the surface charges to a positive or negative potential. The charged surface can cause interferences with scientific instruments on board. In this thesis, we investigate how spacecraft charging affects low-energy ion measurements. The Rosetta spacecraft visited comet 67P/Churyumov-Gerasimenko between the years 2014-2016. On board the spacecraft, the Ion Composition Analyzer (ICA) was measuring positive ions in the environment around the comet with the aim of investigating the interaction between cometary particles and the solar wind. Important for this interaction is ions with a low energy. Measuring these ions is, however, difficult due to the charged spacecraft surface. Rosetta was commonly charged to a negative potential, and consequently the measured positive ions were accelerated toward the surface before detection, affecting both their energy and travel direction. In this thesis, we study how the changed travel directions affected the effective field of view (FOV) of the instrument. We use the Spacecraft Plasma Interaction Software (SPIS) to simulate the spacecraft plasma interactions and the ion trajectories around the spacecraft. The results show that the FOV of ICA is severely distorted at low ion energies, but the distortion varies between different viewing directions of the instrument and is dependent on the properties of the surrounding plasma.
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| 4. |
- Bergman, Sofia, 1991-, et al.
(författare)
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The Influence of Spacecraft Charging on Low‐Energy Ion Measurements Made by RPC‐ICA on Rosetta
- 2020
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Ingår i: Journal of Geophysical Research - Space Physics. - : John Wiley & Sons. - 2169-9380 .- 2169-9402. ; 125:1
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Tidskriftsartikel (refereegranskat)abstract
- Spacecraft charging is problematic for low‐energy plasma measurements. The charged particles are attracted to or repelled from the charged spacecraft, affecting both the energy and direction of travel of the particles. The Ion Composition Analyzer (RPC‐ICA) on board the Rosetta spacecraft is suffering from this effect. RPC‐ICA was measuring positive ions in the vicinity of comet 67P/Churyumov‐Gerasimenko, covering an energy range of a few eV/q to 40 keV/q. The low‐energy part of the data is, however, heavily distorted by the negatively charged spacecraft. In this study we use the Spacecraft Plasma Interaction Software to model the influence of the spacecraft potential on the ion trajectories and the corresponding distortion of the field of view (FOV) of the instrument. The results show that the measurements are not significantly distorted when the ion energy corresponds to at least twice the spacecraft potential. Below this energy the FOV is often heavily distorted, but the distortion differs between different viewing directions. Generally, ions entering the instrument close to the aperture plane are less affected than those entering with extreme elevation angles.
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| 5. |
- Bergman, Sofia, 1991-, et al.
(författare)
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The Influence of Varying Spacecraft Potentials and Debye Lengths on In Situ Low-Energy Ion Measurements
- 2020
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 125:4
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Tidskriftsartikel (refereegranskat)abstract
- Low‐energy ions are difficult to measure, mainly due to spacecraft charging. The ions areattracted to or repelled from the charged surface prior to detection, which changes both the energy andtravel direction of the ions. This results in distortions of the data, and the changed travel directions distort the effective field of view (FOV) of the instrument performing the measurements. The ion composition analyzer (RPC‐ICA) was measuring positive ions down to an energy of a few eV around comet67P/Churyumov‐Gerasimenko. Low‐energy ions play important parts in processes in the cometary environment, but the FOV of RPC‐ICA has been shown to get severely distorted at low ion energies. Several factors are believed to affect the distortion level. In this study we use the Spacecraft Plasma Interaction Software (SPIS) to investigate the influence of varying spacecraft potentials and Debye lengths on the FOV distortion of RPC‐ICA. We show that the distortion level is dependent on the Debye length of the surrounding plasma, but the sensitivity varies substantially between different viewing directions of the instrument. We also show that a small nonlinearity exists in the relation between FOV distortion, ion energy, and spacecraft potential, mainly caused by the photoemission and bulk flow of the cometary plasma.
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| 6. |
- Canu Blot, Romain, et al.
(författare)
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Upper limit of the solar wind protons backscattering efficiency from Comet 67P/Churyumov-Gerasimenko
- 2024
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 683
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Tidskriftsartikel (refereegranskat)abstract
- 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. |
- Nilsson, Hans, et al.
(författare)
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Average cometary ion flow pattern in the vicinity of comet 67P from moment data
- 2020
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 498:4, s. 5263-5272
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Tidskriftsartikel (refereegranskat)abstract
- Average flow patterns of ions around comet 67P detected by the RPC-ICA instrument onboard Rosetta are presented both as a time series and as a spatial distribution of the average flow in the plane perpendicular to the comet - Sun direction (Y-Z plane in the coordinate systems used). Cometary ions in the energy range up to 60 eV flow radially away from the nucleus in the Y-Z plane, irrespective of the direction of the magnetic field, throughout the mission. These ions may however be strongly affected by the spacecraft potential, the uncertainty due to this is briefly discussed. Inside the solar wind ion cavity and in the periods just before and after, the cometary pick up ions moving antisunward are deflected against the inferred solar wind electric field direction. This is opposite to what is observed for lower levels of mass-loading. These pick up ions are behaving in a similar way to the solar wind ions and are deflected due to mass-loading. A spatial asymmetry can be seen in the observations of deflected pick up ions, with motion against the electric field primarily within a radius of 200 km of the nucleus and also in the negative electric field hemisphere. Cometary ions observed by RPC-ICA typically move in the antisunward direction throughout the mission. These are average patterns, full-resolution data show very much variability.
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| 8. |
- Nilsson, Hans, et al.
(författare)
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Birth of a comet magnetosphere : A spring of water ions
- 2015
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 347:6220
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Tidskriftsartikel (refereegranskat)abstract
- The Rosetta mission shall accompany comet 67P/Churyumov-Gerasimenko from a heliocentric distance of >3.6 astronomical units through perihelion passage at 1.25 astronomical units, spanning low and maximum activity levels. Initially, the solar wind permeates the thin comet atmosphere formed from sublimation, until the size and plasma pressure of the ionized atmosphere define its boundaries: A magnetosphere is born. Using the Rosetta Plasma Consortium ion composition analyzer, we trace the evolution from the first detection of water ions to when the atmosphere begins repelling the solar wind (~3.3 astronomical units), and we report the spatial structure of this early interaction. The near-comet water population comprises accelerated ions (
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| 9. |
- Nilsson, Hans, et al.
(författare)
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Evolution of the ion environment of comet 67P during the Rosetta mission as seen by RPC-ICA
- 2017
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 469:Suppl_2, s. S252-S261
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Nilsson, Hans, et al.
(författare)
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Solar cycle variation of ion escape from Mars
- 2023
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Ingår i: Icarus. - : Academic Press. - 0019-1035 .- 1090-2643. ; 393
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Tidskriftsartikel (refereegranskat)abstract
- 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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