| 1. |
- Bergman, Sofia, 1991-
(author)
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The effect of spacecraft charging on low-energy ion measurements around comet 67P/Churyumov-Gerasimenko
- 2020
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Licentiate thesis (other academic/artistic)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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| 2. |
- Bergman, Sofia, 1991-, et al.
(author)
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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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In: Journal of Geophysical Research - Space Physics. - : John Wiley & Sons. - 2169-9380 .- 2169-9402. ; 125:1
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Journal article (peer-reviewed)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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| 3. |
- Bergman, Sofia, 1991-, et al.
(author)
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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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In: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 125:4
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Journal article (peer-reviewed)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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| 4. |
- Nilsson, Hans, et al.
(author)
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Average cometary ion flow pattern in the vicinity of comet 67P from moment data
- 2020
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 498:4, s. 5263-5272
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Journal article (peer-reviewed)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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