| 1. |
- Futaana, Yoshifumi, et al.
(author)
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Corotation Plasma Environment Model : An Empirical Probability Model of the Jovian Magnetosphere
- 2018
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In: IEEE Transactions on Plasma Science. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 0093-3813 .- 1939-9375. ; 46:6, s. 2126-2145
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Journal article (peer-reviewed)abstract
- We developed a new empirical model for corotating plasma in the Jovian magnetosphere. The model, named the corotation plasma environment model version 2 (CPEMv2), considers the charge density, velocity vector, and ion temperature based on Galileo/plasma system (PLS) ion data. In addition, we develop hot electron temperature and density models based on Galileo/PLS electron data. All of the models provide respective quantities in the magnetic equator plane of 9-30RJ, while the charge density model can be extended to 3-D space. A characteristic feature of the CPEM is its support of the percentile as a user input. This feature enables us to model extreme conditions in addition to normal states. In this paper, we review the foundations of the new empirical model, present a general derivation algorithm, and offer a detailed formulation of each parameter of the CPEMv2. As all CPEM parameters are of the analytical form, their implementation is straightforward, and execution involves the use of a small number of computational resources. The CPEM is flexible; for example, it can be extended, as new data (from observations or simulation results) become available. The CPEM can be used for the mission operation of the European Space Agency's mission to Jupiter, JUpiter ICy moons Explorer (JUICE), and for future data analyses.
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| 2. |
- Mannesson, Karin, 1980-, et al.
(author)
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Abnormal grain growth in cemented carbides - Experiments and simulations
- 2011
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In: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368. ; 29:4, s. 488-494
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Journal article (peer-reviewed)abstract
- The grain size and the grain size distribution are two of the most important factors when tailoring the mechanical properties of cemented carbides. In the present work the effect on the growth behavior when adding some abnormal grains in an initial fine grained powder is studied. It is clearly seen that abnormal grains in a fine grained matrix lead to faster grain growth and a higher average grain size.
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| 3. |
- Mannesson, Karin, 1980-, et al.
(author)
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Carbide grain growth : the effect of grain size distributions
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Other publication (other academic/artistic)abstract
- It is important to understand the WC grain growth behavior in order to obtain good mechanical properties when manufacturing cemented carbides. In the present study the WC grain growth for dierent initial grain size distributions during sintering is studied theoretically using a model based on three processes; 2-D nucleation of atomic planes, long-range diusion and interface friction. Simulations clearly show that the initial grain size distribution inuences the growth behavior significantly. Bimodal powders show a more dramatic grain growth than powders having a lognormalgrain size distribution.
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| 4. |
- Mannesson, Karin, 1980-
(author)
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WC grain growth during sintering of cemented carbides : Experiments and simulations
- 2011
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Doctoral thesis (other academic/artistic)abstract
- Cemented carbides are composite materials consisting of a hard carbide and a ductile binder. They are powdermetallurgically manufactured, where liquid-phase sintering is one of the main steps. The most common cemented carbide consists of WC and Co and it is widely used for cutting tools. Two of the most important factors controlling the mechanical properties are the WC grain size and the grain size distribution and thus it is of great interest to understand the grain growth behavior. In this thesis the grain growth during sintering at 1430 °C is studied both experimentally and through computer simulations. The grain growth behavior in cemented carbides cannot be explained from the classical LSW-theory. The WC grains have a faceted shape necessitating growth by 2-D nucleation of new atomic layers or surface defects. A new model based on 2-D nucleation, long-range diffusion and interface friction is formulated. Three powders having different average sizes are studied and both experiments and simulations show that a fine-grained powder may grow past a coarse-grained powder, indicating that abnormal grain growth has taken place in the fine-grained powder. Fine-grained powders with various fractions of large grains are also studied and it is seen that a faster growth is obtained with increasing fraction of large grains and that an initially slightly bimodal powder can approach the logaritmic normal distribution after long sintering times. The grain size measurements are performed on 2-D sections using image analysis on SEM images or EBSD analysis. Since the growth model is based on 3-D size distributions the 2-D size distributions have to be transformed to 3-D, and a new method, Inverse Saltykov, is proposed. The 2-D size distribution is first represented with kernel estimators and the 3-D size distribution is optimized in an iterative manner. In this way both negative values in the 3-D size distribution and modifications of the raw data are avoided.
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| 5. |
- Ågren, Karin, 1980-, et al.
(author)
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Detection of negative ions in the deep ionosphere of Titan during the Cassini T70 flyby
- 2012
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 39, s. L10201-
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Journal article (peer-reviewed)abstract
- We present radio and plasma wave science (RPWS) Langmuir probe (LP) observations that give evidence for a population of heavy, negative ions at altitudes below 900 km in Titan's ionosphere during the Cassini T70 flyby. The negative ion density in this region is comparable to, or higher than, the electron density of 760 cm(-3). Both positive and negative ions are moving with a velocity of at least a few hundred m s(-1) relative to Titan. We show two limiting cases where we have analysed RPWS/LP ion measurements. The data can be interpreted as either that a population of negative ions with density comparable to the electron density is present, moving at a very high (>2 km s(-1)) velocity, or that the ion population is moving at a few hundred m s(-1), but with a density an order of magnitude larger than the electron density in the same region.
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| 6. |
- Ågren, Karin, 1980-
(author)
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On the Formation and Structure of the Ionosphere of Titan
- 2012
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Doctoral thesis (other academic/artistic)abstract
- We present results on the ionospheric structure around Titan observed during numerous deep (<1000 km) flybys by the Cassini spacecraft. Our results are based on measurements by the radio and plasma wave science instrument, in particular the Langmuir probe. In addition, data from the magnetometer and electron spectrometer have contributed.The ionosphere of Titan is created when the atmosphere of the moon becomes ionised. There are several mechanisms that contribute to this, the most important of which are considered to be photoionisation by EUV from the Sun with associated photoelectron ionisation, and particle impact ionisation by electrons and ions from Saturn’s corotating magnetosphere.We investigate the influence of the solar zenith angle on the electron number density at the ionospheric peak. The results show on average four times more plasma on the dayside compared to the nightside, with typical densities of 2500 – 3500 cm-3 and 400 – 1000 cm-3, respectively. In a complementary study, we make a case study of a nightside flyby and show that the altitude structure of the deep ionosphere is reproducible by a simple electron impact ionisation model. Taken together, this leads to the conclusion that solar photons are the main ionisation source of the dayside ionosphere. However, magnetospheric particle precipitation also contributes and can explain the electron densities seen on the nightside.As Titan does not exhibit any large intrinsic magnetic field, the fact that it is embedded in the magnetosphere of Saturn means that the Kronian field drapes around the moon and gives rise to an induced magnetosphere. We show that there are currents of the order of 10 – 100 nA m-2 flowing in the ionosphere of the moon. Associated with the currents are perpendicular electric fields ranging from 0.5 to 3 µV m-1.Finally, we investigate measurements obtained during T70, the deepest Titan flyby performed to date. We show that there is a substantial amount of negative ions present below an altitude of 900 km. This confirms previous result by the electron spectrometer, showing negative ions at higher altitudes in Titan’s ionosphere.
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