| 1. |
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| 2. |
- Backrud-Ivgren, Marie, et al.
(författare)
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Cluster observations and theoretical identification of broadband waves in the auroral region
- 2005
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 23:12, s. 3739-3752
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Tidskriftsartikel (refereegranskat)abstract
- Broadband waves are common on auroral field lines. We use two different methods to study the polarization of the waves at 10 to 180 Hz observed by the Cluster spacecraft at altitudes of about 4 Earth radii in the nightside auroral region. Observations of electric and magnetic wave fields, together with electron and ion data, are used as input to the methods. We find that much of the wave emissions are consistent with linear waves in homogeneous plasma. Observed waves with a large electric field perpendicular to the geomagnetic field are more common (electrostatic ion cyclotron waves), while ion acoustic waves with a large parallel electric field appear in smaller regions without suprathermal (tens of eV) plasma. The regions void of suprathermal plasma are interpreted as parallel potential drops of a few hundred volts.
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| 3. |
- Eriksson, Jonas, et al.
(författare)
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Rhinitis phenotypes correlate with different symptom presentation and risk factor patterns of asthma
- 2011
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Ingår i: Respiratory Medicine. - London : Baillière Tindall. - 0954-6111 .- 1532-3064. ; 105:11, s. 1611-1621
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Tidskriftsartikel (refereegranskat)abstract
- Background: Asthma and rhinitis frequently coexist, but no population study has previously determined the relationship between nasal comorbidities and symptom expression and risk factors of asthma.Methods: In 2008, a postal questionnaire on respiratory health was sent to 30 000 randomly selected subjects aged 16-75 years in West Sweden; 29218 could be traced and 18 087 (62%) responded. The questionnaire included questions on asthma, rhinitis, chronic rhinosinusitis, respiratory symptoms and possible determinants.Results: Prevalence of allergic rhinitis in asthma was 63.9% and of asthma in allergic rhinitis 19.8%. Prevalence of chronic rhinosinusitis in asthma was 8.4% and of asthma in chronic rhinosinusitis 24.4%. Asthma subjects with chronic rhinitis, or chronic rhinosinusitis, had more symptoms of asthma and bronchitis than those without rhinitis (p < 0.001). There was an obvious trend of higher ORs for various environmental exposures including occupational exposure to dust, gases and fumes (OR 2.32 vs. OR 1.44), visible mould at home (OR 1.72 vs. OR 1.27) and water damage at home (OR 1.82 vs. OR 1.06) for asthma with chronic rhinosinusitis than for asthma with allergic rhinitis. Family history of allergy yielded a higher OR for asthma with allergic rhinitis than with asthma with chronic rhinosinusitis (OR 7.15 vs. OR 4.48).Conclusion: Considerable overlap between asthma and nasal comorbidities was documented, confirming a close relationship between nasal disease and asthma. Allergic rhinitis, chronic rhinitis and chronic rhinosinusitis were associated with different risk factor patterns and symptom expression of asthma. Thus, different nasal comorbidities may reflect different phenotypes of asthma. (C) 2011 Elsevier Ltd. All rights reserved.
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| 4. |
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| 5. |
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| 6. |
- Hamrin, Maria, et al.
(författare)
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GALS : gradient analysis by least squares
- 2006
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Ingår i: Proceedings of the 2006 AGU Fall Meeting. - : American Geophysical Union. ; , s. SM53A-1442-SM53A-1442
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 7. |
- Hamrin, Maria, et al.
(författare)
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GALS - Gradient Analysis by Least Squares
- 2008
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 26:11, s. 3491-3499
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Tidskriftsartikel (refereegranskat)abstract
- We present a method, GALS (Gradient Analysis by Least Squares) for estimating the gradient of a physical field from multi-spacecraft observations. To obtain the best possible spatial resolution, the gradient is estimated in the frame of reference where structures in the field are essentially locally stationary. The estimates are refined iteratively by a least squares method. We show that GALS is not very sensitive to the spacecraft configuration and resolves structures much smaller than the characteristic size of the spacecraft distribution. Furthermore, GALS requires little user input. GALS has been tested on synthetic magnetic field data and data from the Cluster FGM instrument. GALS will also be useful for other types of data. The results indicate that GALS is robust and superior to the curlometer method for estimating the current from magnetic field measurements.
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| 8. |
- Hamrin, Maria, et al.
(författare)
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Observations of concentrated generator regions in the nightside magnetosphere by Cluster/FAST conjunctions
- 2006
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 24, s. 637-49
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Tidskriftsartikel (refereegranskat)abstract
- Here and in the companion paper, Marghitu et al. (2006), we investigate plausible auroral generator regions in the nightside auroral magnetosphere. In this article we use magnetically conjugate data from the Cluster and the FAST satellites during a 3.5-h long event from 19-20 September 2001. Cluster is in the Southern Hemisphere close to apogee, where it probes the plasma sheet and lobe at an altitude of about 18 RE. FAST is below the acceleration region at approximately 0.6 RE. Searching for clear signatures of negative power densities, E(.)J < O, in the Cluster data we can identify three concentrated generator regions (CGRs) during our event. From the magnetically conjugate FAST data we see that the observed generator regions in the Cluster data correlate with auroral precipitation. The downward Poynting flux observed by Cluster, as well as the scale size of the CGRs, are consistent with the electron energy flux and the size of the inverted-V regions observed by FAST. To our knowledge, these are the first in-situ observations of the crossing of an auroral generator region. The main contribution to E(.)J < O comes from the GSE E(y)J(y). The electric field E-y is weakly negative during most of our entire event and we conclude that the CGRs occur when the duskward current J(y) grows large and positive. We find that our observations are consistent with a local southward expansion of the plasma sheet and/or rather complicated, 3-D wavy structures propagating over the Cluster satellites. We find that the plasma is working against the magnetic field, and that kinetic energy is being converted into electromagnetic energy. Some of the energy is transported away as Poynting flux.
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| 9. |
- Hamrin, Maria, et al.
(författare)
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The importance of a dark ionosphere for ion heating and auroral arc formation
- 2000
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 27:11, s. 1635-1638
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Tidskriftsartikel (refereegranskat)abstract
- We present observations from the Freja Satellite to show that density reductions and ion heating at Freja heights are anticorrelated with solar illumination of the ionosphere. When the ionospheric foot-point of a flux-tube is in shadow, the ambient density is lower, transverse ion energization is more common and more intense, and the associated density cavities are deeper. In combination with the suggestion that the electrons must be accelerated to keV energies to carry an imposed current in a low density auroral cavity, these observations may explain the recent observation that auroras are more common when the ionosphere below is in darkness.
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| 10. |
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| 11. |
- Marghitu, Octav, et al.
(författare)
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Cluster observations of energy conversion regions in the plasma sheet
- 2010
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Ingår i: The Cluster Active Archive. - Dordrecht : Springer Netherlands. - 9789048134984 - 9789048134991 ; , s. 453-459
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Konferensbidrag (refereegranskat)abstract
- Cluster allows for the first time a systematic examination of energy conversion, by the evaluation of the power density, E · J, with E the electric field and J the current density. Following a careful inspection of the Cluster plasma sheet data from the summer and fall of 2001, we selected 43 energy conversion regions (ECRs), out of which 26 concentrated load regions (CLRs, E · J > 0) and 17 concentrated generator regions (CGRs, E · J < 0). As expected in the tail, at about 19 RE geocentric distance, the energy conversion is more intense for CLRs, on average some 25 pW∕m3, compared to some 5 pW∕m3 for CGRs. The CLRs are located closer to the neutral sheet and dominated by E and J in the GSE y direction, unlike the CGRs, that prefer locations towards the plasma sheet boundary layer, where the deviations of E and J from the GSE y direction can be significant. The ECRs are often associated with high speed bulk flows, on average faster and hotter for CLRs. The CLRs appear to be associated also with density drop and sometimes with temperature anisotropy, T∥ > T⊥, features which are observed less frequently for CGRs.
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| 12. |
- Markidis, Stefano, et al.
(författare)
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The Fluid-Kinetic Particle-in-Cell method for plasma simulations
- 2014
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Ingår i: Journal of Computational Physics. - San Diego : Elsevier BV. - 0021-9991 .- 1090-2716. ; 271, s. 415-429
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Tidskriftsartikel (refereegranskat)abstract
- A method that solves concurrently the multi-fluid and Maxwell's equations has been developed for plasma simulations. By calculating the stress tensor in the multi-fluid momentum equation by means of computational particles moving in a self-consistent electromagnetic field, the kinetic effects are retained while solving the multi-fluid equations. The Maxwell's and multi-fluid equations are discretized implicitly in time enabling kinetic simulations over time scales typical of the fluid simulations. The Fluid-Kinetic Particle-in-Cell method has been implemented in a three-dimensional electromagnetic code, and tested against the two-stream instability, the Weibel instability, the ion cyclotron resonance and magnetic reconnection problems. The method is a promising approach for coupling fluid and kinetic methods in a unified framework.
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| 13. |
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| 14. |
- Rönnmark, Kjell, et al.
(författare)
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Auroral Electron acceleration by Alfven waves and electrostatic fields
- 2000
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 105:A11, s. 25333-25344
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Tidskriftsartikel (refereegranskat)abstract
- We present a two-dimensional numerical model for the formation of discrete auroral arcs. This model describes the evolution of shear Alfven waves generated by a growing force near the equatorial plane, and the transition to electrostatic fields when the force becomes stationary. The parallel electric fields on auroral field lines may be regarded as shear Alfven waves driven by a magnetospheric generator at zero frequency. In our collisionless model, precipitating auroral electrons are accelerated to an energy of 350 eV when the upward current is 3.1 mu Am-2. We also find that the electrostatic potential drop is proportional to the square of the current density.
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| 15. |
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| 16. |
- Vedin, Jörgen, et al.
(författare)
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A linear auroral current-voltage relation in fluid theory
- 2004
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Ingår i: Annales Geophysicae. - Paris : Gauthier-Villars. - 0992-7689 .- 1432-0576. ; 22:5, s. 1719-1728
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Tidskriftsartikel (refereegranskat)abstract
- Progress in our understanding of auroral currents and auroral electron acceleration has for decades been hampered by an apparent incompatibility between kinetic and fluid models of the physics involved. A well established kinetic model predicts that steady upward field-aligned currents should be linearly related to the potential drop along the field line, but collisionless fluid models that reproduce this linear current-voltage relation have not been found. Using temperatures calculated from the kinetic model in the presence of an upward auroral current, we construct here approximants for the parallel and perpendicular temperatures. Although our model is rather simplified, we find that the fluid equations predict a realistic large-scale parallel electric field and a linear current-voltage relation when these approximants are employed as nonlocal equations of state. This suggests that the concepts we introduce can be applied to the development of accurate equations of state for fluid simulations of auroral flux tubes.
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| 17. |
- Vedin, Jörgen, et al.
(författare)
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Electron pressure effects on driven auroral Alfvén waves
- 2005
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Ingår i: Journal of Geophysical Research. - Washington : American Geophysical Union.. - 0148-0227 .- 2156-2202. ; 110:A01214, s. 11-
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Tidskriftsartikel (refereegranskat)abstract
- Fluid models for the auroral electron acceleration processes have almost exclusively been derived by assuming cold or isothermal electrons. The consequences of these assumptions have never been thoroughly analyzed. In this study we compare results from an isothermal simulation with those obtained when the pressure is calculated from a double adiabatic approximation and from stationary kinetic theory. We find that the reflection of shear Alfvén waves, as well as the current-voltage relation, is very sensitive to the description of the electron pressure variations. Using pressures calculated from steady-state kinetic theory, we find that driven shear Alfvén waves can build up auroral currents and fields that are consistent with a linear current-voltage relation.
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| 18. |
- Vedin, Jörgen, et al.
(författare)
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Electrostatic potentials in the downward auroral current region
- 2005
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Ingår i: Journal of Geophysical Research. - Washington : American Geophysical Union. - 0148-0227 .- 2156-2202. ; 110:A08207, s. 1-9
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Tidskriftsartikel (refereegranskat)abstract
- Assuming a fixed ion density, adiabatic electron motion, and quasi-neutrality, we use the stationary Vlasov equation to derive the self-consistent potential in an auroral flux tube that carries downward current. Our model predicts downward electric fields ∼5 mV/m at an altitude near 2000 km, and around 4000 km the potential reaches ∼2.5 kV. A weak upward electric field at high altitudes reduces the potential, and the potential difference between the ionosphere and magnetosphere is much smaller.
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| 19. |
- Vedin, Jörgen, et al.
(författare)
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Estimating properties of concentrated parallel electric fields from electron velocity distributions
- 2007
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Ingår i: Geophysical Research Letters. - Washington : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 34:16
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Tidskriftsartikel (refereegranskat)abstract
- Information about the magnitude of the field-aligned potential drop along auroral field lines is usually derived from the velocity distribution of the particles. When the electrons are accelerated by a strong double layer their velocity distribution will have features different from those produced by a weak, spread-out, electric field. Quantifying these features, we obtain information about the strength and thickness of the double layer.
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| 20. |
- Vedin, Jörgen, et al.
(författare)
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Implementing a particle-fluid model of auroral electrons
- 2007
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Ingår i: Applied Parallel Computing. State of the Art in Scientific Computing. - Heidelberg : Springer Berlin/Heidelberg. - 9783540757542 ; , s. 371-379
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Konferensbidrag (refereegranskat)abstract
- The particle-fluid model of auroral electrons that is presented in [1] is a major step forward within the field of dynamic models of the auroral generation mechanisms. The model is, however, also an example where the implementation of a physical model requires a lot of knowledge from the field of computer science. Therefore, this paper contains a detailed description of the implementation behind the particle-fluid model. We present how the particles are implemented in doubly linked lists, how the fluid equations are solved in a time-efficient algorithm, and how these two parts are coupled into a single framework. We also describe how the code is parallelized with an efficiency of nearly 100%.
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| 21. |
- Vedin, Jörgen, 1978-
(författare)
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Numerical modeling of auroral processes
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- One of the most conspicuous problems in space physics for the last decades has been to theoretically describe how the large parallel electric fields on auroral field lines can be generated. There is strong observational evidence of such electric fields, and stationary theory supports the need for electric fields accelerating electrons to the ionosphere where they generate auroras. However, dynamic models have not been able to reproduce these electric fields. This thesis sheds some light on this incompatibility and shows that the missing ingredient in previous dynamic models is a correct description of the electron temperature. As the electrons accelerate towards the ionosphere, their velocity along the magnetic field line will increase. In the converging magnetic field lines, the mirror force will convert much of the parallel velocity into perpendicular velocity. The result of the acceleration and mirroring will be a velocity distribution with a significantly higher temperature in the auroral acceleration region than above. The enhanced temperature corresponds to strong electron pressure gradients that balance the parallel electric fields. Thus, in regions with electron acceleration along converging magnetic field lines, the electron temperature increase is a fundamental process and must be included in any model that aims to describe the build up of parallel electric fields. The development of such a model has been hampered by the difficulty to describe the temperature variation. This thesis shows that a local equation of state cannot be used, but the electron temperature variations must be descibed as a nonlocal response to the state of the auroral flux tube. The nonlocal response can be accomplished by the particle-fluid model presented in this thesis. This new dynamic model is a combination of a fluid model and a Particle-In-Cell (PIC) model and results in large parallel electric fields consistent with in-situ observations.
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| 22. |
- Vedin, Jörgen, et al.
(författare)
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Parallel electric fields : variations in space and time on auroral field lines
- 2008
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Ingår i: Journal of Plasma Physics. - : Cambridge University Press. - 0022-3778 .- 1469-7807. ; 74:1, s. 53-64
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Tidskriftsartikel (refereegranskat)abstract
- We present results from a particle–fluid simulation of auroral electrons and discuss the distribution of parallel electric fields along auroral field lines and the processes occurring during the build up of these electric fields. Neglecting field-aligned ion dynamics, the main potential drop has a width of about 5000, km and is centered at an altitude of roughly 5000, km. We find that the gradient in the potential becomes steeper and the build up of the potential drop becomes faster if the temperature of the magnetospheric electrons is lower.
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| 23. |
- Vedin, Jörgen, et al.
(författare)
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Particle-fluid simulation of the auroral current circuit
- 2006
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Ingår i: Journal of Geophysical Research. - Washington : American Geophysical Union.. - 0148-0227 .- 2156-2202. ; 111:A12201, s. 12-
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Tidskriftsartikel (refereegranskat)abstract
- The incompatibility between stationary kinetic and dynamic fluid descriptions of auroral electron acceleration has been an outstanding problem in space physics for decades. In this study we introduce a new numerical simulation model that provides a unified picture by including electron temperature variations consistent with collisionless kinetic theory in the fluid description. We demonstrate that this new particle-fluid model can describe the partial reflection of Alfvén waves from the acceleration region, as well as the formation of a field-aligned potential drop proportional to the upward current. This study also suggests that for example ion dynamics and high-frequency waves must be added to the model before it properly can describe the return current region. Simulations based on the particle-fluid concept can be applied to various processes in space physics and astrophysics where strong currents flowing along an inhomogeneous magnetic field will cause temperature increases and field-aligned electric fields.
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