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  • 1. Backrud-Ivgren, M.
    et al.
    Stenberg, Gabriella
    Umeå University, Faculty of Science and Technology, Physics.
    André, Mats
    Morooka, M.
    Hobara, Y.
    Joko, S.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Physics.
    Cluster observations and theoretical identification of broadband waves in the auroral region2005In: Annales Geophysicae, Vol. 23, p. 3739-3752Article in journal (Refereed)
  • 2.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Andre, M
    Norqvist, Patrik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    The importance of a dark ionosphere for ion heating and auroral arc formation2000In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 27, no 11, p. 1635-1638Article in journal (Refereed)
    Abstract [en]

    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.

  • 3.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marghitu, Octav
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Klecker, Berndt
    André, Mats
    Buchert, S
    Kistler, L
    McFadden, J
    Rème, H
    Vaivads, Andris
    Observations of concentrated generator regions in the nightside magnetosphere by Cluster/FAST conjunctions2006In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 24, p. 637-49Article in journal (Refereed)
  • 4.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Norqvist, Patrik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Fellgård, D
    The importance of solar illumination for discrete and diffuse aurora2005In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 23, p. 3481-3486Article in journal (Refereed)
  • 5.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Börlin, Niclas
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Vaivads, Andris
    GALS: gradient analysis by least squares2008In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 26, no 11, p. 3491-3499Article in journal (Refereed)
  • 6.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Vedin, Jörgen
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Börlin, Niclas
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Stenberg, Gabriella
    IRF, Kiruna.
    GALS: gradient analysis by least squares2006In: Proceedings of the 2006 AGU Fall Meeting, American Geophysical Union , 2006, p. SM53A-1442-SM53A-1442Conference paper (Other academic)
  • 7.
    Marghitu, Octav
    et al.
    Institute for Space Sciences, Bucharest, Romania.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Klecker, Berndt
    Max-Planck-Institut für extraterrestrische Physik, Garching, Germany.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Buchert, Stephan
    Swedish Institute of Space Physics, Uppsala, Sweden.
    Kistler, Lynn M.
    Space Science Center, University of New Hampshire, Durham, USA.
    André, Mats
    Swedish Institute of Space Physics, Uppsala, Sweden.
    Rème, Henri
    CESR-CNRS, Toulouse, France.
    Cluster observations of energy conversion regions in the plasma sheet2010In: The Cluster Active Archive: Studying the Earth's Space Plasma Environment / [ed] Harri Laakso, Matthew Taylor, C. Philippe Escoubet, Springer Netherlands , 2010, p. 453-459Conference paper (Refereed)
    Abstract [en]

    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.

  • 8. Markidis, Stefano
    et al.
    Henri, Pierre
    Lapenta, Giovanni
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Meliani, Zakaria
    Laure, Erwin
    The fluid-kinetic particle-in-cell method for plasma simulations2014In: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 271, p. 415-429Article in journal (Refereed)
    Abstract [en]

    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.

  • 9.
    Reiniusson, Annie
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Stenberg, Gabriella
    Umeå University, Faculty of Science and Technology, Physics.
    Norqvist, Patrik
    Umeå University, Faculty of Science and Technology, Physics.
    Eriksson, A.I.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Physics.
    Enhancement of electric and magnetic wave fields at density gradients2006In: Annales Geophysicae, Vol. 24, p. 367-79Article in journal (Refereed)
  • 10.
    Rönnmark, Kjell
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Auroral Electron acceleration by Alfven waves and electrostatic fields2000In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 105, no A11, p. 25333-25344Article in journal (Refereed)
    Abstract [en]

    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.

  • 11. Singh, A.K.
    et al.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Physics.
    Generation mechanism for VLF chorus emissions observed at a low latitude ground station2004In: Annales Geophysicae, Vol. 22, p. 2067-72Article in journal (Refereed)
  • 12.
    Vedin, Jörgen
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    A linear auroral current-voltage relation in fluid theory2004In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 22, no 5, p. 1719-1728Article in journal (Refereed)
    Abstract [en]

    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.

  • 13.
    Vedin, Jörgen
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Electron pressure effects on driven auroral Alfvén waves2005In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 110, no A01214, p. 11-Article in journal (Refereed)
    Abstract [en]

    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.

     

  • 14.
    Vedin, Jörgen
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Implementing a particle-fluid model of auroral electrons2007In: Applied Parallel Computing. State of the Art in Scientific Computing: 8th International Workshop, PARA 2006, Umeå, Sweden, June 18-21, 2006, Revised Selected Papers / [ed] Bo Kågström, Erik Elmroth, Jack Dongarra, Jerzy Wasniewski, Heidelberg: Springer Berlin/Heidelberg, 2007, p. 371-379Conference paper (Refereed)
    Abstract [en]

    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%.

  • 15.
    Vedin, Jörgen
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Parallel electric fields: variations in space and time on auroral field lines2008In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 74, no 1, p. 53-64Article in journal (Refereed)
    Abstract [en]

    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.

  • 16.
    Vedin, Jörgen
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Particle-fluid simulation of the auroral current circuit2006In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 111, no A12201, p. 12-Article in journal (Refereed)
    Abstract [en]

    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.

  • 17.
    Vedin, Jörgen
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bunescu, Costel
    Marghitu, Octav
    Estimating properties of concentrated parallel electric fields from electron velocity distributions2007In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 34, no 16, article id L16107Article in journal (Refereed)
    Abstract [en]

    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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