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  • 1.
    Markidis, Stefano
    et al.
    KTH, School of Computer Science and Communication (CSC), Centres, Centre for High Performance Computing, PDC.
    Henri, P.
    Lapenta, G.
    Divin, A.
    Goldman, M. V.
    Newman, D.
    Eriksson, S.
    Collisionless magnetic reconnection in a plasmoid chain2012In: Nonlinear processes in geophysics, ISSN 1023-5809, E-ISSN 1607-7946, Vol. 19, no 1, p. 145-153Article in journal (Refereed)
    Abstract [en]

    The kinetic features of plasmoid chain formation and evolution are investigated by two dimensional Particlein-Cell simulations. Magnetic reconnection is initiated in multiple X points by the tearing instability. Plasmoids form and grow in size by continuously coalescing. Each chain plasmoid exhibits a strong out-of plane core magnetic field and an out-of-plane electron current that drives the coalescing process. The disappearance of the X points in the coalescence process are due to anti-reconnection, a magnetic reconnection where the plasma inflow and outflow are reversed with respect to the original reconnection flow pattern. Anti-reconnection is characterized by the Hall magnetic field quadrupole signature. Two new kinetic features, not reported by previous studies of plasmoid chain evolution, are here revealed. First, intense electric fields develop in-plane normally to the separatrices and drive the ion dynamics in the plasmoids. Second, several bipolar electric field structures are localized in proximity of the plasmoid chain. The analysis of the electron distribution function and phase space reveals the presence of counter-streaming electron beams, unstable to the two stream instability, and phase space electron holes along the reconnection separatrices.

  • 2.
    Marklund, Göran T.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    On the ionospheric coupling of auroral electric fields2009In: Nonlinear processes in geophysics, ISSN 1023-5809, E-ISSN 1607-7946, Vol. 16, no 2, p. 365-372Article in journal (Refereed)
    Abstract [en]

    The quasi-static coupling of high-altitude potential structures and electric fields to the ionosphere is discussed with particular focus on the downward field-aligned current (FAC) region. Results are presented from a preliminary analysis of a selection of electric field events observed by Cluster above the acceleration region. The degree of coupling is here estimated as the ratio between the magnetic field-aligned potential drop,Delta Phi(II), as inferred from the characteristic energy of upward ion (electron) beams for the upward (downward) current region and the high-altitude perpendicular (to B) potential, Delta Phi(perpendicular to), as calculated by integrating the perpendicular electric field across the structure. For upward currents, the coupling can be expressed analytically, using the linear current-voltage relation, as outlined by Weimer et al. (1985). This gives a scale size dependent coupling where structures are coupled (decoupled) above (below) a critical scale size. For downward currents, the current-voltage relation is highly non-linear which complicates the understanding of how the coupling works. Results from this experimental study indicate that small-scale structures are decoupled, similar to small-scale structures in the upward current region. There are, however, exceptions to this rule as illustrated by Cluster results of small-scale intense electric fields, correlated with downward currents, indicating a perfect coupling between the ionosphere and Cluster altitude.

  • 3.
    Marklund, Göran T.
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Karlsson, Tomas
    KTH, Superseded Departments, Alfvén Laboratory.
    Figueiredo, Sonia
    KTH, Superseded Departments, Alfvén Laboratory.
    Johansson, Tommy
    KTH, Superseded Departments, Alfvén Laboratory.
    Lindqvist, Per-Arne
    KTH, Superseded Departments, Alfvén Laboratory.
    André, M.
    Buchert, S.
    Kistler, L. M.
    Fazakerley, A.
    Characteristics of quasi-static potential structures observed in the auroral return current region by Cluster2004In: Nonlinear processes in geophysics, ISSN 1023-5809, E-ISSN 1607-7946, Vol. 11, no 5-6, p. 709-720Article in journal (Refereed)
    Abstract [en]

    Temporal and spatial characteristics of intense quasi-static electric fields and associated electric potential structures in the return current region are discussed using Cluster observations at geocentric distances of about 5 Earth radii. Results are presented from four Cluster encounters with such acceleration structures to illustrate common as well as different features of such structures. The electric field structures are characterized by (all values are projected to 100 cm altitude) peak amplitudes of approximate to 1V/m, bipolar or unipolar profiles, perpendicular scale sizes of approximate to 10km, occurrence at auroral plasma boundaries associated with plasma density gradients, downward field-aligned currents of approximate to 10 muA/m(2), and upward electron beams with characteristic energies of a few hundred eV to a few keV. Two events illustrate he temporal evolution of bipolar, diverging electric field strictures, indicative of positive U-shaped potentials increasing in magnitude from less than 1 kV to a few kV on a few 100s time scale. This is also the typical formation time for ionospheric plasma cavities, which are connected to the potential structure and suggested to evolve hand-in-hand with these. In one of these events an energy decay of inverted-V ions was observed in the upward field-aligned current region prior to the acceleration potential increase in the adjacent downward current region, possibly suggesting that a potential redistribution took place between the two current branches. The other two events were characterized by intense unipolar electric fields, indicative of S-shaped potential contours and were encountered at the polar cap boundary. The total observation time for these events was typically 10-20 s, too short for monitoring the evolution of the structure, bui yet of interest for revealing their short term stability. The locations of the two bipolar events at the poleward boundary of the central plasma sheet and of the two unipolar events at the polar cap boundary, suggest that the special profile shape depends on whether plasma populations, dense enough to support upward field-aligned currents and closure of the return current, exist on both sides, or on one side only, of the boundary.

  • 4. Rypdal, K.
    et al.
    Paulsen, J. V.
    Garcia, O. E.
    Ratynskaia, Svetlana V.
    Demidov, V. I.
    Non-equilibrium quasi-stationary states in a magnetized plasma2003In: Nonlinear processes in geophysics, ISSN 1023-5809, E-ISSN 1607-7946, Vol. 10, no 02-jan, p. 139-149Article in journal (Refereed)
    Abstract [en]

    Non-equilibrium quasi-stationary states resulting from curvature driven interchange instabilities and driftwave instabilities in a low beta, weakly ionized, magnetized plasma are investigated in the context of laboratory experiments in a toroidal configuration. Analytic modelling, numerical simulations and experimental results are discussed with emphasis on identifying the unstable modes and understanding the physics of anomalous particle and energy fluxes and their linkage to self-organized pressure profiles.

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