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  • 1.
    Brandenburg, Axel
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy.
    Haugen, Nils Erland L.
    Babkovskaia, Natalia
    Turbulent front speed in the Fisher equation: Dependence on Damkohler number2011In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 83, no 1, p. 016304-Article in journal (Refereed)
    Abstract [en]

    Direct numerical simulations and mean-field theory are used to model reactive front propagation in a turbulent medium. In the mean-field approach, memory effects of turbulent diffusion are taken into account to estimate the front speed in cases in which the Damkohler number is large. This effect is found to saturate the front speed to values comparable with the speed of the turbulent motions. By comparing with direct numerical simulations, it is found that the effective correlation time is much shorter than for nonreacting flows. The nonlinearity of the reaction term is found to make the front speed slightly faster.

  • 2. Dieckmann, M. E.
    et al.
    Frederiksen, Jacob
    Stockholm University, Faculty of Science, Department of Astronomy.
    Bret, A.
    Shukla, P. K.
    Evolution of the fastest-growing relativistic mixed mode instability driven by a tenuous plasma beam in one and two dimensions2006In: Physics of Plasmas, ISSN 1070-664X, Vol. 13, no 11, p. 112110-Article in journal (Refereed)
    Abstract [en]

    Particle-in-cell simulations confirm here that a mixed plasma mode is the fastest growing when a highly relativistic tenuous electron-proton beam interacts with an unmagnetized plasma. The mixed modes grow faster than the filamentation and two-stream modes in simulations with beam Lorentz factors Gamma of 4, 16, and 256, and are responsible for thermalizing the electrons. The mixed modes are followed to their saturation for the case of Gamma=4 and electron phase space holes are shown to form in the bulk plasma, while the electron beam becomes filamentary. The initial saturation is electrostatic in nature in the considered one- and two-dimensional geometries. Simulations performed with two different particle-in-cell simulation codes evidence that a finite grid instability couples energy into high-frequency electromagnetic waves, imposing simulation constraints.

  • 3. Fine, B. V.
    et al.
    Elsayed, T. A.
    Kropf, C. M.
    de Wijn, Astrid S.
    Stockholm University, Faculty of Science, Department of Physics.
    Absence of exponential sensitivity to small perturbations in nonintegrable systems of spins 1/22014In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 89, no 1, p. 012923-Article in journal (Refereed)
    Abstract [en]

    We show that macroscopic nonintegrable lattices of spins 1/2, which are often considered to be chaotic, do not exhibit the basic property of classical chaotic systems, namely, exponential sensitivity to small perturbations. We compare chaotic lattices of classical spins and nonintegrable lattices of spins 1/2 in terms of their magnetization responses to an imperfect reversal of spin dynamics known as Loschmidt echo. In the classical case, magnetization is exponentially sensitive to small perturbations with a characteristic exponent equal to twice the value of the largest Lyapunov exponent of the system. In the case of spins 1/2, magnetization is only power-law sensitive to small perturbations.

  • 4.
    Neau, Anita
    Stockholm University.
    Dissociative processes of relevance in low temperature plasmas2002Doctoral thesis, comprehensive summary (Other academic)
  • 5. Paardekooper, Sijme-Jan
    et al.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy.
    Halting type I planet migration in non-isothermal disks2006In: Astronomy and Astrophysics, Vol. 459, no 1, p. L17-L20Article in journal (Refereed)
    Abstract [en]

    Aims: We investigate the effect of including a proper energy balance on the interaction of a low-mass planet with a protoplanetary disk.

    Methods: We use a three-dimensional version of the RODEO method to perform hydrodynamical simulations including the energy equation. Radiation is included in the flux-limited diffusion approach.

    Results: The sign of the torque is sensitive to the ability of the disk to radiate away the energy generated in the immediate surroundings of the planet. In the case of high opacity, corresponding to the dense inner regions of protoplanetary disks, migration is directed outward, instead of the usual inward migration that was found in locally isothermal disks. For low values of the opacity we recover inward migration and show that torques originating in the coorbital region are responsible for the change in migration direction.

  • 6.
    Scharmer, Göran
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Institutet för solfysik.
    Spruit, Henk
    Magnetostatic penumbra models with field-free gaps2006In: Astronomy & Astrophysics, ISSN 0004-6361, Vol. 460, no 2, p. 605-615Article in journal (Refereed)
    Abstract [en]

    We present numerical 2D magnetostatic models for sunspot penumbrae consisting of radially aligned field-free gaps in a potential magnetic field, as proposed by Spruit & Scharmer (2006, A&A, 447, 343). The shape of the gaps and the field configurations around them are computed consistently from the condition of magnetostatic pressure balance between the gap and the magnetic field. The results show that field-free gaps in the inner penumbra are cusp-shaped and bounded by a magnetic field inclined by about 70° from the vertical. Here, the magnetic component has a Wilson depression on the order 200-300 km relative to the top of the field-free gap; the gaps should thus appear as noticeably elevated features. This structure explains the large variations in field strength in the inner penumbra inferred from magnetograms and two-component inversions, and the varying appearance of the inner penumbra with viewing angle. In the outer penumbra, on the other hand, the gaps are flat-topped with a horizontal magnetic field above the middle of the gap. The magnetic field has large inclination variations horizontally, but only small fluctuations in field strength, in agreement with observations.

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