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Vlasis A.,Catholic University of Leuven | Vlasis A.,Leuven Mathematical Modeling and Computational Science Center | van Eerten H.J.,New York University | Meliani Z.,Catholic University of Leuven | And 5 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

Strong optical and radio flares often appear in the afterglow phase of gamma-ray bursts (GRBs). It has been proposed that colliding ultrarelativistic shells can produce these flares. Such consecutive shells can be formed due to the variability in the central source of a GRB. We perform high-resolution 1D numerical simulations of late collisions between two ultrarelativistic shells in order to explore these events. We examine the case where a cold uniform shell collides with a self-similar Blandford & McKee shell in a constant density environment and consider cases with different Lorentz factor and energy for the uniform shell. We produce the corresponding on-axis light curves and emission images for the afterglow phase and examine the occurrence of optical and radio flares, assuming a spherical explosion and a hard-edged jet scenario. For our simulations, we use the Adaptive Mesh Refinement version of the Versatile Advection Code coupled to a linear radiative transfer code to calculate synchrotron emission. We find steeply rising flares like the behaviour of small jet opening angles and more gradual rebrightenings for large opening angles. Synchrotron self-absorption is found to strongly influence the onset and shape of the radio flare. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Lapenta G.,Catholic University of Leuven | Lapenta G.,Leuven Mathematical Modeling and Computational Science Center | Markidis S.,Catholic University of Leuven | Divin A.,Catholic University of Leuven | And 2 more authors.
Physics of Plasmas | Year: 2010

We analyze the signatures of component reconnection for a Harris current sheet with a guide field using the physical mass ratio of hydrogen. The study uses the fully kinetic particle in cell code IPIC3D to investigate the scaling with mass ratio of the following three main component reconnection features: electron density cavities along the separatrices, channels of fast electron flow within the cavities, and electron phase space holes due to the Buneman instability in the electron high speed channels. The width and strength of the electron holes and of the electron cavities are studied up the mass ratio proper of hydrogen, considering the effect of the simulation box size, and of the boundary conditions. The results compare favorably with the existing data from the Cluster and Themis missions and provide quantitative predictions for realistic conditions to be encountered by the planned magnetospheric multiscale mission. © 2010 American Institute of Physics.


Delmont P.,Center for Plasma Astrophysics | Delmont P.,Leuven Mathematical Modeling and Computational Science Center | Keppens R.,Center for Plasma Astrophysics | Keppens R.,Leuven Mathematical Modeling and Computational Science Center | And 2 more authors.
Journal of Physics: Conference Series | Year: 2010

We study the classical problem of planar shock refraction at an oblique density discontinuity, separating two gases at rest, in planar ideal (magneto)hydrodynamics. In the hydrodynamical case, 3 signals arise and the interface becomes Richtmyer-Meshkov unstable due to vorticity deposition on the shocked contact. In the magnetohydrodynamical case, on the other hand, when the normal component of the magnetic field does not vanish, 5 signals will arise. The interface then typically remains stable, since the Rankine-Hugoniot jump conditions in ideal MHD do not allow for vorticity deposition on a contact discontinuity. We present an exact Riemann solver based solution strategy to describe the initial self similar refraction phase. Using grid-adaptive MHD simulations, we show that after reflection from the top wall, the interface remains stable. © 2010 IOP Publishing Ltd.


Vranjes J.,Center for Plasma Astrophysics | Vranjes J.,Leuven Mathematical Modeling and Computational Science Center | Poedts S.,Center for Plasma Astrophysics | Poedts S.,Leuven Mathematical Modeling and Computational Science Center
Astrophysical Journal | Year: 2010

The solar atmosphere is structured and inhomogeneous, both horizontally and vertically. The omnipresence of coronal magnetic loops implies gradients of the equilibrium plasma quantities such as the density, magnetic field, and temperature. These gradients are responsible for the excitation of drift waves that grow both within the twocomponent fluid description (both in the presence of collisions and without it) and within the two-component kinetic descriptions (due to purely kinetic effects). In this work, the effects of the density gradient in the direction perpendicular to the magnetic field vector are investigated within the kinetic theory, in both electrostatic (ES) and electromagnetic (EM) regimes. The EM regime implies the coupling of the gradient-driven drift wave with the Alfvén wave. The growth rates for the two cases are calculated and compared. It is found that, in general, the ES regime is characterized by stronger growth rates, as compared with the EM perturbations. Also discussed is the stochastic heating associated with the drift wave. The released amount of energy density due to this heating should be more dependent on the magnitude of the background magnetic field than on the coupling of the drift and Alfvén waves. The stochastic heating is expected to be much higher in regions with a stronger magnetic field. On the whole, the energy release rate caused by the stochastic heating can be several orders of magnitude above the value presently accepted as necessary for a sustainable coronal heating. The vertical stratification and the very long wavelengths along the magnetic loops imply that a drift-Alfvén wave, propagating as a twisted structure along the loop, in fact occupies regions with different plasma-β and, therefore, may have different (EM-ES) properties, resulting in different heating rates within just one or two wavelengths. © 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.


Jacobs C.,Centrum voor Plasma Astrofysica | Jacobs C.,Leuven Mathematical Modeling and Computational Science Center | Poedts S.,Centrum voor Plasma Astrofysica | Poedts S.,Leuven Mathematical Modeling and Computational Science Center
Advances in Space Research | Year: 2011

The solar wind fills the heliosphere and is the background medium in which coronal mass ejections propagate. A realistic modelling of the solar wind is therefore essential for space weather research and for reliable predictions. Although the solar wind is highly anisotropic, magnetohydrodynamic (MHD) models are able to reproduce the global, average solar wind characteristics rather well. The modern computer power makes it possible to perform full three dimensional (3D) simulations in domains extending beyond the Earth's orbit, to include observationally driven boundary conditions, and to implement even more realistic physics in the equations. In general, MHD models for the solar wind often make use of additional source and sink terms in order to mimic the observed solar wind parameters and/or they hide the not-explicitly modelled physical processes in a reduced or variable adiabatic index. Even the models that try to take as much as possible physics into account, still need additional source terms and fine tuning of the parameters in order to produce realistic results. In this paper we present a new and simple polytropic model for the solar wind, incorporating data from the ACE spacecraft to set the model parameters. This approach allows to reproduce the different types of solar wind, where the simulated plasma variables are in good correspondence with the observed solar wind plasma near 1 AU. © 2011 COSPAR. Published by Elsevier Ltd. All rights reserved.

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