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Romashets E.,Prairie View A&M University | Vandas M.,Czech Republic Astronomical Institute | Poedts S.,Center for Plasma Astrophysics
Solar Physics | Year: 2010

To model and study local magnetic-field enhancements in a solar flux rope we consider the magnetic field in its interior as a superposition of two linear (constant α) force-free magnetic-field distributions, viz. a global one, which is locally similar to a part of the cylinder, and a local torus-shaped magnetic distribution. The newly derived solution for a toroid with an aspect ratio close to unity is applied. The symmetry axis of the toroid and that of the cylinder may or may not coincide. Both the large and small radii of the toroid are set equal to the cylinder's radius. The total magnetic field distribution yields a flux tube which has a variable diameter with local minima and maxima. In principle, this approach can be used for the interpretation and analysis of solar-limb observations of coronal loops. © Springer Science+Business Media B.V. 2010.

Lazar M.,Ruhr University Bochum | Schlickeiser R.,Ruhr University Bochum | Poedts S.,Center for Plasma Astrophysics
Physics of Plasmas | Year: 2010

The kinetic instabilities of the Weibel type are presently invoked in a large variety of astrophysical scenarios because anisotropic plasma structures are ubiquitous in space. The Weibel instability is driven by a temperature anisotropy which is commonly modeled by a bi-axis distribution function, such as a bi-Maxwellian or a generalized bi-Kappa. Previous studies have been limited to a bi-Kappa distribution and found a suppression of this instability in the presence of suprathermal tails. In the present paper it is shown that the Weibel growth rate is rather more sensitive to the shape of the anisotropic distribution function. In order to illustrate the distinguishing properties of this instability a product-bi-Kappa distribution is introduced, with the advantage that this distribution function enables the use of different values of the spectral index in the two directions, κ κ⊥. The growth rates and the instability threshold are derived and contrasted with those for a simple bi-Kappa and a bi-Maxwellian. Thus, while the maximum growth rates reached at the saturation are found to be higher, and the threshold is drastically reduced making the anisotropic product bi-Kappa (with small Kappas) highly susceptible to the Weibel instability. This effect could also raise questions on the temperature or the temperature anisotropy that seems to be not an exclusive source of free energy for this instability, and definition of these notions for such Kappa distributions must probably be reconsidered. © 2010 American Institute of Physics.

Xia C.,Nanjing University | Chen P.F.,Nanjing University | Keppens R.,Center for Plasma Astrophysics | Keppens R.,FOM Institute for Atomic and Molecular Physics | Van Marle A.J.,Center for Plasma Astrophysics
Astrophysical Journal | Year: 2011

It has been established that cold plasma condensations can form in a magnetic loop subject to localized heating of its footpoints. In this paper, we use grid-adaptive numerical simulations of the radiative hydrodynamic equations to investigate the filament formation process in a pre-shaped loop with both steady and finite-time chromospheric heating. Compared to previous works, we consider low-lying loops with shallow dips and use a more realistic description for radiative losses. We demonstrate for the first time that the onset of thermal instability satisfies the linear instability criterion. The onset time of the condensation is roughly 2 hr or more after the localized heating at the footpoint is effective, and the growth rate of the thread length varies from 800 km hr-1 to 4000 km hr-1, depending on the amplitude and the decay length scale characterizing this localized chromospheric heating. We show how single or multiple condensation segments may form in the coronal portion. In the asymmetric heating case, when two segments form, they approach and coalesce, and the coalesced condensation later drains down into the chromosphere. With steady heating, this process repeats with a periodicity of several hours. While our parametric survey confirms and augments earlier findings, we also point out that steady heating is not necessary to sustain the condensation. Once the condensation is formed, it keeps growing even after the localized heating ceases. In such a finite-heating case, the condensation instability is maintained by chromospheric plasma that gets continuously siphoned into the filament thread due to the reduced gas pressure in the corona. Finally, we show that the condensation can survive the continuous buffeting of perturbations from photospheric p-mode waves. © 2011. The American Astronomical Society. All rights reserved..

Copil P.,Center for Plasma Astrophysics | Voitenko Y.,Belgian Institute for Space Aeronomy | Goossens M.,Center for Plasma Astrophysics
Astronomy and Astrophysics | Year: 2010

Context. The magnetic field structuring in the solar corona occurs on large scales (loops and funnels), but also on small scales. For instance, coronal loops are made up of thin strands with different densities and magnetic fields across the loop. Aims. We consider a thin current thread and model it as a magnetic flux tube with twisted magnetic field inside the tube and straight field outside. We prove the existence of trapped Alfvén modes in twisted magnetic flux tubes (current threads) and we calculate the wave profile in the radial direction for two different magnetic twist models. Methods. We used the Hall MHD equations that we linearized in order to derive and solve the eigenmode equation for the torsional Alfvén waves. Results. We show that the trapped Alfv én eigenmodes do exist and are localized in thin current threads where the magnetic field is twisted. The wave spectrum is discrete in phase velocity, and the number of modes is finite and depends on the amount of the magnetic field twist. The phase speeds of the modes are between the minimum of the Alfvén speed in the interior and the exterior Alfén speed. Conclusions. Torsional Alfvén waves can be guided by thin twisted magnetic flux-tubes (current threads) in the solar corona. We suggest that the current threads guiding torsional Alfvén waves, are subject to enhanced plasma heating due to wave dissipation. © ESO, 2010.

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.

Lazar M.,Ruhr University Bochum | Lazar M.,Center for Plasma Astrophysics | Poedts S.,Ruhr University Bochum | Schlickeiser R.,Ruhr University Bochum
Monthly Notices of the Royal Astronomical Society | Year: 2011

The electron cyclotron emissions represent a useful tool in the diagnostics of fusion plasmas and space plasma fluctuations. The instability which enhances the whistler-cyclotron modes is driven in the presence of an ambient regular magnetic field by an excess of transverse kinetic energy of plasma particles. Previous studies have modelled the anisotropic particles with a bi-Maxwellian or a bi-Kappa distribution function and found a suppression of this instability in the presence of suprathermal tails. Here, the anisotropic plasma is modelled with a product-bi-Kappa distribution, with the advantage that this distribution function enables the use of two different spectral indices in the main directions, κ ||≠κ {box drawings light up and horizontal}, and permits further characterization of kappa populations and their excitations. The exact numerical values of the growth rates and the instability threshold are derived and contrasted with those for a simple bi-Kappa and a bi-Maxwellian, using plasma parameters and magnetic fields relevant for the solar and terrestrial environments. © 2010 The Authors. Journal compilation © 2010 RAS.

Meliani Z.,Center for Plasma Astrophysics | Keppens R.,Center for Plasma Astrophysics | Keppens R.,FOM Institute for Atomic and Molecular Physics | Keppens R.,University Utrecht
Astronomy and Astrophysics | Year: 2010

Aims. In gamma-ray-bursts (GRBs), ultra-relativistic blast waves are ejected into the circumburst medium. We analyse in unprecedented detail the deceleration of a self-similar Blandford-McKee blast wave from a Lorentz factor 25 to the nonrelativistic Sedov phase. Our goal is to determine the stability properties of its frontal shock. Methods. We carried out a grid-adaptive relativistic 2D hydro-simulation at extreme resolving power, following the GRB jet during the entire afterglow phase. We investigate the effect of the finite initial jet opening angle on the deceleration of the blast wave, and identify the growth of various instabilities throughout the coasting shock front. Results. We find that during the relativistic phase, the blast wave is subject to pressure-ram pressure instabilities that ripple and fragment the frontal shock. These instabilities manifest themselves in the ultra-relativistic phase alone, remain in full agreement with causality arguments, and decay slowly to finally disappear in the near-Newtonian phase as the shell Lorentz factor drops below 3. From then on, the compression rate decreases to levels predicted to be stable by a linear analysis of the Sedov phase. Our simulations confirm previous findings that the shell also spreads laterally because a rarefaction wave slowly propagates to the jet axis, inducing a clear shell deformation from its initial spherical shape. The blast front becomes meridionally stratified, with decreasing speed from axis to jet edge. In the wings of the jetted flow, Kelvin-Helmholtz instabilities occur, which are of negligible importance from the energetic viewpoint. Conclusions. Relativistic blast waves are subject to hydrodynamical instabilities that can significantly affect their deceleration properties. Future work will quantify their effect on the afterglow light curves. © 2010 ESO.

Van Marle A.J.,Center for Plasma Astrophysics | Meliani Z.,Center for Plasma Astrophysics | Keppens R.,Center for Plasma Astrophysics | Decin L.,Catholic University of Leuven
Astrophysical Journal Letters | Year: 2011

We study the hydrodynamical behavior occurring in the turbulent interaction zone of a fast-moving red supergiant star, where the circumstellar and interstellar material collide. In this wind-interstellar-medium collision, the familiar bow shock, contact discontinuity, and wind termination shock morphology form, with localized instability development. Our model includes a detailed treatment of dust grains in the stellar wind and takes into account the drag forces between dust and gas. The dust is treated as pressureless gas components binned per grain size, for which we use 10 representative grain size bins. Our simulations allow us to deduce how dust grains of varying sizes become distributed throughout the circumstellar medium. We show that smaller dust grains (radius <0.045 μm) tend to be strongly bound to the gas and therefore follow the gas density distribution closely, with intricate fine structure due to essentially hydrodynamical instabilities at the wind-related contact discontinuity. Larger grains which are more resistant to drag forces are shown to have their own unique dust distribution, with progressive deviations from the gas morphology. Specifically, small dust grains stay entirely within the zone bound by shocked wind material. The large grains are capable of leaving the shocked wind layer and can penetrate into the shocked or even unshocked interstellar medium. Depending on how the number of dust grains varies with grain size, this should leave a clear imprint in infrared observations of bow shocks of red supergiants and other evolved stars. © 2011. The American Astronomical Society. All rights reserved.

Pierrard V.,Belgian Institute for Space Aeronomy | Pierrard V.,Catholic University of Louvain | Lazar M.,Ruhr University Bochum | Lazar M.,Center for Plasma Astrophysics
Solar Physics | Year: 2010

The plasma particle velocity distributions observed in the solar wind generally show enhanced (non-Maxwellian) suprathermal tails, decreasing as a power law of the velocity and well described by the family of Kappa distribution functions. The presence of non-thermal populations at different altitudes in space plasmas suggests a universal mechanism for their creation and important consequences concerning plasma fluctuations, the resonant and nonresonant wave - particle acceleration and plasma heating. These effects are well described by the kinetic approaches where no closure requires the distributions to be nearly Maxwellian. This paper summarizes and analyzes the various theories proposed for the Kappa distributions and their valuable applications in coronal and space plasmas. © 2010 Springer Science+Business Media B.V.

Lazar M.,Center for Plasma Astrophysics | Lazar M.,Ruhr University Bochum | Poedts S.,Center for Plasma Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2013

The low-frequency fluctuations of the interplanetary magnetic field are frequently attributed to electromagnetic ion-cyclotron (EMIC) waves generated either locally and self-consistently by the temperature anisotropy of ions, or in the corona and transported by the super-Alfvénic solar wind. This paper conducts a refined analysis of the EMIC instability in the presence of suprathermal populations. The anisotropic distributions are modelled with two different power-law distributions functions, the additive bi-Kappa (BK) and the more general productbi- Kappa (PBK) distribution function. EMIC solutions are derived exactly numerically for the full range of the plasma parameters, including conditions relevant for the solar wind and magnetospheric plasmas. Accurate physical correlations are provided between the maximum growth rates and the instability threshold conditions. The expectation that the instability might be stimulated by the suprathermals is confirmed by both Kappa models, but in a complementary way: while the instability thresholds are lowered by the BK model, at higher anisotropies the growth rates are enhanced only by the PBK model. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

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