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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.

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.

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.

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..

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