Entity

Time filter

Source Type


Vasheghani Farahani S.,University of Warwick | Nakariakov V.M.,University of Warwick | Nakariakov V.M.,Russian Academy of Sciences | Van Doorsselaere T.,Centrum voor Plasma Astrofysica | Verwichte E.,University of Warwick
Astronomy and Astrophysics | Year: 2011

Aims. We investigate the nonlinear phenomena accompanying long-wavelength torsional waves in solar and stellar coronae. Methods. The second order thin flux-tube approximation is used to determine perturbations of a straight untwisted and non-rotating magnetic flux-tube, nonlinearly induced by long-wavelength axisymmetric magnetohydrodynamic waves of small, but finite amplitude. Results. Propagating torsional waves induce compressible perturbations oscillating with double the frequency of the torsional waves. In contrast with plane shear Alfvén waves, the amplitude of compressible perturbations is independent of the plasma-β and is proportional to the torsional wave amplitude squared. Standing torsional waves induce compressible perturbations of two kinds, that grow with the characteristic time inversely proportional to the sound speed, and that oscillate at double the frequency of the inducing torsional wave. The growing density perturbation saturates at the level, inversely proportional to the sound speed. © 2010 ESO. Source


Meliani Z.,Centrum voor Plasma Astrofysica | Sauty C.,Observatoire de Paris | Tsinganos K.,National and Kapodistrian University of Athens | Trussoni E.,National institute for astrophysics | Cayatte V.,Observatoire de Paris
Astronomy and Astrophysics | Year: 2010

Context.The two types of Fanaroff-Riley (FR) radio-loud galaxies, FR I and FR II, exhibit strong jets which have different properties. These differences may be associated to the central engine and/or the external medium. Aims. The AGN classification FR I and FR II can be linked to the rate of electromagnetic Poynting flux extraction from the inner corona of the central engine by the jet. The collimation results from the distribution of the total electromagnetic energy across the jet, as compared to the corresponding distribution of the thermal and gravitational energies. Methods. We use exact solutions of the fully relativistic magnetohydrodynamical (GRMHD) equations obtained by a nonlinear separation of the variables to study outflows from a Schwarzschild black hole corona. Results.A strong correlation is found between the jet features and the energetic distribution of the plasma of the inner corona, which may be related to the efficiency of the magnetic rotator. Conclusions. It is shown that observations of FR I and FR II jets may be partially constrained by our model for spine jets. The deceleration observed in FR I jets may be associated with a low magnetic efficiency of the central magnetic rotator and an important thermal confinement by the hot surrounding medium. Conversely, the strongly collimated and accelerated FR II outflows may be self-collimated by their own magnetic field because of the high efficiency of the central magnetic rotator. © 2010 ESO. Source


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. Source


Jacobs C.,Centrum voor Plasma Astrofysica | Jacobs C.,Computational Science Center | Poedts S.,Centrum voor Plasma Astrofysica | Poedts S.,Computational Science Center
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2011

Coronal mass ejections (CMEs) play a key role in space weather. The mathematical modelling of these violent solar phenomena can contribute to a better understanding of their origin and evolution and as such improve space weather predictions. We review the state-of-the-art in CME simulations, including a brief overview of current models for the background solar wind as it has been shown that the background solar wind affects the onset and initial evolution of CMEs quite substantially. We mainly focus on the attempt to retrieve the initiation and propagation of CMEs in the framework of computational magnetofluid dynamics (CMFD). Advanced numerical techniques and large computer resources are indispensable when attempting to reconstruct an event from Sun to Earth. Especially the simulations developed in dedicated event studies yield very realistic results, comparable with the observations. However, there are still a lot of free parameters in these models and ad hoc source terms are often added to the equations, mimicking the physics that is not really understood yet in detail. © 2010 Elsevier Ltd. Source

Discover hidden collaborations