Index Introduction The Research at Rijnhuizen Results in 2008 Education, Training, Outreach and Public Information Output Appendix website Rijnhuizen 3.8 | Astrophysical Plasma Physics Coordinator: Prof. dr. R. Keppens Funding: NWO-E, Transition Programme Objective: To study the magnetohydrodynamics of plasmas in astrophysical systems 3.8.1 Grid-adaptive multi-dimensional relativistic HD and MHD simulations In many gas- and magnetofluid computations of astrophysical interest, a hierarchy of spatial scales needs to be resolved simultaneously for extended time periods. This justifies the need for parallel computing, allowing high resolution studies. The AMRVAC code is currently used routinely on supercomputers in Europe, and its extension to relativistic regimes has led to applications where we numerically investigate the magnetic collimation of relativistic jet flows in Active Galactic Nuclei (AGN). We investigated the influence of helical magnetic fields on jet beam propagation in an overdense external interstellar medium. We adopt a special relativistic magnetohydrodynamic viewpoint on the shock-dominated AGN jet evolution. We find that the propagation speed of the bow shock systematically exceeds the value expected from estimates using beam-average parameters, in accord with the centrally peaked Lorentz factor variation. The helicity of the beam magnetic field is effectively transported down the beam, with compression zones in between diagonal internal cross-shocks showing stronger toroidal field regions. These results were selected by the editor of Astronomy and Astrophysics as a Highlight of this issue (volume 486-3). Figure 3.33. From a special relativistic magnetised jet simulation: the flow topology on top of density plot (left panel), along with a quantification of the inverse pitch throughout the jet: values above 1 are colored black. In the right panel, a translucent Schlieren plot of the density is combined with field lines. 3.8.2 Accretion disk dynamics in young star environments Finally, in the context of accretion disk physics, we studied the conditions required to steadily deviate an accretion flow from a circumstellar disc into a magnetospheric funnel flow onto a slow rotating young forming star. Accretion funnels are found to be robust features which occur below the co-rotation radius, where the stellar poloidal magnetic pressure becomes both at equipartition with the disc thermal pressure and is comparable to the disc poloidal ram pressure. Weak dipole fields, similar in magnitude to those observed, lead to the development of accretion funnel flows in weakly accreting T Tauri stars. An example is shown in Figure 3.34. Figure 3.34: Resistive MHD simulation for a 5-day period rotating magnetic young star. We show the density distribution in the computational domain using a log scale. The black lines represent the magnetic field lines and the black arrows represent the velocity field. The white line in the first snapshot represents an initial magnetic field line anchored at the corotation radius. We superimpose a part of the computational grid to show the good resolution near the truncation radius. An accretion column is formed and one observes the expansion of the poloidal magnetic field and transient disc ejecta.