Rony Keppens

Magnetized Accretion-Ejection Structures: 2.5-dimensional Magnetohydrodynamic Simulations of Continuous Ideal Jet Launching from Resistive Accretion Disks

We present numerical magnetohydrodynamic (MHD) simulations of a magnetized accretion disk launching trans-Alfve·nic jets. These simulations, performed in a 2.5-dimensional time-dependent polytropic resistive MHD framework, model a resistive accretion disk threaded by an initial vertical magnetic ield. The resistivity is only important inside the disk and is prescribed as … mVAH expufdff 2Z 2 =H 2 fi, where VA stands for Alfve·n speed, H is the disk scale height, and the coecien t m is smaller than unity. By performing the simulations over several tens of dynamical disk timescales, we show that the launching of a collimated outiow occurs self-consistently and the ejection of matter is continuous and quasi-stationary. These are the irst eversimulationsofresistiveaccretion disks launchingnontransient ideal MHD jets. Roughly15% ofaccreted mass is persistently ejected. This outiow is safely characterized as a jet since the iow becomes superfast magnetosonic, well collimated, and reaches a quasi-stationary state. We present a complete illustration and explanation of the eeaccretion-ejectionee mechanism that leads to jet formation from a magnetized accretion disk. In particular, the magnetic torque inside the disk brakes the matter azimuthally and allows for accretion, while it is responsible for an eecti ve magnetocentrifugal acceleration in the jet. As such, the magnetic ield channels the disk angular momentum and powers the jet acceleration and collimation. The jet originates from the inner disk region where equipartition between thermal and magnetic forces is achieved. A hollow, superfast magnetosonicshellof densematerial isthenatural outcome of theinward advectionofaprimordial ield. Subject headings: accretion, accretion disks N galaxies:jets N ISM:jets and outiows N MHD

Adaptive Mesh Refinement for conservative systems: multi-dimensional efficiency evaluation

Abstract Obtainable computational efficiency is evaluated when using an Adaptive Mesh Refinement (AMR) strategy in time accurate simulations governed by sets of conservation laws. For a variety of 1D, 2D, and 3D hydro- and magnetohydrodynamic simulations, AMR is used in combination with several shock-capturing, conservative discretization schemes. Solution accuracy and execution times are compared with static grid simulations at the corresponding high resolution and time spent on AMR overhead is reported. Our examples reach corresponding efficiencies of 5 to 20 in multi-dimensional calculations and only 1.5–8% overhead is observed. For AMR calculations of multi-dimensional magnetohydrodynamic problems, several strategies for controlling the ∇· B =0 constraint are examined. Three source term approaches suitable for cell-centered B representations are shown to be effective. For 2D and 3D calculations where a transition to a more globally turbulent state takes place, it is advocated to use an approximate Riemann solver based discretization at the highest allowed level(s), in combination with the robust Total Variation Diminishing Lax–Friedrichs method on the coarser levels. This level-dependent use of the spatial discretization acts as a computationally efficient, hybrid scheme.

Radiatively inefficient magnetohydrodynamic accretion-ejection structures

We present magnetohydrodynamic simulations of a resistive accretion disk continuously launching transmagnetosonic, collimated jets. We time-evolve the full set of magnetohydrodynamic equations but neglect radiative losses in the energetics (radiatively inefficient). Our calculations demonstrate that a jet is self-consistently produced by the interaction of an accretion disk with an open, initially bent large-scale magnetic field. A constant fraction of heated disk material is launched in the inner equipartition disk regions, leading to the formation of a hot corona and a bright collimated, superfast magnetosonic jet. We illustrate the complete dynamics of the hot near-steady state outflow (where thermal pressure magnetic pressure) by showing force balance, energy budget, and current circuits. The evolution to this near-stationary state is analyzed in terms of the temporal variation of energy fluxes controlling the energetics of the accretion disk. We find that unlike advection-dominated accretion flow, the energy released by accretion is mainly sent into the jet rather than transformed into disk enthalpy. These magnetized, radiatively inefficient accretion-ejection structures can account for underluminous thin disks supporting bright fast collimated jets as seen in many systems displaying jets (for instance, M87).

Extrapolation of a nonlinear force-free field containing a highly twisted magnetic loop

The stress-and-relax method for the extrapolation of nonlinear force-free coronal magnetic fields from photospheric vector magnetograms is formulated and implemented in a manner analogous to the evolutionary extrapolation method. The technique is applied to a numerically constructed force-free equilibrium that has a simple bipolar structure of the normal field component in the bottom (magnetogram) plane but contains a highly twisted loop and a shear (current) layer, with a smooth but strong variation of the force-free parameter α in the magnetogram. A standard linear force-free extrapolation of this magnetogram, using the so-called αbest value, is found to fail in reproducing the twisted loop (or flux rope) and the shear layer; it yields a loop pair instead and the shear is not concentrated in a layer. With the nonlinear extrapolation technique, the given equilibrium is readily reconstructed to a high degree of accuracy if the magnetogram is sufficiently resolved. A parametric study quantifies the requirements on the resolution for a successful nonlinear extrapolation. Permitting magnetic reconnection by a controlled use of resistivity improved the extrapolation at a resolution comparable to the smallest structures in the magnetogram.

Waves and instabilities in accretion disks: Magnetohydrodynamic spectroscopic analysis

A complete analytical and numerical treatment of all magnetohydrodynamic waves and instabilities for radially stratified, magnetized accretion disks is presented. The instabilities are a possible source of anomalous transport. While recovering results on known hydrodynamic and both weak- and strong-field magnetohydrodynamic perturbations, the full magnetohydrodynamic spectra for a realistic accretion disk model demonstrate a much richer variety of instabilities accessible to the plasma than previously realized. We show that both weakly and strongly magnetized accretion disks are prone to strong nonaxisymmetric instabilities. The ability to characterize all waves arising in accretion disks holds great promise for magnetohydrodynamic spectroscopic analysis.

STELLAR WINDS, DEAD ZONES, AND CORONAL MASS EJECTIONS

Axisymmetric stellar wind solutions are presented that were obtained by numerically solving the ideal magnetohydrodynamic (MHD) equations. Stationary solutions are critically analyzed using the knowledge of the flux functions. These flux functions enter in the general variational principle governing all axisymmetric stationary ideal MHD equilibria. The magnetized wind solutions for (differentially) rotating stars contain both a wind and a dead zone. We illustrate the influence of the magnetic field topology on the wind acceleration pattern by varying the coronal field strength and the extent of the dead zone. This is evident from the resulting variations in the location and appearance of the critical curves for which the wind speed equals the slow, Alfven, and fast speed. Larger dead zones cause effective, fairly isotropic acceleration to super-Alfvenic velocities as the polar, open field lines are forced to fan out rapidly with radial distance. A higher field strength moves the Alfven transition outward. In the ecliptic, the wind outflow is clearly modulated by the extent of the dead zone. The combined effect of a fast stellar rotation and an equatorial dead zone in a bipolar field configuration can lead to efficient thermocentrifugal equatorial winds. Such winds show both a strong poleward collimation and some equatorward streamline bending due to significant toroidal field pressure at midlatitudes. We discuss how coronal mass ejections are then simulated on top of the transonic outflows.

Interplay between Kelvin-Helmholtz and current-driven instabilities in jets

We investigate, by means of three-dimensional compressible magnetohydrodynamic numerical simulations, the interaction of Kelvin-Helmholtz (KH) and current-driven (CD) instabilities in a magnetized cylindrical jet configuration. The jet has a supersonic axial flow, sheared in the radial direction, and is embedded in a helical magnetic field. The strength of the axial magnetic field component is chosen to be weak, in accord with the weak field regime previously defined by Ryu, Jones, & Frank for uniformly magnetized configurations. We follow the time evolution of a periodic section where the jet surface is perturbed at m = ±1 azimuthal mode numbers. A m = -1 KH surface mode linearly develops dominating the m = +1 KH one, in agreement with results obtained using an independent ideal stability code. This lifted degeneracy, because of the presence of the helical field, leads nonlinearly to clear morphological differences in the jet deformation as compared to uniformly magnetized configurations. As predicted by stability results, a m = -1 CD instability also develops linearly inside the jet core for configurations having a small enough magnetic pitch length. As time proceeds, this magnetic mode interacts with the KH vortical structures and significantly affects the further nonlinear evolution. The magnetic field deformation induced by the CD instability provides a stabilizing effect through its azimuthal component Bθ. This helps to saturate the KH vortices in the vicinity of the jet surface. Beyond saturation, the subsequent disruptive effect on the flow is weaker than in cases having similar uniform and helical magnetic field configurations without the CD mode. We discuss the implications of this stabilizing mechanism for the stability of astrophysical jets.

Interaction of high-velocity pulsars with supernova remnant shells

Hydrodynamical simulations are presented of a pulsar wind emitted by a supersonically moving pulsar. The pulsar moves through the interstellar medium or, in the more interesting case, through the supernova remnant createdat its birth event. In both cases there exists a three-fold structure consisting of the wind termination shock, contact discontinuity and a bow shock bounding the pulsar wind nebula. Using hydrodynamical simulations we study the behaviour of the pulsar wind nebula inside a supernova remnant, and in particular the interaction with the outer shell of swept up interstellar matter and the blast wave surrounding the remnant. This interaction occurs when the pulsar breaks out of the supernova remnant. We assume the remnant is in the Sedov stage of its evolution. Just before break-through, the Mach number associated with the pulsar motion equals M p s r = 7/ 5, independent of the supernova explosion energy and pulsar velocity. The bow shock structure is shown to survive this break-through event.

Unstable continuous spectra of transonic axisymmetric plasmas

In transonically rotating toroidal plasmas a new class of local magnetohydrodynamic instabilities is found, called trans-slow Alfven continuum modes, that are due to poloidal flows exceeding the critical slow magnetosonic speed. When this condition is satisfied, virtually the whole plasma becomes unstable with modes localized at, or close to, rational surfaces with approximately the same growth rates. The instabilities are studied from a general point of view, treating magnetically dominated plasmas (tokamaks) and gravitationally dominated plasmas (accretion disks) on an equal footing. In the first kind of plasmas, rotating overstable modes are found with growth rates that are a fraction of the Alfven frequency, determined by the poloidal Alfven Mach number. When the mass of the central object is increased, these modes lock to become explosively unstable with growth rates that may exceed the Alfven frequency. The instabilities are localized on the magnetic/flow surfaces to which the flows and magnetic fie...

Nonlinear dynamics of Kelvin–Helmholtz unstable magnetized jets: Three-dimensional effects

A numerical study of the Kelvin–Helmholtz instability in compressible magnetohydrodynamics is presented. The three-dimensional simulations consider shear flow in a cylindrical jet configuration, embedded in a uniform magnetic field directed along the jet axis. The growth of linear perturbations at specified poloidal and axial mode numbers demonstrate intricate nonlinear coupling effects. The physical mechanisms leading to induced secondary Kelvin–Helmholtz instabilities at higher mode numbers are identified. The initially weak magnetic field becomes locally dominant in the nonlinear dynamics before and during saturation. Thereby, it controls the jet deformation and eventual breakup. The results are obtained using the Versatile Advection Code [G. Toth, Astrophys. Lett. Commun. 34, 245 (1996)], a software package designed to solve general systems of conservation laws. An independent calculation of the same Kelvin–Helmholtz unstable jet configuration using a three-dimensional pseudospectral code gives important insights into the coupling and excitation events of the various linear mode numbers.A numerical study of the Kelvin–Helmholtz instability in compressible magnetohydrodynamics is presented. The three-dimensional simulations consider shear flow in a cylindrical jet configuration, embedded in a uniform magnetic field directed along the jet axis. The growth of linear perturbations at specified poloidal and axial mode numbers demonstrate intricate nonlinear coupling effects. The physical mechanisms leading to induced secondary Kelvin–Helmholtz instabilities at higher mode numbers are identified. The initially weak magnetic field becomes locally dominant in the nonlinear dynamics before and during saturation. Thereby, it controls the jet deformation and eventual breakup. The results are obtained using the Versatile Advection Code [G. Toth, Astrophys. Lett. Commun. 34, 245 (1996)], a software package designed to solve general systems of conservation laws. An independent calculation of the same Kelvin–Helmholtz unstable jet configuration using a three-dimensional pseudospectral code gives import...