MHD instabilities at the disk-magnetosphere boundary: 3D simulations (original) (raw)
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Accretion to magnetized stars through the Rayleigh–Taylor instability: global 3D simulations
Monthly Notices of the Royal Astronomical Society, 2008
We present results of 3D simulations of magnetohydrodynamics (MHD) instabilities at the accretion disc-magnetosphere boundary. The instability is Rayleigh-Taylor, and develops for a fairly broad range of accretion rates and stellar rotation rates and magnetic fields. It manifests itself in the form of tall, thin tongues of plasma that penetrate the magnetosphere in the equatorial plane. The shape and number of the tongues changes with time on the inner disc dynamical timescale. In contrast with funnel flows, which deposit matter mainly in the polar region, the tongues deposit matter much closer to the stellar equator. The instability appears for relatively small misalignment angles, 30 • , between the star's rotation and magnetic axes, and is associated with higher accretion rates. The hotspots and light curves during accretion through instability are generally much more chaotic than during stable accretion. The unstable state of accretion has possible implications for quasi-periodic oscillations and intermittent pulsations from accreting systems, as well as planet migration.
AIP Conference Proceedings, 2008
We present results of 3D simulations of MHD instabilities at the accretion diskmagnetosphere boundary. The instability is Rayleigh-Taylor, and develops for a fairly broad range of accretion rates and stellar rotation rates and magnetic fields. It produces tall, thin tongues of plasma that penetrate the magnetosphere in the equatorial plane. The shape and number of the tongues changes with time on the inner-disk dynamical timescale. In contrast with funnel flows, which deposit matter mainly in the polar region, the tongues deposit matter much closer to the stellar equator. The instability appears for relatively small misalignment angles, Θ 30 • , between the star's rotation and magnetic axes, and is associated with higher accretion rates. The hot spots and light curves during accretion through instability are generally much more chaotic than during stable accretion. The unstable state of accretion has possible implications for quasi-periodic oscillations and intermittent pulsations from accreting systems.
The Astrophysical …, 2008
We report on the first global three-dimensional (3D) MHD simulations of disk accretion onto a rotating magnetized star through the Rayleigh-Taylor instability. The star has a dipole field misaligned relative to the rotation axis by a small angle V. Simulations show that, depending on the accretion rate, a star may be in the stable or unstable regime of accretion. In the unstable regime, matter penetrates deep into the magnetosphere through several elongated "tongues" which deposit matter at random places on the surface of the star, leading to stochastic light curves. In the stable regime, matter accretes in ordered funnel streams and the light curves are almost periodic. A star may switch between these two regimes depending on the accretion rate and may thus show alternate episodes of ordered pulsations and stochastic light curves. In the intermediate regime, both stochastic and ordered pulsations are observed. For , the instability is suppressed and stable accretion V 1 30Њ through funnel streams dominates.
MHD simulations of disk-star interaction
Proceedings of the International Astronomical Union, 2007
We discuss a number of topics relevant to disk-magnetosphere interaction and how numerical simulations illuminate them. The topics include: (1) disk-magnetosphere interaction and the problem of disk-locking; (2) the wind problem; (3) structure of the magnetospheric flow, hot spots at the star's surface, and the inner disk region; (4) modeling of spectra from 3D funnel streams; (5) accretion to a star with a complex magnetic field; (6) accretion through 3D instabilities; (7) magnetospheric gap and survival of protoplanets. Results of both 2D and 3D simulations are discussed.
Unstable magnetohydrodynamical continuous spectrum of accretion disks
Astronomy and Astrophysics, 2007
Context. We present a detailed study of localised magnetohydrodynamical (MHD) instabilities occuring in twodimensional magnetized accretion disks. Aims. We model axisymmetric MHD disk tori, and solve the equations governing a two-dimensional magnetized accretion disk equilibrium and linear wave modes about this equilibrium. We show the existence of novel MHD instabilities in these two-dimensional equilibria which do not occur in an accretion disk in the cylindrical limit. Methods. The disk equilibria are numerically computed by the FINESSE code. The stability of accretion disks is investigated analytically as well as numerically. We use the PHOENIX code to compute all the waves and instabilities accessible to the computed disk equilibrium. Results. We concentrate on strongly magnetized disks and sub-Keplerian rotation in a large part of the disk. These disk equilibria show that the thermal pressure of the disk can only decrease outwards if there is a strong gravitational potential. Our theoretical stability analysis shows that convective continuum instabilities can only appear if the density contours coincide with the poloidal magnetic flux contours. Our numerical results confirm and complement this theoretical analysis. Furthermore, these results show that the influence of gravity can either be stabilizing or destabilizing on this new kind of MHD instability. In the likely case of a non-constant density, the height of the disk should exceed a threshold before this type of instability can play a role.
2D and 3D MHD simulations of disk accretion by rotating magnetized stars: Search for variability
Advances in Space Research, 2006
We performed 2D and full 3D magnetohydrodynamic simulations of disk accretion to a rotating star with an aligned or misaligned dipole magnetic field. We investigated the rotational equilibrium state and derived from simulations the ratio between two main frequencies: the spin frequency of the star and the orbital frequency at the inner radius of the disk. In 3D simulations we observed different features related to the non-axisymmetry of the magnetospheric flow. These features may be responsible for high-frequency quasi-periodic oscillations (QPOs). Variability at much lower frequencies may be connected with restructuring of the magnetic flux threading the inner regions of the disk. Such variability is specifically strong at the propeller stage of evolution.
Magnetohydrodynamic Simulations of Global Accretion Disks with Vertical Magnetic Fields
The Astrophysical Journal, 2014
We report results of three dimensional mangetohydrodynamical (MHD) simulations of global accretion disks threaded with weak vertical magnetic fields. We perform the simulations in the spherical coordinates with different temperature profiles and accordingly different rotation profiles. In the cases with a spatially constant temperature, because the rotation frequency is vertically constant in the equilibrium condition, general properties of the turbulence excited by magnetorotational instability (MRI) are quantitatively similar to those obtained in local shearing box simulations. On the other hand, in the cases with a radially variable temperature profile, the vertical differential rotation, which is inevitable in the equilibrium condition, winds up the magnetic field lines, in addition to the usual radial differential rotation. As a result, the coherent wound magnetic fields contribute to the Maxwell stress in the surface regions. Our global simulations give somewhat larger density fluctuation, δρ/ρ = 0.1 − 0.2, near the midplane than the values obtained in previous local shearing box simulations and global simulations without net vertical magnetic field. The velocity fluctuations, dominated by the radial component, are ≈ 0.1 − 0.2 of the local sound speed. The azimuthal power spectra of the magnetic fields show shallow slopes, ∝ m 0 ∼ m −1 , where m is an azimuthal mode number, which might be related to the energy injection by MRI from small scales. On the other hand, the power spectra of the velocities and density show steeper slopes, ∝ m −1 ∼ m −2 . We observe intermittent and structured disk winds driven by the Poynting flux associated with the MHD turbulence, with the slightly smaller mass fluxes than that obtained in our local simulations. The Poynting flux originating from magnetic tension is injected from the regions above a scale height toward both the midplane and the surfaces. Related to this, sound waves are directed to the midplane from the surface regions. The mass accretion mainly occurs near the surfaces and the gas near the midplane slowly moves outward in the time domain of the present simulations. The vertical magnetic fields are also dragged inward in the surface regions, while they stochastically move outward and inward around the midplane. The difference of the velocities at the midplane and the surfaces might cause large-scale meridional circulations. Applying to protoplanetary disks, these waves and circulation are supposed to play an important role in the dynamics of solid particles. We also discuss an observational implication of induced spiral structure in the simulated turbulent disks.
Three-dimensional simulations of MHD disk winds to hundred AU scale from the protostar
EPJ Web of Conferences, 2014
We present the results of four, large scale, three-dimensional magnetohydrodynamics simulations of jets launched from a Keplerian accretion disk. The jets are followed from the source out to 90 AU, a scale that covers several pixels of HST images of nearby protostellar jets. The four simulations analyzed are for four different initial magnetic field configuration threading the surface of the accretion disk with varying degree of openness of the field lines. Our simulations show that jets are heated along their length by many shocks and we compute the line emission that is produced. We find excellent agreement with the observations and use these diagnostics to discriminate between different magnetic field configurations. A two-component jet emerges in simulations with less open field lines along the disk surface. The two-components are physically and dynamically separated with an inner fast and rotating jet and an outer slow jet. The second component weakens and eventually only one-component jet (i.e. only the inner jet) is obtained for the most open field configurations. In all of our simulations we find that the faster inner component inherits the Keplerian profile and preserves it to large distances from the source. On the other hand, the outer component is associated with velocity gradients mimicking rotation.
Instability, turbulence, and enhanced transport in accretion disks
Reviews of Modern Physics, 1998
Recent years have witnessed dramatic progress in our understanding of how turbulence arises and transports angular momentum in astrophysical accretion disks. The key conceptual point has its origins in work dating from the 1950s, but its implications have been fully understood only in the last several years: the combination of a subthermal magnetic field (any nonpathological configuration will do) and outwardly decreasing differential rotation rapidly generates magnetohydrodynamic (MHD) turbulence via a remarkably simple linear instability. The result is a greatly enhanced effective viscosity, the origin of which had been a long-standing problem. The MHD nature of disk turbulence has linked two broad domains of magnetized fluid research: accretion theory and dynamos. The understanding that weak magnetic fields are not merely passively acted upon by turbulence, but actively generate it, means that the assumptions of classical dynamo theory break down in disks. Paralleling the new conceptual understanding has been the development of powerful numerical MHD codes. These have taught us that disks truly are turbulent, transporting angular momentum at greatly enhanced rates. We have also learned, however, that not all forms of disk turbulence do this. Purely hydrodynamic turbulence, when it is imposed, simply causes fluctuations without a significant increase in transport. The interplay between numerical simulation and analytic arguments has been particularly fruitful in accretion disk theory and is a major focus of this article. The authors conclude with a summary of what is now known of disk turbulence and mention some knotty outstanding questions (e.g., what is the physics behind nonlinear field saturation?) for which we may soon begin to develop answers. [S0034-6861(98)00501-7] CONTENTS 42 5. The evolution of an initially random field 42 6. Shear vs vorticity 43 7. Density stratification 44 C. MHD simulations: a summary 45 VI. Accretion Disk Dynamos 45 A. The dynamo-electric machine 45 B. A brief review of mean-field dynamo theory 46 C. Mean-field theory and nonlinear evolution of the magnetorotational instability 47 D. Saturation 49 VII. Summary 50 Acknowledgments 50 References 51
Astronomy & Astrophysics, 2009
Aims. We investigate the accretion process from an accretion disk onto a magnetized rotating star with a purely dipolar magnetic field. Our main aim is to study the mechanisms that regulate the stellar angular momentum. In this work, we consider two effects that can contrast with the spin-up torque normally associated with accretion: (1) the spin-down torque exerted by an extended magnetosphere connected to the disk beyond the corotation radius; (2) the spin-down torque determined by a stellar wind flowing along the opened magnetospheric field lines. Methods. Our study is based on time-dependent axisymmetric magnetohydrodynamic numerical simulations of the interaction between a viscous and resistive accretion disk with the dipolar magnetosphere of a rotating star. We present the first example of a numerical experiment able to model at the same time the formation of accretion curtains, the effects of an extended stellar magnetosphere and the launching of a stellar wind.