Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userLocal Study of Accretion Disks with a Strong Vertical Magnetic Field: Magnetorotational Instability and Disk Outflow
167
0
0.0
(
0
)
Authors
Xue-Ning Bai
Ask ChatGPT about the research
No Arabic abstract
We perform 3D vertically-stratified local shearing-box ideal MHD simulations of the magnetorotational instability (MRI) that include a net vertical magnetic flux, which is characterized by beta_0 (ratio of gas pressure to magnetic pressure of the net vertical field at midplane). We have considered beta_0=10^2, 10^3 and 10^4 and in the first two cases the most unstable linear MRI modes are well resolved in the simulations. We find that the behavior of the MRI turbulence strongly depends on beta_0: The radial transport of angular momentum increases with net vertical flux, achieving alpha=0.08 for beta_0=10^4 and alpha>1.0 for beta_0=100, where alpha is the Shakura-Sunyaev parameter. A critical value lies at beta_0=10^3: For beta_0>10^3, the disk consists of a gas pressure dominated midplane and a magnetically dominated corona. The turbulent strength increases with net flux, and angular momentum transport is dominated by turbulent fluctuations. The magnetic dynamo that leads to cyclic flips of large-scale fields still exists, but becomes more sporadic as net flux increases. For beta_0<10^3, the entire disk becomes magnetic dominated. The turbulent strength saturates, and the magnetic dynamo is quenched. Stronger large-scale fields are generated with increasing net flux, which dominates angular momentum transport. A strong outflow is launched from the disk by the magnetocentrifugal mechanism, and the mass flux increases linearly with net vertical flux and shows sign of saturation at beta_0=10^2. However, the outflow is unlikely to be directly connected to a global wind: for beta_0>10^3, the large-scale field has no permanent bending direction due to dynamo activities, while for beta_0<10^3, the outflows from the top and bottom sides of the disk bend towards opposite directions, inconsistent with a physical disk wind geometry. Global simulations are needed to address the fate of the outflow.
rate research
Read More
Axisymmetric magnetorotational instability (MRI) in viscous accretion disks is investigated by linear analysis and two-dimensional nonlinear simulations. The linear growth of the viscous MRI is characterized by the Reynolds number defined as $R_{rm MRI} equiv v_A^2/ uOmega $, where $v_A$ is the Alfv{e}n velocity, $ u$ is the kinematic viscosity, and $Omega$ is the angular velocity of the disk. Although the linear growth rate is suppressed considerably as the Reynolds number decreases, the nonlinear behavior is found to be almost independent of $R_{rm MRI}$. At the nonlinear evolutionary stage, a two-channel flow continues growing and the Maxwell stress increases until the end of calculations even though the Reynolds number is much smaller than unity. A large portion of the injected energy to the system is converted to the magnetic energy. The gain rate of the thermal energy, on the other hand, is found to be much larger than the viscous heating rate. Nonlinear behavior of the MRI in the viscous regime and its difference from that in the highly resistive regime can be explained schematically by using the characteristics of the linear dispersion relation. Applying our results to the case with both the viscosity and resistivity, it is anticipated that the critical value of the Lundquist number $S_{rm MRI} equiv v_A^2/etaOmega$ for active turbulence depends on the magnetic Prandtl number $S_{{rm MRI},c} propto Pm^{1/2}$ in the regime of $Pm gg 1$ and remains constant when $Pm ll 1$, where $Pm equiv S_{rm MRI}/R_{rm MRI} = u/eta$ and $eta$ is the magnetic diffusivity.
By performing local three-dimensional MHD simulations of stratified accretion disks, we investigate disk winds driven by MHD turbulence. Initially given weak vertical magnetic fields are effectively amplified by magnetorotational instability and winding due to differential rotation. Large scale channel flows develop most effectively at 1.5 - 2 times the scale heights where the magnetic pressure is comparable to but slightly smaller than the gas pressure. The breakup of these channel flows drives structured disk winds by transporting the Poynting flux to the gas. These features are universally observed in the simulations of various initial fields. This disk wind process should play an essential role in the dynamical evaporation of proto-planetary disks. The breakup of channel flows also excites the momentum fluxes associated with Alfvenic and (magneto-)sonic waves toward the mid-plane, which possibly contribute to the sedimentation of small dust grains in protoplanetary disks.
We report new global ideal MHD simulations for thin accretion disks (with thermal scale height H/R=0.1 and 0.05) threaded by net vertical magnetic fields. Our computations span three orders of magnitude in radius, extend all the way to the pole, and are evolved for more than one viscous time over the inner decade in radius. Static mesh refinement is used to properly resolve MRI. We find that:(1) inward accretion occurs mostly in the upper magnetically dominated regions of the disk, similar to the predictions from some previous analytical work and the coronal accretion in previous GRMHD simulations. Rapid inflow in the upper layers combined with slow outflow at the midplane creates strong $Rphi$ and $zphi$ stresses in the mean field; the vertically integrated $alphasim 0.5-1$ when the initial field has $beta_{0}=10^3$ at the midplane. (2) A quasi-static global field geometry is established in which flux transport by inflows at the surface is balanced by turbulent diffusion. The field is strongly pinched inwards at the surface. A steady-state advection-diffusion model, with turbulent magnetic Prandtl number of order unity, reproduces this geometry well. (3) Weak unsteady disk winds are launched at $z/Rsim1$ with the Alfven radius $R_{A}/R_{0}sim3$. Although the wind is episodic, the time averaged properties are well described by steady wind theory. Wind is not efficient at transporting angular momentum. Even with $beta_{0}=10^3$, only 5% of the angular momentum transport is driven by torque from the wind, and the wind mass flux from the inner decade of radius is only $sim$ 0.4% of the mass accretion rate. With weaker fields or thinner disks, the wind contributes even less. (4) Most of the disk accretion is driven by the $Rphi$ stress from the MRI and global magnetic fields. Our simulations have many applications to astrophysical accretion disk systems.
The time-scales of the variabilities in changing look (CL) active galactic nuclei (AGNs) are usually at the order of years to tens of years (some of them are even shorter than one year), which are much shorter than the viscous timescale of a standard thin accretion disk. It implies that the variabilities of CL AGNs cannot be reproduced by varying the mass accretion rate of the thin disk. In this work, we employ a magnetic accretion disk-outflow model to calculate the inflow time of the disk predominantly driven by magnetic outflows. In this model, most angular momentum of the gas in the disk is carried away by the outflows, and therefore its radial velocity can be substantially higher than that of a conventional viscous disk. Our calculations show that the inflow time of such a disk with outflows can be around several years to tens years. The calculated spectra of the disk with outflows can fit the observed spectra of a CL AGN Mrk 1018 quite well both in the low and high states. The derived inflow time of such a disk with outflows is around 5 years in the high state, while it becomes $sim 20$ years in the low state, which is roughly consistent with the observations of the variabilities in Mrk 1018.
141 -
Nagendra Kumar
, Banibrata Mukhopadhyay (Department of Physics
, n Indian Institute of Science
2021
Many astrophysical sources, e.g., cataclysmic variables, X-ray binaries, active galactic nuclei, exhibit a wind outflow, when they reveal a multicolor blackbody spectrum, hence harboring a geometrically thin Keplerian accretion disk. Unlike an advective disk, in the thin disk, the physical environment, like, emission line, external heating, is expected to play a key role to drive the wind outflow. We show the wind outflow in a thin disk attributing a disk irradiation effect, probably from the inner to outer disks. We solve the set of steady, axisymmetric disk model equations in cylindrical coordinates along the vertical direction for a given launching radius $(r)$ from the midplane, introducing irradiation as a parameter. We obtain an acceleration solution, for a finite irradiation in the presence of a fixed but tiny initial vertical velocity (hence thin disk properties practically do not alter) at the midplane, upto a maximum height ($z^{max}$). We find that wind outflow mainly occurs from the outer region of the disk and its density decreases with increasing launching radius, and for a given launching radius with increasing ejection height. Wind power decreases with increasing ejection height. For $z^{max} < 2r$, wind outflow is ejected tangentially (or parallel to the disk midplane) in all directions with the fluid speed same as the azimuthal speed. This confirms mainly, for low mass X-ray binaries, (a) wind outflow should be preferentially observed in high-inclination sources, (b) the expectation of red and blue shifted absorption lines.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
