Local magnetohydrodynamic instabilities and the wave-driven dynamo in accretion disks

The stability properties of magnetized discs rotating with angular velocity Ω = Ω(s, z), dependent on both the radial and the vertical coordinates s and z, are considered. Such a rotation law is adequate for many astrophysical discs (e.g., galactic and protoplanetary discs, as well as accretion discs in binaries). In general, the angular velocity depends on height, even in thin accretion discs. A linear stability analysis is performed in the Boussinesq approximation, and the dispersion relation is obtained for short-wavelength perturbations. Any dependence of Ω on z can destabilize the flow. This concerns primarily small-scale perturbations for which the stabilizing effect of buoyancy is strongly suppressed due to the energy exchange with the surrounding plasma. For a weak magnetic field, instability of discs is mainly associated with vertical shear, whilst for an intermediate magnetic field the magnetic shear instability, first considered by Chandrasekhar and Velikhov, is more efficient. This instability is caused by the radial shear which is typically much stronger than the vertical shear. Therefore the growth time for the magnetic shear instability is much shorter than for the vertical shear instability. A relatively strong magnetic field can suppress both these instabilities. The vertical shear instability could be the source of turbulence in protoplanetary discs, where the conductivity is low.

view Abstract Citations (44) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dynamo Action by Internal Waves in Accretion Disks Vishniac, Ethan T. ; Jin, Liping ; Diamond, Patrick Abstract An analysis is conducted of the ability of low frequency, slightly nonaxisymmetric waves in thin accretion disks to drive an alpha-Omega dynamo in a disk. The asymmetry in the wave distribution which creates a nonzero helicity follows from the fact that the fundamental waves all have a positive angular momentum flux. As a result there will be a large-scale magnetic field driven by an alpha-Omega dynamo. It is also likely that small-scale fields, driven by higher-order wave modes, will contribute significantly to the local value of B(r)B(phi). It is argued that the magnetic field saturates when its pressure is comparable to the thermal pressure, and a crude model of the nonlinear of transfer of power to small-scale turbulence is presented. Publication: The Astrophysical Journal Pub Date: December 1990 DOI: 10.1086/169518 Bibcode: 1990ApJ...365..648V Keywords: Accretion Disks; Black Holes (Astronomy); Dynamo Theory; Internal Waves; Stellar Magnetic Fields; Angular Momentum; Low Frequencies; Wave Propagation; Astrophysics; BLACK HOLES; MAGNETIC FIELDS; STARS: ACCRETION; WAVE MOTIONS full text sources ADS | data products SIMBAD (3)

Local shear and convective instabilities of internal inertia-gravity waves (IGW) are examined assuming a steady, plane-parallel flow with vertical profiles of horizontal velocity and static stability resembling an IGW packet in a basic state at rest, without mean vertical shear. The eigenproblem can be described in terms of a nondimensional rotation rate R = f/ω0 < 1, where f is the Coriolis parameter, ω0 is IGW intrinsic frequency, and IGW amplitude is a, such that a = 1 for convectively neutral waves. In the nonrotating case, shear instability is possible only for convectively supercritical waves, with horizontal wavevector aligned parallel or nearly parallel to the plane of IGW propagation. Transverse convection, with wavevector aligned perpendicular to the plane of IGW propagation, displays faster growth than parallel shear or convective instability at any horizontal wavenumber. For intermediate R, eigenmodes in supercritical IGW are characterized at small horizontal wavenumber k by a trans...

Millisecond X-ray pulsars have weak magnetic dipole moments of ∼1016 T m3 compared to ordinary X-ray pulsars with dipole moments of 1020 T m3. For this reason a surrounding accretion disc can extend closer to the millisecond X-ray pulsars, and thus reach a higher temperature, at which the opacity is dominated by electron scattering and radiation pressure is strong. We compute the self-similar structure of such a geometrically thin axisymmetric accretion disc with an internal dynamo. Such models produce significantly stronger torques on the neutron star than models without dynamos, and can explain the strong spin variations in some millisecond X-ray pulsars.

view Abstract Citations (62) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Self-consistent Model of Mass and Angular Momentum Transport in Accretion Disks Vishniac, Ethan T. ; Diamond, Patrick Abstract The nature of linear perturbations to a thin, shearing accretion disk composed of compressible material is reexamined. Modes that can carry angular momentum throughout the disk are identified, and it is shown how the subsequent dissipation of these waves through nonlinear processes can form the basis of a viscosity mechanism for realistic accretion disks. It is suggested that the impact of accreting material at the outer edge of the disk provides a natural source of excitation for waves that drive angular momentum transport within the disk. Publication: The Astrophysical Journal Pub Date: December 1989 DOI: 10.1086/168131 Bibcode: 1989ApJ...347..435V Keywords: Accretion Disks; Angular Momentum; Computational Astrophysics; Internal Waves; Sound Waves; Stellar Mass Accretion; Astronomical Models; Energy Dissipation; Optical Thickness; Perturbation Theory; Viscosity; Wave Packets; Astrophysics; STARS: ACCRETION; WAVE MOTIONS full text sources ADS |

On the basis of data from a towed thermistor chain collected near 140°W on the equator during April 1987, the zonal wavenumber and vertical structure of internal gravity waves were observed to vary significantly between wave events. Our hypothesis is that this variability is due to changes in the vertical structure of mean horizontal velocity and density. Assuming that the observed waves were the fastest growing modes for shear instability, we solve the Taylor‐Goldstein equation, using different analytical basic states, including a zonal and meridional flow, to simulate the different conditions during 4 nights of intense internal wave activity. We find that while the observed waves are of finite amplitude, linear shear instability is sufficient to explain the wavelength and vertical structure of vertical displacement for most of the waves. The fastest growing, unstable, mode‐one solutions have e‐folding growth times of less than 10 min. These solutions show wave phase speeds and vertical structures to be highly dependent upon the velocity structure of the uppermost 40 m. Near the base of the mixed layer at a flow inflection point the kinetic energy of the mean flow is extracted for wave growth. Wave vertical displacement is maximum near this inflection point. Zonal phase speeds range from −0.8 to −0.1 m/s. The propagation direction of waves with growth rates of 75% of the maximum growth rate can range from about 45° north to 45° south of the zonal direction. The vertical wave‐induced Reynolds stress divergence could explain a discrepancy in zonal momentum budgets of the upper 90 m of this region. Estimates of this stress divergence show that only about 100 days of intense internal wave activity is needed per year for these internal waves to explain estimated residuals of the mean zonal momentum budgets of this region at 50‐ to 100‐m depth.

We examined the self-gravitation-dominant thin accretion disk and its stability. A self-consistent outer disk model for quasar is given. We derive the instability of the self-gravitation in the region dominated by the self-gravitation. It can explain satisfactorily the optical violent variations in this kind of the quasar.

The line profiles like that of the fluorescent Fe K or Co K lines in the X-ray spectra of the active galactic nuclei (AGN) reflect characteristics of the central regions of these objects. These lines can be formed in the accretion disks around central supermassive black holes and their shapes are connected with the central black hole spin and the accretion disk inclination angle to the line-of-the-sight. If an AGN is a source of a gravitational lens system with microlensing events, one can get an additional important information about both the accretion disk parameters and gravitational lens parameters as well. Microlensing processes were observed in such gravitational lens systems, as PKS 1830-211, B0218+357, RX J1131-1231 i HE1104-1805, Q2237+0305 and we can suspect to observe there also the spectral appearances of microlensing. Here we performed the numerical simulations of the microlensed relativistic spectral line profiles formed in the AGN accretion disks. Using the inear caustic model we show that the time dependence of the profile is determined essentially by the angle between to the disk axis and the caustic. This gives us an opportunity to assess this orientation. Microlens caustics magnify some parts of the accretion disk more prominently than others. Due to the Doppler effects and differences in the rotation direction this leads to the frequency-dependent magnification which distorts the profile of a relativistic spectral line. Such deformations are variable with time due to relative motions of the source and the microlens, and they can give us possibility to obtain some additional information about the disk brightness profile and caustic orientation relatively to the disk. Here we consider the thin disk model, Schwarzschild black hole, and the linear caustic approximation as well. The numerical simulations of the relativistic emission line profiles distorted by strong gravitational microlensing effect were performed for several different orientations of the linear caustic relatively to the disk, as well as several inclinations of the disk to the line-of-the-sight. Basic presumptions for the numerical modeling were the following: (a) AGN is a source in the gravitational lens system and it its inner parts the luminescent emission lines with relativistic profiles are being emitted; (b) this line is formed in the thin accretion disk quite far away from the central black hole and can be calculated with no taking into account the relativistic effects; (c) the caustic can be considered as a linear one. We show that the relative orientation of the caustic and the disk can be determined from emission lines profiles. Our numerical simulations demonstrate that the difference between profiles corresponding to different caustic orientations appears to be more prominent during the first half of the strong microlensing event, namely, before the crossing the disk center, and this dependence is irrespective to the accretion disk brightness profile. We show that for the spectral accuracy level high enough we have a perspective to determine the caustic orientation from the observational data.

High‐resolution thermistor chain data collected between 5 and 20 m in a large stratified freshwater lake (Lake Constance) at a water depth of 60 m reveal the frequent occurrence of large‐amplitude (≥ 1‐m) vertical density inversions that indicate overturns in the pelagic thermocline. Velocity data collected simultaneously with the temperature measurements indicate that the density inversions are mainly produced by shear instabilities. A comparison between the timing of the passage of the basin‐scale internal Kelvin wave and the density inversions demonstrates a pronounced phase relationship, implying that the processes leading to the occurrence of turbulence and mixing are connected to the passage of the Kelvin wave. Two different processes are identified during which density inversions were particularly common. An increased number of density inversions and especially high dissipation rates of turbulent kinetic energy were observed when Kelvin wave–induced critical shear supported the generation of large Kelvin–Helmholtz billows. A particularly large number of density inversions was also associated with the passage of nonlinear highfrequency waves of large amplitudes. The density inversions typically occurred at the wave troughs, which indicates breaking of these waves. These observations indicate that self‐induced shear generated by the basinscale seiche and by high‐frequency internal waves leads to a significant amount of turbulence and mixing in the pelagic thermocline.

Oceanic internal gravity waves span a range between mesoscale eddies and smallscale turbulence, providing an important link in the overall energy cascade from the large scales of generation to the small scales of dissipation. Progress in the understanding of internal‐wave dynamics was discussed at the sixth ‘Aha Huliko'a Hawaiian Winter Workshop, held January 15–18, 1991, in Honolulu. The workshop brought together oceanographers and meteorologists, who came to the following conclusions on the topic. Considerable progress is being made in predicting the diapycnal mixing rates associated with the shear instability of small‐scale internal waves (Figure 1). Large‐scale internal waves are forced by the atmosphere and (possibly) other oceanic flows. These waves are the most energetic and can propagate over distances of O(1000 km).

Abstract. A new method for the reconstruction of internal gravity wave (IGW) parameters from a single vertical temperature profile measurement in the Earth's atmosphere has been developed. This method does not require any additional information not contained in the profile and may be used for the analysis of profiles measured by various techniques. The criterion for the IGW identification has been formulated and argued. In the case when this criterion is satisfied, then analyzed temperature fluctuations can be considered as wave-induced. The method is based on the analysis of relative amplitude thresholds of the temperature wave field and on the linear IGW saturation theory in which amplitude thresholds are restricted by dynamical (shear) instability processes in the atmosphere. When the amplitude of an internal gravity wave reaches the shear instability limit, energy is assumed to be dissipated in such a way that the amplitude is maintained at the instability limit as the wave propagates upwards. In order to approbate the method we have used data of simultaneous high-resolution balloon measurements of the temperature and wind velocity in the Earth's stratosphere over France where a long-period inertia-gravity wave has been detected. Using the radiosonde temperature data only, we have reconstructed all wave parameters, which were determined by radiosondes, with relative deviations not larger than 30%. An application of the method to the radio occultation (RO) data has given the possibility to identify the IGWs in the Earth's stratosphere and to determine the magnitudes of key wave parameters such as the intrinsic frequency, amplitudes of vertical and horizontal perturbations of the wind velocity, vertical and horizontal wavelengths, intrinsic vertical and horizontal phase (and group) speeds, kinetic and potential energy, vertical fluxes of the wave energy and horizontal momentum. The obtained results of internal wave studies in the Earth's stratosphere deduced from the COSMIC and CHAMP GPS occultation temperature profiles are presented and discussed.

The possibility of observationally discriminating between various types of neutron stars, described by different equations of state of the nuclear matter, as well as differentiating neutron stars from other types of exotic objects, like, for example, quark stars, is one of the fundamental problems in contemporary astrophysics. We consider and investigate carefully the possibility that different types of rapidly rotating neutron stars, as well as other type of compact general relativistic objects, can be differentiated from the study of the emission properties of the accretion disks around them. We obtain the energy flux, the temperature distribution and the emission spectrum from the accretion disks around several classes of rapidly rotating neutron stars, described by different equations of state of the neutron matter, and for quark stars, described by the MIT bag model equation of state and in the CFL (Color-Flavor-Locked) phase, respectively. Particular signatures appear in the electromagnetic spectrum, thus leading to the possibility of directly testing the equation of state of the dense matter by using astrophysical observations of the emission spectra from accretion disks.

view Abstract Citations (223) References (44) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Irradiation of Accretion Disks around Young Objects. I. Near-Infrared CO Bands Calvet, Nuria ; Patino, Alberto ; Magris, Gladis C. ; D'Alessio, Paola Abstract The effect of irradiation by the central star on the atmospheres of optically thick physically thin accretion disks around young objects are approximated, assuming radiative equilibrium. Irradiation is found to increase the temperature in the atmosphere of a viscous accretion disk relative to the case when only the viscous flux goes through it. Its effect on the near-infrared spectrum of the star-disk configuration is to decrease the strength of the absorption in the CO bands or turn them into emission, depending on the irradiation rate and on the mass accretion rate. As the stellar effect temperature increases for a given mass accretion rate, the disk surface temperature increases and the CO bands turn into emission. Observed spectra of young objects such as T Tauri stars, FU Ori subjects, and Herbig Be/Ae stars are in qualitative agreement with those predicted using the corresponding stellar parameters and disk mass accretion rates. It is inferred that the near-infrared CO bands can be used as indicators of the rate of mass accretion in the disk around young objects. Publication: The Astrophysical Journal Pub Date: October 1991 DOI: 10.1086/170618 Bibcode: 1991ApJ...380..617C Keywords: Accretion Disks; Carbon Monoxide; Near Infrared Radiation; Radiative Transfer; Stellar Atmospheres; A Stars; B Stars; Computational Astrophysics; Stellar Mass Accretion; Stellar Temperature; T Tauri Stars; Astrophysics; RADIATIVE TRANSFER; STARS: ACCRETION; STARS: PRE--MAIN-SEQUENCE full text sources ADS | data products SIMBAD (6) Related Materials (1) Part 2: 1992RMxAA..24...27C

The long time effect of weak rotation on an internal solitary wave is the decay into inertia-gravity waves and the eventual formation of a localised wavepacket. Here this initial value problem is considered within the context of the Ostrovsky, or the rotation-modified Korteweg-de Vries (KdV), equation and a numerical method for obtaining accurate wavepacket solutions is presented. The flow evolutions are described in the regimes of relatively-strong and relatively-weak rotational effects. When rotational effects are relatively strong a second-order soliton solution of the nonlinear Schrödinger equation accurately predicts the shape, and phase and group velocities of the numerically determined wavepackets. It is suggested that these solitons may form from a local Benjamin-Feir instability in the inertia-gravity wave-train radiated when a KdV solitary wave rapidly adjusts to the presence of strong rotation. When rotational effects are relatively weak the initial KdV solitary wave remains coherent longer, decaying only slowly due to weak radiation and modulational instability is no longer relevant. Wavepacket solutions in this regime appear to consist of a modulated KdV soliton wavetrain propagating on a slowly varying background of finite extent.

We study magnetic field topologies and magnetic field strength in a thin accretion disk around a rotating black hole. The magnetic field is assumed to enter the disk at the outer edge and is amplified in the accretion process by differential rotation. This scenario seems likely for AGN, where magnetized plasma from a molecular torus flows into an inner accretion disk.