The Experts below are selected from a list of 6303 Experts worldwide ranked by ideXlab platform
Dong Lai - One of the best experts on this subject based on the ideXlab platform.
-
Corotational instability magnetic resonances and global inertial acoustic oscillations in magnetized black hole accretion discs
Monthly Notices of the Royal Astronomical Society, 2011Co-Authors: Dong LaiAbstract:Low-order, non-axisymmetric p-modes (also referred as inertial-acoustic modes) in hydrodynamic accretion discs around black holes are plausible candidates for high- frequency quasi-periodic oscillations (QPOs) observed in a number of accreting black- hole systems. These modes are trapped in the inner-most region of the accretion disc, and are subject to global instabilities due to wave absorption at the Corotation reso- nance (where the wave pattern frequency !=m equals the disc rotation rate ), when the fluid vortensity, � = � 2 =(2�) (whereandare the radial epicyclic frequency and disc surface density, respectively), has a positive gradient. We investigate the effects of disc magnetic fields on the wave absorption at Corotation and the related wave super-reflection of the Corotation barrier, and on the overstability of disc p- modes. In general, in the presence of magnetic fields, the p-modes have the character of inertial-fast magnetosonic waves in their propagation zone. For discs with a pure toroidal field, the Corotation resonance is split into two magnetic resonances, where the wave frequency in the corotating frame of the fluid, ˜ ! = ! m, matches the slow magnetosonic wave frequency. Significant wave energy/angular momentum absorption occurs at both magnetic resonances, but with opposite signs, such that one of them enhances the super-reflection while the other diminishes it. The combined effect of the two magnetic resonances is to reduce the super-reflection and the growth rate of the overstable p-modes. Our calculations show that even a subthermal toroidal field (with the magnetic pressure less than the gas pressure) may suppress the overstabil- ity of hydrodynamic (B = 0) p-modes. For accretion discs with mixed (toroidal and vertical) magnetic fields, two additional Alfven resonances appear, where ˜ ! matches the local Alfven wave frequency. The effect of these additional resonances is to fur- ther reduce or diminish the growth rate of p-modes. Our results suggest that in order for the non-axisymmetric p-modes to be a viable candidate for the observed high- frequency QPOs, the disc magnetic field must be appreciably subthermal, or other mode excitation mechanisms are at work.
-
Corotational instability magnetic resonances and global inertial acoustic oscillations in magnetized black hole accretion discs
arXiv: High Energy Astrophysical Phenomena, 2010Co-Authors: Dong LaiAbstract:Low-order, non-axisymmetric p-modes (also referred as inertial-acoustic modes) trapped in the inner-most region of hydrodynamic accretion discs around black holes, are plausible candidates for high-frequency quasi-periodic oscillations (QPOs) observed in a number of accreting black-hole systems. These modes are subject to global instabilities due to wave absorption at the Corotation resonance (where the wave pattern frequency $\omega/m$ equals the disc rotation rate $\Omega$), when the fluid vortensity, $\zeta=\kappa^2/(2\Omega\Sigma)$ (where $\kappa$ and $\Sigma$ are the radial epicyclic frequency and disc surface density, respectively), has a positive gradient. We investigate the effects of disc magnetic fields on the wave absorption at Corotation and the related wave super-reflection of the Corotation barrier, and on the overstability of disc p-modes. For discs with a pure toroidal field, the Corotation resonance is split into two magnetic resonances, where the wave frequency in the corotating frame of the fluid, $\tomega=\omega-m\Omega$, matches the slow magnetosonic wave frequency. Significant wave energy/angular momentum absorption occurs at both magnetic resonances, but with opposite signs. The combined effect of the two magnetic resonances is to reduce the super-reflection and the growth rate of the overstable p-modes. We show that even a subthermal toroidal field may suppress the overstability of hydrodynamic (B=0) p-modes. For accretion discs with mixed (toroidal and vertical) magnetic fields, two additional Alfven resonances appear, where $\tomega$ matches the local Alfven wave frequency. They further reduce the growth rate of p-modes. Our results suggest that in order for the non-axisymmetric p-modes to be a viable candidate for the observed high-frequency QPOs, the disc magnetic field must be appreciably subthermal, or other mode excitation mechanisms are at work.
-
Corotational instability of inertial acoustic modes in black hole accretion discs non barotropic flows
Monthly Notices of the Royal Astronomical Society, 2009Co-Authors: David Tsang, Dong LaiAbstract:We study the effect of Corotation resonance on the inertial-acoustic oscillations (p-modes) of black hole accretion discs. Previous works have shown that for barotropic flows (where the pressure depends only on the density), wave absorption at the Corotation resonance can lead to mode growth when the disc vortensity, ζ = κ 2 /(2ΩΣ) (where Ω, κ, Σ are the rotation rate, radial epicyclic frequency and surface density of the disc, respectively), has a positive gradient at the Corotation radius. Here, we generalize the analysis of the Corotation resonance effect to non-barotropic fluids. We show that the mode instability criterion is modified by the finite radial Brunt―Vaisala frequency of the disc. We derive an analytic expression for the reflectivity when a density wave impinges upon the Corotation barrier, and calculate the frequencies and growth rates of global p-modes for disc models with various α-viscosity parametrizations. We find that for disc fluids with constant adiabatic index r, super-reflection and mode growth depend on the gradient of the effective vortensity, ζ eff = ζ/S 2/Γ (where S ≡ P/Σ Γ measures the entropy): when dζ eff /dr > 0 at the Corotation radius, wave absorption leads to amplification of the p-mode. Our calculations show that the lowest order p-modes with azimuthal wave number m = 2, 3, 4, ... have the largest growth rates, with the frequencies approximately in (but distinct from) the 2:3:4... commensurate ratios. We discuss the implications of our results for the high-frequency quasi-periodic oscillations observed in accreting black hole systems.
-
Corotational instability of inertial acoustic modes in black hole accretion discs non barotropic flows
arXiv: High Energy Astrophysical Phenomena, 2009Co-Authors: David Tsang, Dong LaiAbstract:We study the effect of Corotation resonance on the inertial-acoustic oscillations (p-modes) of black-hole accretion discs. Previous works have shown that for barotropic flows (where the pressure depends only on the density), wave absorption at the Corotation resonance can lead to mode growth when the disc vortensity, $\zeta=\kappa^2/(2\Omega\Sigma)$ (where $\Omega, \kappa, \Sigma$ are the rotation rate, radial epicyclic frequency and surface density of the disc, respectively), has a positive gradient at the Corotation radius. Here we generalize the analysis of the Corotation resonance effect to non-barotropic fluids. We show that the mode instability criterion is modified by the finite radial Brunt-V\"as\"al\"a frequency of the disc. We derive an analytic expression for the reflectivity when a density wave impinges upon the Corotation barrier, and calculate the frequencies and growth rates of global p-modes for disc models with various $\alpha$-viscosity parameterizations. We find that for disc fluids with constant adiabatic index $\Gamma$, super-reflection and mode growth depend on the gradient of the effective vortensity, $\zeta_{\rm eff} = \zeta/S^{2/\Gamma}$ (where $S \equiv P/\Sigma^{\Gamma}$ measures the entropy): when $d\zeta_{\rm eff}/dr > 0$ at the Corotation radius, wave absorption leads to amplification of the p-mode. Our calculations show that the lowest-order p-modes with azimuthal wave number $m=2, 3, 4,...$ have the largest growth rates, with the frequencies approximately in (but distinct from) the $2:3:4...$ commensurate ratios. We discuss the implications of our results for the high-frequency quasi-periodic oscillations observed in accreting black-hole systems.
Sijmejan Paardekooper - One of the best experts on this subject based on the ideXlab platform.
-
low mass planet migration in magnetically torqued dead zones ii flow locked and runaway migration and a torque prescription
Monthly Notices of the Royal Astronomical Society, 2018Co-Authors: Colin P Mcnally, Richard P Nelson, Sijmejan PaardekooperAbstract:We examine the migration of low mass planets in laminar protoplanetary discs, threaded by large scale magnetic fields in the dead zone that drive radial gas flows. As shown in Paper I, a dynamical Corotation torque arises due to the flow-induced asymmetric distortion of the Corotation region and the evolving vortensity contrast between the librating horseshoe material and background disc flow. Using simulations of laminar torqued discs containing migrating planets, we demonstrate the existence of the four distinct migration regimes predicted in Paper I. In two regimes, the migration is approximately locked to the inward or outward radial gas flow, and in the other regimes the planet undergoes outward runaway migration that eventually settles to fast steady migration. In addition, we demonstrate torque and migration reversals induced by midplane magnetic stresses, with a bifurcation dependent on the disc surface density. We develop a model for fast migration, and show why the outward runaway saturates to a steady speed, and examine phenomenologically its termination due to changing local disc conditions. We also develop an analytical model for the Corotation torque at late times that includes viscosity, for application to discs that sustain modest turbulence. Finally, we use the simulation results to develop torque prescriptions for inclusion in population synthesis models of planet formation.
-
low mass planet migration in hall affected disks
Journal of Physics: Conference Series, 2018Co-Authors: Colin P Mcnally, Richard P Nelson, Sijmejan Paardekooper, Oliver Gressel, Wladimir LyraAbstract:Recent developments in non-ideal magnetohydrodynamic simulations of protoplanetary disks suggest that instead of being traditional turbulent (viscous) accretion disks, they have a largely laminar flow with accretion driven by large-scale wind torques. These disks are possibly threaded by Hall-effect generated large-scale horizontal magnetic fields. We have examined the dynamics of the Corotation region of a low mass planet embedded in such a disk and the evolution of the associated migration torque. These disks lack strong turbulence and associated turbulent diffusion, and the presence of a magnetic field and radial gas flow presents a situation outside the applicability of previous Corotation torque theory. We summarize the analytical analysis of the Corotation torque, give details on the numerical methods used, and in particular the relative merits of different numerical schemes for the inviscid problem.
-
low mass planet migration in magnetically torqued dead zones i static migration torque
Monthly Notices of the Royal Astronomical Society, 2017Co-Authors: Colin P Mcnally, Richard P Nelson, Sijmejan Paardekooper, Oliver Gressel, Wladimir LyraAbstract:Motivated by models suggesting that the inner planet forming regions of protoplanetary discs are predominantly lacking in viscosity-inducing turbulence, and are possibly threaded by Hall-effect generated large-scale horizontal magnetic fields, we examine the dynamics of the Corotation region of a low-mass planet in such an environment. The Corotation torque in an inviscid, isothermal, dead zone ought to saturate, with the libration region becoming both symmetrical and of a uniform vortensity, leading to fast inward migration driven by the Lindblad torques alone. However, in such a low viscosity situation, the material on librating streamlines essentially preserves its vortensity. If there is relative radial motion between the disc gas and the planet, the librating streamlines will no longer be symmetrical. Hence, if the gas is torqued by a large scale magnetic field so that it undergoes a net inflow or outflow past the planet, driving evolution of the vortensity and inducing asymmetry of the Corotation region, the Corotation torque can grow, leading to a positive torque. In this paper we treat this effect by applying a symmetry argument to the previously studied case of a migrating planet in an inviscid disc. Our results show that the Corotation torque due to a laminar Hall-induced magnetic field in a dead zone behaves quite differently from that studied previously for a viscous disc. Furthermore, the magnetic field induced Corotation torque and the dynamical Corotation torque in a low viscosity disc can be regarded as one unified effect.
-
a torque formula for non isothermal type i planetary migration ii effects of diffusion
Monthly Notices of the Royal Astronomical Society, 2011Co-Authors: Sijmejan Paardekooper, Clement Baruteau, Willy KleyAbstract:We study the effects of diffusion on the non-linear Corotation torque, or horseshoe drag, in the two-dimensional limit, focusing on low-mass planets for which the width of the horseshoe region is much smaller than the scaleheight of the disc. In the absence of diffusion, the non-linear Corotation torque saturates, leaving only the Lindblad torque. Diffusion of heat and momentum can act to sustain the Corotation torque. In the limit of very strong diffusion, the linear Corotation torque is recovered. For the case of thermal diffusion, this limit corresponds to having a locally isothermal equation of state. We present some simple models that are able to capture the dependence of the torque on diffusive processes to within 20 per cent of the numerical simulations.
-
a torque formula for non isothermal type i planetary migration ii effects of diffusion
arXiv: Earth and Planetary Astrophysics, 2010Co-Authors: Sijmejan Paardekooper, Clement Baruteau, Willy KleyAbstract:We study the effects of diffusion on the non-linear Corotation torque, or horseshoe drag, in the two-dimensional limit, focusing on low-mass planets for which the width of the horseshoe region is much smaller than the scale height of the disc. In the absence of diffusion, the non-linear Corotation torque saturates, leaving only the Lindblad torque. Diffusion of heat and momentum can act to sustain the Corotation torque. In the limit of very strong diffusion, the linear Corotation torque is recovered. For the case of thermal diffusion, this limit corresponds to having a locally isothermal equation of state. We present some simple models that are able to capture the dependence of the torque on diffusive processes to within 20% of the numerical simulations.
S M Krimigis - One of the best experts on this subject based on the ideXlab platform.
-
drift resonant relativistic electron acceleration at the outer planets insights from the response of saturn s radiation belts to magnetospheric storms
Icarus, 2018Co-Authors: Elias Roussos, D C Hamilton, P Kollmann, N Krupp, C Paranicas, K Dialynas, N Sergis, D G Mitchell, S M KrimigisAbstract:Abstract The short, 7.2-day orbital period of Cassini’s Ring Grazing Orbits (RGO) provided an opportunity to monitor how fast the effects of an intense magnetospheric storm-time period (days 336–343/2016) propagated into Saturn’s electron radiation belts. Following the storms, Cassini’s MIMI/LEMMS instrument detected a transient extension of the electron radiation belts that in subsequent orbits moved towards the inner belts, intensifying them in the process. This intensification was followed by an equally fast decay, possibly due to the rapid absorption of MeV electrons by the planet’s main rings. Surprisingly, all this cycle was completed within four RGOs, effectively in less than a month. That is considerably faster than the year-long time scales of Saturn’s proton radiation belt evolution. In order to explain this difference, we propose that electron radial transport is partly controlled by the variability of global scale electric fields which have a fixed local time pointing. Such electric fields may distort significantly the orbits of a particular class of energetic electrons that cancel out magnetospheric Corotation due to their westward gradient and curvature drifts (termed “Corotation-resonant” or “local-time stationary” electrons) and transport them radially between the ring current and the radiation belts within several days and few weeks. The significance of the proposed process is highlighted by the fact that Corotation resonance at Saturn occurs for electrons of few hundred keV to several MeV. These are the characteristic energies of seed electrons from the ring current that sustain the radiation belts of the planet. Our model’s feasibility is demonstrated through the use of a simple test-particle simulation, where we estimate that uniform but variable electric fields with magnitudes lower that 1.0 mV/m can lead to a very efficient transport of Corotation resonant electrons. Such electric fields have been consistently measured in the magnetosphere, and here we provide additional evidence showing that they may be constantly present all the way down to the outer edge of Saturn’s main rings, further supporting our model. The implications of our findings are not limited to Saturn. Corotation resonance at Jupiter occurs for electrons with energies above about 10 MeV throughout the quasi-dipolar, energetic particle-trapping region of the magnetosphere. The proposed process could in principle then lead to rapid transport and adiabatic acceleration electrons into ultra-relativistic energies. The observation by Galileo’s EPD/LEMMS instrument of an intense Jovian acceleration event at the orbital distance of Ganymede during the mission’s C22 orbit, when > 11 MeV electron fluxes were preferentially enhanced, provides additional support to our transport model and insights on the origin of that orbit’s extreme energetic electron environment. Finally, if the mode of radial transport that we describe here is a dominant one, radial diffusion coefficients (DLL) would be subject to strong energy, pitch angle and species dependencies.
Wouter Bleeker - One of the best experts on this subject based on the ideXlab platform.
-
paleomagnetism and rock magnetism of the ca 1 87 ga pearson formation northwest territories canada a test of vertical axis rotation within the great slave basin
Precambrian Research, 2018Co-Authors: Zheng Gong, David A D Evans, Paul F Hoffman, Ross N Mitchell, Wouter BleekerAbstract:Abstract A geometrically quantitative plate-kinematic model, based on paleomagnetism, for the initial assembly of Laurentia has taken form in the past few decades. Within this framework, there remains but one problematic interval of data predominantly from the Slave craton, which is the 1.96–1.87 Ga Coronation apparent polar wander path (APWP). The Coronation APWP shows large (∼110°) back-and-forth oscillations that are difficult to explain in terms of plate motion. Nonetheless, poles from the Coronation APWP have been incorporated in various paleogeographic reconstructions of Laurentia and the supercontinent Nuna, pointing to the importance of testing its veracity. In this study, we conducted a detailed paleomagnetic and rock magnetic study of the ca. 1.87 Ga Pearson Formation, East Arm of Great Slave Lake, Northwest Territories, Canada. Our results show that Pearson Formation yields a characteristic remanent magnetization carried by single-domain or small pseudo-single-domain magnetite. The age of the magnetization is constrained to be older than Paleoproterozoic deformation and is interpreted as primary. Paleomagnetic declinations reveal a one-to-one correlation with local structural attitudes, indicating that some small blocks in the fold belt likely experienced significant (∼60°) vertical-axis rotations, presumably related to large dextral displacements along the McDonald Fault system. Alternative explanations, such as true polar wander or a non-dipole magnetic field, are considered less parsimonious for the data presented here. It is suspected that some existing Christie Bay Group poles (the Stark and Tochatwi Formations), which were sampled in areas with anomalous structural attitudes and differ from time-equivalent poles obtained from areas of the Slave craton far from major transcurrent faults, may similarly suffer from vertical-axis rotation. We suggest further study before using possibly rotated Christie Bay Group poles for paleogeographic reconstructions.
Paul F Hoffman - One of the best experts on this subject based on the ideXlab platform.
-
paleomagnetism and rock magnetism of the ca 1 87 ga pearson formation northwest territories canada a test of vertical axis rotation within the great slave basin
Precambrian Research, 2018Co-Authors: Zheng Gong, David A D Evans, Paul F Hoffman, Ross N Mitchell, Wouter BleekerAbstract:Abstract A geometrically quantitative plate-kinematic model, based on paleomagnetism, for the initial assembly of Laurentia has taken form in the past few decades. Within this framework, there remains but one problematic interval of data predominantly from the Slave craton, which is the 1.96–1.87 Ga Coronation apparent polar wander path (APWP). The Coronation APWP shows large (∼110°) back-and-forth oscillations that are difficult to explain in terms of plate motion. Nonetheless, poles from the Coronation APWP have been incorporated in various paleogeographic reconstructions of Laurentia and the supercontinent Nuna, pointing to the importance of testing its veracity. In this study, we conducted a detailed paleomagnetic and rock magnetic study of the ca. 1.87 Ga Pearson Formation, East Arm of Great Slave Lake, Northwest Territories, Canada. Our results show that Pearson Formation yields a characteristic remanent magnetization carried by single-domain or small pseudo-single-domain magnetite. The age of the magnetization is constrained to be older than Paleoproterozoic deformation and is interpreted as primary. Paleomagnetic declinations reveal a one-to-one correlation with local structural attitudes, indicating that some small blocks in the fold belt likely experienced significant (∼60°) vertical-axis rotations, presumably related to large dextral displacements along the McDonald Fault system. Alternative explanations, such as true polar wander or a non-dipole magnetic field, are considered less parsimonious for the data presented here. It is suspected that some existing Christie Bay Group poles (the Stark and Tochatwi Formations), which were sampled in areas with anomalous structural attitudes and differ from time-equivalent poles obtained from areas of the Slave craton far from major transcurrent faults, may similarly suffer from vertical-axis rotation. We suggest further study before using possibly rotated Christie Bay Group poles for paleogeographic reconstructions.
