Turbulent Transport in Global Models of Magnetized Accretion Disks

The modern theory of accretion disks is dominated by the discovery of the magnetorotational instability (MRI). While hydrodynamic disks satisfy Rayleigh's criterion and there exists no known unambiguous route to turbulence in such disks, a weakly magnetized disk of plasma is subject to the MRI and will become turbulent. This MRI-driven magnetohydrodnamic turbulence generates a strong anisotropic correlation between the radial and azimuthal magnetic fields which drives angular momentum outwards. Accretion disks perform two vital functions in various astrophysical systems: an intermediate step in the gravitational collapse of a rotating gas, where the disk transfers angular momentum outwards and allows material to fall inwards; and as a power source, where the gravitational potential energy of infalling matter can be converted to luminosity. Accretion disks are important in astrophysical processes at all scales in the universe. Studying accretion from first principles is difficult, as analytic treatments of turbulent systems have proven quite limited. As such, computer simulations are at the forefront of studying systems this far into the non-linear regime. While computational work is necessary to study accretion disks, it is no panacea. Fully three-dimensional simulations of turbulent astrophysical systems require an enormous amount of computational power that is inaccessible even to sophisticated modern supercomputers. These limitations have necessitated the use of local models, in which a small spatial region of the full disk is simulated, and constrain numerical resolution to what is feasible. These compromises, while necessary, have the potential to introduce numerical artifacts in the resulting simulations. Understanding how to disentangle these artifacts from genuine physical phenomena and to minimize their effect is vital to constructing simulations that can make reliable astrophysical predictions and is the primary concern of the work presented here. The use of local models is predicated on the assumption that these models accurately capture the dynamics of a small patch of a global astrophysical disk. This assumption is tested in detail through the study of local regions of global simulations. To reach resolutions comparable to those used in local simulations an orbital advection algorithm, a semi-Lagrangian reformulation of the fluid equations, is used which allows an order of magnitude increase in computational efficiency. It is found that the turbulence in global simulations agrees at intermediate- and small-scales with local models and that the presence of magnetic flux stimulates angular momentum transport in global simulations in a similar manner to that observed for local ones. However, the importance of this flux-stress connection is shown to cast doubt on the validity of local models due to their inability to accurately capture the temporal evolution of the magnetic flux seen in global simulations. The use of orbital advection allows the ability to probe previously-inaccessible resolutions in global simulations and is the basis for a rigorous resolution study presented here. Included are the results of a study utilizing a series of global simulations of varying resolutions and initial magnetic field topologies where a collection of proposed metrics of numerical convergence are explored. The resolution constraints necessary to establish numerical convergence of astrophysically-important measurements are presented along with evidence suggesting that the use of proper azimuthal resolution, while computationally-demanding, is vital to achieving convergence. The majority of the proposed metrics are found to be useful diagnostics of MRI-driven turbulence, however they suffer as metrics of convergence due to their dependence on the initial magnetic field topology. In contrast to this, the magnetic tilt angle, a measure of the planar anisotropy of the magnetic field, is found to be a powerful tool for diagnosing convergence independent of initial magnetic field topology

Connections Between Local and Global Turbulence in Accretion Disks

We analyze a suite of global magnetohydrodynamic (MHD) accretion disk simulations in order to determine whether scaling laws for turbulence driven by the magnetorotational instability, discovered via local shearing box studies, are globally robust. The simulations model geometrically-thin disks with zero net magnetic flux and no explicit resistivity or viscosity. We show that the local Maxwell stress is correlated with the self-generated local vertical magnetic field in a manner that is similar to that found in local simulations. Moreover, local patches of vertical field are strong enough to stimulate and control the strength of angular momentum transport across much of the disk. We demonstrate the importance of magnetic linkages (through the low-density corona) between different regions of the disk in determining the local field, and suggest a new convergence requirement for global simulations -- the vertical extent of the corona must be fully captured and resolved. Finally, we examine the temporal convergence of the average stress, and show that an initial long-term secular drift in the local flux-stress relation dies away on a time scale that is consistent with turbulent mixing of the initial magnetic field.Comment: 8 Pages, 7 Figures ApJ, In Pres

Low-Frequency Oscillations in Global Simulations of Black Hole Accretion

We have identified the presence of large-scale, low-frequency dynamo cycles in a long-duration, global, magnetohydrodynamic (MHD) simulation of black hole accretion. Such cycles had been seen previously in local shearing box simulations, but we discuss their evolution over 1,500 inner disk orbits of a global pi/4 disk wedge spanning two orders of magnitude in radius and seven scale heights in elevation above/below the disk midplane. The observed cycles manifest themselves as oscillations in azimuthal magnetic field occupying a region that extends into a low-density corona several scale heights above the disk. The cycle frequencies are ten to twenty times lower than the local orbital frequency, making them potentially interesting sources of low-frequency variability when scaled to real astrophysical systems. Furthermore, power spectra derived from the full time series reveal that the cycles manifest themselves at discrete, narrow-band frequencies that often share power across broad radial ranges. We explore possible connections between these simulated cycles and observed low-frequency quasi-periodic oscillations (LFQPOs) in galactic black hole binary systems, finding that dynamo cycles have the appropriate frequencies and are located in a spatial region associated with X-ray emission in real systems. Derived observational proxies, however, fail to feature peaks with RMS amplitudes comparable to LFQPO observations, suggesting that further theoretical work and more sophisticated simulations will be required to form a complete theory of dynamo-driven LFQPOs. Nonetheless, this work clearly illustrates that global MHD dynamos exhibit quasi-periodic behavior on timescales much longer than those derived from test particle considerations.Comment: Version accepted to The Astrophysical Journal, 8 pages, 7 figure

Oxygen Ion Escape at Venus Associated With Three-Dimensional Kelvin-Helmholtz Instability

How oxygens escape from Venus has long been a fundamental but controversial topic in the planetary research. Among various key mechanisms, the Kelvin-Helmholtz instability (KHI) has been suggested to play an important role in the oxygen ion escape from Venus. Limited by either scarce in-situ observations or simplified theoretical estimations, the mystery of oxygen ion escape process associated with KHI is still unsettled. Here we present the first three-dimensional configuration of KHI at Venus with a global multifluid magnetohydrodynamics model, showing a significantly fine structure and evolution of the KHI. KHI mainly occurred at the low latitude boundary layer if defining the interplanetary magnetic field-perpendicular plane as the equatorial plane, resulting in escaping oxygen ions through mixing with the solar wind at the Venusian boundary layer, with an escape rate around 4 × 1024 s−1. The results provide new insights into the basic physical process of atmospheric loss at other unmagnetized planet

How Jupiter's Unusual Magnetospheric Topology Structures Its Aurora

Jupiter's bright persistent polar aurora and Earth's dark polar region indicate that the planets' magnetospheric topologies are very different. High-resolution global simulations show that the reconnection rate at the interface between the interplanetary and jovian magnetic fields is too slow to generate a magnetically open, Earth-like polar cap on the timescale of planetary rotation, resulting in only a small crescent-shaped region of magnetic flux interconnected with the interplanetary magnetic field. Most of the jovian polar cap is threaded by helical magnetic flux that closes within the planetary interior, extends into the outer magnetosphere and piles-up near its dawnside flank where fast differential plasma rotation pulls the field lines sunward. This unusual magnetic topology provides new insights into Jupiter's distinctive auroral morphology

Mesoscale phenomena and their contribution to the global response: a focus on the magnetotail transition region and magnetosphere-ionosphere coupling

An important question that is being increasingly studied across subdisciplines of Heliophysics is “how do mesoscale phenomena contribute to the global response of the system?” This review paper focuses on this question within two specific but interlinked regions in Near-Earth space: the magnetotail’s transition region to the inner magnetosphere and the ionosphere. There is a concerted effort within the Geospace Environment Modeling (GEM) community to understand the degree to which mesoscale transport in the magnetotail contributes to the global dynamics of magnetic flux transport and dipolarization, particle transport and injections contributing to the storm-time ring current development, and the substorm current wedge. Because the magnetosphere-ionosphere is a tightly coupled system, it is also important to understand how mesoscale transport in the magnetotail impacts auroral precipitation and the global ionospheric system response. Groups within the Coupling, Energetics and Dynamics of Atmospheric Regions Program (CEDAR) community have also been studying how the ionosphere-thermosphere responds to these mesoscale drivers. These specific open questions are part of a larger need to better characterize and quantify mesoscale “messengers” or “conduits” of information—magnetic flux, particle flux, current, and energy—which are key to understanding the global system. After reviewing recent progress and open questions, we suggest datasets that, if developed in the future, will help answer these questions