A self-sustaining nonlinear dynamo process in Keplerian shear flows

A three-dimensional nonlinear dynamo process is identified in rotating plane Couette flow in the Keplerian regime. It is analogous to the hydrodynamic self-sustaining process in non-rotating shear flows and relies on the magneto-rotational instability of a toroidal magnetic field. Steady nonlinear solutions are computed numerically for a wide range of magnetic Reynolds numbers but are restricted to low Reynolds numbers. This process may be important to explain the sustenance of coherent fields and turbulent motions in Keplerian accretion disks, where all its basic ingredients are present.Comment: 4 pages, 7 figures, accepted for publication in Physical Review Letter

Dissipative effects on the sustainment of a magnetorotational dynamo in Keplerian shear flow

The magnetorotational (MRI) dynamo has long been considered one of the possible drivers of turbulent angular momentum transport in astrophysical accretion disks. However, various numerical results suggest that this dynamo may be difficult to excite in the astrophysically relevant regime of magnetic Prandtl number (Pm) significantly smaller than unity, for reasons currently not well understood. The aim of this article is to present the first results of an ongoing numerical investigation of the role of both linear and nonlinear dissipative effects in this problem. Combining a parametric exploration and an energy analysis of incompressible nonlinear MRI dynamo cycles representative of the transitional dynamics in large aspect ratio shearing boxes, we find that turbulent magnetic diffusion makes the excitation and sustainment of this dynamo at moderate magnetic Reynolds number (Rm) increasingly difficult for decreasing Pm. This results in an increase in the critical Rm of the dynamo for increasing kinematic Reynolds number (Re), in agreement with earlier numerical results. Given its very generic nature, we argue that turbulent magnetic diffusion could be an important determinant of MRI dynamo excitation in disks, and may also limit the efficiency of angular momentum transport by MRI turbulence in low Pm regimes.Comment: 7 pages, 6 figure

Magnetorotational dynamo chimeras. The missing link to turbulent accretion disk dynamo models?

In Keplerian accretion disks, turbulence and magnetic fields may be jointly excited through a subcritical dynamo process involving the magnetorotational instability (MRI). High-resolution simulations exhibit a tendency towards statistical self-organization of MRI dynamo turbulence into large-scale cyclic dynamics. Understanding the physical origin of these structures, and whether they can be sustained and transport angular momentum efficiently in astrophysical conditions, represents a significant theoretical challenge. The discovery of simple periodic nonlinear MRI dynamo solutions has recently proven useful in this respect, and has notably served to highlight the role of turbulent magnetic diffusion in the seeming decay of the dynamics at low magnetic Prandtl number Pm (magnetic diffusivity larger than viscosity), a common regime in accretion disks. The connection between these simple structures and the statistical organization reported in turbulent simulations remained elusive, though. Here, we report the numerical discovery in moderate aspect ratio Keplerian shearing boxes of new periodic, incompressible, three-dimensional nonlinear MRI dynamo solutions with a larger dynamical complexity reminiscent of such simulations. These "chimera" cycles are characterized by multiple MRI-unstable dynamical stages, but their basic physical principles of self-sustainment are nevertheless identical to those of simpler cycles found in azimuthally elongated boxes. In particular, we find that they are not sustained at low Pm either due to subcritical turbulent magnetic diffusion. These solutions offer a new perspective into the transition from laminar to turbulent instability-driven dynamos, and may prove useful to devise improved statistical models of turbulent accretion disk dynamos.Comment: 12 pages, 8 figures, submitted to A&

Corruption and Development: The Need for International Investigations with a Multijurisdictional Approach Involving Multilateral Development Banks and National Authorities

We argue that while Multilateral Development Banks (“MDBs”) and national governments have mechanisms to fight corruption, the objectives and outcomes of these enforcement mechanisms diverge. MDBs are interested in the causes and effects of corruption from a development perspective and, as such, tend to sanction small and medium enterprises and individuals, while national governments are focused on a more punitive outcome, targeting larger multinational corporations. This article examines the enforcement objectives articulated in national legislation, namely the US Foreign and Corrupt Practices Act and its Canadian counterpart, the Corruption of Foreign Public Officials Act, as well as several Canadian cases, on the one hand, and the tools and outcomes of MDBs’ sanctions systems on the other. We conclude that national enforcement efforts and MDBs’ sanctions outcomes intersect in their fight against international corruption in that their results are complementary; the former punishing large-scale offenders while the latter ensuring the integrity of development projects

Corruption and Development: The Need for International Investigations with a Multijurisdictional Approach Involving Multilateral Development Banks and National Authorities

We argue that while Multilateral Development Banks (“MDBs”) and national governments have mechanisms to fight corruption, the objectives and outcomes of these enforcement mechanisms diverge. MDBs are interested in the causes and effects of corruption from a development perspective and, as such, tend to sanction small and medium enterprises and individuals, while national governments are focused on a more punitive outcome, targeting larger multinational corporations. This article examines the enforcement objectives articulated in national legislation, namely the US Foreign and Corrupt Practices Act and its Canadian counterpart, the Corruption of Foreign Public Officials Act, as well as several Canadian cases, on the one hand, and the tools and outcomes of MDBs’ sanctions systems on the other. We conclude that national enforcement efforts and MDBs’ sanctions outcomes intersect in their fight against international corruption in that their results are complementary; the former punishing large-scale offenders while the latter ensuring the integrity of development projects

Low-Income Engineering Students: Considering Financial Aid and Differential Tuition

This paper explores the relationship between tuition differentials and low-income students in Engineering fields at two public, research intensive universities. Although current reports indicate the need for increased participation within the Science, Technology, Engineering, and Mathematics (STEM) fields, rising tuition prices at the university and program levels may deter low-income students to enroll and persist within STEM, specifically Engineering. The findings reveal that increased costs due to tuition differentials policies are initially offset by financial aid, but over time costs increase, particularly for low-income students. The results highlight the need for comprehensive, time-sensitive financial aid packages that provide students opportunities to complete their postsecondary degrees, particularly in fields with higher tuition rates

Periodic magnetorotational dynamo action as a prototype of nonlinear magnetic field generation in shear flows

The nature of dynamo action in shear flows prone to magnetohydrodynamic instabilities is investigated using the magnetorotational dynamo in Keplerian shear flow as a prototype problem. Using direct numerical simulations and Newton's method, we compute an exact time-periodic magnetorotational dynamo solution to the three-dimensional dissipative incompressible magnetohydrodynamic equations with rotation and shear. We discuss the physical mechanism behind the cycle and show that it results from a combination of linear and nonlinear interactions between a large-scale axisymmetric toroidal magnetic field and non-axisymmetric perturbations amplified by the magnetorotational instability. We demonstrate that this large scale dynamo mechanism is overall intrinsically nonlinear and not reducible to the standard mean-field dynamo formalism. Our results therefore provide clear evidence for a generic nonlinear generation mechanism of time-dependent coherent large-scale magnetic fields in shear flows and call for new theoretical dynamo models. These findings may offer important clues to understand the transitional and statistical properties of subcritical magnetorotational turbulence.Comment: 10 pages, 6 figures, accepted for publication in Physical Review