Visco-rotational shear instability of Keplerian granular flows [EPA]

We present the linear rheological instability triggered by the interplay of the shear rheology and Keplerian differential rotation of incompressible dense granular fluids. Instability sets in granular fluids, where the viscosity parameter grows faster than the square of the local shear rate (strain rate) at constant pressure. Found instability can play a crucial role in the formation of observed structures in planetary rings, as well as promote structure formation in protoplanetary disks dense granular material.

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L. Poniatowski and A. Tevzadze
Fri, 24 Feb 17

Comments: 5 pages, 3 figures. Comments welcome

Universal small-scale structure in turbulence driven by magnetorotational instability [HEAP]

The intermittent small-scale structure of turbulence governs energy dissipation in many astrophysical plasmas and is often believed to have universal properties for sufficiently large systems. In this work, we argue that small-scale turbulence in accretion disks is universal in the sense that it is insensitive to the magnetorotational instability (MRI) and background shear, and therefore indistinguishable from standard homogeneous magnetohydrodynamic (MHD) turbulence at small scales. We investigate the intermittency of current density, vorticity, and energy dissipation in numerical simulations of incompressible MHD turbulence driven by the MRI in a shearing box. We find that the simulations exhibit a similar degree of intermittency as in standard MHD turbulence. We perform a statistical analysis of intermittent dissipative structures and find that energy dissipation is concentrated in thin sheet-like structures that span a wide range of scales up to the box size. We show that these structures exhibit strikingly similar statistical properties to those in standard MHD turbulence. Additionally, the structures are oriented in the toroidal direction with a characteristic tilt of approximately 17.5 degrees, implying an effective guide field in that direction.

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V. Zhdankin, J. Walker, S. Boldyrev, et. al.
Fri, 10 Feb 17

Comments: 9 pages, 11 figures, to appear in Monthly Notices of the Royal Astronomical Society

Meridional Circulation Dynamics in a Cyclic Convective Dynamo [SSA]

Surface observations indicate that the speed of the solar meridional circulation in the photosphere varies in anti-phase with the solar cycle. The current explanation for the source of this variation is that inflows into active regions alter the global surface pattern of the meridional circulation. When these localized inflows are integrated over a full hemisphere, they contribute to the slow down of the axisymmetric poleward horizontal component. The behavior of this large scale flow deep inside the convection zone remains largely unknown. Present helioseismic techniques are not sensitive enough to capture the dynamics of this weak large scale flow. Moreover, the large time of integration needed to map the meridional circulation inside the convection zone, also masks some of the possible dynamics on shorter timescales. In this work we examine the dynamics of the meridional circulation that emerges from a 3D MHD global simulation of the solar convection zone. Our aim is to assess and quantify the behavior of meridional circulation deep inside the convection zone, where the cyclic large-scale magnetic field can reach considerable strength. Our analyses indicate that the meridional circulation morphology and amplitude are both highly influenced by the magnetic field, via the impact of magnetic torques on the global angular momentum distribution. A dynamic feature induced by these magnetic torques is the development of a prominent upward flow at mid latitudes in the lower convection zone that occurs near the equatorward edge of the toroidal bands and that peaks during cycle maximum. Globally, the dynamo-generated large-scale magnetic field drives variations in the meridional flow, in stark contrast to the conventional kinematic flux transport view of the magnetic field being advected passively by the flow.

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D. Passos, M. Miesch, G. Guerrero, et. al.
Thu, 9 Feb 17

Comments: 26 pages, 16 figures, submitted to A&A

On the Helium fingers in the intracluster medium [GA]

The primary goal of this paper is to build upon the machinery usually employed to study the salt finger instability in order to address the onset of the similar double diffusive convection phenomenon in the intracluster medium — the weakly-collisional magnetized inhomogeneous plasma permeating galaxy clusters; and subsequently, to investigate the nature of the width of the analogous Helium fingers in the supercritical regime of the instability. Specifically, we conclude that the width of the Helium fingers scales as one-fourth power of the radius of the inner region of the ICM. In the process, we also find out the explicit mathematical expression of the criterion for the onset of the heat-flux-driven buoyancy instability modified by the presence of inhomogeneously distributed Helium ions in the galaxy cluster. This criterion incorporates the contribution of the magnetic tension into it.

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S. Sadhukhan, H. Gupta and S. Chakraborty
Tue, 7 Feb 17

Comments: 7 pages, 1 figure

Torsional Alfvén resonances as an efficient damping mechanism for non-radial oscillations in red giant stars [SSA]

Stars are self-gravitating fluids in which pressure, buoyancy, rotation and magnetic fields provide the restoring forces for global modes of oscillation. Pressure and buoyancy energetically dominate, while rotation and magnetism are generally assumed to be weak perturbations and often ignored. However, observations of anomalously weak dipole mode amplitudes in red giant stars suggest that a substantial fraction of these are subject to an additional source of damping localised to their core region, with indirect evidence pointing to the role of a deeply buried magnetic field. It is also known that in many instances the gravity-mode character of affected modes is preserved, but so far no effective damping mechanism has been proposed that accommodates this aspect. Here we present such a mechanism, which damps the oscillations of stars harbouring magnetised cores via resonant interactions with standing Alfv\’en modes of high harmonic index. The damping rates produced by this mechanism are quantitatively on par with those associated with turbulent convection, and in the range required to explain observations, for realistic stellar models and magnetic field strengths. Our results suggest that magnetic fields can provide an efficient means of damping stellar oscillations without needing to disrupt the internal structure of the modes, and lay the groundwork for an extension of the theory of global stellar oscillations that incorporates these effects.

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S. Loi and J. Papaloizou
Wed, 1 Feb 17

Comments: 15 pages, 10 figures. Accepted for publication in MNRAS

The onset of turbulent rotating dynamos at the low $Pm$ limit [CL]

We demonstrate that the critical magnetic Reynolds number $Rm_c$ for a turbulent non-helical dynamo in the low magnetic Prandtl number $Pm$ limit (i.e. $Pm = Rm/Re \ll 1$) can be significantly reduced if the flow is submitted to global rotation. Even for moderate rotation rates the required energy injection rate can be reduced by a factor more than $10^3$. This strong decrease of the onset is attributed to the reduction of the turbulent fluctuations that makes the flow to have a much larger cut-off length-scale compared to a non-rotating flow of the same Reynolds number. The dynamo thus behaves as if it is driven by laminar behaviour (i.e. high $Pm$ behaviour) even at high values of the Reynolds number (i.e. at low values of $Pm$). Our finding thus points into a new paradigm for the design of new liquid metal dynamo experiments.

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K. Seshasayanan, V. Dallas and A. Alexakis
Wed, 1 Feb 17

Comments: 5 pages, 6 figures

Numerical aspects of Giant Impact simulations [EPA]

In this paper we present solutions to three short comings of Smoothed Particles Hydrodynamics (SPH) encountered in previous work when applying it to Giant Impacts. First we introduce a novel method to obtain accurate SPH representations of a planet’s equilibrium initial conditions based on equal area tessellations of the sphere. This allows one to imprint an arbitrary density and internal energy profile with very low noise which substantially reduces computation because these models require no relaxation prior to use. As a consequence one can significantly increase the resolution and more flexibly change the initial bodies to explore larger parts of the impact parameter space in simulations. The second issue addressed is the proper treatment of the matter/vacuum boundary at a planet’s surface with a modified SPH density estimator that properly calculates the density stabilizing the models and avoiding an artificially low density atmosphere prior to impact. Further we present a novel SPH scheme that simultaneously conserves both energy and entropy for an arbitrary equation of state. This prevents loss of entropy during the simulation and further assures that the material does not evolve into unphysical states. Application of these modifications to impact simulations for different resolutions up to $6.4 \cdot 10^6$ particles show a general agreement with prior result. However, we observe resolution dependent differences in the evolution and composition of post collision ejecta. This strongly suggests that the use of more sophisticated equations of state also demands a large number of particles in such simulations.

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C. Reinhardt and J. Stadel
Tue, 31 Jan 17

Comments: N/A