193 research outputs found

### Electromotive force due to magnetohydrodynamic fluctuations in sheared rotating turbulence

This article presents a calculation of the mean electromotive force arising
from general small-scale magnetohydrodynamical turbulence, within the framework
of the second-order correlation approximation. With the goal of improving
understanding of the accretion disk dynamo, effects arising through small-scale
magnetic fluctuations, velocity gradients, density and turbulence
stratification, and rotation, are included. The primary result, which
supplements numerical findings, is that an off-diagonal turbulent resistivity
due to magnetic fluctuations can produce large-scale dynamo action -- the
magnetic analogue of the "shear-current" effect. In addition, consideration of
$\alpha$ effects in the stratified regions of disks gives the puzzling result
that there is no strong prediction for a sign of $\alpha$, since the effects
due to kinetic and magnetic fluctuations, as well as those due to shear and
rotation, are each of opposing signs and tend to cancel each other

### Coherent nonhelical shear dynamos driven by magnetic fluctuations at low Reynolds numbers

Nonhelical shear dynamos are studied with a particular focus on the
possibility of coherent dynamo action. The primary results -- serving as a
follow up to the results of Squire & Bhattacharjee [arXiv:1506.04109 (2015)] --
pertain to the "magnetic shear-current effect" as a viable mechanism to drive
large-scale magnetic field generation. This effect raises the interesting
possibility that the saturated state of the small-scale dynamo could drive
large-scale dynamo action, and is likely to be important in the unstratified
regions of accretion disk turbulence. In this paper, the effect is studied at
low Reynolds numbers, removing the complications of small-scale dynamo
excitation and aiding analysis by enabling the use of quasi-linear statistical
simulation methods. In addition to the magnetically driven dynamo, new results
on the kinematic nonhelical shear dynamo are presented. These illustrate the
relationship between coherent and incoherent driving in such dynamos,
demonstrating the importance of rotation in determining the relative dominance
of each mechanism

### Generation of large-scale magnetic fields by small-scale dynamo in shear flows

We propose a new mechanism for turbulent mean-field dynamo in which the
magnetic fluctuations resulting from a small-scale dynamo drive the generation
of large-scale magnetic fields. This is in stark contrast to the common idea
that small-scale magnetic fields should be harmful to large-scale dynamo
action. These dynamos occur in the presence of large-scale velocity shear and
do not require net helicity, resulting from off-diagonal components of the
turbulent resistivity tensor as the magnetic analogue of the "shear-current"
effect. Given the inevitable existence of non-helical small-scale magnetic
fields in turbulent plasmas, as well as the generic nature of velocity shear,
the suggested mechanism may help to explain generation of large-scale magnetic
fields across a wide range of astrophysical objects

### The magnetic shear-current effect: generation of large-scale magnetic fields by the small-scale dynamo

A novel large-scale dynamo mechanism, the magnetic shear-current effect, is
discussed and explored. The effect relies on the interaction of magnetic
fluctuations with a mean shear flow, meaning the saturated state of the
small-scale dynamo can drive a large-scale dynamo -- in some sense the inverse
of dynamo quenching. The dynamo is nonhelical, with the mean-field $\alpha$
coefficient zero, and is caused by the interaction between an off-diagonal
component of the turbulent resistivity and the stretching of the large-scale
field by shear flow. Following up on previous numerical and analytic work, this
paper presents further details of the numerical evidence for the effect, as
well as an heuristic description of how magnetic fluctuations can interact with
shear flow to produce the required electromotive force. The pressure response
of the fluid is fundamental to this mechanism, which helps explain why the
magnetic effect is stronger than its kinematic cousin, and the basic idea is
related to the well-known lack of turbulent resistivity quenching by magnetic
fluctuations. As well as being interesting for its applications to general high
Reynolds number astrophysical turbulence, where strong small-scale magnetic
fluctuations are expected to be prevalent, the magnetic shear-current effect is
a likely candidate for large-scale dynamo in the unstratified regions of
ionized accretion disks. Evidence for this is discussed, as well as future
research directions and the challenges involved with understanding details of
the effect in astrophysically relevant regimes

### Resonant drag instability of grains streaming in fluids

We show that grains streaming through a fluid are generically unstable if
their velocity, projected along some direction, matches the phase velocity of a
fluid wave (linear oscillation). This can occur whenever grains stream faster
than any fluid wave. The wave itself can be quite general--sound waves,
magnetosonic waves, epicyclic oscillations, and Brunt-V\"ais\"al\"a
oscillations each generate instabilities, for example. We derive a simple
expression for the growth rates of these "resonant drag instabilities" (RDI).
This expression (i) illustrates why such instabilities are so virulent and
generic, and (ii) allows for simple analytic computation of RDI growth rates
and properties for different fluids. As examples, we introduce several new
instabilities, which could see application across a variety of physical systems
from atmospheres to protoplanetary disks, the interstellar medium, and galactic
outflows. The matrix-based resonance formalism we introduce can also be applied
more generally in other (nonfluid) contexts, providing a simple means for
calculating and understanding the stability properties of interacting systems.Comment: 5 Pages. Published in ApJ

### Physical models of streaming instabilities in protoplanetary discs

We develop simple, physically motivated models for drag-induced dust–gas streaming instabilities, which are thought to be crucial for clumping grains to form planetesimals in protoplanetary discs. The models explain, based on the physics of gaseous epicyclic motion and dust–gas drag forces, the most important features of the streaming instability and its simple generalization, the disc settling instability. Some of the key properties explained by our models include the sudden change in the growth rate of the streaming instability when the dust-to-gas mass ratio surpasses one, the slow growth rate of the streaming instability compared to the settling instability for smaller grains, and the main physical processes underlying the growth of the most unstable modes in different regimes. As well as providing helpful simplified pictures for understanding the operation of an interesting and fundamental astrophysical fluid instability, our models may prove useful for analysing simulations and developing non-linear theories of planetesimal growth in discs

### The distribution of density in supersonic turbulence

We propose a model for the density statistics in supersonic turbulence, which
play a crucial role in star-formation and the physics of the interstellar
medium (ISM). Motivated by [Hopkins, MNRAS, 430, 1880 (2013)], the model
considers the density to be arranged into a collection of strong shocks of
width $\sim\! \mathcal{M}^{-2}$, where $\mathcal{M}$ is the turbulent Mach
number. With two physically motivated parameters, the model predicts all
density statistics for $\mathcal{M}>1$ turbulence: the density probability
distribution and its intermittency (deviation from log-normality), the density
variance-Mach number relation, power spectra, and structure functions. For the
proposed model parameters, reasonable agreement is seen between model
predictions and numerical simulations, albeit within the large uncertainties
associated with current simulation results. More generally, the model could
provide a useful framework for more detailed analysis of future simulations and
observational data. Due to the simple physical motivations for the model in
terms of shocks, it is straightforward to generalize to more complex physical
processes, which will be helpful in future more detailed applications to the
ISM. We see good qualitative agreement between such extensions and recent
simulations of non-isothermal turbulence

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