178 research outputs found
Enhancement of small-scale turbulent dynamo by large-scale shear
Small-scale dynamos are ubiquitous in a broad range of turbulent flows with
large-scale shear, ranging from solar and galactic magnetism to accretion
disks, cosmology and structure formation. Using high-resolution direct
numerical simulations we show that in non-helically forced turbulence with zero
mean magnetic field, large-scale shear supports small-scale dynamo action,
i.e., the dynamo growth rate increases with shear and shear enhances or even
produces turbulence, which, in turn, further increases the dynamo growth rate.
When the production rates of turbulent kinetic energy due to shear and forcing
are comparable, we find scalings for the growth rate of the
small-scale dynamo and the turbulent velocity with shear rate
that are independent of the magnetic Prandtl number: and
. For large fluid and magnetic Reynolds numbers,
, normalized by its shear-free value, depends only on shear. Having
compensated for shear-induced effects on turbulent velocity, we find that the
normalized growth rate of the small-scale dynamo exhibits the scaling,
, arising solely from the induction
equation for a given velocity field.Comment: Improved version submitted to the Astrophysical Journal Letters, 6
pages, 5 figure
Generation of large-scale magnetic fields due to fluctuating in shearing systems
We explore the growth of large-scale magnetic fields in a shear flow, due to
helicity fluctuations with a finite correlation time, through a study of the
Kraichnan-Moffatt model of zero-mean stochastic fluctuations of the
parameter of dynamo theory. We derive a linear integro-differential equation
for the evolution of large-scale magnetic field, using the first-order
smoothing approximation and the Galilean invariance of the -statistics.
This enables construction of a model that is non-perturbative in the shearing
rate and the -correlation time . After a brief review
of the salient features of the exactly solvable white-noise limit, we consider
the case of small but non-zero . When the large-scale magnetic
field varies slowly, the evolution is governed by a partial differential
equation. We present modal solutions and conditions for the exponential growth
rate of the large-scale magnetic field, whose drivers are the Kraichnan
diffusivity, Moffatt drift, Shear and a non-zero correlation time. Of
particular interest is dynamo action when the -fluctuations are weak;
i.e. when the Kraichnan diffusivity is positive. We show that in the absence of
Moffatt drift, shear does not give rise to growing solutions. But shear and
Moffatt drift acting together can drive large scale dynamo action with growth
rate .Comment: 19 pages, 4 figures, Accepted in Journal of Plasma Physic
Fanning out of the -mode in presence of nonuniform magnetic fields
We show that in the presence of a harmonically varying magnetic field the
fundamental or -mode in a stratified layer is altered in such a way that it
fans out in the diagnostic diagram, but with mode power also within
the fan. In our simulations, the surface is defined by a temperature and
density jump in a piecewise isothermal layer. Unlike our previous work (Singh
et al. 2014) where a uniform magnetic field was considered, we employ here a
nonuniform magnetic field together with hydromagnetic turbulence at length
scales much smaller than those of the magnetic fields. The expansion of the
-mode is stronger for fields confined to the layer below the surface. In
some of those cases, the diagram also reveals a new class of low
frequency vertical stripes at multiples of twice the horizontal wavenumber of
the background magnetic field. We argue that the study of the -mode
expansion might be a new and sensitive tool to determining subsurface magnetic
fields with longitudinal periodicity.Comment: 6 pages, 4 figures, submitted to Astrophysical Journal Letter
Properties of - and -modes in hydromagnetic turbulence
With the ultimate aim of using the fundamental or -mode to study
helioseismic aspects of turbulence-generated magnetic flux concentrations, we
use randomly forced hydromagnetic simulations of a piecewise isothermal layer
in two dimensions with reflecting boundaries at top and bottom. We compute
numerically diagnostic wavenumber-frequency diagrams of the vertical velocity
at the interface between the denser gas below and the less dense gas above. For
an Alfv\'en-to-sound speed ratio of about 0.1, a 5% frequency increase of the
-mode can be measured when -, where is the
horizontal wavenumber and is the pressure scale height at the
surface. Since the solar radius is about 2000 times larger than ,
the corresponding spherical harmonic degree would be 6000-8000. For weaker
fields, a -dependent frequency decrease by the turbulent motions becomes
dominant. For vertical magnetic fields, the frequency is enhanced for
, but decreased relative to its nonmagnetic value for
.Comment: 17 pages, 22 figures, Version accepted in MNRA
Mean field dynamo action in shearing flows. II: fluctuating kinetic helicity with zero mean
Here we explore the role of temporal fluctuations in kinetic helicity on the
generation of large-scale magnetic fields in presence of a background linear
shear flow. Key techniques involved here are same as in our earlier work
\citep[][hereafter paper~I]{JS20}, where we have used the renovating flow based
model with shearing waves. Both, the velocity and the helicity fields, are
treated as stochastic variables with finite correlation times, and
, respectively. Growing solutions are obtained when ,
even when this time-scale separation, characterised by , remains
below the threshold for causing the turbulent diffusion to turn negative. In
regimes when turbulent diffusion remains positive, and is on the order
of eddy turnover time , the axisymmetric modes display non-monotonic
behaviour with shear rate : both, the growth rate and the
wavenumber corresponding to the fastest growing mode, first increase,
reach a maximum and then decrease with , with being always
smaller than eddy-wavenumber, thus boosting growth of magnetic fields at large
length scales. The cycle period of growing dynamo wave is
inversely proportional to at small shear, exactly as in the fixed kinetic
helicity case of paper~I. This dependence becomes shallower at larger shear.
Interestingly enough, various curves corresponding to different choices of
collapse on top of each other in a plot of with .Comment: 14 pages, 10 figures, Submitted to MNRA
Binary systems: implications for outflows & periodicities relevant to masers
Bipolar molecular outflows have been observed and studied extensively in the
past, but some recent observations of periodic variations in maser intensity
pose new challenges. Even quasi-periodic maser flares have been observed and
reported in the literature. Motivated by these data, we have tried to study
situations in binary systems with specific attention to the two observed
features, i.e., the bipolar flows and the variabilities in the maser intensity.
We have studied the evolution of spherically symmetric wind from one of the
bodies in the binary system, in the plane of the binary. Our approach includes
the analytical study of rotating flows with numerical computation of
streamlines of fluid particles using PLUTO code. We present the results of our
findings assuming simple configurations, and discuss the implications.Comment: 5 pages, 3 figures, Proceedings IAU Symposium No. 287, 2012, Cosmic
masers - from OH to H
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