104 research outputs found
Understanding Helical Magnetic Dynamo Spectra with a Nonlinear Four-Scale Theory
Recent MHD dynamo simulations for magnetic Prandtl number demonstrate
that when MHD turbulence is forced with sufficient kinetic helicity, the
saturated magnetic energy spectrum evolves from having a single peak below the
forcing scale to become doubly peaked with one peak at the system (=largest)
scale and one at the forcing scale. The system scale field growth is well
modeled by a recent nonlinear two-scale nonlinear helical dynamo theory in
which the system and forcing scales carry magnetic helicity of opposite sign.
But a two-scale theory cannot model the shift of the small-scale peak toward
the forcing scale. Here I develop a four-scale helical dynamo theory which
shows that the small-scale helical magnetic energy first saturates at very
small scales, but then successively saturates at larger values at larger
scales, eventually becoming dominated by the forcing scale. The transfer of the
small scale peak to the forcing scale is completed by the end of the kinematic
growth regime of the large scale field, and does not depend on magnetic
Reynolds number for large . The four-scale and two-scale theories
subsequently evolve almost identically, and both show significant field growth
on the system and forcing scales that is independent of . In the present
approach, the helical and nonhelical parts of the spectrum are largely
decoupled. Implications for fractionally helical turbulence are discussed.Comment: 19 Pages, LaTex, (includes 4 figs at the end), in press, MNRA
Planetesimal growth in turbulent discs before the onset of gravitational instability
It is difficult to imagine a planet formation model that does not at some
stage include a gravitationally unstable disc. Initially unstable gas-dust
discs may form planets directly, but the high surface density required has
motivated the alternative that gravitational instability occurs in a dust
sub-layer only after grains have grown large enough by electrostatic sticking.
Although such growth up to the instability stage is efficient for laminar
discs, concern has mounted as to whether realistic disc turbulence
catastrophically increases the settling time, thereby requiring additional
processes to facilitate planet formation on the needed time scales. To evaluate
this concern, we develop a model for grain growth that accounts for the
influence of turbulence on the collisional velocity of grains and on the scale
height of the dust layer. The relative effect on these quantities depends on
the grain size. The model produces a disc-radius dependent time scale to reach
a gravitationally unstable phase of planet formation. For a range of dust
sticking and disc parameters, we find that for viscosity parameters , this time scale is short enough over a significant range in radii
that turbulence does not catastrophically slow the early phases of planet
formation, even in the absence of agglomeration enhancement agents like
vortices.Comment: Submitted to New Astronom
Comparing Poynting flux dominated magnetic tower jets with kinetic-energy dominated jets
Magnetic Towers represent one of two fundamental forms of MHD outflows.
Driven by magnetic pressure gradients, these flows have been less well studied
than magneto-centrifugally launched jets even though magnetic towers may well
be as common. Here we present new results exploring the behavior and evolution
of magnetic tower outflows and demonstrate their connection with pulsed power
experimental studies and purely hydrodynamic jets which might represent the
asymptotic propagation regimes of magneto-centrifugally launched jets.
High-resolution AMR MHD simulations (using the AstroBEAR code) provide insights
into the underlying physics of magnetic towers and help us constrain models of
their propagation. Our simulations have been designed to explore the effects of
thermal energy losses and rotation on both tower flows and their hydro
counterparts. We find these parameters have significant effects on the
stability of magnetic towers, but mild effects on the stability of hydro jets.
Current-driven perturbations in the Poynting Flux Dominated (PDF) towers are
shown to be amplified in both the cooling and rotating cases. Our studies of
the long term evolution of the towers show that the formation of weakly
magnetized central jets within the tower are broken up by these instabilities
becoming a series of collimated clumps which magnetization properties vary over
time. In addition to discussing these results in light of laboratory
experiments, we address their relevance to astrophysical observations of young
star jets and outflow from highly evolved solar type stars.Comment: 11 pages, 4 figures, accepted for publication in the High Energy
Density Physics Journal corresponding to the proceedings of the 9th
International Conference on High Energy Density Laboratory Astrophysics, May
4, 2012, Tallahassee Florid
Empirical relation between angular momentum transport and thermal-to-magnetic pressure ratio in shearing box simulations
By combining data from different published 3-D simulations of Keplerian
shearing boxes unstable to the magnetorotational instability (MRI), we
highlight tight anti-correlations between the total effective inferred angular
momentum transport parameter, , its separate Maxwell and Reynolds
contributions and , and the kinetic to magnetic
pressure ratio , defined with the initial or saturated (when available)
thermal pressure.
Plots of , and
vs are well fit by straight lines even as ,
,and vary by four orders of magnitude over the
simulations included. The ratio and the product
are quite constant and largely independent of the presence
or absence of weak mean fields, the choice of initial and boundary conditions,
and the resolution. In short, simulations have more strongly constrained the
product than itself.Comment: 22 pages (includes 10 tables and 3 figs.), accepted by New Astronom
Simulations of galactic dynamos
We review our current understanding of galactic dynamo theory, paying
particular attention to numerical simulations both of the mean-field equations
and the original three-dimensional equations relevant to describing the
magnetic field evolution for a turbulent flow. We emphasize the theoretical
difficulties in explaining non-axisymmetric magnetic fields in galaxies and
discuss the observational basis for such results in terms of rotation measure
analysis. Next, we discuss nonlinear theory, the role of magnetic helicity
conservation and magnetic helicity fluxes. This leads to the possibility that
galactic magnetic fields may be bi-helical, with opposite signs of helicity and
large and small length scales. We discuss their observational signatures and
close by discussing the possibilities of explaining the origin of primordial
magnetic fields.Comment: 28 pages, 15 figure, to appear in Lecture Notes in Physics "Magnetic
fields in diffuse media", Eds. E. de Gouveia Dal Pino and A. Lazaria
Review of Speculative "Disaster Scenarios" at RHIC
We discuss speculative disaster scenarios inspired by hypothetical new
fundamental processes that might occur in high energy relativistic heavy ion
collisions. We estimate the parameters relevant to black hole production; we
find that they are absurdly small. We show that other accelerator and
(especially) cosmic ray environments have already provided far more auspicious
opportunities for transition to a new vacuum state, so that existing
observations provide stringent bounds. We discuss in most detail the
possibility of producing a dangerous strangelet. We argue that four separate
requirements are necessary for this to occur: existence of large stable
strangelets, metastability of intermediate size strangelets, negative charge
for strangelets along the stability line, and production of intermediate size
strangelets in the heavy ion environment. We discuss both theoretical and
experimental reasons why each of these appears unlikely; in particular, we know
of no plausible suggestion for why the third or especially the fourth might be
true. Given minimal physical assumptions the continued existence of the Moon,
in the form we know it, despite billions of years of cosmic ray exposure,
provides powerful empirical evidence against the possibility of dangerous
strangelet production.Comment: 28 pages, REVTeX; minor revisions for publication (Reviews of Modern
Physics, ca. Oct. 2000); email to [email protected]
Current status of turbulent dynamo theory: From large-scale to small-scale dynamos
Several recent advances in turbulent dynamo theory are reviewed. High
resolution simulations of small-scale and large-scale dynamo action in periodic
domains are compared with each other and contrasted with similar results at low
magnetic Prandtl numbers. It is argued that all the different cases show
similarities at intermediate length scales. On the other hand, in the presence
of helicity of the turbulence, power develops on large scales, which is not
present in non-helical small-scale turbulent dynamos. At small length scales,
differences occur in connection with the dissipation cutoff scales associated
with the respective value of the magnetic Prandtl number. These differences are
found to be independent of whether or not there is large-scale dynamo action.
However, large-scale dynamos in homogeneous systems are shown to suffer from
resistive slow-down even at intermediate length scales. The results from
simulations are connected to mean field theory and its applications. Recent
work on helicity fluxes to alleviate large-scale dynamo quenching, shear
dynamos, nonlocal effects and magnetic structures from strong density
stratification are highlighted. Several insights which arise from analytic
considerations of small-scale dynamos are discussed.Comment: 36 pages, 11 figures, Spa. Sci. Rev., submitted to the special issue
"Magnetism in the Universe" (ed. A. Balogh
Particle acceleration in three-dimensional tearing configurations
In three-dimensional electromagnetic configurations that result from unstable
resistive tearing modes particles can efficiently be accelerated to
relativistic energies. To prove this resistive magnetohydrodynamic simulations
are used as input configurations for successive test particle simulations. The
simulations show the capability of three-dimensional non-linearly evolved
tearing modes to accelerate particles perpendicular to the plane of the
reconnecting magnetic field components. The simulations differ considerably
from analytical approaches by involving a realistic three-dimensional electric
field with a non-homogenous component parallel to the current direction. The
resulting particle spectra exhibit strong pitch-angle anisotropies. Typically,
about 5-8 % of an initially Maxwellian distribution is accelerated to the
maximum energy levels given by the macroscopic generalized electric potential
structure. Results are shown for both, non-relativistic particle acceleration
that is of interest, e.g., in the context of auroral arcs and solar flares, and
relativistic particle energization that is relevant, e.g., in the context of
active galactic nuclei.Comment: Physics of Plasmas, in prin
Ohm's Law for Plasma in General Relativity and Cowling's Theorem
The general-relativistic Ohm's law for a two-component plasma which includes
the gravitomagnetic force terms even in the case of quasi-neutrality has been
derived. The equations that describe the electromagnetic processes in a plasma
surrounding a neutron star are obtained by using the general relativistic form
of Maxwell equations in a geometry of slow rotating gravitational object. In
addition to the general-relativistic effect first discussed by Khanna \&
Camenzind (1996) we predict a mechanism of the generation of azimuthal current
under the general relativistic effect of dragging of inertial frames on radial
current in a plasma around neutron star. The azimuthal current being
proportional to the angular velocity of the dragging of inertial
frames can give valuable contribution on the evolution of the stellar magnetic
field if exceeds (
is the number density of the charged particles, is the conductivity of
plasma). Thus in general relativity a rotating neutron star, embedded in
plasma, can in principle generate axial-symmetric magnetic fields even in
axisymmetry. However, classical Cowling's antidynamo theorem, according to
which a stationary axial-symmetric magnetic field can not be sustained against
ohmic diffusion, has to be hold in the general-relativistic case for the
typical plasma being responsible for the rotating neutron star.Comment: Accepted for publication in Astrophysics & Space Scienc
Magnetic Reconnection in Extreme Astrophysical Environments
Magnetic reconnection is a basic plasma process of dramatic rearrangement of
magnetic topology, often leading to a violent release of magnetic energy. It is
important in magnetic fusion and in space and solar physics --- areas that have
so far provided the context for most of reconnection research. Importantly,
these environments consist just of electrons and ions and the dissipated energy
always stays with the plasma. In contrast, in this paper I introduce a new
direction of research, motivated by several important problems in high-energy
astrophysics --- reconnection in high energy density (HED) radiative plasmas,
where radiation pressure and radiative cooling become dominant factors in the
pressure and energy balance. I identify the key processes distinguishing HED
reconnection: special-relativistic effects; radiative effects (radiative
cooling, radiation pressure, and Compton resistivity); and, at the most extreme
end, QED effects, including pair creation. I then discuss the main
astrophysical applications --- situations with magnetar-strength fields
(exceeding the quantum critical field of about 4 x 10^13 G): giant SGR flares
and magnetically-powered central engines and jets of GRBs. Here, magnetic
energy density is so high that its dissipation heats the plasma to MeV
temperatures. Electron-positron pairs are then copiously produced, making the
reconnection layer highly collisional and dressing it in a thick pair coat that
traps radiation. The pressure is dominated by radiation and pairs. Yet,
radiation diffusion across the layer may be faster than the global Alfv\'en
transit time; then, radiative cooling governs the thermodynamics and
reconnection becomes a radiative transfer problem, greatly affected by the
ultra-strong magnetic field. This overall picture is very different from our
traditional picture of reconnection and thus represents a new frontier in
reconnection research.Comment: Accepted to Space Science Reviews (special issue on magnetic
reconnection). Article is based on an invited review talk at the
Yosemite-2010 Workshop on Magnetic Reconnection (Yosemite NP, CA, USA;
February 8-12, 2010). 30 pages, no figure
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