317 research outputs found
Nonlinear Evolution of Hydrodynamical Shear Flows in Two Dimensions
We examine how perturbed shear flows evolve in two-dimensional,
incompressible, inviscid hydrodynamical fluids, with the ultimate goal of
understanding the dynamics of accretion disks. To linear order, vorticity waves
are swung around by the background shear, and their velocities are amplified
transiently before decaying. It has been speculated that sufficiently amplified
modes might couple nonlinearly, leading to turbulence. Here we show how
nonlinear coupling occurs in two dimensions. This coupling is remarkably simple
because it only lasts for a short time interval, when one of the coupled modes
is in mid-swing. We focus on the interaction between a swinging and an
axisymmetric mode. There is instability provided that k_{y,swing}/k_{x,axi} <
omega/q, i.e., that the ratio of wavenumbers is less than the ratio of the
axisymmetric mode's vorticity to the background vorticity. If this is the case,
then when the swinging mode is in mid-swing it couples with the axisymmetric
mode to produce a new leading swinging mode that has larger vorticity than
itself; this new mode in turn produces an even larger leading mode, etc.
Therefore all axisymmetric modes, regardless of how small in amplitude, are
unstable to perturbations with sufficiently large azimuthal wavelength. We show
that this shear instability occurs whenever the momentum transported by a
perturbation has the sign required for it to diminish the background shear;
only when this occurs can energy be extracted from the mean flow and hence
added to the perturbation. For an accretion disk, this means that the
instability transports angular momentum outwards while it operates.Comment: published versio
Dynamical constraints on the origin of hot and warm Jupiters with close friends
Gas giants orbiting their host star within the ice line are thought to have
migrated to their current locations from farther out. Here we consider the
origin and dynamical evolution of observed Jupiters, focusing on hot and warm
Jupiters with outer friends. We show that the majority of the observed Jupiter
pairs (twenty out of twenty-four) will be dynamically unstable if the inner
planet was placed at >~1AU distance from the stellar host. This finding is at
odds with formation theories that invoke the migration of such planets from
semi-major axes >~1AU due to secular dynamical processes (e.g., secular chaos,
Lidov-Kozai oscillations) coupled with tidal dissipation. In fact, the results
of N-body integrations show that the evolution of dynamically unstable systems
does not lead to tidal migration but rather to planet ejections and collisions
with the host star. This and other arguments lead us to suggest that most of
the observed planets with a companion could not have been transported from
further out through secular migration processes. More generally, by using a
combination of numerical and analytic techniques we show that the high-e
Lidov-Kozai migration scenario can only account for less than 10% of all gas
giants observed between 0.1-1 AU. Simulations of multi-planet systems support
this result. Our study indicates that rather than starting on highly eccentric
orbits with orbital periods above one year, these "warm" Jupiters are more
likely to have reached the region where they are observed today without having
experienced significant tidal dissipation.Comment: Accepted to AAS journals (AJ). 15 pages, 9 figure
Imbalanced Strong MHD Turbulence
We present a phenomenological model of imbalanced MHD turbulence in an
incompressible magnetofluid. The steady-state cascades, of waves traveling in
opposite directions along the mean magnetic field, carry unequal energy fluxes
to small length scales, where they decay due to viscous and resistive
dissipation. The inertial-range scalings are well-understood when both cascades
are weak. We study the case when both cascades are, in a sense, strong. The
inertial-range of this imbalanced cascade has the following properties: (i) the
ratio of the r.m.s. Elsasser amplitudes is independent of scale, and is equal
to the ratio of the corresponding energy fluxes; (ii) in common with the
balanced strong cascade, the energy spectra of both Elsasser waves are of the
anisotropic Kolmogorov form, with their parallel correlation lengths equal to
each other on all scales, and proportional to the two-thirds power of the
transverse correlation length; (iii) the equality of cascade time and
waveperiod (critical balance) that characterizes the strong balanced cascade
does not apply to the Elsasser field with the larger amplitude. Instead, the
more general criterion that always applies to both Elsasser fields is that the
cascade time is equal to the correlation time of the straining imposed by
oppositely-directed waves. Our results are particularly relevant for turbulence
in the solar wind. Spacecraft measurements have established that, in the
inertial range of solar wind turbulence, waves travelling away from the sun
have higher amplitudes than those travelling towards it. Result (i) allows us
to infer the turbulent flux ratios from the amplitude ratios, thus providing
insight into the origin of the turbulence
REBOUND: An open-source multi-purpose N-body code for collisional dynamics
REBOUND is a new multi-purpose N-body code which is freely available under an
open-source license. It was designed for collisional dynamics such as planetary
rings but can also solve the classical N-body problem. It is highly modular and
can be customized easily to work on a wide variety of different problems in
astrophysics and beyond.
REBOUND comes with three symplectic integrators: leap-frog, the symplectic
epicycle integrator (SEI) and a Wisdom-Holman mapping (WH). It supports open,
periodic and shearing-sheet boundary conditions. REBOUND can use a Barnes-Hut
tree to calculate both self-gravity and collisions. These modules are fully
parallelized with MPI as well as OpenMP. The former makes use of a static
domain decomposition and a distributed essential tree. Two new collision
detection modules based on a plane-sweep algorithm are also implemented. The
performance of the plane-sweep algorithm is superior to a tree code for
simulations in which one dimension is much longer than the other two and in
simulations which are quasi-two dimensional with less than one million
particles.
In this work, we discuss the different algorithms implemented in REBOUND, the
philosophy behind the code's structure as well as implementation specific
details of the different modules. We present results of accuracy and scaling
tests which show that the code can run efficiently on both desktop machines and
large computing clusters.Comment: 10 pages, 9 figures, accepted by A&A, source code available at
https://github.com/hannorein/reboun
Using Synthetic Spacecraft Data to Interpret Compressible Fluctuations in Solar Wind Turbulence
Kinetic plasma theory is used to generate synthetic spacecraft data to
analyze and interpret the compressible fluctuations in the inertial range of
solar wind turbulence. The kinetic counterparts of the three familiar linear
MHD wave modes---the fast, Alfven, and slow waves---are identified and the
properties of the density-parallel magnetic field correlation for these kinetic
wave modes is presented. The construction of synthetic spacecraft data, based
on the quasi-linear premise---that some characteristics of magnetized plasma
turbulence can be usefully modeled as a collection of randomly phased, linear
wave modes---is described in detail. Theoretical predictions of the
density-parallel magnetic field correlation based on MHD and Vlasov-Maxwell
linear eigenfunctions are presented and compared to the observational
determination of this correlation based on 10 years of Wind spacecraft data. It
is demonstrated that MHD theory is inadequate to describe the compressible
turbulent fluctuations and that the observed density-parallel magnetic field
correlation is consistent with a statistically negligible kinetic fast wave
energy contribution for the large sample used in this study. A model of the
solar wind inertial range fluctuations is proposed comprised of a mixture of a
critically balanced distribution of incompressible Alfvenic fluctuations and a
critically balanced or more anisotropic than critical balance distribution of
compressible slow wave fluctuations. These results imply that there is little
or no transfer of large scale turbulent energy through the inertial range down
to whistler waves at small scales.Comment: Accepted to Astrophysical Journal. 28 pages, 7 figure
Impact of observational uncertainties on universal scaling of MHD turbulence
Scaling exponents are the central quantitative prediction of theories of
turbulence and in-situ satellite observations of the high Reynolds number solar
wind flow have provided an extensive testbed of these. We propose a general,
instrument independent method to estimate the uncertainty of velocity field
fluctuations. We obtain the systematic shift that this uncertainty introduces
into the observed spectral exponent. This shift is essential for the correct
interpretation of observed scaling exponents. It is sufficient to explain the
contradiction between spectral features of the Elsasser fields observed in the
solar wind with both theoretical models and numerical simulations of
Magnetohydrodynamic turbulence
Interesting dynamics at high mutual inclination in the framework of the Kozai problem with an eccentric perturber
We study the dynamics of the 3-D three-body problem of a small body moving
under the attractions of a star and a giant planet which orbits the star on a
much wider and elliptic orbit. In particular, we focus on the influence of an
eccentric orbit of the outer perturber on the dynamics of a small highly
inclined inner body. Our analytical study of the secular perturbations relies
on the classical octupole hamiltonian expansion (third-order theory in the
ratio of the semi-major axes), as third-order terms are needed to consider the
secular variations of the outer perturber and potential secular resonances
between the arguments of the pericenter and/or longitudes of the node of both
bodies. Short-period averaging and node reduction (Laplace plane) reduce the
problem to two degrees of freedom. The four-dimensional dynamics is analyzed
through representative planes which identify the main equilibria of the
problem. As in the circular problem (i.e. perturber on a circular orbit), the
"Kozai-bifurcated" equilibria play a major role in the dynamics of an inner
body on quasi-circular orbit: its eccentricity variations are very limited for
mutual inclination between the orbital planes smaller than ~40^{\deg}, while
they become large and chaotic for higher mutual inclination. Particular
attention is also given to a region around 35^{\deg} of mutual inclination,
detected numerically by Funk et al. (2011) and consisting of long-time stable
and particularly low eccentric orbits of the small body. Using a 12th-order
Hamiltonian expansion in eccentricities and inclinations, in particular its
action-angle formulation obtained by Lie transforms in Libert & Henrard (2008),
we show that this region presents an equality of two fundamental frequencies
and can be regarded as a secular resonance. Our results also apply to binary
star systems where a planet is revolving around one of the two stars.Comment: 12 pages, 9 figures, accepted for publication in MNRA
Evolution of plasma turbulence excited with particle beams
Particles ejected from the Sun that stream through the surrounding plasma of
the solar wind are causing instabilities. These generate wavemodes in a certain
frequency range especially within shock regions, where particles are
accelerated. The aim of this paper is to investigate of amplified Alfvenic
wavemodes in driven incompressible magnetohydrodynamic turbulence. Results of
different heliospheric scenarios from isotropic and anisotropic plasmas, as
well as turbulence near the critical balance are shown. The energy transport of
the amplified wavemode is governed by the mechanisms of diffusion, convection
and dissipation of energy in wavenumber space. The strength of these effects
varies with energy and wavenumber of the mode in question. Two-dimensional
energy spectra of spherical k-space integration that permit detailed insight
into the parallel and perpendicular development are presented. The evolution of
energy injected through driving shows a strong energy transfer to perpendicular
wavemodes. The main process at parallel wavemodes is the dissipation of energy
in wavenumber space. The generation of higher harmonics along the parallel
wavenumber axis is observed. We find evidence for a critical balance in our
simulations.Comment: Accepted for publication in A&
Magnetic Lensing near Ultramagnetized Neutron Stars
Extremely strong magnetic fields change the vacuum index of refraction. This
induces a lensing effect that is not unlike the lensing phenomenon in strong
gravitational fields. The main difference between the two is the polarization
dependency of the magnetic lensing, a behaviour that induces a handful of
interesting effects. The main prediction is that the thermal emission of
neutron stars with extremely strong magnetic fields is polarized - up to a few
percent for the largest fields known. This potentially allows a direct method
for measuring their magnetic fields.Comment: To appear in MNRAS, 12 pages, 9 figure
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