Author name code: nordlund
ADS astronomy entries on 2022-09-14
author:"Nordlund, Ake"
------------------------------------------------------------------------
Title: The dynamical state of massive clumps
Authors: Lu, Zu-Jia; Pelkonen, Veli-Matti; Juvela, Mika; Padoan,
Paolo; Haugbølle, Troels; Nordlund, Åke
Bibcode: 2022MNRAS.509.5589L
Altcode: 2021arXiv211108887L
The dynamical state of massive clumps is key to our understanding of
the formation of massive stars. In this work, we study the kinematic
properties of massive clumps using synthetic observations. We have
previously compiled a very large catalogue of synthetic dust-continuum
compact sources from our 250 pc, SN-driven, star formation
simulation. Here, we compute synthetic $\rm N_{2}H^{+}$ line profiles
for a subsample of those sources and compare their properties with the
observations and with those of the corresponding three-dimensional (3D)
clumps in the simulation. We find that the velocity dispersion of the
sources estimated from the $\rm N_{2}H^{+}$ line is a good estimate of
that of the 3D clumps, although its correlation with the source size is
weaker than the velocity-size correlation of the 3D clumps. The relation
between the mass of the 3D clumps, Mmain, and that of the
corresponding synthetic sources, MSED, has a large scatter
and a slope of 0.5, $M_{\rm main} \propto M_{\rm SED}^{0.5}$, due to
uncertainties arising from the observational band-merging procedure
and from projection effects along the line of sight. As a result, the
virial parameters of the 3D clumps are not correlated with the clump
masses, even if a negative correlation is found for the compact sources,
and the virial parameter of the most massive sources may significantly
underestimate that of the associated clumps.
Title: Physical properties and real nature of massive clumps in
the galaxy
Authors: Lu, Zu-Jia; Pelkonen, Veli-Matti; Juvela, Mika; Padoan,
Paolo; Haugbølle, Troels; Nordlund, Åke
Bibcode: 2022MNRAS.510.1697L
Altcode: 2021MNRAS.tmp.3186L; 2021arXiv210808981L
Systematic surveys of massive clumps have been carried out to study
the conditions leading to the formation of massive stars. These
clumps are typically at large distances and unresolved, so
their physical properties cannot be reliably derived from the
observations alone. Numerical simulations are needed to interpret the
observations. To this end, we generate synthetic Herschel observations
using our large-scale star-formation simulation, where massive stars
explode as supernovae driving the interstellar-medium turbulence. From
the synthetic observations, we compile a catalogue of compact
sources following the exact same procedure as for the Hi-GAL compact
source catalogue. We show that the sources from the simulation have
observational properties with statistical distributions consistent with
the observations. By relating the compact sources from the synthetic
observations to their 3D counterparts in the simulation, we find that
the synthetic observations overestimate the clump masses by about an
order of magnitude on average due to line-of-sight projection, and
projection effects are likely to be even worse for Hi-GAL Inner Galaxy
sources. We also find that a large fraction of sources classified as
protostellar are likely to be starless, and propose a new method to
partially discriminate between true and false protostellar sources.
Title: From the CMF to the IMF: beyond the core-collapse model
Authors: Pelkonen, V. -M.; Padoan, P.; Haugbølle, T.; Nordlund, Å.
Bibcode: 2021MNRAS.504.1219P
Altcode: 2021MNRAS.tmp..844P; 2020arXiv200802192P
Observations have indicated that the pre-stellar core mass function
(CMF) is similar to the stellar initial mass function (IMF), except
for an offset towards larger masses. This has led to the idea that
there is a one-to-one relation between cores and stars, such that the
whole stellar mass reservoir is contained in a gravitationally bound
pre-stellar core, as postulated by the core-collapse model, and assumed
in recent theoretical models of the stellar IMF. We test the validity of
this assumption by comparing the final mass of stars with the mass of
their progenitor cores in a high-resolution star formation simulation
that generates a realistic IMF under physical condition characteristic
of observed molecular clouds. Using a definition of bound cores similar
to previous works we obtain a CMF that converges with increasing
numerical resolution. We find that the CMF and the IMF are closely
related in a statistical sense only; for any individual star there is
only a weak correlation between the progenitor core mass and the final
stellar mass. In particular, for high-mass stars only a small fraction
of the final stellar mass comes from the progenitor core, and even for
low-mass stars the fraction is highly variable, with a median fraction
of only about 50 per cent. We conclude that the core-collapse scenario
and related models for the origin of the IMF are incomplete. We also
show that competitive accretion is not a viable alternative.
Title: A pebble accretion model for the formation of the terrestrial
planets in the Solar System
Authors: Johansen, Anders; Ronnet, Thomas; Bizzarro, Martin; Schiller,
Martin; Lambrechts, Michiel; Nordlund, Åke; Lammer, Helmut
Bibcode: 2021SciA....7..444J
Altcode: 2021arXiv210208611J
Pebbles of millimeter sizes are abundant in protoplanetary discs around
young stars. Chondrules inside primitive meteorites - formed by melting
of dust aggregate pebbles or in impacts between planetesimals - have
similar sizes. The role of pebble accretion for terrestrial planet
formation is nevertheless unclear. Here we present a model where
inwards-drifting pebbles feed the growth of terrestrial planets. The
masses and orbits of Venus, Earth, Theia (which later collided with
the Earth to form the Moon) and Mars are all consistent with pebble
accretion onto protoplanets that formed around Mars' orbit and migrated
to their final positions while growing. The isotopic compositions of
Earth and Mars are matched qualitatively by accretion of two generations
of pebbles, carrying distinct isotopic signatures. Finally, we show
that the water and carbon budget of Earth can be delivered by pebbles
from the early generation before the gas envelope became hot enough
to vaporise volatiles.
Title: The Effect of Supernovae on the Turbulence and Dispersal of
Molecular Clouds
Authors: Lu, Zu-Jia; Pelkonen, Veli-Matti; Padoan, Paolo; Pan, Liubin;
Haugbølle, Troels; Nordlund, Åke
Bibcode: 2020ApJ...904...58L
Altcode: 2020arXiv200709518L
We study the impact of supernovae on individual molecular clouds,
using a high-resolution magnetohydrodynamic simulation of a 250
pc region where we resolve the formation of individual massive
stars. The supernova feedback is implemented with real supernovae,
meaning supernovae that are the natural evolution of the resolved
massive stars, so their position and timing are self-consistent. We
select a large sample of molecular clouds from the simulation to
investigate the supernova energy injection and the resulting properties
of molecular clouds. We find that molecular clouds have a lifetime of
a few dynamical times, less than half of them contract to the point
of becoming gravitationally bound, and the dispersal time of bound
clouds of order one dynamical time is a factor of 2 shorter than that
of unbound clouds. We stress the importance of internal supernovae,
that is, massive stars that explode inside their parent cloud, in
setting the cloud dispersal time, and their huge overdensity compared
to models where the supernovae are randomly distributed. We also
quantify the energy injection efficiency of supernovae as a function of
supernova distance to the clouds. We conclude that intermittent driving
by supernovae can maintain molecular cloud turbulence and may be the
main process for cloud dispersal and that the full role of supernovae
in the evolution of molecular clouds cannot be fully accounted for
without a self-consistent implementation of the supernova feedback.
Title: Transport, Destruction, and Growth of Pebbles in the Gas
Envelope of a Protoplanet
Authors: Johansen, Anders; Nordlund, Åke
Bibcode: 2020ApJ...903..102J
Altcode: 2020arXiv200907837J
We analyze the size evolution of pebbles accreted into the gaseous
envelope of a protoplanet growing in a protoplanetary disk, taking into
account collisions driven by the relative sedimentation speed as well
as the convective gas motion. Using a simple estimate of the convective
gas speed based on the pebble accretion luminosity, we find that the
speed of the convective gas is higher than the sedimentation speed for
all particles smaller than 1 mm. This implies that both pebbles and
pebble fragments are strongly affected by the convective gas motion and
will be transported by large-scale convection cells both toward and
away from the protoplanet's surface. We present a simple scheme for
evolving the characteristic size of the pebbles, taking into account
the effects of erosion, mass transfer, and fragmentation. Including the
downwards motion of convective cells for the transport of pebbles with
an initial radius of 1 mm, we find pebble sizes between 100 μm and
1 mm near the surface of the protoplanet. These sizes are generally
amenable to accretion at the base of the convection flow. Small
protoplanets far from the star (>30 au) nevertheless erode their
pebbles to sizes below 10 μm; future hydrodynamical simulations will
be needed to determine whether such small fragments can detach from
the convection flow and become accreted by the protoplanet.
Title: The structure and characteristic scales of molecular clouds
Authors: Dib, Sami; Bontemps, Sylvain; Schneider, Nicola; Elia, Davide;
Ossenkopf-Okada, Volker; Shadmehri, Mohsen; Arzoumanian, Doris; Motte,
Frédérique; Heyer, Mark; Nordlund, Åke; Ladjelate, Bilal
Bibcode: 2020A&A...642A.177D
Altcode: 2020arXiv200708533D
The structure of molecular clouds holds important clues regarding the
physical processes that lead to their formation and subsequent dynamical
evolution. While it is well established that turbulence imprints a
self-similar structure onto the clouds, other processes, such as gravity
and stellar feedback, can break their scale-free nature. The break of
self-similarity can manifest itself in the existence of characteristic
scales that stand out from the underlying structure generated
by turbulent motions. In this work, we investigate the structure
of the Cygnus-X North and Polaris Flare molecular clouds, which
represent two extremes in terms of their star formation activity. We
characterize the structure of the clouds using the delta-variance
(Δ-variance) spectrum. In the Polaris Flare, the structure of the
cloud is self-similar over more than one order of magnitude in spatial
scales. In contrast, the Δ-variance spectrum of Cygnus-X North exhibits
an excess and a plateau on physical scales of ≈0.5-1.2 pc. In order
to explain the observations for Cygnus-X North, we use synthetic maps
where we overlay populations of discrete structures on top of a fractal
Brownian motion (fBm) image. The properties of these structures, such
as their major axis sizes, aspect ratios, and column density contrasts
with the fBm image, are randomly drawn from parameterized distribution
functions. We are able to show that, under plausible assumptions, it
is possible to reproduce a Δ-variance spectrum that resembles that
of the Cygnus-X North region. We also use a "reverse engineering"
approach in which we extract the compact structures in the Cygnus-X
North cloud and reinject them onto an fBm map. Using this approach,
the calculated Δ-variance spectrum deviates from the observations and
is an indication that the range of characteristic scales (≈0.5-1.2 pc)
observed in Cygnus-X North is not only due to the existence of compact
sources, but is a signature of the whole population of structures that
exist in the cloud, including more extended and elongated structures.
Title: The Origin of Massive Stars: The Inertial-inflow Model
Authors: Padoan, Paolo; Pan, Liubin; Juvela, Mika; Haugbølle, Troels;
Nordlund, Åke
Bibcode: 2020ApJ...900...82P
Altcode: 2019arXiv191104465P
We address the problem of the origin of massive stars, namely the
origin, path, and timescale of the mass flows that create them. Based
on extensive numerical simulations, we propose a scenario where massive
stars are assembled by large-scale, converging, inertial flows that
naturally occur in supersonic turbulence. We refer to this scenario
of massive-star formation as the inertial-inflow model. This model
stems directly from the idea that the mass distribution of stars is
primarily the result of turbulent fragmentation. Under this hypothesis,
the statistical properties of turbulence determine the formation
timescale and mass of prestellar cores, posing definite constraints on
the formation mechanism of massive stars. We quantify such constraints
by analyzing a simulation of supernova-driven turbulence in a 250
pc region of the interstellar medium, describing the formation of
hundreds of massive stars over a time of approximately 30 Myr. Due
to the large size of our statistical sample, we can say with full
confidence that massive stars in general do not form from the collapse
of massive cores nor from competitive accretion, as both models are
incompatible with the numerical results. We also compute synthetic
continuum observables in the Herschel and ALMA bands. We find that,
depending on the distance of the observed regions, estimates of core
mass based on commonly used methods may exceed the actual core masses
by up to two orders of magnitude and that there is essentially no
correlation between estimated and real core masses.
Title: Simulating rocky protoplanet growth via pebble accretion and
the DISPATCH framework
Authors: Ramsey, J.; Nordlund, Å.; Popovas, A.
Bibcode: 2020AAS...23522005R
Altcode:
With more than 3000 confirmed exoplanets, evidently, planet formation is
common. There is also mounting evidence from astronomical observations
and meteorites that planet formation starts very early. How do we
then form and grow planets frequently and quickly enough to match
the evidence? One popular paradigm is pebble accretion. "Pebbles" are
mm-to-cm sized particles which should be abundant in protoplanetary
disks, but are not necessarily well-coupled to the disk gas. It is for
this reason, in fact, that pebbles are prime targets for accretion
by planetary embryos. I will present 3D hydrodynamical simulations
of pebble accretion onto "rocky" planetary embryos that resolve the
disk scale height and the atmosphere of the embryo, but (importantly)
also follow pebble trajectories over time. Our results reinforce the
efficiency and robustness of pebble accretion, even when a convective
atmosphere or radiative cooling is included. From our measured accretion
rates, we find that is possible to grow an Earth-mass planetary embryo
in roughly 150,000 yr. I will also present ongoing work to determine
if growth via pebble accretion does, in fact, transpire that quickly
under realistic conditions.
Title: The Probability Distribution of Density Fluctuations in
Supersonic Turbulence
Authors: Pan, Liubin; Padoan, Paolo; Nordlund, Åke
Bibcode: 2019ApJ...881..155P
Altcode: 2019arXiv190500923P
A theoretical formulation is developed for the probability
distribution function (pdf) of gas density in supersonic turbulence
at steady state, connecting it to the conditional statistics of the
velocity divergence. Two sets of numerical simulations are carried
out, using either a Riemann solver to evolve the Euler equations
or a finite-difference method to evolve the Navier-Stokes (N-S)
equations. After confirming the validity of our theoretical formulation
with the N-S simulations, we examine the effects of dynamical processes
on the pdf, showing that the nonlinear term in the divergence equation
amplifies the right pdf tail and reduces the left one, the pressure
term reduces both the right and left tails, and the viscosity term,
counterintuitively, broadens the right tail of the pdf. Despite
the inaccuracy of the velocity divergence from the Riemann runs, we
show that the density pdf from the Riemann runs is consistent with
that from the N-S runs. Taking advantage of their higher effective
resolution, we use Riemann runs with resolution up to 20483
to study the dependence of the pdf on the Mach number, { \mathcal M },
up to { \mathcal M }∼ 30. The pdf width, σ s , follows
the relation {σ }s2={ln}(1+{b}2{{
\mathcal M }}2), with b ≈ 0.38. However, the pdf exhibits
a negative skewness that increases with increasing { \mathcal M },
as the growth of the right tail with increasing { \mathcal M } tends
to saturate. Thus, the usual prescription that combines a lognormal
shape with a variance-Mach number relation greatly overestimates the
right pdf tail at large { \mathcal M }, with important consequences
for star formation models.
Title: Task Based Radiative Transfer Methods and the DISPATCH Code
Framework
Authors: Nordlund, Å.
Bibcode: 2019ASPC..519...93N
Altcode:
Task based execution, where one avoids global constraint such as a
common timestep or synchronous iterations is arguably the future of
computational astrophysics, in the context of grid-based simulations of
hydrodynamics and magneto-hydrodynamics as well as in the context of
particle-in-cell and pure particle methods. With respect to radiation
transport the same applies, both for direct use in simulations, and for
expensive post-processing, such as multi-level spectral line diagnostic
with partial redistribution in three spatial dimensions and time.
Title: Inaccuracy of Spatial Derivatives in Riemann Solver Simulations
of Supersonic Turbulence
Authors: Pan, Liubin; Padoan, Paolo; Nordlund, Åke
Bibcode: 2019ApJ...876...90P
Altcode: 2019arXiv190200079P
We examine the accuracy of spatial derivatives computed from numerical
simulations of supersonic turbulence. Two sets of simulations, carried
out using a finite-volume code that evolves the hydrodynamic equations
with an approximate Riemann solver and a finite-difference code that
solves the Navier-Stokes (N-S) equations, are tested against a number
of criteria based on the continuity equation, including exact results
at statistically steady state. We find that the spatial derivatives in
the N-S runs are accurate and satisfy all the criteria. In particular,
they satisfy our exact results that, at steady state, the average
of the velocity divergence conditioned on the flow density and the
conditional average of the advection of density both vanish at all
density levels. On the other hand, the Riemann solver simulations fail
all the tests that require accurate evaluation of spatial derivatives,
resulting in apparent violation of the continuity equation, even if
the solver enforces mass conservation. In particular, analysis of the
Riemann simulations may lead to the incorrect conclusion that the p {dV}
work tends to preferentially convert kinetic energy into thermal energy,
which is inconsistent with the exact result that the energy exchange by
p {dV} work is symmetric in barotropic supersonic turbulence at steady
state. The inaccuracy of spatial derivatives is a general problem
in the post-processing of simulations of supersonic turbulence with
Riemann solvers. Solutions from such simulations must be used with
caution in post-processing studies concerning the spatial gradients.
Title: Pebble dynamics and accretion on to rocky planets -
II. Radiative models
Authors: Popovas, Andrius; Nordlund, Åke; Ramsey, Jon P.
Bibcode: 2019MNRAS.482L.107P
Altcode: 2018arXiv181007048P; 2018MNRAS.tmpL.196P
We investigate the effects of radiative energy transfer on a series
of nested-grid, high-resolution hydrodynamic simulations of gas and
particle dynamics in the vicinity of an Earth-mass planetary embryo. We
include heating due to the accretion of solids and the subsequent
convective motions. Using a constant embryo surface temperature, we
show that radiative energy transport results in a tendency to reduce the
entropy in the primordial atmosphere, but this tendency is alleviated by
an increase in the strength of convective energy transport, triggered
by a correspondingly increased superadiabatic temperature gradient. As
a consequence, the amplitude of the convective motions increase by
roughly an order of magnitude in the vicinity of the embryo. In the
cases investigated here, where the optical depth towards the disc
surface is larger than unity, the reduction of the temperature in
the outer parts of the Hill sphere relative to cases without radiative
energy transport is only ∼100 K, while the mass density increase is of
the order of a factor of two in the inner parts of the Hill sphere. Our
results demonstrate that, unless unrealistically low dust opacities are
assumed, radiative cooling in the context of primordial rocky planet
atmospheres can only become important after the disc surface density
has dropped significantly below minimum-mass-solar-nebula values.
Title: Detailed Balance and Exact Results for Density Fluctuations
in Supersonic Turbulence
Authors: Pan, Liubin; Padoan, Paolo; Nordlund, Åke
Bibcode: 2018ApJ...866L..17P
Altcode: 2018arXiv180808302P
The probabilistic approach to turbulence is applied to investigate
density fluctuations in supersonic turbulence. We derive kinetic
equations for the probability distribution function (PDF) of the
logarithm of the density field, s, in compressible turbulence in
two forms: a first-order partial differential equation involving
the average divergence conditioned on the flow density, < {{\nabla
}}\cdot {\boldsymbol{u}}| s> , and a Fokker-Planck equation with the
drift and diffusion coefficients equal to -< {\boldsymbol{u}}\cdot
{{\nabla }}s| s> and < {\boldsymbol{u}}\cdot {{\nabla }}s| s>
, respectively. Assuming statistical homogeneity only, the detailed
balance at steady state leads to two exact results, < {{\nabla
}}\cdot {\boldsymbol{u}}| s> =0, and < {\boldsymbol{u}}\cdot
{{\nabla }}s| s> =0. The former indicates a balance of the flow
divergence over all expanding and contracting regions at each given
density. The exact results provide an objective criterion to judge the
accuracy of numerical codes with respect to the density statistics
in supersonic turbulence. We also present a method to estimate the
effective numerical diffusion as a function of the flow density and
discuss its effects on the shape of the density PDF.
Title: Pebble dynamics and accretion on to rocky planets -
I. Adiabatic and convective models
Authors: Popovas, Andrius; Nordlund, Åke; Ramsey, Jon P.; Ormel,
Chris W.
Bibcode: 2018MNRAS.479.5136P
Altcode: 2018MNRAS.tmp.1678P; 2018arXiv180107707P
We present nested-grid, high-resolution hydrodynamic simulations of
gas and particle dynamics in the vicinity of Mars- to Earth-mass
planetary embryos. The simulations extend from the surface of
the embryos to a few vertical disc scale heights, with a spatial
dynamic range of ∼1.4 × 105. Our results confirm that
`pebble'-sized particles are readily accreted, with accretion rates
continuing to increase up to metre-size `boulders' for a 10 per
cent MMSN surface density model. The gas mass flux in and out of the
Hill sphere is consistent with the Hill rate, Σ Ω R_H^2=4 10^{-3}
M_\oplus yr^{-1}. While smaller size particles mainly track the gas,
a net accretion rate of {≈ } 2 10^{-5} M_\oplus yr^{-1} is reached
for 0.3-1 cm particles, even though a significant fraction leaves the
Hill sphere again. Effectively, all pebble-sized particles that cross
the Bondi sphere are accreted. The resolution of these simulations
is sufficient to resolve accretion-driven convection. Convection
driven by a nominal accretion rate of 10^{-6} M_\oplus yr^{-1} does
not significantly alter the pebble accretion rate. We find that, due
to cancellation effects, accretion rates of pebble-sized particles are
nearly independent of disc surface density. As a result, we can estimate
accurate growth times for specified particle sizes. For 0.3-1 cm size
particles, the growth time from a small seed is ∼0.15 million years
for an Earth-mass planet at 1 au and ∼0.1 million years for a Mars
mass planet at 1.5 au.
Title: DISPATCH: a numerical simulation framework for the exa-scale
era - I. Fundamentals
Authors: Nordlund, Åke; Ramsey, Jon P.; Popovas, Andrius; Küffmeier,
Michael
Bibcode: 2018MNRAS.477..624N
Altcode: 2017arXiv170510774N; 2018MNRAS.tmp..602N
We introduce a high-performance simulation framework that permits the
semi-independent, task-based solution of sets of partial differential
equations, typically manifesting as updates to a collection of `patches'
in space-time. A hybrid MPI/OpenMP execution model is adopted, where
work tasks are controlled by a rank-local `dispatcher' which selects,
from a set of tasks generally much larger than the number of physical
cores (or hardware threads), tasks that are ready for updating. The
definition of a task can vary, for example, with some solving the
equations of ideal magnetohydrodynamics (MHD), others non-ideal MHD,
radiative transfer, or particle motion, and yet others applying
particle-in-cell (PIC) methods. Tasks do not have to be grid based,
while tasks that are, may use either Cartesian or orthogonal curvilinear
meshes. Patches may be stationary or moving. Mesh refinement can be
static or dynamic. A feature of decisive importance for the overall
performance of the framework is that time-steps are determined and
applied locally; this allows potentially large reductions in the total
number of updates required in cases when the signal speed varies greatly
across the computational domain, and therefore a corresponding reduction
in computing time. Another feature is a load balancing algorithm
that operates `locally' and aims to simultaneously minimize load and
communication imbalance. The framework generally relies on already
existing solvers, whose performance is augmented when run under the
framework, due to more efficient cache usage, vectorization, local
time-stepping, plus near-linear and, in principle, unlimited OpenMP
and MPI scaling.
Title: A simple and efficient solver for self-gravity in the DISPATCH
astrophysical simulation framework
Authors: Ramsey, J. P.; Haugbølle, T.; Nordlund, Å.
Bibcode: 2018JPhCS1031a2021R
Altcode: 2018arXiv180610098R
We describe a simple and effective algorithm for solving Poisson’s
equation in the context of self-gravity within the DISPATCH
astrophysical fluid framework. The algorithm leverages the fact that
DISPATCH stores multiple time slices and uses asynchronous time-stepping
to produce a scheme that does not require any explicit global
communication or sub-cycling, only the normal, local communication
between patches and the iterative solution to Poisson’s equation. We
demonstrate that the implementation is suitable for both collections
of patches of a single resolution and for hierarchies of adaptively
resolved patches. Benchmarks are presented that demonstrate the
accuracy, effectiveness and efficiency of the scheme.
Title: The benchmark halo giant HD 122563: CNO abundances revisited
with three-dimensional hydrodynamic model stellar atmospheres
Authors: Collet, R.; Nordlund, Å.; Asplund, M.; Hayek, W.;
Trampedach, R.
Bibcode: 2018MNRAS.475.3369C
Altcode: 2017arXiv171208099C
We present an abundance analysis of the low-metallicity benchmark
red giant star HD 122563 based on realistic, state-of-the-art,
high-resolution, three-dimensional (3D) model stellar atmospheres
including non-grey radiative transfer through opacity binning with
4, 12, and 48 bins. The 48-bin 3D simulation reaches temperatures
lower by ∼300-500 K than the corresponding 1D model in the upper
atmosphere. Small variations in the opacity binning, adopted line
opacities, or chemical mixture can cool the photospheric layers by
a further ∼100-300 K and alter the effective temperature by ∼100
K. A 3D local thermodynamic equilibrium (LTE) spectroscopic analysis
of Fe I and Fe II lines gives discrepant results in terms of derived
Fe abundance, which we ascribe to non-LTE effects and systematic errors
on the stellar parameters. We also determine C, N, and O abundances by
simultaneously fitting CH, OH, NH, and CN molecular bands and lines
in the ultraviolet, visible, and infrared. We find a small positive
3D-1D abundance correction for carbon (+0.03 dex) and negative ones for
nitrogen (-0.07 dex) and oxygen (-0.34 dex). From the analysis of the [O
I] line at 6300.3 Å, we derive a significantly higher oxygen abundance
than from molecular lines (+0.46 dex in 3D and +0.15 dex in 1D). We rule
out important OH photodissociation effects as possible explanation for
the discrepancy and note that lowering the surface gravity would reduce
the oxygen abundance difference between molecular and atomic indicators.
Title: The Stellar IMF from Isothermal MHD Turbulence
Authors: Haugbølle, Troels; Padoan, Paolo; Nordlund, Åke
Bibcode: 2018ApJ...854...35H
Altcode: 2017arXiv170901078H
We address the turbulent fragmentation scenario for the origin of the
stellar initial mass function (IMF), using a large set of numerical
simulations of randomly driven supersonic MHD turbulence. The turbulent
fragmentation model successfully predicts the main features of the
observed stellar IMF assuming an isothermal equation of state without
any stellar feedback. As a test of the model, we focus on the case of a
magnetized isothermal gas, neglecting stellar feedback, while pursuing
a large dynamic range in both space and timescales covering the full
spectrum of stellar masses from brown dwarfs to massive stars. Our
simulations represent a generic 4 pc region within a typical Galactic
molecular cloud, with a mass of 3000 M ⊙ and an rms
velocity 10 times the isothermal sound speed and 5 times the average
Alfvén velocity, in agreement with observations. We achieve a maximum
resolution of 50 au and a maximum duration of star formation of 4.0
Myr, forming up to a thousand sink particles whose mass distribution
closely matches the observed stellar IMF. A large set of medium-size
simulations is used to test the sink particle algorithm, while larger
simulations are used to test the numerical convergence of the IMF and
the dependence of the IMF turnover on physical parameters predicted
by the turbulent fragmentation model. We find a clear trend toward
numerical convergence and strong support for the model predictions,
including the initial time evolution of the IMF. We conclude that
the physics of isothermal MHD turbulence is sufficient to explain the
origin of the IMF.
Title: Accounting for the diversity in stellar environments
Authors: Küffmeier, Michael; Haugbølle, Troels; Nordlund, Åke
Bibcode: 2017arXiv171008900K
Altcode:
Stars and their corresponding protoplanetary disks form in diverse
environments. To account for these natural variations, we investigate
the formation process around nine solar mass stars with a maximum
resolution of 2 AU in a Giant Molecular Cloud of (40 pc)$^3$
in volume by using the adaptive mesh refinement code \ramses. The
magnetohydrodynamic simulations reveal that the accretion process is
heterogeneous in time, in space, and among protostars of otherwise
similar mass. During the first roughly 100 kyr of a protostar evolving
to about a solar mass, the accretion rates peak around $10^{-5}$ to
$10^{-4}$ M$_{\odot}$ yr$^{-1}$ shortly after its birth, declining
with time after that. The different environments also affect the
spatial accretion, and infall of material to the star-disk system
is mostly through filaments and sheets. Furthermore, the formation
and evolution of disks varies significantly from star to star. We
interpret the variety in disk formation as a consequence of the
differences in the combined effects of magnetic fields and turbulence
that may cause differences in the efficiency of magnetic braking,
as well as differences in the strength and distribution of specific
angular momentum.
Title: Microscopic Processes in Global Relativistic Jets Containing
Helical Magnetic Fields: Dependence on Jet Radius
Authors: Nishikawa, Ken-Ichi; Mizuno, Yosuke; Gómez, Jose; Duţan,
Ioana; Meli, Athina; White, Charley; Niemiec, Jacek; Kobzar, Oleh;
Pohl, Martin; Pe'er, Asaf; Frederiksen, Jacob; Nordlund, Åke; Sol,
Helene; Hardee, Philip; Hartmann, Dieter
Bibcode: 2017Galax...5...58N
Altcode: 2017arXiv170807740N
In this study, we investigate the interaction of jets with their
environment at a microscopic level, which is a key open question in the
study of relativistic jets. Using small simulation systems during past
research, we initially studied the evolution of both electron-proton
and electron-positron relativistic jets containing helical magnetic
fields, by focusing on their interactions with an ambient plasma. Here,
using larger jet radii, we have performed simulations of global jets
containing helical magnetic fields in order to examine how helical
magnetic fields affect kinetic instabilities, such as the Weibel
instability, the kinetic Kelvin-Helmholtz instability (kKHI) and the
mushroom instability (MI). We found that the evolution of global jets
strongly depends on the size of the jet radius. For example, phase
bunching of jet electrons, in particular in the electron-proton
jet, is mixed with a larger jet radius as a result of the more
complicated structures of magnetic fields with excited kinetic
instabilities. In our simulation, these kinetic instabilities led to
new types of instabilities in global jets. In the electron-proton jet
simulation, a modified recollimation occurred, and jet electrons were
strongly perturbed. In the electron-positron jet simulation, mixed
kinetic instabilities occurred early, followed by a turbulence-like
structure. Simulations using much larger (and longer) systems are
required in order to further thoroughly investigate the evolution of
global jets containing helical magnetic fields.
Title: Zoom-in Simulations of Protoplanetary Disks Starting from
GMC Scales
Authors: Kuffmeier, Michael; Haugbølle, Troels; Nordlund, Åke
Bibcode: 2017ApJ...846....7K
Altcode: 2016arXiv161110360K
We investigate the formation of protoplanetary disks around nine
solar-mass stars formed in the context of a (40 pc)3
Giant Molecular Cloud model, using ramses adaptive mesh refinement
simulations extending over a scale range of about 4 million, from an
outer scale of 40 pc down to cell sizes of 2 au. Our most important
result is that the accretion process is heterogeneous in multiple
ways: in time, in space, and among protostars of otherwise similar
mass. Accretion is heterogeneous in time, in the sense that accretion
rates vary during the evolution, with generally decreasing profiles,
whose slopes vary over a wide range, and where accretion can increase
again if a protostar enters a region with increased density and
low speed. Accretion is heterogeneous in space, because of the mass
distribution, with mass approaching the accreting star-disk system
in filaments and sheets. Finally, accretion is heterogeneous among
stars, since the detailed conditions and dynamics in the neighborhood
of each star can vary widely. We also investigate the sensitivity of
disk formation to physical conditions and test their robustness by
varying numerical parameters. We find that disk formation is robust
even when choosing the least favorable sink particle parameters,
and that turbulence cascading from larger scales is a decisive
factor in disk formation. We also investigate the transport of
angular momentum, finding that the net inward mechanical transport
is compensated for mainly by an outward-directed magnetic transport,
with a contribution from gravitational torques usually subordinate to
the magnetic transport.
Title: Early formation of planetary building blocks inferred from
Pb isotopic ages of chondrules
Authors: Bollard, Jean; Connelly, James N.; Whitehouse, Martin J.;
Pringle, Emily A.; Bonal, Lydie; Jørgensen, Jes K.; Nordlund, Åke;
Moynier, Frédéric; Bizzarro, Martin
Bibcode: 2017SciA....3E0407B
Altcode: 2017arXiv170802631B
The most abundant components of primitive meteorites (chondrites)
are millimeter-sized glassy spherical chondrules formed by transient
melting events in the solar protoplanetary disk. Using Pb-Pb dates of
22 individual chondrules, we show that primary production of chondrules
in the early solar system was restricted to the first million years
after formation of the Sun and that these existing chondrules were
recycled for the remaining lifetime of the protoplanetary disk. This
is consistent with a primary chondrule formation episode during
the early high-mass accretion phase of the protoplanetary disk that
transitions into a longer period of chondrule reworking. An abundance
of chondrules at early times provides the precursor material required
to drive the efficient and rapid formation of planetary objects via
chondrule accretion.
Title: Nucleosynthetic Diversity of Chondrites and Their Components
— Tracking Disk Mass Transport Processes and the Early Formation
of Large-Scale Solar System Reservoirs
Authors: Bizzarro, M.; Wielandt, D.; Haugbølle, T.; Nordlund, A.
Bibcode: 2017LPICo1975.2008B
Altcode:
We review the state-of-the-art data with respect to the chronology and
stable isotopic data of individual chondrules from various chondrite
groups and discuss how these can be used to provide new insights disk
mass transport processes and storage.
Title: A grid of MARCS model atmospheres for late-type stars. II. S
stars and their properties
Authors: Van Eck, Sophie; Neyskens, Pieter; Jorissen, Alain; Plez,
Bertrand; Edvardsson, Bengt; Eriksson, Kjell; Gustafsson, Bengt;
Jørgensen, Uffe Gråe; Nordlund, Åke
Bibcode: 2017A&A...601A..10V
Altcode:
S-type stars are late-type giants whose atmospheres are enriched in
carbon and s-process elements because of either extrinsic pollution
by a binary companion or intrinsic nucleosynthesis and dredge-up on
the thermally-pulsing asymptotic giant branch. A grid of MARCS model
atmospheres has been computed for S stars, covering the range 2700 ≤
Teff(K) ≤ 4000, 0.50 ≤ C/O ≤ 0.99, 0 ≤ log g ≤ 5,
[Fe/H] = 0., -0.5 dex, and [s/Fe] = 0, 1, and 2 dex (where the latter
quantity refers to the global overabundance of s-process elements). The
MARCS models make use of a new ZrO line list. Synthetic spectra
computed from these models are used to derive photometric indices in
the Johnson and Geneva systems, as well as TiO and ZrO band strengths. A
method is proposed to select the model best matching any given S star,
a non-trivial operation since the grid contains more than 3500 models
covering a five-dimensional parameter space. The method is based on
the comparison between observed and synthetic photometric indices and
spectral band strengths, and has been applied on a vast subsample of
the Henize sample of S stars. Our results confirm the old claim by
Piccirillo (1980, MNRAS, 190, 441) that ZrO bands in warm S stars
(Teff>3200 K) are not caused by the C/O ratio being
close to unity, as traditionally believed, but rather by some Zr
overabundance. The TiO and ZrO band strengths, combined with V-K and
J-K photometric indices, are used to select Teff, C/O, [Fe/H]
and [s/Fe]. The Geneva U-B1 and B2-V1
indices (or any equivalent) are good at selecting the gravity. The
defining spectral features of dwarf S stars are outlined, but none is
found among the Henize S stars. More generally, it is found that, at
Teff = 3200 K, a change of C/O from 0.5 to 0.99 has a strong
impact on V-K (2 mag). Conversely, a range of 2 mag in V-K corresponds
to a 200 K shift along the (Teff, V-K) relationship
(for a fixed C/O value). Hence, the use of a (Teff, V-K)
calibration established for M stars will yield large errors for S stars,
so that a specific calibration must be used, as provided in the present
paper. Using the atmospheric parameters derived by our method for the
sample of Henize S stars, we show that the extrinsic-intrinsic dichotomy
among S stars reveals itself very clearly as a bimodal distribution in
the effective temperatures. Moreover, the increase of s-process element
abundances with increasing C/O ratios and decreasing temperatures is
apparent among intrinsic stars, confirming theoretical expectations. Based on observations carried out at the European Southern Observatory
(ESO, La Silla, Chile; program 58.E-0942), on the Swiss 70 cm telescope
(La Silla, Chile) and on the Mercator telescope (La Palma, Spain).The
MARCS S star model atmospheres will be archived on the MARCS website:
http://marcs.astro.uu.seFull
Tables 2 and 3 are only available at the CDS via anonymous ftp to
http://cdsarc.u-strasbg.fr
(http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/601/A10
Title: Supernova Driving. IV. The Star-formation Rate of Molecular
Clouds
Authors: Padoan, Paolo; Haugbølle, Troels; Nordlund, Åke; Frimann,
Søren
Bibcode: 2017ApJ...840...48P
Altcode: 2017arXiv170207270P
We compute the star-formation rate (SFR) in molecular clouds (MCs)
that originate ab initio in a new, higher-resolution simulation
of supernova-driven turbulence. Because of the large number of
well-resolved clouds with self-consistent boundary and initial
conditions, we obtain a large range of cloud physical parameters with
realistic statistical distributions, which is an unprecedented sample of
star-forming regions to test SFR models and to interpret observational
surveys. We confirm the dependence of the SFR per free-fall time,
SFRff, on the virial parameter, α vir, found in
previous simulations, and compare a revised version of our turbulent
fragmentation model with the numerical results. The dependences on
Mach number, { M }, gas to magnetic pressure ratio, β, and compressive
to solenoidal power ratio, χ at fixed α vir are not well
constrained, because of random scatter due to time and cloud-to-cloud
variations in SFRff. We find that SFRff in MCs
can take any value in the range of 0 ≤ SFRff ≲ 0.2,
and its probability distribution peaks at a value of SFRff
≈ 0.025, consistent with observations. The values of SFRff
and the scatter in the SFRff-α vir relation are
consistent with recent measurements in nearby MCs and in clouds near
the Galactic center. Although not explicitly modeled by the theory,
the scatter is consistent with the physical assumptions of our revised
model and may also result in part from a lack of statistical equilibrium
of the turbulence, due to the transient nature of MCs.
Title: VizieR Online Data Catalog: A grid of MARCS model atmospheres
for S stars (Van Eck+, 2017)
Authors: van Eck, S.; Neyskens, P.; Jorissen, A.; Plez, B.; Edvardsson,
B.; Eriksson, K.; Gustafsson, B.; Jorgensen, U. G.; Nordlund, A.
Bibcode: 2017yCat..36010010V
Altcode:
Johnson and Geneva (G) photometric indices and band indices measured
on the observed Henize S stars. (3 data files).
Title: Particle-in-cell Simulations of Global Relativistic Jets with
Helical Magnetic Fields
Authors: Duţan, Ioana; Nishikawa, Ken-Ichi; Mizuno, Yosuke; Niemiec,
Jacek; Kobzar, Oleh; Pohl, Martin; Gómez, Jose L.; Pe'er, Asaf;
Frederiksen, Jacob T.; Nordlund, Åke; Meli, Athina; Sol, Helene;
Hardee, Philip E.; Hartmann, Dieter H.
Bibcode: 2017IAUS..324..199D
Altcode: 2016arXiv161102882D
We study the interaction of relativistic jets with their environment,
using 3-dimen- sional relativistic particle-in-cell simulations for
two cases of jet composition: (i) electron-proton (e - -
p +) and (ii) electron-positron (e +/-) plasmas
containing helical magnetic fields. We have performed simulations of
``global'' jets containing helical magnetic fields in order to examine
how helical magnetic fields affect kinetic instabilities such as the
Weibel instability, the kinetic Kelvin-Helmholtz instability and the
Mushroom instability. We have found that these kinetic instabilities
are suppressed and new types of instabilities can grow. For the e
- - p + jet, a recollimation-like instability
occurs and jet electrons are strongly perturbed, whereas for the e
+/- jet, a recollimation-like instability occurs at early
times followed by kinetic instability and the general structure is
similar to a simulation without a helical magnetic field. We plan to
perform further simulations using much larger systems to confirm these
new findings.
Title: Particle-in-Cell Simulations of Global Relativistic Jets with
Helical Magnetic Fields
Authors: Nishikawa, K. -I.; Mizuno, Y.; Niemiec, J.; Kobzar, O.;
Pohl, M.; Gomez, J. L.; Dutam, I.; Pe'er, A.; Frederiksen, J. T.;
Nordlund, A.; Meli, A.; Sol, H.; Hardee, P. E.; Hartmann, D. H.
Bibcode: 2016LPICo1962.4035N
Altcode:
We study the interaction of relativistic jets with their environment,
using 3-dimensional relativistic particle-in-cell simulations for two
cases of jet composition. We have found that new types of instabilities
(kink instability; reconnection) grow.
Title: Microscopic Processes in Global Relativistic Jets Containing
Helical Magnetic Fields
Authors: Nishikawa, Ken-Ichi; Mizuno, Yosuke; Niemiec, Jacek;
Kobzar, Oleh; Pohl, Martin; Gómez, Jose; Duţan, Ioana; Pe'er, Asaf;
Frederiksen, Jacob; Nordlund, Åke; Meli, Athina; Sol, Helene; Hardee,
Philip; Hartmann, Dieter
Bibcode: 2016Galax...4...38N
Altcode: 2016arXiv160909363N
In the study of relativistic jets one of the key open questions is
their interaction with the environment on the microscopic level. Here,
we study the initial evolution of both electron-proton ( e - - p + )
and electron-positron ( e ± ) relativistic jets containing helical
magnetic fields, focusing on their interaction with an ambient
plasma. We have performed simulations of "global" jets containing
helical magnetic fields in order to examine how helical magnetic
fields affect kinetic instabilities such as the Weibel instability, the
kinetic Kelvin-Helmholtz instability (kKHI) and the Mushroom instability
(MI). In our initial simulation study these kinetic instabilities are
suppressed and new types of instabilities can grow. In the e - - p +
jet simulation a recollimation-like instability occurs and jet electrons
are strongly perturbed. In the e ± jet simulation a recollimation-like
instability occurs at early times followed by a kinetic instability
and the general structure is similar to a simulation without helical
magnetic field. Simulations using much larger systems are required
in order to thoroughly follow the evolution of global jets containing
helical magnetic fields.
Title: Supernova Driving. III. Synthetic Molecular Cloud Observations
Authors: Padoan, Paolo; Juvela, Mika; Pan, Liubin; Haugbølle, Troels;
Nordlund, Åke
Bibcode: 2016ApJ...826..140P
Altcode: 2016arXiv160503917P
We present a comparison of molecular clouds (MCs) from a simulation of
supernova (SN) driven interstellar medium (ISM) turbulence with real
MCs from the Outer Galaxy Survey. The radiative transfer calculations
to compute synthetic CO spectra are carried out assuming that the
CO relative abundance depends only on gas density, according to
four different models. Synthetic MCs are selected above a threshold
brightness temperature value, T B,min = 1.4 K, of the J = 1 -
0 12CO line, generating 16 synthetic catalogs (four different
spatial resolutions and four CO abundance models), each containing
up to several thousands MCs. The comparison with the observations
focuses on the mass and size distributions and on the velocity-size and
mass-size Larson relations. The mass and size distributions are found
to be consistent with the observations, with no significant variations
with spatial resolution or chemical model, except in the case of the
unrealistic model with constant CO abundance. The velocity-size relation
is slightly too steep for some of the models, while the mass-size
relation is a bit too shallow for all models only at a spatial
resolution dx ≈ 1 pc. The normalizations of the Larson relations
show a clear dependence on spatial resolution, for both the synthetic
and the real MCs. The comparison of the velocity-size normalization
suggests that the SN rate in the Perseus arm is approximately 70%
or less of the rate adopted in the simulation. Overall, the realistic
properties of the synthetic clouds confirm that SN-driven turbulence
can explain the origin and dynamics of MCs.
Title: Supernova Driving. II. Compressive Ratio in Molecular-cloud
Turbulence
Authors: Pan, Liubin; Padoan, Paolo; Haugbølle, Troels; Nordlund, Åke
Bibcode: 2016ApJ...825...30P
Altcode: 2015arXiv151004742P
The compressibility of molecular cloud (MC) turbulence plays a crucial
role in star formation models, because it controls the amplitude
and distribution of density fluctuations. The relation between the
compressive ratio (the ratio of powers in compressive and solenoidal
motions) and the statistics of turbulence has been previously studied
systematically only in idealized simulations with random external
forces. In this work, we analyze a simulation of large-scale turbulence
(250 pc) driven by supernova (SN) explosions that has been shown
to yield realistic MC properties. We demonstrate that SN driving
results in MC turbulence with a broad lognormal distribution of the
compressive ratio, with a mean value ≈0.3, lower than the equilibrium
value of ≈0.5 found in the inertial range of isothermal simulations
with random solenoidal driving. We also find that the compressibility
of the turbulence is not noticeably affected by gravity, nor are the
mean cloud radial (expansion or contraction) and solid-body rotation
velocities. Furthermore, the clouds follow a general relation between
the rms density and the rms Mach number similar to that of supersonic
isothermal turbulence, though with a large scatter, and their average
gas density probability density function is described well by a
lognormal distribution, with the addition of a high-density power-law
tail when self-gravity is included.
Title: Tracking the Distribution of 26Al and 60Fe during the Early
Phases of Star and Disk Evolution
Authors: Kuffmeier, Michael; Frostholm Mogensen, Troels; Haugbølle,
Troels; Bizzarro, Martin; Nordlund, Åke
Bibcode: 2016ApJ...826...22K
Altcode: 2016arXiv160505008K
The short-lived 26Al and 60Fe radionuclides
are synthesized and expelled into the interstellar medium by
core-collapse supernova events. The solar system’s first solids,
calcium-aluminum refractory inclusions (CAIs), contain evidence for the
former presence of the 26 Al nuclide defining the canonical
26Al/27 Al ratio of ∼ 5× {10}-5. A
different class of objects temporally related to canonical CAIs
are CAIs with fractionation and unidentified nuclear effects (FUN
CAIs), which record a low initial 26Al/27Al
of 10-6. The contrasting level of 26Al between
these objects is often interpreted as reflecting the admixing of the
26Al nuclides during the early formative phase of the
Sun. We use giant molecular cloud scale adaptive mesh-refinement
numerical simulations to trace the abundance of 26Al
and 60Fe in star-forming gas during the early stages
of accretion of individual low-mass protostars. We find that the
26Al/27Al and 60Fe/56Fe
ratios of accreting gas within a vicinity of 1000 au of the stars follow
the predicted decay curves of the initial abundances at the time of star
formation without evidence of spatial or temporal heterogeneities for
the first 100 kyr of star formation. Therefore, the observed differences
in 26Al/27Al ratios between FUN and canonical
CAIs are likely not caused by admixing of supernova material during
the early evolution of the proto-Sun. Selective thermal processing of
dust grains is a more viable scenario to account for the heterogeneity
in 26Al/27Al ratios at the time of solar system
formation.
Title: The impact of supernova remnants on interstellar turbulence
and star formation
Authors: Pan, Liubin; Padoan, Paolo; Haugboelle, Troels; Nordlund, Ake
Bibcode: 2016sros.confE.146P
Altcode:
The explosion energy of supernovae is believed to be a major energy
source to drive and maintain turbulent motions in the interstellar
gas. The interaction of supernova remnants with the interstellar
medium plays a crucial role in shaping the statistics of interstellar
turbulence, and has important effects on physical properties
of molecular clouds. To investigate supernova-driven turbulence
in molecular clouds and the implications for star formation, we
conducted a large-scale MHD simulation, keeping track of the evolution
of supernova remnants and their interactions with the interstellar
gas in a region of 250 pc. The simulation accounts for the effects
of gas heating and cooling, the magnetic fields and self-gravity,
and the explosion energy of supernovae is injected as thermal energy
at randomly selected locations in the simulation box. We analyzed
the dense molecular clouds formed in our simulation, and showed that
their properties, including the mass-size, velocity-size relations,
mass and size probability distributions, and magnetic field-density
relation, are all consistent with observational results, suggesting
that the dynamics and structure of molecular clouds are the natural
result of supernova-driven turbulence. We also found that, at the
scale of molecular clouds, turbulent motions contain more power
in solenoidal modes than in compressive modes. This suggests that
the effective driving force for interstellar turbulence is largely
solenoidal, in contrast to the recenthypothesis that supernova driving
is purely compressive. The physical reason is that, as a supernova
remnant impacts the ambient interstellar gas, the baroclinic effect
arises immediately, which preferentially converts compressive motions
to solenoidal modes throughout the evolution of the remnant in the
interstellar medium. The implications of our results concerning the
statistics of supernova-driven turbulence in molecular clouds on
theoretical modeling of star formation will be discussed.
Title: Supernova Driving. I. The Origin of Molecular Cloud Turbulence
Authors: Padoan, Paolo; Pan, Liubin; Haugbølle, Troels; Nordlund, Åke
Bibcode: 2016ApJ...822...11P
Altcode: 2015arXiv150904663P
Turbulence is ubiquitous in molecular clouds (MCs), but its origin
is still unclear because MCs are usually assumed to live longer than
the turbulence dissipation time. Interstellar medium (ISM) turbulence
is likely driven by supernova (SN) explosions, but it has never been
demonstrated that SN explosions can establish and maintain a turbulent
cascade inside MCs consistent with the observations. In this work,
we carry out a simulation of SN-driven turbulence in a volume of (250
pc)3, specifically designed to test if SN driving alone can
be responsible for the observed turbulence inside MCs. We find that SN
driving establishes a velocity scaling consistent with the usual scaling
laws of supersonic turbulence, suggesting that previous idealized
simulations of MC turbulence, driven with a random, large-scale volume
force, were correctly adopted as appropriate models for MC turbulence,
despite the artificial driving. We also find that the same scaling laws
extend to the interiors of MCs, and that the velocity-size relation
of the MCs selected from our simulation is consistent with that of
MCs from the Outer-Galaxy Survey, the largest MC sample available. The
mass-size relation and the mass and size probability distributions also
compare successfully with those of the Outer Galaxy Survey. Finally,
we show that MC turbulence is super-Alfvénic with respect to both the
mean and rms magnetic-field strength. We conclude that MC structure
and dynamics are the natural result of SN-driven turbulence.
Title: Evolution of Global Relativistic Jets: Collimations and
Expansion with kKHI and the Weibel Instability
Authors: Nishikawa, K. -I.; Frederiksen, J. T.; Nordlund, Å.; Mizuno,
Y.; Hardee, P. E.; Niemiec, J.; Gómez, J. L.; Pe'er, A.; Duţan,
I.; Meli, A.; Sol, H.; Pohl, M.; Hartmann, D. H.
Bibcode: 2016ApJ...820...94N
Altcode: 2015arXiv151103581N
In the study of relativistic jets one of the key open questions is their
interaction with the environment. Here we study the initial evolution
of both electron-proton ({e}{--}-{p}+) and
electron-positron (e±) relativistic jets, focusing on their
lateral interaction with ambient plasma. We follow the evolution of
toroidal magnetic fields generated by both the kinetic Kelvin-Helmholtz
and Mushroom instabilities. For an {e}{--}-{p}+
jet, the induced magnetic field collimates the jet and electrons
are perpendicularly accelerated. As the instabilities saturate and
subsequently weaken, the magnetic polarity switches from clockwise to
counterclockwise in the middle of the jet. For an e± jet,
we find strong mixing of electrons and positrons with the ambient
plasma, resulting in the creation of a bow shock. The merging of
current filaments generates density inhomogeneities that initiate
a forward shock. Strong jet-ambient plasma mixing prevents a full
development of the jet (on the scale studied), revealing evidence
for both jet collimation and particle acceleration in the forming
bow shock. Differences in the magnetic field structure generated
by {e}{--}-{p}+ and e± jets may
contribute to the polarization properties of the observed emission in
AGN jets and gamma-ray bursts.
Title: The Surface of Stellar Models - Now with more 3D simulations!
Authors: Trampedach, Regner; Christensen-Dalsgaard, Jørgen; Asplund,
Martin; Stein, Robert F.; Nordlund, Åke
Bibcode: 2015EPJWC.10106064T
Altcode:
We have constructed a grid of 3D hydrodynamic simulations of deep
convective and line-blanketed atmospheres. We have developed a
new consistent method for computing and employing T(τ) relations
from these simulations, as surface boundary conditions for 1D
stellar structure models. These 1D models have, in turn, had their
mixing-length, α, calibrated against the averaged structure of
each of the simulations. Both α and T(τ) vary significantly with
Teff and log g.
Title: Radiation from Particles Accelerated in Relativistic Jet
Shocks and Shear-flows
Authors: Nishikawa, K. -I.; Hardee, P.; Dutan, I.; Zhang, B.; Meli,
A.; Choi, E. J.; Min, K.; Niemiec, J.; Mizuno, Y.; Medvedev, M.;
Nordlund, A.; Frederiksen, J. T.; Sol, H.; Pohl, M.; Hartmann, D.
Bibcode: 2014arXiv1412.7064N
Altcode:
We have investigated particle acceleration and emission from shocks
and shear flows associated with an unmagnetized relativistic jet plasma
propagating into an unmagnetized ambient plasma. Strong electro-magnetic
fields are generated in the jet shock via the filamentation (Weibel)
instability. Shock field strength and structure depend on plasma
composition (($e^{\pm}$ or $e^-$- $p^+$ plasmas) and Lorentz factor. In
the velocity shear between jet and ambient plasmas, strong AC ($e^{\pm}$
plasmas) or DC ($e^-$- $p^+$ plasmas) magnetic fields are generated via
the kinetic Kelvin-Helmholtz instability (kKHI), and the magnetic field
structure also depends on the jet Lorentz factor. We have calculated,
self-consistently, the radiation from electrons accelerated in
shock generated magnetic fields. The spectra depend on the jet's
initial Lorentz factor and temperature via the resulting particle
acceleration and magnetic field generation. Our ongoing "Global" jet
simulations containing shocks and velocity shears will provide us
with the ability to calculate and model the complex time evolution
and/or spectral structure observed from gamma-ray bursts, AGN jets,
and supernova remnants.
Title: Improvements to stellar structure models, based on a grid of 3D
convection simulations - II. Calibrating the mixing-length formulation
Authors: Trampedach, Regner; Stein, Robert F.; Christensen-Dalsgaard,
Jørgen; Nordlund, Åke; Asplund, Martin
Bibcode: 2014MNRAS.445.4366T
Altcode: 2014arXiv1410.1559T
We perform a calibration of the mixing length of convection in stellar
structure models against realistic 3D radiation-coupled hydrodynamics
simulations of convection in stellar surface layers, determining
the adiabat deep in convective stellar envelopes. The mixing-length
parameter α is calibrated by matching averages of the 3D simulations
to 1D stellar envelope models, ensuring identical atomic physics
in the two cases. This is done for a previously published grid of
solar-metallicity convection simulations, covering from 4200 to 6900 K
on the main sequence, and from 4300 to 5000 K for giants with log g =
2.2. Our calibration results in an α varying from 1.6 for the warmest
dwarf, which is just cool enough to admit a convective envelope, and
up to 2.05 for the coolest dwarfs in our grid. In between these is a
triangular plateau of α ∼ 1.76. The Sun is located on this plateau
and has seen little change during its evolution so far. When stars
ascend the giant branch, they largely do so along tracks of constant
α, with α decreasing with increasing mass.
Title: Infall-driven Protostellar Accretion and the Solution to the
Luminosity Problem
Authors: Padoan, Paolo; Haugbølle, Troels; Nordlund, Åke
Bibcode: 2014ApJ...797...32P
Altcode: 2014arXiv1407.1445P
We investigate the role of mass infall in the formation and evolution of
protostars. To avoid ad hoc initial and boundary conditions, we consider
the infall resulting self-consistently from modeling the formation of
stellar clusters in turbulent molecular clouds. We show that infall
rates in turbulent clouds are comparable to accretion rates inferred
from protostellar luminosities or measured in pre-main-sequence
stars. They should not be neglected in modeling the luminosity
of protostars and the evolution of disks, even after the embedded
protostellar phase. We find large variations of infall rates from
protostar to protostar, and large fluctuations during the evolution
of individual protostars. In most cases, the infall rate is initially
of order 10-5 M ⊙ yr-1, and may
either decay rapidly in the formation of low-mass stars, or remain
relatively large when more massive stars are formed. The simulation
reproduces well the observed characteristic values and scatter of
protostellar luminosities and matches the observed protostellar
luminosity function. The luminosity problem is therefore solved once
realistic protostellar infall histories are accounted for, with no need
for extreme accretion episodes. These results are based on a simulation
of randomly driven magnetohydrodynamic turbulence on a scale of 4 pc,
including self-gravity, adaptive-mesh refinement to a resolution of
50 AU, and accreting sink particles. The simulation yields a low
star formation rate, consistent with the observations, and a mass
distribution of sink particles consistent with the observed stellar
initial mass function during the whole duration of the simulation,
forming nearly 1300 sink particles over 3.2 Myr.
Title: Improvements to stellar structure models, based on a grid of
3D convection simulations - I. T(τ) relations
Authors: Trampedach, Regner; Stein, Robert F.; Christensen-Dalsgaard,
Jørgen; Nordlund, Åke; Asplund, Martin
Bibcode: 2014MNRAS.442..805T
Altcode: 2014arXiv1405.0236T
Relations between temperature, T, and optical depth, τ, are often
used for describing the photospheric transition from optically thick
to optically thin in stellar structure models. We show that this is
well justified, but also that currently used T(τ) relations are often
inconsistent with their implementation. As an outer boundary condition
on the system of stellar structure equations, T(τ) relations have an
undue effect on the overall structure of stars. In this age of precision
asteroseismology, we need to re-assess both the method for computing
and for implementing T(τ) relations, and the assumptions they rest
on. We develop a formulation for proper and consistent evaluation
of T(τ) relations from arbitrary 1D or 3D stellar atmospheres,
and for their implementation in stellar structure and evolution
models. We extract radiative T(τ) relations, as described by our
new formulation, from 3D simulations of convection in deep stellar
atmospheres of late-type stars from dwarfs to giants. These simulations
employ realistic opacities and equation of state, and account for
line blanketing. For comparison, we also extract T(τ) relations from
1DMARCSmodel atmospheres using the same formulation. T(τ) relations
from our grid of 3D convection simulations display a larger range of
behaviours with surface gravity, compared with those of conventional
theoretical 1D hydrostatic atmosphere models based on the mixing-length
theory for convection. The 1D atmospheres show little dependence on
gravity. 1D atmospheres of main-sequence stars also show an abrupt
transition to the diffusion approximation at τ ≃ 2.5, whereas the
3D simulations exhibit smooth transitions that occur at the same depth
for M ≃ 0.8 M⊙, and higher in the atmosphere for both
more and less massive main-sequence stars. Based on these results,
we recommend no longer using scaled solar T(τ) relations. Files with
T(τ) relations for our grid of simulations are made available to the
community, together with routines for interpolating in this irregular
grid. We also provide matching tables of atmospheric opacity, for
consistent implementation in stellar structure models.
Title: VizieR Online Data Catalog: T(tau) relations code (Trampedach+,
2014)
Authors: Trampedach, R.; Stein, R. F.; Christensen-Dalsgaard, J.;
Nordlund, A.; Asplund, M.
Bibcode: 2014yCat..74420805T
Altcode:
Radiative T({tau})-relations, in the form of generalised Hopf functions,
computed from a grid of 37, solar metallicity, realistic, 3D convection
simulations with radiative transfer. (6 data files).
Title: Zooming in on the Formation of Protoplanetary Disks
Authors: Nordlund, Åke; Haugbølle, Troels; Küffmeier, Michael;
Padoan, Paolo; Vasileiades, Aris
Bibcode: 2014IAUS..299..131N
Altcode:
We use the adaptive mesh refinement code RAMSES to model the
formation of protoplanetary disks in realistic star formation
environments. The resolution scales over up to 29 powers of two (~
9 orders of magnitude) covering a range from outer scales of 40 pc
to inner scales of 0.015 AU. The accretion rate from a 1.5 solar mass
envelope peaks near 10-4 M⊙ about 6 kyr after
sink particle formation and then decays approximately exponentially,
reaching 10-6 M⊙ in 100 kyr. The models suggest
universal scalings of physical properties with radius during the main
accretion phase, with kinetic and / or magnetic energy in approximate
balance with gravitational energy. Efficient accretion is made possible
by the braking action of the magnetic field, which nevertheless allows
a near-Keplerian disk to grow to a 100 AU size. The magnetic field
strength ranges from more than 10 G at 0.1 AU to less than 1 mG at
100 AU, and drives a time dependent bipolar outflow, with a collimated
jet and a broader disk wind.
Title: The Star Formation Rate of Molecular Clouds
Authors: Padoan, P.; Federrath, C.; Chabrier, G.; Evans, N. J., II;
Johnstone, D.; Jørgensen, J. K.; McKee, C. F.; Nordlund, Å.
Bibcode: 2014prpl.conf...77P
Altcode: 2013arXiv1312.5365P
We review recent advances in the analytical and numerical modeling
of the star formation rate in molecular clouds and discuss the
available observational constraints. We focus on molecular clouds as
the fundamental star formation sites, rather than on the larger-scale
processes that form the clouds and set their properties. Molecular
clouds are shaped into a complex filamentary structure by supersonic
turbulence, with only a small fraction of the cloud mass channeled
into collapsing protostars over a free-fall time of the system. In
recent years, the physics of supersonic turbulence has been widely
explored with computer simulations, leading to statistical models
of this fragmentation process, and to the prediction of the star
formation rate as a function of fundamental physical parameters of
molecular clouds, such as the virial parameter, the root mean square
(rms) Mach number, the compressive fraction of the turbulence driver,
and the ratio of gas to magnetic pressure. Infrared space telescopes,
as well as groundbased observatories, have provided unprecedented probes
of the filamentary structure of molecular clouds and the location of
forming stars within them.
Title: Radiation from accelerated particles in relativistic jets
with shocks, shear-flow, and reconnection
Authors: Nishikawa, K. -I.; Hardee, P.; Mizuno, Y.; Duţan, I.; Zhang,
B.; Medvedev, M.; Choi, E. J.; Min, K. W.; Niemiec, J.; Nordlund,
Å.; Frederiksen, J.; Sol, H.; Pohl, M.; Hartmann, D. H.; Marscher,
A.; Gómez, J. L.
Bibcode: 2013EPJWC..6102003N
Altcode:
We have investigated particle acceleration and shock structure
associated with an unmagnetized relativistic jet propagating into
an unmagnetized plasma for electron-positron and electron-ion
plasmas. Strong magnetic fields generated in the trailing jet shock
lead to transverse deflection and acceleration of the electrons. We
have self-consistently calculated the radiation from the electrons
accelerated in the turbulent magnetic fields for different jet
Lorentz factors. We find that the synthetic spectra depend on the bulk
Lorentz factor of the jet, the jet temperature, and the strength of
the magnetic fields generated in the shock. We have investigated the
generation of magnetic fields associated with velocity shear between
an unmagnetized relativistic (core) jet and an unmagnetized sheath
plasma. We discuss particle acceleration in collimation shocks for AGN
jets formed by relativistic MHD simulations. Our calculated spectra
should lead to a better understanding of the complex time evolution
and/or spectral structure from gamma-ray bursts, relativistic jets,
and supernova remnants.
Title: Nonlinear evolution of the magnetized Kelvin-Helmholtz
instability: From fluid to kinetic modeling
Authors: Henri, P.; Cerri, S. S.; Califano, F.; Pegoraro, F.; Rossi,
C.; Faganello, M.; Šebek, O.; Trávníček, P. M.; Hellinger,
P.; Frederiksen, J. T.; Nordlund, A.; Markidis, S.; Keppens, R.;
Lapenta, G.
Bibcode: 2013PhPl...20j2118H
Altcode: 2013arXiv1310.7707H
The nonlinear evolution of collisionless plasmas is typically a
multi-scale process, where the energy is injected at large, fluid
scales and dissipated at small, kinetic scales. Accurately modelling
the global evolution requires to take into account the main micro-scale
physical processes of interest. This is why comparison of different
plasma models is today an imperative task aiming at understanding
cross-scale processes in plasmas. We report here the first comparative
study of the evolution of a magnetized shear flow, through a variety of
different plasma models by using magnetohydrodynamic (MHD), Hall-MHD,
two-fluid, hybrid kinetic, and full kinetic codes. Kinetic relaxation
effects are discussed to emphasize the need for kinetic equilibriums
to study the dynamics of collisionless plasmas in non trivial
configurations. Discrepancies between models are studied both in the
linear and in the nonlinear regime of the magnetized Kelvin-Helmholtz
instability, to highlight the effects of small scale processes on
the nonlinear evolution of collisionless plasmas. We illustrate how
the evolution of a magnetized shear flow depends on the relative
orientation of the fluid vorticity with respect to the magnetic field
direction during the linear evolution when kinetic effects are taken
into account. Even if we found that small scale processes differ
between the different models, we show that the feedback from small,
kinetic scales to large, fluid scales is negligible in the nonlinear
regime. This study shows that the kinetic modeling validates the use
of a fluid approach at large scales, which encourages the development
and use of fluid codes to study the nonlinear evolution of magnetized
fluid flows, even in the collisionless regime.
Title: Magnetic field generation in a jet-sheath plasma via the
kinetic Kelvin-Helmholtz instability
Authors: Nishikawa, K. -I.; Hardee, P.; Zhang, B.; Duţan, I.;
Medvedev, M.; Choi, E. J.; Min, K. W.; Niemiec, J.; Mizuno, Y.;
Nordlund, A.; Frederiksen, J. T.; Sol, H.; Pohl, M.; Hartmann, D. H.
Bibcode: 2013AnGeo..31.1535N
Altcode: 2013arXiv1307.2928N
We have investigated the generation of magnetic fields associated
with velocity shear between an unmagnetized relativistic jet and an
unmagnetized sheath plasma. We have examined the strong magnetic fields
generated by kinetic shear (Kelvin-Helmholtz) instabilities. Compared
to the previous studies using counter-streaming performed by Alves et
al. (2012), the structure of the kinetic Kelvin-Helmholtz instability
(KKHI) of our jet-sheath configuration is slightly different,
even for the global evolution of the strong transverse magnetic
field. In our simulations the major components of growing modes
are the electric field Ez, perpendicular to the flow
boundary, and the magnetic field By, transverse to the
flow direction. After the By component is excited, an
induced electric field Ex, parallel to the flow direction,
becomes significant. However, other field components remain small. We
find that the structure and growth rate of KKHI with mass ratios
mi/me = 1836 and mi/me =
20 are similar. In our simulations saturation in the nonlinear stage
is not as clear as in counter-streaming cases. The growth rate for
a mildly-relativistic jet case (γj = 1.5) is larger than
for a relativistic jet case (γj = 15).
Title: The Stagger-grid: A grid of 3D stellar atmosphere
models. I. Methods and general properties
Authors: Magic, Z.; Collet, R.; Asplund, M.; Trampedach, R.; Hayek,
W.; Chiavassa, A.; Stein, R. F.; Nordlund, Å.
Bibcode: 2013A&A...557A..26M
Altcode: 2013arXiv1302.2621M
Aims: We present the Stagger-grid, a comprehensive grid of
time-dependent, three-dimensional (3D), hydrodynamic model atmospheres
for late-type stars with realistic treatment of radiative transfer,
covering a wide range in stellar parameters. This grid of 3D models is
intended for various applications besides studies of stellar convection
and atmospheres per se, including stellar parameter determination,
stellar spectroscopy and abundance analysis, asteroseismology,
calibration of stellar evolution models, interferometry, and extrasolar
planet search. In this introductory paper, we describe the methods
we applied for the computation of the grid and discuss the general
properties of the 3D models as well as of their temporal and spatial
averages (here denoted ⟨3D⟩ models).
Methods: All our models
were generated with the Stagger-code, using realistic input physics for
the equation of state (EOS) and for continuous and line opacities. Our ~
220 grid models range in effective temperature, Teff, from
4000 to 7000 K in steps of 500 K, in surface gravity, log g, from 1.5
to 5.0 in steps of 0.5 dex, and metallicity, [Fe/H], from - 4.0 to +
0.5 in steps of 0.5 and 1.0 dex.
Results: We find a tight scaling
relation between the vertical velocity and the surface entropy jump,
which itself correlates with the constant entropy value of the adiabatic
convection zone. The range in intensity contrast is enhanced at lower
metallicity. The granule size correlates closely with the pressure
scale height sampled at the depth of maximum velocity. We compare the
⟨3D⟩ models with currently widely applied one-dimensional (1D)
atmosphere models, as well as with theoretical 1D hydrostatic models
generated with the same EOS and opacity tables as the 3D models, in
order to isolate the effects of using self-consistent and hydrodynamic
modeling of convection, rather than the classical mixing length theory
approach. For the first time, we are able to quantify systematically
over a broad range of stellar parameters the uncertainties of 1D
models arising from the simplified treatment of physics, in particular
convective energy transport. In agreement with previous findings,
we find that the differences can be rather significant, especially
for metal-poor stars. Appendices A-C are available in electronic
form at http://www.aanda.orgFull
Table C.1 is available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/557/A26
Title: Zooming in on Protoplanetary Disks
Authors: Küffmeier, Michael; Nordlund, Åke; Haugbølle, Troels;
Padoan, Paolo
Bibcode: 2013prpl.conf1H003K
Altcode:
We show visualizations of simulations obtained by the adaptive
mesh-refinement (AMR) code RAMSES. Our visualization illustrates
a zoom from models of ∼ 1e5 solar mass Giant Molecular Clouds to
protoplanetary disk scales of about 0.015 AU, by selectively following
the collapse of a few 1-2 solar mass protostellar systems. The models
realized by the adaptive mesh-refinement (AMR) code RAMSES and their
visualizations demonstrate the extremely important roles played by
magnetic fields in this process. We show that magnetic fields carry
away a lot of the excess angular momentum during the collapse phase
and provide the ingredients necessary to create bipolar outflows,
namely collimated jets and wider angle disk winds. Furthermore,
the presence of magnetic fields in wound up, nearly force-free form
in the protoplanetary disks can strongly influence and suppress the
Kelvin-Helmholtz instability driven turbulence. This turbulence is
otherwise associated with the speed difference between a dust layer
concentrated near the disk midplane and the vertically more extended
gas, rotating slower than the dust layer, because of the partial support
by radial gas pressure gradients. Subsequently, this may allow the
original Goldreich-Ward mechanism to work, thus enabling a fast path
to planetesimal formation.
Title: Radiation from accelerated particles in relativistic jets
with shocks, shear-flow, and reconnection
Authors: Nishikawa, K. -I.; Zhang, B.; Dutan, I.; Medvedev, M.; Hardee,
P.; Choi, E. J.; Min, K. W.; Niemiec, J.; Mizuno, Y.; Nordlund, A.;
Frederiksen, J. T.; Sol, H.; Pohl, M.; Hartmann, D. H.
Bibcode: 2013EAS....61..177N
Altcode: 2013arXiv1303.2569N
We investigated particle acceleration and shock structure associated
with an unmagnetized relativistic jet propagating into an unmagnetized
plasma. Strong magnetic fields generated in the trailing shock
contribute to the electrons transverse deflection and acceleration. We
have calculated, self-consistently, the radiation from electrons
accelerated in these turbulent magnetic fields. We found that the
synthetic spectra depend on the bulk Lorentz factor of the jet, its
temperature and strength of the generated magnetic fields. We have also
investigated accelerated electrons in strong magnetic fields generated
by kinetic shear (Kelvin-Helmholtz) instabilities. The calculated
properties of the emerging radiation will guide our understanding of the
complex time evolution and/or spectral structure in gamma-ray bursts,
relativistic jets in general, and supernova remnants.
Title: Zooming in on the Formation of Protoplanetary Disks
Authors: Nordlund, Åke; Küffmeier, Michael; Haugbølle, Troels;
Padoan, Paolo
Bibcode: 2013prpl.conf1H002N
Altcode: 2013arXiv1309.2278N
We use the adaptive mesh refinement computer code RAMSES to model
the formation of protoplanetary disks in realistic star formation
environments, with resolution scaling over 30 powers of two (about 9
powers of ten), covering a range from outer scales of about 50 pc to
inner scales of about 0.01 AU. The simulations are done in three steps,
with the first step covering 16 powers of two, following individual star
formation in a 50 pc GMC model. In the 2nd step, the neighborhoods of
stars with a final system mass of about two solar masses are followed
during the accretion process, with a smallest mesh size of 2.5 AU,
sufficient to follow the development of the large scale structure
of their accretion disks. Finally, a selection of these disks are
studied over shorter time intervals, with cell sizes ranging down to
0.01 AU, sufficient to resolve the vertical structure of a significant
radius fraction of the disks. The purpose of this procedure is
to characterize the typical properties of accretion disks around
solar mass protostars, with as few free parameters as possible,
and to gather a statistical sample of such conditions, to quantify
the extent of statistical variation of properties. This is a vast
improvement over models where initial and boundary conditions have to
be chosen arbitrarily. Here, the initial and boundary conditions follow
instead from the statistical properties of the interstellar medium,
which are reasonably well established, as per for example the "Larson
relations" and the "B-n" relation, which provide typical values for the
velocity and magnetic field RMS values on different scales. As a
byproduct of this type of modeling, which starts out from a supernova
driven interstellar medium, we can follow the transport of short-lived
radioactive nuclides (SLRs), from the time of ejection from supernovae
and until they become part of the proto-planetary disks. As shown in
a recent paper (arXiv: astro-ph/1302.0843) the transport time is on
average short enough to be consistent with initial abundance of 26Al in
the Solar System derived from cosmochemistry. Of particular interest is
to characterize the amount of variation with time of the SLR abundance
during the lifetime of PP-disks surrounding solar mass stars.
Title: Accretion and Formation of Nascent Solar Systems
Authors: Haugbølle, Troels; Padoan, Paolo; Küffmeier, Michael;
Nordlund, Åke
Bibcode: 2013prpl.conf1H004H
Altcode:
Traditionally, models of the formation and accretion of protostellar
systems have considered isolated stars embedded in a small symmetric
envelope. However, by using a locally optimized version of the adaptive
mesh refinement code RAMSES, we can simultaneously resolve the molecular
cloud dynamics and the evolution of the protostellar disk with a
dynamic range of more than 4 million. We will present novel results
that include the formation and evolution of protostellar systems with
realistic in-fall rates and magnetic field structures, consistently
determined from the collapse of the molecular cloud core from parsec to
sub-AU scales. We demonstrate that the resulting accretion rate to the
disk is an unsteady, non-symmetric process, and that the filamentary
structure of the molecular cloud prevails to scales of thousands
of astronomical units. Finally, because our simulations represent a
statistically significant fraction of the interstellar medium with
thousands of stars, our results can be compared to observations on a
statistically sound basis, and we can account for the cosmic variance
in star formation in a natural way, including isolated low mass star
formation, binaries, and rich star clusters.
Title: Kinetic Modeling of Particle Acceleration in a Solar Null-point
Reconnection Region
Authors: Baumann, G.; Haugbølle, T.; Nordlund, Å.
Bibcode: 2013ApJ...771...93B
Altcode: 2012arXiv1204.4947B
The primary focus of this paper is on the particle acceleration
mechanism in solar coronal three-dimensional reconnection null-point
regions. Starting from a potential field extrapolation of a Solar and
Heliospheric Observatory (SOHO) magnetogram taken on 2002 November
16, we first performed magnetohydrodynamics (MHD) simulations with
horizontal motions observed by SOHO applied to the photospheric boundary
of the computational box. After a build-up of electric current in the
fan plane of the null point, a sub-section of the evolved MHD data was
used as initial and boundary conditions for a kinetic particle-in-cell
model of the plasma. We find that sub-relativistic electron acceleration
is mainly driven by a systematic electric field in the current sheet. A
non-thermal population of electrons with a power-law distribution in
energy forms in the simulated pre-flare phase, featuring a power-law
index of about -1.78. This work provides a first step toward bridging
the gap between macroscopic scales on the order of hundreds of Mm
and kinetic scales on the order of centimeter in the solar corona,
and explains how to achieve such a cross-scale coupling by utilizing
either physical modifications or (equivalent) modifications of the
constants of nature. With their exceptionally high resolution—up to
135 billion particles and 3.5 billion grid cells of size 17.5 km—these
simulations offer a new opportunity to study particle acceleration in
solar-like settings.
Title: VizieR Online Data Catalog: STAGGER-grid of 3D stellar
models. I. (Magic+, 2013)
Authors: Magic, Z.; Collet, R.; Asplund, M.; Trampedach, R.; Hayek,
W.; Chiavassa, A.; Stein, R. F.; Nordlund, A.
Bibcode: 2013yCat..35570026M
Altcode: 2013yCat..35579026M
The 3D model atmospheres presented here were constructed with
a custom version of the Stagger-code, a state-of-the-art,
multipurpose, radiative-magnetohydrodynamics (R-MHD)
code originally developed by Nordlund & Galsgaard (1995,
http://www.astro.ku.dk/~kg/Papers/MHD_code.ps.gz), and continuously
improved over the years by its user community. (1 data file).
Title: photon-plasma: A modern high-order particle-in-cell code
Authors: Haugbølle, Troels; Frederiksen, Jacob Trier; Nordlund,
A. ˚ke
Bibcode: 2013PhPl...20f2904H
Altcode: 2012arXiv1211.4575H
We present the photon-plasma code, a modern high order charge conserving
particle-in-cell code for simulating relativistic plasmas. The code is
using a high order implicit field solver and a novel high order charge
conserving interpolation scheme for particle-to-cell interpolation
and charge deposition. It includes powerful diagnostics tools with
on-the-fly particle tracking, synthetic spectra integration, 2D volume
slicing, and a new method to correctly account for radiative cooling
in the simulations. A robust technique for imposing (time-dependent)
particle and field fluxes on the boundaries is also presented. Using
a hybrid OpenMP and MPI approach, the code scales efficiently from 8
to more than 250.000 cores with almost linear weak scaling on a range
of architectures. The code is tested with the classical benchmarks
particle heating, cold beam instability, and two-stream instability. We
also present particle-in-cell simulations of the Kelvin-Helmholtz
instability, and new results on radiative collisionless shocks.
Title: 3D Solar Null Point Reconnection MHD Simulations
Authors: Baumann, G.; Galsgaard, K.; Nordlund, Å.
Bibcode: 2013SoPh..284..467B
Altcode: 2012SoPh..tmp..291B; 2012arXiv1203.1018B; 2012SoPh..tmp..270B
Numerical MHD simulations of 3D reconnection events in the solar
corona have improved enormously over the last few years, not only
in resolution, but also in their complexity, enabling more and more
realistic modeling. Various ways to obtain the initial magnetic
field, different forms of solar atmospheric models as well as diverse
driving speeds and patterns have been employed. This study considers
differences between simulations with stratified and non-stratified solar
atmospheres, addresses the influence of the driving speed on the plasma
flow and energetics, and provides quantitative formulas for mapping
electric fields and dissipation levels obtained in numerical simulations
to the corresponding solar quantities. The simulations start out from
a potential magnetic field containing a null-point, obtained from a
Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager
(MDI) magnetogram magnetogram extrapolation approximately 8 hours
before a C-class flare was observed. The magnetic field is stressed
with a boundary motion pattern similar to - although simpler than -
horizontal motions observed by SOHO during the period preceding the
flare. The general behavior is nearly independent of the driving speed,
and is also very similar in stratified and non-stratified models,
provided only that the boundary motions are slow enough. The boundary
motions cause a build-up of current sheets, mainly in the fan-plane
of the magnetic null-point, but do not result in a flare-like energy
release. The additional free energy required for the flare could have
been partly present in non-potential form at the initial state, with
subsequent additions from magnetic flux emergence or from components
of the boundary motion that were not represented by the idealized
driving pattern.
Title: A Grid of Three-dimensional Stellar Atmosphere Models of Solar
Metallicity. I. General Properties, Granulation, and Atmospheric
Expansion
Authors: Trampedach, Regner; Asplund, Martin; Collet, Remo; Nordlund,
Åke; Stein, Robert F.
Bibcode: 2013ApJ...769...18T
Altcode: 2013arXiv1303.1780T
Present grids of stellar atmosphere models are the workhorses in
interpreting stellar observations and determining their fundamental
parameters. These models rely on greatly simplified models of
convection, however, lending less predictive power to such models of
late-type stars. We present a grid of improved and more reliable stellar
atmosphere models of late-type stars, based on deep, three-dimensional
(3D), convective, stellar atmosphere simulations. This grid is to be
used in general for interpreting observations and improving stellar
and asteroseismic modeling. We solve the Navier Stokes equations in
3D and concurrent with the radiative transfer equation, for a range
of atmospheric parameters, covering most of stellar evolution with
convection at the surface. We emphasize the use of the best available
atomic physics for quantitative predictions and comparisons with
observations. We present granulation size, convective expansion of the
acoustic cavity, and asymptotic adiabat as functions of atmospheric
parameters.
Title: Abundance of 26 Al and 60 Fe in
Evolving Giant Molecular Clouds
Authors: Vasileiadis, Aristodimos; Nordlund, Åke; Bizzarro, Martin
Bibcode: 2013ApJ...769L...8V
Altcode: 2013arXiv1302.0843V
The nucleosynthesis and ejection of radioactive 26Al (t
1/2 ~ 0.72 Myr) and 60Fe, (t 1/2
~ 2.5 Myr) into the interstellar medium is dominated by the stellar
winds of massive stars and supernova type II explosions. Studies of
meteorites and their components indicate that the initial abundances
of these short-lived radionuclides in the solar protoplanetary disk
were higher than the background levels of the galaxy inferred from
γ-ray astronomy and models of the galactic chemical evolution. This
observation has been used to argue for a late-stage addition of stellar
debris to the solar system's parental molecular cloud or, alternatively,
the solar protoplanetary disk, thereby requiring a special scenario
for the formation of our solar system. Here, we use supercomputers
to model—from first principles—the production, transport, and
admixing of freshly synthesized 26Al and 60Fe in
star-forming regions within giant molecular clouds. Under typical star
formation conditions, the levels of 26Al in most star-forming
regions are comparable to that deduced from meteorites, suggesting that
the presence of short-lived radionuclides in the early solar system is a
generic feature of the chemical evolution of giant molecular clouds. The
60Fe/26Al yield ratio of ≈0.2 calculated from
our simulations is consistent with the galactic value of 0.15 ± 0.06
inferred from γ-ray astronomy but is significantly higher than most
current solar system measurements indicate. We suggest that estimates
based on differentiated meteorites and some chondritic components may
not be representative of the initial 60Fe abundance of the
bulk solar system.
Title: SWIFF: Space weather integrated forecasting framework
Authors: Lapenta, Giovanni; Pierrard, Viviane; Keppens, Rony; Markidis,
Stefano; Poedts, Stefaan; Šebek, Ondřej; Trávníček, Pavel M.;
Henri, Pierre; Califano, Francesco; Pegoraro, Francesco; Faganello,
Matteo; Olshevsky, Vyacheslav; Restante, Anna Lisa; Nordlund, Åke;
Trier Frederiksen, Jacob; Mackay, Duncan H.; Parnell, Clare E.;
Bemporad, Alessandro; Susino, Roberto; Borremans, Kris
Bibcode: 2013JSWSC...3A..05L
Altcode:
SWIFF is a project funded by the Seventh Framework Programme of the
European Commission to study the mathematical-physics models that
form the basis for space weather forecasting. The phenomena of space
weather span a tremendous scale of densities and temperature with
scales ranging 10 orders of magnitude in space and time. Additionally
even in local regions there are concurrent processes developing at
the electron, ion and global scales strongly interacting with each
other. The fundamental challenge in modelling space weather is the
need to address multiple physics and multiple scales. Here we present
our approach to take existing expertise in fluid and kinetic models to
produce an integrated mathematical approach and software infrastructure
that allows fluid and kinetic processes to be modelled together. SWIFF
aims also at using this new infrastructure to model specific coupled
processes at the Solar Corona, in the interplanetary space and in the
interaction at the Earth magnetosphere.
Title: Ab Initio Active Region Formation
Authors: Stein, Robert F.; Nordlund, A.
Bibcode: 2013AAS...22141502S
Altcode:
The tachocline is not necessary to produce active regions with their
global properties. Dynamo action within the convection zone can produce
large scale reversing polarity magnetic fields as shown by ASH code
and Charboneau et al simulations. Magneto-convection acting on this
large scale field produces Omega-loops which emerge through the surface
to produce active regions. The field first emerges as small bipoles
with horizontal field over granules anchored in vertical fields in the
intergranular lanes. The fields are quickly swept into the intergranular
lanes and produce a mixed polarity "pepper and salt" pattern. The
opposite polarities then migrate toward separate unipolar regions
due to the underlying large scale loop structure. When sufficient
flux concentrates, pores and sunspots form. We will show movies of
magneto-convection simulations of the emerging flux, its migration,
and concentration to form pores and spots, as well as the underlying
magnetic field evolution. In addition, the same atmospheric data has
been used as input to the LILIA Stokes Inversion code to calculate
Stokes spectra for the Fe I 630 nm lines and then invert them to
determine the magnetic field. Comparisons of the inverted field with the
simulation field shows that small-scale, weak fields, less than 100 G,
can not be accurately determined because of vertical gradients that are
difficult to match in fitting the line profiles. Horizontal smoothing
by telescope diffraction further degrades the inversion accuracy.
Title: Current Fragmentation and Particle Acceleration in Solar Flares
Authors: Cargill, P. J.; Vlahos, L.; Baumann, G.; Drake, J. F.;
Nordlund, Å.
Bibcode: 2013pacp.book..223C
Altcode:
No abstract at ADS
Title: Flux Emergence and Pore Formation: What ATST can See
Authors: Stein, R. F.; Nordlund, Å.
Bibcode: 2012ASPC..463...83S
Altcode:
Pores form spontaneously in flux emergence simulations where minimally
structured (uniform, untwisted, horizontal) magnetic field rises from
a depth of 20 Mm. With 1 kG incident field pores formed after about
a turnover time (2 days). To compare what ATST will see with current
telescopes a very high resolution (6 km) magneto-convection simulation
was carried out with an initially uniform, vertical field. Stokes
V-profiles were compared for the simulation and as modified for the
diffraction pattern for the ATST and the SST.
Title: Solar Fe abundance and magnetic fields. Towards a consistent
reference metallicity
Authors: Fabbian, D.; Moreno-Insertis, F.; Khomenko, E.; Nordlund, Å.
Bibcode: 2012A&A...548A..35F
Altcode: 2012arXiv1209.2771F
Aims: We investigate the impact on Fe abundance determination of
including magnetic flux in series of 3D radiation-magnetohydrodynamics
(MHD) simulations of solar convection, which we used to synthesize
spectral intensity profiles corresponding to disc centre.
Methods: A differential approach is used to quantify the changes
in theoretical equivalent width of a set of 28 iron spectral lines
spanning a wide range in wavelength, excitation potential, oscillator
strength, Landé factor, and formation height. The lines were computed
in local thermodynamic equilibrium (LTE) using the spectral synthesis
code LILIA. We used input magnetoconvection snapshots covering 50 min
of solar evolution and belonging to series having an average vertical
magnetic flux density of ⟨ Bvert ⟩ = 0,50,100, and
200 G. For the relevant calculations we used the Copenhagen Stagger
code.
Results: The presence of magnetic fields causes both a
direct (Zeeman-broadening) effect on spectral lines with non-zero
Landé factor and an indirect effect on temperature-sensitive
lines via a change in the photospheric T - τ stratification. The
corresponding correction in the estimated atomic abundance ranges
from a few hundredths of a dex up to |Δlog ɛ(Fe)⊙|
~ 0.15 dex, depending on the spectral line and on the amount of
average magnetic flux within the range of values we considered. The
Zeeman-broadening effect gains relatively more importance in the
IR. The largest modification to previous solar abundance determinations
based on visible spectral lines is instead due to the indirect effect,
i.e., the line-weakening caused by a warmer stratification as seen on
an optical depth scale. Our results indicate that the average solar
iron abundance obtained when using magnetoconvection models can be ~
0.03-0.11 dex higher than when using the simpler hydrodynamics (HD)
convection approach.
Conclusions: We demonstrate that accounting
for magnetic flux is important in state-of-the-art solar photospheric
abundance determinations based on 3D convection simulations.
Title: Realistic numerical simulations of solar convection: emerging
flux, pores, and Stokes spectra
Authors: Georgobiani, D.; Stein, R.; Nordlund, A.
Bibcode: 2012IAUSS...6E.102G
Altcode:
We report on magneto-convection simulations of magnetic flux
emerging through the upper layers of the solar convection zone into
the photosphere. Simulations by Georgobiani, Stein and Nordlund start
from minimally structured, uniform, untwisted horizontal field advected
into the computational domain by supergranule scale inflows at 20 Mm
depth. At the opposite extreme, simulations by Cheung (2007, 2008,
2011) start with a coherent flux tube inserted into or forced into
the bottom of the computational domain. Several robust results have
emerged from the comparison of results from these two very different
initial states. First, rising magnetic flux gets deformed into
undulating, serpentine shapes by the influence of the convective up-
and down-flows. The flux develops fine structure and appears at the
surface first as a "pepper and salt" pattern of mixed polarity. Where
magnetic flux approaches the surface, granules become darker and
elongated in the direction of the field. Subsequently, the underlying
large scale magnetic structures make the field collect into unipolar
regions. Magneto-convection produces a complex, small-scale magnetic
field topology, whatever the initial state. A heirarchy of magnetic
loops corresponding to the different scales of convective motions are
produced. Vertical vortex tubes form at intergranule lane vertices which
can lead to tornado-like magnetic fields in the photosphere. Gradients
in field strength and velocity produce asymmetric Stokes spectra. Where
emerging Omega loops leave behind nearly vertical legs, long lived
pores can spontaneously form. The field in the pores first becomes
concentrated and evacuated near the surface and the evacuated flux
concentration then extends downward.
Title: Current Fragmentation and Particle Acceleration in Solar Flares
Authors: Cargill, P. J.; Vlahos, L.; Baumann, G.; Drake, J. F.;
Nordlund, Å.
Bibcode: 2012SSRv..173..223C
Altcode: 2012SSRv..tmp...36C
Particle acceleration in solar flares remains an outstanding problem in
plasma physics and space science. While the observed particle energies
and timescales can perhaps be understood in terms of acceleration
at a simple current sheet or turbulence site, the vast number of
accelerated particles, and the fraction of flare energy in them, defies
any simple explanation. The nature of energy storage and dissipation
in the global coronal magnetic field is essential for understanding
flare acceleration. Scenarios where the coronal field is stressed by
complex photospheric motions lead to the formation of multiple current
sheets, rather than the single monolithic current sheet proposed by
some. The currents sheets in turn can fragment into multiple, smaller
dissipation sites. MHD, kinetic and cellular automata models are used to
demonstrate this feature. Particle acceleration in this environment thus
involves interaction with many distributed accelerators. A series of
examples demonstrate how acceleration works in such an environment. As
required, acceleration is fast, and relativistic energies are readily
attained. It is also shown that accelerated particles do indeed
interact with multiple acceleration sites. Test particle models also
demonstrate that a large number of particles can be accelerated, with a
significant fraction of the flare energy associated with them. However,
in the absence of feedback, and with limited numerical resolution,
these results need to be viewed with caution. Particle in cell models
can incorporate feedback and in one scenario suggest that acceleration
can be limited by the energetic particles reaching the condition for
firehose marginal stability. Contemporary issues such as footpoint
particle acceleration are also discussed. It is also noted that the
idea of a "standard flare model" is ill-conceived when the entire
distribution of flare energies is considered.
Title: A Simple Law of Star Formation
Authors: Padoan, Paolo; Haugbølle, Troels; Nordlund, Åke
Bibcode: 2012ApJ...759L..27P
Altcode: 2012arXiv1208.3758P
We show that supersonic MHD turbulence yields a star formation rate
(SFR) as low as observed in molecular clouds, for characteristic
values of the free-fall time divided by the dynamical time, t
ff/t dyn, the Alfvénic Mach number, {\cal M}_a,
and the sonic Mach number, {\cal M}_s. Using a very large set of deep
adaptive-mesh-refinement simulations, we quantify the dependence
of the SFR per free-fall time, epsilonff, on the above
parameters. Our main results are (1) that epsilonff decreases
exponentially with increasing t ff/t dyn, but
is insensitive to changes in {\cal M}_s, for constant values of t
ff/t dyn and {\cal M}_a. (2) Decreasing values
of {\cal M}_a (stronger magnetic fields) reduce epsilonff,
but only to a point, beyond which epsilonff increases
with a further decrease of {\cal M}_a. (3) For values of {\cal
M}_a characteristic of star-forming regions, epsilonff
varies with {\cal M}_a by less than a factor of two. We propose
a simple star formation law, based on the empirical fit to the
minimum epsilonff, and depending only on t ff/t
dyn: epsilonff ≈ epsilonwindexp
(- 1.6 t ff/t dyn). Because it only depends on
the mean gas density and rms velocity, this law is straightforward
to implement in simulations and analytical models of galaxy formation
and evolution.
Title: The Absolute Chronology and Thermal Processing of Solids in
the Solar Protoplanetary Disk
Authors: Connelly, James N.; Bizzarro, Martin; Krot, Alexander N.;
Nordlund, Åke; Wielandt, Daniel; Ivanova, Marina A.
Bibcode: 2012Sci...338..651C
Altcode:
Transient heating events that formed calcium-aluminum-rich inclusions
(CAIs) and chondrules are fundamental processes in the evolution of the
solar protoplanetary disk, but their chronology is not understood. Using
U-corrected Pb-Pb dating, we determined absolute ages of individual
CAIs and chondrules from primitive meteorites. CAIs define a brief
formation interval corresponding to an age of 4567.30 ± 0.16 million
years (My), whereas chondrule ages range from 4567.32 ± 0.42 to 4564.71
± 0.30 My. These data refute the long-held view of an age gap between
CAIs and chondrules and, instead, indicate that chondrule formation
started contemporaneously with CAIs and lasted ~3 My. This time scale
is similar to disk lifetimes inferred from astronomical observations,
suggesting that the formation of CAIs and chondrules reflects a process
intrinsically linked to the secular evolution of accretionary disks.
Title: Particle-in-cell Simulation of Electron Acceleration in Solar
Coronal Jets
Authors: Baumann, G.; Nordlund, Å.
Bibcode: 2012ApJ...759L...9B
Altcode: 2012arXiv1205.3486B
We investigate electron acceleration resulting from three-dimensional
magnetic reconnection between an emerging, twisted magnetic flux rope
and a pre-existing weak, open magnetic field. We first follow the rise
of an unstable, twisted flux tube with a resistive MHD simulation where
the numerical resolution is enhanced by using fixed mesh refinement. As
in previous MHD investigations of similar situations, the rise of
the flux tube into the pre-existing inclined coronal magnetic field
results in the formation of a solar coronal jet. A snapshot of the
MHD model is then used as an initial and boundary condition for a
particle-in-cell simulation, using up to half a billion cells and over
20 billion charged particles. Particle acceleration occurs mainly in
the reconnection current sheet, with accelerated electrons displaying
a power law in the energy probability distribution with an index of
around -1.5. The main acceleration mechanism is a systematic electric
field, striving to maintaining the electric current in the current
sheet against losses caused by electrons not being able to stay in
the current sheet for more than a few seconds at a time.
Title: Radiation from accelerated particles in shocks
Authors: Nishikawa, K. -I.; Zhang, B.; Choi, E. J.; Min, K. W.;
Niemiec, J.; Medvedev, M.; Hardee, P.; Mizuno, Y.; Nordlund, A.;
Frederiksen, J.; Sol, H.; Pohl, M.; Hartmann, D. H.; Fishman, G. J.
Bibcode: 2012IAUS..279..371N
Altcode:
Recent PIC simulations of relativistic electron-positron (electron-ion)
jets injected into a stationary medium show that particle
acceleration occurs in the shocked regions. Simulations show that
the Weibel instability is responsible for generating and amplifying
highly nonuniform, small-scale magnetic fields and for particle
acceleration. These magnetic fields contribute to the electron's
transverse deflection behind the shock. The ``jitter'' radiation
from deflected electrons in turbulent magnetic fields has properties
different from synchrotron radiation calculated in a uniform magnetic
field. This jitter radiation may be important for understanding the
complex time evolution and/or spectral structure of gamma-ray bursts,
relativistic jets in general, and supernova remnants. In order to
calculate radiation from first principles and go beyond the standard
synchrotron model, we have used PIC simulations. We present synthetic
spectra to compare with the spectra obtained from Fermi observations.
Title: Helioseismic Data from Emerging Flux Simulations
Authors: Stein, R. F.; Lagerfjärd, A.; Nordlund, Å.; Georgobiani, D.
Bibcode: 2012ASPC..462..345S
Altcode:
Data from solar magneto-convection emerging flux simulations is
available for validating helioseismic inversion procedures. In these
simulations a uniform, untwisted, horizontal magnetic field is advected
by inflows at the bottom of the domain 48 Mm wide by 20 Mm deep and
rises to the surface. The evolution for different field strengths at
20 Mm depth has been investigated. The field emerges first in a mixed
polarity pepper and salt pattern, but then collects into separate,
unipolar concentrations and when enough flux has reached the surface,
pores are produced. In one case the field strength was artificially
increased and then the pores grew into spot-like structures with
penumbral-like borders. The online data consists of slices of vertical
and horizontal velocity and magnetic field strength at continuum
optical depths of 0.01, 0.1 and 1 as well as the emergent intensity
at one minute intervals plus four hour averages (with 2 hour cadence)
of the three-dimensional (3D) density, velocity, temperature, energy,
sound speed and magnetic field. The data can be found as links from the
web page: http://steinr.pa.msu.edu/∼bob/data.html. These calculation
were performed on the supercomputers of the NASA Advanced Supercomputing
Division and were supported by grants from NASA and NSF.
Title: Emerging Flux Simulations
Authors: Stein, R. F.; Lagerfjärd, A.; Nordlund, Å.; Georgobiani, D.
Bibcode: 2012ASPC..454..193S
Altcode:
We simulate the rise through the upper convection zone and emergence
through the solar surface of initially uniform, untwisted, horizontal
magnetic flux that is advected into a domain 48 Mm wide by 20 Mm deep,
with the same entropy as the non-magnetic plasma. The magnetic field
is transported upward by the diverging upflows and pulled down in
the downdrafts, which produces a hierarchy of loop-like structures of
increasingly smaller scale as the surface is approached. 20 kG fields at
the bottom significantly modify the convective flows, leading to long
thin cells of ascending fluid aligned with the magnetic field. Their
magnetic buoyancy makes them rise to the surface faster than the
fluid rise time. A large scale magnetic loop is produced that, as
it emerges through the surface, leads to the formation of a bipolar
pore-like structure.
Title: Erratum: "Ambipolar Drift Heating in Turbulent Molecular
Clouds" (2000, ApJ, 540, 332)
Authors: Padoan, Paolo; Zweibel, Ellen; Nordlund, Åke
Bibcode: 2012ApJ...755..182P
Altcode:
No abstract at ADS
Title: Simulation of Relativistic Jets and Associated Self-consistent
Radiation
Authors: Nishikawa, K. -I.; Choi, E. -J.; Min, K.; Hardee, P.; Mizuno,
Y.; Zhang, B.; Niemiec, J.; Medvedev, M.; Nordlund, A.; Frederiksen,
J.; Sol, H.; Pohl, M.; Hartmann, D. H.; Fishman, J. F.
Bibcode: 2012ASPC..459..143N
Altcode:
Plasma instabilities excited in collisionless shocks are responsible
for particle acceleration. We have investigated the particle
acceleration and shock structure associated with an unmagnetized
relativistic electron-positron jet propagating into an unmagnetized
electron-positron plasma. Cold jet electrons are thermalized and
slowed while the ambient electrons are swept up to create a partially
developed hydrodynamic-like shock structure. In the leading shock,
electron density increases by a factor of about 3.5 in the simulation
frame. Strong electromagnetic fields are generated in the trailing shock
and provide an emission site. These magnetic fields contribute to the
electron's transverse deflection behind the shock. Our initial results
of a jet-ambient interaction with anti-parallel magnetic fields show
pile-up of magnetic fields at the colliding shock, which may lead to
reconnection and associated particle acceleration. We will investigate
the radiation in a transient stage as a possible generation mechanism
of precursors of prompt emission. In our simulations we calculate the
radiation from electrons in the shock region. The detailed properties
of this radiation are important for understanding the complex time
evolution and spectral structure in gamma-ray bursts, relativistic jets,
and supernova remnants.
Title: On the Formation of Active Regions
Authors: Stein, Robert F.; Nordlund, Åke
Bibcode: 2012ApJ...753L..13S
Altcode: 2012arXiv1207.4248S
Magnetoconvection can produce an active region without an initial
coherent flux tube. A simulation was performed where a uniform,
untwisted, horizontal magnetic field of 1 kG strength was advected into
the bottom of a computational domain 48 Mm wide by 20 Mm deep. The
up and down convective motions produce a hierarchy of magnetic loops
with a wide range of scales, with smaller loops riding "piggy-back"
in a serpentine fashion on larger loops. When a large loop approaches
the surface, it produces a small active region with a compact leading
spot and more diffuse following spots.
Title: Emerging Flux Simulations and Proto-Active Regions
Authors: Stein, R. F.; Lagerfjärd, A.; Nordlund, &.; Georgobiani, D.
Bibcode: 2012ASPC..455..133S
Altcode: 2011arXiv1102.1049S
The emergence of minimally structured (uniform and horizontal) magnetic
field from a depth of 20 Mm has been simulated. The field emerges first
in a mixed polarity pepper and salt pattern, but then collects into
separate, unipolar concentrations and produces pores. The field strength
was then artificially increased to produce spot-like structures. The
field strength at continuum optical depth unity peaks at 1 kG, with
a maximum of 4 kG. Where the vertical field is strong, the spots
persist (at present an hour of solar time has been simulated). Where
the field is weak, the spot gets filled in and disappears. Stokes
profiles have been calculated and processed with the Hinode annular
Modulation Transfer Function, the slit diffraction, and frequency
smoothing. These data are available at steinr.pa.msu.edu/∼bob/stokes.
Title: Spontaneous Pore Formation in Magneto-Convection Simulations
Authors: Stein, R.; Nordlund, A.
Bibcode: 2012ASPC..456...39S
Altcode:
Pores form spontaneously in simulations of minimaly structured (uniform,
untwisted, horizontal) magnetic field emerging from a depth of 20
Mm in a 48 Mm wide domain. The input field strength at the bottom
was slowly increased from 200 G to 1 kG with an e-folding time of 5
hours and thereafter held constant. After about a turnover time (2
days) pores formed. The pore's magnetic concentration first developed
near the surface when magnetic loops passed into the solar atmosphere
(through the upper boundary at the temperature minimum) leaving behind
their vertical legs. The magnetic concentration then extended downward
all the way to the bottom at 20 Mm depth. The minimum intensity in the
pore is 20% of the average intensity. The magnetic flux has reached
about 2×1020 Mx and the field is nearly vertical in the
pore interior and inclined more than 45o to the vertical
at the edges. The pores have existed for 10 hours so far.
Title: Pore Formation and Evolution
Authors: Stein, Robert F.; Nordlund, A.
Bibcode: 2012AAS...22020620S
Altcode:
Pores form spontaneously in magneto-convection simulations over a
wide range of initial conditions. These simulations were initiated
by convective inflows at the bottom advecting minimally structured,
uniform, untwisted, horizontal field into the computational
domain. Typically a pore forms when a magnetic loop rises through
the upper boundary of the simulation domain leaving behind its two
nearly vertical legs. In one case the pore formed directly in one of
the legs and in another it assembled from smaller individual magnetic
flux concentrations. The flux concentration that becomes a pore first
forms near the surface and then extends downwards. The cooling and
evacuation of the flux concentration also begin near the surface and
extend downward. Eventually, the entire 20 Mm depth of the box was
included. The turnover time at 20 Mm depth is about 2 days. So far
the longest lived pore has existed for about half a day. One of the
pores is slowly rotating. Supported by NSF grant AGS 1141921 and NASA
grant NNX08AF44G.
Title: Scaling of turbulent and hierarchical reconnection
Authors: Nordlund, A.; Galsgaard, K.
Bibcode: 2012EGUGA..1412646N
Altcode:
We investigate the relation between the theories and scaling formulae
for turbulent and hierarchical reconnection proposed by various authors;
Parker (1972, ApJ 174, 499; 1988, ApJ 330, 474), van Ballegooijen
(1986, ApJ 311, 1001), Galsgaard & Nordlund (1996, JGR 101, 13445),
and Lazarian & Vishniac (1999; ApJ 517, 700), considering also
the results of Lapenta (2008, PhRvL 100, 235001) and Bettarini &
Lapenta (2010, A&A 518, 57).
Title: Radiation from shock-accelerated particles
Authors: Nishikawa, K. I.; Choi, E. J.; Min, K. W.; Niemiec, J.; Zhang,
B.; Hardee, P.; Mizuno, Y.; Medvedev, M.; Nordlund, A.; Frederiksen,
J.; Sol, H.; Pohl, M.; Hartmann, D. H.; Fishman, G. J.
Bibcode: 2012grb..confE..28N
Altcode: 2012PoS...152E..28N
No abstract at ADS
Title: Simulation of Relativistic Jets and Associated Self-Consistent
Radiation
Authors: Nishikawa, K. -I.; Niemiec, J.; Zhang, B.; Medvedev, M.;
Hardee, P.; Mizuno, Y.; Nordlund, Å.; Frederiksen, J.; Sol, H.;
Pohl, M.; Hartmann, D. H.; Fishman, G. J.
Bibcode: 2012IJMPS...8..259N
Altcode: 2011arXiv1111.3622N
Plasma instabilities are responsible not only for the onset and
mediation of collisionless shocks but also for the associated
acceleration of particles. We have investigated particle acceleration
and shock structure associated with an unmagnetized relativistic
electron-positron jet propagating into an unmagnetized electron-positron
plasma. Cold jet electrons are thermalized and slowed while the ambient
electrons are swept up to create a partially developed hydrodynamic-like
shock structure. In the leading shock, electron density increases by
a factor of about 3.5 in the simulation frame. Strong electromagnetic
fields are generated in the trailing shock and provide an emission
site. These magnetic fields contribute to the electrons transverse
deflection and, more generally, relativistic acceleration behind
the shock. We have calculated, self-consistently, the radiation from
electrons accelerated in the turbulent magnetic fields. We found that
the synthetic spectra depend on the Lorentz factor of the jet, its
thermal temperature and strength of the generated magnetic fields. The
properties of the radiation may be important for understanding the
complex time evolution and/or spectral structure in gamma-ray bursts,
relativistic jets in general, and supernova remnants.
Title: A grid of S stars MARCS model atmospheres
Authors: Van Eck, Sophie; Neyskens, Pieter; Plez, Bertrand; Jorissen,
Alain; Edvardsson, Bengt; Eriksson, Kjell; Gustafsson, Bengt; Gråe
Jørgensen, Uffe; Nordlund, Åke
Bibcode: 2011JPhCS.328a2009V
Altcode:
S stars are cool stars of temperatures similar to those of M giants,
but their atmospheres are enriched in carbon and s-process elements
because of either extrinsic pollution by a binary companion or intrinsic
nucleosynthesis and dredge-up on the thermally-pulsing AGB. Despite
numerous attempts to link phenomenological spectral classification
criteria to physical parameters (Teff, gravity, C/O, [s/Fe],
[Fe/H]), the parameter space of S stars is poorly known and this has
prevented accurate abundance analysis of S stars until now. Here we
present a large grid of S-star model atmospheres. ZrO and TiO band
strength indices as well as VJHKL photometry are needed to disentangle
the effective temperature, C/O and [s/Fe]. The stellar parameters
derived on the basis of low-resolution spectra and photometry are
shown to be fairly accurate when compared to high-resolution data
of the same stars. The C/O ratio of S stars is found to be between
the solar value (0.5) and 0.99, and not 1 as often claimed in the
literature. Consistently with stellar evolution expectations, the C/O
ratio increases as the effective temperature decreases.
Title: Enhanced Abundance of 26Al and 60Fe in Giant Molecular Clouds
Authors: Vasileiadis, A.; Nordlund, Å.; Bizzarro, M.
Bibcode: 2011LPICo1639.9101V
Altcode:
No abstract at ADS
Title: Heterogeneous Distribution of 26Al in the Solar Protoplanetary
Disk
Authors: Larsen, K. K.; Trinquier, A.; Paton, C.; Schiller, M.;
Wielandt, D.; Ivanova, M. A.; Connelly, J. N.; Nordlund, Å.; Krot,
A. N.; Bizzarro, M.
Bibcode: 2011LPICo1639.9053L
Altcode:
No abstract at ADS
Title: Zooming in on Star Formation
Authors: Haugboelle, T.; Nordlund, A.; Padoan, P.
Bibcode: 2011LPICo1639.9116H
Altcode:
No abstract at ADS
Title: Formation of brown dwarfs and planets
Authors: Nordlund, Åke
Bibcode: 2011IAUS..276..105N
Altcode:
Brown dwarfs and massive planets have similar structures, and there is
probably an overlap in mass between the most massive planets and the
lowest mass brown dwarfs. This raises questions as to what extent the
structures of the most massive planets and lowest mass brown dwarfs
differ, and what similarities (or not) there might be between their
formation mechanisms. Here I discuss these issues on the background
of recent numerical simulations of star formation, new evidence from
cosmochemistry about the conditions in the early solar system, and
recently discovered mechanisms that can expedite planetesimal and
possibly planet formation greatly.
Title: The Observable Prestellar Phase of the Initial Mass Function
Authors: Padoan, Paolo; Nordlund, Åke
Bibcode: 2011ApJ...741L..22P
Altcode: 2011arXiv1108.2543P
The observed similarities between the mass function of prestellar
cores (CMF) and the stellar initial mass function (IMF) have led to
the suggestion that the IMF is already largely determined in the gas
phase. However, theoretical arguments show that the CMF may differ
significantly from the IMF. In this Letter, we study the relation
between the CMF and the IMF, as predicted by the IMF model of Padoan
and Nordlund. We show that (1) the observed mass of prestellar cores is
on average a few times smaller than that of the stellar systems they
generate; (2) the CMF rises monotonically with decreasing mass, with
a noticeable change in slope at approximately 3-5 M sun,
depending on mean density; (3) the selection of cores with masses
larger than half their Bonnor-Ebert mass yields a CMF approximately
consistent with the system IMF, rescaled in mass by the same factor as
our model IMF, and therefore suitable to estimate the local efficiency
of star formation, and to study the dependence of the IMF peak on cloud
properties; and (4) only one in five pre-brown-dwarf core candidates
is a true progenitor to a brown dwarf.
Title: Short-Lived Radionuclide Abundances and Nucleosynthetic
Isotope Anomalies in Bulk Planetary Materials: Is There a Connection?
Authors: Bizzarro, M.; Nordlund, Å.
Bibcode: 2011LPICo1639.9052B
Altcode:
No abstract at ADS
Title: Simulation of Relativistic Shocks and Associated Radiation
from Turbulent Magnetic Fields
Authors: Nishikawa, K. -I.; Niemiec, J.; Medvedev, M.; Zhang, B.;
Hardee, P.; Mizuno, Y.; Nordlund, A.; Frederiksen, J.; Sol, H.; Pohl,
M.; Hartmann, D. H.; Fishman, G. J.
Bibcode: 2011ASPC..444...81N
Altcode:
Using our new 3-D relativistic particle-in-cell (PIC) code, we
investigated long-term particle acceleration associated with a
relativistic electron-positron jet propagating in an unmagnetized
ambient electron-positron plasma. The simulations were performed using
a much longer simulation system than our previous simulations in order
to investigate the full nonlinear stage of the Weibel instability
and its particle acceleration mechanism. Cold jet electrons are
thermalized and ambient electrons are accelerated in the resulting
shocks. Acceleration of ambient electrons leads to a maximum ambient
electron density three times larger than the original value as
predicted by the hydrodynamic compression. Behind the bow shock,
in the jet shock, strong electromagnetic fields are generated. These
fields may lead to time dependent afterglow emission. In order to go
beyond the standard synchrotron model used in astrophysical objects we
have used PIC simulations and calculated radiation based on the first
principles. We calculated radiation from electrons propagating in a
uniform parallel magnetic field to verify the technique. We also used
the technique to calculate emission from electrons based on simulations
with a small system. We obtained spectra which are consistent with those
generated from electrons propagating in turbulent magnetic fields. This
turbulent magnetic field is similar to the magnetic field generated at
an early nonlinear stage of the Weibel instability. A fully developed
shock within a larger system may generate a jitter/synchrotron spectrum.
Title: Simulation of Relativistic Shocks and Associated
Self-consistent Radiation
Authors: Nishikawa, K. -I.; Niemiec, J.; Medvedev, M.; Zhang, B.;
Hardee, P.; Nordlund, Å.; Frederiksen, J.; Mizuno, Y.; Sol, H.;
Pohl, M.; Hartmann, D. H.; Fishman, G. J.
Bibcode: 2011AIPC.1366..163N
Altcode:
Using our new 3-D relativistic particle-in-cell (PIC) code parallelized
with MPI, we investigated long-term particle acceleration associated
with a relativistic electron-positron jet propagating into an
unmagnetized ambient electron-positron plasma. The simulations were
performed using a much longer simulation system than our previous
simulations in order to investigate the full nonlinear stage of the
Weibel instability and its particle acceleration mechanism. Cold
jet electrons are thermalized and ambient electrons are accelerated
in the resulting shocks. Acceleration of ambient electrons leads
to a maximum ambient electron density three times larger than the
original value as predicted by hydrodynamic shock compression. In
the jet (reverse) shock behind the bow (forward) shock the strongest
electromagnetic fields are generated. These fields may lead to time
dependent afterglow emission. In order to calculate radiation from
first principles that goes beyond the standard synchrotron model used
in astrophysical objects we have used PIC simulations. Initially we
calculated radiation from electrons propagating in a uniform parallel
magnetic field to verify the technique. We then used the technique to
calculate emission from electrons in a small simulation system. From
these simulations we obtained spectra which are consistent with those
generated from electrons propagating in turbulent magnetic fields with
red noise. This turbulent magnetic field is similar to the magnetic
field generated at an early nonlinear stage of the Weibel instability. A
fully developed shock within a larger simulation system may generate
a jitter/synchrotron spectrum.
Title: A Grid of MARCS Model Atmospheres for S Stars
Authors: van Eck, S.; Neyskens, P.; Plez, B.; Jorissen, A.; Edvardsson,
B.; Eriksson, K.; Gustafsson, B.; Jørgensen, U. G.; Nordlund, Å.
Bibcode: 2011ASPC..445...71V
Altcode: 2010arXiv1011.2092V
S-type stars are late-type giants whose atmospheres are enriched in
carbon and s-process elements because of either extrinsic pollution by
a binary companion or intrinsic nucleosynthesis and dredge-up on the
thermally-pulsing AGB. A large grid of S-star model atmospheres has
been computed covering the range 2700 ≤ Teff(K) ≤ 4000
with 0.5 ≤ C/O ≤ 0.99. ZrO and TiO band strength indices as well
as VJHKL photometry are needed to disentangle Teff, C/O and
[s/Fe]. A “best-model finding tool” has been developed using a set
of well-chosen indices and checked against photometry as well as low-
and high-resolution spectroscopy. It is found that applying M-star
model atmospheres (i.e., with a solar C/O ratio) to S stars can lead
to errors in Teff up to 400 K. We constrain the parameter
space occupied by the S stars of the vast Henize sample in terms of
Teff, [C/O] and [s/Fe].
Title: Simulation of relativistic shocks and associated radiation
from turbulent magnetic fields
Authors: Nishikawa, K. -I.; Niemiec, J.; Medvedev, M.; Zhang, B.;
Hardee, P.; Nordlund, A. ˚.; Frederiksen, J.; Mizuno, Y.; Sol, H.;
Pohl, M.; Hartmann, D. H.; Fishman, G. J.
Bibcode: 2011AIPC.1358...87N
Altcode:
Using our new 3-D relativistic particle-in-cell (PIC) code, we
investigated long-term particle acceleration associated with a
relativistic electron-positron jet propagating in an unmagnetized
ambient electron-positron plasma. The simulations were performed using
a much longer simulation system than our previous simulations in order
to investigate the full nonlinear stage of the Weibel instability
and its particle acceleration mechanism. Cold jet electrons are
thermalized and ambient electrons are accelerated in the resulting
shocks. Acceleration of ambient electrons leads to a maximum ambient
electron density three times larger than the original value as predicted
by hydrodynamic compression. Behind the bow shock, in the jet shock,
strong electromagnetic fields are generated. These fields may lead to
time dependent afterglow emission. In order to go beyond the standard
synchrotron model used in astrophysical objects we have used PIC
simulations and calculated radiation based on first principles. We
calculated radiation from electrons propagating in a uniform parallel
magnetic field to verify the technique. We also used the technique to
calculate emission from electrons based on simulations with a small
system. We obtain spectra which are consistent with those generated
from electrons propagating in turbulent magnetic fields. This turbulent
magnetic field is similar to the magnetic field generated at an early
nonlinear stage of the Weibel instability. A fully developed shock
within a larger system may generate a jitter/synchrotron spectrum.
Title: Radiation Signatures of Sub-Larmor Scale Magnetic Fields
Authors: Medvedev, Mikhail V.; Frederiksen, Jacob Trier; Haugbølle,
Troels; Nordlund, Åke
Bibcode: 2011ApJ...737...55M
Altcode: 2010arXiv1003.0063M
Spontaneous rapid growth of strong magnetic fields is rather ubiquitous
in high-energy density environments ranging from astrophysical sources
(e.g., gamma-ray bursts and relativistic shocks), to reconnection, to
laser-plasma interaction laboratory experiments, where they are produced
by kinetic streaming instabilities of the Weibel type. Relativistic
electrons propagating through these sub-Larmor-scale magnetic
fields radiate in the jitter regime, in which the anisotropy of the
magnetic fields and the particle distribution have a strong effect
on the produced radiation. Here we develop the general theory of
jitter radiation, which (1) includes anisotropic magnetic fields and
electron velocity distributions, (2) accounts for the effects of trapped
electrons, and (3) extends the description to large deflection angles of
radiating particles thus establishing a cross-over between the classical
jitter and synchrotron regimes. Our results are in remarkable agreement
with the radiation spectra obtained from particle-in-cell simulations of
the classical Weibel instability. Particularly interesting is the onset
of the field growth, when the transient hard synchrotron-violating
spectra are common as a result of the dominant role of the trapped
population. This effect can serve as a distinct observational
signature of the violent field growth in astrophysical sources and
lab experiments. It is also interesting that a system with small-scale
fields tends to evolve toward the small-angle jitter regime, which can,
under certain conditions, dominate the overall emission of a source.
Title: Comparing Numerical Methods for Isothermal Magnetized
Supersonic Turbulence
Authors: Kritsuk, Alexei G.; Nordlund, Åke; Collins, David; Padoan,
Paolo; Norman, Michael L.; Abel, Tom; Banerjee, Robi; Federrath,
Christoph; Flock, Mario; Lee, Dongwook; Li, Pak Shing; Müller,
Wolf-Christian; Teyssier, Romain; Ustyugov, Sergey D.; Vogel,
Christian; Xu, Hao
Bibcode: 2011ApJ...737...13K
Altcode: 2011arXiv1103.5525K
Many astrophysical applications involve magnetized turbulent flows
with shock waves. Ab initio star formation simulations require a robust
representation of supersonic turbulence in molecular clouds on a wide
range of scales imposing stringent demands on the quality of numerical
algorithms. We employ simulations of supersonic super-Alfvénic
turbulence decay as a benchmark test problem to assess and compare
the performance of nine popular astrophysical MHD methods actively
used to model star formation. The set of nine codes includes: ENZO,
FLASH, KT-MHD, LL-MHD, PLUTO, PPML, RAMSES, STAGGER, and ZEUS. These
applications employ a variety of numerical approaches, including both
split and unsplit, finite difference and finite volume, divergence
preserving and divergence cleaning, a variety of Riemann solvers, and
a range of spatial reconstruction and time integration techniques. We
present a comprehensive set of statistical measures designed to quantify
the effects of numerical dissipation in these MHD solvers. We compare
power spectra for basic fields to determine the effective spectral
bandwidth of the methods and rank them based on their relative effective
Reynolds numbers. We also compare numerical dissipation for solenoidal
and dilatational velocity components to check for possible impacts of
the numerics on small-scale density statistics. Finally, we discuss the
convergence of various characteristics for the turbulence decay test and
the impact of various components of numerical schemes on the accuracy
of solutions. The nine codes gave qualitatively the same results,
implying that they are all performing reasonably well and are useful
for scientific applications. We show that the best performing codes
employ a consistently high order of accuracy for spatial reconstruction
of the evolved fields, transverse gradient interpolation, conservation
law update step, and Lorentz force computation. The best results are
achieved with divergence-free evolution of the magnetic field using
the constrained transport method and using little to no explicit
artificial viscosity. Codes that fall short in one or more of these
areas are still useful, but they must compensate for higher numerical
dissipation with higher numerical resolution. This paper is the largest,
most comprehensive MHD code comparison on an application-like test
problem to date. We hope this work will help developers improve their
numerical algorithms while helping users to make informed choices about
choosing optimal applications for their specific astrophysical problems.
Title: Magnetic Fields in Molecular Clouds
Authors: Padoan, Paolo; Lunttila, Tuomas; Juvela, Mika; Nordlund, Åke;
Collins, David; Kritsuk, Alexei; Normal, Michael; Ustyugov, Sergey
Bibcode: 2011IAUS..271..187P
Altcode:
Supersonic magneto-hydrodynamic (MHD) turbulence in molecular clouds
(MCs) plays an important role in the process of star formation. The
effect of the turbulence on the cloud fragmentation process depends
on the magnetic field strength. In this work we discuss the idea
that the turbulence is super-Alfvénic, at least with respect to
the cloud mean magnetic field. We argue that MCs are likely to be
born super-Alfvénic. We then support this scenario based on a recent
simulation of the large-scale warm interstellar medium turbulence. Using
small-scale isothermal MHD turbulence simulation, we also show that
MCs may remain super-Alfvénic even with respect to their rms magnetic
field strength, amplified by the turbulence. Finally, we briefly discuss
the comparison with the observations, suggesting that super-Alfvénic
turbulence successfully reproduces the Zeeman measurements of the
magnetic field strength in dense MC clouds.
Title: Evidence for Magnesium Isotope Heterogeneity in the Solar
Protoplanetary Disk
Authors: Larsen, Kirsten K.; Trinquier, Anne; Paton, Chad; Schiller,
Martin; Wielandt, Daniel; Ivanova, Marina A.; Connelly, James N.;
Nordlund, Åke; Krot, Alexander N.; Bizzarro, Martin
Bibcode: 2011ApJ...735L..37L
Altcode:
With a half-life of 0.73 Myr, the 26Al-to-26Mg
decay system is the most widely used short-lived chronometer
for understanding the formation and earliest evolution of
the solar protoplanetary disk. However, the validity of
26Al-26Mg ages of meteorites and their
components relies on the critical assumption that the canonical
26Al/27Al ratio of ~5 × 10-5
recorded by the oldest dated solids, calcium-aluminium-rich inclusions
(CAIs), represents the initial abundance of 26Al for the
solar system as a whole. Here, we report high-precision Mg-isotope
measurements of inner solar system solids, asteroids, and planets
demonstrating the existence of widespread heterogeneity in the
mass-independent 26Mg composition (μ26Mg*)
of bulk solar system reservoirs with solar or near-solar Al/Mg
ratios. This variability may represent heterogeneity in the initial
abundance of 26Al across the solar protoplanetary disk at
the time of CAI formation and/or Mg-isotope heterogeneity. By comparing
the U-Pb and 26Al-26Mg ages of pristine solar
system materials, we infer that the bulk of the μ26Mg*
variability reflects heterogeneity in the initial abundance of
26Al across the solar protoplanetary disk. We conclude that
the canonical value of ~5 × 10-5 represents the average
initial abundance of 26Al only in the CAI-forming region,
and that large-scale heterogeneity—perhaps up to 80% of the canonical
value—may have existed throughout the inner solar system. If correct,
our interpretation of the Mg-isotope composition of inner solar system
objects precludes the use of the 26Al-26Mg system
as an accurate early solar system chronometer.
Title: 3D LTE spectral line formation with scattering in red giant
stars
Authors: Hayek, W.; Asplund, M.; Collet, R.; Nordlund, Å.
Bibcode: 2011A&A...529A.158H
Altcode: 2011arXiv1108.3366H
Aims: We investigate the effects of coherent isotropic continuum
scattering on the formation of spectral lines in local thermodynamic
equilibrium (LTE) using 3D hydrodynamical and 1D hydrostatic model
atmospheres of red giant stars.
Methods: Detailed radiative
transfer with coherent and isotropic continuum scattering is computed
for 3D hydrodynamical and 1D hydrostatic models of late-type stellar
atmospheres using the SCATE code. Opacities are computed in LTE, while
a coherent and isotropic scattering term is added to the continuum
source function. We investigate the effects of scattering by comparing
continuum flux levels, spectral line profiles and curves of growth
for different species with calculations that treat scattering as
absorption.
Results: Rayleigh scattering is the dominant source
of scattering opacity in the continuum of red giant stars. Photons
may escape from deeper, hotter layers through scattering, resulting in
significantly higher continuum flux levels beneath a wavelength of λ
≲ 5000 Å. The magnitude of the effect is determined by the importance
of scattering opacity with respect to absorption opacity; we observe
the largest changes in continuum flux at the shortest wavelengths
and lowest metallicities; intergranular lanes of 3D models are more
strongly affected than granules. Continuum scattering acts to increase
the profile depth of LTE lines: continua gain more brightness than line
cores due to their larger thermalization depth in hotter layers. We
thus observe the strongest changes in line depth for high-excitation
species and ionized species, which contribute significantly to photon
thermalization through their absorption opacity near the continuum
optical surface. Scattering desaturates the line profiles, leading to
larger abundance corrections for stronger lines, which reach -0.5 dex
at 3000 Å for Fe ii lines in 3D with excitation potential χ = 2 eV at
[Fe/H] = -3.0. The corrections are less severe for low-excitation lines,
longer wavelengths, and higher metallicity. Velocity fields increase
the effects of scattering by separating emission from granules and
intergranular lanes in wavelength. 1D calculations exhibit similar
scattering abundance corrections for weak lines, but those for
strong lines are generally smaller compared to 3D models and depend
on the choice of microturbulence.
Conclusions: Continuum
scattering should be taken into account for computing realistic
spectral line profiles at wavelengths λ ≲ 4000 Å in metal-poor
giant stars. Profile shapes are strongly affected by velocity fields
and horizontal inhomogeneities, requiring a treatment based on 3D
hydrodynamical rather than classical 1D hydrostatic model atmospheres.
Title: Magnetic Fields: Modeling And ATST Observations
Authors: Stein, Robert F.; Georgobiani, D.; Nordlund, A.; Lagerfjard,
A.
Bibcode: 2011SPD....42.0804S
Altcode: 2011BAAS..43S.0804S
We have performed magneto-convection simulations starting from
snapshots of hydrodynamic convection with initial conditions both
of uniform vertical magnetic field and with minimally structured
(uniform, untwisted), horizontal magnetic field advected into
the computational domain from a depth of 20 Mm. One clear result
is that while the magnetic field can collect into large-scale
concentrations - pores and sunspots - most of the magnetic flux is
in small concentrations with steep horizontal gradients in the field
and plasma properties. Furthermore, the field strength distribution
is a power law with slope between -1 and -2, so most of the field
at the surface is weak. A large aperture telescope, such as ATST, is
needed both to collect sufficient photons to measure the ubiquitous
weak fields and to resolve the small-scale magnetic features. We present results on flux emergence, pore formation, and Stokes
spectra as they would appear in Hinode and ATST compared with the
raw simulation.For those interested in analyzing the simulation data,
it is available online at steinr.pa.msu.edu/ bob/data.html. There are
slices of the velocity and magnetic field vectors at continuum optical
depths of 1, 0.1, and 0.01 and the emergent intensity have been saved
at 1 minute intervals. Four hour averages, with 2 hour cadence for the
3D cube for variables: velocity, magnetic field, density, temperature,
sound speed, and internal energy have been computed. Stokes spectra
have been computed for the Hinode FeI 630 nm lines, processed with the
Hinode annular mtf, the slit diffraction and frequency smoothing. This work has been supported by NASA grants NNX07AO71G, NNX07AH79G and
NNX08AH44G and NSF grant AST0605738. The simulations where performed
on the Pleiades cluster of the NASA Advanced Supercomputing Division
at the Ames Research Center.
Title: Radiation from relativistic shocks in turbulent magnetic fields
Authors: Nishikawa, K. -I.; Niemiec, J.; Medvedev, M.; Zhang, B.;
Hardee, P.; Nordlund, A.; Frederiksen, J.; Mizuno, Y.; Sol, H.; Pohl,
M.; Hartmann, D. H.; Oka, M.; Fishman, G. J.
Bibcode: 2011AdSpR..47.1434N
Altcode:
Using our new 3-D relativistic particle-in-cell (PIC) code parallelized
with MPI, we investigated long-term particle acceleration associated
with a relativistic electron-positron jet propagating in an
unmagnetized ambient electron-positron plasma. The simulations were
performed using a much longer simulation system than our previous
simulations in order to investigate the full nonlinear stage of the
Weibel instability and its particle acceleration mechanism. Cold
jet electrons are thermalized and ambient electrons are accelerated
in the resulting shocks. Acceleration of ambient electrons leads
to a maximum ambient electron density three times larger than the
original value as predicted by hydrodynamic shock compression. In
the jet (reverse) shock behind the bow (forward) shock the strongest
electromagnetic fields are generated. These fields may lead to time
dependent afterglow emission. In order to calculate radiation from
first principles that goes beyond the standard synchrotron model used
in astrophysical objects we have used PIC simulations. Initially we
calculated radiation from electrons propagating in a uniform parallel
magnetic field to verify the technique. We then used the technique to
calculate emission from electrons in a small simulation system. From
these simulations we obtained spectra which are consistent with those
generated from electrons propagating in turbulent magnetic fields with
red noise. This turbulent magnetic field is similar to the magnetic
field generated at an early nonlinear stage of the Weibel instability. A
fully developed shock within a larger simulation system may generate
a jitter/synchrotron spectrum.
Title: Three-dimensional surface convection simulations of metal-poor
stars. The effect of scattering on the photospheric temperature
stratification
Authors: Collet, R.; Hayek, W.; Asplund, M.; Nordlund, Å.; Trampedach,
R.; Gudiksen, B.
Bibcode: 2011A&A...528A..32C
Altcode: 2011arXiv1101.3265C
Context. Three-dimensional (3D) radiative hydrodynamic model atmospheres
of metal-poor late-type stars are characterized by cooler upper
photospheric layers than their one-dimensional counterparts. This
property of 3D model atmospheres can dramatically affect the
determination of elemental abundances from temperature-sensitive
spectral features, with profound consequences on galactic chemical
evolution studies.
Aims: We investigate whether the cool surface
temperatures predicted by 3D model atmospheres of metal-poor stars
can be ascribed to approximations in the treatment of scattering
during the modelling phase.
Methods: We use the Bifrost
code to construct 3D model atmospheres of metal-poor stars and test
three different ways to handle scattering in the radiative transfer
equation. As a first approach, we solve iteratively the radiative
transfer equation for the general case of a source function with
a coherent scattering term, treating scattering in a correct and
consistent way. As a second approach, we solve the radiative transfer
equation in local thermodynamic equilibrium approximation, neglecting
altogether the contribution of continuum scattering to extinction in the
optically thin layers; this has been the default mode in our previous
3D modelling as well as in present Stagger-Code models. As our third
and final approach, we treat continuum scattering as pure absorption
everywhere, which is the standard case in the 3D modelling by the
CO5BOLD collaboration.
Results: For all simulations,
we find that the second approach produces temperature structures
with cool upper photospheric layers very similar to the case in which
scattering is treated correctly. In contrast, treating scattering as
pure absorption leads instead to significantly hotter and shallower
temperature stratifications. The main differences in temperature
structure between our published models computed with the Stagger-
and Bifrost codes and those generated with the CO5BOLD
code can be traced to the different treatments of scattering.
Conclusions: Neglecting the contribution of continuum scattering to
extinction in optically thin layers provides a good approximation
to the full, iterative solution of the radiative transfer equation
in metal-poor stellar surface convection simulations, and at a much
lower computational cost. Our results also demonstrate that the cool
temperature stratifications predicted for metal-poor late-type stars
by previous models by our collaboration are not an artifact of the
approximated treatment of scattering.
Title: Ray Casting and Flux Limited Diffusion
Authors: Nordlund, Åke
Bibcode: 2011IAUS..270..207N
Altcode:
Solving radiative transfer problems with ray casting methods is compared
with the commonly used `Flux Limited Diffusion' approximation. Whereas
ray casting produces solutions that converge to the exact one as the
number of rays is increased, flux-limited-diffusion is fundamentally a
`look-alike' method, which produces solutions that are reminiscent of
the correct solution but which cannot be made to converge to it.
Title: Astrophysical turbulence modeling
Authors: Brandenburg, Axel; Nordlund, Åke
Bibcode: 2011RPPh...74d6901B
Altcode: 2009arXiv0912.1340B
The role of turbulence in various astrophysical settings
is reviewed. Among the differences to laboratory and atmospheric
turbulence we highlight the ubiquitous presence of magnetic fields that
are generally produced and maintained by dynamo action. The extreme
temperature and density contrasts and stratifications are emphasized
in connection with turbulence in the interstellar medium and in stars
with outer convection zones, respectively. In many cases turbulence
plays an essential role in facilitating enhanced transport of mass,
momentum, energy and magnetic fields in terms of the corresponding
coarse-grained mean fields. Those transport properties are usually
strongly modified by anisotropies and often completely new effects
emerge in such a description that have no correspondence in terms of
the original (non-coarse-grained) fields.
Title: Theory of the Star Formation Rate
Authors: Padoan, Paolo; Nordlund, Åke
Bibcode: 2011IAUS..270..347P
Altcode:
This work presents a new physical model of the star formation
rate (SFR), tested with a large set of numerical simulations of
driven, supersonic, self-gravitating, magneto-hydrodynamic (MHD)
turbulence, where collapsing cores are captured with accreting
sink particles. The model depends on the relative importance of
gravitational, turbulent, magnetic, and thermal energies, expressed
through the virial parameter, αvir, the rms sonic Mach
number, S,0, and the ratio of mean gas pressure to mean
magnetic pressure, β0. The SFR is predicted to decrease
with increasing αvir (stronger turbulence relative to
gravity), and to depend weakly on S,0 and β0,
for values typical of star forming regions (S,0~4-20 and
β0~1-20). The star-formation simulations used to test
the model result in an approximately constant SFR, after an initial
transient phase. Both the value of the SFR and its dependence on the
virial parameter found in the simulations agree very well with the
theoretical predictions.
Title: The Star Formation Rate of Supersonic Magnetohydrodynamic
Turbulence
Authors: Padoan, Paolo; Nordlund, Åke
Bibcode: 2011ApJ...730...40P
Altcode: 2009arXiv0907.0248P
This work presents a new physical model of the star formation
rate (SFR), which is verified with an unprecedented set of large
numerical simulations of driven, supersonic, self-gravitating,
magneto-hydrodynamic (MHD) turbulence, where collapsing cores are
captured with accreting sink particles. The model depends on the
relative importance of gravitational, turbulent, magnetic, and thermal
energies, expressed through the virial parameter, αvir,
the rms sonic Mach number, M_S,0, and the ratio of mean gas pressure
to mean magnetic pressure, β0. The SFR is predicted to
decrease with increasing αvir (stronger turbulence relative
to gravity), to increase with increasing M_S,0 (for constant values of
αvir), and to depend weakly on β0 for values
typical of star forming regions (M_S,0≈ 4-20 and β0
≈ 1-20). In the unrealistic limit of β0 → ∞, that
is, in the complete absence of a magnetic field, the SFR increases
approximately by a factor of three, which shows the importance of
magnetic fields in the star formation process, even when they are
relatively weak (super-Alfvénic turbulence). The star-formation
simulations used to test the model result in an approximately constant
SFR, after an initial transient phase. The dependence of the SFR on
the virial parameter is shown to agree very well with the theoretical
predictions.
Title: Explosive Outflows Powered by the Decay of Non-hierarchical
Multiple Systems of Massive Stars: Orion BN/KL
Authors: Bally, John; Cunningham, Nathaniel J.; Moeckel, Nickolas;
Burton, Michael G.; Smith, Nathan; Frank, Adam; Nordlund, Ake
Bibcode: 2011ApJ...727..113B
Altcode: 2010arXiv1011.5512B
The explosive Becklin-Neugebauer (BN)/Kleinman-Low (KL) outflow emerging
from OMC1 behind the Orion Nebula may have been powered by the dynamical
decay of a non-hierarchical multiple system ~500 years ago that ejected
the massive stars I, BN, and source n, with velocities of about 10-30
km s-1. New proper-motion measurements of H2
features show that within the errors of measurement, the outflow
originated from the site of stellar ejection. Combined with published
data, these measurements indicate an outflow age of ~500 years,
similar to the time since stellar ejection. The total kinetic energy
of the ejected stars and the outflow is about 2 to 6 × 1047
erg. It is proposed that the gravitational potential energy released by
the formation of a short-period binary, most likely source I, resulted
in stellar ejection and powered the outflow. A scenario is presented
for the formation of a compact, non-hierarchical multiple star system,
its decay into an ejected binary and two high-velocity stars, and launch
of the outflow. Three mechanisms may have contributed to the explosion
in the gas: (1) unbinding of the circumcluster envelope following
stellar ejection, (2) disruption of circumstellar disks and high-speed
expulsion of the resulting debris during the final stellar encounter,
and (3) the release of stored magnetic energy. Plausible protostellar
disk end envelope properties can produce the observed outflow mass,
velocity, and kinetic energy distributions. The ejected stars may
have acquired new disks by fall-back or Bondi-Hoyle accretion with
axes roughly orthogonal to their velocities. The expulsion of gas and
stars from OMC1 may have been driven by stellar interactions.
Title: Solar Flux Emergence Simulations
Authors: Stein, R. F.; Lagerfjärd, A.; Nordlund, Å.; Georgobiani, D.
Bibcode: 2011SoPh..268..271S
Altcode: 2009arXiv0912.4938S; 2010SoPh..tmp...34S
We simulate the rise through the upper convection zone and emergence
through the solar surface of initially uniform, untwisted, horizontal
magnetic flux, with the same entropy as the nonmagnetic plasma,
that is advected into a domain 48 Mm wide by 20 Mm deep. The magnetic
field is advected upward by the diverging upflows and pulled down in
the downdrafts, which produces a hierarchy of loop-like structures
of increasingly smaller scale as the surface is approached. There are
significant differences between the behavior of fields of 10 kG and 20
or 40 kG strength at 20 Mm depth. The 10 kG fields have little effect
on the convective flows and show small magnetic-buoyancy effects,
reaching the surface in the typical fluid rise time from 20 Mm depth
of 32 hours. 20 and 40 kG fields significantly modify the convective
flows, leading to long, thin cells of ascending fluid aligned with
the magnetic field and their magnetic buoyancy makes them rise to the
surface faster than the fluid rise time. The 20 kG field produces a
large-scale magnetic loop that as it emerges through the surface leads
to the formation of a bipolar, pore-like structure.
Title: Simulation of relativistic shocks and associated radiation
from turbulent magnetic fields
Authors: Nishikawa, K. -I.; Niemiec, J.; Medvedev, M.; Zhang, B.;
Hardee, P.; Mizuno, Y.; Nordlund, A.; Frederiksen, J.; Sol, H.; Pohl,
M.; Hartmann, D. H.; Fishman, G. J.
Bibcode: 2011IAUS..275..354N
Altcode:
Recent PIC simulations of relativistic electron-positron (electron-ion)
jets injected into a stationary medium show that particle
acceleration occurs in the shocked regions. Simulations show that
the Weibel instability is responsible for generating and amplifying
highly nonuniform, small-scale magnetic fields and for particle
acceleration. These magnetic fields contribute to the electron's
transverse deflection behind the shock. The ``jitter'' radiation
from deflected electrons in turbulent magnetic fields has different
properties from synchrotron radiation calculated in a uniform magnetic
field. This jitter radiation may be important for understanding the
complex time evolution and/or spectral structure of gamma-ray bursts,
relativistic jets in general, and supernova remnants. In order to
calculate radiation from first principles and go beyond the standard
synchrotron model, we have used PIC simulations. We will present
detailed spectra for conditions relevant to various astrophysical
sites of collisionless shock formation. In particular we will discuss
application to GRBs and SNRs.
Title: Solar Abundance Corrections Derived Through Three-dimensional
Magnetoconvection Simulations
Authors: Fabbian, D.; Khomenko, E.; Moreno-Insertis, F.; Nordlund, Å.
Bibcode: 2010ApJ...724.1536F
Altcode: 2010arXiv1006.0231F
We explore the effect of the magnetic field when using realistic
three-dimensional convection experiments to determine solar element
abundances. By carrying out magnetoconvection simulations with a
radiation-hydro code (the Copenhagen stagger code) and through a
posteriori spectral synthesis of three Fe I lines, we obtain evidence
that moderate amounts of mean magnetic flux cause a noticeable
change in the derived equivalent widths compared with those for a
non-magnetic case. The corresponding Fe abundance correction for a
mean flux density of 200 G reaches up to ~0.1 dex in magnitude. These
results are based on space- and time-averaged line profiles over a time
span of 2.5 solar hours in the statistically stationary regime of the
convection. The main factors causing the change in equivalent widths,
namely the Zeeman broadening and the modification of the temperature
stratification, act in different amounts and, for the iron lines
considered here, in opposite directions; yet, the resulting |Δlog
epsilonsun(Fe)| coincides within a factor of 2 in all
of them, even though the sign of the total abundance correction
is different for the visible and infrared lines. We conclude that
magnetic effects should be taken into account when discussing precise
values of the solar and stellar abundances and that an extended study
is warranted.
Title: Simulation of Relativistic Shocks and Associated
Self-consistent Radiation
Authors: Nishikawa, K. -I.; Niemiec, J.; Medvedev, M.; Zhang, B.;
Hardee, P.; Mizuno, Y.; Nordlund, Å.; Frederiksen, J.; Sol, H.;
Pohl, M.; Hartmann, D. H.; Fishman, G. J.
Bibcode: 2010AIPC.1279..261N
Altcode: 2009arXiv0912.1583N
We calculated radiation from electrons propagating in a uniform
parallel magnetic field to verify our technique. We also used our new
technique to calculate emission from electrons in small simulation
systems with three different Lorentz factors and ambient parallel
magnetic fields. We obtained spectra which are consistent with those
generated by electrons propagating in turbulent magnetic fields, that
are generated at an early nonlinear stage of the Weibel instability.
Title: Radiation Spectral Synthesis of Relativistic Filamentation
Authors: Frederiksen, Jacob Trier; Haugbølle, Troels; Medvedev,
Mikhail V.; Nordlund, Åke
Bibcode: 2010ApJ...722L.114F
Altcode: 2010arXiv1003.1140T
Radiation from many astrophysical sources, e.g., gamma-ray
bursts and active galactic nuclei, is believed to arise from
relativistically shocked collisionless plasmas. Such sources often
exhibit highly transient spectra evolving rapidly compared with
source lifetimes. Radiation emitted from these sources is typically
associated with nonlinear plasma physics, complex field topologies,
and non-thermal particle distributions. In such circumstances, a
standard synchrotron paradigm may fail to produce accurate conclusions
regarding the underlying physics. Simulating spectral emission and
spectral evolution numerically in various relativistic shock scenarios
is then the only viable method to determine the detailed physical
origin of the emitted spectra. In this Letter, we present synthetic
radiation spectra representing the early stage development of the
filamentation (streaming) instability of an initially unmagnetized
plasma, which is relevant for both collisionless shock formation
and reconnection dynamics in relativistic astrophysical outflows
as well as for laboratory astrophysics experiments. Results were
obtained using a highly efficient in situ diagnostics method, based
on detailed particle-in-cell modeling of collisionless plasmas. The
synthetic spectra obtained here are compared with those predicted by
a semi-analytical model for jitter radiation from the filamentation
instability, the latter including self-consistent generated field
topologies and particle distributions obtained from the simulations
reported upon here. Spectra exhibit dependence on the presence—or
the absence—of an inert plasma constituent, when comparing baryonic
plasmas (i.e., containing protons) with pair plasmas. The results
also illustrate that considerable care should be taken when using
lower-dimensional models to obtain information about the astrophysical
phenomena generating observed spectra.
Title: Simulation of Relativistic Shocks and Associated Radiation
Authors: Nishikawa, K.; Niemiec, J.; Medvedev, M.; Zhang, B.; Hardee,
P.; Mizuno, Y.; Nordlund, A.; Frederiksen, J.; Sol, H.; Pohl, M.;
Hartmann, D. H.; Oka, M.; Fishman, J. F.
Bibcode: 2010ASPC..429..127N
Altcode:
We investigated long-term particle acceleration associated with
a relativistic electron-positron (ion) jet propagating in an
unmagnetized ambient electron-positron (ion) plasma using our 3-D
relativistic particle-in-cell (PIC) code parallelized with MPI. Cold
jet electrons are thermalized and ambient electrons are accelerated in
the resulting shocks for both cases. Acceleration of ambient electrons
leads to a maximum ambient electron density three times larger than
the original value for pair plasmas. We calculated radiation from
electrons propagating in a uniform parallel magnetic field to verify the
technique. We also used the new technique to calculate emission from
electrons based on simulations with a small system with two different
case for Lorentz factors. We obtained spectra which are consistent
with those generated from electrons propagating in turbulent magnetic
fields, which are generated at an early nonlinear stage of the Weibel
instability.
Title: Heterogeneous Distribution of 26Al in the Solar Protoplanetary
Disk
Authors: Larsen, K.; Trinquier, A.; Paton, C.; Ivanova, M.; Nordlund,
Å.; Krot, A. N.; Bizzarro, M.
Bibcode: 2010M&PSA..73.5202L
Altcode:
No abstract at ADS
Title: Flux Emergence Simulations
Authors: Stein, Robert F.; Lagerfjard, Anders; Nordlund, Ake;
Giorgobiani, Dali
Bibcode: 2010shin.confE..82S
Altcode:
In a supergranule scale domain (48 Mm wide by 20 Mm deep) we have
simulated the rise and emergence through the solar surface of initially
minimally structured (uniform and untwisted) horizontal magnetic flux
with the same entropy as the non-magnetic surrounding plasma. We have
studied two cases with field strengths of 20 and 5 kG are advected into
the domain at 20 Mm depth. The stronger field has significant buoyancy,
while the weaker does not. The 20 kG field significantly modifies the
convection, the 5 kG field does not. The fields initially emerge in a
mixed polarity salt and pepper pattern. Subsequently, the different
polarities collect in isolated, unipolar regions due to the action
of underlying, large scale magnetic loop structures. The vertical
field distribution has peaks at 0 and 2 kG at continuum optical depth
0.1. Where the field is strong it tends to be vertical and where
it is weak it tends to be horizontal. Pores are produced and as the
unsigned vertical flux increases they become larger. Stokes profiles
have been calculated.
Title: MHD Turbulence In Star-Forming Clouds
Authors: Padoan, P.; Kritsuk, A. G.; Lunttila, T.; Juvela, M.;
Nordlund, A.; Norman, M. L.; Ustyugov, S. D.
Bibcode: 2010AIPC.1242..219P
Altcode:
Supersonic magneto-hydrodynamic (MHD) turbulence in molecular clouds
(MCs) plays an important role in the process of star formation. The
effect of the turbulence on the cloud fragmentation process depends
on the magnetic field strength. In this work we discuss the idea
that the turbulence is super-Alfvénic, at least with respect to
the cloud mean magnetic field. We argue that MCs are likely to be
born super-Alfvénic. We then support this scenario based on a recent
simulation of the large-scale warm interstellar medium turbulence. Using
small-scale isothermal MHD turbulence simulation, we also show that
MCs may remain super-Alfvénic even with respect to their rms magnetic
field strength, amplified by the turbulence. Finally, we briefly discuss
the comparison with the observations, suggesting that super-Alfvénic
turbulence successfully reproduces the Zeeman measurements of the
magnetic field strength in dense MC clouds.
Title: Supergranule Scale Flux Emergence Simulations
Authors: Stein, Robert F.; Lagerfjard, A.; Nordlund, A.; Georgobiani,
D.
Bibcode: 2010AAS...21621103S
Altcode:
We simulate the rise of initially horizontal, untwisted magnetic flux
from 20 Mm depth through the near surface convection to the solar
surface in a domain 48 Mm wide. The magnetic field is transported
upward by diverging upflows aided by magnetic buoyancy, and pushed
down by downdrafts, which produces a hierarchy of loop like structures,
of increasingly smaller scale as the surface is approached. We compare
two cases with field strengths of 5 and 20 kG at 20 Mm depth. In the
stronger field strength case, the magnetic field significantly disturbs
the convection below 3 Mm, inhibiting the vertical motion, shutting
off convective energy transport and producing elongated cellular
structures in the field direction. Shallower than 3 Mm the convection
appears normal, but with concentrated vertical magnetic concentrations
("flux tubes") extending through the surface and producing pores where
the field is greatest. Even in the weaker field case, the magnetic
field inhibits vertical motions and the convective transport of
energy although the convective cellular pattern is not significantly
distorted. This work was supported by NSF grant AST065738 and NASA
grants NNX08AH44G, NNX07AH79G and NNX07AO71G. The simulations were
performed at the NASA Advanced Supercomputing Division of the Ames
Research Center.
Title: The Thermal Relaxation Time
Authors: Stein, Robert F.; Nordlund, A.
Bibcode: 2010AAS...21631302S
Altcode: 2010BAAS...41Q.888S
The thermal relaxation time for an atmosphere is the ratio of the
thermal energy content to the energy flux, which can be much longer
than the dynamic turn over time. We will discuss this issue and provide
examples from simulations of solar convection in a domain extending
from the surface to 20 Mm below the surface. At 20 Mm the turnover time
is 2 days. The thermal relaxation time at 10 Mm depth is 2 years and
at 20 Mm depth it is 19 years. This work was supported by NASA grants
NNX07AH79G and NNX08AH44G and NSF grant AST0605738.
Title: Simulation Of Relativistic Shocks And Associated
Self-consistent Radiation
Authors: Nishikawa, Ken-Ichi; Niemiec, J.; M; Medvedev, M.; M; Zhang,
B.; M; Hardee, P.; M; Mizuno, Y.; M; Nordlund, A.; M; Frederiksen,
J.; M; Sol, H.; F; Pohl, M.; M; Hartmann, D. H.; M; Fishman, G. J.; M
Bibcode: 2010HEAD...11.1403N
Altcode: 2010BAAS...42..675N
Plasma instabilities excited in collisionless shocks are responsible for
particle acceleration. We have investigated the particle acceleration
and shock structure associated with an unmagnetized relativistic
electron-positron jet propagating into an unmagnetized electron-positron
plasma. Cold jet electrons are thermalized and slowed while the ambient
electrons are swept up to create a partially developed hydrodynamic-like
shock structure. In the leading shock, electron density increases by
a factor of about 3.5 in the simulation frame. Strong electromagnetic
fields are generated in the trailing shock and provide an emission
site. These magnetic fields contribute to the electrons transverse
deflection behind the shock. We calculate the radiation from deflected
electrons in the turbulent magnetic fields. The properties of this
radiation may be important for understanding the complex time evolution
and/or spectral structure in gamma-ray bursts, relativistic jets in
general, and supernova remnants.
Title: Convection and the Origin of Evershed Flows
Authors: Nordlund, Å.; Scharmer, G. B.
Bibcode: 2010ASSP...19..243N
Altcode: 2009arXiv0905.0918N; 2010mcia.conf..243N
Numerical simulations have by now revealed that the fine scale structure
of the penumbra in general and the Evershed effect in particular is due
to overturning convection, mainly confined to gaps with strongly reduced
magnetic field strength. The Evershed flow is the radial component of
the overturning convective flow visible at the surface. It is directed
outwards - away from the umbra - because of the broken symmetry due
to the inclined magnetic field. The dark penumbral filament cores
visible at high resolution are caused by the "cusps" in the magnetic
field that form above the gaps. Still remaining to be established are
the details of what determines the average luminosity of penumbrae,
the widths, lengths, and filling factors of penumbral filaments, and
the amplitudes and filling factors of the Evershed flow. These are
likely to depend at least partially also on numerical aspects such as
limited resolution and model size, but mainly on physical properties
that have not yet been adequately determined or calibrated, such as
the plasma beta profile inside sunspots at depth and its horizontal
profile, the entropy of ascending flows in the penumbra, etc.
Title: Radiation from Relativistic Shocks with Turbulent Magnetic
Fields
Authors: Nishikawa, K. -I.; Nimiec, J.; Medvedev, M.; Zhang, B.;
Hardee, P.; Mizuno, Y.; Nordlund, Å.; Frederiksen, J.; Sol, H.;
Pohl, M.; Hartmann, D. H.; Oka, M.; Fishman, J. F.
Bibcode: 2010IJMPD..19..715N
Altcode: 2009arXiv0906.5018N
Using our new 3D relativistic electromagnetic particle (REMP) code
parallelized with MPI, we investigated long-term particle acceleration
associated with a relativistic electron-positron jet propagating in an
unmagnetized ambient electron-positron plasma. We have also performed
simulations with electron-ion jets. The simulations were performed using
a much longer simulation system than our previous simulations in order
to investigate the full nonlinear stage of the Weibel instability for
electron-positron jets and its particle acceleration mechanism. Cold
jet electrons are thermalized and ambient electrons are accelerated
in the resulting shocks for pair plasma case. Acceleration of ambient
electrons leads to a maximum ambient electron density three times larger
than the original value for pair plasmas. Behind the bow shock in the
jet shock strong electromagnetic fields are generated. These fields may
lead to time-dependent afterglow emission. We calculated radiation from
electrons propagating in a uniform parallel magnetic field to verify
the technique. We also used the new technique to calculate emission
from electrons based on simulations with a small system with two
different cases for Lorentz factors (15 and 100). We obtained spectra
which are consistent with those generated from electrons propagating
in turbulent magnetic fields with red noise. This turbulent magnetic
field is similar to the magnetic field generated at an early nonlinear
stage of the Weibel instability.
Title: Supergranulation-Scale Convection Simulations
Authors: Stein, R. F.; Nordlund, Å.; Georgoviani, D.; Benson, D.;
Schaffenberger, W.
Bibcode: 2009ASPC..416..421S
Altcode:
Results of realistic simulations of solar surface convection on the
scale of supergranules (96 Mm wide by 20 Mm deep) are presented. The
simulations cover only 10% of the geometric depth of the solar
convection zone, but half its pressure scale heights. They include the
hydrogen ionization zone, and the first and most of the second helium
ionization zones. The horizontal velocity spectrum is a power law,
and the horizontal size of the dominant convective cells increases
with increasing depth. Convection is driven by buoyancy work, which
is largest close to the surface, but significant over the entire
domain. Close to the surface, buoyancy driving is balanced by the
divergence of the kinetic energy flux, but deeper down it is balanced
by dissipation. The damping length of the turbulent kinetic energy
is 4 pressure scale heights. The mass mixing length is 1.8 scale
heights. Two thirds of the area is upflowing fluid except very close
to the surface. The internal (ionization) energy flux is the largest
contributor to the convective flux for temperatures less than 40,000
K and the thermal energy flux is the largest contributor at higher
temperatures. This data set is useful for validating local helioseismic
inversion methods. Sixteen hours of data are available as four hour
averages, with two hour cadence, at steinr.msu.edu/~bob/96averages,
as idl save files. The variables stored are the density, temperature,
sound speed, and three velocity components. In addition, the three
velocity components at 200 km above mean continuum optical depth unity
are available at 30 second cadence.
Title: Solar Magneto-Convection Simulations of Emergin Flux
Authors: Stein, R. F.; Lagerfjard, A.; Nordlund, A.; Geogobiani, D.;
Benson, D.
Bibcode: 2009AGUFMSH11B..05S
Altcode:
We present preliminary results of magneto-convection simulations
of the rise of initially horizontal magnetic flux from 20 Mm deep
through the solar surface in a domain 48 Mm wide. The magnetic field
is advected upward by the diverging upflows and pulled down in the
downdrafts which produces a hierarchy of loop like structures, of
increasingly smaller scale as the surface is approached. Stronger
fields rise faster due to magnetic buoyancy (lower density in the
strong field region). Slow, large scale, diverging motions sweep the
magnetic field to the boundaries of supergranular like structures to
form a magnetic network. The field strength varies with depth as the
cube root of the density.
Title: Comparing the Hinode and SOHO/MDI Data to the Simulated Large
Scale Solar Convection
Authors: Georgobiani, D.; Zhao, J.; Kosovichev, A.; Benson, D.; Stein,
R. F.; Nordlund, Å.
Bibcode: 2009ASPC..415..421G
Altcode:
Large-scale simulations of solar turbulent convection produce realistic
data and provide a unique opportunity to study solar oscillations
and test various techniques commonly used for the analysis of solar
observations. We applied helioseismic methods to the sets of simulated
as well as observed data and find remarkable similarities. Power
spectra, k-ν diagrams, time-distance diagrams exhibit similar details,
although sometimes subtle differences are present.
Title: Supergranulation Scale Convection Simulations
Authors: Stein, R. F.; Lagerfjård, A.; Nordlund, Å.; Georgobiani,
D.; Benson, D.; Schaffenberger, W.
Bibcode: 2009ASPC..415...63S
Altcode:
Results of realistic simulations of solar surface convection on
the scale of supergranules (48 and 96 Mm wide by 20 Mm deep) are
presented. The simulations include the hydrogen, first and most
of the second helium ionization zones. Horizontal magnetic field is
advected into the domain by upflows at the bottom. Upflows stretch the
field lines upward, while downflows push them down, thus producing
loop like magnetic structures. The mass mixing length is 1.8 scale
heights. Two thirds of the area is upflowing fluid except very close
to the surface. The internal (ionization) energy flux is the largest
contributor to the convective flux for temperatures less than 40,000
K and the thermal energy flux is the largest contributor at higher
temperatures. The data is available for evaluating local helioseismic
procedures.
Title: Solar Surface Convection
Authors: Nordlund, Åke; Stein, Robert F.; Asplund, Martin
Bibcode: 2009LRSP....6....2N
Altcode:
We review the properties of solar convection that are directly
observable at the solar surface, and discuss the relevant underlying
physics, concentrating mostly on a range of depths from the temperature
minimum down to about 20 Mm below the visible solar surface.
Title: The Super-Alfvénic Model of Molecular Clouds: Predictions
for Mass-to-Flux and Turbulent-to-Magnetic Energy Ratios
Authors: Lunttila, Tuomas; Padoan, Paolo; Juvela, Mika; Nordlund, Åke
Bibcode: 2009ApJ...702L..37L
Altcode: 2009arXiv0907.0587L
Recent measurements of the Zeeman effect in dark-cloud cores provide
important tests for theories of cloud dynamics and prestellar core
formation. In this Letter, we report results of simulated Zeeman
measurements, based on radiative transfer calculations through
a snapshot of a simulation of supersonic and super-Alfvénic
turbulence. We have previously shown that the same simulation yields
a relative mass-to-flux ratio (core versus envelope) in agreement
with the observations (and in contradiction with the ambipolar-drift
model of core formation). Here, we show that the mass-to-flux and
turbulent-to-magnetic-energy ratios in the simulated cores agree
with the observed values as well. The mean magnetic field strength in
the simulation is very low, \bar{B}=0.34 μG, presumably lower than
the mean field in molecular clouds. Nonetheless, high magnetic field
values are found in dense cores, in agreement with the observations
(the rms field, amplified by the turbulence, is B rms =
3.05 μG). We conclude that a strong large-scale mean magnetic field
is not required by Zeeman effect measurements to date, although it is
not ruled out by this work.
Title: Accurate Radiation Hydrodynamics and MHD Modeling of 3-D
Stellar Atmospheres
Authors: Nordlund, Å.; Stein, R. F.
Bibcode: 2009AIPC.1171..242N
Altcode:
Stellar atmospheres provide a unique and valuable testing ground
for radiation hydrodynamics and MHD. Spectral line synthesis based
on reasonably affordable 3-D models can potentially reach very high
accuracy, with widths, strengths, and shapes of photospheric spectral
lines matching observations to within fractions of a percent, with ``no
free parameters'' i.e., using only the effective temperature, surface
acceleration of gravity, and element abundances as input parameters,
and without the need for artificial fitting parameters such as micro-
and macro-turbulence. When combined with accurate atomic parameters
the results can be used to determine the abundance of individual
chemical elements more accurately than was possible in the past,
when spectral line synthesis was based on one-dimensional modeling
and artificial fitting parameters. A necessary condition for reaching
the desired accuracy is that the radiative energy transfer in the
photosphere is treated with sufficient accuracy. Since at different
levels in stellar atmospheres different wavelength regions dominate
the energy exchange between the gas and the radiation field this is
a non-trivial and potentially very computer intensive problem. We
review the computationally efficient methods that are being used to
achieve accurate solutions to this problem, addressing in particular
the relation to the solar ``oxygen abundance problem.'' In this context
we also briefly comment on ``look-alike'' radiative transfer methods
such as Flux Limited Diffusion.
Title: Solar Twins and Possible Solutions of the Solar and Jupiter
Abundance Problems
Authors: Nordlund, A.
Bibcode: 2009arXiv0908.3479N
Altcode:
Implications of the recently discovered systematic abundance difference
between the Sun and two collections of `solar twins' are discussed. The
differences can be understood as an imprint on the abundances of the
solar convection zone caused by the lock-up of heavy elements in the
planets. Such a scenario also leads naturally to possible solutions
of two other abundance peculiarities; 1) the discrepancy between
photospheric abundances derived from accurate 3-D models of the solar
photosphere and the abundance of heavy elements in the solar interior
deduced from helioseismology, and 2) the abundance pattern of Jupiter,
which can either--with great difficulty--be interpreted as a general
and similar overabundance of both common elements such as carbon,
nitrogen and sulphur and rare inert gases such as argon, krypton and
xenon, or--much more simply--as an under-abundance of hydrogen.
Title: Solar Magneto-Convection Simulations
Authors: Stein, Robert F.; Lagerfjard, A.; Nordlund, A.; Benson, D.;
Georgobiani, D.; Schaffenberger, W.
Bibcode: 2009SPD....40.0401S
Altcode:
We present preliminary results of magneto-convection simulations
of the rise of initially horizontal magnetic flux from 20 Mm deep
through the solar surface in a domain 48 Mm wide. The magnetic field
is stretched upward in the diverging upflows and pulled down in the
downdrafts which produces a hierarchy of loop like structures. The
strength varies with depth as the square root of the density. The field
is swept to the boundaries of small supergranular like structures to
form a magnetic network.
Title: Simulated Large Scale Solar Convection Versus Observations:
A Multiwavelength Approach
Authors: Georgobiani, Dali; Zhao, J.; Kosovichev, A. G.; Benson, D.;
Stein, R. F.; Nordlund, A.
Bibcode: 2009SPD....40.0301G
Altcode:
The realistic 3D radiative-hydrodynamic simulations of the upper layers
of solar convection provide a perfect opportunity to validate various
techniques, widely used in solar physics and helioseismology. Our
aim is to perform multiwavelength analysis of large scale flows. We
analyze the simulated intensity and velocities at certain heights
in the solar atmosphere, and compare our results with the outcome
of the similar analysis of the SOHO/MDI and Hinode observations. To
fine tune the comparison, we use the instrumental response functions
to weigh the simulated parameters at different heights to emulate
the observational lines. We find the remarkable similarity between
the simulated and observed power spectra, their spatial parts, and
time-distance diagrams. This demonstrates one more time that the
simulations can be efficiently used to perform and validate local
helioseismology techniques, and to study solar flows and structures
beneath the surface, inaccessible for direct observations.
Title: Radiation from relativistic jets in turbulent magnetic fields
Authors: Nishikawa, K. -I.; Medvedev, M.; Zhang, B.; Hardee, P.;
Niemiec, J.; Nordlund, Å.; Frederiksen, J.; Mizuno, Y.; Sol, H.;
Fishman, G. J.
Bibcode: 2009AIPC.1133..235N
Altcode: 2009arXiv0901.4058N
Using our new 3-D relativistic electromagnetic particle (REMP)
code parallelized with MPI, we have investigated long-term particle
acceleration associated with an relativistic electron-positron jet
propagating in an unmagnetized ambient electron-positron plasma. The
simulations have been performed using a much longer simulation
system than our previous simulations in order to investigate the full
nonlinear stage of the Weibel instability and its particle acceleration
mechanism. Cold jet electrons are thermalized and ambient electrons
are accelerated in the resulting shocks. The acceleration of ambient
electrons leads to a maximum ambient electron density three times
larger than the original value. Behind the bow shock in the jet shock
strong electromagnetic fields are generated. These fields may lead to
the afterglow emission. We have calculated the time evolution of the
spectrum from two electrons propagating in a uniform parallel magnetic
field to verify the technique.
Title: Coupling from the Photosphere to the Chromosphere and the
Corona
Authors: Wedemeyer-Böhm, S.; Lagg, A.; Nordlund, Å.
Bibcode: 2009SSRv..144..317W
Altcode: 2008SSRv..tmp..171W; 2008arXiv0809.0987W
The atmosphere of the Sun is characterized by a complex interplay of
competing physical processes: convection, radiation, conduction, and
magnetic fields. The most obvious imprint of the solar convection
and its overshooting in the low atmosphere is the granulation
pattern. Beside this dominating scale there is a more or less smooth
distribution of spatial scales, both towards smaller and larger
scales, making the Sun essentially a multi-scale object. Convection and
overshooting give the photosphere its face but also act as drivers for
the layers above, namely the chromosphere and corona. The magnetic field
configuration effectively couples the atmospheric layers on a multitude
of spatial scales, for instance in the form of loops that are anchored
in the convection zone and continue through the atmosphere up into
the chromosphere and corona. The magnetic field is also an important
structuring agent for the small, granulation-size scales, although
(hydrodynamic) shock waves also play an important role—especially in
the internetwork atmosphere where mostly weak fields prevail. Based on
recent results from observations and numerical simulations, we attempt
to present a comprehensive picture of the atmosphere of the quiet Sun
as a highly intermittent and dynamic system.
Title: Supergranulation Scale Convection Simulations
Authors: Stein, Robert F.; Georgobiani, Dali; Schafenberger, Werner;
Nordlund, Åke; Benson, David
Bibcode: 2009AIPC.1094..764S
Altcode: 2009csss...15..764S
Results of realistic simulations of solar surface convection on the
scale of supergranules (96 Mm wide by 20 Mm deep) are presented. The
simulations cover only 10% of the geometric depth of the solar
convection zone, but half its pressure scale heights. They include the
hydrogen, first and most of the second helium ionization zones. The
horizontal velocity spectrum is a power law and the horizontal
size of the dominant convective cells increases with increasing
depth. Convection is driven by buoyancy work which is largest close
to the surface, but significant over the entire domain. Close to the
surface buoyancy driving is balanced by the divergence of the kinetic
energy flux, but deeper down it is balanced by dissipation. The
damping length of the turbulent kinetic energy is 4 pressure scale
heights. The mass mixing length is 1.8 scale heights. Two thirds of the
area is upflowing fluid except very close to the surface. The internal
(ionization) energy flux is the largest contributor to the convective
flux for temperatures less than 40,000 K and the thermal energy flux
is the largest contributor at higher temperatures.
Title: Microscpic Processes On Radiation From Accelerated Particles
In Relativiatic Jets
Authors: Nishikawa, Ken-Ichi; Hardee, P.; Niemiec, J.; Mizuno, Y.;
Medvedev, M.; Zhang, B.; Sol, H.; Nordlund, A.; Freddriksen, J.;
Lyubarsky, Y.; Hartmann, D.; Fishman, J.
Bibcode: 2009AAS...21332608N
Altcode: 2009BAAS...41..383N
Nonthermal radiation observed from astrophysical systems containing
relativistic jets and shocks, e.g., gamma-ray bursts (GRBs),
active galactic nuclei (AGNs), and Galactic microquasar systems
usually have power-law emission spectra. Recent PIC simulations of
relativistic electron-ion (electro-positron) jets injected into a
stationary medium show that particle acceleration occurs within
the downstream jet. In the collisionless relativistic shock particle
acceleration is due to plasma waves and their associated instabilities
(e.g., the Buneman instability, other two-streaming instability,
and the Weibel (filamentation) instability) created in the
shocks are responsible for particle (electron, positron, and ion)
acceleration. The simulation results show that the Weibel instability
is responsible for generating and amplifying highly nonuniform,
small-scale magnetic fields. These magnetic fields contribute to the
electron's transverse deflection behind the jet head. The ``jitter''
radiation from deflected electrons has different properties than
synchrotron radiation which is calculated in a uniform magnetic
field. This jitter radiation may be important to understanding the
complex time evolution and/or spectral structure in gamma-ray bursts,
relativistic jets, and supernova remnants.
Title: Abundance analysis of the halo giant HD 122563 with
three-dimensional model stellar atmospheres
Authors: Collet, R.; Nordlund, Å.; Asplund, M.; Hayek, W.;
Trampedach, R.
Bibcode: 2009MmSAI..80..719C
Altcode: 2009arXiv0909.0690C
We present a preliminary local thermodynamic equilibrium (LTE) abundance
analysis of the template halo red giant HD122563 based on a realistic,
three-dimensional (3D), time-dependent, hydrodynamical model atmosphere
of the very metal-poor star. We compare the results of the 3D analysis
with the abundances derived by means of a standard LTE analysis based
on a classical, 1D, hydrostatic model atmosphere of the star. Due to
the different upper photospheric temperature stratifications predicted
by 1D and 3D models, we find large, negative, 3D-1D LTE abundance
differences for low-excitation OH and Fe I lines. We also find trends
with lower excitation potential in the derived Fe LTE abundances
from Fe I lines, in both the 1D and 3D analyses. Such trends may be
attributed to the neglected departures from LTE in the spectral line
formation calculations.
Title: The Origin and Dynamics of Solar Magnetism
Authors: Thompson, M. J.; Balogh, A.; Culhane, J. L.; Nordlund, Å.;
Solanki, S. K.; Zahn, J. -P.
Bibcode: 2009odsm.book.....T
Altcode:
No abstract at ADS
Title: Solar Dynamo and Magnetic Self-Organization
Authors: Kosovichev, A. G.; Arlt, R.; Bonanno, A.; Brandenburg,
A.; Brun, A. S.; Busse, F.; Dikpati, M.; Hill, F.; Gilman, P. A.;
Nordlund, A.; Ruediger, G.; Stein, R. F.; Sekii, T.; Stenflo, J. O.;
Ulrich, R. K.; Zhao, J.
Bibcode: 2009astro2010S.160K
Altcode:
No abstract at ADS
Title: The formation of distributed and clustered stars in molecular
clouds
Authors: Megeath, S. T.; Li, Z. -Y.; Nordlund, Å.
Bibcode: 2009sfa..book..228M
Altcode: 2008arXiv0801.0492M
During the last two decades, the focus of star formation research has
shifted from understanding the collapse of a single dense core into
a star to studying the formation hundreds to thousands of stars in
molecular clouds. In this chapter, we overview recent observational
and theoretical progress toward understanding star formation on the
scale of molecular clouds and complexes, i.e the macrophysics of
star formation. We begin with an overview of recent surveys of young
stellar objects (YSOs) in molecular clouds and embedded clusters,
and we outline an emerging picture of cluster formation. We then
discuss the role of turbulence to both support clouds and create dense,
gravitationally unstable structures, with an emphasis on the role of
magnetic fields (in the case of distributed stars) and feedback (in
the case of clusters) to slow turbulent decay and mediate the rate and
density of star formation. The discussion is followed by an overview
of how gravity and turbulence may produce observed scaling laws for
the properties of molecular clouds, stars and star clusters, and how
the observed, low star formation rate may result from self regulated
star formation. We end with some concluding remarks, including a number
of questions to be addressed by future observations and simulations.
Title: Coupling from the Photosphere to the Chromosphere and the
Corona
Authors: Wedemeyer-Böhm, S.; Lagg, A.; Nordlund, Å.
Bibcode: 2009odsm.book..317W
Altcode:
The atmosphere of the Sun is characterized by a complex interplay of
competing physical processes: convection, radiation, conduction, and
magnetic fields. The most obvious imprint of the solar convection
and its overshooting in the low atmosphere is the granulation
pattern. Beside this dominating scale there is a more or less smooth
distribution of spatial scales, both towards smaller and larger
scales, making the Sun essentially a multi-scale object. Convection and
overshooting give the photosphere its face but also act as drivers for
the layers above, namely the chromosphere and corona. The magnetic field
configuration effectively couples the atmospheric layers on a multitude
of spatial scales, for instance in the form of loops that are anchored
in the convection zone and continue through the atmosphere up into
the chromosphere and corona. The magnetic field is also an important
structuring agent for the small, granulation-size scales, although
(hydrodynamic) shock waves also play an important role—especially in
the internetwork atmosphere where mostly weak fields prevail. Based on
recent results from observations and numerical simulations, we attempt
to present a comprehensive picture of the atmosphere of the quiet Sun
as a highly intermittent and dynamic system.
Title: New Relativistic Particle-In-Cell Simulation Studies of Prompt
and Early Afterglows from GRBs
Authors: Nishikawa, K. -I.; Niemiec, J.; Sol, H.; Medvedev, M.; Zhang,
B.; Nordlund, Å.; Frederiksen, J.; Hardee, P.; Mizuno, Y.; Hartmann,
D. H.; Fishman, G. J.
Bibcode: 2008AIPC.1085..589N
Altcode: 2008arXiv0809.5067N
Nonthermal radiation observed from astrophysical systems containing
relativistic jets and shocks, e.g., gamma-ray bursts (GRBs), active
galactic nuclei (AGNs), and microquasars commonly exhibit power-law
emission spectra. Recent PIC simulations of relativistic electron-ion
(or electron-positron) jets injected into a stationary medium show
that particle acceleration occurs within the downstream jet. In
collisionless, relativistic shocks, particle (electron, positron,
and ion) acceleration is due to plasma waves and their associated
instabilities (e.g., the Weibel (filamentation) instability) created
in the shock region. The simulations show that the Weibel instability
is responsible for generating and amplifying highly nonuniform,
small-scale magnetic fields. These fields contribute to the electron's
transverse deflection behind the jet head. The resulting ``jitter''
radiation from deflected electrons has different properties compared to
synchrotron radiation, which assumes a uniform magnetic field. Jitter
radiation may be important for understanding the complex time evolution
and/or spectra in gamma-ray bursts, relativistic jets in general,
and supernova remnants.
Title: Stellar (magneto-)convection
Authors: Nordlund, Å.
Bibcode: 2008PhST..133a4002N
Altcode:
The importance of convection as an energy transport mechanism, as an
agent in the generation of stellar magnetic fields, and as a driver
of chromospheric and corona activity in stars is discussed. The
multiscale nature of stellar convection is illustrated with results
from simulations and with observations of the Sun, showing that
there is a hierarchy of velocity patterns in the solar convection
zone, directly visible in the horizontal velocity field at the solar
surface. The velocity amplitude spectrum is smooth, with amplitudes
decaying approximately inversely proportionally to the horizontal scale,
and with at most a minor enhancement at super-granulation scales. The
multiscale (magneto-)convection is in a generalized sense the driver
of all chromospheric and coronal activity. One of the more striking
aspects of solar magneto-convection is the creation of sunspots; the
modelling of sunspots and their structural details are also discussed.
Title: Supergranulation Scale Connection Simulations
Authors: Stein, R. F.; Nordlund, A.; Georgobiani, D.; Benson, D.;
Schaffenberger, W.
Bibcode: 2008arXiv0811.0472S
Altcode:
Results of realistic simulations of solar surface convection on the
scale of supergranules (96 Mm wide by 20 Mm deep) are presented. The
simulations cover only 10% of the geometric depth of the solar
convection zone, but half its pressure scale heights. They include the
hydrogen, first and most of the second helium ionization zones. The
horizontal velocity spectrum is a power law and the horizontal
size of the dominant convective cells increases with increasing
depth. Convection is driven by buoyancy work which is largest close
to the surface, but significant over the entire domain. Close to the
surface buoyancy driving is balanced by the divergence of the kinetic
energy flux, but deeper down it is balanced by dissipation. The
damping length of the turbulent kinetic energy is 4 pressure scale
heights. The mass mixing length is 1.8 scale heights. Two thirds of the
area is upflowing fluid except very close to the surface. The internal
(ionization) energy flux is the largest contributor to the convective
flux for temperatures less than 40,000 K and the thermal energy flux is
the largest contributor at higher temperatures. This data set is useful
for validating local helioseismic inversion methods. Sixteen hours
of data are available as four hour averages, with two hour cadence,
at steinr.msu.edu/~bob/96averages, as idl save files. The variables
stored are the density, temperature, sound speed, and three velocity
components. In addition, the three velocity components at 200 km above
mean continuum optical depth unity are available at 30 sec. cadence.
Title: The Super-Alfvénic Model of Molecular Clouds: Predictions
for Zeeman Splitting Measurements
Authors: Lunttila, Tuomas; Padoan, Paolo; Juvela, Mika; Nordlund, Åke
Bibcode: 2008ApJ...686L..91L
Altcode: 2008arXiv0806.3854L
We present synthetic OH Zeeman splitting measurements of a
super-Alfvénic molecular cloud model. We select dense cores from
synthetic13CO maps computed from the largest simulation
to date of supersonic and super-Alfvénic turbulence. The synthetic
Zeeman splitting measurements in the cores yield a relation between the
magnetic field strength, B, and the column density, N, in good agreement
with the observations. The large scatter in B at a fixed value of N is
partly due to intrinsic variations in the magnetic field strength from
core to core. We also compute the relative mass-to-flux ratio between
the center of the cores and their envelopes, \mathstrutR} μ
, and show that super-Alfvénic turbulence produces a significant
scatter also in \mathstrutR} μ , including negative values
(field reversal between core center and envelope). We find \mathstrutR}
μ < 1 for 70% of the cores, and \mathstrutR} μ
< 0 for 12%. Of the cores with | BLOS| > 10
μG, 81% have \mathstrutR} μ < 1. These predictions
of the super-Alfvénic model are in stark contrast to the ambipolar
drift model of core formation, where only \mathstrutR} μ
> 1 is allowed.
Title: Trans-Debye Scale Plasma Modeling & Stochastic GRB
Wakefield Plasma Processes
Authors: Frederiksen, Jacob Trier; Haugbølle, Troels; Nordlund, Åke
Bibcode: 2008AIPC.1054...87F
Altcode: 2008arXiv0808.0710T
Modeling plasma physical processes in astrophysical context demands
for both detailed kinetics and large scale development of the
electromagnetic field densities. We present a new framework for
modeling plasma physics of hot tenuous plasmas by a two-split scheme, in
which the large scale fields are modeled by means of a particle-in-cell
(PIC) code, and in which binary collision processes and single-particle
processes are modeled through a Monte-Carlo approach. Our novel
simulation tool-the PHOTONPLASMA code-is a unique hybrid model; it
combines a highly parallelized (Vlasov) particle-in-cell approach
with continuous weighting of particles and a sub-Debye Monte-Carlo
binary particle interaction framework. As an illustration of the
capabilities we present results from a numerical study [1] of gamma-ray
burst-circumburst medium interaction and plasma preconditioning via
Compton scattering. We argue that important microphysical processes
can only viably be investigated by means of such ``trans-Debye scale''
hybrid codes. Our first results from 3D simulations with this
new simulation tool suggest that magnetic fields and plasma filaments
are created in the wakefield of prompt gamma-ray bursts. Furthermore,
the photon flux density gradient impacts on particle acceleration in
the burst head and wakefield. We discuss some possible implications of
the circumburst medium being preconditioned for a trailing afterglow
shock front. We also discuss important improvements for future studies
of GRB wakefields processes, using the PHOTONPLASMA code.
Title: A grid of MARCS model atmospheres for late-type
stars. I. Methods and general properties
Authors: Gustafsson, B.; Edvardsson, B.; Eriksson, K.; Jørgensen,
U. G.; Nordlund, Å.; Plez, B.
Bibcode: 2008A&A...486..951G
Altcode: 2008arXiv0805.0554G
Context: In analyses of stellar spectra and colours, and for the
analysis of integrated light from galaxies, a homogeneous grid
of model atmospheres of late-type stars and corresponding flux
spectra is needed.
Aims: We construct an extensive grid of
spherically-symmetric models (supplemented with plane-parallel ones
for the highest surface gravities), built on up-to-date atomic and
molecular data, and make it available for public use.
Methods:
The most recent version of the MARCS program is used.
Results:
We present a grid of about 104 model atmospheres for stars
with 2500 K ≤ T_eff ≤ 8000 K, -1 ≤ log g = log (GM/R^2) ≤ 5
(cgs) with various masses and radii, -5 ≤ [Me/H] ≤ +1, with [
α/Fe] = 0.0 and 0.4 and different choices of C and N abundances. This
includes “CN-cycled” models with C/N = 4.07 (solar), 1.5 and 0.5,
C/O ranging from 0.09 to (normally) 5.0 to also represent stars of
spectral types R, S and N, and with 1.0 ≤ ξt ≤ 5 km
s-1. We also list thermodynamic quantities (T, P_g, P_e, ρ,
partial pressures of molecules, etc.) and provide them on the World Wide
Web, as well as calculated fluxes in approximately 108 000 wavelength
points. Underlying assumptions in addition to 1D stratification
(spherical or plane-parallel) include hydrostatic equilibrium,
mixing-length convection and local thermodynamic equilibrium. We
discuss a number of general properties of the models, in particular
in relation to the effects of changing abundances, of blanketing, and
of sphericity. We illustrate positive and negative feedbacks between
sphericity and molecular blanketing. We compare the models with those of
other available grids and find excellent agreement with plane-parallel
models of Castelli & Kurucz (if convection is treated consistently)
within the overlapping parameter range. Although there are considerable
departures from the spherically-symmetric NextGen models, the agreement
with more recent PHOENIX models is gratifying.
Conclusions: The
models of the grid show considerable regularities, but some interesting
departures from general patterns occur for the coolest models due
to the molecular opacities. We have tested a number of approximate
“rules of thumb” concerning effects of blanketing and sphericity
and often found them to be astonishingly accurate. Some interesting
new phenomena have been discovered and explored, such as the intricate
coupling between blanketing and sphericity, and the strong effects of
carbon enhancement on metal-poor models. We give further details of
line absorption data for molecules, as well as details of models and
comparisons with observations in subsequent papers.
Title: Convection and the Origin of Evershed Flows in Sunspot
Penumbrae
Authors: Scharmer, G. B.; Nordlund, Å.; Heinemann, T.
Bibcode: 2008ApJ...677L.149S
Altcode: 2008arXiv0802.1927S
We discuss a numerical 3D radiation-MHD simulation of penumbral fine
structure in a small sunspot. This simulation shows the development of
short filamentary structures with horizontal flows, similar to observed
Evershed flows, and an inward propagation of these structures at a
speed compatible with observations. Although the lengths of these
filaments are much shorter than observed, we conjecture that this
simulation qualitatively reproduces the mechanisms responsible for
filament formation and Evershed flows in penumbrae. We conclude that the
Evershed flow represents the horizontal-flow component of overturning
convection in gaps with strongly reduced field strength. The top of
the flow is always directed outward—away from the umbra—because of
the broken symmetry due to the inclined magnetic field. Upflows occur
in the inner parts of the gaps and most of the gas turns over radially
(outward and sideways), and descends back down again. The ascending,
cooling, and overturning flow tends to bend magnetic field lines down,
forcing a weakening of the field that makes it easier for gas located
in an adjacent layer—farther in—to initiate a similar sequence of
motion, aided by lateral heating, thus causing the inward propagation
of the filament.
Title: Radiation from relativistic jets
Authors: Nishikawa, K. I.; Mizuno, Y.; Hardee, P.; Sol, H.; Medvedev,
M.; Zhang, B.; Nordlund, Å.; Frederiksen, J. T.; Fishman, G. J.;
Preece, R.
Bibcode: 2008bves.confE..53N
Altcode: 2008arXiv0808.3781N; 2008PoS....63E..53N
Nonthermal radiation observed from astrophysical systems containing
relativistic jets and shocks, e.g., gamma-ray bursts (GRBs), active
galactic nuclei (AGNs), and Galactic microquasar systems usually have
power-law emission spectra. Recent PIC simulations of relativistic
electron-ion (electron-positron) jets injected into a stationary
medium show that particle acceleration occurs within the downstream
jet. In the presence of relativistic jets, instabilities such as
the Buneman instability, other two-streaming instability, and the
Weibel (filamentation) instability create colli- sionless shocks,
which are responsible for particle (electron, positron, and ion)
acceleration. The simulation results show that the Weibel instability
is responsible for generating and amplifying highly nonuniform,
small-scale magnetic fields. These magnetic fields contribute to the
electron's transverse deflection behind the jet head. The "jitter"
radiation from deflected electrons in small- scale magnetic fields has
different properties than synchrotron radiation which is calculated in
a uniform magnetic field. This jitter radiation, a case of diffusive
synchrotron radiation, may be im- portant to understand the complex time
evolution and/or spectral structure in gamma-ray bursts, relativistic
jets, and supernova remnants.
Title: Surface Convection
Authors: Stein, Robert F.; Benson, David; Georgobiani, Dali; Nordlund,
Åke; Schaffenberger, Werner
Bibcode: 2007AIPC..948..111S
Altcode:
What are supergranules? Why do they stand out? Preliminary results from
realistic simulations of solar convection on supergranule scales (96 Mm
wide by 20 Mm deep) are presented. The solar surface velocity amplitude
is a decreasing power law from the scale of granules up to giant cells
with a slight enhancement at supergranule scales. The simulations show
that the size of the horizontal convective cells increases gradually
and continuously with increasing depth. Without magnetic fields
present there is, as yet, no enhancement at supergranule scales at the
surface. A hypothesis is presented that it is the balance between the
rate of magnetic flux emergence and the horizontal sweeping of magnetic
flux by convective motions that determines the size of the magnetic
network and produces the extra power at supergranulation scales.
Title: MHD Simulations of Penumbra Fine Structure
Authors: Heinemann, T.; Nordlund, Å.; Scharmer, G. B.; Spruit, H. C.
Bibcode: 2007ApJ...669.1390H
Altcode: 2006astro.ph.12648H
We present the results of numerical 3D magnetohydrodynamic (MHD)
simulations with radiative energy transfer of fine structure in a small
sunspot of about 4 Mm width. The simulations show the development
of filamentary structures and flow patterns that are, except for
the lengths of the filaments, very similar to those observed. The
filamentary structures consist of gaps with reduced field strength
relative to their surroundings. Calculated synthetic images show dark
cores like those seen in the observations; the dark cores are the result
of a locally elevated τ=1 surface. The magnetic field in these cores
is weaker and more horizontal than for adjacent brighter structures,
and the cores support a systematic outflow. Accompanying animations
show the migration of the dark-cored structures toward the umbra,
and fragments of magnetic flux that are carried away from the spot by
a large-scale ``moat flow.'' We conclude that the simulations are in
qualitative agreement with observed penumbra filamentary structures,
Evershed flows, and moving magnetic features.
Title: Helioseismic Holography of Simulated Solar Convection and
Prospects for the Detection of Small-Scale Subsurface Flows
Authors: Braun, D. C.; Birch, A. C.; Benson, D.; Stein, R. F.;
Nordlund, Å.
Bibcode: 2007ApJ...669.1395B
Altcode: 2007arXiv0708.0214B
We perform helioseismic holography on realistic solar convection
simulations and compare the observed travel-time perturbations
with the expected travel times from the horizontal flows in the
simulations computed from forward models under the assumption of
the Born approximation. We demonstrate reasonable agreement between
the observed and model travel times, which reinforces the validity
of helioseismic holography in the detection of subsurface horizontal
flows. An assessment is made of the uncertainty of the measured p-mode
travel times from the rms of the residuals. From the variation of the
signal-to-noise ratio with depth we conclude that the helioseismic
detection of individual flow structures with spatial scales of
supergranulation or smaller is not possible for depths below about 5 Mm
below the surface over timescales of less than a day. The travel-time
noise estimated from these simulations appears to be similar to noise
in measurements made using solar observations. We therefore suggest
that similar limitations exist regarding the detection of analogous
subsurface flows in the Sun. A study of the depth dependence of
the contribution to the travel-time perturbations for focus depths
between 3 and 7 Mm is made, showing that approximately half of the
observed signal originates within the first 2 Mm below the surface. A
consequence of this is a rapid decrease (and reversal in some cases)
of the travel-time perturbations with depth due to the contribution to
the measurements of oppositely directed surface flows in neighboring
convective cells. This confirms an earlier interpretation of similar
effects reported from observations of supergranulation.
Title: Solar Magneto-Convection Simulations
Authors: Stein, R. F.; Benson, D.; Nordlund, A.
Bibcode: 2007ASPC..369...87S
Altcode:
We review recent realistic simulations of solar surface
magneto-convection in small meso-granule scale Cartesian domains
and global scale interior magneto-convection in spherical
shells. Implications for the solar dynamo are also discussed.
Title: Nonlinear MHD dynamo operating at equipartition
Authors: Archontis, V.; Dorch, S. B. F.; Nordlund, Å.
Bibcode: 2007A&A...472..715A
Altcode:
Context: We present results from non linear MHD dynamo experiments with
a three-dimensional steady and smooth flow that drives fast dynamo
action in the kinematic regime. In the saturation regime, the system
yields strong magnetic fields, which undergo transitions between an
energy-equipartition and a turbulent state. The generation and evolution
of such strong magnetic fields is relevant for the understanding of
dynamo action that occurs in stars and other astrophysical objects.
Aims: We study the mode of operation of this dynamo, in the linear
and non-linear saturation regimes. We also consider the effect of
varying the magnetic and fluid Reymolds number on the non-linear
behaviour of the system.
Methods: We perform three-dimensional
non-linear MHD simulations and visualization using a high resolution
numerical scheme.
Results: We find that this dynamo has a
high growth rate in the linear regime, and that it can saturate at
a level significantly higher than intermittent turbulent dynamos,
namely at energy equipartition, for high values of the magnetic and
fluid Reynolds numbers. The equipartition solution however does not
remain time-independent during the simulation but exhibits a much
more intricate behaviour than previously thought. There are periods
in time where the solution is smooth and close to energy-equipartition
and others where it becomes turbulent. Similarities and differences in
the way the magnetic field is amplified and sustained for experiments
with varying Reynolds numbers are discussed.
Conclusions: Strong
magnetic fields, in near equipartition, can be generated also by a
non-turbulent dynamo. A striking result is that the saturation state
of this dynamo reveals interesting transitions between turbulent and
laminar states.
Title: Fundamental differences between SPH and grid methods
Authors: Agertz, Oscar; Moore, Ben; Stadel, Joachim; Potter, Doug;
Miniati, Francesco; Read, Justin; Mayer, Lucio; Gawryszczak, Artur;
Kravtsov, Andrey; Nordlund, Åke; Pearce, Frazer; Quilis, Vicent;
Rudd, Douglas; Springel, Volker; Stone, James; Tasker, Elizabeth;
Teyssier, Romain; Wadsley, James; Walder, Rolf
Bibcode: 2007MNRAS.380..963A
Altcode: 2007MNRAS.tmp..726A; 2006astro.ph.10051A
We have carried out a comparison study of hydrodynamical codes by
investigating their performance in modelling interacting multiphase
fluids. The two commonly used techniques of grid and smoothed particle
hydrodynamics (SPH) show striking differences in their ability to
model processes that are fundamentally important across many areas of
astrophysics. Whilst Eulerian grid based methods are able to resolve
and treat important dynamical instabilities, such as Kelvin-Helmholtz
or Rayleigh-Taylor, these processes are poorly or not at all resolved
by existing SPH techniques. We show that the reason for this is that
SPH, at least in its standard implementation, introduces spurious
pressure forces on particles in regions where there are steep density
gradients. This results in a boundary gap of the size of an SPH
smoothing kernel radius over which interactions are severely damped.
Title: Gravitational Collapse in Turbulent Clouds
Authors: Nordlund, Åke
Bibcode: 2007sftn.confE..18N
Altcode:
No abstract at ADS
Title: Three-Dimensional Radiative Hydrodynamics for Disk Stability
Simulations: A Proposed Testing Standard and New Results
Authors: Boley, Aaron C.; Durisen, Richard H.; Nordlund, Åke;
Lord, Jesse
Bibcode: 2007ApJ...665.1254B
Altcode: 2007arXiv0704.2532B
Recent three-dimensional radiative hydrodynamics simulations of
protoplanetary disks report disparate disk behaviors, and these
differences involve the importance of convection to disk cooling, the
dependence of disk cooling on metallicity, and the stability of disks
against fragmentation and clump formation. To guarantee trustworthy
results, a radiative physics algorithm must demonstrate the capability
to handle both the high and low optical depth regimes. We develop a
test suite that can be used to demonstrate an algorithm's ability to
relax to known analytic flux and temperature distributions, to follow a
contracting slab, and to inhibit or permit convection appropriately. We
then show that the radiative algorithm employed by Mejía and Boley
et al. and the algorithm employed by Cai et al. pass these tests with
reasonable accuracy. In addition, we discuss a new algorithm that
couples flux-limited diffusion with vertical rays, we apply the test
suite, and we discuss the results of evolving the Boley et al. disk
with this new routine. Although the outcome is significantly different
in detail with the new algorithm, we obtain the same qualitative
answers. Our disk does not cool fast due to convection, and it is stable
to fragmentation. We find an effective α~10-2. In addition,
transport is dominated by low-order modes.
Title: Two Regimes of Turbulent Fragmentation and the Stellar Initial
Mass Function from Primordial to Present-Day Star Formation
Authors: Padoan, Paolo; Nordlund, Åke; Kritsuk, Alexei G.; Norman,
Michael L.; Li, Pak Shing
Bibcode: 2007ApJ...661..972P
Altcode: 2007astro.ph..1795P
The Padoan and Nordlund model of the stellar initial mass function
(IMF) is derived from low-order statistics of supersonic turbulence,
neglecting gravity (e.g., gravitational fragmentation, accretion,
and merging). In this work, the predictions of that model are tested
using the largest numerical experiments of supersonic hydrodynamic (HD)
and magnetohydrodynamic (MHD) turbulence to date (~10003
computational zones) and three different codes (Enzo, Zeus, and
the Stagger code). The model predicts a power-law distribution for
large masses, related to the turbulence-energy power-spectrum slope
and the shock-jump conditions. This power-law mass distribution is
confirmed by the numerical experiments. The model also predicts a sharp
difference between the HD and MHD regimes, which is recovered in the
experiments as well, implying that the magnetic field, even below
energy equipartition on the large scale, is a crucial component of
the process of turbulent fragmentation. These results suggest that
the stellar IMF of primordial stars may differ from that in later
epochs of star formation, due to differences in both gas temperature
and magnetic field strength. In particular, we find that the IMF of
primordial stars born in turbulent clouds may be narrowly peaked around
a mass of order 10 Msolar, as long as the column density
of such clouds is not much in excess of 1022 cm-2.
Title: Helioseismic Holography of Simulated Solar Convection and
Prospects for the Detection of Small-Scale Subsurface Flows
Authors: Braun, Douglas; Birch, A. C.; Benson, D.; Stein, R. F.;
Nordlund, A.
Bibcode: 2007AAS...210.2201B
Altcode: 2007BAAS...39..124B
We perform helioseismic holography on the solar convection simulations
of Benson, Stein, and Nordlund and compare the observed acoustic
travel-time perturbations with the expected travel times from the
horizontal flows in the simulations computed from forward models under
the assumption of the Born approximation. The agreement between the
observed and model travel times reinforces the validity of helioseismic
holography in the detection of subsurface horizontal flows. However,
from the variation of the signal-to-noise ratio with depth, we conclude
that the local helioseismic detection of individual supergranule-size
(or smaller) flow patterns is not possible for depths below about
5 Mm below the surface over time scales less than a day. We suggest
that similar limitations exist regarding the detection of analogous
subsurface flows in the Sun. We also study the depth dependence of
the contribution to the travel-time perturbations for the simulated
flows. For holography measurements focused down to 7 Mm, we find
that approximately half of the observed signal originates within
the first 2 Mm below the surface. A consequence of this is a a rapid
decrease (and possible reversal) of the travel-time perturbations with
increasing focus depth due to the contribution to the measurements of
oppositely directed surface flows in neighboring convective cells. This
confirms an earlier interpretation of similar effects reported from
holographic analyses of observations of supergranulation. This
work is supported by NASA contracts NNH05CC76C and NNH04CC05C, NSF
grant AST-0406225 , and a subcontract through the HMI project at
Stanford University awarded to NWRA, and by NASA grant NNG04GB92G and
NSF grant AST-0605738 to MSU.
Title: Validating Time-Distance Helioseismology by Use of Realistic
Simulations of Solar Convection
Authors: Zhao, Junwei; Georgobiani, D.; Kosovichev, A. G.; Benson,
D.; Stein, R. F.; Nordlund, A.
Bibcode: 2007AAS...210.2203Z
Altcode: 2007BAAS...39..124Z
Recent progress in realistic simulations of solar convection have
enabled us to evaluate the robustness of solar interior structures
and dynamics obtained by methods of local helioseismology. We
present results of testing the time-distance method using realistic
simulations. By computing acoustic wave propagation time and distance
relations for different depths of the simulated data, we confirm that
acoustic waves propagate into the interior and then turn back to the
photosphere. For the surface gravity waves (f-mode), we calculate
perturbations of their travel times, caused by localized downdrafts,
and demonstrate that the spatial pattern of these perturbations
(representing so-called sensitivity kernels) is similar to the
patterns obtained from the real Sun, displaying characteristic
hyperbolic structures. We then test the time-distance measurements
and inversions by calculating acoustic travel times from a sequence
of vertical velocities at the photosphere of the simulated data, and
inferring a mean 3D flow fields by performing inversion based on the
ray approximation. The inverted horizontal flow fields agree very well
with the simulated data in subsurface areas up to 3 Mm deep, but differ
in deeper areas. These initial tests provide important validation of
time-distance helioseismology measurements of supergranular-scale
convection, illustrate limitations of this technique, and provide
guidance for future improvements.
Title: Application of convection simulations to oscillation excitation
and local helioseismology
Authors: Stein, Robert F.; Benson, David; Georgobiani, Dali; Nordlund,
Åke
Bibcode: 2007IAUS..239..331S
Altcode:
No abstract at ADS
Title: Realistic Solar Convection Simulations
Authors: Stein, Robert F.; Nordlund, A.
Bibcode: 2007AAS...210.2205S
Altcode: 2007BAAS...39..125S
We report on the progress of our supergranule scale realistic solar
convection simulations with horizontal dimensions of 96 Mm and 48 Mm (57
hours) and a depth of 20 Mm. Snapshots are saved at 1 min intervals. The
results from these simulations are available to the community. They
are especially useful for testing local helioseismic techniques as
is reported elsewhere at this meeting. The simulations were performed
on the NASA Advanced Supercomputing Division "Columbia" computer and
was supported by NASA grant NNG04GB92G and NSF grant AST 0605738.
Title: Validation of Time-Distance Helioseismology by Use of Realistic
Simulations of Solar Convection
Authors: Zhao, Junwei; Georgobiani, Dali; Kosovichev, Alexander G.;
Benson, David; Stein, Robert F.; Nordlund, Åke
Bibcode: 2007ApJ...659..848Z
Altcode: 2006astro.ph.12551Z
Recent progress in realistic simulations of solar convection have
given us an unprecedented opportunity to evaluate the robustness of
solar interior structures and dynamics obtained by methods of local
helioseismology. We present results of testing the time-distance
method using realistic simulations. By computing acoustic wave
propagation time and distance relations for different depths of the
simulated data, we confirm that acoustic waves propagate into the
interior and then turn back to the photosphere. This demonstrates
that in numerical simulations properties of acoustic waves (p-modes)
are similar to the solar conditions, and that these properties can be
analyzed by the time-distance technique. For surface gravity waves
(f-modes), we calculate perturbations of their travel times caused
by localized downdrafts and demonstrate that the spatial pattern of
these perturbations (representing so-called sensitivity kernels)
is similar to the patterns obtained from the real Sun, displaying
characteristic hyperbolic structures. We then test time-distance
measurements and inversions by calculating acoustic travel times from
a sequence of vertical velocities at the photosphere of the simulated
data and inferring mean three-dimensional flow fields by performing
inversion based on the ray approximation. The inverted horizontal
flow fields agree very well with the simulated data in subsurface
areas up to 3 Mm deep, but differ in deeper areas. Due to the cross
talk effects between the horizontal divergence and downward flows,
the inverted vertical velocities are significantly different from the
mean convection velocities of the simulation data set. These initial
tests provide important validation of time-distance helioseismology
measurements of supergranular-scale convection, illustrate limitations
of this technique, and provide guidance for future improvements.
Title: Local Helioseismology and Correlation Tracking Analysis of
Surface Structures in Realistic Simulations of Solar Convection
Authors: Georgobiani, Dali; Zhao, Junwei; Kosovichev, Alexander G.;
Benson, David; Stein, Robert F.; Nordlund, Åke
Bibcode: 2007ApJ...657.1157G
Altcode: 2006astro.ph..8204G
We apply time-distance helioseismology, local correlation tracking, and
Fourier spatial-temporal filtering methods to realistic supergranule
scale simulations of solar convection and compare the results with
high-resolution observations from the Solar and Heliospheric Observatory
(SOHO) Michelson Doppler Imager (MDI). Our objective is to investigate
the surface and subsurface convective structures and test helioseismic
measurements. The size and grid of the computational domain are
sufficient to resolve various convective scales from granulation to
supergranulation. The spatial velocity spectrum is approximately a
power law for scales larger than granules, with a continuous decrease
in velocity amplitude with increasing size. Aside from granulation
no special scales exist, although a small enhancement in power at
supergranulation scales can be seen. We calculate the time-distance
diagram for f- and p-modes and show that it is consistent with the SOHO
MDI observations. From the simulation data we calculate travel-time
maps for surface gravity waves (f-mode). We also apply correlation
tracking to the simulated vertical velocity in the photosphere to
calculate the corresponding horizontal flows. We compare both of these
to the actual large-scale (filtered) simulation velocities. All three
methods reveal similar large-scale convective patterns and provide an
initial test of time-distance methods.
Title: Excitation of solar-like oscillations across the HR diagram
Authors: Samadi, R.; Georgobiani, D.; Trampedach, R.; Goupil, M. J.;
Stein, R. F.; Nordlund, Å.
Bibcode: 2007A&A...463..297S
Altcode: 2006astro.ph.11762S
Aims:We extend semi-analytical computations of excitation rates for
solar oscillation modes to those of other solar-like oscillating stars
to compare them with recent observations
Methods: Numerical
3D simulations of surface convective zones of several solar-type
oscillating stars are used to characterize the turbulent spectra
as well as to constrain the convective velocities and turbulent
entropy fluctuations in the uppermost part of the convective zone of
such stars. These constraints, coupled with a theoretical model for
stochastic excitation, provide the rate P at which energy is injected
into the p-modes by turbulent convection. These energy rates are
compared with those derived directly from the 3D simulations.
Results: The excitation rates obtained from the 3D simulations
are systematically lower than those computed from the semi-analytical
excitation model. We find that Pmax, the P maximum, scales as
(L/M)s where s is the slope of the power law and L and M are
the mass and luminosity of the 1D stellar model built consistently
with the associated 3D simulation. The slope is found to depend
significantly on the adopted form of χ_k, the eddy time-correlation;
using a Lorentzian, χ_k^L, results in s=2.6, whereas a Gaussian,
χ_k^G, gives s=3.1. Finally, values of V_max, the maximum in the mode
velocity, are estimated from the computed power laws for P_max and we
find that Vmax increases as (L/M)sv. Comparisons
with the currently available ground-based observations show that the
computations assuming a Lorentzian χk yield a slope, sv,
closer to the observed one than the slope obtained when assuming a
Gaussian. We show that the spatial resolution of the 3D simulations
must be high enough to obtain accurate computed energy rates.
Title: The Formation of Brown Dwarfs: Theory
Authors: Whitworth, A.; Bate, M. R.; Nordlund, Å.; Reipurth, B.;
Zinnecker, H.
Bibcode: 2007prpl.conf..459W
Altcode:
We review five mechanisms for forming brown dwarfs: (1) turbulent
fragmentation of molecular clouds, producing very-low-mass prestellar
cores by shock compression; (2) collapse and fragmentation of more
massive prestellar cores; (3) disk fragmentation; (4) premature ejection
of protostellar embryos from their natal cores; and (5) photoerosion
of pre-existing cores overrun by Hii regions. These mechanisms are
not mutually exclusive. Their relative importance probably depends on
environment, and should be judged by their ability to reproduce the
brown dwarf IMF, the distribution and kinematics of newly formed brown
dwarfs, the binary statistics of brown dwarfs, the ability of brown
dwarfs to retain disks, and hence their ability to sustain accretion and
outflows. This will require more sophisticated numerical modeling than
is presently possible, in particular more realistic initial conditions
and more realistic treatments of radiation transport, angular momentum
transport, and magnetic fields. We discuss the minimum mass for brown
dwarfs, and how brown dwarfs should be distinguished from planets.
Title: The mass distribution of unstable cores in turbulent magnetized
clouds
Authors: Padoan, Paolo; Nordlund, Åke; Kritsuk, Alexei G.; Norman,
Michael L.; Li, Pak Shing
Bibcode: 2007IAUS..237..283P
Altcode:
No abstract at ADS
Title: A Test Suite for 3D Radiative Hydrodynamics Simulations of
Protoplanetary Disks
Authors: Boley, Aaron C.; Durisen, R. H.; Nordlund, A.; Lord, J.
Bibcode: 2006AAS...209.7606B
Altcode: 2006BAAS...38..995B
Radiative hydrodynamics simulations of protoplanetary disks with
different treatments for radiative cooling demonstrate disparate
evolutions (see Durisen et al. 2006, PPV chapter). Some of these
differences include the effects of convection and metallicity on disk
cooling and the susceptibility of the disk to fragmentation. Because a
principal reason for these differences may be the treatment of radiative
cooling, the accuracy of cooling algorithms must be evaluated. In this
paper we describe a radiative transport test suite, and we challenge
all researchers who use radiative hydrodynamics to study protoplanetary
disk evolution to evaluate their algorithms with these tests. The test
suite can be used to demonstrate an algorithm's accuracy in transporting
the correct flux through an atmosphere and in reaching the correct
temperature structure, to test the algorithm's dependence on resolution,
and to determine whether the algorithm permits of inhibits convection
when expected. In addition, we use this test suite to demonstrate the
accuracy of a newly developed radiative cooling algorithm that combines
vertical rays with flux-limited diffusion. This research was
supported in part by a Graduate Student Researchers Program fellowship.
Title: Supergranulation Scale Convection Simulations
Authors: Benson, D.; Stein, R.; Nordlund, Å.
Bibcode: 2006ASPC..354...92B
Altcode:
Initial results are reported for 3D simulations of solar convection on
a supergranular scale (48 Mm wide by 20 Mm deep). Results from several
solar hours of simulation at the 48 Mm scale are available as well as 24
solar hours on the 24 Mm scale. Relaxation is rapid near the surface,
but very slow at large depths and large horizontal scales. These
simulations will help separate the role of the second Helium ionization
zone from the effect of the increasing scale height with depth and will
be of use in analyzing local helioseismic inversion techniques. Since
Coriolis forces become significant on these spatio-temporal scales,
f-plane rotation will be added to investigate the nature of the surface
shear layer. Magnetic fields will also be added to study the development
and maintenance of the magnetic network.
Title: Spatial and Temporal Spectra of Solar Convection
Authors: Georgobiani, D.; Stein, R. F.; Nordlund, Å.
Bibcode: 2006ASPC..354..109G
Altcode:
Recent observations support the theory that solar-type oscillations
are stochastically excited by turbulent convection in the outer
layers of the solar-like stars. The acoustic power input rates
depend on the details of the turbulent energy spectrum. We
use numerical simulations to study the spectral properties of solar
convection. We find that spatial turbulent energy spectra vary at
different temporal frequencies, while temporal turbulent spectra show
various features at different spatial wavenumbers, and their best fit
at all frequencies is a generalized power law Power = Amplitude ×
(frequency^2 + width^2)^{-n(k)}, where n(k) depends on the spatial
wavenumber. Therefore, it is impossible to separate the spatial and
temporal components of the turbulent spectra.
Title: The Future: Where are We Headed
Authors: Nordlund, Å.
Bibcode: 2006ASPC..354..353N
Altcode:
I discuss what the future may bring in some of the topics of this
meeting. In particular I discuss near-surface dynamics and the solar
surface velocity spectrum, sub-surface dynamics and dynamo action,
emerging flux, sunspots, and the chromosphere and corona. For each
topic, a prediction and/or provocation is made, as a challenge towards
bringing about progress in that topic area of solar physics.
Title: Supergranule scale convection simulations
Authors: Stein, R. F.; Benson, D.; Georgobiani, D.; Nordlund, Å.
Bibcode: 2006ESASP.624E..79S
Altcode: 2006soho...18E..79S
No abstract at ADS
Title: Rapid Temporal Variability of Faculae: High-Resolution
Observations and Modeling
Authors: De Pontieu, B.; Carlsson, M.; Stein, R.; Rouppe van der Voort,
L.; Löfdahl, M.; van Noort, M.; Nordlund, Å.; Scharmer, G.
Bibcode: 2006ApJ...646.1405D
Altcode:
We present high-resolution G-band observations (obtained with the
Swedish 1 m Solar Telescope) of the rapid temporal variability of
faculae, which occurs on granular timescales. By combining these
observations with magnetoconvection simulations of a plage region, we
show that much of this variability is not intrinsic to the magnetic
field concentrations that are associated with faculae, but rather
a phenomenon associated with the normal evolution and splitting of
granules. We also show examples of facular variability caused by
changes in the magnetic field, with movies of dynamic behavior of
the striations that dominate much of the facular appearance at 0.1"
resolution. Examples of these dynamics include merging, splitting,
rapid motion, apparent fluting, and possibly swaying.
Title: Solar supergranulation-scale simulations
Authors: Stein, R. F.; Benson, D.; Nordlund, A.
Bibcode: 2006IAUJD..17E..15S
Altcode:
In order to understand the nature of supergranulation and provide a
test bed for calibrating local helioseismic methods we have performed
a realistic solar surface convection simulation on supergranulation
scales (48 Mm wide by 20 Mm deep), whose duration is currently
48 hours. The simulation includes f-plane rotation and develops a
surface shear layer. There is a gradual increase in the horizontal
scale of upflows with increasing depth due to merging of downflows
advected by the larger scale diverging upflows from below. There is
a rich spectrum of p-modes excited in the simulation. This data set
is available for studying solar oscillations and local helioseismic
inversion techniques. We will shortly be initiating an even larger-
scale simulation, 96 Mm wide, containing an active region.
Title: Supergranulation-Scale Simulations of the Solar Convection Zone
Authors: Benson, David; Stein, R. F.; Nordlund, A.
Bibcode: 2006SPD....37.3003B
Altcode: 2006BAAS...38..256B
We report on the status of solar surface supergranulation scale
simulations (48Mm x 48Mm x 20Mm (deep)). Effects of f-plane rotation
at a latitude of 30 degrees are included. These simulations were
bootstrapped from smaller width calculations which were relaxed for 3
turnover times (6 days) and have now relaxed for another turnover time
at the full width. The size of dominant structures increases with depth,
due to the halting of some downdrafts and the merging of others as they
descend, to form the boundaries of the larger horizontal upflows. These
large scale structures are also visible at the surface with a velocity
amplitude that decreases linearly with increasing size. We thank NASA
and NSF for their support of this work.
Title: Time-Distance and Correlation Tracking Analysesof Convective
Structures using Realistic Large-ScaleSimulations of Solar Convection
Authors: Georgobiani, Dali; Zhao, J.; Kosovichev, A. G.; Benson, D.;
Stein, R. F.; Nordlund, A.
Bibcode: 2006SPD....37.0509G
Altcode: 2006BAAS...38..224G
Recent large-scale simulations of solar turbulentconvection and
oscillations produce a wealth of realisticdata and provide a great
opportunity to study solaroscillations and test various techniques,
such aslocal helioseismology or local correlation trackingmethods,
widely used for the analysis of the realobserved solar data.The
application of the time-distance analysis to theartificial data produced
with a realistic 3D radiativehydrodynamic code successfully reproduces
thetime-distance diagram and travel time maps. Resultingtravel times are
similar to the travel times obtainedfrom the SOHO/MDI observations. To
further validatethe model, the inversion will be performed in
orderto infer the interior velocities at various depthsand compare
them with the simulated data.f-mode time-distanceanalysis as well as
local correlation tracking can be usedto study the morphology of the
simulated convection. Bothmethods reveal the large-scale convective
structures, whichare also directly visible in the time-averaged
simulatedflow fields.
Title: Solar Small-Scale Magnetoconvection
Authors: Stein, R. F.; Nordlund, Å.
Bibcode: 2006ApJ...642.1246S
Altcode:
Magnetoconvection simulations on mesogranule and granule scales near
the solar surface are used to study the effect of convective motions on
magnetic fields: the sweeping of magnetic flux into downflow lanes, the
twisting of magnetic field lines, and the emergence and disappearance
of magnetic flux tubes. From weak seed fields, convective motions
produce highly intermittent magnetic fields in the intergranular lanes
that collect over the boundaries of the underlying mesogranular scale
cells. Instances of both emerging magnetic flux loops and magnetic
flux disappearing from the surface occur in the simulations. We show
an example of a flux tube collapsing to kilogauss field strength and
a case of flux disappearance due to submergence of the flux. We note
that observed Stokes profiles of small magnetic structures are severely
distorted by telescope diffraction and seeing, so caution is needed
in interpreting low-resolution vector magnetograms of small-scale
magnetic structures.
Title: Stable magnetic fields in stellar interiors
Authors: Braithwaite, J.; Nordlund, Å.
Bibcode: 2006A&A...450.1077B
Altcode: 2005astro.ph.10316B
We investigate the 50-year old hypothesis that the magnetic fields
of the Ap stars are stable equilibria that have survived in these
stars since their formation. With numerical simulations we find that
stable magnetic field configurations indeed appear to exist under
the conditions in the radiative interior of a star. Confirming a
hypothesis by Prendergast (1956, ApJ, 123, 498), the configurations
have roughly equal poloidal and toroidal field strengths. We find
that tori of such twisted fields can form as remnants of the decay
of an unstable random initial field. In agreement with observations,
the appearance at the surface is an approximate dipole with smaller
contributions from higher multipoles, and the surface field strength
can increase with the age of the star. The results of this paper were
summarised by Braithwaite & Spruit (2004, Nature, 431, 891).
Title: Radiative transfer in decomposed domains
Authors: Heinemann, T.; Dobler, W.; Nordlund, Å.; Brandenburg, A.
Bibcode: 2006A&A...448..731H
Altcode: 2005astro.ph..3510H
Aims. An efficient algorithm for calculating radiative transfer
on massively parallel computers using domain decomposition is
presented. Methods. The integral formulation of the transfer equation
is used to divide the problem into a local but compute-intensive
part for calculating the intensity and optical depth integrals,
and a nonlocal part for communicating the intensity between adjacent
processors. Results. The waiting time of idle processors during the
nonlocal communication part does not have a severe impact on the
scaling. The wall clock time thus scales nearly linearly with the
inverse number of processors.
Title: Forward Modeling of the Corona of the Sun and Solar-like Stars:
From a Three-dimensional Magnetohydrodynamic Model to Synthetic
Extreme-Ultraviolet Spectra
Authors: Peter, Hardi; Gudiksen, Boris V.; Nordlund, Åke
Bibcode: 2006ApJ...638.1086P
Altcode: 2005astro.ph..3342P
A forward model is described in which we synthesize spectra from an ab
initio three-dimensional MHD simulation of an outer stellar atmosphere,
where the coronal heating is based on braiding of magnetic flux due to
photospheric footpoint motions. We discuss the validity of assumptions
such as ionization equilibrium and investigate the applicability of
diagnostics like the differential emission measure inversion. We find
that the general appearance of the synthesized corona is similar to
the solar corona and that, on a statistical basis, integral quantities
such as average Doppler shifts or differential emission measures are
reproduced remarkably well. The persistent redshifts in the transition
region, which have puzzled theorists since their discovery, are
explained by this model as caused by the flows induced by the heating
through braiding of magnetic flux. While the model corona is only
slowly evolving in intensity, as is observed, the amount of structure
and variability in Doppler shift is very large. This emphasizes the need
for fast coronal spectroscopic observations, as the dynamical response
of the corona to the heating process manifests itself in a comparably
slow evolving coronal intensity but rapid changes in Doppler shift.
Title: Simulated Solar Plages
Authors: Stein, R. F.; Carlsson, M.; de Pontieu, B.; Scharmer, G.;
Nordlund, Å.; Benson, D.
Bibcode: 2006apri.meet...30S
Altcode:
No abstract at ADS
Title: Time-distance analysis of realistic simulations of solar
convection
Authors: Georgobiani, D.; Zhao, J.; Benson, D.; Stein, R. F.;
Kosovichev, A. G.; Nordlund, A.
Bibcode: 2005AGUFMSH41A1117G
Altcode:
The results of the new realistic large-scale simulations of solar
turbulent convection provide an unprecedented opportunity to study
solar oscillations and perform similar local helioseismology techniques
as for the real solar data. The results offer an unique opportunity
to compare the simulated flow fields with the flows and sounds speed
variations inferred from the time-distance analysis. Applying some
of the existing local helioseismology methods to the simulated solar
convection and comparing to the observed results, one can validate
the accuracy of these methods. We apply the time-distance analysis
to the simulated data and successfully obtain the time-distance
curve and travel time maps. Our travel times are consistent with the
SOHO/MDI observations. The next step is to perform inversion to infer
the interior flow fields at various depths and compare them with the
simulated data in order to validate the model. This work is currently
in progress.
Title: Gamma-Ray Burst Synthetic Spectra from Collisionless Shock
PIC Simulations
Authors: Busk Hededal, Christian; Nordlund, Åke
Bibcode: 2005astro.ph.11662B
Altcode:
The radiation from afterglows of gamma-ray bursts is generated in the
collisionless plasma shock interface between a relativistic outflow and
a quiescent circum-burst medium. The two main ingredients responsible
for the radiation are high-energy, non-thermal electrons and a strong
magnetic field. In this Letter we present, for the first time, synthetic
spectra extracted directly from first principles particle-in-cell
simulations of relativist collisionless plasma shocks. The spectra
are generated by a numerical Fourier transformation of the electrical
far-field from each of a large number of particles, sampled directly
from the particle-in-cell simulations. Both the electromagnetic field
and the non-thermal particle acceleration are self-consistent products
of the Weibel two-stream instability. We find that the radiation
spectrum from a $\Gamma=15$ shock simulation show great resemblance with
observed GRB spectra -- we compare specifically with that of GRB000301C.
Title: Coronal Heating Through Braiding of Magnetic Field Lines
Synthesized Coronal EUV Emission and Magnetic Structure
Authors: Peter, H.; Gudiksen, B. V.; Nordlund, A.
Bibcode: 2005ESASP.596E..14P
Altcode: 2005ccmf.confE..14P
No abstract at ADS
Title: Effect of the radiative background flux in convection
Authors: Brandenburg, A.; Chan, K. L.; Nordlund, Å.; Stein, R. F.
Bibcode: 2005AN....326..681B
Altcode: 2005astro.ph..8404B
Numerical simulations of turbulent stratified convection are used
to study models with approximately the same convective flux, but
different radiative fluxes. As the radiative flux is decreased, for
constant convective flux: the entropy jump at the top of the convection
zone becomes steeper, the temperature fluctuations increase and the
velocity fluctuations decrease in magnitude, and the distance that
low entropy fluid from the surface can penetrate increases. Velocity
and temperature fluctuations follow mixing length scaling laws.
Title: EUV Emission from a 3D MHD Coronal Model: Temporal Variability
in a Synthesized Corona
Authors: Peter, H.; Gudiksen, B. V.; Nordlund, Å.
Bibcode: 2005ESASP.592..527P
Altcode: 2005soho...16E..98P; 2005ESASP.592E..98P
No abstract at ADS
Title: The Structure of the Base of the Corona
Authors: Bingert, S.; Peter, H.; Gudiksen, B.; Nordlund, Ake
Bibcode: 2005ESASP.592..471B
Altcode: 2005ESASP.592E..84B; 2005soho...16E..84B
No abstract at ADS
Title: Spectrum and amplitudes of internal gravity waves excited by
penetrative convection in solar-type stars
Authors: Dintrans, B.; Brandenburg, A.; Nordlund, Å.; Stein, R. F.
Bibcode: 2005A&A...438..365D
Altcode: 2005astro.ph..2138D
The excitation of internal gravity waves by penetrative convective
plumes is investigated using 2-D direct simulations of compressible
convection. The wave generation is quantitatively studied from the
linear response of the radiative zone to the plumes penetration,
using projections onto the g-modes solutions of the associated linear
eigenvalue problem for the perturbations. This allows an accurate
determination of both the spectrum and amplitudes of the stochastically
excited modes. Using time-frequency diagrams of the mode amplitudes,
we then show that the lifetime of a mode is around twice its period
and that during times of significant excitation up to 40% of the total
kinetic energy may be contained into g-modes.
Title: Collisionless Shocks: Dynamics and Synthetic Spectra
Authors: Nordlund, Åke
Bibcode: 2005paoa.progE..18N
Altcode:
No abstract at ADS
Title: Excitation of Solar-like Oscillations: From PMS to MS Stellar
Models
Authors: Samadi, R.; Goupil, M. -J.; Alecian, E.; Baudin, F.;
Georgobiani, D.; Trampedach, R.; Stein, R.; Nordlund, Å.
Bibcode: 2005JApA...26..171S
Altcode:
The amplitude of solar-like oscillations results from a balance between
excitation and damping. As in the sun, the excitation is attributed
to turbulent motions that stochastically excite the p modes in the
upper-most part of the convective zone. We present here a model for the
excitation mechanism. Comparisons between modeled amplitudes and helio
and stellar seismic constraints are presented and the discrepancies
discussed. Finally the possibility and the interest of detecting
such stochastically excited modes in pre-main sequence stars are
also discussed.
Title: In situ particle acceleration in collisionless shocks
Authors: Hededal, C. B.; Haugbølle, T.; Frederiksen, J. T.;
Nordlund, Å.
Bibcode: 2005NCimC..28..411H
Altcode: 2005astro.ph..2372H
The outflows from gamma-ray bursts, active galactic nuclei and
relativistic jets in general interact with the surrounding media
through collisionless shocks. With three dimensional relativistic
particle-in-cell simulations we investigate such shocks. The results
from these experiments show that small-scale magnetic filaments
with strengths of up to percents of equipartition are generated and
that electrons are accelerated to power law distributions N(γ)
∝ γ-p in the vicinity of the filaments through a
new acceleration mechanism. The acceleration is locally confined,
instantaneous and differs from recursive acceleration processes
such as Fermi acceleration. We find that the proposed acceleration
mechanism competes with thermalization and becomes important at high
Lorentz factors.
Title: Supergranulation Scale Solar Convection Simulations
Authors: Benson, D.; Stein, R.; Nordlund, A.
Bibcode: 2005AGUSMSP11C..05B
Altcode:
Supergranulation scale (50 Mm wide by 20 Mm deep) simulations of solar
convection are being relaxed thermally and dynamically. The initial
state was made by duplicating a periodic smaller simulation of 24
Mm wide by 9 Mm deep and extending it in depth assuming constant
entropy upflows and extrapolating the downflows. Relaxation is
rapid near the surface, but very slow at large depths and large
horizontal scales. Initial results are reported. These simulations
will help separate the role of the second helium ionization zone from
the effect of the increasing scale height with depth. This large
size is also necessary for analyzing local helioseismic inversion
techniques. Coriolis forces becomes significant on these spatio-temporal
scales and we have added f-plane rotation to investigate the nature
of the surface shear layer. Eventually, magnetic fields will be added
to study the development and maintenance of the magnetic network.
Title: Erratum: ``An AB Initio Approach to Solar Coronal Loops''
(ApJ, 618, 1031 [2005])
Authors: Gudiksen, Boris Vilhelm; Nordlund, Åke
Bibcode: 2005ApJ...623..597G
Altcode:
Because of an error at the Press, incorrect versions of Figures 4 (top
and bottom panels), 5, 7, and 9 were published. In all these figures, a
dotted or dash-dotted line appeared as a solid line. The correct figures
appear below. Figures 4 (middle panel), 6, 8, and 10 are also reproduced
here for comparison purposes. The Press sincerely regrets these errors.
Title: Erratum: ``An AB Initio Approach to the Solar Coronal
Heating Problem'' (ApJ, 618, 1020
[2005])
Authors: Gudiksen, Boris Vilhelm; Nordlund, Åke
Bibcode: 2005ApJ...623..600G
Altcode:
Because of an error at the Press, an incorrect version of Figure 5
was published, in which what should be a dash-dotted line (showing
convective flux) appears as a solid line. The correct version appears
below. The Press sincerely regrets the error.
Title: Tackling the coronal heating problem using 3D MHD coronal
simulations with spectral synthesis
Authors: Peter, H.; Gudiksen, B. V.; Nordlund, A.
Bibcode: 2005ESASP.560...59P
Altcode: 2005csss...13...59P
No abstract at ADS
Title: A Solution to the Pre-Main-Sequence Accretion Problem
Authors: Padoan, Paolo; Kritsuk, Alexei; Norman, Michael L.; Nordlund,
Åke
Bibcode: 2005ApJ...622L..61P
Altcode: 2004astro.ph.11129P
Accretion rates of order 10-8 Msolar
yr-1 are observed in young pre-main-sequence (PMS) stars of
approximately a solar mass with evidence of circumstellar disks. The
accretion rate is significantly lower for PMS stars of smaller mass,
approximately proportional to the second power of the stellar mass,
M˙accr~M2. The traditional view is that the
observed accretion is the consequence of the angular momentum transport
in isolated circumstellar disks, controlled by disk turbulence or
self-gravity. However, these processes are not well understood and the
observed accretion, a fundamental aspect of star formation, remains an
unsolved problem. In this Letter, we propose the stellar accretion rate
is controlled by accretion from the large-scale gas distribution in the
parent cloud, not by the isolated disk evolution. Approximating this
process as Bondi-Hoyle accretion onto the star-disk system, we obtain
accretion rates comparable to the observed ones. We also reproduce the
observed dependence of the accretion rate on the stellar mass. These
results are based on realistic values of the ambient gas density and
velocity, as inferred from numerical simulations of star formation in
self-gravitating turbulent clouds.
Title: Collisionless Plasma Shocks Field Generation and Particle
Acceleration
Authors: Haugbøelle, Troels; Hededal, Christian; Nordlund, Åke;
Frederiksen, Jacob Trier
Bibcode: 2005tsra.conf..684H
Altcode: 2005astro.ph..3332H
Gamma ray bursts are among the most energetic events in the known
universe. A highly relativistic fireball is ejected. In most cases the
burst itself is followed by an afterglow, emitted under deceleration
as the fireball plunges through the circum-stellar media. To interpret
the observations of the afterglow emission, two physical aspects need
to be understood: 1) The origin and nature of the magnetic field
in the fireball and 2) the particle velocity distribution function
behind the shock. Both are necessary in existing afterglow models to
account for what is believed to be synchrotron radiation. To answer
these questions, we need to understand the microphysics at play in
collisionless shocks. Using 3D particle-in-cell simulations we can
gain insight in the microphysical processes that take place in such
shocks. We discuss the results of such computer experiments. It is
shown how a Weibel-like two-stream plasma instability is able to
create a strong transverse intermittent magnetic field and how this
points to a connected mechanism for in situ particle acceleration in
the shock region.
Title: The Stellar IMF as a Property of Turbulence
Authors: Padoan, Paolo; Nordlund, Åke
Bibcode: 2005ASSL..327..357P
Altcode: 2004astro.ph.11474P; 2005imf..conf..357P
We propose to interpret the stellar IMF as a property of the turbulence
in the star--forming gas. Gravitationally unstable density enhancements
in the turbulent flow collapse and form stars. Their mass distribution
can be derived analytically from the power spectrum of the turbulent
flow and the isothermal shock jump conditions in the magnetized
gas. For a power spectrum index \beta=1.74, consistent with Larson's
velocity dispersion--size relation as well as with new numerical and
analytic results on supersonic turbulence, we obtain a power law mass
distribution of dense cores with a slope equal to 3/(4-\beta)=1.33,
consistent with the slope of Salpeter's stellar IMF. Below one solar
mass, the mass distribution flattens and turns around at a fraction
of a solar mass, as observed for the stellar IMF in a number of
stellar clusters, because only the densest cores are gravitationally
unstable. The mass distribution at low masses is determined by the
Log--Normal distribution of the gas density. The intermittent nature
of this distribution is responsible for the generation of a significant
number of collapsing cores of brown dwarf mass.
Title: Brown dwarfs from turbulent fragmentation
Authors: Padoan, Paolo; Kritsuk, Alexei; Michael; Norman, L.;
Nordlund, Åke
Bibcode: 2005MmSAI..76..187P
Altcode: 2004astro.ph.11480P
The origin of brown dwarfs (BDs) is an important component of the
theory of star formation, because BDs are approximately as numerous as
solar mass stars. It has been suggested that BDs originate from the
gravitational fragmentation of protostellar disks, a very different
mechanism from the formation of hydrogen burning stars. We propose
that BDs are instead formed by the process of turbulent fragmentation,
like more massive stars. In numerical simulations of turbulence and
star formation we find that gravitationally unstable density peaks
of BD mass are commonly formed by the turbulent flow. These density
peaks collapse into BD mass objects with circumstellar disks, like more
massive protostars. We rely on numerical experiments with very large
resolution, achieved with adaptive mesh refinement (AMR). The turbulence
simulation presented here is the first AMR turbulence experiment ever
attempted and achieves an effective resolution of 10243
computational zones. The star formation simulation achieves an effective
resolution of (106)3 computational zones, from
a cloud size of 5 pc to protostellar disks resolved down to 1 AU.
Title: Old and New Paradigms for Planet Formation
Authors: Nordlund, Å.
Bibcode: 2005prpl.conf.8618N
Altcode: 2005LPICo1286.8618N
No abstract at ADS
Title: An AB Initio Approach to Solar Coronal Loops
Authors: Gudiksen, Boris Vilhelm; Nordlund, Åke
Bibcode: 2005ApJ...618.1031G
Altcode: 2004astro.ph..7267G
Data from recent numerical simulations of the solar corona and
transition region are analyzed, and the magnetic field connections
between the low corona and the photosphere are found to be close to
those of a potential field. The field line-to-field line displacements
follow a power-law distribution with typical displacements of just a
few Mm. Three loops visible in simulated TRACE filters are analyzed
in detail and found to have significantly different heating rates
and distributions thereof, one of them showing a small-scale heating
event. The dynamical structure is complicated, even though all the
loops are visible in a single filter along most of their lengths. The
loops are nonstatic and are in the process of evolving into loops with
very different characteristics. Differential emission measure (DEM)
curves along one of the loops illustrate that DEM curves have to be
treated carefully if physical characteristics are to be extracted.
Title: Excitation of P-Modes in the Sun and Stars
Authors: Stein, Robert; Georgobiani, Dali; Trampedach, Regner; Ludwig,
Hans-Günter; Nordlund, Åke
Bibcode: 2005HiA....13..411S
Altcode:
We describe the stochastic excitation of p-mode oscillations by solar
convection. We discuss the role of Reynolds stresses and entropy
fluctuations what controls the excitation spectrum the depth of the
driving and the location of the driving. We then present results for
a range of other stars and discuss the similarities and differences
with the Sun.
Title: An Ab Initio Approach to the Solar Coronal Heating Problem
Authors: Gudiksen, Boris Vilhelm; Nordlund, Åke
Bibcode: 2005ApJ...618.1020G
Altcode: 2004astro.ph..7266G
We present an ab initio approach to the solar coronal heating problem by
modeling a small part of the solar corona in a computational box using
a three-dimensional MHD code including realistic physics. The observed
solar granular velocity pattern and its amplitude and vorticity power
spectra, as reproduced by a weighted Voronoi tessellation method,
are used as a boundary condition that generates a Poynting flux in
the presence of a magnetic field. The initial magnetic field is a
potential extrapolation of a SOHO/MDI high-resolution magnetogram,
and a standard stratified atmosphere is used as a thermal initial
condition. Except for the chromospheric temperature structure, which
is kept nearly fixed, the initial conditions are quickly forgotten
because the included Spitzer conductivity and radiative cooling
function have typical timescales much shorter than the time span of the
simulation. After a short initial start-up period, the magnetic field is
able to dissipate (3-4)×106ergscm-2s-1
in a highly intermittent corona, maintaining an average temperature
of ~106 K, at coronal density values for which simulated
images of the TRACE 171 and 195 Å passbands reproduce observed photon
count rates.
Title: Magnetohydrodynamics of the Solar Atmosphere
Authors: Nordlund, Å.
Bibcode: 2004ASPC..325..165N
Altcode:
I review recent progress in the understanding of solar atmospheric
magnetohydrodynamics, triggered by new high resolution observations and
by recent advances in 3-D numerical modeling. I discuss particularly the
requirements and capabilities for realistic numerical modeling of solar
magnetohydrodynamics, and outline the primary requirements for further
progress, in particular in the direction of chromospheric modeling.
Title: Analysis of Synthetic EUV Spectra from 3d Models of the Corona
Authors: Bingert, S.; Peter, H.; Gudiksen, B.; Nordlund, A.; Dobler, W.
Bibcode: 2004ESASP.575..348B
Altcode: 2004soho...15..348B
No abstract at ADS
Title: The ``Mysterious'' Origin of Brown Dwarfs
Authors: Padoan, Paolo; Nordlund, Åke
Bibcode: 2004ApJ...617..559P
Altcode: 2002astro.ph..5019P
Hundreds of brown dwarfs (BDs) have been discovered in the last few
years in stellar clusters and among field stars. BDs are almost as
numerous as hydrogen-burning stars, and so a theory of star formation
should also explain their origin. The ``mystery'' of the origin of BDs
is that their mass is 2 orders of magnitude smaller than the average
Jeans mass in star-forming clouds, and yet they are so common. In this
work we investigate the possibility that gravitationally unstable
protostellar cores of BD mass are formed directly by the process of
turbulent fragmentation. Supersonic turbulence in molecular clouds
generates a complex density field with a very large density contrast. As
a result, a fraction of BD mass cores formed by the turbulent flow are
dense enough to be gravitationally unstable. We find that with density,
temperature, and rms Mach number typical of cluster-forming regions,
turbulent fragmentation can account for the observed BD abundance.
Title: Excitation rates of p modes: mass luminosity relation across
the HR diagram
Authors: Samadi, R.; Georgobiani, D.; Trampedach, R.; Goupil, M. J.;
Stein, R. F.; Nordlund, Å.
Bibcode: 2004sf2a.conf..323S
Altcode: 2004astro.ph.10043S
We compute the rates P at which energy is injected into the p modes for
a set of 3D simulations of outer layers of stars. We found that Pmax
- the maximum in P - scales as (L/M)^s where s is the slope of the
power law, L and M are the luminosity and the mass of the 1D stellar
models associated with the simulations. The slope is found to depend
significantly on the adopted representation for the turbulent eddy-time
correlation function, chi_k. According to the expected performances
of COROT, it will likely be possible to measure Pmax as a function
of L/M and to constrain the properties of stellar turbulence as the
turbulent eddy time-correlation.
Title: Non-Fermi Power-Law Acceleration in Astrophysical Plasma Shocks
Authors: Hededal, C. B.; Haugbølle, T.; Frederiksen, J. Trier;
Nordlund, Å.
Bibcode: 2004ApJ...617L.107H
Altcode: 2004astro.ph..8558H
Collisionless plasma shock theory, which applies, for example, to
the afterglow of gamma-ray bursts, still contains key issues that
are poorly understood. In this Letter, we study charged particle
dynamics in a highly relativistic collisionless shock numerically
using ~109 particles. We find a power-law distribution
of accelerated electrons, which upon detailed investigation turns
out to originate from an acceleration mechanism that is decidedly
different from Fermi acceleration. Electrons are accelerated by strong
filamentation instabilities in the shocked interpenetrating plasmas and
coincide spatially with the power-law-distributed current filamentary
structures. These structures are an inevitable consequence of the
now well-established Weibel-like two-stream instability that operates
in relativistic collisionless shocks. The electrons are accelerated
and decelerated instantaneously and locally: a scenery that differs
qualitatively from recursive acceleration mechanisms such as Fermi
acceleration. The slopes of the electron distribution power laws
are in concordance with the particle power-law spectra inferred from
observed afterglow synchrotron radiation in gamma-ray bursts, and the
mechanism can possibly explain more generally the origin of nonthermal
radiation from shocked interstellar and circumstellar regions and from
relativistic jets.
Title: Synthetic EUV Spectra from 3D MHD Coronal Simulations:
Coronal Heating Through Magnetic Braiding
Authors: Peter, H.; Gudiksen, B. V.; Nordlund, Å.
Bibcode: 2004ESASP.575...50P
Altcode: 2004soho...15...50P
No abstract at ADS
Title: Coronal Heating through Braiding of Magnetic Field Lines
Authors: Peter, Hardi; Gudiksen, Boris V.; Nordlund, Åke
Bibcode: 2004ApJ...617L..85P
Altcode: 2004astro.ph..9504P
Cool stars such as our Sun are surrounded by a million degree hot outer
atmosphere, the corona. For more than 60 years, the physical nature
of the processes heating the corona to temperatures well in excess of
those on the stellar surface have remained puzzling. Recent progress in
observational techniques and numerical modeling now opens a new window
to approach this problem. We present the first coronal emission-line
spectra synthesized from three-dimensional numerical models describing
the evolution of the dynamics and energetics as well as of the magnetic
field in the corona. In these models the corona is heated through
motions on the stellar surface that lead to a braiding of magnetic
field lines inducing currents that are finally dissipated. These
forward models enable us to synthesize observed properties such as
(average) emission-line Doppler shifts or emission measures in the outer
atmosphere, which until now have not been understood theoretically,
even though many suggestions have been made in the past. As our model
passes these observational tests, we conclude that the flux braiding
mechanism is a prime candidate for being the dominant heating process
of the magnetically closed corona of the Sun and solar-like stars.
Title: Supergranulation Scale Solar Convection Simulations
Authors: Benson, D.; Stein, R.; Nordlund, A.
Bibcode: 2004AAS...20517401B
Altcode: 2005BAAS...37..377B
Solar convection simulations have been started on small supergranulation
scales of 24 x 24 Mm x 9 Mm deep. The initial state was made by
duplicating a periodic smaller simulation of 12 x 12 Mm x 9 Mm
deep and adding a small velocity perturbation. This state has now
relaxed for about 2 hours. Near the surface, the initial pattern
has disappeared, but in deeper layers the predominant duplication
in each horizontal direction is still present. We estimate it will
take about width/horizontal velocity at depth = 24 Mm / (0.15 km/s) =
43 hours to dynamically relax and develop structures on the scale of
24 Mm. Since this is the size of small supergranules, we expect that
one of these will eventually develop after a few turnover times. The
evolution of the convective structure at various depths is shown. Eventually, a region 48 x 48 Mm x 18 Mm deep will be simulated. This
will help separate the role of the second helium ionization zone from
the effect of the increasing scale height with depth. This large
size is also necessary for analyzing local helioseismic inversion
techniques. Coriolis forces become significant on these spatio-temporal
scales. We will investigate the surface shear layer that should develop
with the inclusion of f-plane rotation. Finally, magnetic fields will
be added to study the development and maintenance of the magnetic
network. This work is supported by NASA grants NAG 512450 and
NNG046-B92G and NSF grant AST0205500.
Title: High Degree Solar Oscillations in 3d Numerical Simulations
Authors: Georgobiani, D.; Stein, R. F.; Nordlund, Å.; Kosovichev,
A. G.; Mansour, N. N.
Bibcode: 2004ESASP.559..267G
Altcode: 2004soho...14..267G
No abstract at ADS
Title: Oscillation Power Spectra of the Sun and of CEN a: Observations
Versus Models
Authors: Samadi, R.; Goupil, M. J.; Baudin, F.; Georgobiani, D.;
Trampedach, R.; Stein, R.; Nordlund, A.
Bibcode: 2004ESASP.559..615S
Altcode: 2004astro.ph..9325S; 2004soho...14..615S
Hydrodynamical, 3D simulations of the outer layers of the Sun and Alpha
Cen A are used to obtain constraints on the properties of turbulent
convection in such stars. These constraints enable us to compute -
on the base of a theoretical model of stochastic excitation - the
rate P at which p modes are excited by turbulent convection in those
two stars. Results are then compared with solar seismic observations
and recent observations of Alpha Cen A. For the Sun, a good agreement
between observations and computed P is obtained. For Alpha Cen A a
large discrepancy is obtained which origin cannot be yet identified:
it can either be caused by the present data quality which is not
sufficient for our purpose or by the way the intrinsic amplitudes and
the life-times of the modes are determined or finally attributed to
our present modelling. Nevertheless, data with higher quality or/and
more adapted data reductions will likely provide constraints on the
p-mode excitation mechanism in Alpha Cen A.
Title: Self-Regulating Supernova Heating in Interstellar Medium
Simulations
Authors: Sarson, Graeme R.; Shukurov, Anvar; Nordlund, Åke; Gudiksen,
Boris; Brandenburg, Axel
Bibcode: 2004Ap&SS.292..267S
Altcode: 2003astro.ph..7013S
Numerical simulations of the multi-phase interstellar medium have been
carried out, using a 3D, nonlinear, magnetohydrodynamic, shearing-box
model, with random motions driven by supernova explosions. These
calculations incorporate the effects of magnetic fields and rotation
in 3D; these play important dynamical roles in the galaxy, but are
neglected in many other simulations. The supernovae driving the motions
are not arbitrarily imposed, but occur where gas accumulates into cold,
dense clouds; their implementation uses a physically motivated model
for the evolution of such clouds. The process is self-regulating, and
produces mean supernova rates as part of the solution. Simulations with
differing mean density show a power law relation between the supernova
rate and density, with exponent 1.7; this value is within the range
suggested from observations (taking star formation rate as a proxy for
supernova rate). The global structure of the supernova driven medium
is strongly affected by the presence of magnetic fields; e.g. for one
solution the filling factor of hot gas is found to vary from 0.19 (with
no field) to 0.12 (with initial mid-plane field B 0= 6 μG).
Title: Observational Manifestations of Solar Magnetoconvection:
Center-to-Limb Variation
Authors: Carlsson, Mats; Stein, Robert F.; Nordlund, Åke; Scharmer,
Göran B.
Bibcode: 2004ApJ...610L.137C
Altcode: 2004astro.ph..6160C
We present the first center-to-limb G-band images synthesized from
high-resolution simulations of solar magnetoconvection. Toward the
limb the simulations show ``hilly'' granulation with dark bands on
the far side, bright granulation walls, and striated faculae, similar
to observations. At disk center G-band bright points are flanked
by dark lanes. The increased brightness in magnetic elements is due
to their lower density compared with the surrounding intergranular
medium. One thus sees deeper layers where the temperature is higher. At
a given geometric height, the magnetic elements are cooler than the
surrounding medium. In the G band, the contrast is further increased
by the destruction of CH in the low-density magnetic elements. The
optical depth unity surface is very corrugated. Bright granules have
their continuum optical depth unity 80 km above the mean surface,
the magnetic elements 200-300 km below. The horizontal temperature
gradient is especially large next to flux concentrations. When viewed
at an angle, the deep magnetic elements' optical surface is hidden by
the granules and the bright points are no longer visible, except where
the ``magnetic valleys'' are aligned with the line of sight. Toward
the limb, the low density in the strong magnetic elements causes unit
line-of-sight optical depth to occur deeper in the granule walls behind
than for rays not going through magnetic elements, and variations
in the field strength produce a striated appearance in the bright
granule walls.
Title: Scaling Relations of Supersonic Turbulence in Molecular Clouds
Authors: Boldyrev, S.; Padoan, P.; Jimenez, R.; Nordlund, Å.
Bibcode: 2004Ap&SS.292...61B
Altcode:
We discuss a model for driven supersonic, super-Alfvénic MHD
turbulence that is believed to govern the structure of molecular
clouds. Such turbulence is highly intermittent; we describe its
statistical properties by obtaining scaling of velocity-difference
structure functions. This scaling was analytically predicted in Boldyrev
(2002), confirmed in numerical simulations by Boldyrev et al. (2002),
and discovered in observations by Padoan et al. (2003).
Title: Magnetic Field Generation in Collisionless Shocks: Pattern
Growth and Transport
Authors: Frederiksen, J. Trier; Hededal, C. B.; Haugbølle, T.;
Nordlund, Å.
Bibcode: 2004ApJ...608L..13F
Altcode: 2003astro.ph..8104T; 2003astro.ph..8104F
We present results from three-dimensional particle simulations of
collisionless shock formation, with relativistic counterstreaming
ion-electron plasmas. Particles are followed over many skin depths
downstream of the shock. Open boundaries allow the experiments to
be continued for several particle crossing times. The experiments
confirm the generation of strong magnetic and electric fields by a
Weibel-like kinetic streaming instability and demonstrate that the
electromagnetic fields propagate far downstream of the shock. The
magnetic fields are predominantly transversal and are associated
with merging ion current channels. The total magnetic energy grows as
the ion channels merge and as the magnetic field patterns propagate
downstream. The electron populations are quickly thermalized, while the
ion populations retain distinct bulk speeds in shielded ion channels
and thermalize much more slowly. The results help reveal processes of
importance in collisionless shocks and may help to explain the origin
of the magnetic fields responsible for afterglow synchrotron/jitter
radiation from gamma-ray bursts.
Title: Structure Function Scaling in Compressible Super-Alfvénic
MHD Turbulence
Authors: Padoan, Paolo; Jimenez, Raul; Nordlund, Åke; Boldyrev,
Stanislav
Bibcode: 2004PhRvL..92s1102P
Altcode: 2003astro.ph..1026P
Supersonic turbulent flows of magnetized gas are believed to
play an important role in the dynamics of star-forming clouds in
galaxies. Understanding statistical properties of such flows is crucial
for developing a theory of star formation. In this Letter we propose
a unified approach for obtaining the velocity scaling in compressible
and super-Alfvénic turbulence, valid for the arbitrary sonic Mach
number, MS. We demonstrate with numerical simulations that
the scaling can be described with the She-Lévêque formalism, where
only one parameter, interpreted as the Hausdorff dimension of the most
intense dissipative structures, needs to be varied as a function of
MS. Our results thus provide a method for obtaining the
velocity scaling in interstellar clouds once their Mach numbers have
been inferred from observations.
Title: G-band Images from MHD Convection Simulations
Authors: Stein, R. F.; Carlsson, M.; Nordlund, A.; Scharmer, G.
Bibcode: 2004AAS...204.8804S
Altcode: 2004BAAS...36..820S
High resolution magneto-convection simulations are used to calculate
G-band and G-continuum images at various angles. Towards the limb
the simulations show "hilly" granulation, bright granulation walls,
intergranular striations and "sticking out" G-band bright features
similar to observations. The increased brightness in magnetic
elements is due to their lower density compared with the surrounding
intergranular medium, so that one sees deeper layers where the
temperature is higher. At a given geometric height, the magnetic
elements are not hotter than the surrounding medium. In the G-band,
the contrast is further increased by the destruction of CH in the
low density magnetic elements. The optical depth unity surface is
very corrugated. Bright granules have their continuum optical depth
unity 80 km above the mean surface, the magnetic elements 200-300 km
below. At large angles, the deep lying magnetic elements are hidden
by the granules and the bright points are no longer visible. Where
the "magnetic valleys" are aligned with the line of sight, they are
visible as elongated structures seemingly "sticking out". Even when
the deep hot surface is hidden, the low density in the strong magnetic
elements causes unit line-of-sight optical depth to occur deeper in
the granule walls behind then for rays not going through magnetic
elements. Flux concentrations in intergranular lanes therefore cause
a striped intensity pattern. This work is funded by NSF grants AST
0205500 and ATM 99881112 and NASA grants NAG 5 12450 and NNGO4GB92G.
Title: Excitation of Radial P-Modes in the Sun and Stars
Authors: Stein, Robert; Georgobiani, Dali; Trampedach, Regner; Ludwig,
Hans-Günter; Nordlund, Åke
Bibcode: 2004SoPh..220..229S
Altcode:
P-mode oscillations in the Sun and stars are excited stochastically
by Reynolds stress and entropy fluctuations produced by convection in
their outer envelopes. The excitation rate of radial oscillations of
stars near the main sequence from K to F and a subgiant K IV star have
been calculated from numerical simulations of their surface convection
zones. P-mode excitation increases with increasing effective temperature
(until envelope convection ceases in the F stars) and also increases
with decreasing gravity. The frequency of the maximum excitation
decreases with decreasing surface gravity.
Title: The Average Magnetic Field Strength in Molecular Clouds:
New Evidence of Super-Alfvénic Turbulence
Authors: Padoan, Paolo; Jimenez, Raul; Juvela, Mika; Nordlund, Åke
Bibcode: 2004ApJ...604L..49P
Altcode: 2003astro.ph.11349P
The magnetic field strength in molecular clouds is a fundamental
quantity for theories of star formation. It is estimated by Zeeman
splitting measurements in a few dense molecular cores, but its
volume-averaged value within large molecular clouds (over several
parsecs) is still uncertain. In this work, we provide a new method to
constrain the average magnetic field strength in molecular clouds. We
compare the power spectrum of gas density of molecular clouds with
that of two 3503 numerical simulations of supersonic MHD
turbulence. The numerical simulation with approximate equipartition
of kinetic and magnetic energies (model A) yields the column density
power spectrum P(k)~k-2.25+/-0.01, the super-Alfvénic
simulation (model B) P(k)~k-2.71+/-0.01. The column
density power spectrum of the Perseus, Taurus, and Rosetta molecular
cloud complexes is found to be well approximated by a power law,
P0(k)~k-a, with a=2.74+/-0.07, 2.74+/-0.08,
and 2.76+/-0.08, respectively. We conclude that the observations
are consistent with the presence of super-Alfvénic turbulence in
molecular clouds (model B), while model A is inconsistent (more than
99% confidence) with the observations.
Title: Stochastic excitation of gravity waves by overshooting
convection in solar-type stars
Authors: Dintrans, Boris; Brandenburg, Axel; Nordlund, Ake; Stein,
R. F.
Bibcode: 2004astro.ph..3093D
Altcode:
The excitation of gravity waves by penetrative convective plumes is
investigated using 2D direct simulations of compressible convection. The
oscillation field is measured by a new technique based on the projection
of our simulation data onto the theoretical g-modes solutions of the
associated linear eigenvalue problem. This allows us to determine both
the excited modes and their corresponding amplitudes accurately.
Title: The effects of spiral arms on the multi-phase ISM
Authors: Shukurov, Anvar; Sarson, Graeme R.; Nordlund, Åke; Gudiksen,
Boris; Brandenburg, Axel
Bibcode: 2004Ap&SS.289..319S
Altcode: 2002astro.ph.12260S
Statistical parameters of the ISM driven by thermal energy
injectionsfrom supernova explosions have been obtained from 3D,
nonlinear,magnetohydrodynamic, shearing-box simulations for spiral
arm andinterarm regions. The density scale height obtained for the
interarm regionsis 50% larger than within the spiral arms because
of thehigher gas temperature. The filling factorof the hot gas is
also significantly larger between the armsand depends sensitively on
magnetic field strength.
Title: An Ab Initio Approach to the Solar Coronal Heating Problem
Authors: Gudiksen, B. V.; Nordlund, Å.
Bibcode: 2004IAUS..219..488G
Altcode:
No abstract at ADS
Title: High resolution limb images synthesized from 3D MHD simulations
Authors: Carlsson, Mats; Stein, Robert F.; Nordlund, Åke; Scharmer,
Göran B.
Bibcode: 2004IAUS..223..233C
Altcode: 2005IAUS..223..233C
We present the first center-to-limb G-band images synthesized from
high resolution simulations of solar magneto-convection. Towards the
limb the simulations show "hilly" granulation with dark bands on the
far side, bright granulation walls and striated faculae, similar
to observations. At disk center G-band bright points are flanked
by dark lanes. The increased brightness in magnetic elements is due
to their lower density compared with the surrounding intergranular
medium. One thus sees deeper layers where the temperature is higher. At
a given geometric height, the magnetic elements are cooler than the
surrounding medium. In the G-band, the contrast is further increased
by the destruction of CH in the low density magnetic elements. The
optical depth unity surface is very corrugated. Bright granules have
their continuum optical depth unity 80 km above the mean surface,
the magnetic elements 200-300 km below. The horizontal temperature
gradient is especially large next to flux concentrations. When viewed
at an angle, the deep magnetic elements optical surface is hidden by
the granules and the bright points are no longer visible, except where
the "magnetic valleys" are aligned with the line of sight. Towards
the limb, the low density in the strong magnetic elements causes
unit line-of-sight optical depth to occur deeper in the granule
walls behind than for rays not going through magnetic elements and
variations in the field strength produce a striated appearance in the
bright granule walls.
Title: Magneto-Convection: Structure and Dynamics
Authors: Stein, Robert F.; Nordlund, Åke
Bibcode: 2004IAUS..223..179S
Altcode: 2005IAUS..223..179S
We present results from realistic, high resolution, simulations of solar
magneto-convection. Simulations were run with both a mean vertical and
a mean horizontal field. The magnetic field is quickly swept out of the
granules and meso-granules and concentrated in the intergranular lanes.
Title: Theory and Simulations of Solar Atmosphere Dynamics
Authors: Stein, R. F.; Bogdan, T. J.; Carlsson, M.; Hansteen, V.;
McMurry, A.; Rosenthal, C. S.; Nordlund, Å.
Bibcode: 2004ESASP.547...93S
Altcode: 2004soho...13...93S
Numerical simulations are used to study the generation and propagation
of waves in the solar atmosphere. Solar p-mode oscillations are excited
by turbulent pressure work and entropy fluctuations (non-adiabatic gas
pressure work) near the solar surface. Interactions between short and
long period waves and radiative energy transfer control the formation of
shocks. The magnetic structure of the atmosphere induces coupling among
various MHD wave modes, with intense coupling and wave transformation
at the beta equal one surface, which likely is the location of the
so-called "magnetic canopy".
Title: Waves in the Magnetized Solar Atmosphere. II. Waves from
Localized Sources in Magnetic Flux Concentrations
Authors: Bogdan, T. J.; Carlsson, M.; Hansteen, V. H.; McMurry, A.;
Rosenthal, C. S.; Johnson, M.; Petty-Powell, S.; Zita, E. J.; Stein,
R. F.; McIntosh, S. W.; Nordlund, Å.
Bibcode: 2003ApJ...599..626B
Altcode:
Numerical simulations of wave propagation in a two-dimensional
stratified magneto-atmosphere are presented for conditions that
are representative of the solar photosphere and chromosphere. Both
the emergent magnetic flux and the extent of the wave source are
spatially localized at the lower photospheric boundary of the
simulation. The calculations show that the coupling between the
fast and slow magneto-acoustic-gravity (MAG) waves is confined to
thin quasi-one-dimensional atmospheric layers where the sound speed
and the Alfvén velocity are comparable in magnitude. Away from this
wave conversion zone, which we call the magnetic canopy, the two MAG
waves are effectively decoupled because either the magnetic pressure
(B2/8π) or the plasma pressure (p=NkBT)
dominates over the other. The character of the fluctuations observed
in the magneto-atmosphere depend sensitively on the relative location
and orientation of the magnetic canopy with respect to the wave source
and the observation point. Several distinct wave trains may converge
on and simultaneously pass through a given location. Their coherent
superposition presents a bewildering variety of Doppler and intensity
time series because (1) some waves come directly from the source while
others emerge from the magnetic canopy following mode conversion, (2)
the propagation directions of the individual wave trains are neither
co-aligned with each other nor with the observer's line of sight, and
(3) the wave trains may be either fast or slow MAG waves that exhibit
different characteristics depending on whether they are observed in
high-β or low-β plasmas (β≡8πp/B2). Through the
analysis of four numerical experiments a coherent and physically
intuitive picture emerges of how fast and slow MAG waves interact
within two-dimensional magneto-atmospheres.
Title: Dynamo action in turbulent flows
Authors: Archontis, V.; Dorch, S. B. F.; Nordlund, Å.
Bibcode: 2003A&A...410..759A
Altcode: 2003astro.ph..6069A
We present results from numerical simulations of nonlinear MHD dynamo
action produced by three-dimensional flows that become turbulent for
high values of the fluid Reynolds number. The magnitude of the forcing
function driving the flow is allowed to evolve with time in such way as
to maintain an approximately constant velocity amplitude (and average
kinetic energy) when the flow becomes hydrodynamically unstable. It
is found that the saturation level of the dynamo increases with the
fluid Reynolds number (at constant magnetic Prandtl number), and that
the average growth rate approaches an asymptotic value for high fluid
Reynolds number. The generation and destruction of magnetic field is
examined during the laminar and turbulent phase of the flow and it is
found that in the neighborhood of strong magnetic flux ``cigars" Joule
dissipation is balanced by the work done against the Lorentz force,
while the steady increase of magnetic energy occurs mainly through
work done in the weak part of the magnetic field.
Title: What Causes p-Mode Asymmetry Reversal?
Authors: Georgobiani, Dali; Stein, Robert F.; Nordlund, Åke
Bibcode: 2003ApJ...596..698G
Altcode: 2002astro.ph..5141G
The solar acoustic p-mode line profiles are asymmetric. Velocity spectra
have more power on the low-frequency sides, whereas intensity profiles
show the opposite sense of asymmetry. Numerical simulations of the
upper convection zone have resonant p-modes with the same asymmetries
and asymmetry reversal as the observed modes. The temperature
and velocity power spectra at optical depth τcont=1
have the opposite asymmetry, as is observed for the intensity and
velocity spectra. At a fixed geometrical depth, corresponding to
<τcont>=1, however, the temperature and velocity
spectra have the same asymmetry. This indicates that the asymmetry
reversal in the simulation is produced by radiative transfer effects and
not by correlated noise. The cause of this reversal is the nonlinear
amplitude of the displacements in the simulation and the nonlinear
dependence of the H- opacity on temperature. Where the
temperature is hotter the opacity is larger and photons escape from
higher, cooler layers. This reduces the fluctuations in the radiation
temperature compared to the gas temperature. The mode asymmetry reversal
in the simulation is a small frequency-dependent differential effect
within this overall reduction. Because individual solar modes have
smaller amplitudes than the simulation modes, this effect will be
smaller on the Sun.
Title: Numerical 3D constraints on convective eddy time-correlations:
Consequences for stochastic excitation of solar p modes
Authors: Samadi, R.; Nordlund, Å.; Stein, R. F.; Goupil, M. J.;
Roxburgh, I.
Bibcode: 2003A&A...404.1129S
Altcode: 2003astro.ph..4457S
A 3D simulation of the upper part of the solar convective zone is used
to obtain information on the frequency component, chik
, of the correlation product of the turbulent velocity field. This
component plays an important role in the stochastic excitation of
acoustic oscillations. A time analysis of the solar simulation shows
that a Gaussian function does not correctly reproduce the nu -dependency
of chik inferred from the 3D simuation in the frequency range
where the acoustic energy injected into the solar p modes is important
(nu =~ 2 - 4 mHz). The nu -dependency of chik is fitted
with different analytical functions which can then conveniently be
used to compute the acoustic energy supply rate P injected into the
solar radial oscillations. With constraints from a 3D simulation,
adjustment of free parameters to solar data is no longer necessary
and is not performed here. The result is compared with solar seismic
data. Computed values of P obtained with the analytical function
which fits best chik are found ~ 2.7 times larger than
those obtained with the Gaussian model and reproduce better the solar
seismic observations. This non-Gaussian description also leads to
a Reynolds stress contribution of the same order as the one arising
from the advection of the turbulent fluctuations of entropy by the
turbulent motions. Some discrepancy between observed and computed
P values still exist at high frequency and possible causes for this
discrepancy are discussed.
Title: Numerical constraints on the model of stochastic excitation
of solar-type oscillations
Authors: Samadi, R.; Nordlund, Å.; Stein, R. F.; Goupil, M. J.;
Roxburgh, I.
Bibcode: 2003A&A...403..303S
Altcode: 2003astro.ph..3198S
Analyses of a 3D simulation of the upper layers of a solar convective
envelope provide constraints on the physical quantities which enter
the theoretical formulation of a stochastic excitation model of solar
p modes, for instance the convective velocities and the turbulent
kinetic energy spectrum. These constraints are then used to compute
the acoustic excitation rate for solar p modes, P. The resulting
values are found ~ 5 times larger than the values resulting from a
computation in which convective velocities and entropy fluctuations are
obtained with a 1D solar envelope model built with the time-dependent,
nonlocal Gough (\cite{Gough77}) extension of the mixing length
formulation for convection (GMLT). This difference is mainly due to
the assumed mean anisotropy properties of the velocity field in the
excitation region. The 3D simulation suggests much larger horizontal
velocities compared to vertical ones than in the 1D GMLT solar
model. The values of P obtained with the 3D simulation constraints
however are still too small compared with the values inferred from
solar observations. Improvements in the description of the turbulent
kinetic energy spectrum and its depth dependence yield further increased
theoretical values of P which bring them closer to the observations. It
is also found that the source of excitation arising from the advection
of the turbulent fluctuations of entropy by the turbulent movements
contributes ~ 65-75 % to the excitation and therefore remains dominant
over the Reynolds stress contribution. The derived theoretical values
of P obtained with the 3D simulation constraints remain smaller by a
factor ~ 3 compared with the solar observations. This shows that the
stochastic excitation model still needs to be improved.
Title: Magnetoconvection and micropores
Authors: Bercik, D. J.; Nordlund, A.; Stein, R. F.
Bibcode: 2003ESASP.517..201B
Altcode: 2003soho...12..201B
We report on results from a series of radiative magnetoconvection
simulations in a 12 Mm×12 Mm×3 Mm near-surface solar layer. Initially
unipolar, vertical magnetic field at average field strengths of 0 G,
200 G and 400 G is imposed on a fully relaxed hydrodynamic convective
state. Magnetic field is swept to the intergranular boundaries by
the convective flows, where it is compressed to kilogauss field
strenghts. The shapes and intensities of magnetic features typically
evolve on the same time scale as the granulation pattern; however,
the underlying magnetic structure evolves on a much longer time
scale. Occasionally, dark, high field strength features form that have
properties consistent with observed micropores. The micropores primarily
form when a small granule submerges and the surrounding magnetic field
moves into the resulting dark "hole". The fluid flow inside micropores
is suppressed by the strong magnetic field. The surrounding walls of
a micropore experiences a net cooling through vertical radiation. The
resulting thermodynamic structure of micropores stabilize them against
destruction, allowing some micropores to exist for many granulation
time scales.
Title: Understanding the convective Sun
Authors: Trampedach, Regner; Georgobiani, Dali; Stein, Robert F.;
Nordlund, Åke
Bibcode: 2003ESASP.517..195T
Altcode: 2003soho...12..195T
Hydrodynamical simulations of the surface layers of the Sun, has greatly
improved our understanding and interpretation of solar observations. I
review some past successes in matching spectral lines, improving the
agreement with high-degree p-mode frequencies and matching the depth of
the solar convection zone without adjustable convection-parameters. Our
solar simulations contain p-modes, and are used for studying the
asymmetry of p-mode peaks and to calibrate the conversion between the
observed velocity proxies and the actual velocities.
Title: Asymmetry reversal in solar acoustic modes
Authors: Georgobiani, Dali; Stein, Robert F.; Nordlund, Åke
Bibcode: 2003ESASP.517..279G
Altcode: 2003soho...12..279G
The power spectra of solar acoustic modes are asymmetric, with velocity
having more power on the low frequency side of the peak and intensity
having more power on the high frequency side. This effect exists in both
observations and simulations, and it is believed to be caused by the
correlated background noise. We study the temperature near the solar
surface by means of a 3D hydrodynamic simulation of convection with a
detailed treatment of radiation. The temperature spectrum at optical
depth τcont = 1 has opposite asymmetry to the velocity
spectrum, whereas the temperature measured at a fixed geometrical depth,
corresponding to <τcont> = 1, has the same asymmetry
as velocity. We believe that the asymmetry reversal in temperature
at τcont = 1 (and therefore in intensity) occurs partly
because of the radiative transfer effects. High temperature sensitivity
of the opacity suppresses temperature fluctuations on opposite sides
of the mode peaks differently, thus causing the asymmetry reversal.
Title: Seismic Diagnostics on Stellar Convection Treatment from
Oscillation Amplitudes of p-modes
Authors: Samadi, R.; Goupil, M. J.; Lebreton, Y.; Nordlund, Å.;
Baudin, F.
Bibcode: 2003Ap&SS.284..221S
Altcode: 2002astro.ph.10036S
The excitation rate P of solar p-modes is computed with a model
of stochastic excitation which involves constraints on the averaged
properties of the solar turbulence. These constraints are obtained from
a 3D simulation. Resulting values for P are found ~ 4.5 times larger
than when the calculation assumes properties of turbulent convection
which are derived from an 1D solar model based on Gough (1977)'s
formulation of the mixing-length theory (GMLT). This difference
is mainly due to the assumed values for the mean anisotropy of the
velocity field in each case. Calculations based on 3D constraints
bring the P maximum closer to the observational one. We also compute
P for several models of intermediate mass stars (1 <~ M <~ 2
Msolar). Differences in the values of Pmax between models
computed with the classical mixing-length theory and GMLT models are
found large enough for main sequence stars to suggest that measurements
of P in this mass range will be able to discriminate between different
models of turbulent convection.
Title: Radiative Transfer in 3D Numerical Simulations
Authors: Stein, R. F.; Nordlund, Å.
Bibcode: 2003ASPC..288..519S
Altcode: 2002astro.ph..9510S; 2003sam..conf..519S
We simulate convection near the solar surface, where the continuum
optical depth is of order unity. Hence, to determine the radiative
heating and cooling in the energy conservation equation, we must solve
the radiative transfer equation (instead of using the diffusion or
optically thin cooling approximations). A method efficient enough to
calculate the radiation for thousands of time steps is needed. We
explicitly solve the Feautrier equation along a vertical and four
straight, slanted, rays (at four azimuthal angles which are rotated
every time step) assuming LTE and using a 4 bin opacity distribution
function. We will discuss details of our approach. We also present some
results showing comparison of simulated and observed line profiles in
the Sun, the importance of 3D transfer, stokes profiles for intergranule
magnetic fields and micropores, and the effect of radiation on p-mode
asymmetries.
Title: Characterizing the Dynamic Properties of the Solar Turbulence
with 3-D Simulations: Consequences in Term of p-mode Excitation
Authors: Samadi, R.; Nordlund, Å.; Stein, R. F.; Goupil, M. -J.;
Roxburgh, I.
Bibcode: 2003IAUS..210P..C2S
Altcode: 2002astro.ph..8577S
A 3D simulation of the upper part of the solar convective zone is used
to derive constraints about the averaged and dynamic properties of
solar turbulent convection. Theses constraints are then used to compute
the acoustic energy supply rate P(nu) injected into the solar radial
oscillations according to the theoretical expression in Samadi &
Goupil (2001). The result is compared with solar seismic data. Assuming,
as it is usually done, a gaussian model for the frequency (nu)
component chi_k(nu) of the model of turbulence, it is found that the
computed P(nu) is underestimated compared with the solar seismic data
by a factor ~ 2.5. A frequency analysis of the solar simulation shows
that the gaussian model indeed does not correctly model chi_k(nu) in
the frequency range where the acoustic energy injected into the solar
p-modes is important (nu ~ 2 - 4 mHz). One must consider an additional
non-gaussian component for chi_k(nu) to reproduce its behavior. Computed
values of P obtained with this non-gaussian component reproduce better
the solar seismic observations. This non-gaussian component leads to
a Reynolds stress contribution of the same order than the one arising
from the advection of the turbulent fluctuations of entropy by the
turbulent motions.
Title: Numerical simulations of kinematic dynamo action
Authors: Archontis, V.; Dorch, S. B. F.; Nordlund, Å.
Bibcode: 2003A&A...397..393A
Altcode: 2002astro.ph..4208A
Numerical simulations of kinematic dynamo action in steady and
three-dimensional ABC flows are presented with special focus on the
difference in growth rates between cases with single and multiple
periods of the prescribed velocity field. It is found that the
difference in growth rate (apart from a trivial factor stemming from
a scaling of the rate of strain with the wavenumber of the velocity
field) is due to differences in the recycling of the weakest part of the
magnetic field. The single wavelength classical ABC-flow experiments
impose stronger symmetry requirements, which results in a suppression
of the growth rate. The experiments with larger wave number achieve
growth rates that are more compatible with the turn-over time scale
by breaking the symmetry of the resulting dynamo-generated magnetic
field. Differences in topology in cases with and without stagnation
points in the imposed velocity field are also investigated, and it is
found that the cigar-like structures that develop in the classical
A=B=C dynamos are replaced by ribbon structures in cases where the
flow is without stagnation points.
Title: Solar Surface Magneto-Convection
Authors: Stein, R. F.; Bercik, D.; Nordlund, Å.
Bibcode: 2003ASPC..286..121S
Altcode: 2003ctmf.conf..121S; 2002astro.ph..9470S
Magneto-convection simulations on meso-granule and granule scales near
the solar surface are used to study small scale dynamo activity, the
emergence and disappearance of magnetic flux tubes, and the formation
and evolution of micropores. From weak seed fields, convective motions
produce highly intermittent magnetic fields in the intergranular lanes
which collect over the boundaries of the underlying meso-granular scale
cells. Instances of both emerging magnetic flux loops and magnetic
flux disappearing from the surface occur in the simulations. We show
an example of a flux tube collapsing to kG field strength and discuss
how the nature of flux disappearance can be investigated. Observed
stokes profiles of small magnetic structures are severely distorted by
telescope diffraction and seeing. Because of the strong stratification,
there is little recycling of plasma and field in the surface
layers. Recycling instead occurs by exchange with the deep layers of
the convection zone. Plasma and field from the surface descend through
the convection zone and rise again toward the surface. Because only
a tiny fraction of plasma rising up from deep in the convection zone
reaches the surface due to mass conservation, little of the magnetic
energy resides in the near surface layers. Thus the dynamo acting on
weak incoherent fields is global, rather than a local surface dynamo.
Title: Star Formation and the Initial Mass Function
Authors: Nordlund, Å.; Padoan, P.
Bibcode: 2003LNP...614..271N
Altcode: 2003tmfa.conf..271N; 2002astro.ph..9244N
Supersonic turbulence fragments the interstellar medium into dense
sheets, filaments, cores and large low-density voids, thanks to a
complex network of highly radiative shocks. The turbulence is driven on
large scales, predominantly by supernovae. While on scales of the order
of the galactic disk thickness the magnetic energy is in approximate
equipartition with the kinetic energy of the turbulence, on scales
of a few pc the turbulent kinetic energy significantly exceeds the
magnetic energy. The scaling properties of supersonic turbulence are
well described by a new analytical theory, which allows to predict
the structure functions of the density and velocity distributions in
star-forming clouds up to very high order. The distribution of core
masses depends primarily on the power spectrum of the turbulent flow,
and on the jump conditions for isothermal shocks in a magnetized
gas. For the predicted velocity power spectrum index beta=1.74,
consistent with results of numerical experiments of supersonic
turbulence as well as with Larson's velocity-size relation, one obtains
by scaling arguments a power law mass distribution of dense cores with
a slope equal to 3/(4-beta) = 1.33, consistent with the slope of the
Salpeter stellar initial mass function (IMF). Results from numerical
simulations confirm this scaling. The analytical model for the stellar
IMF and its numerical estimate show that turbulent fragmentation may
explain the origin of brown dwarfs, but only if the critical mass
for collapse under dynamical conditions is an order of magnitude
smaller than the Jeans' mass from a linear stability analysis. The
main conclusion is that the stellar IMF directly reflects the mass
distribution of prestellar cores, due predominantly to the process of
turbulent fragmentation.
Title: Solar Surface Magnetoconvection
Authors: Stein, R. F.; Nordlund, Å.
Bibcode: 2003IAUS..210..169S
Altcode:
No abstract at ADS
Title: Solar photosphere and convection
Authors: Nordlund, Å.
Bibcode: 2003dysu.book..148N
Altcode:
An abrupt transition from convective to radiative energy transport at
the solar surface results in a spatially and temporally very complex
photosphere. The properties of the solar photosphere as well as its
importance for both the sub-surface layers and for the chromosphere and
corona above are now beginning to be understood in some detail. Progress
has been made largely through the use and interpretation of numerical
simulations of this region. Comparisons are made in a forward sense;
synthetic observational data are generated from the numerical models,
and are compared directly with corresponding observational data.
Title: Particle Acceleration in Relativistic Collisionless Shocks
Authors: Hededal, Christian B.; Nordlund, Åke; Frederiksen, Jacob T.
Bibcode: 2003IAUJD...1E..12H
Altcode:
Through relativistic charged particle-in-cell simulations we show
how plasma instabilities can create strong particle acceleration and
magnetic field generation in collisionless shocks.
Title: On the generation of internal gravity waves by penetrative
convection
Authors: Dintrans, B.; Brandenburg, A.; Nordlund, Å.; Stein, R. F.
Bibcode: 2003sf2a.conf..511D
Altcode: 2003sf2a.confE.216D
Gravity waves propagating in the radiative zones of solar-type stars are
suspected to play a major role in the transport processes. However, the
problem of their excitation remains open as a simple kappa-mechanism
based on hydrogen and helium ionization zones is not applicable
here. One possibility concerns the excitation by overshooting convection
from neighboring convection zones. Strong downward plumes are known
to penetrate substantial distances into the adjacent stable zone so
that internal gravity waves can be randomly generated. We will present
results coming from 2D-simulations of overshooting convection, for
which a new detection method based on the anelastic subspace allows
us to precisely measure internal waves which are stochastically excited.
Title: Stochastic Excitation of Gravity Waves by Overshooting
Convection in Solar-Type Stars
Authors: Dintrans, Boris; Brandenburg, Axel; Nordlund, Åke; Stein,
Robert F.
Bibcode: 2003Ap&SS.284..237D
Altcode:
The excitation of gravity waves by penetrative convective plumes is
investigated using 2D direct simulations of compressible convection. The
oscillation field is measured by a new technique based on the projection
of our simulation data onto the theoretical g-modes solutions of the
associated linear eigenvalue problem. This allows us to determine both
the excited modes and their corresponding amplitudes accurately.
Title: Structure Function Scaling in the Taurus and Perseus Molecular
Cloud Complexes
Authors: Padoan, Paolo; Boldyrev, Stanislav; Langer, William;
Nordlund, Åke
Bibcode: 2003ApJ...583..308P
Altcode: 2002astro.ph..7568P
We compute the structure function scaling of the integrated intensity
images of two J=1-0 13CO maps of Taurus and Perseus. The
scaling exponents of the structure functions, normalized to the third
order, follow the velocity scaling of supersonic turbulence, suggesting
that turbulence plays an important role in the fragmentation of cold
interstellar clouds. The data also allow one to verify the validity
of the two basic assumptions of the hierarchical symmetry model,
originally proposed for the derivation of the velocity structure
function scaling. This shows that the same hierarchical symmetry holds
for the projected density field of cold interstellar clouds.
Title: Dynamo action in turbulent flows
Authors: Archontis, V.; Nordlund, Å.
Bibcode: 2002ESASP.505...95A
Altcode: 2002solm.conf...95A; 2002IAUCo.188...95A
We present results from numerical simulations of kinematic and nonlinear
MHD dynamo action produced by turbulent flows. Traditionally, turbulence
was thought to be essential to dynamo action. There is new evidence
that indicates that laminar and turbulent dynamos are surprisingly
similar, with growth rates similar to large scale turn-over time in
both cases. An analysis of the Lorentz work and Joule dissipation
shows that dynamo occurs primarily in regions where the field is weak
by bending and stretching the magnetic field lines.
Title: Solar Surface Magneto-Convection and Dynamo Action
Authors: Stein, Robert F.; Nordlund, Åke
Bibcode: 2002ESASP.505...83S
Altcode: 2002IAUCo.188...83S; 2002solm.conf...83S
Magneto-convection simulations on meso-granule and granule scales near
the solar surface are used to study small scale dynamo activity and the
emergence and disappearance of magnetic flux tubes. Convective motions
produce highly intermittent magnetic fields in the intergranular lanes
which collect over the boundaries of the underlying meso-granular scale
cells. When observing these magnetic fields, it is important to note
that the telescope point spread function and seeing significantly
reduce the amplitudes of the observed Stokes profiles. Because of
the strong stratification, there is little recycling of plasma and
field in these surface layers. Recycling instead occurs by exchange
with the deep layers of the convection zone. Plasma and field from
the surface descend to the bottom of the convection zone, where they
rise again toward the surface. Because the turnover time in the deep
convection zone is of order a month, and because only a tiny fraction
of plasma rising up from the bottom of the convection zone reaches the
surface due to mass conservation, the time constant for this dynamo
is long and little of the magnetic energy resides in the near surface
layers. Thus the dynamo acting on weak incoherent fields is global,
rather than a local surface dynamo.
Title: A simulation of solar convection at supergranulation scale
Authors: Rieutord, M.; Ludwig, H. -G.; Roudier, T.; Nordlund, .;
Stein, R.
Bibcode: 2002NCimC..25..523R
Altcode: 2001astro.ph.10208R
We present here numerical simulations of surface solar convection
which cover a box of 30$\times30\times$3.2 Mm$^3$ with a resolution of
315$\times315\times$82, which is used to investigate the dynamics of
scales larger than granulation. No structure resembling supergranulation
is present; possibly higher Reynolds numbers (i.e. higher numerical
resolution), or magnetic fields, or greater depth are necessary. The
results also show interesting aspects of granular dynamics which are
briefly presented, like extensive p-mode ridges in the k-$\omega$
diagram and a ringlike distribution of horizontal vorticity around
granules. At large scales, the horizontal velocity is much larger
than the vertical velocity and the vertical motion is dominated by
p-mode oscillations.
Title: Solar convection and magneto-convection simulations
Authors: Stein, R. F.; Bercik, D.; Nordlund, A.
Bibcode: 2002NCimC..25..513S
Altcode: 2001astro.ph.12117S
Magneto-convection simulations with two scenarios have been
performed: In one, horizontal magnetic field is advected into the
computational domain by fluid entering at the bottom. In the other,
an initially uniform vertical magnetic field is imposed on a snapshot
of non-magnetic convection and allowed to evolve. In both cases,
the field is swept into the intergranular lanes and the boundaries
of the underlying mesogranules. The largest field concentrations at
the surface reach pressure balance with the surrounding gas. They
suppress both horizontal and vertical flows, which reduces the heat
transport. They cool, become evacuated and their optical depth unity
surface is depressed by several hundred kilometers. Micropores form,
typically where a small granule disappears and surrounding flux tubes
squeeze into its previous location.
Title: The Stellar Initial Mass Function from Turbulent Fragmentation
Authors: Padoan, Paolo; Nordlund, Åke
Bibcode: 2002ApJ...576..870P
Altcode: 2000astro.ph.11465P
The morphology and kinematics of molecular clouds (MCs) are best
explained as the consequence of supersonic turbulence. Supersonic
turbulence fragments MCs into dense sheets, filaments, and cores
and large low-density ``voids,'' via the action of highly radiative
shocks. We refer to this process as turbulent fragmentation. In this
work we derive the mass distribution of gravitationally unstable
cores generated by the process of turbulent fragmentation. The mass
distribution above 1 Msolar depends primarily on the power
spectrum of the turbulent flow and on the jump conditions for isothermal
shocks in a magnetized gas. For a power spectrum index β=1.74,
consistent with Larson's velocity dispersion-size relation as well as
with new numerical and analytic results on supersonic turbulence, we
obtain a power-law mass distribution of dense cores with a slope equal
to 3/(4-β)=1.33, consistent with the slope of the stellar initial
mass function (IMF). Below 1 Msolar, the mass distribution
flattens and turns around at a fraction of 1 Msolar,
as observed for the stellar IMF in a number of stellar clusters,
because only the densest cores are gravitationally unstable. The
mass distribution at low masses is determined by the probability
distribution of the gas density, which is known to be approximately
lognormal for an isothermal turbulent gas. The intermittent nature of
the turbulent density distribution is thus responsible for the existence
of a significant number of small collapsing cores, even of substellar
mass. Since turbulent fragmentation is unavoidable in supersonically
turbulent molecular clouds, and given the success of the present model
in predicting the observed shape of the stellar IMF, we conclude that
turbulent fragmentation is essential to the origin of the stellar IMF.
Title: Supersonic Turbulence and Structure of Interstellar Molecular
Clouds
Authors: Boldyrev, Stanislav; Nordlund, Åke; Padoan, Paolo
Bibcode: 2002PhRvL..89c1102B
Altcode: 2002astro.ph..3452B
The interstellar medium provides a unique laboratory for highly
supersonic, driven hydrodynamic turbulence. We propose a theory of
such turbulence, test it by numerical simulations, and use the results
to explain observational scaling properties of interstellar molecular
clouds, the regions where stars are born.
Title: Scaling Relations of Supersonic Turbulence in Star-forming
Molecular Clouds
Authors: Boldyrev, Stanislav; Nordlund, Åke; Padoan, Paolo
Bibcode: 2002ApJ...573..678B
Altcode: 2001astro.ph.11345B
We present a direct numerical and analytical study of driven
supersonic magnetohydrodynamic turbulence that is believed to
govern the dynamics of star-forming molecular clouds. We describe
statistical properties of the turbulence by measuring the velocity
difference structure functions up to the fifth order. In particular,
the velocity power spectrum in the inertial range is found to be
close to Ek~k-1.74, and the velocity difference
scales as <|Δu|>~L0.42. The results agree well with
the Kolmogorov-Burgers analytical model suggested for supersonic
turbulence.We then generalize the model to more realistic, fractal
structure of molecular clouds and show that depending on the fractal
dimension of a given molecular cloud, the theoretical value for
the velocity spectrum spans the interval [-1.74, -1.89], while the
corresponding window for the velocity difference scaling exponent is
[0.42, 0.78].
Title: Waves in magnetic flux concentrations: The critical role of
mode mixing and interference
Authors: Bogdan, T. J.; Rosenthal, C. S.; Carlsson, M.; Hansteen, V.;
McMurry, A.; Zita, E. J.; Johnson, M.; Petty-Powell, S.; McIntosh,
S. W.; Nordlund, Å.; Stein, R. F.; Dorch, S. B. F.
Bibcode: 2002AN....323..196B
Altcode:
Time-dependent numerical simulations of nonlinear wave propagation
in a two-dimensional (slab) magnetic field geometry show wave mixing
and interference to be important aspects of oscillatory phenomena in
starspots and sunspots. Discrete sources located within the umbra
generate both fast and slow MHD waves. The latter are compressive
acoustic waves which are guided along the magnetic field lines and
steepen into N-waves with increasing height in the spot atmosphere. The
former are less compressive, and accelerate rapidly upward through the
overlying low-beta portion of the umbral photosphere and chromosphere
(beta equiv 8pi p/ B2). As the fast wave fronts impinge
upon the beta ~ 1 penumbral ``magnetic canopy" from above, they
interfere with the outward-propagating field-guided slow waves, and
they also mode convert to (non-magnetic) acoustic-gravity waves as
they penetrate into the weak magnetic field region which lies between
the penumbral canopy and the base of the surrounding photosphere. In
a three-dimensional situation, one expects additional generation,
mixing and interference with the remaining torsional Alfvén waves.
Title: Consequences of the non gaussian character of the stochastic
excitation for solar-type oscillations
Authors: Samadi, R.; Nordlund, A.; Stein, R. F.; Goupil, M. -J.;
Roxburgh, I.
Bibcode: 2002sf2a.conf..489S
Altcode: 2002astro.ph.10028S
Stochastic excitation of stellar p-modes of low massive stars (M <
2Mo) are attribued to regular turbulent cells moving in the upper
convective zone. The current calculation of the acoustic energy supply
rate P - which ensures the p-modes excitation - is mainly based on this
simplifying picture and thus assume a crude description of the static
and dynamic properties of the turbulent medium. With the help of a 3D
simulation of the solar convective zone, we show that the gaussian model
does not sastisfactory model the dynamical behavior of the turbulent
medium in the frequency range where the acoustic energy injected into
the solar p-modes is important (frequency : 2 - 4 mHz). Instead, one
has to consider an additionnal component - which slowly decreases with
frequency - to reproduce better the dynamic of the turbulence. This
non-gaussian component is suggested arising from presence of plumes
in the solar convection region. Inclusion of it leads to a Reynolds
stress contribution of the same order than the one arising from the
advection of the turbulent fluctuations of entropy by the turbulent
movements. In the present work we investigate some consequences of this
non-gaussian component for the p-modes excitation in low massive stars
(M < 2Mo) and compare our computations of P with previous estimates.
Title: Bulk Heating and Slender Magnetic Loops in the Solar Corona
Authors: Gudiksen, Boris Vilhelm; Nordlund, Åke
Bibcode: 2002ApJ...572L.113G
Altcode:
The heating of the solar corona and the puzzle of the slender high
reaching magnetic loops seen in observations from the Transition
Region and Coronal Explorer (TRACE) has been investigated through
three-dimensional numerical simulations and found to be caused by
the well-observed plasma flows in the photosphere displacing the
footpoints of magnetic loops in a nearly potential configuration. It
is found that even the small convective displacements cause magnetic
dissipation sufficient to heat the corona to temperatures of the
order of a million K. The heating is intermittent in both space and
time-at any one height and time it spans several orders of magnitude,
and localized heating causes transonic flows along field lines, which
explains the observed nonhydrostatic stratification of loops that are
bright in emission measure.
Title: Stability and heating of magnetically driven jets from
Keplerian accretion discs
Authors: Thomsen, F.; Nordlund, A.
Bibcode: 2002astro.ph..6394T
Altcode:
We have performed 3-D numerical magnetohydrodynamic (MHD) jet
experiments to study the instabilities associated with strongly toroidal
magnetic fields and determine if such magnetic configurations in jets
are as unstable as similar situations are found to be in the laboratory
and in analytical estimates. A perfectly conducting Keplerian disc
with fixed density, rotational velocity and pressure is used as
a lower boundary for the jet. The energy equation is solved, with
the inclusion of self-consistently computed heating by viscous and
magnetic dissipation. The resulting jets evolve into time-dependent,
non-axisymmetric configurations, but we find only minor disruption of
the jets by for example the kink instability. We find that magnetic
dissipation may have profound effects on the jet flow as: 1) it turns on
in highly wound up magnetic field regions and helps to prevent critical
kink situations; 2) it influences jet dynamics by re-organizing the
magnetic field structure and increasing thermal pressure in the jet;
and 3) it influences mass loading by increasing temperature and pressure
at the base of the jet.
Title: Supersonic turbulence and structure of interstellar molecular
clouds
Authors: Padoan, P.; Boldyrev, S.; Nordlund, A.
Bibcode: 2002AAS...200.7319P
Altcode: 2002BAAS...34..769P
The interstellar medium provides unique laboratory for highly
supersonic, driven hydrodynamics turbulence. We present a theory of such
turbulence, confirm it by numerical simulations, and use the results
to explain observational properties of interstellar molecular clouds,
the regions where stars are born.
Title: Observational Signatures of a Solar Small-Scale Global Dynamo
Authors: Keller, C. U.; Stein, R. F.; Nordlund, A.
Bibcode: 2002AAS...200.8908K
Altcode: 2002BAAS...34..792K
There is ample theoretical and observational evidence for the existence
of a dynamo operating in the solar convection zone that produces
small-scale, weak magnetic fields. The next generation of solar
telescopes such as the 4-m Advanced Technology Solar Telescope and
the 1.5-m GREGOR will be able to provide observational data on these
magnetic fields. In order to guide the development of instruments and
observational procedures to investigate these small-scale magnetic
fields, we have calculated polarized spectral line profiles from
numerical simulations of a small-scale global dynamo and analyzed
them as if they were actual observations of the Sun. The simulated
observations include realistic noise, spatial smearing from a partially
correcting AO system, and spectral smearing and scattered light from a
spectrograph. We identify the unique signatures of these magnetic fields
and relate them to the physical conditions in the numerical simulations.
Title: The Structure and Dissipation of Hierarchial Current Sheets;
When and How to Apply Adaptive Meshes, and When Not
Authors: Nordlund, Åke
Bibcode: 2002smra.progE..18N
Altcode:
No abstract at ADS
Title: Waves in the Magnetized Solar Atmosphere. I. Basic Processes
and Internetwork Oscillations
Authors: Rosenthal, C. S.; Bogdan, T. J.; Carlsson, M.; Dorch,
S. B. F.; Hansteen, V.; McIntosh, S. W.; McMurry, A.; Nordlund, Å.;
Stein, R. F.
Bibcode: 2002ApJ...564..508R
Altcode:
We have modeled numerically the propagation of waves through magnetic
structures in a stratified atmosphere. We first simulate the propagation
of waves through a number of simple, exemplary field geometries in
order to obtain a better insight into the effect of differing field
structures on the wave speeds, amplitudes, polarizations, direction
of propagation, etc., with a view to understanding the wide variety of
wavelike and oscillatory processes observed in the solar atmosphere. As
a particular example, we then apply the method to oscillations in the
chromospheric network and internetwork. We find that in regions where
the field is significantly inclined to the vertical, refraction by
the rapidly increasing phase speed of the fast modes results in total
internal reflection of the waves at a surface whose altitude is highly
variable. We conjecture a relationship between this phenomenon and the
observed spatiotemporal intermittancy of the oscillations. By contrast,
in regions where the field is close to vertical, the waves continue
to propagate upward, channeled along the field lines but otherwise
largely unaffected by the field.
Title: Magnetic fields in young galaxies
Authors: Nordlund, Åke; Rögnvaldsson, Örnólfur
Bibcode: 2002HiA....12..706N
Altcode:
We have studied the fate of initial magnetic fields in the hot halo gas
out of which the visible parts of galaxies form, using three-dimensional
numerical MHD-experiments. The halo gas undergoes compression by
several orders of magnitude in the subsonic cooling flow that forms
the cold disk. The magnetic field is carried along and is amplified
considerably in the process, reaching μG levels for reasonable values
of the initial ratio of magnetic to thermal energy density.
Title: Collisionless Shocks - Magnetic Field Generation and Particle
Acceleration
Authors: Frederiksen, J. T.; Hededal, C. B.; Haugbølle, T.;
Nordlund, Å.
Bibcode: 2002bjgr.conf..115F
Altcode: 2003astro.ph..3360F; 2003astro.ph..3360T
We present numerical results from plasma particle simulations
of collisionless shocks and ultra-relativistic counter-streaming
plasmas. We demonstrate how the field-particle interactions lead to
particle acceleration behind the shock-front. Further, we demonstrate
how ultra relativistic counter-streaming plasmas create large scale
patchy magnetic field structures and that these field structures
propagate down-stream of the shock front. These results may help
explain the origin of the magnetic fields and accelerated electrons
responsible for afterglow synchrotron radiation from gamma ray bursts.
Title: Kolmogorov-Burgers Model for Turbulence in Molecular Clouds
Authors: Boldyrev, S.; Nordlund, A.; Padoan, P.
Bibcode: 2001AAS...19914902B
Altcode: 2001BAAS...33R1528B
The process of star formation in interstellar molecular clouds is
believed to be controlled by driven supersonic magnetohydrodynamic
turbulence. We suggest that in the inertial range such turbulence
obeys the Kolmogorov law, while in the dissipative range it
behaves as Burgers turbulence developing shock singularities. On
the base of the She--Leveque analytical model we then predict
the velocity power spectrum in the inertial range to be
Ek~k-1.74. This result reproduces
the observational Larson law, < u2_l > ~
l0.74\cdots0.76, [Larson, MNRAS 194 (1981) 809] and
agrees well with recent numerical findings by Padoan and Nordlund
[astro-ph/0011465]. The application of the model to more general
dissipative structures, with higher fractal dimensionality, leads to
better agreement with recent observational results.
Title: Cooling Rates of Molecular Clouds Based on Numerical
Magnetohydrodynamic Turbulence and Non-LTE Radiative Transfer
Authors: Juvela, Mika; Padoan, Paolo; Nordlund, Åke
Bibcode: 2001ApJ...563..853J
Altcode: 2001astro.ph..4280J
We have computed line-emission cooling rates for the main cooling
species in models of interstellar molecular clouds. The models are
based on numerical simulations of supersonic magnetohydrodynamic (MHD)
turbulence. Non-LTE radiative transfer calculations have been performed
to properly account for the complex density and velocity structures
in the MHD simulations. Three models are used. Two of the models
are based on MHD simulations with different magnetic field strength
(one model is super-Alfvénic, while the other has equipartition of
magnetic and kinetic energy). The third model includes the computation
of self-gravity (in the super-Alfvénic regime of turbulence). The
density and velocity fields in the simulations are determined
self-consistently by the dynamics of supersonic turbulence. The models
are intended to represent molecular clouds with linear size L~6 pc and
mean density <n>~300 cm-3, with the density exceeding
104 cm-3 in the densest cores. We present
12CO, 13CO, C18O, O2, O I,
C I, and H2O cooling rates in isothermal clouds with kinetic
temperatures 10-80 K. Analytical approximations are derived for the
cooling rates. The inhomogeneity of the models reduces photon trapping
and enhances the cooling in the densest parts of the clouds. Compared
with earlier models, the cooling rates are less affected by optical
depth effects. The main effects come, however, from the density
variation, since cooling efficiency increases with density. This is
very important for the cooling of the clouds as a whole, since most
cooling is provided by gas with density above the average.
Title: Flux-loss of buoyant ropes interacting with convective flows
Authors: Dorch, S. B. F.; Gudiksen, B. V.; Abbett, W. P.; Nordlund, Å.
Bibcode: 2001A&A...380..734D
Altcode: 2001astro.ph.10205D
We present 3-d numerical magneto-hydrodynamic simulations of a buoyant,
twisted magnetic flux rope embedded in a stratified, solar-like model
convection zone. The flux rope is given an initial twist such that it
neither kinks nor fragments during its ascent. Moreover, its magnetic
energy content with respect to convection is chosen so that the flux
rope retains its basic geometry while being deflected from a purely
vertical ascent by convective flows. The simulations show that magnetic
flux is advected away from the core of the flux rope as it interacts
with the convection. The results thus support the idea that the amount
of toroidal flux stored at or near the bottom of the solar convection
zone may currently be underestimated.
Title: Are granules good tracers of solar surface velocity fields?
Authors: Rieutord, M.; Roudier, T.; Ludwig, H. -G.; Nordlund, Å.;
Stein, R.
Bibcode: 2001A&A...377L..14R
Altcode: 2001astro.ph..8284R
Using a numerical simulation of compressible convection with radiative
transfer mimicking the solar photosphere, we compare the velocity
field derived from granule motions to the actual velocity field of
the plasma. We thus test the idea that granules may be used to trace
large-scale velocity fields at the sun's surface. Our results show that
this is indeed the case provided the scale separation is sufficient. We
thus estimate that neither velocity fields at scales less than 2500
km nor time evolution at scales shorter than 0.5 hr can be faithfully
described by granules. At larger scales the granular motions correlate
linearly with the underlying fluid motions with a slope of ≲2 reaching
correlation coefficients up to ~ 0.9.
Title: Magnetohydrodynamic turbulence in warped accretion discs
Authors: Torkelsson, Ulf; Ogilvie, Gordon I.; Brandenburg, Axel;
Pringle, James E.; Nordlund, Åke; Stein, Robert F.
Bibcode: 2001AIPC..586..681T
Altcode: 2001tsra.conf..681T; 2001astro.ph..3057T
Warped, precessing accretion discs appear in a range of astrophysical
systems, for instance the X-ray binary Her X-1 and in the active
nucleus of NGC4258. In a warped accretion disc there are horizontal
pressure gradients that drive an epicyclic motion. We have studied
the interaction of this epicyclic motion with the magneto-hydrodynamic
turbulence in numerical simulations. We find that the turbulent stress
acting on the epicyclic motion is comparable in size to the stress that
drives the accretion, however an important ingredient in the damping
of the epicyclic motion is its parametric decay into inertial waves. .
Title: Theoretical Models of Polarized Dust Emission from Protostellar
Cores
Authors: Padoan, Paolo; Goodman, Alyssa; Draine, B. T.; Juvela, Mika;
Nordlund, Åke; Rögnvaldsson, Örnólfur Einar
Bibcode: 2001ApJ...559.1005P
Altcode: 2001astro.ph..4231P
We model the polarized thermal dust emission from protostellar
cores that are assembled by supersonic turbulent flows in molecular
clouds. Self-gravitating cores are selected from a three-dimensional
simulation of supersonic and super-Alfvénic magnetohydrodynamic
(MHD) turbulence. The polarization is computed in two ways. In model
A it is assumed that dust properties and grain alignment efficiency
are uniform; in model B it is assumed that grains are not aligned at
visual extinction larger than AV,0=3 mag, consistent with
theoretical expectations for grain alignment mechanisms. Instead of
using a specific set of grain properties, we adopt a maximum degree
of polarization Pmax=15%. Results are therefore sensitive
mainly to the topology of the magnetic field (model A) and to the
gas distribution that determines the distribution of AV
(model B). Furthermore, the radiative transfer in the MHD model is
solved with a non-LTE Monte Carlo method, to compute spectral maps of
the J=1-0 transition of CS. The CS spectral maps are used to estimate
the turbulent velocity, as in the observations. The main results of
this work are the following: (1) Values of P between 1% and 10% (up
to almost Pmax) are typical, despite the super-Alfvénic
nature of the turbulence. (2) A steep decrease of P with increasing
values of the submillimeter dust continuum intensity I is always
found in self-gravitating cores selected from the MHD simulations
if grains are not aligned above a certain value of visual extinction
AV,0 (model B). (3) The same behavior is hard to reproduce
if grains are aligned independently of AV (model A). (4)
The Chandrasekhar-Fermi formula, corrected by a factor f~0.4, provides
an approximate estimate of the average magnetic field strength in the
cores. Submillimeter dust continuum polarization maps of quiescent
protostellar cores and Bok globules have recently been obtained. They
always show a decrease in P with increasing value of I consistent with
the predictions of our model B. We therefore conclude that submillimeter
polarization maps of quiescent cores do not map the magnetic field
inside the cores at visual extinction larger than AV,0~3
mag. The use of such maps to constrain models of protostellar core
formation and evolution is questionable. This conclusion suggests that
there is no inconsistency between the results from optical and near-IR
polarized absorption of background stars and the observed polarization
of submillimeter dust continuum from quiescent cores. In both cases,
grains at large visual extinction appear to be virtually unaligned.
Title: Wave Propagation in a Magnetized Atmosphere
Authors: Bogdan, T. J.; Rosenthal, C. S.; Carlsson, M.; McIntosh,
S.; Dorch, S.; Hansteen, V.; McMurry, A.; Nordlund, Å; Stein, R. F.
Bibcode: 2001AGUSM..SH41A01B
Altcode:
Numerical simulations of MHD wave propagation in plane-parallel
atmospheres threaded by non-trivial potential magnetic fields will be
presented, and their implications for understanding distinctions between
intranetwork and internetwork oscillations will be discussed. Our
findings basically confirm the conjecture of McIntosh et al. (2001,
ApJ 548, L237), that the two-dimensional surface where the Alfvén
and sound speeds coincide (i.e., where the plasma-β , the ratio of
gas to magnetic pressure, is of order unity) plays a fundamental
role in mediating the conversion between the fast-, intermediate-
(Alfvén), and slow-Magneto-Atmospheric-Gravity (MAG) waves. For
example, upward-propagating acoustic waves generated at the base of
the internetwork photosphere suffer significant downward reflection
when they encounter this β ≈ 1 surface. Close to the network, this
surface descends from the upper chromosphere and low corona (which
pertains in the internetwork cell interiors) down into the photosphere,
and so chromospheric oscillation `shadows' are predicted to surround
the network. In the network, strong vertical magnetic fields further
depress the β ≈ 1 surface below the surface layers where the
(magnetic field-aligned) acoustic waves (i.e., slow MAG-waves) are
generated. For frequencies in excess of the cutoff frequency, these
acoustic waves suffer little reflection from the overlying atmosphere
and they steepen as they progress upward.
Title: The Turbulent Shock Origin of Proto-Stellar Cores
Authors: Padoan, Paolo; Juvela, Mika; Goodman, Alyssa A.; Nordlund,
Åke
Bibcode: 2001ApJ...553..227P
Altcode: 2000astro.ph.11122P
The fragmentation of molecular clouds (MC) into proto-stellar cores
is a central aspect of the process of star formation. Because of the
turbulent nature of supersonic motions in MCs, it has been suggested
that dense structures such as filaments and clumps are formed by shocks
in a turbulent flow. In this work we present strong evidence in favor
of the turbulent origin of the fragmentation of MCs. The most generic
result of turbulent fragmentation is that dense postshock gas traces
a gas component with a smaller velocity dispersion than lower density
gas, since shocks correspond to regions of converging flows, where the
kinetic energy of the turbulent motion is dissipated. Using synthetic
maps of spectra of molecular transitions, computed from the results
of numerical simulations of supersonic turbulence, we show that the
dependence of velocity dispersion on gas density generates an observable
relation between the rms velocity centroid and the integrated intensity
(column density), σ(V0)-I, which is indeed found in the
observational data. The comparison between the theoretical model (maps
of synthetic 13CO spectra) with 13CO maps from
the Perseus, Rosette, and Taurus MC complexes shows excellent agreement
in the σ(V0)-I relation. The σ(V0)-I relation
of different observational maps with the same total rms velocity are
remarkably similar, which is a strong indication of their origin from
a very general property of the fluid equations, such as the turbulent
fragmentation process.
Title: Waves in the Magnetised Solar Atmosphere
Authors: Rosenthal, C. S.; Carlsson, M.; Hansteen, V.; McMurry,
A.; Bogdan, T. J.; McIntosh, S.; Nordlund, A.; Stein, R. F.; Dorch,
S. B. F.
Bibcode: 2001IAUS..203..170R
Altcode:
We have simulated the propagation of magneto-acoustic disturbances
through various magneto-hydrostatic structures constructed to mimic
the solar magnetic field. As waves propagate from regions of strong
to weak magnetic field and vice-versa different types of wave modes
(transverse and longitudinal) are coupled. In closed-field geometries
we see the trapping of wave energy within loop-like structures. In
open-field regions we see wave energy preferentially focussed away
from strong-field regions. We discuss these oscillations in terms
of various wave processes seen on the Sun - umbral oscillations,
penumbral running waves, internetwork oscillations etc.
Title: Solar Oscillations and Convection. I. Formalism for Radial
Oscillations
Authors: Nordlund, Å.; Stein, R. F.
Bibcode: 2001ApJ...546..576N
Altcode: 2000astro.ph..6336N
We present a formalism for investigating the interaction between p-mode
oscillations and convection by analyzing realistic, three-dimensional
simulations of the near-surface layers of the solar convection
zone. By choosing suitable definitions for fluctuations and averages,
we obtain a separation that retains exact equations. The equations for
the horizontal averages contain one part that corresponds directly to
the wave equations for a one-dimensional medium, plus additional terms
that arise from the averaging and correspond to the turbulent pressure
gradient in the momentum equation and the divergence of the convective
and kinetic energy fluxes in the internal energy equation. These
terms cannot be evaluated in closed form, but they may be measured
in numerical simulations. The additional terms may cause the mode
frequencies to shift, relative to what would be obtained if only the
terms corresponding to a one-dimensional medium were retained-most
straightforwardly by changing the mean stratification and more subtly by
changing the effective compressibility of the medium. In the presence of
time-dependent convection, the additional terms also have a stochastic
time dependence, which acts as a source of random excitation of the
coherent modes. In the present paper, we derive an expression for the
excitation power and test it by applying it to a numerical experiment
of sufficient duration for the excited modes to be spectrally resolved.
Title: Convective Pumping of Magnetic Fields: On the Flux Storage
Problem for Solar-like Dynamos
Authors: Dorch, S. B. F.; Nordlund, Å.
Bibcode: 2001IAUS..203..186D
Altcode:
A long standing issue in the theory of stellar dynamos is the problem
of keeping the magnetic field within the convection zone long enough
for the dynamo to operate: A magnetic flux rope is typically assumed
to escape the convection zone in a month or so, while the dynamo
is thought to operate on a longer time scale of decades. We present
results from three-dimensional numerical simulations, of the interaction
of stratified over-turning solar-like convection with a large-scale
magnetic field: By the very topology of stellar convection, even a
formally super-equipartion field may be held down at the bottom of
the convection zone, rendering the storage problem obsolete. This
effect might also explain the observations of magnetically active
but fully convective late type dwarf stars. Several simulations have
been performed, with both open and closed upper boundary conditions,
as well as including differential rotation: Inclusion of an open upper
boundary may lead to a considerable flux loss unless the boundary is
placed close to the physical boundary.
Title: Solar Oscillations and Convection. II. Excitation of Radial
Oscillations
Authors: Stein, R. F.; Nordlund, Å.
Bibcode: 2001ApJ...546..585S
Altcode: 2000astro.ph..8048S
Solar p-mode oscillations are excited by the work of stochastic,
nonadiabatic, pressure fluctuations on the compressive modes. We
evaluate the expression for the radial mode excitation rate derived
by Nordlund & Stein using numerical simulations of near-surface
solar convection. We first apply this expression to the three radial
modes of the simulation and obtain good agreement between the predicted
excitation rate and the actual mode damping rates as determined from
their energies and the widths of their resolved spectral profiles. These
radial simulation modes are essentially the same as the solar modes
at the resonant frequencies, where the solar modes have a node at
the depth of the bottom of the simulation domain. We then apply this
expression for the mode excitation rate to the solar modes and obtain
excellent agreement with the low l damping rates determined from data
obtained by the ``global oscillations at low frequencies'' (GOLF)
instrument on SOHO. Excitation occurs close to the surface, mainly
in the intergranular lanes and near the boundaries of granules (where
turbulence and radiative cooling are large). The nonadiabatic pressure
fluctuations near the surface are produced by small instantaneous local
imbalances between the divergence of the radiative and convective fluxes
near the solar surface. Below the surface, the nonadiabatic pressure
fluctuations are produced primarily by turbulent-pressure fluctuations
(Reynolds stresses). The frequency dependence of the mode excitation
is due to effects of the mode structure and the pressure fluctuation
spectrum. Excitation is small at low frequencies because of mode
properties-the mode compression decreases and the mode mass increases
at low frequency. Excitation is small at high frequencies because of
the pressure fluctuation spectrum-pressure fluctuations become small
at high frequencies because they are due to convection, which is a
long-timescale phenomenon compared with the dominant p-mode periods.
Title: Turbulent Fragmentation and the Initial Conditions for Star
Formation
Authors: Padoan, P.; Nordlund, Å.; Rögnvaldsson, Ö. E.; Goodman, A.
Bibcode: 2001ASPC..243..279P
Altcode: 2001fdtl.conf..279P
No abstract at ADS
Title: R200,000 Spectroscopic Observations of Procyon. The Surface
Convection and Radial Velocity (CD-ROM Directory: contribs/allende2)
Authors: Allende Prieto, C.; Asplund, M.; García López, R. J.;
Lambert, D. L.; Nordlund, Å.
Bibcode: 2001ASPC..223..760A
Altcode: 2001csss...11..760A
No abstract at ADS
Title: On the transport of magnetic fields by solar-like stratified
convection
Authors: Dorch, S. B. F.; Nordlund, Å.
Bibcode: 2001A&A...365..562D
Altcode:
The interaction of magnetic fields and stratified convection was studied
in the context of the solar and late type stellar dynamos by using
numerical 3D MHD simulations. The topology of stratified asymmetric
and over-turning convection enables a pumping mechanism that may
render the magnetic flux storage problem obsolete. The inclusion of
open boundary conditions leads to a considerable flux loss unless the
open boundary is placed close to the physical boundary. Simulations
including solar-like latitudinal shear indicates that a toroidal field
of several tens of kilo-Gauss may be held down by the pumping mechanism.
Title: Magneto-Convection in Micropores
Authors: Bercik, D. J.; Stein, R. F.; Nordlund, A.
Bibcode: 2000AAS...197.3105B
Altcode: 2000BAAS...32.1447B
We report results from a series of magneto-convection simulations. An
initially vertical magnetic field is evolved in a 12 Mm x 12 Mm x 3
Mm solar near-surface layer at average field strengths of 0 G, 200 G
and 400 G. Small dark features develop, that have sizes and lifetimes
comparable to micropores observed on the solar surface. We present the
properties of these micropore features, including structure, formation
and evolution. This work is supported by grants from NASA and NSF.
Title: Solar Interior: Convection Theory
Authors: Nordlund, Å.
Bibcode: 2000eaa..bookE1996N
Altcode:
Convection is the transport of energy by hot fluid moving upwards
and cold fluid moving downwards. A familiar example is the upwards
and downwards motions one may observe (best just before the boiling
point) when heating water in a casserole. In stars, convection is
often responsible for transporting heat up to the visible surface,
or for transporting heat away from the central parts of the star,...
Title: Magnetic Fields in Young Galaxies
Authors: . Nordlund, A; Rognvaldsson, O. E.
Bibcode: 2000astro.ph.10499.
Altcode: 2000astro.ph.10499N
We have studied the fate of initial magnetic fields in the hot halo gas
out of which the visible parts of galaxies form, using three-dimensional
numerical MHD-experiments. The halo gas undergoes compression by
several orders of magnitude in the subsonic cooling flow that forms
the cold disk. The magnetic field is carried along and is amplified
considerably in the process, reaching muG levels for reasonable values
of the initial ratio of magnetic to thermal energy density.
Title: The response of a turbulent accretion disc to an imposed
epicyclic shearing motion
Authors: Torkelsson, Ulf; Ogilvie, Gordon I.; Brandenburg, Axel;
Pringle, James E.; Nordlund, Åke; Stein, Robert F.
Bibcode: 2000MNRAS.318...47T
Altcode: 2000astro.ph..5199T
We excite an epicyclic motion, the amplitude of which depends on the
vertical position, z, in a simulation of a turbulent accretion disc. An
epicyclic motion of this kind may be caused by a warping of the disc. By
studying how the epicyclic motion decays, we can obtain information
about the interaction between the warp and the disc turbulence. A
high-amplitude epicyclic motion decays first by exciting inertial
waves through a parametric instability, but its subsequent exponential
damping may be reproduced by a turbulent viscosity. We estimate the
effective viscosity parameter, αv, pertaining to such a
vertical shear. We also gain new information on the properties of the
disc turbulence in general, and measure the usual viscosity parameter,
αh, pertaining to a horizontal (Keplerian) shear. We
find that, as is often assumed in theoretical studies, αv
is approximately equal to αh and both are much less than
unity, for the field strengths achieved in our local box calculations
of turbulence. In view of the smallness (~0.01) of αv and
αh we conclude that for βpgaspmag~10
the time-scale for diffusion or damping of a warp is much shorter than
the usual viscous time-scale. Finally, we review the astrophysical
implications.
Title: Excitation of Chromospheric Wave Transients by Collapsing
Granules
Authors: Skartlien, R.; Stein, R. F.; Nordlund, Å.
Bibcode: 2000ApJ...541..468S
Altcode:
The excitation of acoustic waves is studied using three-dimensional
numerical simulations of the nonmagnetic solar atmosphere and the
upper convection zone. Transient acoustic waves in the atmosphere
are excited at the top of the convective zone (the cooling layer) and
immediately above in the convective overshoot zone, by small granules
that undergo a rapid collapse, in the sense that upflow reverses to
downflow, on a timescale shorter than the atmospheric acoustic cutoff
period (3 minutes). These collapsing granules tend to be located above
downflows at the boundaries of mesogranules where the upward enthalpy
flux is smaller than average. An extended downdraft between larger
cells is formed at the site of the collapse. The waves produced are
long wavelength, gravity modified acoustic waves with periods close to
the 3 minute cutoff period of the solar atmosphere. The oscillation
is initially horizontally localized with a size of about 1 Mm. The
wave amplitude decays in time as energy is transported horizontally and
vertically away from the site of the event. Observed ``acoustic events''
and darkening of intergranular lanes could be explained by this purely
hydrodynamical process. Furthermore, the observed ``internetwork bright
grains'' in the Ca II H and K line cores and associated shock waves
in the chromosphere may also be linked to such wave transients.
Title: Ambipolar Drift Heating in Turbulent Molecular Clouds
Authors: Padoan, Paolo; Zweibel, Ellen; Nordlund, Åke
Bibcode: 2000ApJ...540..332P
Altcode: 1999astro.ph.10147P
We present calculations of frictional heating by ion-neutral
drift in three-dimensional simulations of turbulent, magnetized
molecular clouds. We show that ambipolar drift heating is a strong
function of position in a turbulent cloud, and its average value
can be significantly larger than the average cosmic-ray heating
rate. The heating rate per unit volume due to ambipolar drift,
HAD=|JXB|2/ρiνin~
B4/(16π2L2Bρi
νin), is found to depend on the rms Alfvénic Mach
number, MA, and on the average field strength, as
HAD~M2A<|B|>4. This
implies that the typical scale of variation of the magnetic field,
LB, is inversely proportional to MA, which we
also demonstrate.
Title: Astrophysical convection and dynamos
Authors: Brandenburg, A.; Nordlund, A.; Stein, R. F.
Bibcode: 2000gac..conf...85B
Altcode:
Convection can occur in various astrophysical settings. In this review
some aspects of solar convection are highlighted. In deeper layers
of the solar convection zone, rotation becomes important and can
lead to effects such as downward pumping of vorticity and magnetic
fields. Rotation has the tendency to partially evacuate vortex tubes
making them lighter. This effect can sometimes reverse the core of
a downdraft and make it buoyant. The problem of different thermal
and dynamic a time scales is addressed and finally the formation of
magnetic structures by convection is discussed.
Title: Line formation in solar granulation. II. The photospheric
Fe abundance
Authors: Asplund, M.; Nordlund, Å.; Trampedach, R.; Stein, R. F.
Bibcode: 2000A&A...359..743A
Altcode: 2000astro.ph..5321A
The solar photospheric Fe abundance has been determined using realistic
ab initio 3D, time-dependent, hydrodynamical model atmospheres. The
study is based on the excellent agreement between the predicted
and observed line profiles directly rather than equivalent widths,
since the intrinsic Doppler broadening from the convective motions and
oscillations provide the necessary non-thermal broadening. Thus, three
of the four hotly debated parameters (equivalent widths, microturbulence
and damping enhancement factors) in the center of the recent solar Fe
abundance dispute regarding Fe i lines no longer enter the analysis,
leaving the transition probabilities as the main uncertainty. Both Fe i
(using the samples of lines of both the Oxford and Kiel studies) and
Fe ii lines have been investigated, which give consistent results:
log epsilon_FeI = 7.44 +/- 0.05 and log epsilon_FeII = 7.45 +/-
0.10. Also the wings of strong Fe i lines return consistent abundances,
log epsilon_FeII = 7.42 +/- 0.03, but due to the uncertainties inherent
in analyses of strong lines we give this determination lower weight
than the results from weak and intermediate strong lines. In view of
the recent slight downward revision of the meteoritic Fe abundance
log epsilon_Fe = 7.46 +/- 0.01, the agreement between the meteoritic
and photospheric values is very good, thus appearingly settling the
debate over the photospheric Fe abundance from Fe i lines.
Title: The effects of numerical resolution on hydrodynamical surface
convection simulations and spectral line formation
Authors: Asplund, M.; Ludwig, H. -G.; Nordlund, Å.; Stein, R. F.
Bibcode: 2000A&A...359..669A
Altcode: 2000astro.ph..5319A
The computationally demanding nature of radiative-hydrodynamical
simulations of stellar surface convection warrants an investigation
of the sensitivity of the convective structure and spectral synthesis
to the numerical resolution and dimension of the simulations, which
is presented here. With too coarse a resolution the predicted spectral
lines tend to be too narrow, reflecting insufficient Doppler broadening
from the convective motions, while at the currently highest affordable
resolution the line shapes have converged essentially perfectly to
the observed profiles. Similar conclusions are drawn from the line
asymmetries and shifts. Due to the robustness of the pressure and
temperature structures with respect to the numerical resolution, strong
Fe lines with pronounced damping wings and H i lines are essentially
immune to resolution effects, and can therefore be used for improved
T_eff and log g determinations even at very modest resolutions. In
terms of abundances, weak Fe i and Fe ii lines show a very small
dependence ( =~ 0.02 dex) while for intermediate strong lines with
significant non-thermal broadening the sensitivity increases (<~ 0.10
dex). Problems arise when using 2D convection simulations to describe
an inherent 3D phenomenon, which translates to inaccurate atmospheric
velocity fields and temperature and pressure structures. In 2D the
theoretical line profiles tend to be too shallow and broad compared with
the 3D calculations and observations, in particular for intermediate
strong lines. In terms of abundances, the 2D results are systematically
about 0.1 dex lower than for the 3D case for Fe i lines. Furthermore,
the predicted line asymmetries and shifts are much inferior in 2D
with discrepancies amounting to ~ 200 m s-1. Given these
shortcomings and computing time considerations it is better to use
3D simulations of even modest resolution than high-resolution 2D
simulations.
Title: Line formation in solar granulation. I. Fe line shapes,
shifts and asymmetries
Authors: Asplund, M.; Nordlund, Å.; Trampedach, R.; Allende Prieto,
C.; Stein, R. F.
Bibcode: 2000A&A...359..729A
Altcode: 2000astro.ph..5320A
Realistic ab-initio 3D, radiative-hydrodynamical convection simulations
of the solar granulation have been applied to Fe i and Fe ii line
formation. In contrast to classical analyses based on hydrostatic 1D
model atmospheres the procedure contains no adjustable free parameters
but the treatment of the numerical viscosity in the construction
of the 3D, time-dependent, inhomogeneous model atmosphere and the
elemental abundance in the 3D spectral synthesis. However, the numerical
viscosity is introduced purely for numerical stability purposes and is
determined from standard hydrodynamical test cases with no adjustments
allowed to improve the agreement with the observational constraints
from the solar granulation. The non-thermal line broadening is mainly
provided by the Doppler shifts arising from the convective flows in
the solar photosphere and the solar oscillations. The almost perfect
agreement between the predicted temporally and spatially averaged
line profiles for weak Fe lines with the observed profiles and the
absence of trends in derived abundances with line strengths, seem to
imply that the micro- and macroturbulence concepts are obsolete in
these 3D analyses. Furthermore, the theoretical line asymmetries and
shifts show a very satisfactory agreement with observations with an
accuracy of typically 50-100 m s-1 on an absolute velocity
scale. The remaining minor discrepancies point to how the convection
simulations can be refined further.
Title: Ambipolar Drift Heating in Turbulent Molecular Clouds
Authors: Padoan, P.; Zweibel, E.; Nordlund, Å.
Bibcode: 2000ESASP.445..479P
Altcode: 2000sfsl.conf..479P
No abstract at ADS
Title: Magnetoconvection and the Solar Dynamo
Authors: Nordlund, Å.; Dorch, S. B. F.; Stein, R. F.
Bibcode: 2000JApA...21..307N
Altcode:
No abstract at ADS
Title: Magnetohydrodynamic Turbulence in Accretion Discs
Authors: Torkelsson, U.; Brandenburg, A.; Nordlund, A.; Stein, R. F.
Bibcode: 2000IAUS..195..241T
Altcode:
We present results from numerical simulations of magnetohydrodynamic
turbulence in accretion discs. Our simulations show that the turbulent
stresses that drive the accretion are less stratified than the matter;
thus, the surface layers are more strongly heated than the interior
of the disc.
Title: Dynamo Mechanisms: Continued Discussion
Authors: Nordlund, Åke
Bibcode: 2000astu.progE..19N
Altcode:
No abstract at ADS
Title: Star Formation and Supersonic Turbulence
Authors: Nordlund, Åke
Bibcode: 2000astu.progE..20N
Altcode:
No abstract at ADS
Title: Three-dimensional Separator Reconnection - How Does It Occur?
Authors: Galsgaard, K.; Priest, E. R.; Nordlund, Å.
Bibcode: 2000SoPh..193....1G
Altcode:
In two dimensions magnetic energy release takes place at locations
where the magnetic field strength becomes zero and has an x-point
topology. The x-point topology can collapse into two y-points connected
by a current sheet when the advection of magnetic flux into the x-point
is larger than the dissipation of magnetic flux at the x-point. In
three dimensions magnetic fields may also contain singularities in the
form of three-dimensional null points. Three-dimensional nulls are
created in pairs and are therefore, at least in the initial stages,
always connected by at least one field line - the separator. The
separator line is defined by the intersection of the fan planes of the
two nulls. In the plane perpendicular to a single separator the field
line topology locally has a two dimensional x-point structure. Using
a numerical approach we find that the collapse of the separator can be
initiated at the two nulls by a velocity shear across the fan plane. It
is found that for a current concentration to connect the two nulls
along the separator, the current sheet can only obtain two different
orientations relative to the field line structure of the nulls. The
sheet has to have an orientation midway between the fan plane and the
spine axis of each null. As part of this process the spine axes are
found to lose their identity by transforming into an integrated part
of the separator surfaces that divide space into four magnetically
independent regions around the current sheet.
Title: Realistic Solar Convection Simulations
Authors: Stein, Robert F.; Nordlund, Åke
Bibcode: 2000SoPh..192...91S
Altcode:
We report on realistic simulations of solar surface convection that
are essentially parameter-free, but include detailed physics in the
equation of state and radiative energy exchange. The simulation results
are compared quantitatively with observations. Excellent agreement is
obtained for the distribution of the emergent continuum intensity,
the profiles of weak photospheric lines, the p-mode frequencies,
the asymmetrical shape of the mode velocity and intensity spectra,
the p-mode excitation rate, and the depth of the convection zone. We
describe how solar convection is non-local. It is driven from a thin
surface thermal boundary layer where radiative cooling produces low
entropy gas which forms the cores of the downdrafts in which most of the
buoyancy work occurs. Turbulence and vorticity are mostly confined to
the intergranular lanes and underlying downdrafts. Finally, we present
some preliminary results on magneto-convection.
Title: Numerical Simulations of Oscillation Modes of the Solar
Convection Zone
Authors: Georgobiani, D.; Kosovichev, A. G.; Nigam, R.; Nordlund,
Å.; Stein, R. F.
Bibcode: 2000ApJ...530L.139G
Altcode: 1999astro.ph.12485G
We use the three-dimensional hydrodynamic code of Stein & Nordlund
to realistically simulate the upper layers of the solar convection zone
in order to study physical characteristics of solar oscillations. Our
first result is that the properties of oscillation modes in the
simulation closely match the observed properties. Recent observations
from the Solar and Heliospheric Observatory (SOHO)/Michelson Doppler
Imager (MDI) and Global Oscillations Network Group have confirmed the
asymmetry of solar oscillation line profiles, initially discovered
by Duvall et al. In this Letter, we compare the line profiles in
the power spectra of the Doppler velocity and continuum intensity
oscillations from the SOHO/MDI observations with the simulation. We
also compare the phase differences between the velocity and intensity
data. We have found that the simulated line profiles are asymmetric
and have the same asymmetry reversal between velocity and intensity
as observed. The phase difference between the velocity and intensity
signals is negative at low frequencies, and phase jumps in the vicinity
of modes are also observed. Thus, our numerical model reproduces the
basic observed properties of solar oscillations and allows us to study
the physical properties which are not observed.
Title: Realistic Solar Surface Convection Simulations
Authors: Stein, Robert F.; Nordlund, Åke
Bibcode: 2000NYASA.898...21S
Altcode:
We perform essentially parameter free simulations with realistic physics
of convection near the solar surface. We summarize the physics that is
included and compare the simulation results with observations. Excellent
agreement is obtained for the depth of the convection zone, the
p-mode frequencies, the p-mode excitation rate, the distribution of
the emergent continuum intensity, and the profiles of weak photospheric
lines. We describe how solar convection is nonlocal. It is driven from a
thin surface thermal boundary layer where radiative cooling produces low
entropy gas which forms the cores of the downdrafts in which most of the
buoyancy work occurs. We show that turbulence and vorticity are mostly
confined to the intergranular lanes and underlying downdrafts. Finally,
we illustrate our current work on magneto-convection.
Title: The atmospheric dynamics in 2D and 3D simulations of stellar
surface convection
Authors: Ludwig, Hans-Guenter; Nordlund, Ake
Bibcode: 2000ASSL..254...37L
Altcode: 2000stas.conf...37L
No abstract at ADS
Title: A Comparison of 13CO Local Thermodynamic Equilibrium
and True Column Densities in Molecular Cloud Models
Authors: Padoan, Paolo; Juvela, Mika; Bally, John; Nordlund, Åke
Bibcode: 2000ApJ...529..259P
Altcode:
In this work we use models of molecular clouds and non-LTE radiative
transfer calculations to compare the column densities of molecular
clouds with their LTE 13CO column densities. The cloud
models consist of three-dimensional grids of density and velocity
fields obtained as solutions of the compressible magnetohydrodynamic
equations in a 1283 periodic grid in both the supersonic
and super-Alfvénic regimes. Because of the random nature of the
velocity field and the presence of shocks, the densities span a
continuous range of values covering about 6 orders of magnitude (from
~0.1 to ~105 cm-3). As a result, the LTE column
densities can be calculated over 3 orders of magnitude. We find that
LTE column densities of molecular clouds typically underestimate the
mean 13CO true column densities by factors ranging from
1.3 to 7. These results imply that the standard LTE methods for the
derivation of column densities from CO data systematically underestimate
the true values independent of other major sources of uncertainty such
as the relative abundance of CO.
Title: The Solar Dynamo: Flux Pumping by Stratified Convection
Authors: Dorch, S. B. F.; Nordlund, Å.
Bibcode: 2000ESASP.463..305D
Altcode: 2000sctc.proc..305D
A longstanding issue in the theory of the solar dynamo is the ``flux
storage problem'' i.e. the problem of explaining how the magnetic field
may be kept within the solar convection zone long enough for the dynamo
to replenish it. We present results from numerical 3-D MHD simulations
of deep solar-like magneto-convection (both with and without open upper
boundary conditions) that show that by the very nature of stratified
over-turning convection, magnetic flux of a considerable strength may be
`pumped' downwards and kept in the lower part of the solar convection
zone. As a consequence even a formally super-equipartition magnetic
field may be effectively stored in and below the solar convection zone
thereby rendering the flux storage problem obsolete.
Title: Erratum: Numerical 3D simulations of buoyant magnetic flux
tubes
Authors: Dorch, S. B. F.; Nordlund, Å.
Bibcode: 2000A&A...353.1139D
Altcode:
As the result of an unfortunate error, the names of authors were
missing from the references in the text. We reprint the article with its
original page numbers to avoid confusion in citations of the article.
Title: Magnetic Fields in Young Galaxies
Authors: Nordlund, A.; Rögnvaldsson, Ö. E.
Bibcode: 2000IAUJD..14E...2N
Altcode:
The hot cluster gas out of which the visible parts of galaxies form
by cooling and contraction is known to contain some magnetic fields,
presumably of a random, turbulent nature. We have studied the fate of
such random magnetic fields during the formation of disk galaxies,
with three-dimensional numerical MHD-models. The hot gas undergoes
a compression by several orders of magnitude in the subsonic cooling
flow that forms the cold disk. The magnetic field is carried along and
is amplified considerably in the process, reaching microgauss field
strengths for reasonable values of the initial field strength.
Title: 3-D Convection Models: Are They Compatible with 1-D Models?
Authors: Nordlund, Å.; Stein, R. F.
Bibcode: 2000ASPC..203..362N
Altcode: 2000ilss.conf..362N; 2000IAUCo.176..362N
We review properties of stellar convection, as derived from detailed 3-D
numerical modeling, and assess to what extent 1-D models are able to
provide a fair representation of stellar structure in various regions
of the HR diagram. We point out a number of problems and discrepancies
that are inevitable when using conventional 1-D models. The problems
originate mainly in the surface layers,where horizontal fluctuations
become particularly large, and where convective energy transport gives
way to radiation. We conclude that it is necessary (and possible)
to use three-dimensional models of these layers, in order to avoid
the uncertainties and inaccuracies associated with 1-D representations.
Title: 3D simulations of twisted magnetic flux ropes
Authors: Dorch, S. B. F.; Archontis, V.; Nordlund, Å.
Bibcode: 1999A&A...352L..79D
Altcode:
Several numerical simulations of buoyant 2D and 3D twisted flux ropes
have been performed. It is found that the apex region of an anchored
3D flux rope behaves similarly to the simpler case of a 2D horizontal
twisted flux tube while the overall structure of such a 3D flux rope
developes quite differently. Upon emergence a characteristic S-shape
of the magnetic field lines is displayed in agreement with observations
in soft X-ray.
Title: Convective contributions to the frequencies of solar
oscillations
Authors: Rosenthal, C. S.; Christensen-Dalsgaard, J.; Nordlund, Å.;
Stein, R. F.; Trampedach, R.
Bibcode: 1999A&A...351..689R
Altcode: 1998astro.ph..3206R
Differences between observed and theoretical eigenfrequencies of the Sun
have characteristics which identify them as arising predominantly from
properties of the oscillations in the vicinity of the solar surface:
in the super-adiabatic, convective boundary layer and above. These
frequency differences may therefore provide useful information about
the structure of these regions, precisely where the theory of solar
structure is most uncertain. In the present work we use numerical
simulations of the outer part of the Sun to quantify the influence
of turbulent convection on solar oscillation frequencies. Separating
the influence into effects on the mean model and effects on the
physics of the modes, we find that the main model effects are due
to the turbulent pressure that provides additional support against
gravity, and thermal differences between average 3-D models and 1-D
models. Surfaces of constant pressure in the visible photosphere are
elevated by about 150 km, relative to a standard envelope model. As a
result, the turning points of high-frequency modes are raised, while
those of the low-frequency modes remain essentially unaffected. The
corresponding gradual lowering of the mode frequencies accounts for
most of the frequency difference between observations and standard
solar models. Additional effects are expected to come primarily from
changes in the physics of the modes, in particular from the modulation
of the turbulent pressure by the oscillations.
Title: A Super-Alfvénic Model of Dark Clouds
Authors: Padoan, Paolo; Nordlund, Åke
Bibcode: 1999ApJ...526..279P
Altcode: 1999astro.ph..1288P
Supersonic random motions are observed in dark clouds and are
traditionally interpreted as Alfvén waves, but the possibility that
these motions are super-Alfvénic has not been ruled out. In this
work we report the results of numerical experiments in two opposite
regimes: \MscrA~1 and \MscrA>>1, where
\MscrA is the initial Alfvénic Mach number--the ratio of
the rms velocity to the Alfvén speed. Our results show that models
with \MscrA>>1 are consistent with the observed
properties of molecular clouds that we have tested (statistics of
extinction measurements, distribution of integrated antenna temperature,
Zeeman-splitting measurements of magnetic field strength, line width
versus integrated antenna temperature of molecular emission-line
spectra, statistical B-n relation, and scatter in that relation),
while models with \MscrA~1 have properties that are in
conflict with the observations. We find that both the density and
the magnetic field in molecular clouds may be very intermittent. The
statistical distributions of the magnetic field and gas density are
related by a power law, with an index that decreases with time in
experiments with decaying turbulence. After about one dynamical time
it stabilizes at B~n0.4. Magnetically dominated cores form
early in the evolution, while later on the intermittency in the density
field wins out, and also cores with a weak field can be generated by
mass accretion along magnetic field lines.
Title: Supersonic Turbulence in the Perseus Molecular Cloud
Authors: Padoan, Paolo; Bally, John; Billawala, Youssef; Juvela,
Mika; Nordlund, Åke
Bibcode: 1999ApJ...525..318P
Altcode: 1999astro.ph..5383P
We compare the statistical properties of J=1-->0 13CO
spectra observed in the Perseus molecular cloud with synthetic
J=1-->0 13CO spectra, computed solving the non-LTE
radiative transfer problem for a model cloud obtained as solutions of
the three-dimensional magnetohydrodynamic (MHD) equations. The model
cloud is a randomly forced super-Alfvénic and highly supersonic
turbulent isothermal flow. The purpose of the present work is to
test if idealized turbulent flows, without self-gravity, stellar
radiation, stellar outflows, or any other effect of star formation,
are inconsistent or not with statistical properties of star-forming
molecular clouds. We present several statistical results that
demonstrate remarkable similarity between real data and the synthetic
cloud. Statistical properties of molecular clouds like Perseus are
appropriately described by random supersonic and super-Alfvénic
MHD flows. Although the description of gravity and stellar radiation
are essential to understand the formation of single protostars and
the effects of star formation in the cloud dynamics, the overall
description of the cloud and of the initial conditions for star
formation can apparently be provided on intermediate scales without
accounting for gravity, stellar radiation, and a detailed modeling of
stellar outflows. We also show that the relation between equivalent
line width and integrated antenna temperature indicates the presence
of a relatively strong magnetic field in the core B1, in agreement
with Zeeman splitting measurements.
Title: On the location of energy release and temperature profiles
along coronal loops
Authors: Galsgaard, K.; Mackay, D. H.; Priest, E. R.; Nordlund, Å.
Bibcode: 1999SoPh..189...95G
Altcode:
Several mechanisms have been suggested to contribute to the heating
of the solar corona, each of which deposits energy along coronal
loops in a characteristic way. To compare the theoretical models
with observations one has to derive observable quantities from the
models. One such parameter is the temperature profile along a loop. Here
numerical experiments of flux braiding are used to provide the spatial
distribution of energy deposition along a loop. It is found that
braiding produces a heat distribution along the loop which has slight
peaks near the footpoints and summit and whose magnitude depends on
the driving time. Using different examples of the heat deposition,
the temperature profiles along the loop are determined assuming a
steady state. Along with this, different methods for providing average
temperature profiles from the time-series have been investigated. These
give summit temperatures within approximately 10% of each other. The
distribution of the heating has a significant impact on both the summit
temperature and the temperature distribution along the loop. In each
case the ratio between the heat deposited and radiation provides a
scaling for the summit temperature.
Title: 3D hydrodynamical model atmospheres of metal-poor
stars. Evidence for a low primordial Li abundance
Authors: Asplund, Martin; Nordlund, Åke; Trampedach, Regner; Stein,
Robert F.
Bibcode: 1999A&A...346L..17A
Altcode: 1999astro.ph..5059A
Realistic 3-dimensional (3D), radiative hydrodynamical surface
convection simulations of the metal-poor halo stars HD 140283 and
HD 84937 have been performed. Due to the dominance of adiabatic
cooling over radiative heating very low atmospheric temperatures are
encountered. The lack of spectral lines in these metal-poor stars
thus causes much steeper temperature gradients than in classical 1D
hydrostatic model atmospheres where the temperature of the optically
thin layers is determined by radiative equilibrium. The modified
atmospheric structures cause changes in the emergent stellar spectra. In
particular, the primordial Li abundances may have been overestimated
by 0.2-0.35 dex with 1D model atmospheres. However, we caution that
our result assumes local thermodynamic equilibrium (LTE), while the
steep temperature gradients may be prone to e.g. over-ionization.
Title: The gravity-brightening effect and stellar atmospheres. II
Results for illuminated models with 3 700 K < T_eff < 7 000 K
Authors: Alencar, S. H. P.; Vaz, L. P. R.; Nordlund, Å.
Bibcode: 1999A&A...346..556A
Altcode:
The influence of the so-called ``reflection effect'' (mutual
illumination in a close binary) on the gravity-brightening exponent
(beta ) is studied using the UMA (Uppsala Model Atmosphere)
code. The model is applied to convective grey (in the sense of
continuum-only-opacity) and non-grey (line-blanketed) atmospheres with
3 700 K < T_eff < 7 000 K, illuminated by grey and non-grey
fluxes. The results for grey atmospheres illuminated by grey or
non-grey fluxes are very similar. In this case beta mostly depends
on the amount of incident energy and on the illumination direction,
apart from the dependence on the effective temperature already
discussed for non-illuminated models in a previous work (Alencar &
Vaz \cite{paper1}). The existence of a maximum in the beta (T_eff)
relation is due to the interplay between the convection and opacity
properties of the models. The external illumination increases the
values of beta , that is, the larger the amount of incident flux
the larger the value of the exponent. This effect is caused by the
``quenching'' of convection as the external illumination heats the
surface layers of the illuminated star, thus bringing it closer to
radiative equilibrium, where beta is close to unity. We provide a
polynomial fit to the variation of beta with the fundamental parameters,
in order to make it possible to easily account for the effect in light
curve synthesis programs. For line-blanketed illuminated atmospheres
there is an additional dependence on the effective temperature of the
incident flux (the heating temperature). This is related to the overall
wavelength dependence of the spectral line opacity. Particularly in
the UV, the line opacity is so strong that it prevents a significant
amount of the incident flux from penetrating to the continuum formation
layers. The quenching of convection by the external illumination and
the related increase of beta are thus partly prevented.
Title: Near Surface Magneto-Convection
Authors: Bercik, D. J.; Stein, R. F.; Nordlund, A.
Bibcode: 1999AAS...194.5501B
Altcode: 1999BAAS...31..910B
The emergence of magnetic flux alters the structure of the solar
surface. We use simulations of magneto-convection of a near surface
layer to investigate the dynamical interaction between magnetic fields
and convection. The results of these simulations are presented to show
the behavior of emerging flux tubes as well as the three dimensional
structure and evolution of bright points and small pores.
Title: Realistic Simulations of Solar Surface Convection
Authors: Stein, R. F.; Bercik, D.; Georgobiani, D.; Nordlund, A.
Bibcode: 1999AAS...194.2104S
Altcode: 1999BAAS...31R.858S
Results from realistic simulations of near surface solar convection
will be summarized and compared with observations. Solar convection
is driven by radiative cooling from an extremely thin surface thermal
boundary layer, which produces low entropy fluid. Its topology is
controlled by mass conservation and consists of turbulent downdrafts
penetrating nearly laminar upflows. The horizontal scales increase with
depth. Good agreement is found with the of the depth of the convection
zone, p-mode frequencies, excitation, line asymmetries and intensity -
velocity phase differences from helioseismology; with observations of
granulation and profiles of weak Fe lines. This work was supported by
grants from NSF, NASA, and the Danish Research Council. The calculations
were performed at NCSA, MSU and UNIC.
Title: Three-dimensional simulations of solar oscillations: line
profiles and asymmetries
Authors: Georgobiani, D. G.; Nigam, R.; Kosovichev, A. G.; Stein,
R. F.; Nordlund, A.
Bibcode: 1999AAS...194.5605G
Altcode: 1999BAAS...31..912G
In order to study spectral characteristics of the solar oscillations,
we use the Stein-Nordlund 3d hydrodynamic code to generate lond
temporal sequencies of realistically simulated upper layers of the
solar convective zone. The simulation domain ranges from 0.5 Mm above
the surface of tau =1 to 2.5 Mm below this surface, and is 6 Mm by
6 Mm wide. We have generated 24 hours of solar time. We calculate
power spectra of the vertical velocity and temperature at different
heights and the emergent intensity at the surface. Here, we present the
profiles of velocity, intensity and temperature for both radial (l = 0)
and first nonradial (l = 700) mode. We compare line profiles from the
simulation with the power spectra of the Doppler velocity and continuum
intensity from the SOHO/MDI observations. Both simulated and observed
profiles demonstrate similar types of asymmetry, and the asymmetry
reversal between the local quantities like velocity and temperature, and
emergent intensity profiles is also present in the simulated data. The
preliminary results are promising as they allow us to establish a
connection between the observational data and realistic simulations,
and enable us to understand better the physics of solar oscillations.
Title: A Supernova-regulated Interstellar Medium: Simulations of
the Turbulent Multiphase Medium
Authors: Korpi, M. J.; Brandenburg, A.; Shukurov, A.; Tuominen, I.;
Nordlund, Å.
Bibcode: 1999ApJ...514L..99K
Altcode:
The dynamic state of the interstellar medium, heated and stirred by
supernovae (SNe), is simulated using a three-dimensional, nonideal
MHD model in a domain extended 0.5×0.5 kpc horizontally and 2 kpc
vertically, with the gravitational field symmetric about the midplane
of the domain, z=0. We include both Type I and Type II SNe, allowing
the latter to cluster in regions with enhanced gas density. The
system segregates into two main phases: a warm, denser phase and a
hot, dilute gas in global pressure equilibrium; there is also dense,
cool gas compressed into filaments, shells, and clumps by expanding
SN remnants. The filling factor of the hot phase grows with height,
so it dominates at z>~0.5 kpc. The multicomponent structure persists
throughout the simulation, and its statistical parameters show little
time variation. The warm gas is in hydrostatic equilibrium, which is
supported by thermal and turbulent pressures. The multiphase gas is in
a state of developed turbulence. The rms random velocity is different
in the warm and hot phases, 10 and 40 km s-1, respectively,
at z<~1 kpc; the turbulent cell size (twice the velocity correlation
scale) is about 60 pc in the warm phase.
Title: Confrontation of Stellar Surface Convection Simulations with
Stellar Spectroscopy
Authors: Asplund, M.; Nordlund, Å.; Trampedach, R.
Bibcode: 1999ASPC..173..221A
Altcode: 1999sstt.conf..221A
No abstract at ADS
Title: Magneto-Convection
Authors: Stein, R. F.; Georgobiani, D.; Bercik, D. J.; Brandenburg,
A.; Nordlund, Å.
Bibcode: 1999ASPC..173..193S
Altcode: 1999sstt.conf..193S
No abstract at ADS
Title: Solar Convection and MHD
Authors: Nordlund, Å.; Stein, R. F.
Bibcode: 1999ASSL..240..293N
Altcode: 1999numa.conf..293N
No abstract at ADS
Title: Stellar Evolution with a Variable Mixing-Length Parameter
Authors: Trampedach, R.; Stein, R. F.; Christensen-Dalsgaard, J.;
Nordlund, Å.
Bibcode: 1999ASPC..173..233T
Altcode: 1999sstt.conf..233T
No abstract at ADS
Title: The Excitation of Solar Oscillations -- Observations and
Simulations
Authors: Goode, P.; Strous, L.; Rimmele, T.; Stein, R.; Nordlund, Å.
Bibcode: 1999ASPC..183..456G
Altcode: 1999hrsp.conf..456G
No abstract at ADS
Title: The Dynamics of Turbulent Viscosity
Authors: Torkelsson, U.; Ogilvie, G. I.; Pringle, J. E.; Brandenburg,
A.; Nordlund, Å.; Stein, R. F.
Bibcode: 1999ASPC..161..422T
Altcode: 1999hepa.conf..422T
No abstract at ADS
Title: Super-Alfvénic Turbulent Fragmentation in Molecular Clouds
Authors: Padoan, Paolo; Nordlund, Å. Ke
Bibcode: 1999intu.conf..248P
Altcode:
No abstract at ADS
Title: Convection Simulations
Authors: Nordlund, Å.; Stein, R. F.
Bibcode: 1999ASPC..173...91N
Altcode: 1999sstt.conf...91N
No abstract at ADS
Title: The Density PDFs of Supersonic Random Flows
Authors: Nordlund, Å. Ke; Padoan, Paolo
Bibcode: 1999intu.conf..218N
Altcode: 1998astro.ph.10074N
The question of the shape of the density PDF for supersonic turbulence
is adressed, using both analytical and numerical methods. For isothermal
supersonic turbulence, the PDF is Log-Normal, with a width that scales
approximately linearly with the Mach number. For a polytropic equation
of state, with an effective gamma smaller than one, the PDF becomes
skewed and becomes reminiscent of (but not equal to) a power law on
the high density side.
Title: Dynamics of Magnetic Flux Elements in the Solar Photosphere
Authors: van Ballegooijen, A. A.; Nisenson, P.; Noyes, R. W.; Löfdahl,
M. G.; Stein, R. F.; Nordlund, Å.; Krishnakumar, V.
Bibcode: 1998ApJ...509..435V
Altcode: 1998astro.ph..2359V
The interaction of magnetic fields and convection is investigated in
the context of the coronal heating problem. We study the motions of
photospheric magnetic elements using a time series of high-resolution
G-band and continuum filtergrams obtained at the Swedish Vacuum
Solar Telescope at La Palma. The G-band images show bright points
arranged in linear structures (``filigree'') located in the lanes
between neighboring granule cells. We measure the motions of these
bright points using an object tracking technique, and we determine
the autocorrelation function describing the temporal variation of
the bright point velocity. The correlation time of the velocity is
about 100 s. To understand the processes that determine the spatial
distribution of the bright points, we perform simulations of horizontal
motions of magnetic flux elements in response to solar granulation
flows. Models of the granulation flow are derived from the observed
granulation intensity images using a simple two-dimensional model
that includes both inertia and horizontal temperature gradients; the
magnetic flux elements are assumed to be passively advected by this
granulation flow. The results suggest that this passive advection model
is in reasonable agreement with the observations, indicating that on
a timescale of 1 hr the flux tubes are not strongly affected by their
anchoring at large depth. Finally, we use potential-field modeling
to extrapolate the magnetic and velocity fields to larger height. We
find that the velocity in the chromosphere can be locally enhanced at
the separatrix surfaces between neighboring flux tubes. The predicted
velocities are several km s-1, significantly larger than
those of the photospheric flux tubes. The implications of these results
for coronal heating are discussed.
Title: Stellar background power spectra from hydrodynamical
simulations of stellar atmospheres
Authors: Trampedach, R.; Christensen-Dalsgaard, J.; Nordlund, A.;
Stein, R. F.
Bibcode: 1998mons.proc...59T
Altcode:
The non-p-mode contribution to the temporal irradiance or velocity
spectra of the Sun has for a long time been considered as noise,
but in recent years it has gradually been appreciated as the signal of
granulation. Accordingly these spectra are now referred to as background
spectra. We hope that further analysis of these background spectra
will serve two purposes: to provide information about convection in
other stars; and, as the background still constitutes a noise source
when looking for p- and in particular g-modes of solar type stars,
to provide us with stricter limits as to what is observable. Based on
hydrodynamical simulations of convection in the atmospheres of the Sun,
alpha Cen A and Procyon, we calculate irradiance and velocity spectra
and infer a few properties of these spectra. Due to the limited
horizontal extent of the simulations (covering 6-8 granules each)
we only get a signal from the granulation, whereas effects of meso-
and supergranulation are missing in our signal. At the high-frequency
end we are limited by the horizontal resolution of the simulations.
Title: Numerical 3D simulations of buoyant magnetic flux tubes
Authors: Dorch, S. B. F.; Nordlund, A.
Bibcode: 1998A&A...338..329D
Altcode:
We have examined instabilities of non-thin buoyant magnetic flux tubes
ascending through a solar convection zone model using numerical 3D MHD
experiments. The experiments show that the fate of the flux tubes is
entirely dependent on the internal topology of the magnetic field lines
in the flux tube; if the initial topology is too simple the tube is
quickly disrupted by a Rayleigh-Taylor like instability. The disruption
is prevented or delayed if the field has a component that makes the
topology non-trivial. Even a weak random or twisting component, an order
of magnitude weaker than the longitudinal magnetic field, is sufficient
to let the tube ascend as a more or less coherent structure. These 3D
results may resolve the apparent contradiction between the success
of experiments using the thin flux tube approximation to study the
buoyant rise of magnetic flux tubes, and the rapid break-up of flux
tubes found in 2D experiments.
Title: Synthetic Molecular Clouds from Supersonic Magnetohydrodynamic
and Non-LTE Radiative Transfer Calculations
Authors: Padoan, Paolo; Juvela, Mike; Bally, John; Nordlund, Åke
Bibcode: 1998ApJ...504..300P
Altcode: 1997astro.ph.11051P
The dynamics of molecular clouds is characterized by supersonic
random motions in the presence of a magnetic field. We study
this situation using numerical solutions of the three-dimensional
compressible magnetohydrodynamic (MHD) equations in a regime of
highly supersonic random motions. The non-LTE radiative transfer
calculations are performed through the complex density and velocity
fields obtained as solutions of the MHD equations, and more than 5 ×
105 spectra of 12CO, 13CO, and CS are
obtained. In this way we build synthetic molecular clouds of 5 and 20
pc diameter, evolved for about one dynamical time from their initial
configuration. We use a numerical flow without gravity or external
forcing. The flow is super-Alfvénic. Synthetic data consist
of sets of 90 × 90 synthetic spectra with 60 velocity channels,
in five molecular transitions: J = 1 --> 0 and J = 2 -->
1 for 12CO and 13CO, and J = 1 --> 0 for
CS. Although we do not consider the effects of stellar radiation,
gravity, or mechanical energy input from discrete sources, our models
do contain the basic physics of magnetofluid dynamics and non-LTE
radiation transfer and are therefore more realistic than previous
calculations. As a result, these synthetic maps and spectra bear a
remarkable resemblance to the corresponding observations of real clouds.
Title: Simulations of Solar Granulation. I. General Properties
Authors: Stein, R. F.; Nordlund, Å.
Bibcode: 1998ApJ...499..914S
Altcode:
Numerical simulations provide information on solar convection not
available by direct observation. We present results of simulations of
near surface solar convection with realistic physics: an equation of
state including ionization and three-dimensional, LTE radiative transfer
using a four-bin opacity distribution function. Solar convection is
driven by radiative cooling in the surface thermal boundary layer,
producing the familiar granulation pattern. In the interior of granules,
warm plasma ascends with ~10% ionized hydrogen. As it approaches and
passes through the optical surface, the plasma cools, recombines,
and loses entropy. It then turns over and converges into the dark
intergranular lanes and further into the vertices between granulation
cells. These vertices feed turbulent downdrafts below the solar surface,
which are the sites of buoyancy work that drives the convection. Only
a tiny fraction of the fluid ascending at depth reaches the surface to
cool, lose entropy, and form the cores of these downdrafts. Granules
evolve by pushing out against and being pushed in by their neighboring
granules, and by being split by overlying fluid that cools and is
pulled down by gravity. Convective energy transport properties that
are closely related to integral constraints such as conservation
of energy and mass are exceedingly robust. Other properties, which
are less tightly constrained and/or involve higher order moments or
derivatives, are found to depend more sensitively on the numerical
resolution. At the highest numerical resolution, excellent agreement
between simulated convection properties and observations is found. In
interpreting observations it is crucial to remember that surfaces of
constant optical depth are corrugated. The surface of unit optical
depth in the continuum is higher above granules and lower in the
intergranular lanes, while the surface of optical depth unity in
a spectral line is corrugated in ways that are influenced by both
thermal and Doppler effects.
Title: Exploring magnetohydrodynamic turbulence on the computer
Authors: Torkelsson, Ulf; Ogilvie, Gordon I.; Brandenburg, Axel;
Nordlund, A. ˚Ke; Stein, Robert F.
Bibcode: 1998AIPC..431...69T
Altcode: 1998apas.conf...69T
Although numerical simulations have established magnetohydrodynamic
turbulence as a possible candidate for the angular momentum transport
mechanism in accretion discs there is still a need for a deeper
understanding of the physics of the shear-induced turbulence. There
are two complementary pathways to this goal, to analyze the results of
a simulation at depth or to start from a simple state, whose evolution
can be understood by semi-analytical methods and `extrapolate' to the
turbulent state that we want to understand. We will show examples of
these two approaches.
Title: The excitation and damping of p-modes
Authors: Nordlund, A.; Stein, R. F.
Bibcode: 1998IAUS..185..199N
Altcode:
Numerical simulations of convection in the surface layers of the Sun
may be used to study the excitation and damping of p-modes. This may
be done in two ways: either passively, by looking at the modes that
spontaneously develop in the numerical simulations, or actively, by
performing numerical experiments specifically aimed at measuring the
excitation and damping of the oscillations. Because the simulation
boxes have smaller ``mode mass'' than the real Sun, the time scales
for growth and decay are correspondingly smaller, and because of the
smaller volumes, the mode spectrum is much sparser, with only a few
modes spanning the 3 mHz band that contains millions of modes in the
Sun. The total rms amplitude of the modes is expected to be similar
to that of the Sun, though, since the ratio of excitation to damping
remains the same. We report on the results of both passive measurements
and active experiments. We find that the main source of excitation
is the entropy fluctions associated with the convective downdrafts,
and that the main damping mechanism is that part of the turbulent
pressure that is in quadrature with the mode, and from the point of
view of the p-modes acts as a turbulent diffusion of momentum.
Title: Convection and p-modes
Authors: Stein, R. F.; Nordlund, Å.
Bibcode: 1998ESASP.418..693S
Altcode: 1998soho....6..693S
The solar p-modes are driven (and damped) and have their resonant
frequencies altered by interaction with the turbulent solar
convection. We present results on both the eigenfrequency modification
and mode driving derived from realistic 3D simulations of the upper
solar convection zone. Convection enlarges the resonant cavity for high
frequency modes, thereby lowering their frequencies, in improving the
agreement with the observed modes (Rosenthal et al./ 1998). This is
due to (i) turbulent pressure raising the layers above the region of
large superadiabatic gradient, and (ii) the average plasma temperature
is higher than predicted by 1D calculations for the same effective
temperature, which increases the scale height, because we do not see the
high temperatures in the granules due to the temperature sensitivity of
the H- opacity, yet they contribute to the average stratification. The
p-modes are driven by non-adiabatic pressure fluctuations (entropy
fluctuations) producing a net stochastic PdV work (Stein and Norlund
1991, Nordlund and Stein 1998). At low frequencies, the total pressure
fluctuation is very small since hydrostatic equilibrium must be
maintained. Both gas and turbulent pressure fluctuations are large,
but are out of phase and cancel each other. With increasing frequency
the magnitude of the pressure fluctuations decrease as approximately
nu-4. The peak in the total pressure fluctuation occurs at ~4
mHz, and in this range the gas pressure fluctuations dominate over the
turbulent pressure fluctuations. This work was supported by NASA grant
NAG5-4031, NSF grant AST 9521785 and the Danish Research Foundation,
through its establishment of the Theoretical Astrophysics Center.
Title: Solar Magneto-Convection
Authors: Stein, R. F.; Bercik, D. J.; Brandenburg, A.; Georgobiani,
D.; Nordlund, A.
Bibcode: 1998AAS...191.7417S
Altcode: 1998BAAS...30..758S
We present results of realistic simulations of magneto-convection near
the solar surface. The simulations were performed with two magnetic
field topologies - (1) a unipolar, initially vertical field, and (2)
a bipolar field, where fluid entering at the base of the computational
domain advects in horizontal field. As the unipolar flux is increased,
the magnetic field concentrates in the intergranule lanes and develops
large, dark, cool regions. These regions surround smaller areas where
convection has not been suppressed. In contrast, for the bipolar case,
the strongest fields appear as bright points in the intergranule lanes.
Title: Tests of Convective Frequency Effects with SOI/MDI High-Degree
Data
Authors: Rosenthal, C. S.; Christensen-Dalsgaard, J.; Kosovichev,
A. G.; Nordlund, A. A.; Reiter, J.; Rhodes, E. J., Jr.; Schou, J.;
Stein, R. F.; Trampedach, R.
Bibcode: 1998ESASP.418..521R
Altcode: 1998astro.ph..7066R; 1998soho....6..521R
Advances in hydrodynamical simulations have provided new insight into
the effects of convection on the frequencies of solar oscillations. As
more accurate observations become available, this may lead to an
improved understanding of the dynamics of convection and the interaction
between convection and pulsation (Rosenthal et al. 1999). Recent
high-resolution observations from the SOI/MDI instrument on the
SOHO spacecraft have provided the so-far most-detailed observations
of high-degree modes of solar oscillations, which are particularly
sensitive to the near-surface properties of the Sun. Here we present
preliminary results of a comparison between these observations and
frequencies computed for models based on realistic simulations of
near-surface convection. Such comparisons may be expected to help
in identifying the causes for the remaining differences between the
observed frequencies and those of solar models.
Title: Solar Magneto-Convection
Authors: Bercik, David J.; Basu, Shantanu; Georgobiani, Dali; Nordlund,
Ake; Stein, Robert F.
Bibcode: 1998ASPC..154..568B
Altcode: 1998csss...10..568B
We have simulated magneto-convection near the solar surface
with two topologies: (1) an initial vertical field; and (2) a
horizontal field carried in with the fluid entering at the base
of the computational domain. We report results on the interaction
of convection and magnetic fields. An MPEG video is viewable at:
http://www.pa.msu.edu/~steinr/images/bhoriz.mpg The MPEG video is also
included on the CS10 CD ROM.
Title: Near-surface constraints on the structure of stellar convection
zones
Authors: Trampedach, R.; Christensen-Dalsgaard, J.; Nordlund, A.;
Stein, R.
Bibcode: 1997ASSL..225...73T
Altcode: 1997scor.proc...73T
By simulating the convection in the upper layers of six different stars
and matching these simulations to 1D-mixing length models using the
same input physics, we have been able to infer the behaviour of the
mixing-length parameter, $\alpha$, as the stellar parameters changes.
Title: Is stellar granulation turbulence?
Authors: Nordlund, A.; Spruit, H. C.; Ludwig, H. -G.; Trampedach, R.
Bibcode: 1997A&A...328..229N
Altcode:
We show that power spectra of granulation images or velocity fields
cannot be compared meaningfully with spectra from theoretical models
based on turbulent cascades. The small scale power in these images
is due almost entirely to the sharp edges between granules and
intergranular lanes, not to turbulence in the usual sense. This is
demonstrated with a number of experiments with result from numerical
simulations, and with simpler synthetic data with power spectra similar
to that of granulation. The reason for the seemingly laminar behavior of
the granulation flow, in spite of the high Reynolds numbers involved,
is the influence of stratification on the local ratio of turbulence
to bulk flow. The rapid expansion of upflows, their deep origin and
near-adiabatic stratification lead to low levels of turbulence in
the overturning fluid at the surface. Higher levels of turbulence
are expected in the converging flows near downdrafts, but mostly at
scales that are below current observational resolution limits, and
contributing relatively little to the total convective flux and to
spectral line broadening.
Title: Stellar Convection; general properties
Authors: Nordlund, A.; Stein, R.
Bibcode: 1997ASSL..225...79N
Altcode: 1997scor.proc...79N
We review the properties of stellar convection zones, in
particular with respect to issues of relevance to helio- and
astero-seismology. Convection is responsible both for establishing
the one-dimensional average structure on top of which the waves are
propagating and for maintaining large amplitude three-dimensional
fluctuations that interact with the wave mode fluctuations. We discuss
qualitative and quantitative aspects of these interactions on the
background of numerical simulations of convection. We conclude that
the average properties obtained from numerical simulations are quite
robust and that the main uncertainties in applying these results to
helio- and astero-seismology lie in evaluating the effects of the
convective fluctuations on the wave propagation. One of the main
structure effects is the elevation of the photosphere caused by the
turbulent pressure. An important wave-convection interaction effect
is the contribution of the fluctuations in the turbulent pressure to
the effective gamma of the turbulent gas.
Title: The universality of the stellar initial mass function
Authors: Padoan, Paolo; Nordlund, Ake; Jones, Bernard J. T.
Bibcode: 1997MNRAS.288..145P
Altcode: 1997astro.ph..3110P
We propose that the stellar initial mass function (IMF) is universal
in the sense that its functional form arises as a consequence of the
statistics of random supersonic flows. A model is developed for the
origin of the stellar IMF that contains a dependence on the average
physical parameters (temperature, density and velocity dispersion) of
the large-scale site of star formation. The model is based on recent
numerical experiments of highly supersonic random flows that have a
strong observational counterpart. It is shown that a Miller-Scalo-like
IMF is naturally produced by the model for the typical physical
conditions in molecular clouds. A more `massive' IMF in star-bursts
is also predicted.
Title: A physical model for the stellar IMF
Authors: Padoan, Paolo; Nordlund, A. ˚Ke; Jones, Bernard J. T.
Bibcode: 1997AIPC..393..101P
Altcode: 1997sfnf.conf..101P
We propose that the stellar initial mass function (IMF) arises as a
consequence of the existence of random supersonic flows in molecular
clouds. A Miller-Scalo like IMF is predicted for the typical physical
conditions in molecular clouds, and a more ``massive'' one in star
bursts.
Title: Structure of dark clouds from stellar extinction
Authors: Padoan, Paolo; Jones, Bernard J. T.; Nordlund, Åke
Bibcode: 1997AIPC..393...97P
Altcode: 1997sfnf.conf...97P
We show that the 3D density field of the cloud IC5146 is well described
by a Log-Normal distribution down to very small scales; the power
spectrum and the standard deviation of the 3D density field can be
constrained; the cloud structure is likely to be determined by the
random supersonic motions present in the gas.
Title: Supersonic Turbulence in the Interstellar Medium: Stellar
Extinction Determinations as Probes of the Structure and Dynamics
of Dark Clouds
Authors: Padoan, Paolo; Jones, Bernard J. T.; Nordlund, Åke P.
Bibcode: 1997ApJ...474..730P
Altcode: 1996astro.ph..3061P
Lada et al. have described a method for studying the distribution of
dust in dark clouds using infrared imaging surveys. In particular,
they show that the method provides some information about the structure
of the gas (dust) on scales smaller than their resolution. In
the present work we clarify the nature of the information provided
by their method. We show that: 1. The three-dimensional
density field of the gas is well described by a lognormal distribution
down to very small scales. 2. The power spectrum and the standard
deviation of the three-dimensional density field can be constrained. 3. Such a structure of the density field is likely to be the effect
of random supersonic motions present in the gas. In fact, we find
a qualitative and quantitative agreement between the predictions based
on recent numerical simulations of randomly forced supersonic flows
by Nordlund & Padoan and by Padoan, Nordlund, & Jones and the
constraints given by the infrared dust extinction measurements.
Title: Heating and activity of the solar corona. 3. Dynamics of a
low beta plasma with three-dimensional null points
Authors: Galsgaard, Klaus; Nordlund, Åke
Bibcode: 1997JGR...102..231G
Altcode:
We investigate the self-consistent nonlinear evolution of an initially
force-free three-dimensional magnetic field subjected to stress on
two boundaries. The results illustrate how complicated magnetic field
structures, such as those found in the solar corona, evolve dynamically
when forced by stress from boundaries and how the energy which is
temporarily stored in the magnetic field may be converted into other
forms of energy such as heat, flow energy, and fast particles. The
initial model state is triple periodic and contains eight magnetic null
points. During the time evolution, the current density concentrates
near particular locations in space that can be identified with the
singular field lines connecting pairs of null points of the initial
state. Current sheets are found to grow out of the singular lines formed
by the intersection of surfaces across which the magnetic connectivity
is discontinuous. Jets of plasma shoot out from the edges of the
currents sheets, driven by the ``sling-shot'' Lorentz force created
by reconnecting magnetic field lines. As a result of the reconnection,
most of the magnetic connectivity between the two boundaries is lost,
and the remaining magnetic field develops arcade-like structures along
the boundaries. These arcade structures are long-lived, and the system
enters a quasi-stationary state, where small-scale current sheets are
continually appearing and disappearing. The distribution of size of
these current sheets is limited only by the numerical resolution. The
current sheets dissipate the energy supplied by the boundary driving
and also slowly deplete the magnetic energy from the initial constant
alpha magnetic field. The dissipation occurs in an increasing number
of current sheets of decreasing size at higher numerical resolution,
which keeps the overall reconnection rate nearly independent of the
numerical resolution. This suggests that ``fast reconnection'' may occur
through the collaborative effort of a large number of many small-scale
current sheets, rather than in the very large magnetic Reynolds number
limit of single current sheets that has been so extensively discussed
in the literature. This has important applications to both the problem
of understanding coronal heating and the search for efficient flare
energy release mechanisms.
Title: The nonlinear evolution of a single mode of the magnetic
shearing instability
Authors: Torkelsson, U.; Ogilvie, G. I.; Brandernburg, A.; Nordlund,
Å.; Stein, R. F.
Bibcode: 1997LNP...487..135T
Altcode: 1997adna.conf..135T
We simulate in one dimension the magnetic shearing instability for
a vertical magnetic field penetrating a Keplerian accretion disc. An
initial equilibrium state is perturbed by adding a single eigenmode of
the shearing instability and the subsequent evolution is followed into
the nonlinear regime. Assuming that the perturbation is the most rapidly
growing eigenmode, the linear theory remains applicable until the
magnetic pressure perturbation is strong enough to induce significant
deviations from the original density. If the initial perturbation is
not the fastest growing mode, the faster growing modes will appear
after some time.
Title: Topologically Forced Reconnection
Authors: Nordlund, A.; Galsgaard, K.
Bibcode: 1997LNP...489..179N
Altcode: 1997shpp.conf..179N
A magnetically dominated plasma driven by braiding motions on boundaries
at which magnetic field lines are anchored is forced to dissipate the
work being done upon it, no matter how small the electrical resistivity
may be. Recent numerical experiments have clarified the mechanisms
through which balance between the boundary work and the dissipation in
the interior is achieved. The results largely confirm Parker's (1972)
idea of "topological dissipation"; dissipation is achieved through the
formation of a hierarchy of electrical current sheets. Current sheets
form as a result of the topological interlocking of individual strands
of magnetic field. The average level of dissipation is well described
by a scaling law that is independent of the electrical resistivity.
Title: Heating and activity of the solar corona. 2. Kink instability
in a flux tube
Authors: Galsgaard, Klaus; Nordlund, Åke
Bibcode: 1997JGR...102..219G
Altcode:
The development of kink instability in a flux tube is investigated
numerically, by solving the resistive MHD equations in three dimensions
for a setup where a flux tube is stressed by rotating both ends in
opposite directions. Two cases are investigated: one where the tube is
initially isolated and in pressure equilibrium with surrounding plasma
(external kink) and another with an initially uniform magnetic field,
where only a smaller part of the boundaries are used to twist the field
(internal kink). The twist angle at the onset of the kink instability
depends on several parameters, such as rotation velocity, tube diameter,
field strength, and magnetic resistivity, but is generally in the
range 4π-8π. Both sets of experiments are followed beyond the
point where they become kink unstable into the regime of nonlinear
evolution. Of particular interest is the topological evolution. As
magnetic dissipation becomes significant, the connectivity between
the two boundaries changes from ordered to chaotic, and small-scale
current sheets develop. Even though the gross features of the external
kink appear to saturate, the total magnetic energy continues to grow,
by a steady increase of the free energy in the chaotic region that
develops as a result of the kink and by a secular spreading of the
magnetic field into the initially field-free region. The internal
kink is confined to the cylinder defined by the boundary driving and
has only limited influence on the external magnetic field. After the
kink, the twist of the magnetic field is reduced, and the internal
kink settles into a quasi-steady state where the dissipation on the
average balances the Poynting flux input. The average Poynting flux
is similar in the external and internal kinks, with a magnitude that
corresponds to local winding numbers of the order of unity. Scaling of
these results to values characteristic of the solar corona indicate
that systematic rotation or shear of the endpoints could be a source
of quasi-steady heating in coronal loops.
Title: Magnetohydrodynamic Turbulence in Accretion Discs: Towards
More Realistic Models
Authors: Torkelsson, U.; Brandenburg, A.; Nordlund, A.; Stein, R. F.
Bibcode: 1997ASPC..121..210T
Altcode: 1997apro.conf..210T; 1997IAUCo.163..210T
No abstract at ADS
Title: Numerical Simulations Can Lead to New Insights
Authors: Stein, Robert F.; Carlsson, Mats; Nordlund, Ake
Bibcode: 1997ASPC..123...72S
Altcode: 1997taca.conf...72S
No abstract at ADS
Title: Double null points and magnetic reconnection
Authors: Galsgaard, K.; Rickard, G. J.; Reddy, R. V.; Nordlund, Å.
Bibcode: 1997AdSpR..19.1785G
Altcode:
2D reconnection is possible only in connection with the existence
of a singularity in the magnetic field line topology, associated
with a magnetic null point or a current sheet. Both of these provide
an X-type structure of the magnetic field where fields of opposite
polarity meet and reconnect. In 3D a similar topology is found in a
null point pair, when the null points are connected by a separator
line. The separator is defined as the intersection line of the two
null-point fan planes. This paper reports on the topological evolution
of this configuration with respect to different perturbations emerging
from imposed boundary velocities, using a nonlinear numerical approach.
Title: Heating and activity of the solar corona 1. Boundary shearing
of an initially homogeneous magnetic field
Authors: Galsgaard, Klaus; Nordlund, Åke
Bibcode: 1996JGR...10113445G
Altcode:
To contribute to the understanding of heating and dynamic activity
in boundary-driven, low-beta plasmas such as the solar corona, we
investigate how an initially homogeneous magnetic field responds
to random large-scale shearing motions on two boundaries, by
numerically solving the dissipative MHD equations, with resolutions
ranging from 243 to 1363. We find that even a
single application of large-scale shear, in the form of orthogonal
sinusoidal shear on two boundaries, leads to the formation of
tangential discontinuities (current sheets). The formation time
scales logarithmically with the resistivity and is of the order of a
few times the inverse shearing rate for any reasonable resistivity,
even though no mathematical discontinuity would form in a finite
time in the limit of vanishing resistivity. The reason for the
formation of the current sheets is the interlocking of two magnetic
flux systems. Reconnection in the current sheets is necessary for the
field lines to straighten out. The formation of current sheets causes
a transition to a very dynamic plasma state, where reconnection drives
supersonic and super-Alfvénic jet flows and where these, in turn,
cause the formation of smaller-scale current sheets. A statistically
steady state level for the average Poynting flux and the average Joule
dissipation is reached after a few correlation times, but both boundary
work and Joule dissipation are highly fluctuating in time and space
and are only weakly correlated. Strong and bursty Joule dissipation
events are favored when the volume has a large length/diameter ratio
and is systematically driven for periods longer than the Alfvèn
crossing time. The understanding of the reason for the current sheet
formation allows a simple scaling law to be constructed for the average
boundary work. Numerical experiments over a range of parameter values,
covering over 3 orders of magnitude in average dissipation, obey the
scaling law to within a factor of 2. The heating rate depends on the
boundary velocity amplitude and correlation time, the Alfvén speed,
and the initial magnetic field strength but appears to be independent
of the resistivity because of the formation of a hierarchy of current
sheets. Estimates of the photospheric boundary work on the solar coronal
magnetic field using the scaling law are consistent with estimates of
the required coronal heating rates. We therefore conclude that the
work supplied to the solar corona as a consequence of the motion of
the magnetic foot points in the solar photosphere and the emergence of
new flux is a significant contributor to coronal heating and flaring
and that it quite plausibly is the dominant one.
Title: Supercomputer windows into the solar convection zone
Authors: Nordlund, Å.; Stein, R. F.; Brandenburg, A.
Bibcode: 1996BASI...24..261N
Altcode:
No abstract at ADS
Title: The Disk Accretion Rate for Dynamo-generated Turbulence
Authors: Brandenburg, Axel; Nordlund, Ake; Stein, Robert F.;
Torkelsson, Ulf
Bibcode: 1996ApJ...458L..45B
Altcode:
Dynamo-generated turbulence is simulated in a modified shearing
box approximation that removes scale invariance and allows finite
accretion rates for a given distance from the central object. The
effective Shakura-Sunyaev viscosity parameter, alpha SS, is estimated
in three different ways using the resulting mass accretion rate, the
heating rate, and the horizontal components of the Maxwell and Reynolds
stress tensors. The results are still resolution dependent: doubling
the resolution leads to 1.4--1.6 times larger values for the viscosity
parameter. For 63 x 127 x 64 meshpoints we find that alpha SS = 0.007.
Title: Dynamical Properties of Single and Double 3D Null Points
Authors: Galsgaard, K.; Rickard, G. J.; Reddy, R. V.; Nordlund, A.
Bibcode: 1996ASPC..111...82G
Altcode: 1997ASPC..111...82G
The dynamical reconnection properties of three-dimensional single and
double nulls are investigated using nonlinear simulations. The authors
confirm the importance of the three-dimensional topological structures
- the spine, fan, and separator - in the reconnection process. In
particular, they highlight the accumulated current structures in
relation to the underlying magnetic field topology as reconnection
proceeds.
Title: Magnetic structures in a dynamo simulation
Authors: Brandenburg, A.; Jennings, R. L.; Nordlund, Å.; Rieutord,
M.; Stein, R. F.; Tuominen, I.
Bibcode: 1996JFM...306..325B
Altcode:
We use three-dimensional simulations to study compressible convection
in a rotating frame with magnetic fields and overshoot into surrounding
stable layers. The, initially weak, magnetic field is amplified and
maintained by dynamo action and becomes organized into flux tubes
that are wrapped around vortex tubes. We also observe vortex buoyancy
which causes upward flows in the cores of extended downdraughts. An
analysis of the angles between various vector fields shows that there
is a tendency for the magnetic field to be parallel or antiparallel
to the vorticity vector, especially when the magnetic field is
strong. The magnetic energy spectrum has a short inertial range with
a slope compatible with k(+1/3) during the early growth phase of the
dynamo. During the saturated state the slope is compatible with k(-1). A
simple analysis based on various characteristic timescales and energy
transfer rates highlights important qualitative ideas regarding the
energy budget of hydromagnetic dynamos.
Title: Coronal Heating by Flux Braiding
Authors: Galsgaard, K.; Nordlund, Å.
Bibcode: 1996ApL&C..34..175G
Altcode:
No abstract at ADS
Title: The Turbulent Viscosity in Accretion Discs
Authors: Torkelsson, U.; Brandenburg, A.; Nordlund, Å.; Stein, R. F.
Bibcode: 1996ApL&C..34..383T
Altcode:
No abstract at ADS
Title: Dynamo-generated turbulence in disks: value and variability
of alpha.
Authors: Brandenburg, A.; Nordlund, Å.; Stein, R. F.; Torkelsson, U.
Bibcode: 1996bpad.conf..285B
Altcode: 1996pada.conf..285B
Dynamo-generated turbulence seems to be a universal mechanism for
angular momentum transport in accretion disks. The authors discuss the
resulting value of the viscosity parameter alpha and emphasize that this
value is in general not constant. Alpha varies with the magnetic field
strength which, in turn, can vary in an approximately cyclic manner. The
authors also show that the stress does not vary significantly with
depth, even though the density drops by a factor of about 30.
Title: Solar Convection and Magneto-Convection
Authors: Basu, S.; Bercik, D. J.; Nordlund, A.; Stern, R. F.
Bibcode: 1996mpsa.conf..621B
Altcode: 1996IAUCo.153..621B
No abstract at ADS
Title: 3D non-LTE line formation in the solar photosphere and the
solar oxygen abundance.
Authors: Kiselman, D.; Nordlund, A.
Bibcode: 1995A&A...302..578K
Altcode: 1995astro.ph..5037K
We study the formation of OI and OH spectral lines in three-dimensional
hydrodynamic models of the solar photosphere. The line source function
of the OI 777nm triplet is allowed to depart from local thermodynamic
equilibrium (lte), within the two-level-atom approximation. Comparison
with results from 1D models show that the 3D models alleviate, but
do not remove, the discrepancy between the oxygen abundances reported
from non-lte work on the 777nm triplet and from the [OI] 630nm and OH
lines. Results for the latter two could imply that the solar oxygen
abundance is below 8.8. If this is confirmed, the discrepancy between
theory and observation for the 777nm triplet lines might fall within
the range of errors in equivalent-width measurements and f-values. The
line source function of the 777nm triplet in the 1.5D approximation
is shown to differ insignificantly from the full 3D non-lte result.
Title: Convective Perturbations to Solar Oscillations: the f-MODE
Authors: Rosenthal, C. S.; Christensen-Dalsgaard, Jorgen; Nordlund,
Ake; Trampedach, Regner
Bibcode: 1995ESASP.376b.453R
Altcode: 1995soho....2..453R; 1995help.confP.453R
No abstract at ADS
Title: Seismology of the Solar Surface Regions
Authors: Rosenthal, C. S.; Christensen-Dalsgaard, J.; Houdek, G.;
Monteiro, M. J. P. F. G.; Nordlund, A.; Trampedach, R.
Bibcode: 1995ESASP.376b.459R
Altcode: 1995soho....2..459R; 1995help.confP.459R
We investigate the influence of dynamical and nonadiabatic affects
ocurring in the superadiabatic region near the top of the solar
convetcion zone on the frequencies of solar p modes. Taking as our
baseline a standard hydrostatic solar model, we calculate frequency
changes resulting from a number of different formalisms, involving
modifications of the superadiabatic temperature gradient, turbulent
pressure and/or nonadiabatic effects. We compare these various
methods of calculating the effect of convection on solar-oscillation
eigenfrequencies with each other and with the measured frequency
residuals.
Title: Dynamo-generated Turbulence and Large-Scale Magnetic Fields
in a Keplerian Shear Flow
Authors: Brandenburg, Axel; Nordlund, Ake; Stein, Robert F.;
Torkelsson, Ulf
Bibcode: 1995ApJ...446..741B
Altcode:
The nonlinear evolution of magnetized Keplerian shear flows is
simulated in a local, three-dimensional model, including the effects
of compressibility and stratification. Supersonic flows are initially
generated by the Balbus-Hawley magnetic shear instability. The
resulting flows regenerate a turbulent magnetic field which, in
turn, reinforces the turbulence. Thus, the system acts like a dynamo
that generates its own turbulence. However, unlike usual dynamos,
the magnetic energy exceeds the kinetic energy of the turbulence by
a factor of 3-10. By assuming the field to be vertical on the outer
(upper and lower) surfaces we do not constrain the horizontal magnetic
flux. Indeed, a large-scale toroidal magnetic field is generated,
mostly in the form of toroidal flux tubes with lengths comparable
to the toroidal extent of the box. This large-scale field is mainly
of even (i.e., quadrupolar) parity with respect to the midplane and
changes direction on a timescale of ∼30 orbits, in a possibly cyclic
manner. The effective Shakura-Sunyaev alpha viscosity parameter is
between 0.001 and 0.005, and the contribution from the Maxwell stress
is ∼3-7 times larger than the contribution from the Reynolds stress.
Title: Near-surface Effects in Modelling Oscillations of Eta Boo
Authors: Christensen-Dalsgaard, J.; Bedding, T. R.; Houdek, G.;
Kjeldsen, H.; Rosenthal, C.; Trampedach, H.; Monteiro, M. J. P. F. G.;
Nordlund, A.
Bibcode: 1995ASPC...83..447C
Altcode: 1995IAUCo.155..447C; 1995aasp.conf..447C; 1995astro.ph..3106C
Following the report of solar-like oscillations in the G0 V star eta
Boo (Kjeldsen et al. 1995, AJ 109, 1313), a first attempt to model the
observed frequencies was made by Christensen-Dalsgaard et al. (1995, ApJ
Letters, in press). This attempt succeeded in reproducing the observed
frequency separations, although there remained a difference of about
10 microHz between observed and computed frequencies. In those models,
the near-surface region of the star was treated rather crudely. Here we
consider more sophisticated models that include non-local mixing-length
theory, turbulent pressure and nonadiabatic oscillations.
Title: Convection; Significance for Stellar Structure and Evolution
Authors: Nordlund, A.; Stein, R. F.
Bibcode: 1995LIACo..32...75N
Altcode: 1995sews.book...75N
No abstract at ADS
Title: Dynamo Generated Turbulence in Discs
Authors: Brandenburg, A.; Nordlund, Å.; Stein, R. F.; Torkelsson, U.
Bibcode: 1995LNP...462..385B
Altcode: 1995ssst.conf..385B
The magnetic shear instability appears to be a workable mechanism
for generating turbulence in accretion discs. The magnetic field,
in turn, is generated by a dynamo process that taps energy from the
Keplerian shear flow. Large scale magnetic fields are generated, whose
strength is comparable with, or in excess of, the turbulent kinetic
energy. Such models enable us to investigate the detailed nature
of turbulence in discs. We discuss in particular the possibility of
generating convection, where the heat source is viscous and magnetic
heating in the bulk of the disc.
Title: Modeling of the Solar Convection Zone
Authors: Basu, S.; Bercik, D. J.; Nordlund, A.; Stein, R. F.
Bibcode: 1994AAS...185.4402B
Altcode: 1994BAAS...26Q1377B
We present results from a simulation of a 6 x 6 x 3 Mm region
of the upper solar convection zone at twice the resolution (25 km
horizontally and 15-35 km vertically) of our previous calculation. We
compare identical times at the two resolutions to show the effect on
downdrafts and other properties of convection.
Title: Dynamic behavior and topology of 3D magnetic fields
Authors: Galsgaard, K.; Nordlund, Å.
Bibcode: 1994SSRv...68...75G
Altcode:
We investigate numerically the dynamical evolution of a boundary
driven, topologically complex low β plasma. The initial state is
a simple, but topologically nontrivial 3D magnetic field, and the
evolution is driven by forced motions on two opposite boundaries of
the computational domain. A large X-type reconnection event with a
supersonic one-sided jet occurs as part of a process that brakes down
the large scale topology of the initial field. An energetically steady
state is reached, with a double arcade overall topology, in which the
driving causes continuous creation of small scale thin current sheets
at various locations in the arcade structures.
Title: Magnetoconvection and magnetoturbulence
Authors: Nordlund, Å.; Galsgaard, K.; Stein, R. F.
Bibcode: 1994ASIC..433..471N
Altcode:
No abstract at ADS
Title: Subphotospheric Convection
Authors: Stein, R. F.; Nordlund, A.
Bibcode: 1994IAUS..154..225S
Altcode:
No abstract at ADS
Title: Ionization Effects in Three-dimensional Solar Granulation
Simulations
Authors: Rast, Mark P.; Nordlund, Ake; Stein, Robert F.; Toomre, Juri
Bibcode: 1993ApJ...408L..53R
Altcode:
These numerical studies show that ionization influences both the
transport and dynamical properties of compressible convection
near the surface of the Sun. About two-thirds of the enthalpy
transported by convective motions in the region of partial hydrogen
ionization is carried as latent heat. The role of fast downflow
plumes in total convective transport is substantially elevated
by this contribution. Instability of the thermal boundary layer
is strongly enhanced by temperature sensitive variations in the
radiative properties of the fluid, and this provides a mechanism for
plume initiation and cell fragmentation in the surface layers. As
the plumes descend, temperature fluctuations and associated buoyancy
forces are maintained because of the increased specific heat of the
partially ionized material. This can result is supersonic vertical
flows. At greater depths, ionization effects diminish, and the plumes
are decelerated by significant entrainment of surrounding fluid.
Title: Reynolds Stresses Derived from Simulations
Authors: Pulkkinen, P.; Tuominen, I.; Brandenburg, A.; Nordlund, A.
Bibcode: 1993IAUS..157..123P
Altcode:
No abstract at ADS
Title: Ionization Effects on Solar Granulation Dynamics
Authors: Rast, M. P.; Nordlund, A.; Stein, R. F.; Toomre, J.
Bibcode: 1993ASPC...42...57R
Altcode: 1993gong.conf...57R
No abstract at ADS
Title: Rotational effects on convection simulated at different
latitudes
Authors: Pulkkinen, Pentti; Tuominen, Ilkka; Brandenburg, Axel;
Nordlund, Ake; Stein, Robert F.
Bibcode: 1993A&A...267..265P
Altcode:
We simulate numerically convection inside the solar convection
zone under the influence of rotation at different latitudes. The
computational domain is a small rectangular box with stress-free upper
and lower boundaries, and with periodicity assumed in the lateral
directions. We study the transport of angular momentum, which is
important for the generation of differential rotation. The sign and
the latitudinal dependence of the horizontal Reynolds stress component
turn out to be in good agreement with correlation measurements of
sunspot proper motions and with predictions from the theory of the
Lambda effect. We also investigate the other components of the Reynolds
stress as well as the eddy heat flux tensor, both of which are needed
in mean field models of differential rotation.
Title: The Distant Future of Solar Activity: A Case Study of beta
Hydri. I. Stellar Evolution, Lithium Abundance, and Photospheric
Structure
Authors: Dravins, D.; Lindegren, L.; Nordlund, A.; Vandenberg, D. A.
Bibcode: 1993ApJ...403..385D
Altcode:
A detailed comparison of the current sun (G2 V) with the very old
solar-type star Beta Hyi (G2 IV) is presented in order to study the
postmain-sequence evolution of stellar activity and of nonthermal
processes in solar-type atmospheres. Special attention is given to
general stellar properties and the deeper atmosphere of Beta Hyi. A
critical review of data from various sources is presented, and the
age of Beta Hyi is determined from evolutionary models to 9.5 +/-
0.8 Gyr. The relatively high lithium abundance may be a signature of
the early subgiant stage, when lithium that once diffused to beneath
the main-sequence convection zone is dredged up to the surface as the
convection zone deepens. Numerical simulations of the 3D photospheric
hydrodynamics show typical granules to be significantly larger (a
factor of about 5) than solar ones.
Title: Evolution of a magnetic flux tube in two-dimensional
penetrative convection
Authors: Jennings, R. L.; Brandenburg, A.; Nordlund, A.; Stein, R. F.
Bibcode: 1992MNRAS.259..465J
Altcode:
Highly supercritical compressible convection is simulated in a
two-dimensional domain in which the upper half is unstable to convection
while the lower half is stably stratified. This configuration is
an idealization of the layers near the base of the solar convection
zone. Once the turbulent flow is well developed, a toroidal magnetic
field Btor is introduced to the stable layer. The field's
evolution is governed by an advection-diffusion-type equation, and
the Lorentz force does not significantly affect the flow. After many
turnover times the field is stratified such that the absolute value
of Btor/rho is approximately constant in the convective
layer, where rho is density, while in the stable layer this ratio
decreases linearly with depth. Consequently most of the magnetic flux
is stored in the overshoot layer. The inclusion of rotation leads
to travelling waves which transport magnetic flux latitudinally in a
manner reminiscent of the migrations seen during the solar cycle.
Title: Effects of sphericity in carbon star atmospheres.
Authors: Jorgensen, Uffe G.; Johnson, Hollis R.; Nordlund, Ake
Bibcode: 1992A&A...261..263J
Altcode:
The effect of sphericity in carbon star atmospheres is investigated by
comparing results of calculations of more than 200 model atmospheres
for carbon stars in spherical with those in plane-parallel geometry
for various combinations of the parameters Teff = 3400,
3100, 2800, 2500 K; log(g) = 0.5, -0.5, -1.0; C/O = 1.02, 1.35, 2.0;
Z/Z-solar = 1, 1/2, 1/10; and M/M-solar = 0.7, 1.5, 3, 10, and 100. It
is shown that, compared with the corresponding plane-parallel models,
spherical models are cooler in the surface layers and the gas pressure
is higher (e.g., for a 0.7 solar mass spherical model, a cooling of
100 K and an increase in gas pressure by a factor of 3 in the surface
layers is found). The effect is largest for the low-gravity models and
increases somewhat with increasing temperatures; on the other hand, the
C/O and Z values have small effects on the response to sphericity. It
was also found that only HCN line and the strongest lines of CO are
sensitive to the effects of sphericity.
Title: Dynamo Action in Stratified Convection with Overshoot
Authors: Nordlund, Ake; Brandenburg, Axel; Jennings, Richard L.;
Rieutord, Michel; Ruokolainen, Juha; Stein, Robert F.; Tuominen, Ilkka
Bibcode: 1992ApJ...392..647N
Altcode:
Results are presented from direct simulations of turbulent compressible
hydromagnetic convection above a stable overshoot layer. Spontaneous
dynamo action occurs followed by saturation, with most of the generated
magnetic field appearing as coherent flux tubes in the vicinity
of strong downdrafts, where both the generation and destruction of
magnetic field is most vigorous. Whether or not this field is amplified
depends on the sizes of the magnetic Reynolds and magnetic Prandtl
numbers. Joule dissipation is balanced mainly by the work done against
the magnetic curvature force. It is this curvature force which is also
responsible for the saturation of the dynamo.
Title: Spherical opacity sampling model atmospheres for
M-giants. I. Techniques,data and discussion.
Authors: Plez, Bertrand; Brett, John M.; Nordlund, Ake
Bibcode: 1992A&A...256..551P
Altcode:
We present a new code for the calculation of static, spherically
symmetric, opacity-sampling model atmospheres in local thermodynamic
equilibrium for cool giants and supergiants of spectral type
M. Up-to-date data for line opacities have been included, with new
calculated line lists for the TiO, VO, and H2O molecules. This is
the first time both sphericity and a realistic description (opacity
sampling) of all opacity sources are used simultaneously for the
calculation of cool stellar atmospheres and this results in a better
agreement between calculated and observed fluxes and colors.
Title: Spherical Models for Carbon Stars
Authors: Jorgensen, U. G.; Johnson, H. R.; Nordlund, A.
Bibcode: 1992ASPC...26..540J
Altcode: 1992csss....7..540J
No abstract at ADS
Title: Spherical opacity samples of model atmospheres for M giants
and supergiants
Authors: Plez, B.; Brett, J. M.; Nordlund, Å.
Bibcode: 1992iesh.conf..119P
Altcode:
No abstract at ADS
Title: Magnet Convection (Invited Review)
Authors: Stein, R. F.; Brandenburg, A.; Nordlund, A.
Bibcode: 1992ASPC...26..148S
Altcode: 1992csss....7..148S
No abstract at ADS
Title: Deformation of Magnetic Null Points
Authors: Galsgaard, K.; Nordlund, A.
Bibcode: 1992LNP...399..343G
Altcode: 1992esf..coll..343G; 1992IAUCo.133..343G
No abstract at ADS
Title: Large scale simulations
Authors: Nordlund, Ake; Galsgaard, Klaus
Bibcode: 1992AIPC..267...13N
Altcode: 1992ecsa.work...13N
We discuss large scale numerical simulations as a tool for obtaining
qualitative understanding of the processes directly and indirectly
responsible for coronal heating. The actual heating process in the
low beta coronal plasma is most likely driven by transfer of magnetic
energy from the subsurface high beta region, where magnetic energy is
created as an energetically insignificant byproduct of solar convection
and rotation. Based on the results of recent numerical experiments,
we discuss some of the processes involved.
Title: The distant future of solar activity: a case study of beta
Hydri (abstract)
Authors: Dravins, D.; Linde, P.; Ayres, T. R.; Fredga, K.; Gahm, G.;
Lindegren, L.; Linsky, J. L.; Monsignori-Fossi, B.; Nordlund, Å.;
Simon, T.; Vandenberg, D.; Wallinder, F.
Bibcode: 1992sccw.conf..105D
Altcode:
No abstract at ADS
Title: Convection and Its Influence on Oscillations
Authors: Stein, Robert F.; Nordlund, Åke
Bibcode: 1991LNP...388..195S
Altcode: 1991ctsm.conf..195S
We investigate the interaction between p-mode oscillations and
convection using a realistic, three-dimensional simulation of the
upper solar convection zone. P-mode oscillations are excited at the
eigenfrequencies of the simulation volume. Their frequency is different
than that found from one-dimensional mixing length models. Their
resonant cavity becomes larger when overshooting into the photosphere
is possible, which lowers the mode frequencies, while interaction with
the inhomogeneities in the sound speed and the motions generated by the
convection tends to raise the mode frequencies. The modes are excited
stochastically by non-adiabatic fluctuations in the gas pressure caused
by the switch from convective to radiative energy transport at the
solar surface.
Title: NLTE Spectral Line Formation in Three Dimensions
Authors: Nordlund, A.
Bibcode: 1991ASIC..341...61N
Altcode: 1991sabc.conf...61N
No abstract at ADS
Title: Vector Potential Magnetic Null Points
Authors: Galsgaard, Klaus; Nordlund, Åke
Bibcode: 1991LNP...380...89G
Altcode: 1991IAUCo.130...89G; 1991sacs.coll...89G
No abstract at ADS
Title: Granulation: Non-adiabatic Patterns and Shocks
Authors: Nordlund, Åke; Stein, Robert F.
Bibcode: 1991LNP...388..141N
Altcode: 1991ctsm.conf..141N
We present, in graphical form, some results from numerical simulations
of the solar granulation. We compare synthetic granulation images with
observations of the solar granulation, and illustrate the corresponding
pressure and velocity fields. In particular, the non-adiabatic part
of the pressure fluctuation, which is a major source of stochastic
excitation of P-modes, is shown.
Title: The Role of Overshoot in Solar Activity - a Direct Simulation
of the Dynamo
Authors: Brandenburg, A.; Jennings, R. L.; Nordlund, Å.; Stein,
R. F.; Tuominen, I.
Bibcode: 1991LNP...380...86B
Altcode: 1991IAUCo.130...86B; 1991sacs.coll...86B
We investigate convective overshoot in a layer of electrically
conducting fluid. The radiative conductivity is assumed to be larger
in the lower part of the layer which makes it stable to convective
motions, yet penetrative convection from the upper layer can occur. The
numerical resolution is 633 gridpoints. We observe a dynamo effect for
magnetic Reynolds numbers around one thousand when a magnetic seed
field is rapidly concentrated to form flux tubes. Later the average
magnetic field is expelled from the convectively unstable regions,
but it accumulates in the interface between the convection zone and
the radiative interior.
Title: The Significance of Magnetic Null Points (With 1 Figure)
Authors: Galsgaard, K.; Nordlund, Å.
Bibcode: 1991mcch.conf..541G
Altcode:
No abstract at ADS
Title: Magnetic Tubes in Overshooting Compressible Convection
Authors: Jennings, R. L.; Brandenburg, A.; Nordlund, Å.; Stein,
R. F.; Tuominen, I.
Bibcode: 1991LNP...380...92J
Altcode: 1991sacs.coll...92J; 1991IAUCo.130...92J
A magnetic tube is introduced into turbulent compressible penetrative
convection. After being strongly advected, most of the magnetic flux
is stored in the overshoot region. With rotation there are meridional
travelling waves.
Title: Convection and p-modes.
Authors: Nordlund, Å.; Stein, R. F.
Bibcode: 1991dsoo.conf...37N
Altcode:
An introductory overview of the qualitative properties of convection
and p modes in solar type stars is followed by a discussion of how to
obtain a meaningful separation between "wave-motion" and "convection" in
a strongly inhomogeneous medium. For radial waves, a natural separation
is obtained by using certain weighted averages, in a "pseudo-Lagrangian"
coordinate system in which there is no net vertical mass flux. Three
principal influences of the convection on the wave modes are identified:
Frequency shifts due to coherent perturbations in phase with "restoring
force" terms in the wave equations, linear damping or growth due to
coherent perturbations 90 degrees out of phase with restoring force
terms, and stochastic excitation due to incoherent perturbations of
the wave equations. In addition, convection influences p-modes by
cavity changes: i.e., changes of the size of the resonant cavity due
to changes in the mean structure. Numerical illustrations of these
effects are given, using results from supercomputer simulations of
the interaction of solar convection with p-modes.
Title: Recent development in solar convection theory.
Authors: Chan, Kwing L.; Nordlund, A.; Steffen, Matthias; Stein, R. F.
Bibcode: 1991sia..book..223C
Altcode:
In recent years, the theory of solar (and stellar) convection has
made fundamental advances due to the increasing cost effectiveness of
supercomputers and the constant improvement of numerical techniques. It
is expected that the numerical approach will become a dominant trend
for the future. The authors report on these new advances. References
to theoretical studies on phenomena related to solar convection are
compiled. The authors then discuss three numerical studies of solar
convection in greater detail, so as to provide the readers with some
general understanding of the numerical techniques being used and
the results obtained: The discussion starts with a two-dimensional
study of the spectroscopic properties of solar granules. While the
two-dimensional limitation is severely detrimental to some important
hydrodynamical processes, it is both economical and able to provide some
initial understanding of the gross features of solar convection. Next
they discuss the testing of the well-known mixing-length theory with
three-dimensional numerical experiments. An example is also given of
applying the numerically gained knowledge to analytical study, in this
case the behavior of compressible convection as a heat engine. The
third case describes a realistic, three-dimensional simulation of
solar granulation; many observational features of solar granules
are faithfully reproduced. It is the most sophisticated numerical
calculation of this sort today.
Title: Magnetoacoustic Waves and Their Generation by Convection
(With 15 Figures)
Authors: Stein, R. F.; Nordlund, Å.
Bibcode: 1991mcch.conf..386S
Altcode:
No abstract at ADS
Title: Rotational Effects on Reynolds Stresses in the Solar
Convection Zone
Authors: Pulkkinen, P.; Tuominen, I.; Brandenburg, A.; Nordlund, Å.;
Stein, R. F.
Bibcode: 1991LNP...380...98P
Altcode: 1991IAUCo.130...98P; 1991sacs.coll...98P
Three-dimensional hydrodynamic simulations are carried out in a
rectangular box. The angle between gravity and rotation axis is kept
as an external parameter in order to study the latitude-dependence
of convection. Special attention is given to the horizontal Reynolds
stress and the -effect (Rüdiger, 1989). The results of the simulations
are compared with observations and theory and a good agreement is found.
Title: Dynamics of an Radiative Transfer in Inhomogeneous Media
Authors: Nordlund, A.; Stein, R. F.
Bibcode: 1991ASIC..341..263N
Altcode: 1991sabc.conf..263N
No abstract at ADS
Title: The distant future of solar activity - A case study of
Beta Hydri
Authors: Dravins, D.; Linde, P.; Ayres, T. R.; Fredga, K.; Gahm, G.;
Lindegren, L.; Linsky, J. L.; Monsignori-Fossi, B.; Nordlund, A.;
Simon, T.; Vandenberg, D.; Wallinder, F.
Bibcode: 1990ESASP.310..323D
Altcode: 1990eaia.conf..323D
No abstract at ADS
Title: 3-D simulation of turbulent cyclonic magneto-convection.
Authors: Brandenburg, A.; Tuominen, I.; Nordlund, A.; Pulkkinen, P.;
Stein, R. F.
Bibcode: 1990A&A...232..277B
Altcode:
Results are presented of a simulation of turbulent three-dimensional
magnetic convection under the influence of rotation in a fluid layer
whose depth is about 1 pressure-scale hight. The approach is similar
to that of Meneguzzi and Pouquet (1989), except for the assumptions
that the fluid is a compressible conducting gas and there is a
vanishing horizontal magnetic field at the boundaries. The results
demonstrate that topological effects may be of great importance for
MHD convection. It is shown that, as a consequence of topological
effects, anisotropies of the alpha-effect can play a dominant role. In
particular, the sign of alpha(V) can be opposite to that expected from
a first-order smoothing approach.
Title: 3-D simulations of solar and stellar convection and
magnetoconvection
Authors: Nordlund, Å.; Stein, R. F.
Bibcode: 1990CoPhC..59..119N
Altcode:
We present the key components of a 3-D code designed for simulating
the hydrodynamics and magnetohydrodynamics of stellar atmospheres
and envelopes. Some particular properties of the code are: (1) the
ability to handle strong stratification (extensive simulations with
bottom/top pressure ratios of 3×104 have been performed,
and simulations with pressure ratios of 5×106 are being
initiated); (2) a detailed treatment of the radiating surface; (3) a
functional form of the subgrid-scale diffusion designed to minimize
the influence on resolved motions; (4) boundary conditions open
to flows. The top boundary allows the transmission of short period
waves, while the bottom boundary condition was designed to enforce a
displacement node for radial pressure modes.
Title: The reflection effect in model stellar atmospheres. II - The
bolometric reflection albedo in non-grey atmospheres with convection
Authors: Nordlund, A.; Vaz, L. P. R.
Bibcode: 1990A&A...228..231N
Altcode:
The effects of external irradiation on model atmospheres with
convection, and with spectral line absorption represented by opacity
distribution functions are presented. The heating of optically thin
layers associated with external irradiation, and the reduction of the
temperature gradient in the continuum formation layers has noticeable
effects on the spectrum of the illuminated star. The dependence of
this reflection effect on the abundance of heavy elements, on the
strength of convection, and on the properties (spectrum, angle of
inclination) of the incident radiative flux is investigated. It is
noted that the amount of disturbance of the illuminated model depends
rather critically on whether or not there is a detailed match between
the absorption features of the illuminating and illuminated stars.
Title: Stellar granulation. IV. Line formation in inhomogeneous
stellar photospheres.
Authors: Dravins, D.; Nordlund, A.
Bibcode: 1990A&A...228..184D
Altcode:
Synthetic images of stellar granulation and photospheric Fe line
profiles are computed in model atmospheres obtained from detailed
numerical simulations of stellar surface convection. Models
corresponding to Procyon (F5 IV-V), α Cen A (G2V), β Hyi (G2IV),
and β Cen B (K1V) are studied (5200 ≤Teff≤6600 K). The
broadening, wavelength shift and asymmetry of spatially and temporally
resolved line profiles follows from radiative transfer in explicitly
computed three- dimensional and time-variable velocity fields, and
no adjustable - fitting parameters (such as e. g. "turbulence") are
used. Synthetic white-light and monochromatic images illustrate the
intensity contrast on stellar surfaces, its center-to-limb variation
and the morphology of line formation. Spatially resolved and spatially
averaged profiles illustrate line broadening through the Doppler
effect in photospheric velocity fields. An increase in the velocity
spread of spatially resolved lines near the stellar limbs reflects the
larger amplitudes of horizontal velocities in line-forming layers. Time
variability of spatially averaged line profiles and of their continuum
flux levels reflects time evolution of convective patterns larger than
individual granules. Spatially and temporally averaged data identify
how different shapes, asymmetries and shifts among lines of different
strength, excitation potential, ionization level, and wavelength region,
map the detailed physical properties throughout the photo sphere. The
properties of averaged profiles (in particular their asymmetries)
are not at all typical for individual points on the stellar surface,
but rather reflect the statistical distribution of photospheric
inhomogeneities. Only very strong lines have sufficiently extended
depths of formation for their asymmetry to be significantly influenced
also by the depth-variation of photospheric flow velocities. Effects of
the (non-LTE) radiative ionization of iron are not large but visible
as a weakening of blueshifted Fe I line components above especially
hot and bright granules. Convective blueshifts, originating from
correlations between local brightness and local Doppler shift, vary
between ∼=200 and 1000 ms-1 at disk center in different
stars. Since such correlations change throughout the atmosphere, already
small differences in line formation conditions for lines of different
strength or excitation potential may result in different asymmetries
and wavelength shifts. For example, the lower surface gravity on the
solar near-twin α Cen A permits larger velocity amplitudes in the
high photosphere, causing noticeable differences to the Sun in the
asymmetries of its stronger photospheric lines.
Title: Stellar granulation. V. Synthetic spectral lines in
disk-integrated starlight.
Authors: Dravins, D.; Nordlund, A.
Bibcode: 1990A&A...228..203D
Altcode:
Numerical simulations of stellar photo spheric structure have provided
line profiles at different positions across stellar disks. Using
these data, synthetic Fe line profiles in disk-integrated flux are
computed (including their asymmetries and wavelength shifts) for
models corresponding to Procyon (F 5 IV-V), α Cen A (G2V), β Hyi
(G2IV) and α Cen B (K1V). The line profiles are computed without
any adjustable physical parameters besides that of stellar rotation,
and the model atmospheres contain no classical parameters such as
"mixing-length" nor "turbulence". Since line strength, width, asymmetry,
rotational broadening, and limb darkening change with disk position,
the disk-integrated profiles reflect these properties in a complex
manner. This intercoupling might allow determinations of not only
stellar rotation, but also line profile variations across stellar disks,
using observations of similar stars with different rotation. Grids of
"observed" synthetic line profiles and bisectors illustrate effects
of finite spectral resolution. Comparisons with observations show good
agreement, and the stellar rotation can be independently determined from
the symmetric line broadening, and from the bisector patterns. For the
well observed stars Procyon and α Cen A, we estimate V sin i≃2.9
and 1.8 km s-1, respectively. For the solar near-twin α
Cen A, the profile and bisector fits are almost perfect, and permit
the identification of subtle differences against the Sun, apparently
reflecting changes in solar-type granulation during some billion years
of stellar evolution. The bisector fit for Procyon is excellent, but
some absorption is missing in the flanks of the intensity profiles
outside about ±5 km s-1. This, and a similar effect in the
subgiant β Hyi, is believed to be an artifact of the hydrodynamically
anelastic atmospheric model, which excludes sound waves and absorption
by features moving at near-sonic speeds. Different stars have different
line asymmetries, and in each star there is a systematic dependence
on line-strength. The excitation-potential and wavelength-region
dependences are smaller. The convective blueshift of spectral lines
ranges between ≃200 km s-1 in K dwarfs to ≃1000 m
s-1 in F stars. Such effects may limit the accuracies
possible in spectroscopic determinations of stellar radial velocities.
Title: Stellar granulation. III. Hydrodynamic model atmospheres.
Authors: Nordlund, A.; Dravins, D.
Bibcode: 1990A&A...228..155N
Altcode:
Detailed models for the three-dimensional, time-dependent and
radiation-coupled hydrodynamics of solar granular convection have been
adapted to stellar conditions, and extensive numerical simulations have
been carried out to model four different stars in the vicinity of the
sun in the H-R diagram. The results from the simulations, showing the
three-dimensional structure and time evolution of temperature, velocity,
and pressure features in stellar photospheres, are presented. They are
then used as sets of temporally and spatially varying model atmospheres
in which radiative transfer computations are made of the continuum and
line radiation leaving the stars. Synthetic images show the optical
appearance of stellar surface structure at different positions across
stellar disks. Synthetic spectral line profiles are computed for
different locations and times, and the buildup of average line profiles
is examined for lines of different strength, excitation potential,
ionization level, and wavelength region. The average line profiles
are then used as an input to synthesize the disk-integrated flux of
photospheric Fe lines for stars of different rotational velocities
in order to predict observable spectral line shapes, asymmetries,
and wavelength shifts.
Title: Turbulent diffusivities derived from simulations.
Authors: Brandenburg, A.; Nordlund, Å.; Pulkkinen, P.; Stein, R. F.;
Tuominen, I.
Bibcode: 1990fas..conf....1B
Altcode:
By employing direct simulations of turbulent magneto-convection the
authors determine the turbulent diffusivities, such as the turbulent
magnetic diffusivity, the eddy viscosity and the turbulent heat
conductivity.
Title: Solar Magnetoconvection
Authors: Nordlund, Å.; Stein, R. F.
Bibcode: 1990IAUS..138..191N
Altcode:
No abstract at ADS
Title: Model Atmospheres for M Giants
Authors: Plez, B.; Nordlund, A.
Bibcode: 1990fmpn.coll...88P
Altcode:
No abstract at ADS
Title: Driving and Damping of Oscillations
Authors: Stein, Robert F.; Nordlund, Åke
Bibcode: 1990LNP...367...93S
Altcode: 1990psss.conf...93S
We have simulated the upper 2.5 Mm of the solar convection zone using
a realistic, three-dimensional, compressible, hydrodynamic computer
code. P-mode oscillations are excited at the eigenfrequencies of the
simulation volume. We have calculated the time averages of the work
terms in the kinetic energy equation, using the internal energy equation
to evaluate the fluctuations in the gas pressure. This calculation
shows that the modes are excited near the surface by the divergence
of the convective flux and damped by the divergence of the radiative
flux. The fundamental mode is also spuriously driven at the lower
boundary, by density and turbulent pressure fluctuations induced when
downward plunging convective plumes pass through the lower boundary
of the simulation.
Title: Solar convection.
Authors: Spruit, H. C.; Nordlund, A.; Title, A. M.
Bibcode: 1990ARA&A..28..263S
Altcode:
The current understanding of solar convection is examined in connection
with optical observations of the surface, helioseismological
observations of the interior, and theories and simulations of
compressible convection. Recent progress in these fields has been
documented in workshops on solar granulation, the solar photosphere,
and helioseismology.
Title: The distant future of solar activity - a case study of
Beta Hydri.
Authors: Dravins, D.; Linde, P.; Ayres, T. R.; Fredga, K.; Gahm, G.;
Lindegren, L.; Linsky, J. L.; Monsignori-Fossi, B.; Nordlund, Å.;
Simon, T.; Vandenberg, D.; Wallinder, F.
Bibcode: 1990apsu.conf...17D
Altcode:
No abstract at ADS
Title: Topology of Convection beneath the Solar Surface
Authors: Stein, R. F.; Nordlund, A.
Bibcode: 1989ApJ...342L..95S
Altcode:
It is shown that the topology of convection beneath the solar surface
is dominated by effects of stratification. Convection in a strongly
stratified medium has: (1) gentle expanding structureless warm upflows
and (2) strong converging filamentary cool downdrafts. The horizontal
flow topology is cellular, with a hierarchy of cell sizes. The small
density scale height in the surface layers forces the formation of
the solar granulation, which is a shallow surface phenomenon. Deeper
layers support successively larger cells. The downflows of small cells
close to the surface merge into filamentary downdrafts of larger cells
at greater depths, and this process is likely to continue through most
of the convection zone. Radiative cooling at the surface provides the
entropy-deficient material which drives the circulation.
Title: Convection and Waves
Authors: Stein, R. F.; Nordlund, Å.; Kuhn, J. R.
Bibcode: 1989ASIC..263..381S
Altcode: 1989ssg..conf..381S
No abstract at ADS
Title: Simulating Magnetoconvection
Authors: Nordlund, Å.; Stein, R. F.
Bibcode: 1989ASIC..263..453N
Altcode: 1989ssg..conf..453N
No abstract at ADS
Title: Constraints Imposed by Very High Resolution Spectra and Images
on Theoretical Simulations of Granular Convection
Authors: Lites, B. W.; Nordlund, Å.; Scharmer, G. B.
Bibcode: 1989ASIC..263..349L
Altcode: 1989ssg..conf..349L
No abstract at ADS
Title: Convection and p-mode oscillations.
Authors: Stein, R. F.; Nordlund, A.; Kuhn, J. R.
Bibcode: 1988ESASP.286..529S
Altcode: 1988ssls.rept..529S
The authors have simulated the upper 2.5 Mm of the solar convection
zone using a three-dimensional, compressible, hydrodynamic computer
code. Preliminary results show that convection excites p-mode
oscillations. The frequencies of the modes in the numerical simulation
agree well with the eigenfrequencies of our computational box calculated
for the time averaged mean atmosphere. The agreement is excellent at low
frequencies, and diverges at higher frequencies in a manner similar to
the difference between observed and theoretical frequencies for the sun.
Title: What Does the Sun Look Like Beneath the Surface?
Authors: Nordlund, A.; Stein, R. F.
Bibcode: 1988BAAS...20..702N
Altcode:
No abstract at ADS
Title: Stellar Granulation: Photospheric Line Asymmetries and
Hydrodynamic Model Atmospheres
Authors: Dravins, D.; Nordlund, A.
Bibcode: 1986BAAS...18.1002D
Altcode:
No abstract at ADS
Title: Solar Fe II line asymmetries and wavelength shifts.
Authors: Dravins, D.; Larsson, B.; Nordlund, A.
Bibcode: 1986A&A...158...83D
Altcode:
Convective motions of solar granulation are manifest in the spatially
unresolved spectrum as slight asymmetries and wavelength shifts of
photospheric spectral lines. In a previous paper (Dravins et al.,
1981) that dependence for Fe I lines with line strength, excitation
potential and wavelength region was analyzed. This paper extends that
work to Fe II lines, examining bisector shapes and wavelength shifts
of "unblended" Fe II lines both at disk center and in integrated
sunlight. Fe II lines form predominantly in the hotter and denser
regions of the deep photosphere, and these different line formation
conditions for Fe II manifest themselves in well-defined differences
from Fe I: the average Fe II bisectors show a more articulated curvature
and a larger convective blueshift. Synthetic spectral lines, computed
from a three-dimensional time-dependent hydrodynamic simulation of
solar photospheric convection confirm the observed behavior.
Title: The reflection effect in model stellar atmospheres. I. Grey
atmospheres with convection.
Authors: Vaz, L. P. R.; Nordlund, Å.
Bibcode: 1986RMxAA..12..190V
Altcode:
No abstract at ADS
Title: The Reflection Effect in Model Stellar Atmospheres 1: Grey
Atmospheres with Convection
Authors: Ribeiro Vaz, L. P.; Nordlund, A.
Bibcode: 1986RMxAA..12..190R
Altcode:
The effects of the mutual illumination of the components of binary
systems are investigated, by introducing an external radiation field
in a model for plane-parallel stellar atmospheres in radiative +
convective equilibrium. For grey atmospheres in radiative equilibrium,
the results are verified against exact solutions. In general, the
external illumination causes a heating of the atmosphere. For models
in radiative equilibrium, the heating is such that all incident energy
is re-emitted by the atmosphere (bolometric reflection albedo equal to
one). The frequency distribution of the re-erritted energy is described
in terms of a frequency dependent effective reflection albedo, for which
approximate numerical expressions are given. For models in radiative +
convective equilibrium, not all the incident energy is re-emitted. By
requiring the entropy in the deep con vectionzone to be the same in the
illuminated and non-illuminated parts of a reflecting star, bolometric
retiection aibedos for the illuminated parts may be determined. For
the particular case of Algol, good agreement with observational
results are obtained. In some cases ( specially for small angles of
incidence), the bolometric reflection albedo may become negative. This
is shown to be the net result of two competing effects, where the
strong temperature sensitivity of the continuum opacity plays a major
role. For a particular reflecting star (main sequence, Teff = 450Q K),
results are given for the bolometric reflection albedo as a function
of angle of incidence and relative incident energy flux. Changes in
the limb-darkening due to illumination are also. discussed and it
is shown that, at least for grey atmospheres, a convenient numerical
expression may be given for the reduction of the limb- darkening as
a function of frequency, angle of incidence, and relative incident flux.
Title: 3-D Model Calculations
Authors: Nordlund, Å.
Bibcode: 1986ssmf.conf...83N
Altcode:
A qualitative discussion of the structure and evolution of small
scale magnetic flux concentrations is given, based on 3-D model
calculations. The importance of stratification for the topology of the
velocity field is pointed out, and the importance of radiative energy
transport for the energy balance and detailed structure of magnetic
flux concentrations is stressed.
Title: Solar Convection
Authors: Nordlund, A.
Bibcode: 1985SoPh..100..209N
Altcode:
The hydrodynamics of solar convection is reviewed. In particular, a
discussion is given of convection on the scale of granulation; i.e., the
energy carrying convection patterns in the solar surface layers, and its
penetration into the stable layers of the solar photosphere. Convection
on global and intermediate scales, and interaction with rotation and
magnetic fields is discussed briefly.
Title: The 3-D Structure of the Magnetic Field and its Interaction
with Granulation
Authors: Nordlund, A.
Bibcode: 1985tphr.conf..101N
Altcode:
No abstract at ADS
Title: The Dynamics of Granulation and its Interaction with the
Radiation Field
Authors: Nordlund, A.
Bibcode: 1985tphr.conf....1N
Altcode:
No abstract at ADS
Title: A Comparison of Artificial Solar Granules with Real Solar
Granules
Authors: Woehl, H.; Nordlund, A.
Bibcode: 1985SoPh...97..213W
Altcode:
The lifetimes, characteristics of the shapes as well as lengths and
perimeters of artificial solar granules (Nordlund, 1982, 1984a) are
compared with data from the literature and parameters determined from
two different sets of observed granules. No significant differences
of the parameters for these sets of granules are detectable.
Title: The reflection effect in model stellar atmospheres. I -
Grey atmospheres with convection
Authors: Vaz, L. P. R.; Nordlund, A.
Bibcode: 1985A&A...147..281V
Altcode:
The effect of external illumination on a plane-parallel grey stellar
atmosphere in radiative or radiative + convective equilibrium is
investigated theoretically as it applies to models of eclipsing
binaries. The LTE model atmosphere developed by Gustafsson et al. (1975)
for stars with Teff less than 8000 K is employed, and simulation results
for atmospheres with and without convection are presented in extensive
graphs and tables. Consideration is given to the reflection albedo,
the bolometric reflection albedo, the effective reflection albedo
for the convective case, and the effects of illumination on limb
darkening. An expression permitting the estimation of limb darkening
from the frequency, incidence angle, and relative flux of the incident
light is derived, and the results for particular cases are shown to
be in agreement with observations.
Title: The 3-D structure of the magnetic field, and its interaction
with granulation.
Authors: Nordlund, Å.
Bibcode: 1985MPARp.212..101N
Altcode:
The interaction of small scale magnetic flux concentrations and
convection in the solar photosphere is discussed. The applicability and
particular merits of one-, two-, and three-dimensional models of such
magnetic flux concentrations are discussed, with an attempt to focus
on questions of particular relevance for the Solar Optical Telescope.
Title: The dynamics of granulation, and its interaction with the
radiation field.
Authors: Nordlund, Å.
Bibcode: 1985MPARp.212....1N
Altcode:
The hydrodynamics of solar granulation is reviewed. In particular, the
discussion centers on the importance of radiative transfer effects for
the dynamics of granular convection, and on the importance of convective
overshoot for the temperature structure of the upper photosphere and
the temperature minimum region.
Title: NLTE spectral line formation in a three-dimensional atmosphere
with velocity fields.
Authors: Nordlund, A.
Bibcode: 1985ASIC..152..215N
Altcode: 1985pssl.proc..215N
A method to solve the "two-level-atom-with-overlapping-continuum"
problem in a three-dimensional atmosphere is presented. The method
is based on treating the radiative transfer along a number of rays
through the models as separate sub-problems. In each iteration, the
error in the source function is evaluated along all the rays through
the model, and an estimate of the necessary correction is obtained for
each ray. The converged solution is an exact solution to the problem. As
an application, the method is used on the case of a neutral iron line
in the solar photosphere.
Title: Magnetoconvection: the interaction of convection and small
scale magnetic fields.
Authors: Nordlund, A.
Bibcode: 1984ESASP.220...37N
Altcode: 1984ESPM....4...37N
Present consensus about the interaction of convection and magnetic
fields is reviewed, with special emphasis on the interaction of
convection in the solar convection zone with magnetic fields on
the scale of granulation and supergranulation. Computer simulations
of Boussinesq systems are reviewed, and additional effects due to
stratification and radiation are discussed. The particular mechanisms
that determine the degree of evacuation of magnetic fields in
the solar photosphere are discussed, and some limitations of the
"flux tube" concept are pointed out. Finally, some recent results
from numerical simulations of convection in a stratified medium with
magnetic fields are discussed, and possible consequences for activity
in the chromosphere and corona are mentioned.
Title: Book-Review - Guidance and Control 1983
Authors: Brinkman, A. C.; Hovenier, J. W.; Wittenberg, H.; Friedjung,
Michael; 't Hooft, G.; Dworetsky, M. M.; Danziger, I. J.; van de
Hulst, H. C.; Zimmerman, J. T. F.; Fárník, František; Doom,
Cl.; Schuurmans, C. J. E.; Valníček, B.; van Ingen, J. L.; van
Konijnenburg, R.; Schuiling, R. D.; Millman, Peter M.; Nordlund, Åke;
Maeder, André; Habing, H. J.; Dachs, J.; Kleczek, J.; Wakker, K. F.
Bibcode: 1984SSRv...39..215B
Altcode:
No abstract at ADS
Title: Interaction of convection and small-scale magnetic fields:
influence on the solar luminosity.
Authors: Nordlund, A.
Bibcode: 1984NASCP2310..121N
Altcode: 1984siva.work..121N
Changes in the local solar luminosity due to the presence of a small
scale structured (facular) magnetic field in the photosphere are
discussed. The discussion is based on three dimensional numerical
simulations of the magnetohydrodynamics of the top of the convection
zone, and the adjacent stable photosphere. The simulations demonstrate
that practically all of the magnetic flux present is concentrated into
intense magnetic flux structures, such that the magnetic field pressure
is balanced by the gas pressure of the surrounding plasma. The flux
concentration is caused by the convectively unstable stratification. The
average luminosity of the area is influenced by three effects: (1)
the brightness of the flux concentrations, (2) their filling factor,
and (3) the average luminosity of the surrounding plasma.
Title: Effects of HCN molecules in carbon star atmospheres.
Authors: Eriksson, K.; Gustafsson, B.; Jorgensen, U. G.; Nordlund, A.
Bibcode: 1984A&A...132...37E
Altcode:
Existing model atmospheres for carbon stars as well as observations
indicate that some polyatomic molecules may be important opacity
sources in at least the cooler carbon stars. In order to investigate
the importance of the absorption from HCN, which seems to be one
of the most important polyatomic opacity sources, monochromatic
absorption cross sections have been calculated at a great number of
wavelengths for this molecule. These calculations have been based on
the assumption that the ratios between the transition probabilities
of the combination transitions relative to the fundamentals follow
rules, deduced empirically for other molecules. Opacity distribution
functions were calculated and model atmospheres including the HCN
opacity were constructed. The effects of HCN on the models turned out
to be remarkably great.
Title: Modelling of Small-Scale Dynamical Processes: Convection and
Wave Generation (Keynote)
Authors: Nordlund, A.
Bibcode: 1984ssdp.conf..181N
Altcode:
The hydrodynamics of granular convection is discussed, with particular
emphasis on a qualitative understanding of the penetration of convective
motions into the stable photosphere. Generation of pressure and
internal gravity waves is considered. The importance of spatially and
temporally averaged spectral line profiles as diagnostics of motions
in the solar photosphere is discussed, and the calculation of such
synthetic spectral lines for neutral iron is described. Finally,
some possible observational programs are suggested, and the use of
spatially resolved synthetic spectral line profiles in the evaluation
of new observational techniques is encouraged.
Title: Book-Review - the Solar Granulation
Authors: Bray, R. J.; Loughhead, R. E.; Durrant, C. J.; Nordlund, A.
Bibcode: 1984SSRv...39R.222B
Altcode:
No abstract at ADS
Title: Iterative solution of radiative transfer problems with
spherical symmetry using a single-ray approximation.
Authors: Nordlund, A.
Bibcode: 1984mrt..book..211N
Altcode: 1984mrt..conf..211N
An iterative technique for solving radiative transfer problems with
spherical symmetry is described. An approximate relation between changes
in a source function and the resulting changes in the mean intensity
is obtained by using the radiative transfer equation along a single
representative ray, combined with a geometrical factor relating the
mean intensity to the intensity along the representative ray. This
approximate relation is then used iteratively to improve the estimate
of the source function until a prespecified accuracy is obtained. The
convergence properties of this technique are very good, with typically
2 - 3 iterations being sufficient for better than 1% accuracy in the
source function.
Title: HCN and C2H2 in Carbon Stars
Authors: Eriksson, K.; Gustafsson, B.; Jorgensen, U. G.; Nordlund, A.
Bibcode: 1984IAUS..105..199E
Altcode:
HCN and C2H2 have numerous bands at wavelengths
where a major part of the stellar flux is transported. This opacity
could therefore be of great importance when constructing models
for cool carbon star atmospheres, but has not been included in
earlier models. Models without the HCN and C2H2
opacity show a strange transition to "thin", high-pressure atmospheric
structures when Teff is decreased below about 2900K. When
HCN (and C2H2) opacity was added in the model
atmosphere calculations, great effects on the structure were found
for the lower temperatures.
Title: A Re-evaluation of the Granular Δ Irms
Authors: Nordlund, A.
Bibcode: 1984ssdp.conf..174N
Altcode:
The degradation of observed granular intensity fluctuations is
determined by the combined point spread function of the instrument
and the atmosphere, or by its Fourier transform, the modulation
transfer function. Empirically determined point spread functions are
characterized by a narrow "core" determining the size of the smallest
details observable, and extended "wings" influencing the contrast level
of observed small- and intermediate-scale structures. This contribution
discusses the extent to which the presence of these wings has been
properly corrected for in the paper by Deubner and Mattig (1975).
Title: Small Scale Magnetic Field Structure and Evolution
Authors: Nordlund, A.
Bibcode: 1983BAAS...15..710N
Altcode:
No abstract at ADS
Title: Numerical 3-D simulations of the collapse of photospheric
flux tubes
Authors: Nordlund, A.
Bibcode: 1983IAUS..102...79N
Altcode:
The interaction of photospheric granular convection with a small scale
magnetic field has been simulated numerically in a three-dimensional
model, with an extension of techniques recently used to simulate
field-free granulation. The evolution of an initially homogeneous
magnetic field was followed numerically, both in a kinematic
(weak-field limit) description, and in a dynamic description, where
the back-reaction of the field on the motion through the Lorentz force
is taken into account. The simulations illustrate the strong tendency
for the field to be swept up and concentrated in the inter-granular
lanes because of the topology of the granular flow. The convectively
unstable stratification allows field concentration up to a kilogauss
field because of the temperature reduction in the magnetic plasma.
Title: Photospheric sources of magnetic field aligned currents
Authors: Nordlund, A.
Bibcode: 1983ASSL..102..601N
Altcode: 1983IAUCo..71..601N; 1983ards.proc..601N
Distortions of the photospheric magnetic field topology in the
photosphere cause twists (field-aligned currents) to propagate along
field lines up into the coronal magnetic field. It is noted that for
small-scale magnetic loops, these currents have a duration that is
long in comparison with the propagation time of Alfven waves along
the loop. This gives rise to quasi-static twists of the coronal field
lines rather than propagating Alfven waves. The magnetic field-aligned
currents associated with such twisted fields may lead to resistive MHD
instabilities that are similar to Tokamak instabilities (Waddell et
al., 1979; Carreras et al., 1980). For this reason, they may figure
prominently in small-scale chromospheric and coronal activity.
Title: Numerical simulations of the solar granulation. I. Basic
equations and methods.
Authors: Nordlund, A.
Bibcode: 1982A&A...107....1N
Altcode:
Hydrodynamical and radiative transfer equations governing the
evolution of granular convection patterns are discussed. The anelastic
approximation of the continuity equation is used to exclude pressure
waves from the problem, and numerical methods are developed to
simulate the solar granulation, which are based on a two-dimensional
Fourier-series representation of horizontal fluctuations, combined
with a cubic spline representation in the vertical direction. The
simulations are a natural representation for a medium of essentially
infinite horizontal extension with strong vertical stratification,
which allows a simple treatment of the viscosity-like influence
ofgrid-scale motions and a rapid solution of the Poisson-type
equation for the dynamic pressure, giving high numerical accuracy for
a given number of mesh points.
Title: DQPT: a computer program for solving non-LTE problems for
two-level atoms in one-dimensional semi-infinite media with velocity
fields.
Authors: Scharmer, G. B.; Nordlund, Å.
Bibcode: 1982StoOR..19.....S
Altcode:
No abstract at ADS
Title: Solar granulation - Influence of convection on spectral line
asymmetries and wavelength shifts
Authors: Dravins, D.; Lindegren, L.; Nordlund, A.
Bibcode: 1981A&A....96..345D
Altcode:
The observed shapes and shifts of 311 Fe I lines in the spectrum of
solar disk center and also of integrated sunlight are investigated. Line
shapes are described using bisectors, and the dependence of
these on line strength, excitation potential, and wavelength
region is analyzed. A theoretical model atmosphere incorporating
radiation-coupled, time-dependent hydrodymamics of solar convection
is used to compute synthetic photospheric spectral lines. These lines
exhibit asymmetries and wavelength shifts, and the observed bisector
behavior can be closely reproduced. The detailed properties of, for
example, convective motions and changing granulation constrast with
wavelength manifest themselves in the detailed bisector shapes. It is
confirmed that convection is the principal cause of solar line shifts,
and errors in other suggested explanations are pointed out. It is
concluded that the study of line shapes and shifts is a powerful tool
for the analysis of solar photospheric convection.
Title: Numerical simulation of the solar granulation
Authors: Nordlund, Åke
Bibcode: 1980LNP...114R..17N
Altcode: 1980sttu.collR..17N; 1980IAUCo..51R..17N
No abstract at ADS
Title: Numerical simulation of granular convection - Effects on
photospheric spectral line profiles
Authors: Nordlund, A.
Bibcode: 1980LNP...114..213N
Altcode: 1980sttu.coll..213N; 1980IAUCo..51..213N
The results of numerical simulations of the solar granulation are
used to investigate the effects on photospheric spectral lines of the
correlated velocity and temperature fluctuations of the convective
granular motions. It is verified that the granular velocity field
is the main cause for the observed broadening and strengthening
of photospheric spectral lines relative to values expected from
pure thermal and pressure broadening. These effects are normally
referred to as being due to `macro-turbulence' and "micro-turbulence",
respectively. It is also shown that the correlated temperature and
velocity fluctuations produce a "convective blue shift" in agreement
with the observed blue shift of photospheric spectral lines. Reasons
are given for the characteristic shapes of spectral line bisectors, and
the dependence of these shapes on line strength, excitation potential,
and center to limb distance are discussed.
Title: Solar granulation and the nature of "microturbulence".
Authors: Nordlund, A.
Bibcode: 1978bs...symp...95N
Altcode:
The hydrodynamics of solar granulation is discussed, and it is shown
that granular motion is the likely cause of both the strengthening and
the broadening of photospheric spectral lines. A schematic description
of the observed broadening and strengthening of photospheric spectral
lines is given, a simple model granular velocity field based on the
continuity condition is examined, and a method is outlined for solving
the hydrodynamic equations of motion for specified steady-state
temperature fluctuations. This method is then extended to full
self-consistency by the inclusion of an energy equation (including
radiative-transfer effects). Possible applications to chromospheric
heating are considered.
Title: Convective Overshooting in the Solar Photosphere; a Model
Granular Velocity Field
Authors: Nordlund, A.
Bibcode: 1977LNP....71..237N
Altcode: 1977stco.coll..237N; 1977IAUCo..38..237N; 1977psc..conf..237N
No abstract at ADS
Title: A two-component representation of stellar atmospheres with
convection.
Authors: Nordlund, A.
Bibcode: 1976A&A....50...23N
Altcode:
A method is set forth for constructing fully deductive two-stream
models of stellar atmospheres. The two streams are defined by thin
horizontal slabs, where each slab is split into two well defined,
nonstationary components defined according to the sign of the vertical
velocity. Conservation of mass, energy, vertical momentum, and number
of photons, with due account taken of horizontal exchange, provides a
set of differential equations in component mean values. Approximate
relations among these mean values provide the remaining equations
to obtain a closed set. In estimating rates of exchange of energy,
vertical momentum, and photons, it is assumed that the components can be
characterized by a typical linear size. Late-type stellar atmospheres
are calculated numerically. Two-component solar model atmospheres are
in good agreement with recent observations of solar granulation.
Title: Theories for convection in stellar atmospheres
Authors: Nordlund, Åke
Bibcode: 1976PhDT........51N
Altcode:
No abstract at ADS
Title: A grid of model atmospheres for metal-deficient giant
stars. II.
Authors: Bell, R. A.; Eriksson, K.; Gustafsson, B.; Nordlund, A.
Bibcode: 1976A&AS...23...37B
Altcode:
Details are presented for a previously calculated grid of LTE model
atmospheres for yellow and red giant stars. The grid covers the general
range of effective temperatures from 3750 K to 6000 K, log g from 0.75
to 3.0, and (A/H) from -3.0 to 0.0. For each model, numerical values
are given for the optical depth, the Rosseland mean, the corresponding
optical depth, the geometric depth and temperature, the electron
pressure, gas pressure, radiative pressure, density, specific heat,
and flux fraction. A model with typical solar parameters is included.
Title: Theories for convection in stellar atmospheres.
Authors: Nordlund, A.
Bibcode: 1976tcsa.book.....N
Altcode:
No abstract at ADS
Title: A grid of model atmospheres for metal-deficient giant stars. I.
Authors: Gustafsson, B.; Bell, R. A.; Eriksson, K.; Nordlund, A.
Bibcode: 1975A&A....42..407G
Altcode: 2009A&A...500...67G
A grid of flux-constant model atmospheres for stars with effective
temperatures between 3750 and 6000 K, log g between 0.75 and 3.0, and
(A/H) between -3.0 and 0.0 has been constructed. The line absorption
is approximated by opacity distribution functions. Metal lines and
molecular lines, including those from the infrared bands of CO and CN,
are taken into account. The variation of the structure of the models
with metal abundance and microturbulence parameter is found to be
quite regular and not very drastic. The surface cooling produced
by CO is important for all the models with a maximum temperature
of 5000 K, while CN mainly causes a backwarming effect but is not
very important for solar CNO abundances. The effects of convection,
estimated by using the mixing-length approximation, are important only
for the coldest models. The models compare very well with models from
overlapping regions of other grids. A solar model consistent with the
grid models is found to agree satisfactorily with empirical solar model
atmospheres. The careful use of scaled solar models for stars with
(A/H) approximately equal to zero is justified.
Title: On Convection in Stellar Atmospheres
Authors: Nordlund, A.
Bibcode: 1974A&A....32..407N
Altcode:
Summary. Three nonlocal theories of stellar convection are
discussed. Two of these, due to Ulrich and Spiegel, predict convective
fluxes widely different from those predicted by local mixing-length
theory. With the aid of differential equations for the vertical
velocity, excess temperature, and excess energy of fluctuations in
turbulent convective layers, the importance of radiative and turbulent
exchange is discussed. It is found that the amount of radiative
cooling effectively determines the importance of convective flux
relative to radiative flux, and that the turbulent dilution of kinetic
energy determines the convective velocities. A parametrization of the
rate of turbulent dilution in terms of a mixing length allows local
mixing-length theory expressions for the convective flux to be obtained
in the limit of slowly varying conditions. A local approximation for
the ratio between excess energy and vertical velocity is found to give
expressions for the convective flux in close agreement with expressions
given previously by Parsons. The numerical investigation applies
powerful difference equation methods to radiative transfer; this is used
to construct an iterative Newton-Raphson procedure producing realistic
fluxconstant models including nonlocal convection. By comparing models
with different convection theories and by analysing the importance
of radiative cooling, it is concluded that the first two theories
noted above overestimate the importance of convection in the visible
layers of giants and solar type stars. A quantitative measure of the
importance of the convective flux in the visible layers is introduced,
and it is concluded that this quantity is small in main sequence stars
and in giants whose effective temperatures exceed approximately 5000 K
and 4000 K, respectively. Key words: stellar atmospheres - convection -
non-local effects - F-, G- and K-stars