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Author name code: nordlund
ADS astronomy entries on 2022-09-14
author:"Nordlund, Ake" 

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Title: The dynamical state of massive clumps
Authors: Lu, Zu-Jia; Pelkonen, Veli-Matti; Juvela, Mika; Padoan,
   Paolo; Haugbølle, Troels; Nordlund, Åke
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, M<SUB>main</SUB>, and that of the
  corresponding synthetic sources, M<SUB>SED</SUB>, 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.

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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
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.

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Title: From the CMF to the IMF: beyond the core-collapse model
Authors: Pelkonen, V. -M.; Padoan, P.; Haugbølle, T.; Nordlund, Å.
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.

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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
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.

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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
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.

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Title: Transport, Destruction, and Growth of Pebbles in the Gas
    Envelope of a Protoplanet
Authors: Johansen, Anders; Nordlund, Åke
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 (&gt;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.

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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
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.

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Title: The Origin of Massive Stars: The Inertial-inflow Model
Authors: Padoan, Paolo; Pan, Liubin; Juvela, Mika; Haugbølle, Troels;
   Nordlund, Åke
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.

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Title: Simulating rocky protoplanet growth via pebble accretion and
    the DISPATCH framework
Authors: Ramsey, J.; Nordlund, Å.; Popovas, A.
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.

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Title: The Probability Distribution of Density Fluctuations in
    Supersonic Turbulence
Authors: Pan, Liubin; Padoan, Paolo; Nordlund, Åke
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 2048<SUP>3</SUP>
  to study the dependence of the pdf on the Mach number, { \mathcal M },
  up to { \mathcal M }∼ 30. The pdf width, σ <SUB> s </SUB>, follows
  the relation {σ }<SUB>s</SUB><SUP>2</SUP>={ln}(1+{b}<SUP>2</SUP>{{
  \mathcal M }}<SUP>2</SUP>), 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.

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Title: Task Based Radiative Transfer Methods and the DISPATCH Code
    Framework
Authors: Nordlund, Å.
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.

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Title: Inaccuracy of Spatial Derivatives in Riemann Solver Simulations
    of Supersonic Turbulence
Authors: Pan, Liubin; Padoan, Paolo; Nordlund, Åke
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.

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Title: Pebble dynamics and accretion on to rocky planets -
    II. Radiative models
Authors: Popovas, Andrius; Nordlund, Åke; Ramsey, Jon P.
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.

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Title: Detailed Balance and Exact Results for Density Fluctuations
    in Supersonic Turbulence
Authors: Pan, Liubin; Padoan, Paolo; Nordlund, Åke
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, &lt; {{\nabla
  }}\cdot {\boldsymbol{u}}| s&gt; , and a Fokker-Planck equation with the
  drift and diffusion coefficients equal to -&lt; {\boldsymbol{u}}\cdot
  {{\nabla }}s| s&gt; and &lt; {\boldsymbol{u}}\cdot {{\nabla }}s| s&gt;
  , respectively. Assuming statistical homogeneity only, the detailed
  balance at steady state leads to two exact results, &lt; {{\nabla
  }}\cdot {\boldsymbol{u}}| s&gt; =0, and &lt; {\boldsymbol{u}}\cdot
  {{\nabla }}s| s&gt; =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.

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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.
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 × 10<SUP>5</SUP>. 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.

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Title: DISPATCH: a numerical simulation framework for the exa-scale
    era - I. Fundamentals
Authors: Nordlund, Åke; Ramsey, Jon P.; Popovas, Andrius; Küffmeier,
   Michael
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.

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Title: A simple and efficient solver for self-gravity in the DISPATCH
    astrophysical simulation framework
Authors: Ramsey, J. P.; Haugbølle, T.; Nordlund, Å.
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.

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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.
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.

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Title: The Stellar IMF from Isothermal MHD Turbulence
Authors: Haugbølle, Troels; Padoan, Paolo; Nordlund, Åke
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 <SUB>⊙</SUB> 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
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
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
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)<SUP>3</SUP>
  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
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.
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
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 ≤
  T<SUB>eff</SUB>(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
  (T<SUB>eff</SUB>&gt;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 T<SUB>eff</SUB>, C/O, [Fe/H]
  and [s/Fe]. The Geneva U-B<SUB>1</SUB> and B<SUB>2</SUB>-V<SUB>1</SUB>
  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
  T<SUB>eff</SUB> = 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 (T<SUB>eff</SUB>, V-K) relationship
  (for a fixed C/O value). Hence, the use of a (T<SUB>eff</SUB>, 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. <P
  />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:
  <A href="http://marcs.astro.uu.se">http://marcs.astro.uu.se</A>Full
  Tables 2 and 3 are only available at the CDS via anonymous ftp to
  <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
  (<A href="http://130.79.128.5">http://130.79.128.5</A>) or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/601/A10">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/601/A10</A>

---------------------------------------------------------
Title: Supernova Driving. IV. The Star-formation Rate of Molecular
    Clouds
Authors: Padoan, Paolo; Haugbølle, Troels; Nordlund, Åke; Frimann,
   Søren
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,
  SFR<SUB>ff</SUB>, on the virial parameter, α <SUB>vir</SUB>, 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 <SUB>vir</SUB> are not well
  constrained, because of random scatter due to time and cloud-to-cloud
  variations in SFR<SUB>ff</SUB>. We find that SFR<SUB>ff</SUB> in MCs
  can take any value in the range of 0 ≤ SFR<SUB>ff</SUB> ≲ 0.2,
  and its probability distribution peaks at a value of SFR<SUB>ff</SUB>
  ≈ 0.025, consistent with observations. The values of SFR<SUB>ff</SUB>
  and the scatter in the SFR<SUB>ff</SUB>-α <SUB>vir</SUB> 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.
2017yCat..36010010V    Altcode:
  Johnson and Geneva (G) photometric indices and band indices measured
  on the observed Henize S stars. <P />(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.
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 <SUP>-</SUP> -
  p <SUP>+</SUP>) and (ii) electron-positron (e <SUP>+/-</SUP>) 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
  <SUP>-</SUP> - p <SUP>+</SUP> jet, a recollimation-like instability
  occurs and jet electrons are strongly perturbed, whereas for the e
  <SUP>+/-</SUP> 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.
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
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
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 <SUB>B,min</SUB> = 1.4 K, of the J = 1 -
  0 <SUP>12</SUP>CO 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
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
2016ApJ...826...22K    Altcode: 2016arXiv160505008K
  The short-lived <SUP>26</SUP>Al and <SUP>60</SUP>Fe 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 <SUP>26</SUP> Al nuclide defining the canonical
  <SUP>26</SUP>Al/<SUP>27</SUP> Al ratio of ∼ 5× {10}<SUP>-5</SUP>. 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 <SUP>26</SUP>Al/<SUP>27</SUP>Al
  of 10<SUP>-6</SUP>. The contrasting level of <SUP>26</SUP>Al between
  these objects is often interpreted as reflecting the admixing of the
  <SUP>26</SUP>Al 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 <SUP>26</SUP>Al
  and <SUP>60</SUP>Fe in star-forming gas during the early stages
  of accretion of individual low-mass protostars. We find that the
  <SUP>26</SUP>Al/<SUP>27</SUP>Al and <SUP>60</SUP>Fe/<SUP>56</SUP>Fe
  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 <SUP>26</SUP>Al/<SUP>27</SUP>Al 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 <SUP>26</SUP>Al/<SUP>27</SUP>Al 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
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
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)<SUP>3</SUP>, 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.
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}<SUP>{--</SUP>}-{p}<SUP>+</SUP>) and
  electron-positron (e<SUP>±</SUP>) 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}<SUP>{--</SUP>}-{p}<SUP>+</SUP>
  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<SUP>±</SUP> 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}<SUP>{--</SUP>}-{p}<SUP>+</SUP> and e<SUP>±</SUP> 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
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
  T<SUB>eff</SUB> 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.
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
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
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<SUP>-5</SUP> M <SUB>⊙</SUB> yr<SUP>-1</SUP>, 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
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<SUB>⊙</SUB>, 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.
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. <P />(6 data files).

---------------------------------------------------------
Title: Zooming in on the Formation of Protoplanetary Disks
Authors: Nordlund, Åke; Haugbølle, Troels; Küffmeier, Michael;
   Padoan, Paolo; Vasileiades, Aris
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<SUP>-4</SUP> M<SUB>⊙</SUB> about 6 kyr after
  sink particle formation and then decays approximately exponentially,
  reaching 10<SUP>-6</SUP> M<SUB>⊙</SUB> 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.
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.
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.
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.
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 E<SUB>z</SUB>, perpendicular to the flow
  boundary, and the magnetic field B<SUB>y</SUB>, transverse to the
  flow direction. After the B<SUB>y</SUB> component is excited, an
  induced electric field E<SUB>x</SUB>, 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
  m<SUB>i</SUB>/m<SUB>e</SUB> = 1836 and m<SUB>i</SUB>/m<SUB>e</SUB> =
  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 (γ<SUB>j</SUB> = 1.5) is larger than
  for a relativistic jet case (γ<SUB>j</SUB> = 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.
2013A&A...557A..26M    Altcode: 2013arXiv1302.2621M
  <BR /> 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). <BR /> 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, T<SUB>eff</SUB>, 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. <BR /> 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. <P />Appendices A-C are available in electronic
  form at <A href="http://www.aanda.org">http://www.aanda.org</A>Full
  Table C.1 is available at the CDS via anonymous ftp to <A
  href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
  (ftp://130.79.128.5) or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/557/A26">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/557/A26</A>

---------------------------------------------------------
Title: Zooming in on Protoplanetary Disks
Authors: Küffmeier, Michael; Nordlund, Åke; Haugbølle, Troels;
   Padoan, Paolo
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.
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
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. <P />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. <P />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
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.
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.
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 &amp; Galsgaard (1995,
  http://www.astro.ku.dk/~kg/Papers/MHD_code.ps.gz), and continuously
  improved over the years by its user community. <P />(1 data file).

---------------------------------------------------------
Title: photon-plasma: A modern high-order particle-in-cell code
Authors: Haugbølle, Troels; Frederiksen, Jacob Trier; Nordlund,
   A. ˚ke
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.
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.
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 <SUP> 26 </SUP>Al and <SUP> 60 </SUP>Fe in
    Evolving Giant Molecular Clouds
Authors: Vasileiadis, Aristodimos; Nordlund, Åke; Bizzarro, Martin
2013ApJ...769L...8V    Altcode: 2013arXiv1302.0843V
  The nucleosynthesis and ejection of radioactive <SUP>26</SUP>Al (t
  <SUB>1/2</SUB> ~ 0.72 Myr) and <SUP>60</SUP>Fe, (t <SUB>1/2</SUB>
  ~ 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 <SUP>26</SUP>Al and <SUP>60</SUP>Fe in
  star-forming regions within giant molecular clouds. Under typical star
  formation conditions, the levels of <SUP>26</SUP>Al 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
  <SUP>60</SUP>Fe/<SUP>26</SUP>Al 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 <SUP>60</SUP>Fe 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
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.
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.
2013pacp.book..223C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Flux Emergence and Pore Formation: What ATST can See
Authors: Stein, R. F.; Nordlund, Å.
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.
2012A&A...548A..35F    Altcode: 2012arXiv1209.2771F
  <BR /> 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. <BR />
  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 ⟨ B<SUB>vert</SUB> ⟩ = 0,50,100, and
  200 G. For the relevant calculations we used the Copenhagen Stagger
  code. <BR /> 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)<SUB>⊙</SUB>|
  ~ 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. <BR /> 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.
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.
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
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
  <SUB>ff</SUB>/t <SUB>dyn</SUB>, 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, epsilon<SUB>ff</SUB>, on the above
  parameters. Our main results are (1) that epsilon<SUB>ff</SUB> decreases
  exponentially with increasing t <SUB>ff</SUB>/t <SUB>dyn</SUB>, but
  is insensitive to changes in {\cal M}_s, for constant values of t
  <SUB>ff</SUB>/t <SUB>dyn</SUB> and {\cal M}_a. (2) Decreasing values
  of {\cal M}_a (stronger magnetic fields) reduce epsilon<SUB>ff</SUB>,
  but only to a point, beyond which epsilon<SUB>ff</SUB> increases
  with a further decrease of {\cal M}_a. (3) For values of {\cal
  M}_a characteristic of star-forming regions, epsilon<SUB>ff</SUB>
  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 epsilon<SUB>ff</SUB>, and depending only on t <SUB>ff</SUB>/t
  <SUB>dyn</SUB>: epsilon<SUB>ff</SUB> ≈ epsilon<SUB>wind</SUB>exp
  (- 1.6 t <SUB>ff</SUB>/t <SUB>dyn</SUB>). 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.
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.
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.
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.
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.
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" <A href="/abs/2000ApJ...540..332P">(2000, ApJ, 540, 332)</A>
Authors: Padoan, Paolo; Zweibel, Ellen; Nordlund, Åke
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.
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
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.
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.
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 2×10<SUP>20</SUP> Mx and the field is nearly vertical in the
  pore interior and inclined more than 45<SUP>o</SUP> 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.
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.
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 &amp; Nordlund (1996, JGR 101, 13445),
  and Lazarian &amp; Vishniac (1999; ApJ 517, 700), considering also
  the results of Lapenta (2008, PhRvL 100, 235001) and Bettarini &amp;
  Lapenta (2010, A&amp;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.
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.
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
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 (T<SUB>eff</SUB>, 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.
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.
2011LPICo1639.9053L    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Zooming in on Star Formation
Authors: Haugboelle, T.; Nordlund, A.; Padoan, P.
2011LPICo1639.9116H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Formation of brown dwarfs and planets
Authors: Nordlund, Åke
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
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 <SUB>sun</SUB>,
  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.
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.
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.
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.
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 ≤ T<SUB>eff</SUB>(K) ≤ 4000
  with 0.5 ≤ C/O ≤ 0.99. ZrO and TiO band strength indices as well
  as VJHKL photometry are needed to disentangle T<SUB>eff</SUB>, 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 T<SUB>eff</SUB> up to 400 K. We constrain the parameter
  space occupied by the S stars of the vast Henize sample in terms of
  T<SUB>eff</SUB>, [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.
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
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
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
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
2011ApJ...735L..37L    Altcode:
  With a half-life of 0.73 Myr, the <SUP>26</SUP>Al-to-<SUP>26</SUP>Mg
  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
  <SUP>26</SUP>Al-<SUP>26</SUP>Mg ages of meteorites and their
  components relies on the critical assumption that the canonical
  <SUP>26</SUP>Al/<SUP>27</SUP>Al ratio of ~5 × 10<SUP>-5</SUP>
  recorded by the oldest dated solids, calcium-aluminium-rich inclusions
  (CAIs), represents the initial abundance of <SUP>26</SUP>Al 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 <SUP>26</SUP>Mg composition (μ<SUP>26</SUP>Mg*)
  of bulk solar system reservoirs with solar or near-solar Al/Mg
  ratios. This variability may represent heterogeneity in the initial
  abundance of <SUP>26</SUP>Al across the solar protoplanetary disk at
  the time of CAI formation and/or Mg-isotope heterogeneity. By comparing
  the U-Pb and <SUP>26</SUP>Al-<SUP>26</SUP>Mg ages of pristine solar
  system materials, we infer that the bulk of the μ<SUP>26</SUP>Mg*
  variability reflects heterogeneity in the initial abundance of
  <SUP>26</SUP>Al across the solar protoplanetary disk. We conclude that
  the canonical value of ~5 × 10<SUP>-5</SUP> represents the average
  initial abundance of <SUP>26</SUP>Al 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 <SUP>26</SUP>Al-<SUP>26</SUP>Mg 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.
2011A&A...529A.158H    Altcode: 2011arXiv1108.3366H
  <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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.
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. <P
  />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. <P
  />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.
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.
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. <BR /> 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. <BR /> 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
  CO<SUP>5</SUP>BOLD collaboration. <BR /> 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 CO<SUP>5</SUP>BOLD
  code can be traced to the different treatments of scattering. <BR />
  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
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
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
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, α<SUB>vir</SUB>, the rms sonic Mach
  number, <SUB>S,0</SUB>, and the ratio of mean gas pressure to mean
  magnetic pressure, β<SUB>0</SUB>. The SFR is predicted to decrease
  with increasing α<SUB>vir</SUB> (stronger turbulence relative to
  gravity), and to depend weakly on <SUB>S,0</SUB> and β<SUB>0</SUB>,
  for values typical of star forming regions (<SUB>S,0</SUB>~4-20 and
  β<SUB>0</SUB>~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
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, α<SUB>vir</SUB>,
  the rms sonic Mach number, M_S,0, and the ratio of mean gas pressure
  to mean magnetic pressure, β<SUB>0</SUB>. The SFR is predicted to
  decrease with increasing α<SUB>vir</SUB> (stronger turbulence relative
  to gravity), to increase with increasing M_S,0 (for constant values of
  α<SUB>vir</SUB>), and to depend weakly on β<SUB>0</SUB> for values
  typical of star forming regions (M_S,0≈ 4-20 and β<SUB>0</SUB>
  ≈ 1-20). In the unrealistic limit of β<SUB>0</SUB> → ∞, 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
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<SUP>-1</SUP>. New proper-motion measurements of H<SUB>2</SUB>
  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 × 10<SUP>47</SUP>
  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.
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.
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.
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
  epsilon<SUB>sun</SUB>(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.
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
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.
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.
2010M&PSA..73.5202L    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Flux Emergence Simulations
Authors: Stein, Robert F.; Lagerfjard, Anders; Nordlund, Ake;
   Giorgobiani, Dali
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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
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
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 <SUB>rms</SUB> =
  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.
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.
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.
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.
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.
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.
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
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.
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 <P />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 <P />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.
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.
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.
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.
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.
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.
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.
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.
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
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
  synthetic<SUP>13</SUP>CO 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} <SUB>μ
  </SUB>, and show that super-Alfvénic turbulence produces a significant
  scatter also in \mathstrutR} <SUB>μ </SUB>, including negative values
  (field reversal between core center and envelope). We find \mathstrutR}
  <SUB>μ </SUB> &lt; 1 for 70% of the cores, and \mathstrutR} <SUB>μ
  </SUB> &lt; 0 for 12%. Of the cores with | B<SUB>LOS</SUB>| &gt; 10
  μG, 81% have \mathstrutR} <SUB>μ </SUB> &lt; 1. These predictions
  of the super-Alfvénic model are in stark contrast to the ambipolar
  drift model of core formation, where only \mathstrutR} <SUB>μ </SUB>
  &gt; 1 is allowed.

---------------------------------------------------------
Title: Trans-Debye Scale Plasma Modeling &amp; Stochastic GRB
    Wakefield Plasma Processes
Authors: Frederiksen, Jacob Trier; Haugbølle, Troels; Nordlund, Åke
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. <P />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. <P />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. <P />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.
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. <BR />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. <BR />Methods:
  The most recent version of the MARCS program is used. <BR />Results:
  We present a grid of about 10<SUP>4</SUP> 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 ≤ ξ<SUB>t</SUB> ≤ 5 km
  s<SUP>-1</SUP>. 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 &amp; 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. <BR />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.
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.
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
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.
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.
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.
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.
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. <BR
  />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. <BR />Methods: We perform three-dimensional
  non-linear MHD simulations and visualization using a high resolution
  numerical scheme. <BR />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. <BR />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
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
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
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<SUP>-2</SUP>. 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
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 (~1000<SUP>3</SUP>
  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 M<SUB>solar</SUB>, as long as the column density
  of such clouds is not much in excess of 10<SUP>22</SUP> cm<SUP>-2</SUP>.

---------------------------------------------------------
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.
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. <P />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.
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
2007IAUS..239..331S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Realistic Solar Convection Simulations
Authors: Stein, Robert F.; Nordlund, A.
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 <P />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
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
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.
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 <BR />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. <BR
  />Results: The excitation rates obtained from the 3D simulations
  are systematically lower than those computed from the semi-analytical
  excitation model. We find that P<SUB>max</SUB>, the P maximum, scales as
  (L/M)<SUP>s</SUP> 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 V<SUB>max</SUB> increases as (L/M)<SUP>sv</SUP>. Comparisons
  with the currently available ground-based observations show that the
  computations assuming a Lorentzian χ<SUB>k</SUB> 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.
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
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.
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. <P />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.
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.
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. <P />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.
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.
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.
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.
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.
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.
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.
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.
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 &amp; Spruit (2004, Nature, 431, 891).

---------------------------------------------------------
Title: Radiative transfer in decomposed domains
Authors: Heinemann, T.; Dobler, W.; Nordlund, Å.; Brandenburg, A.
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
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.
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.
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
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.
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.
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.
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
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.
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
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.
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.
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<SUP>-p</SUP> 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.
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”
    (<A href="/abs/2005ApJ...618.1031G">ApJ, 618, 1031 [2005]</A>)
Authors: Gudiksen, Boris Vilhelm; Nordlund, Åke
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” (<A href="/abs/2005ApJ...618.1020G">ApJ, 618, 1020
    [2005]</A>)
Authors: Gudiksen, Boris Vilhelm; Nordlund, Åke
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.
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
2005ApJ...622L..61P    Altcode: 2004astro.ph.11129P
  Accretion rates of order 10<SUP>-8</SUP> M<SUB>solar</SUB>
  yr<SUP>-1</SUP> 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˙<SUB>accr</SUB>~M<SUP>2</SUP>. 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
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
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
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 1024<SUP>3</SUP>
  computational zones. The star formation simulation achieves an effective
  resolution of (10<SUP>6</SUP>)<SUP>3</SUP> 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.
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
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
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
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)×10<SUP>6</SUP>ergscm<SUP>-2</SUP>s<SUP>-1</SUP>
  in a highly intermittent corona, maintaining an average temperature
  of ~10<SUP>6</SUP> 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.
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.
2004ESASP.575..348B    Altcode: 2004soho...15..348B
  No abstract at ADS

---------------------------------------------------------
Title: The “Mysterious” Origin of Brown Dwarfs
Authors: Padoan, Paolo; Nordlund, Åke
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.
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.
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 ~10<SUP>9</SUP> 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.
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
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.
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. <P
  />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. <P />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.
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.
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
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 <SUB>0</SUB>= 6 μG).

---------------------------------------------------------
Title: Observational Manifestations of Solar Magnetoconvection:
    Center-to-Limb Variation
Authors: Carlsson, Mats; Stein, Robert F.; Nordlund, Åke; Scharmer,
   Göran B.
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.
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.
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
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, M<SUB>S</SUB>. 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
  M<SUB>S</SUB>. 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.
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
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
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 350<SUP>3</SUP> 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<SUP>-2.25+/-0.01</SUP>, the super-Alfvénic
  simulation (model B) P(k)~k<SUP>-2.71+/-0.01</SUP>. The column
  density power spectrum of the Perseus, Taurus, and Rosetta molecular
  cloud complexes is found to be well approximated by a power law,
  P<SUB>0</SUB>(k)~k<SUP>-a</SUP>, 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.
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
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.
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.
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
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.
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.
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
  (B<SUP>2</SUP>/8π) or the plasma pressure (p=Nk<SUB>B</SUB>T)
  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/B<SUP>2</SUP>). 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.
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
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 τ<SUB>cont</SUB>=1
  have the opposite asymmetry, as is observed for the intensity and
  velocity spectra. At a fixed geometrical depth, corresponding to
  &lt;τ<SUB>cont</SUB>&gt;=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<SUP>-</SUP> 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.
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, chi<SUB>k</SUB>
  , 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 chi<SUB>k</SUB> 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 chi<SUB>k</SUB> 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 chi<SUB>k</SUB> 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.
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.
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
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
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 τ<SUB>cont</SUB> = 1 has opposite asymmetry to the velocity
  spectrum, whereas the temperature measured at a fixed geometrical depth,
  corresponding to &lt;τ<SUB>cont</SUB>&gt; = 1, has the same asymmetry
  as velocity. We believe that the asymmetry reversal in temperature
  at τ<SUB>cont</SUB> = 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.
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 &lt;~ M &lt;~ 2
  Msolar). Differences in the values of P<SUB>max</SUB> 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.
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.
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 &amp;
  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.
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.
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.
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.
2003IAUS..210..169S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Solar photosphere and convection
Authors: Nordlund, Å.
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.
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.
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.
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
2003ApJ...583..308P    Altcode: 2002astro.ph..7568P
  We compute the structure function scaling of the integrated intensity
  images of two J=1-0 <SUP>13</SUP>CO 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.
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
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.
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.
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
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 M<SUB>solar</SUB> 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 M<SUB>solar</SUB>, the mass distribution
  flattens and turns around at a fraction of 1 M<SUB>solar</SUB>,
  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
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
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 E<SUB>k</SUB>~k<SUP>-1.74</SUP>, and the velocity difference
  scales as &lt;|Δu|&gt;~L<SUP>0.42</SUP>. 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.
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/ B<SUP>2</SUP>). 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.
2002sf2a.conf..489S    Altcode: 2002astro.ph.10028S
  Stochastic excitation of stellar p-modes of low massive stars (M &lt;
  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 &lt; 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
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.
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.
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.
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
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.
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
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.
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.
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
  E<SUB>k</SUB>~k<SUP>-1.74</SUP>. This result reproduces
  the observational Larson law, &lt; u<SUP>2_l</SUP> &gt; ~
  l<SUP>0.74\cdots0.76</SUP>, [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
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 &lt;n&gt;~300 cm<SUP>-3</SUP>, with the density exceeding
  10<SUP>4</SUP> cm<SUP>-3</SUP> in the densest cores. We present
  <SUP>12</SUP>CO, <SUP>13</SUP>CO, C<SUP>18</SUP>O, O<SUB>2</SUB>, O I,
  C I, and H<SUB>2</SUB>O 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.
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.
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.
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
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 A<SUB>V,0</SUB>=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 P<SUB>max</SUB>=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 A<SUB>V</SUB>
  (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 P<SUB>max</SUB>) 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
  A<SUB>V,0</SUB> (model B). (3) The same behavior is hard to reproduce
  if grains are aligned independently of A<SUB>V</SUB> (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 A<SUB>V,0</SUB>~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.
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
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(V<SUB>0</SUB>)-I, which is indeed found in the
  observational data. The comparison between the theoretical model (maps
  of synthetic <SUP>13</SUP>CO spectra) with <SUP>13</SUP>CO maps from
  the Perseus, Rosette, and Taurus MC complexes shows excellent agreement
  in the σ(V<SUB>0</SUB>)-I relation. The σ(V<SUB>0</SUB>)-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.
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.
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.
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.
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 &amp; 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.
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.
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.
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.
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.
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.
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.
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, α<SUB>v</SUB>, 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,
  α<SUB>h</SUB>, pertaining to a horizontal (Keplerian) shear. We
  find that, as is often assumed in theoretical studies, α<SUB>v</SUB>
  is approximately equal to α<SUB>h</SUB> 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 α<SUB>v</SUB> and
  α<SUB>h</SUB> we conclude that for βp<SUB>gas</SUB>p<SUB>mag</SUB>~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.
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
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,
  H<SUB>AD</SUB>=|JXB|<SUP>2</SUP>/ρ<SUB>i</SUB>ν<SUB>in</SUB>~
  B<SUP>4</SUP>/(16π<SUP>2</SUP>L<SUP>2</SUP><SUB>B</SUB>ρ<SUB>i</SUB>
  ν<SUB>in</SUB>), is found to depend on the rms Alfvénic Mach
  number, M<SUB>A</SUB>, and on the average field strength, as
  H<SUB>AD</SUB>~M<SUP>2</SUP><SUB>A</SUB>&lt;|B|&gt;<SUP>4</SUP>. This
  implies that the typical scale of variation of the magnetic field,
  L<SUB>B</SUB>, is inversely proportional to M<SUB>A</SUB>, which we
  also demonstrate.

---------------------------------------------------------
Title: Astrophysical convection and dynamos
Authors: Brandenburg, A.; Nordlund, A.; Stein, R. F.
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.
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.
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 (&lt;~ 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<SUP>-1</SUP>. 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.
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<SUP>-1</SUP> 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.
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.
2000JApA...21..307N    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Magnetohydrodynamic Turbulence in Accretion Discs
Authors: Torkelsson, U.; Brandenburg, A.; Nordlund, A.; Stein, R. F.
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
2000astu.progE..19N    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Star Formation and Supersonic Turbulence
Authors: Nordlund, Åke
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.
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
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.
2000ApJ...530L.139G    Altcode: 1999astro.ph.12485G
  We use the three-dimensional hydrodynamic code of Stein &amp; 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
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
2000ASSL..254...37L    Altcode: 2000stas.conf...37L
  No abstract at ADS

---------------------------------------------------------
Title: A Comparison of <SUP>13</SUP>CO Local Thermodynamic Equilibrium
    and True Column Densities in Molecular Cloud Models
Authors: Padoan, Paolo; Juvela, Mika; Bally, John; Nordlund, Åke
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 <SUP>13</SUP>CO 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 128<SUP>3</SUP> 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 ~10<SUP>5</SUP> cm<SUP>-3</SUP>). 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 <SUP>13</SUP>CO 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.
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.
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.
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.
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.
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.
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
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: \Mscr<SUB>A</SUB>~1 and \Mscr<SUB>A</SUB>&gt;&gt;1, where
  \Mscr<SUB>A</SUB> is the initial Alfvénic Mach number--the ratio of
  the rms velocity to the Alfvén speed. Our results show that models
  with \Mscr<SUB>A</SUB>&gt;&gt;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 \Mscr<SUB>A</SUB>~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~n<SUP>0.4</SUP>. 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
1999ApJ...525..318P    Altcode: 1999astro.ph..5383P
  We compare the statistical properties of J=1--&gt;0 <SUP>13</SUP>CO
  spectra observed in the Perseus molecular cloud with synthetic
  J=1--&gt;0 <SUP>13</SUP>CO 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.
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.
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 &lt; T_eff &lt; 7 000 K
Authors: Alencar, S. H. P.; Vaz, L. P. R.; Nordlund, Å.
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 &lt; T_eff &lt; 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 &amp;
  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.
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.
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.
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.
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&gt;~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<SUP>-1</SUP>, respectively,
  at z&lt;~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.
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.
1999ASPC..173..193S    Altcode: 1999sstt.conf..193S
  No abstract at ADS

---------------------------------------------------------
Title: Solar Convection and MHD
Authors: Nordlund, Å.; Stein, R. F.
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.
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.
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.
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
1999intu.conf..248P    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Convection Simulations
Authors: Nordlund, Å.; Stein, R. F.
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
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.
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<SUP>-1</SUP>, 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.
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.
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
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 ×
  10<SUP>5</SUP> spectra of <SUP>12</SUP>CO, <SUP>13</SUP>CO, 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. <P />Synthetic data consist
  of sets of 90 × 90 synthetic spectra with 60 velocity channels,
  in five molecular transitions: J = 1 --&gt; 0 and J = 2 --&gt;
  1 for <SUP>12</SUP>CO and <SUP>13</SUP>CO, and J = 1 --&gt; 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.
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.
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.
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.
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<SUP>-4</SUP>. 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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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. <P />In
  the present work we clarify the nature of the information provided
  by their method. <P />We show that: <P />1. The three-dimensional
  density field of the gas is well described by a lognormal distribution
  down to very small scales. <P />2. The power spectrum and the standard
  deviation of the three-dimensional density field can be constrained. <P
  />3. Such a structure of the density field is likely to be the effect
  of random supersonic motions present in the gas. <P />In fact, we find
  a qualitative and quantitative agreement between the predictions based
  on recent numerical simulations of randomly forced supersonic flows
  by Nordlund &amp; Padoan and by Padoan, Nordlund, &amp; 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
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.
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.
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
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.
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
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.
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
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 24<SUP>3</SUP> to 136<SUP>3</SUP>. 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.
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
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.
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.
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.
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.
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.
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.
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.
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
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.
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
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.
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.
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.
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.
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.
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.
1994ASIC..433..471N    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Subphotospheric Convection
Authors: Stein, R. F.; Nordlund, A.
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
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.
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.
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.
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.
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.
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 B<SUB>tor</SUB> 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 B<SUB>tor/rho</SUB> 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
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 T<SUB>eff</SUB> = 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
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
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.
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.
1992iesh.conf..119P    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Magnet Convection (Invited Review)
Authors: Stein, R. F.; Brandenburg, A.; Nordlund, A.
1992ASPC...26..148S    Altcode: 1992csss....7..148S
  No abstract at ADS

---------------------------------------------------------
Title: Deformation of Magnetic Null Points
Authors: Galsgaard, K.; Nordlund, A.
1992LNP...399..343G    Altcode: 1992esf..coll..343G; 1992IAUCo.133..343G
  No abstract at ADS

---------------------------------------------------------
Title: Large scale simulations
Authors: Nordlund, Ake; Galsgaard, Klaus
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.
1992sccw.conf..105D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Convection and Its Influence on Oscillations
Authors: Stein, Robert F.; Nordlund, Åke
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.
1991ASIC..341...61N    Altcode: 1991sabc.conf...61N
  No abstract at ADS

---------------------------------------------------------
Title: Vector Potential Magnetic Null Points
Authors: Galsgaard, Klaus; Nordlund, Åke
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 3×10<SUP>4</SUP> have been performed,
  and simulations with pressure ratios of 5×10<SUP>6</SUP> 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.
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.
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 ≤T<SUB>eff</SUB>≤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<SUP>-1</SUP> 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.
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<SUP>-1</SUP>, 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<SUP>-1</SUP>. 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<SUP>-1</SUP> in K dwarfs to ≃1000 m
  s<SUP>-1</SUP> 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.
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.
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.
1990IAUS..138..191N    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Model Atmospheres for M Giants
Authors: Plez, B.; Nordlund, A.
1990fmpn.coll...88P    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Driving and Damping of Oscillations
Authors: Stein, Robert F.; Nordlund, Åke
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.
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.
1990apsu.conf...17D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Topology of Convection beneath the Solar Surface
Authors: Stein, R. F.; Nordlund, A.
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.
1989ASIC..263..381S    Altcode: 1989ssg..conf..381S
  No abstract at ADS

---------------------------------------------------------
Title: Simulating Magnetoconvection
Authors: Nordlund, Å.; Stein, R. F.
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.
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.
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.
1988BAAS...20..702N    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Stellar Granulation: Photospheric Line Asymmetries and
    Hydrodynamic Model Atmospheres
Authors: Dravins, D.; Nordlund, A.
1986BAAS...18.1002D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Solar Fe II line asymmetries and wavelength shifts.
Authors: Dravins, D.; Larsson, B.; Nordlund, A.
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.
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.
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.
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.
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.
1985tphr.conf..101N    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Dynamics of Granulation and its Interaction with the
    Radiation Field
Authors: Nordlund, A.
1985tphr.conf....1N    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: A Comparison of Artificial Solar Granules with Real Solar
    Granules
Authors: Woehl, H.; Nordlund, A.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
1984IAUS..105..199E    Altcode:
  HCN and C<SUB>2</SUB>H<SUB>2</SUB> 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 C<SUB>2</SUB>H<SUB>2</SUB>
  opacity show a strange transition to "thin", high-pressure atmospheric
  structures when T<SUB>eff</SUB> is decreased below about 2900K. When
  HCN (and C<SUB>2</SUB>H<SUB>2</SUB>) 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.
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.
1983BAAS...15..710N    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Numerical 3-D simulations of the collapse of photospheric
    flux tubes
Authors: Nordlund, A.
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.
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.
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
  of<SUB>grid-scale</SUB> 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.
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.
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
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.
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.
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.
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.
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
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.
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.
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.
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.
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