R. F. Stein

Michigan State University

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ÅN

Åke Nordlund

University of Copenhagen

122

Axel Brandenburg

Nordic Institute for Theoretical Physics

M. Carlsson

University of Oslo

D. Georgobiani

Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati

Regner Trampedach

Space Science Institute

David Benson

University of Exeter

I. Tuominen

University of Helsinki

J. W. Leibacher

National Solar Observatory

АГ

А. Г. Косовичев

New Jersey Institute of Technology

C. S. Rosenthal

University of Oslo

Cooperative Institutions

Centre National de la Recherche Scientifique

Max Planck Society

High Altitude Observatory

NSF National Center for Atmospheric Research

Université Paris Cité

Max Planck Institute for Solar System Research

Sorbonne Université

Laboratoire d’études spatiales et d’instrumentation en astrophysique

National Solar Observatory

Observatoire de Paris

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Accurate Radiation Hydrodynamics and MHD Modeling of 3-D Stellar Atmospheres

AIP conference proceedings (2009)

Åke Nordlund R. F. Stein I. Hubený James M. Stone Keith MacGregor

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.

Photosphere

Line (geometry)

10.1063/1.3250064

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Citations (9)

Generation of acoustic and gravity waves by turbulence in an isothermal stratified atmosphere

Solar Physics (1967)

R. F. Stein

10.1007/bf00146490

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Citations (216)

Turbulent Diffusion in the Photosphere as Derived from Photospheric Bright Point Motion

arXiv (Cornell University) (2011)

V. I. Abramenko V. Carbone Vasyl Yurchyshyn Philip R. Goode R. F. Stein

On the basis of observations of solar granulation obtained with the New Solar Telescope (NST) of Big Bear Solar Observatory, we explored proper motion of bright points (BPs) in a quiet sun area, a coronal hole, and an active region plage. We automatically detected and traced bright points (BPs) and derived their mean-squared displacements as a function of time (starting from the appearance of each BP) for all available time intervals. In all three magnetic environments, we found the presence of a super-diffusion regime, which is the most pronounced inside the time interval of 10-300 seconds. Super-diffusion, measured via the spectral index, $\gamma$, which is the slope of the mean-squared displacement spectrum, increases from the plage area ($\gamma=1.48$) to the quiet sun area ($\gamma=1.53$) to the coronal hole ($\gamma=1.67$). We also found that the coefficient of turbulent diffusion changes in direct proportion to both temporal and spatial scales. For the minimum spatial scale (22 km) and minimum time scale (10 sec), it is 22 and 19 km$^{2}$ s$^{-1}$ for the coronal hole and the quiet sun area, respectively, whereas for the plage area it is about 12 km$^{2}$ s$^{-1}$ for the minimum time scale of 15 seconds. We applied our BP tracking code to 3D MHD model data of solar convection (Stein et al. 2007) and found the super-diffusion with $\gamma=1.45$. An expression for the turbulent diffusion coefficient as a function of scales and $\gamma$ is obtained.

Photosphere

Coronal hole

Solar observatory

QUIET

Scale height

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Mechanisms for chromospheric heating

R. F. Stein

A nonthermal energy source is required to heat the solar chromosphere and corona. A survey is made of the properties of waves that propagate along magnetic flux tubes, which may transport the needed energy from the convection zone to the chromosphere. It is next explored how convective motions can generate those waves. Finally, various mechanisms by which these waves may develop small scale structures and dissipate via viscosity and resistivity are discussed.

Chromosphere

Corona (planetary geology)

Energy transport

Energy flux

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Magnetic tubes in overshooting compressible convection

Lecture notes in physics (1991)

R. L. Jennings Axel Brandenburg Åke Nordlund R. F. Stein I. Tuominen

Flux tube

Compressible flow

10.1007/3-540-53955-7_111

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Magnetohydrodynamic Turbulence in Accretion Discs

Symposium - International Astronomical Union (2000)

Ulf Torkelsson Axel Brandenburg Åke Nordlund R. F. Stein

We present results from numerical simulations of magneto-hydrodynamic 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.

Magnetohydrodynamic Turbulence

Accretion disc

10.1017/s0074180900162989

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Convective contributions to the frequencies of solar oscillations

arXiv (Cornell University) (1998)

C. S. Rosenthal J. Christensen‐Dalsgaard Åke Nordlund R. F. Stein Regner Trampedach

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.

Oscillation (cell signaling)

Photosphere

Mode (computer interface)

Envelope (radar)

10.48550/arxiv.astro-ph/9803206

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G-band Images from MHD Convection Simulations

American Astronomical Society Meeting Abstracts #204 (2004)

R. F. Stein M. Carlsson Åke Nordlund G. B. Scharmer

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Rapid Temporal Variability of Faculae: High‐Resolution Observations and Modeling

The Astrophysical Journal (2006)

Bart De Pontieu M. Carlsson R. F. Stein L. Rouppe van der Voort M. G. Löfdahl

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 01 resolution. Examples of these dynamics include merging, splitting, rapid motion, apparent fluting, and possibly swaying.

Solar telescope

Plage

Temporal resolution

10.1086/505074

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Citations (42)

On sound generation by turbulent convection: A new look at old results

The Astrophysical Journal (1994)

Z. E. Musielak R. Rosner R. F. Stein P. Ulmschneider

view Abstract Citations (145) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On Sound Generation by Turbulent Convection: A New Look at Old Results Musielak, Z. E. ; Rosner, R. ; Stein, R. F. ; Ulmschneider, P. Abstract We have revisited the problem of acoustic wave generation by turbulent convection in stellar atmospheres. The theory of aerodynamically generated sound, originally developed by Lighthill and later modified by Stein to include the effects of stratification, has been used to estimate the acoustic wave energy flux generated in solar and stellar convection zones. We correct the earlier computations by incorporating an improved description of the spatial and temporal spectrum of the turbulent convection. We show the dependence of the resulting wave fluxes on the nature of the turbulence, and compute the wave energy spectra and wave energy fluxes generated in the Sun on the basis of a mixing-length model of the solar convection zone. In contrast to the previous results, we show that the acoustic energy generation does not depend very sensitively on the turbulent energy spectrum. However, typical total acoustic fluxes of order FA = 5 x 107 ergs/sq cm/s with a peak of the acoustic frequency spectrum near omega = 100 mHz are found to be comparable to those previously calculated. The acoustic flux turns out to be strongly dependent on the solar model, scaling with the mixing-length parameter alpha as alpha3.8. The computed fluxes most likely constitute a lower limit on the acoustic energy produced in the solar convection zone if recent convection simulations suggesting the presence of shocks near the upper layers of the convection zone apply to the Sun. Publication: The Astrophysical Journal Pub Date: March 1994 DOI: 10.1086/173825 Bibcode: 1994ApJ...423..474M Keywords: Sound Waves; Stellar Atmospheres; Stellar Convection; Turbulent Flow; Wave Generation; Energy Spectra; Fourier Transformation; Reynolds Stress; Wave Equations; Astrophysics; CONVECTION; STARS: ATMOSPHERES; SUN: ATMOSPHERE; TURBULENCE full text sources ADS |

Helioseismology

Standard solar model

10.1086/173825

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Citations (116)