Author name code: georgobiani ADS astronomy entries on 2022-09-14 author:"Georgobiani, Dali" ------------------------------------------------------------------------ Title: Realistic numerical simulations of solar convection: emerging flux, pores, and Stokes spectra Authors: Georgobiani, D.; Stein, R.; Nordlund, A. Bibcode: 2012IAUSS...6E.102G Altcode: We report on magneto-convection simulations of magnetic flux emerging through the upper layers of the solar convection zone into the photosphere. Simulations by Georgobiani, Stein and Nordlund start from minimally structured, uniform, untwisted horizontal field advected into the computational domain by supergranule scale inflows at 20 Mm depth. At the opposite extreme, simulations by Cheung (2007, 2008, 2011) start with a coherent flux tube inserted into or forced into the bottom of the computational domain. Several robust results have emerged from the comparison of results from these two very different initial states. First, rising magnetic flux gets deformed into undulating, serpentine shapes by the influence of the convective up- and down-flows. The flux develops fine structure and appears at the surface first as a "pepper and salt" pattern of mixed polarity. Where magnetic flux approaches the surface, granules become darker and elongated in the direction of the field. Subsequently, the underlying large scale magnetic structures make the field collect into unipolar regions. Magneto-convection produces a complex, small-scale magnetic field topology, whatever the initial state. A heirarchy of magnetic loops corresponding to the different scales of convective motions are produced. Vertical vortex tubes form at intergranule lane vertices which can lead to tornado-like magnetic fields in the photosphere. Gradients in field strength and velocity produce asymmetric Stokes spectra. Where emerging Omega loops leave behind nearly vertical legs, long lived pores can spontaneously form. The field in the pores first becomes concentrated and evacuated near the surface and the evacuated flux concentration then extends downward. Title: Helioseismic Data from Emerging Flux Simulations Authors: Stein, R. F.; Lagerfjärd, A.; Nordlund, Å.; Georgobiani, D. Bibcode: 2012ASPC..462..345S Altcode: Data from solar magneto-convection emerging flux simulations is available for validating helioseismic inversion procedures. In these simulations a uniform, untwisted, horizontal magnetic field is advected by inflows at the bottom of the domain 48 Mm wide by 20 Mm deep and rises to the surface. The evolution for different field strengths at 20 Mm depth has been investigated. The field emerges first in a mixed polarity pepper and salt pattern, but then collects into separate, unipolar concentrations and when enough flux has reached the surface, pores are produced. In one case the field strength was artificially increased and then the pores grew into spot-like structures with penumbral-like borders. The online data consists of slices of vertical and horizontal velocity and magnetic field strength at continuum optical depths of 0.01, 0.1 and 1 as well as the emergent intensity at one minute intervals plus four hour averages (with 2 hour cadence) of the three-dimensional (3D) density, velocity, temperature, energy, sound speed and magnetic field. The data can be found as links from the web page: http://steinr.pa.msu.edu/∼bob/data.html. These calculation were performed on the supercomputers of the NASA Advanced Supercomputing Division and were supported by grants from NASA and NSF. Title: Emerging Flux Simulations Authors: Stein, R. F.; Lagerfjärd, A.; Nordlund, Å.; Georgobiani, D. Bibcode: 2012ASPC..454..193S Altcode: We simulate the rise through the upper convection zone and emergence through the solar surface of initially uniform, untwisted, horizontal magnetic flux that is advected into a domain 48 Mm wide by 20 Mm deep, with the same entropy as the non-magnetic plasma. The magnetic field is transported upward by the diverging upflows and pulled down in the downdrafts, which produces a hierarchy of loop-like structures of increasingly smaller scale as the surface is approached. 20 kG fields at the bottom significantly modify the convective flows, leading to long thin cells of ascending fluid aligned with the magnetic field. Their magnetic buoyancy makes them rise to the surface faster than the fluid rise time. A large scale magnetic loop is produced that, as it emerges through the surface, leads to the formation of a bipolar pore-like structure. Title: Emerging Flux Simulations and Proto-Active Regions Authors: Stein, R. F.; Lagerfjärd, A.; Nordlund, &.; Georgobiani, D. Bibcode: 2012ASPC..455..133S Altcode: 2011arXiv1102.1049S The emergence of minimally structured (uniform and horizontal) magnetic field from a depth of 20 Mm has been simulated. The field emerges first in a mixed polarity pepper and salt pattern, but then collects into separate, unipolar concentrations and produces pores. The field strength was then artificially increased to produce spot-like structures. The field strength at continuum optical depth unity peaks at 1 kG, with a maximum of 4 kG. Where the vertical field is strong, the spots persist (at present an hour of solar time has been simulated). Where the field is weak, the spot gets filled in and disappears. Stokes profiles have been calculated and processed with the Hinode annular Modulation Transfer Function, the slit diffraction, and frequency smoothing. These data are available at steinr.pa.msu.edu/∼bob/stokes. Title: Photospheric Magnetic Fields from Magneto-Convection Simulations Authors: Stein, Robert F.; Nordlund, Aake; Georgobiani, Dali Bibcode: 2012decs.confE..95S Altcode: We present the properties of photospheric magnetic fields from magneto-convection simulations and as they would be observed by Hinode, for both quiet Sun and plage regions. This will include statistical properties, morphology, Stokes spectra, energy fluxes and correlations with convection dynamics. The rate of flux emergence will be discussed as a constraint on model parameters. Title: Magnetic Fields: Modeling And ATST Observations Authors: Stein, Robert F.; Georgobiani, D.; Nordlund, A.; Lagerfjard, A. Bibcode: 2011SPD....42.0804S Altcode: 2011BAAS..43S.0804S We have performed magneto-convection simulations starting from snapshots of hydrodynamic convection with initial conditions both of uniform vertical magnetic field and with minimally structured (uniform, untwisted), horizontal magnetic field advected into the computational domain from a depth of 20 Mm. One clear result is that while the magnetic field can collect into large-scale concentrations - pores and sunspots - most of the magnetic flux is in small concentrations with steep horizontal gradients in the field and plasma properties. Furthermore, the field strength distribution is a power law with slope between -1 and -2, so most of the field at the surface is weak. A large aperture telescope, such as ATST, is needed both to collect sufficient photons to measure the ubiquitous weak fields and to resolve the small-scale magnetic features.

We present results on flux emergence, pore formation, and Stokes spectra as they would appear in Hinode and ATST compared with the raw simulation.For those interested in analyzing the simulation data, it is available online at steinr.pa.msu.edu/ bob/data.html. There are slices of the velocity and magnetic field vectors at continuum optical depths of 1, 0.1, and 0.01 and the emergent intensity have been saved at 1 minute intervals. Four hour averages, with 2 hour cadence for the 3D cube for variables: velocity, magnetic field, density, temperature, sound speed, and internal energy have been computed. Stokes spectra have been computed for the Hinode FeI 630 nm lines, processed with the Hinode annular mtf, the slit diffraction and frequency smoothing.

This work has been supported by NASA grants NNX07AO71G, NNX07AH79G and NNX08AH44G and NSF grant AST0605738. The simulations where performed on the Pleiades cluster of the NASA Advanced Supercomputing Division at the Ames Research Center. Title: Solar Flux Emergence Simulations Authors: Stein, R. F.; Lagerfjärd, A.; Nordlund, Å.; Georgobiani, D. Bibcode: 2011SoPh..268..271S Altcode: 2009arXiv0912.4938S; 2010SoPh..tmp...34S We simulate the rise through the upper convection zone and emergence through the solar surface of initially uniform, untwisted, horizontal magnetic flux, with the same entropy as the nonmagnetic plasma, that is advected into a domain 48 Mm wide by 20 Mm deep. The magnetic field is advected upward by the diverging upflows and pulled down in the downdrafts, which produces a hierarchy of loop-like structures of increasingly smaller scale as the surface is approached. There are significant differences between the behavior of fields of 10 kG and 20 or 40 kG strength at 20 Mm depth. The 10 kG fields have little effect on the convective flows and show small magnetic-buoyancy effects, reaching the surface in the typical fluid rise time from 20 Mm depth of 32 hours. 20 and 40 kG fields significantly modify the convective flows, leading to long, thin cells of ascending fluid aligned with the magnetic field and their magnetic buoyancy makes them rise to the surface faster than the fluid rise time. The 20 kG field produces a large-scale magnetic loop that as it emerges through the surface leads to the formation of a bipolar, pore-like structure. Title: Supergranule Scale Flux Emergence Simulations Authors: Stein, Robert F.; Lagerfjard, A.; Nordlund, A.; Georgobiani, D. Bibcode: 2010AAS...21621103S Altcode: We simulate the rise of initially horizontal, untwisted magnetic flux from 20 Mm depth through the near surface convection to the solar surface in a domain 48 Mm wide. The magnetic field is transported upward by diverging upflows aided by magnetic buoyancy, and pushed down by downdrafts, which produces a hierarchy of loop like structures, of increasingly smaller scale as the surface is approached. We compare two cases with field strengths of 5 and 20 kG at 20 Mm depth. In the stronger field strength case, the magnetic field significantly disturbs the convection below 3 Mm, inhibiting the vertical motion, shutting off convective energy transport and producing elongated cellular structures in the field direction. Shallower than 3 Mm the convection appears normal, but with concentrated vertical magnetic concentrations ("flux tubes") extending through the surface and producing pores where the field is greatest. Even in the weaker field case, the magnetic field inhibits vertical motions and the convective transport of energy although the convective cellular pattern is not significantly distorted. This work was supported by NSF grant AST065738 and NASA grants NNX08AH44G, NNX07AH79G and NNX07AO71G. The simulations were performed at the NASA Advanced Supercomputing Division of the Ames Research Center. Title: Comparing the Hinode and SOHO/MDI Data to the Simulated Large Scale Solar Convection Authors: Georgobiani, D.; Zhao, J.; Kosovichev, A.; Benson, D.; Stein, R. F.; Nordlund, Å. Bibcode: 2009ASPC..415..421G Altcode: Large-scale simulations of solar turbulent convection produce realistic data and provide a unique opportunity to study solar oscillations and test various techniques commonly used for the analysis of solar observations. We applied helioseismic methods to the sets of simulated as well as observed data and find remarkable similarities. Power spectra, k-ν diagrams, time-distance diagrams exhibit similar details, although sometimes subtle differences are present. Title: Supergranulation Scale Convection Simulations Authors: Stein, R. F.; Lagerfjård, A.; Nordlund, Å.; Georgobiani, D.; Benson, D.; Schaffenberger, W. Bibcode: 2009ASPC..415...63S Altcode: Results of realistic simulations of solar surface convection on the scale of supergranules (48 and 96 Mm wide by 20 Mm deep) are presented. The simulations include the hydrogen, first and most of the second helium ionization zones. Horizontal magnetic field is advected into the domain by upflows at the bottom. Upflows stretch the field lines upward, while downflows push them down, thus producing loop like magnetic structures. The mass mixing length is 1.8 scale heights. Two thirds of the area is upflowing fluid except very close to the surface. The internal (ionization) energy flux is the largest contributor to the convective flux for temperatures less than 40,000 K and the thermal energy flux is the largest contributor at higher temperatures. The data is available for evaluating local helioseismic procedures. Title: Solar Magneto-Convection Simulations Authors: Stein, Robert F.; Lagerfjard, A.; Nordlund, A.; Benson, D.; Georgobiani, D.; Schaffenberger, W. Bibcode: 2009SPD....40.0401S Altcode: We present preliminary results of magneto-convection simulations of the rise of initially horizontal magnetic flux from 20 Mm deep through the solar surface in a domain 48 Mm wide. The magnetic field is stretched upward in the diverging upflows and pulled down in the downdrafts which produces a hierarchy of loop like structures. The strength varies with depth as the square root of the density. The field is swept to the boundaries of small supergranular like structures to form a magnetic network. Title: Simulated Large Scale Solar Convection Versus Observations: A Multiwavelength Approach Authors: Georgobiani, Dali; Zhao, J.; Kosovichev, A. G.; Benson, D.; Stein, R. F.; Nordlund, A. Bibcode: 2009SPD....40.0301G Altcode: The realistic 3D radiative-hydrodynamic simulations of the upper layers of solar convection provide a perfect opportunity to validate various techniques, widely used in solar physics and helioseismology. Our aim is to perform multiwavelength analysis of large scale flows. We analyze the simulated intensity and velocities at certain heights in the solar atmosphere, and compare our results with the outcome of the similar analysis of the SOHO/MDI and Hinode observations. To fine tune the comparison, we use the instrumental response functions to weigh the simulated parameters at different heights to emulate the observational lines. We find the remarkable similarity between the simulated and observed power spectra, their spatial parts, and time-distance diagrams. This demonstrates one more time that the simulations can be efficiently used to perform and validate local helioseismology techniques, and to study solar flows and structures beneath the surface, inaccessible for direct observations. Title: Supergranulation Scale Convection Simulations Authors: Stein, Robert F.; Georgobiani, Dali; Schafenberger, Werner; Nordlund, Åke; Benson, David Bibcode: 2009AIPC.1094..764S Altcode: 2009csss...15..764S Results of realistic simulations of solar surface convection on the scale of supergranules (96 Mm wide by 20 Mm deep) are presented. The simulations cover only 10% of the geometric depth of the solar convection zone, but half its pressure scale heights. They include the hydrogen, first and most of the second helium ionization zones. The horizontal velocity spectrum is a power law and the horizontal size of the dominant convective cells increases with increasing depth. Convection is driven by buoyancy work which is largest close to the surface, but significant over the entire domain. Close to the surface buoyancy driving is balanced by the divergence of the kinetic energy flux, but deeper down it is balanced by dissipation. The damping length of the turbulent kinetic energy is 4 pressure scale heights. The mass mixing length is 1.8 scale heights. Two thirds of the area is upflowing fluid except very close to the surface. The internal (ionization) energy flux is the largest contributor to the convective flux for temperatures less than 40,000 K and the thermal energy flux is the largest contributor at higher temperatures. Title: Supergranulation Scale Connection Simulations Authors: Stein, R. F.; Nordlund, A.; Georgobiani, D.; Benson, D.; Schaffenberger, W. Bibcode: 2008arXiv0811.0472S Altcode: Results of realistic simulations of solar surface convection on the scale of supergranules (96 Mm wide by 20 Mm deep) are presented. The simulations cover only 10% of the geometric depth of the solar convection zone, but half its pressure scale heights. They include the hydrogen, first and most of the second helium ionization zones. The horizontal velocity spectrum is a power law and the horizontal size of the dominant convective cells increases with increasing depth. Convection is driven by buoyancy work which is largest close to the surface, but significant over the entire domain. Close to the surface buoyancy driving is balanced by the divergence of the kinetic energy flux, but deeper down it is balanced by dissipation. The damping length of the turbulent kinetic energy is 4 pressure scale heights. The mass mixing length is 1.8 scale heights. Two thirds of the area is upflowing fluid except very close to the surface. The internal (ionization) energy flux is the largest contributor to the convective flux for temperatures less than 40,000 K and the thermal energy flux is the largest contributor at higher temperatures. This data set is useful for validating local helioseismic inversion methods. Sixteen hours of data are available as four hour averages, with two hour cadence, at steinr.msu.edu/~bob/96averages, as idl save files. The variables stored are the density, temperature, sound speed, and three velocity components. In addition, the three velocity components at 200 km above mean continuum optical depth unity are available at 30 sec. cadence. Title: Surface Convection Authors: Stein, Robert F.; Benson, David; Georgobiani, Dali; Nordlund, Åke; Schaffenberger, Werner Bibcode: 2007AIPC..948..111S Altcode: What are supergranules? Why do they stand out? Preliminary results from realistic simulations of solar convection on supergranule scales (96 Mm wide by 20 Mm deep) are presented. The solar surface velocity amplitude is a decreasing power law from the scale of granules up to giant cells with a slight enhancement at supergranule scales. The simulations show that the size of the horizontal convective cells increases gradually and continuously with increasing depth. Without magnetic fields present there is, as yet, no enhancement at supergranule scales at the surface. A hypothesis is presented that it is the balance between the rate of magnetic flux emergence and the horizontal sweeping of magnetic flux by convective motions that determines the size of the magnetic network and produces the extra power at supergranulation scales. Title: Validating Time-Distance Helioseismology by Use of Realistic Simulations of Solar Convection Authors: Zhao, Junwei; Georgobiani, D.; Kosovichev, A. G.; Benson, D.; Stein, R. F.; Nordlund, A. Bibcode: 2007AAS...210.2203Z Altcode: 2007BAAS...39..124Z Recent progress in realistic simulations of solar convection have enabled us to evaluate the robustness of solar interior structures and dynamics obtained by methods of local helioseismology. We present results of testing the time-distance method using realistic simulations. By computing acoustic wave propagation time and distance relations for different depths of the simulated data, we confirm that acoustic waves propagate into the interior and then turn back to the photosphere. For the surface gravity waves (f-mode), we calculate perturbations of their travel times, caused by localized downdrafts, and demonstrate that the spatial pattern of these perturbations (representing so-called sensitivity kernels) is similar to the patterns obtained from the real Sun, displaying characteristic hyperbolic structures. We then test the time-distance measurements and inversions by calculating acoustic travel times from a sequence of vertical velocities at the photosphere of the simulated data, and inferring a mean 3D flow fields by performing inversion based on the ray approximation. The inverted horizontal flow fields agree very well with the simulated data in subsurface areas up to 3 Mm deep, but differ in deeper areas. These initial tests provide important validation of time-distance helioseismology measurements of supergranular-scale convection, illustrate limitations of this technique, and provide guidance for future improvements. Title: Application of convection simulations to oscillation excitation and local helioseismology Authors: Stein, Robert F.; Benson, David; Georgobiani, Dali; Nordlund, Åke Bibcode: 2007IAUS..239..331S Altcode: No abstract at ADS Title: Validation of Time-Distance Helioseismology by Use of Realistic Simulations of Solar Convection Authors: Zhao, Junwei; Georgobiani, Dali; Kosovichev, Alexander G.; Benson, David; Stein, Robert F.; Nordlund, Åke Bibcode: 2007ApJ...659..848Z Altcode: 2006astro.ph.12551Z Recent progress in realistic simulations of solar convection have given us an unprecedented opportunity to evaluate the robustness of solar interior structures and dynamics obtained by methods of local helioseismology. We present results of testing the time-distance method using realistic simulations. By computing acoustic wave propagation time and distance relations for different depths of the simulated data, we confirm that acoustic waves propagate into the interior and then turn back to the photosphere. This demonstrates that in numerical simulations properties of acoustic waves (p-modes) are similar to the solar conditions, and that these properties can be analyzed by the time-distance technique. For surface gravity waves (f-modes), we calculate perturbations of their travel times caused by localized downdrafts and demonstrate that the spatial pattern of these perturbations (representing so-called sensitivity kernels) is similar to the patterns obtained from the real Sun, displaying characteristic hyperbolic structures. We then test time-distance measurements and inversions by calculating acoustic travel times from a sequence of vertical velocities at the photosphere of the simulated data and inferring mean three-dimensional flow fields by performing inversion based on the ray approximation. The inverted horizontal flow fields agree very well with the simulated data in subsurface areas up to 3 Mm deep, but differ in deeper areas. Due to the cross talk effects between the horizontal divergence and downward flows, the inverted vertical velocities are significantly different from the mean convection velocities of the simulation data set. These initial tests provide important validation of time-distance helioseismology measurements of supergranular-scale convection, illustrate limitations of this technique, and provide guidance for future improvements. Title: Local Helioseismology and Correlation Tracking Analysis of Surface Structures in Realistic Simulations of Solar Convection Authors: Georgobiani, Dali; Zhao, Junwei; Kosovichev, Alexander G.; Benson, David; Stein, Robert F.; Nordlund, Åke Bibcode: 2007ApJ...657.1157G Altcode: 2006astro.ph..8204G We apply time-distance helioseismology, local correlation tracking, and Fourier spatial-temporal filtering methods to realistic supergranule scale simulations of solar convection and compare the results with high-resolution observations from the Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI). Our objective is to investigate the surface and subsurface convective structures and test helioseismic measurements. The size and grid of the computational domain are sufficient to resolve various convective scales from granulation to supergranulation. The spatial velocity spectrum is approximately a power law for scales larger than granules, with a continuous decrease in velocity amplitude with increasing size. Aside from granulation no special scales exist, although a small enhancement in power at supergranulation scales can be seen. We calculate the time-distance diagram for f- and p-modes and show that it is consistent with the SOHO MDI observations. From the simulation data we calculate travel-time maps for surface gravity waves (f-mode). We also apply correlation tracking to the simulated vertical velocity in the photosphere to calculate the corresponding horizontal flows. We compare both of these to the actual large-scale (filtered) simulation velocities. All three methods reveal similar large-scale convective patterns and provide an initial test of time-distance methods. Title: Excitation of solar-like oscillations across the HR diagram Authors: Samadi, R.; Georgobiani, D.; Trampedach, R.; Goupil, M. J.; Stein, R. F.; Nordlund, Å. Bibcode: 2007A&A...463..297S Altcode: 2006astro.ph.11762S Aims:We extend semi-analytical computations of excitation rates for solar oscillation modes to those of other solar-like oscillating stars to compare them with recent observations
Methods: Numerical 3D simulations of surface convective zones of several solar-type oscillating stars are used to characterize the turbulent spectra as well as to constrain the convective velocities and turbulent entropy fluctuations in the uppermost part of the convective zone of such stars. These constraints, coupled with a theoretical model for stochastic excitation, provide the rate P at which energy is injected into the p-modes by turbulent convection. These energy rates are compared with those derived directly from the 3D simulations.
Results: The excitation rates obtained from the 3D simulations are systematically lower than those computed from the semi-analytical excitation model. We find that Pmax, the P maximum, scales as (L/M)s where s is the slope of the power law and L and M are the mass and luminosity of the 1D stellar model built consistently with the associated 3D simulation. The slope is found to depend significantly on the adopted form of χ_k, the eddy time-correlation; using a Lorentzian, χ_k^L, results in s=2.6, whereas a Gaussian, χ_k^G, gives s=3.1. Finally, values of V_max, the maximum in the mode velocity, are estimated from the computed power laws for P_max and we find that Vmax increases as (L/M)sv. Comparisons with the currently available ground-based observations show that the computations assuming a Lorentzian χk yield a slope, sv, closer to the observed one than the slope obtained when assuming a Gaussian. We show that the spatial resolution of the 3D simulations must be high enough to obtain accurate computed energy rates. Title: Spatial and Temporal Spectra of Solar Convection Authors: Georgobiani, D.; Stein, R. F.; Nordlund, Å. Bibcode: 2006ASPC..354..109G Altcode: Recent observations support the theory that solar-type oscillations are stochastically excited by turbulent convection in the outer layers of the solar-like stars. The acoustic power input rates depend on the details of the turbulent energy spectrum.

We use numerical simulations to study the spectral properties of solar convection. We find that spatial turbulent energy spectra vary at different temporal frequencies, while temporal turbulent spectra show various features at different spatial wavenumbers, and their best fit at all frequencies is a generalized power law Power = Amplitude × (frequency^2 + width^2)^{-n(k)}, where n(k) depends on the spatial wavenumber. Therefore, it is impossible to separate the spatial and temporal components of the turbulent spectra. Title: Supergranule scale convection simulations Authors: Stein, R. F.; Benson, D.; Georgobiani, D.; Nordlund, Å. Bibcode: 2006ESASP.624E..79S Altcode: 2006soho...18E..79S No abstract at ADS Title: Time-Distance and Correlation Tracking Analysesof Convective Structures using Realistic Large-ScaleSimulations of Solar Convection Authors: Georgobiani, Dali; Zhao, J.; Kosovichev, A. G.; Benson, D.; Stein, R. F.; Nordlund, A. Bibcode: 2006SPD....37.0509G Altcode: 2006BAAS...38..224G Recent large-scale simulations of solar turbulentconvection and oscillations produce a wealth of realisticdata and provide a great opportunity to study solaroscillations and test various techniques, such aslocal helioseismology or local correlation trackingmethods, widely used for the analysis of the realobserved solar data.The application of the time-distance analysis to theartificial data produced with a realistic 3D radiativehydrodynamic code successfully reproduces thetime-distance diagram and travel time maps. Resultingtravel times are similar to the travel times obtainedfrom the SOHO/MDI observations. To further validatethe model, the inversion will be performed in orderto infer the interior velocities at various depthsand compare them with the simulated data.f-mode time-distanceanalysis as well as local correlation tracking can be usedto study the morphology of the simulated convection. Bothmethods reveal the large-scale convective structures, whichare also directly visible in the time-averaged simulatedflow fields. Title: Time-distance analysis of realistic simulations of solar convection Authors: Georgobiani, D.; Zhao, J.; Benson, D.; Stein, R. F.; Kosovichev, A. G.; Nordlund, A. Bibcode: 2005AGUFMSH41A1117G Altcode: The results of the new realistic large-scale simulations of solar turbulent convection provide an unprecedented opportunity to study solar oscillations and perform similar local helioseismology techniques as for the real solar data. The results offer an unique opportunity to compare the simulated flow fields with the flows and sounds speed variations inferred from the time-distance analysis. Applying some of the existing local helioseismology methods to the simulated solar convection and comparing to the observed results, one can validate the accuracy of these methods. We apply the time-distance analysis to the simulated data and successfully obtain the time-distance curve and travel time maps. Our travel times are consistent with the SOHO/MDI observations. The next step is to perform inversion to infer the interior flow fields at various depths and compare them with the simulated data in order to validate the model. This work is currently in progress. Title: Excitation of Solar-like Oscillations: From PMS to MS Stellar Models Authors: Samadi, R.; Goupil, M. -J.; Alecian, E.; Baudin, F.; Georgobiani, D.; Trampedach, R.; Stein, R.; Nordlund, Å. Bibcode: 2005JApA...26..171S Altcode: The amplitude of solar-like oscillations results from a balance between excitation and damping. As in the sun, the excitation is attributed to turbulent motions that stochastically excite the p modes in the upper-most part of the convective zone. We present here a model for the excitation mechanism. Comparisons between modeled amplitudes and helio and stellar seismic constraints are presented and the discrepancies discussed. Finally the possibility and the interest of detecting such stochastically excited modes in pre-main sequence stars are also discussed. Title: Excitation of P-Modes in the Sun and Stars Authors: Stein, Robert; Georgobiani, Dali; Trampedach, Regner; Ludwig, Hans-Günter; Nordlund, Åke Bibcode: 2005HiA....13..411S Altcode: We describe the stochastic excitation of p-mode oscillations by solar convection. We discuss the role of Reynolds stresses and entropy fluctuations what controls the excitation spectrum the depth of the driving and the location of the driving. We then present results for a range of other stars and discuss the similarities and differences with the Sun. Title: Excitation rates of p modes: mass luminosity relation across the HR diagram Authors: Samadi, R.; Georgobiani, D.; Trampedach, R.; Goupil, M. J.; Stein, R. F.; Nordlund, Å. Bibcode: 2004sf2a.conf..323S Altcode: 2004astro.ph.10043S We compute the rates P at which energy is injected into the p modes for a set of 3D simulations of outer layers of stars. We found that Pmax - the maximum in P - scales as (L/M)^s where s is the slope of the power law, L and M are the luminosity and the mass of the 1D stellar models associated with the simulations. The slope is found to depend significantly on the adopted representation for the turbulent eddy-time correlation function, chi_k. According to the expected performances of COROT, it will likely be possible to measure Pmax as a function of L/M and to constrain the properties of stellar turbulence as the turbulent eddy time-correlation. Title: High Degree Solar Oscillations in 3d Numerical Simulations Authors: Georgobiani, D.; Stein, R. F.; Nordlund, Å.; Kosovichev, A. G.; Mansour, N. N. Bibcode: 2004ESASP.559..267G Altcode: 2004soho...14..267G No abstract at ADS Title: Oscillation Power Spectra of the Sun and of CEN a: Observations Versus Models Authors: Samadi, R.; Goupil, M. J.; Baudin, F.; Georgobiani, D.; Trampedach, R.; Stein, R.; Nordlund, A. Bibcode: 2004ESASP.559..615S Altcode: 2004astro.ph..9325S; 2004soho...14..615S Hydrodynamical, 3D simulations of the outer layers of the Sun and Alpha Cen A are used to obtain constraints on the properties of turbulent convection in such stars. These constraints enable us to compute - on the base of a theoretical model of stochastic excitation - the rate P at which p modes are excited by turbulent convection in those two stars. Results are then compared with solar seismic observations and recent observations of Alpha Cen A. For the Sun, a good agreement between observations and computed P is obtained. For Alpha Cen A a large discrepancy is obtained which origin cannot be yet identified: it can either be caused by the present data quality which is not sufficient for our purpose or by the way the intrinsic amplitudes and the life-times of the modes are determined or finally attributed to our present modelling. Nevertheless, data with higher quality or/and more adapted data reductions will likely provide constraints on the p-mode excitation mechanism in Alpha Cen A. Title: Turbulence Convection and Oscillations in the Sun Authors: Mansour, N. N.; Kosovichev, A. G.; Georgobiani, D.; Wray, A.; Miesch, M. Bibcode: 2004ESASP.559..164M Altcode: 2004soho...14..164M No abstract at ADS Title: Excitation of Radial P-Modes in the Sun and Stars Authors: Stein, Robert; Georgobiani, Dali; Trampedach, Regner; Ludwig, Hans-Günter; Nordlund, Åke Bibcode: 2004SoPh..220..229S Altcode: P-mode oscillations in the Sun and stars are excited stochastically by Reynolds stress and entropy fluctuations produced by convection in their outer envelopes. The excitation rate of radial oscillations of stars near the main sequence from K to F and a subgiant K IV star have been calculated from numerical simulations of their surface convection zones. P-mode excitation increases with increasing effective temperature (until envelope convection ceases in the F stars) and also increases with decreasing gravity. The frequency of the maximum excitation decreases with decreasing surface gravity. Title: What Causes p-Mode Asymmetry Reversal? Authors: Georgobiani, Dali; Stein, Robert F.; Nordlund, Åke Bibcode: 2003ApJ...596..698G Altcode: 2002astro.ph..5141G The solar acoustic p-mode line profiles are asymmetric. Velocity spectra have more power on the low-frequency sides, whereas intensity profiles show the opposite sense of asymmetry. Numerical simulations of the upper convection zone have resonant p-modes with the same asymmetries and asymmetry reversal as the observed modes. The temperature and velocity power spectra at optical depth τcont=1 have the opposite asymmetry, as is observed for the intensity and velocity spectra. At a fixed geometrical depth, corresponding to <τcont>=1, however, the temperature and velocity spectra have the same asymmetry. This indicates that the asymmetry reversal in the simulation is produced by radiative transfer effects and not by correlated noise. The cause of this reversal is the nonlinear amplitude of the displacements in the simulation and the nonlinear dependence of the H- opacity on temperature. Where the temperature is hotter the opacity is larger and photons escape from higher, cooler layers. This reduces the fluctuations in the radiation temperature compared to the gas temperature. The mode asymmetry reversal in the simulation is a small frequency-dependent differential effect within this overall reduction. Because individual solar modes have smaller amplitudes than the simulation modes, this effect will be smaller on the Sun. Title: Understanding the convective Sun Authors: Trampedach, Regner; Georgobiani, Dali; Stein, Robert F.; Nordlund, Åke Bibcode: 2003ESASP.517..195T Altcode: 2003soho...12..195T Hydrodynamical simulations of the surface layers of the Sun, has greatly improved our understanding and interpretation of solar observations. I review some past successes in matching spectral lines, improving the agreement with high-degree p-mode frequencies and matching the depth of the solar convection zone without adjustable convection-parameters. Our solar simulations contain p-modes, and are used for studying the asymmetry of p-mode peaks and to calibrate the conversion between the observed velocity proxies and the actual velocities. Title: Asymmetry reversal in solar acoustic modes Authors: Georgobiani, Dali; Stein, Robert F.; Nordlund, Åke Bibcode: 2003ESASP.517..279G Altcode: 2003soho...12..279G The power spectra of solar acoustic modes are asymmetric, with velocity having more power on the low frequency side of the peak and intensity having more power on the high frequency side. This effect exists in both observations and simulations, and it is believed to be caused by the correlated background noise. We study the temperature near the solar surface by means of a 3D hydrodynamic simulation of convection with a detailed treatment of radiation. The temperature spectrum at optical depth τcont = 1 has opposite asymmetry to the velocity spectrum, whereas the temperature measured at a fixed geometrical depth, corresponding to <τcont> = 1, has the same asymmetry as velocity. We believe that the asymmetry reversal in temperature at τcont = 1 (and therefore in intensity) occurs partly because of the radiative transfer effects. High temperature sensitivity of the opacity suppresses temperature fluctuations on opposite sides of the mode peaks differently, thus causing the asymmetry reversal. Title: Solar and Stellar Oscillations Authors: Stein, Robert; Nordlund, Aake; Georgobiani, Dali; Trampedach, Regner; Ludwig, Hans-Guenther Bibcode: 2003IAUJD..12E..41S 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: Models of the solar oscillations Authors: Georgobiani, Dali; Stein, Robert F.; Nordlund, Aake Bibcode: 2001ESASP.464..583G Altcode: 2001soho...10..583G The shallow upper layer of the solar convection zone is simulated using the three-dimensional hydrodynamic code of Stein & Nordlund. The simulation oscillation modes behave similarly to the SOHO/MDI observations; namely, they have the same asymmetries and phase relations. Therefore, one can study the properties of the modes from the simulations to investigate behavior below the surface, which cannot be observed directly. The asymmetry of the line profiles varies with depth. At the surface, the velocity asymmetry is the same as in the SOHO/MDI observations, but deeper down it becomes flipped in comparison to the surface asymmetry. This behavior is well represented by the simple model of a potential well with the source inside (or outside). The simulations can be used to determine the depth of the driving at different frequencies, while the simulation modes show a strong correlation of excitation with emergent intensity. Title: A Least-squares Solution for the Effective Conductivity of the Solar Convection Zone Authors: Kuhn, J. R.; Georgobiani, D. Bibcode: 2000SSRv...94..161K Altcode: Here we show how realistic numerical simulations of solar convection can be parameterized with an effective thermal conductivity tensor. We show that this diffusive approximation yields an accurate statistical (in the sense of the χ^2 test) description of the thermal transport properties of a perturbed solar convection zone. This parameterization will allow more accurate large scale solar irradiance and luminosity calculations. Title: Numerical Simulations of Oscillation Modes of the Solar Convection Zone Authors: Georgobiani, D.; Kosovichev, A. G.; Nigam, R.; Nordlund, Å.; Stein, R. F. Bibcode: 2000ApJ...530L.139G Altcode: 1999astro.ph.12485G We use the three-dimensional hydrodynamic code of Stein & Nordlund to realistically simulate the upper layers of the solar convection zone in order to study physical characteristics of solar oscillations. Our first result is that the properties of oscillation modes in the simulation closely match the observed properties. Recent observations from the Solar and Heliospheric Observatory (SOHO)/Michelson Doppler Imager (MDI) and Global Oscillations Network Group have confirmed the asymmetry of solar oscillation line profiles, initially discovered by Duvall et al. In this Letter, we compare the line profiles in the power spectra of the Doppler velocity and continuum intensity oscillations from the SOHO/MDI observations with the simulation. We also compare the phase differences between the velocity and intensity data. We have found that the simulated line profiles are asymmetric and have the same asymmetry reversal between velocity and intensity as observed. The phase difference between the velocity and intensity signals is negative at low frequencies, and phase jumps in the vicinity of modes are also observed. Thus, our numerical model reproduces the basic observed properties of solar oscillations and allows us to study the physical properties which are not observed. Title: A Least-Squares Solution for the Effective Conductivity of the Solar Convection Zone Authors: Kuhn, J. R.; Georgobiani, D. Bibcode: 2000svc..book..161K Altcode: No abstract at ADS Title: Realistic Simulations of Solar Surface Convection Authors: Stein, R. F.; Bercik, D.; Georgobiani, D.; Nordlund, A. Bibcode: 1999AAS...194.2104S Altcode: 1999BAAS...31R.858S Results from realistic simulations of near surface solar convection will be summarized and compared with observations. Solar convection is driven by radiative cooling from an extremely thin surface thermal boundary layer, which produces low entropy fluid. Its topology is controlled by mass conservation and consists of turbulent downdrafts penetrating nearly laminar upflows. The horizontal scales increase with depth. Good agreement is found with the of the depth of the convection zone, p-mode frequencies, excitation, line asymmetries and intensity - velocity phase differences from helioseismology; with observations of granulation and profiles of weak Fe lines. This work was supported by grants from NSF, NASA, and the Danish Research Council. The calculations were performed at NCSA, MSU and UNIC. Title: Three-dimensional simulations of solar oscillations: line profiles and asymmetries Authors: Georgobiani, D. G.; Nigam, R.; Kosovichev, A. G.; Stein, R. F.; Nordlund, A. Bibcode: 1999AAS...194.5605G Altcode: 1999BAAS...31..912G In order to study spectral characteristics of the solar oscillations, we use the Stein-Nordlund 3d hydrodynamic code to generate lond temporal sequencies of realistically simulated upper layers of the solar convective zone. The simulation domain ranges from 0.5 Mm above the surface of tau =1 to 2.5 Mm below this surface, and is 6 Mm by 6 Mm wide. We have generated 24 hours of solar time. We calculate power spectra of the vertical velocity and temperature at different heights and the emergent intensity at the surface. Here, we present the profiles of velocity, intensity and temperature for both radial (l = 0) and first nonradial (l = 700) mode. We compare line profiles from the simulation with the power spectra of the Doppler velocity and continuum intensity from the SOHO/MDI observations. Both simulated and observed profiles demonstrate similar types of asymmetry, and the asymmetry reversal between the local quantities like velocity and temperature, and emergent intensity profiles is also present in the simulated data. The preliminary results are promising as they allow us to establish a connection between the observational data and realistic simulations, and enable us to understand better the physics of solar oscillations. Title: Magneto-Convection Authors: Stein, R. F.; Georgobiani, D.; Bercik, D. J.; Brandenburg, A.; Nordlund, Å. Bibcode: 1999ASPC..173..193S Altcode: 1999sstt.conf..193S No abstract at ADS Title: Solar P-Mode Spectrum Asymmetries: Testing Theories With Numerical Simulations Authors: Georgobiani, Dali; Nigam, Rakesh; Kosovichev, Alexander G.; Stein, Robert F. Bibcode: 1999soho....9E..58G Altcode: We use a 36 hour sequence of 3-D hydrodynamic simulations of solar convection to study the line profiles of the acoustic modes and their asymmetries. We construct power spectra of the emergent intensity and the vertical velocity at a fixed height of 200 km above the t = 1 surface, as well as their phase differences. We compare the synthetic results with those obtained from the SOHO/MDI observations. The simulations and observations show similar direction of asymmetry and reversal of asymmetry between the velocity and intensity. Our preliminary results confirm the theoretical model of Nigam (Nigam et al. 1998). To make the simulation results more realistic, the intensity and velocity will in future be obtained from the synthetic NiI 6768 line used in the observations. Title: Solar Magneto-Convection Authors: Stein, R. F.; Bercik, D. J.; Brandenburg, A.; Georgobiani, D.; Nordlund, A. Bibcode: 1998AAS...191.7417S Altcode: 1998BAAS...30..758S We present results of realistic simulations of magneto-convection near the solar surface. The simulations were performed with two magnetic field topologies - (1) a unipolar, initially vertical field, and (2) a bipolar field, where fluid entering at the base of the computational domain advects in horizontal field. As the unipolar flux is increased, the magnetic field concentrates in the intergranule lanes and develops large, dark, cool regions. These regions surround smaller areas where convection has not been suppressed. In contrast, for the bipolar case, the strongest fields appear as bright points in the intergranule lanes. Title: Solar Magneto-Convection Authors: Bercik, David J.; Basu, Shantanu; Georgobiani, Dali; Nordlund, Ake; Stein, Robert F. Bibcode: 1998ASPC..154..568B Altcode: 1998csss...10..568B We have simulated magneto-convection near the solar surface with two topologies: (1) an initial vertical field; and (2) a horizontal field carried in with the fluid entering at the base of the computational domain. We report results on the interaction of convection and magnetic fields. An MPEG video is viewable at: http://www.pa.msu.edu/~steinr/images/bhoriz.mpg The MPEG video is also included on the CS10 CD ROM. Title: Heat Transport in the Convective Zone and Deviations from the Mixing Length Models Authors: Georgobiani, D.; Kuhn, J. R.; Nordlund, AA.; Stein, R. F. Bibcode: 1998ESASP.418..771G Altcode: 1998soho....6..771G For several decades, the heat transport in the solar convective zone has been thought to be isotropic. Attempts to describe it in terms of the mixing length theory seemed to be quite successful. In contradiction with such an idealized picture, recent numerical and observational data have demonstrated a highly non-isotropic, inhomogeneous structure of the convective zone. This work presents the results of calculations of the thermal conductivity in the convective zone, using the numerical model of Stein-Nordlund. Thermal conductivity is assumed to be a 3D tensor. Its vertical and horizontal diagonal components differ in magnitudes for each given depth. Moreover, the horizontal component stays negative, while increasing with depth. Both features are naturally explained by the physical properties of the solar convective zone. Implications for global questions of solar convection are considered. Title: Sound speed variations near the photosphere due to entropy perturbations in 3d numerical experiments Authors: Georgobiani, D.; Kuhn, J. R.; Stein, R. F. Bibcode: 1997ASSL..225..127G Altcode: 1997scor.proc..127G Results on how the temperature distribution near the solar photosphere is altered by perturbing the entropy of rising fluid in the convection zone several megameters below the surface, are presented. Effects on the emergent intensity and implications for helioseismic observations are described. Title: Solar Convection: Comparison of Numerical Simulations and Mixing-Length Theory Authors: Abbett, William P.; Beaver, Michelle; Davids, Barry; Georgobiani, Dali; Rathbun, Pamela; Stein, Robert F. Bibcode: 1997ApJ...480..395A Altcode: We compare the results of realistic numerical simulations of convection in the superadiabatic layer near the solar surface with the predictions of mixing-length theory. We find that the peak values of such quantities as the temperature gradient, the temperature fluctuations, and the velocity fluctuations, as well as the entropy jump in the simulation, can be reproduced by mixing-length theory for a ratio of mixing length to pressure scale height α ~ 1.5. However, local mixing-length theory neither reproduces the profiles of these variables with depth nor allows penetration of convective motions into the overlying stable photosphere. Title: Sound Speed Variations Near the Photosphere due to Entropy Perturbations in 3D Numerical Experiments Authors: Georgobiani, D.; Kuhn, J. R.; Stein, R. F. Bibcode: 1996AAS...188.6910G Altcode: 1996BAAS...28..937G Results on how the temperature distribution near the solar photosphere is altered by perturbing the entropy of fluid in the convection zone several megameters below the surface are presented. Effects on the emergent intensity and implications for helioseismic observations are described. Title: Using Eclipse Observations to Test Scintillation Models Authors: Georgobiani, D.; Kuhn, J. R.; Beckers, J. M. Bibcode: 1995SoPh..156....1G Altcode: Near second and third contact during a solar eclipse the spatial spectrum of the solar illumination changes as the relative power at high spatial frequencies increases strongly. Since groundlevel atmospheric scintillation depends on a weighted integral of the image power spectrum, we can expect to see a measureable time dependence to solar scintillation during an eclipse. This effect was observed during an annular solar eclipse and quantitatively compared with a scintillation model. Title: Solar corona photometry by electropolarimetric and CCD observations during the eclipse of July 11, 1991 Authors: Kulidzanishvili, V. I.; Georgobiani, D. G. Bibcode: 1995AN....316...23K Altcode: The observational data of the solar corona obtained during the solar eclipse of July 11, 1993, using both a electropolarimeter (EP) and a CCD matrix were processed. Using these data the photometry of the solar corona was carried out. The results of EP data were compared with those of the CCD data. It must be noticed here that the CCD data give us only the characteristics of the inner corona, while the EP data show the features of both the inner and the middle corona up to 4 solar radii. The standard flattening index epsilon was evaluated from both data. The dependence of epsilon on the distance from the solar limb was investigated. Three-dimensional images of the coronal intensity distribution for different spectral lines are shown. Isophotes in Na and Ca lines with unusual features are plotted. Based on these data some ideas and conclusions on the type of the solar corona and the physical conditions in it are presented. Title: July 11, 1991 Eclipse Corona Photometry by the Results of Electropolarimetric and CCD-Matrix Observations Authors: Kulidzaniskvili, V.; Georgobiani, D. Bibcode: 1994scs..conf..567K Altcode: 1994IAUCo.144..567K The observational data of July 11, 1991 eclipse solar corona obtained by both electropolarimeter (EP) and CCD-matrix were processed and compared. The dependence of the flattening index on the distance from the solar limb is investigated. The isophotes in Na and Ca lines are plotted. Some ideas and conclusions on the type of the solar corona are presented.