The Harvard-Smithsonian reference atmosphere

Gingerich, Owen; Noyes, R. W.; Kalkofen, W.; Cuny, Y.

doi:10.1007/bf00149057

Cited by 612 publications

(259 citation statements)

References 23 publications

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“…This has no impact on the current study because we do not make use of the retrieved information on the magnetic field parameters. We chose the Harvard Smithsonian Reference Atmosphere (HSRA; Gingerich et al 1971) as initial temperature model. We used only two nodes (≡variation of the initial model by linear gradients) for the temperature stratification.…”

Section: Inversion Setupmentioning

confidence: 99%

Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations

Beck

Fabbian

Moreno‐Insertis

et al. 2013

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Context. Numerical three-dimensional (3D) radiative (magneto-)hydrodynamical [(M)HD] simulations of solar convection are nowadays used to understand the physical properties of the solar photosphere and convective envelope, and, in particular, to determine the Sun's photospheric chemical abundances. To validate this approach, it is important to check that no excessive thermodynamic fluctuations arise as a consequence of the partially incomplete treatment of radiative transfer causing radiative damping that is too modest. Aims. We investigate the realism of the thermodynamics in recent state-of-the-art 3D convection simulations of the solar atmosphere carried out with the Stagger code.Methods. We compared the characteristic properties of several Fe i lines (557.6 nm, 630 nm, 1565 nm) and one Si i line at 1082.7 nm in solar disc-centre observations of different spatial resolution with spectra synthesized from 3D convection simulations. The observations were taken with ground-based (Echelle spectrograph, Göttingen Fabry-Pérot Interferometer (GFPI), POlarimetric LIttrow Spectrograph, Tenerife Infrared Polarimeter, all at the Vacuum Tower Telescope on Tenerife) and space-based instruments (Hinode/Spectropolarimeter). We degraded the synthetic spectra to the spatial resolution of the observations, based on the distribution of the continuum intensity I c . We estimated the spectral degradation to be applied to the simulation results by comparing atlas spectra with averaged observed spectra. In addition to deriving a set of line parameters directly from the intensity profiles, we used the SIR (Stokes Inversion based on Response functions) code to invert the spectra. Results. The spatial degradation kernels yield a similar generic spatial stray-light contamination of about 30% for all instruments. The spectral stray light inside the different spectrometers is found to be between 2% and 20%. Most of the line parameters from the observational data are matched by the degraded HD simulation spectra. The inversions predict a macroturbulent velocity v mac below 10 m s −1 for the HD simulation spectra at full spatial resolution, whereas they yield v mac 1000 m s −1 at a spatial resolution of 0. 3. The temperature fluctuations in the inversion of the degraded HD simulation spectra do not exceed those from the observational data (of the order of 100−200 K rms for −2 log τ 500 nm −0.5). The comparison of line parameters in spatially averaged profiles with the averaged values of line parameters in spatially resolved profiles indicates a significant change in (average) line properties on a spatial scale between 0. 13 and 0. 3. Conclusions. Up to a spatial resolution of 0. 3 (GFPI spectra), we find no indications of excessive thermodynamic fluctuations in the 3D HD simulation. To definitely confirm that simulations without spatial degradation contain fully realistic thermodynamic fluctuations requires observations at even higher spatial resolution (i.e. <0. 13).

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Section: Inversion Setupmentioning

confidence: 99%

Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations

Beck

Fabbian

Moreno‐Insertis

et al. 2013

A&A

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“…The magnetic field strength of the two atmospheres is assumed to be constant with optical depth. The temperature stratifications are taken to be that of the quiet sun (represented by the Harvard Smithsonian Reference Atmosphere of Gingerich et al 1971). A fraction α of the resolution element is magnetized, with the first component contributing a fraction β and the second the remaining 1 − β.…”

Section: Simulation Of Stokes Profilesmentioning

confidence: 99%

Understanding internetwork magnetic fields as determined from visible and infrared spectral lines

Rubio

Collados

2003

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Abstract. We present numerical experiments aimed at understanding why near-infrared observations systematically deliver weak magnetic fields in the internetwork, whereas analyses based on visible lines indicate that kG fields are ubiquitous. Synthetic noisy Stokes V profiles of the iron lines at 6302 Å and 1.565 µm have been produced under varying conditions in an effort to simulate polarized spectra coming from the internetwork. An inversion technique has been applied to the profiles, as it is usually done with real observations, in order to derive the distribution of magnetic fields in the simulated region. Our results show that infrared lines yield distributions which are very similar to those used as input for the simulation, while visible lines are to a large extent affected by noise. Analyses based on the Fe I lines at 6302 Å may lead to an overabundance of kG fields if the signal-to-noise ratio in Stokes V is poorer than about 10. A particular example is shown where strong fields are retrieved in nearly 30% of the pixels of a simulated internetwork region in which only fields of 200 G exist.

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“…As noted above, the inversion is made differently depending on whether the spatial pixel to invert is magnetic or not. Magnetic pixels are defined as those that exhibit polarization signals above the 3σ noise level in any one of the Stokes profiles Q, U or V. In both cases the model is initialized with the Harvard-Smithsonian Reference Atmosphere (HSRA) as a starting guess (Gingerich et al 1971) and then iterated in two successive cycles. The number of nodes used in the inversions for the various physical parameters is listed in Table 2.…”

Section: Observations and The Modelmentioning

confidence: 99%

The solar oxygen abundance from an empirical three-dimensional model

Socas‐Navarro

2015

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The oxygen abundance in the solar photosphere, and consequently the solar metallicity itself, is still a controversial question with farreaching implications in many areas of astrophysics. This paper presents a new determination obtained by fitting the forbidden O i line at 6300 Å with an observational 3D model. The approach presented here is novel because previous determinations were based either on 1D empirical stratifications or on 3D theoretical models. The resulting best-fit abundances are log (O) = 8.90 and log (Ni) = 6.15. Nevertheless, by introducing minor tweaks in the model and the procedure, it is possible to retrieve very different values, even down to log (O) = 8.70. This extreme sensitivity of the abundance to possible systematic effects is not specific to this particular work, but probably reflects the real uncertainty inherent to all abundance determinations based on a prescribed model atmosphere.

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The Harvard-Smithsonian reference atmosphere

Cited by 612 publications

References 23 publications

Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations

Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations

Understanding internetwork magnetic fields as determined from visible and infrared spectral lines

The solar oxygen abundance from an empirical three-dimensional model

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