Publications (13)14.77 Total impact
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Article: Structure of a simple sunspot from the inversion of IR spectral data
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ABSTRACT: Analysis of spectral data of two neighboring infrared lines, Fe I 15648.5 Å (g = 3) and FeI 15652.9 Å (geff = 1.53) are carried out for a simple sunspot when it was near the solar disk center (μ = 0.92), to understand the basic structure of sunspot magnetic field. Inversions of Stokes profiles are carried out to derive different atmospheric parameters both as a function of location within the sunspot and height in the atmosphere. As a result of the inversion we have obtained maps of magnetic field strength, temperature, line-of-sight velocity, field inclination and azimuth for different optical depth layers between log(τ5) = 0 and log(τ5) = –2.0. In this paper we present few results from our inversion for a layer averaged between log(τ5) from 0.0 to –0.5.Astronomische Nachrichten 05/2003; 324(4):388 - 389. · 1.01 Impact Factor -
Article: Empirical models of solar magnetic flux-tubes and their non-magnetic surroundings
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ABSTRACT: A powerful method for the analysis of the structure of small scale magnetic elements in the solar photosphere is the inversion of Stokes spectra. In previous papers based on such inversions Bellot Rubio et al. (1997, 1999) and Frutiger et al. (1999) have argued in favor of models with rather different dynamic properties. In this paper we return to this debate and compare results returned by inversions based on new multi-component models applied to several Fe i, Fe ii and C i spectral line profiles obtained in active region plage with a Fourier Transform Spectrometer. These inversions differ from earlier ones by the fact that mass conservation is strictly imposed both inside the magnetic elements and on the surrounding external flow field. These flux-tube models are not only able to reproduce the characteristic Stokes V asymmetries and line-shifts observed in active regions plages or network elements, but also the Stokes I line profiles, including line bisectors. It is confirmed that from the quality of the fits alone it is not possible to distinguish between the steady flow proposed by Bellot Rubio et al. (1997) and the oscillatory model of Frutiger & Solanki (1998). If, however, physical constraints are imposed (e.g. mass conservation or that the flow retains the same direction over height in the flux tube) then the oscillatory model is found to be superior. In addition, the current investigation also provides the first inversion-based model of abnormal granulation.A&A. 01/2001; 369:646-659. -
Article: The Molecular Zeeman Effect and Solar Magnetic Fields
12/2000; 236:551. -
Article: Successful spectral synthesis of Zeeman-split molecular bands in sunspot spectra
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ABSTRACT: We present the first spectral synthesis of Zeeman-split Stokes profiles of the MgH A2Pi -X2BLAigma green system and TiO gamma -system. The calculations involve different regimes of the molecular Zeeman effect, up to the complete Paschen-Back effect for individual lines. The synthetic spectra are compared with observations of Stokes I and V in sunspot umbrae. We find that although the Stokes I spectra are reasonably reproduced, some lines are obviously still missing from the employed line lists. The Stokes V spectra turn out to be much cleaner since the missing lines do not appear to be Zeeman-split. We thus provide the first good fit to Zeeman-split molecular lines, including profiles with unconventional Stokes V shapes, determined by the Paschen-Back effect. Based on observations from the Canada-France-Hawaii Telescope operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique de France and the University of HawaiiAstronomy and Astrophysics 11/2000; 364:L101-L104. · 4.59 Impact Factor -
Article: Properties of the solar granulation obtained from the inversion of low spatial resolution spectra
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ABSTRACT: The spectra of cool stars are rich in information on elemental abundances, convection and non-thermal heating. Extracting this information is by no means straightforward, however. Here we demonstrate that an inversion technique may not only provide the stratification of the classical parameters describing a model atmosphere, but can also determine the properties of convection at the stellar surface. The inversion technique is applied to spectra of photospheric lines, one recorded at the quiet solar disk center, the other integrated over the whole disk. We find that a model based on a single plane-parallel atmosphere gives unsatisfactory fits to the spectral lines and suffers from considerable uncertainties in the derived temperature stratification. Also, the elemental abundances returned by the inversion are not particularly reliable. These problems are greatly reduced if two atmospheric components, corresponding to granular up- and downflows are allowed for. The best results are obtained if the line profiles and bisectors of a neutral and ionized species are fit and the results are constrained using a simple mass conservation scheme. We find that inversions based on two- and three-component models of disk-integrated spectra give similar results to inversions of disk-center observations, although with somewhat lower accuracy. This similarity is promising for future applications of line profile inversions to the study of late-type stars and in particular their convection.Astronomy and Astrophysics 05/2000; 358:1109-1121. · 4.59 Impact Factor -
Article: Do solar magnetic elements harbor downflows?
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ABSTRACT: In a recent paper Bellot Rubio et al. (1997) inverted Zeeman split Stokes profiles to infer the stratification of the temperature, velocity and magnetic field in the photospheric layers of solar magnetic elements (modeled as thin flux tubes). One controversial result of their inversions is the presence of a strong downflow within the flux tubes. In the model underlying their inversion such a downflow is necessary to reproduce the asymmetric shape of the observed V profiles. We present inversions based on two different flux-tube models, both of which reproduce the Stokes I and V profiles obtained in plages and the network with high accuracy, including the V profile asymmetry. One model is almost identical to that employed by Bellot Rubio et al. (1997), and results in a significant downflow within the flux tube. The other, although similar in most respects, has mass conservation enforced inside the flux tubes, i.e. they contain both an upflow and a downflow which could arise from oscillations or siphon flows. Hence, current data may not be sufficiently sensitive to distinguish between the two velocity structures, so that there is no compelling evidence for a net downflow of matter inside magnetic elements. From a physical point of view the model incorporating mass conservation is to be preferred.Astronomy and Astrophysics 07/1998; 336:L65-L68. · 4.59 Impact Factor -
Article: Thermal-magnetic relation of a sunspot as inferred from inversion of 1.5 μm spectral data
Sawaya Lacoste, H.: SOLMAG 2002, Proceedings of the Magnetic Coupling of the Solar Atmosphere, Euroconference and IAU Colloquium 188,, ESA Publ. Div., 501-504 (2002). -
Article: Therma-magnetic relation of a sunspot from the inversion of IR spectral data
Strassmeier, K. G.; Washuettl, A.: 1st Potsdam Thinkshop on Sunspots and Starspots Poster Proceedings, Brandenburgische Universitätsdruckerei, 117-118 (2002). -
Article: The molecular Zeeman effect and diagnostics of solar and stellar magnetic fields: II. Synthetic Stokes profiles in the Zeeman regime
Astronomy and Astrophysics, v.412, 513-527 (2003). -
Article: Structure of a simple sunspot from the inversion of IR spectral data
Astron. Nachr., v.324, 388-389 (2003). -
Article: The molecular Zeeman effect and diagnostics of solar and stellar magnetic fields
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ABSTRACT: Recent advances in the computation of the Zeeman splitting of molecular lines have paved the way for their use as diagnostics of solar and stellar magnetic fields. A systematic study of their diagnostic capabilities had not been carried out so far, however. Here we investigate how molecular lines can be used to deduce the magnetic and thermal structure of sunspots, starspots and cool stars. First, we briefly describe the Stokes radiative transfer of Zeeman-split molecular lines. Then, we compute Stokes spectra of TiO, OH, CH and FeH lines and investigate their diagnostic capabilities. We also compare the synthetic profiles with observations. Spectra of TiO, OH and FeH are found to be interesting diagnostics of sunspot magnetic fields. This is also true for cool stars, where, however, the OH Stokes $V$ profiles may require very high S/N data to be reliably employed. Finally we investigate the potential of various molecular bands for high-contrast imaging of the solar surface. The violet CN and CH bands turn out to be most promising for imaging the photosphere, the TiO bands are excellent for imaging sunspot umbrae, while the UV OH band can be used for imaging both the photosphere and sunspots.http://dx.doi.org/10.1051/0004-6361:20031473. -
Article: Three dimensional structure of a regular sunspot from the inversion of IR Stokes profiles
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ABSTRACT: The magnetic, thermal and velocity structure of a regular sunspot, observed close to solar disk center is presented. Spectropolarimetric data obtained with the Tenerife Infrared Polarimeter (TIP) in two infrared Fe I lines at 15 648.5 Åand 15 652.8 Åare inverted employing a technique based on response functions to retrieve the atmospheric stratification at every point in the sunspot. In order to improve the results for the umbra, profiles of Zeeman split OH lines blending the Fe I 15 652.8 Åare also consistently fit. Thus we obtain maps of temperature, line-of-sight velocity, magnetic field strength, inclination, and azimuth, as a function of both location within the sunspot and height in the atmosphere. We present these maps for an optical depth range between log $\tau _5$ = 0 and log $\tau _5 = -1.5$, where these lines provide accurate results. We find decreasing magnetic field strength with increasing height all over the sunspot, with a particularly large vertical field gradient of ~$-4$ G km$^{-1}$ in the umbra. We also observe the so called “spine” structures in the penumbra, i.e. extended radial features with a stronger and more vertical magnetic field than the surroundings. Also we found that the magnetic field zenith angle increases with height. From the velocity map it is clear that the Evershed flow avoids the spines and mostly concentrates in the more inclined intervening field. The field inclination at a few locations in the outer penumbra in lower layers goes beyond 90°. These locations coincide with the strongest flows in the velocity map.http://dx.doi.org/10.1051/0004-6361:20031282. -
Article: Modeling the Fine Structure of a Sunspot Penumbra through the Inversion of Stokes Profiles
Pestov, Alexei A.; Uitenbroek, Han: Current theoretial models and high resolution solar observations: preparing for ATST, 235-242 (2003).
