Author name code: trujillo-bueno ADS astronomy entries on 2022-09-14 author:"Trujillo Bueno, Javier" ------------------------------------------------------------------------ Title: The transfer of polarized radiation in resonance lines with partial frequency redistribution, J-state interference, and arbitrary magnetic fields. A radiative transfer code and useful approximations Authors: Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J. Bibcode: 2022A&A...664A..76A Altcode: 2022arXiv220412523A
Aims: We present the theoretical framework and numerical methods we have implemented to solve the problem of the generation and transfer of polarized radiation in spectral lines without assuming local thermodynamical equilibrium, while accounting for scattering polarization, partial frequency redistribution (due to both the Doppler effect and elastic collisions), J-state interference, and hyperfine structure. The resulting radiative transfer code allows one to model the impact of magnetic fields of an arbitrary strength and orientation through the Hanle, incomplete Paschen-Back, and magneto-optical effects. We also evaluate the suitability of a series of approximations for modeling the scattering polarization in the wings of strong resonance lines at a much lower computational cost, which is particularly valuable for the numerically intensive case of three-dimensional radiative transfer.
Methods: We examine the suitability of the considered approximations by using our radiative transfer code to model the Stokes profiles of the Mg II h & k lines and of the H I Lyman-α line in magnetized one-dimensional models of the solar atmosphere.
Results: Neglecting Doppler redistribution in the scattering processes that are unperturbed by elastic collisions (i.e., treating them as coherent in the observer's frame) produces a negligible error in the scattering polarization wings of the Mg II resonance lines and a minor one in the Lyman-α wings, although it is unsuitable to model the cores of these lines. For both lines, the scattering processes that are perturbed by elastic collisions only give a significant contribution to the intensity component of the emissivity. Neglecting collisional as well as Doppler redistribution (so that all scattering processes are coherent) represents a rough but suitable approximation for the wings of the Mg II resonance lines, but a very poor one for the Lyman-α wings. The magnetic sensitivity in the scattering polarization wings of the considered lines can be modeled by accounting for the magnetic field in only the ηI and ρV coefficients of the Stokes-vector transfer equation (i.e., using the zero-field expression for the emissivity). Title: TIC: A Stokes Inversion Code for Scattering Polarization with Partial Frequency Redistribution and Arbitrary Magnetic Fields Authors: Li, H.; del Pino Alemán, T.; Trujillo Bueno, J.; Casini, R. Bibcode: 2022ApJ...933..145L Altcode: 2022arXiv220515666L We present the Tenerife Inversion Code (TIC), which has been developed to infer the magnetic and plasma properties of the solar chromosphere and transition region via full Stokes inversion of polarized spectral lines. The code is based on the HanleRT forward engine, which takes into account many of the physical mechanisms that are critical for a proper modeling of the Stokes profiles of spectral lines originating in the tenuous and highly dynamic plasmas of the chromosphere and transition region: the scattering polarization produced by quantum level imbalance and interference (atomic polarization), the effects of frequency coherence in polarized resonance scattering (partial redistribution), and the impact of arbitrary magnetic fields on the atomic polarization and the radiation field. We present first results of atmospheric and magnetic inversions, and discuss future developments for the project. Title: CASPER: A mission to study the time-dependent evolution of the magnetic solar chromosphere and transition regions Authors: Orozco Suárez, D.; del Toro Iniesta, J. C.; Bailén, F. J.; López Jiménez, A.; Balaguez Jiménez, M.; Bellot Rubio, L. R.; Ishikawa, R.; Katsukawa, Y.; Kano, R.; Shimizu, T.; Trujillo Bueno, J.; Asensio Ramos, A.; del Pino Alemán, T. Bibcode: 2022ExA...tmp...26O Altcode: Our knowledge about the solar chromosphere and transition region (TR) has increased in the last decade thanks to the huge scientific return of space-borne observatories like SDO, IRIS, and Hinode, and suborbital rocket experiments like CLASP1, CLASP2, and Hi-C. However, the magnetic nature of those solar regions remain barely explored. The chromosphere and TR of the Sun harbor weak fields and are in a low ionization stage both having critical effects on their thermodynamic behavior. Relatively cold gas structures, such as spicules and prominences, are located in these two regions and display a dynamic evolution in high-resolution observations that static and instantaneous 3D-magnetohydrodynamic (MHD) models are not able to reproduce. The role of the chromosphere and TR as the necessary path to a (largely unexplained) very hot corona calls for the generation of observationally based, time-dependent models of these two layers that include essential, up to now disregarded, ingredients in the modeling such as the vector magnetic field. We believe that the community is convinced that the origin of both the heat and kinetic energy observed in the upper layers of the solar atmosphere is of magnetic origin, but reliable magnetic field measurements are missing. The access to sensitive polarimetric measurements in the ultraviolet wavelengths has been elusive until recently due to limitations in the available technology. We propose a low-risk and high-Technology Readiness Level (TRL) mission to explore the magnetism and dynamics of the solar chromosphere and TR. The mission baseline is a low-Earth, Sun-synchronous orbit at an altitude between 600 and 800 km. The proposed scientific payload consists of a 30 cm aperture telescope with a spectropolarimeter covering the hydrogen Ly-alpha and the Mg II h&k ultraviolet lines. The instrument shall record high-cadence, full spectropolarimetric observations of the solar upper atmosphere. Besides the answers to a fundamental solar problem the mission has a broader scientific return. For example, the time-dependent modeling of the chromospheres of stars harboring exoplanets is fundamental for estimating the planetary radiation environment. The mission is based on technologies that are mature enough for space and will provide scientific measurements that are not available by other means. Title: Spectropolarimetric observations of the solar atmosphere in the Hα 6563 Å line Authors: Jaume Bestard, J.; Trujillo Bueno, J.; Bianda, M.; Štěpán, J.; Ramelli, R. Bibcode: 2022A&A...659A.179J Altcode: 2022arXiv220103815J We present novel spectropolarimetric observations of the hydrogen Hα line taken with the Zürich Imaging Polarimeter (ZIMPOL) at the Gregory Coudé Telescope of the Istituto Ricerche Solari Locarno (IRSOL). The linear polarization is clearly dominated by the scattering of anisotropic radiation and the Hanle effect, while the circular polarization is dominated by the Zeeman effect. The observed linear polarization signals show a rich spatial variability, the interpretation of which would open a new window for probing the solar chromosphere. We study their spatial variation within coronal holes, finding a different behaviour for the U/I signals near the north and south solar poles. We identify some spatial patterns, which may facilitate the interpretation of the observations. In close-to-the-limb regions with sizable circular polarization signals, we find similar asymmetric Q/I profiles. We also show examples of net circular polarization profiles (NCP), along with the corresponding linear polarization signals. The application of the weak field approximation to the observed circular polarization signals gives 10 G (40-60 G) close to the limb quiet (plage) regions for the average longitudinal field strength over the spatio-temporal resolution element. Title: Novel framework for the three-dimensional NLTE inverse problem Authors: Štěpán, Jiří; del Pino Alemán, Tanausú; Trujillo Bueno, Javier Bibcode: 2022A&A...659A.137S Altcode: 2022arXiv220101504S The inversion of spectropolarimetric observations of the solar upper atmosphere is one of the most challenging goals in solar physics. If we account for all relevant ingredients of the spectral line formation process, such as the three-dimensional (3D) radiative transfer out of local thermodynamic equilibrium (NLTE), the task becomes extremely computationally expensive. Instead of generalizing 1D methods to 3D, we have developed a new approach to the inverse problem. In our meshfree method, we do not consider the requirement of 3D NLTE consistency as an obstacle, but as a natural regularization with respect to the traditional pixel-by-pixel methods. This leads to more robust and less ambiguous solutions. We solve the 3D NLTE inverse problem as an unconstrained global minimization problem that avoids repetitive evaluations of the Λ operator. Apart from the 3D NLTE consistency, the method allows us to easily include additional conditions of physical consistency such as the zero divergence of the magnetic field. Stochastic ingredients make the method less prone to ending up within the local minima of the loss function. Our method is capable of solving the inverse problem faster by several orders of magnitude than by using grid-based methods. The method can provide accurate and physically consistent results if sufficient computing time is available, along with approximate solutions in the case of very complex plasma structures or limited computing time. Title: The polarization angle in the wings of Ca I 4227: A new observable for diagnosing unresolved photospheric magnetic fields Authors: Capozzi, Emilia; Alsina Ballester, Ernest; Belluzzi, Luca; Trujillo Bueno, Javier Bibcode: 2022A&A...657A..44C Altcode: 2021arXiv211108967C Context. When observed in quiet regions close to the solar limb, many strong resonance lines show conspicuous linear polarization signals, produced by scattering processes (i.e., scattering polarization), with extended wing lobes. Recent studies indicate that, contrary to what was previously believed, the wing lobes are sensitive to the presence of relatively weak longitudinal magnetic fields through magneto-optical (MO) effects.
Aims: We theoretically investigate the sensitivity of the scattering polarization wings of the Ca I 4227 Å line to the MO effects, and we explore its diagnostic potential for inferring information on the longitudinal component of the photospheric magnetic field.
Methods: We calculate the intensity and polarization profiles of the Ca I 4227 Å line by numerically solving the problem of the generation and transfer of polarized radiation under non-local thermodynamic equilibrium conditions in one-dimensional semi-empirical models of the solar atmosphere, taking into account the joint action of the Hanle, Zeeman, and MO effects. We consider volume-filling magnetic fields as well as magnetic fields occupying a fraction of the resolution element.
Results: In contrast to the circular polarization signals produced by the Zeeman effect, we find that the linear polarization angle in the scattering polarization wings of Ca I 4227 presents a clear sensitivity, through MO effects, not only to the flux of the photospheric magnetic field, but also to the fraction of the resolution element that the magnetic field occupies.
Conclusions: We identify the linear polarization angle in the wings of strong resonance lines as a valuable observable for diagnosing unresolved magnetic fields. Used in combination with observables that encode information on the magnetic flux and other properties of the observed atmospheric region (e.g., temperature and density), it can provide constraints on the filling factor of the magnetic field. Title: Empirical relations between the intensities of Lyman lines of H and He+ Authors: Gordino, M.; Auchère, F.; Vial, J. -C.; Bocchialini, K.; Hassler, D. M.; Bando, T.; Ishikawa, R.; Kano, R.; Kobayashi, K.; Narukage, N.; Trujillo Bueno, J.; Winebarger, A. Bibcode: 2022A&A...657A..86G Altcode: 2022arXiv220101519G Context. Empirical relations between major UV and extreme UV spectral lines are one of the inputs for models of chromospheric and coronal spectral radiances and irradiances. They are also needed for the interpretation of some of the observations of the Solar Orbiter mission.
Aims: We aim to determine an empirical relation between the intensities of the H I 121.6 nm and He II 30.4 nm Ly-α lines.
Methods: Images at 121.6 nm from the Chromospheric Lyman-Alpha Spectro Polarimeter (CLASP) and Multiple XUV Imager (MXUVI) sounding rockets were co-registered with simultaneous images at 30.4 nm from the EIT and AIA orbital telescopes in order to derive a spatially resolved relationship between the intensities.
Results: We have obtained a relationship between the H I 121.6 nm and He II 30.4 nm intensities that is valid for a wide range of solar features, intensities, and activity levels. Additional SUMER data have allowed the derivation of another relation between the H I 102.5 nm (Ly-β) and He II 30.4 nm lines for quiet-Sun regions. We combined these two relationships to obtain a Ly-α/Ly-β intensity ratio that is comparable to the few previously published results.
Conclusions: The relationship between the H I 121.6 nm and He II 30.4 nm lines is consistent with the one previously obtained using irradiance data. We have also observed that this relation is stable in time but that its accuracy depends on the spatial resolution of the observations. The derived Ly-α/Ly-β intensity ratio is also compatible with previous results. Title: Demonstration of Chromospheric Magnetic Mapping with CLASP2.1 Authors: McKenzie, David; Ishikawa, Ryohko; Trujillo Bueno, Javier; Auchere, F.; Kobayashi, Ken; Winebarger, Amy; Kano, Ryouhei; Song, Donguk; Okamoto, Joten; Rachmeler, Laurel; De Pontieu, Bart; Vigil, Genevieve; Belluzzi, Luca; Alsina Ballester, Ernest; del Pino Aleman, Tanausu; Bethge, Christian; Sakao, Taro; Stepan, Jiri Bibcode: 2021AGUFMSH52A..06M Altcode: Probing the magnetic nature of the Suns atmosphere requires measurement of the Stokes I, Q, U and V profiles of relevant spectral lines (of which Q, U and V encode the magnetic field information). Many of the magnetically sensitive lines formed in the chromosphere and transition region are in the ultraviolet spectrum, necessitating observations above the absorbing terrestrial atmosphere. The Chromospheric Layer Spectro-Polarimeter (CLASP2) sounding rocket was flown successfully in April 2019, as a follow-on to the successful flight in September 2015 of the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP). Both projects were funded by NASAs Heliophysics Technology and Instrument Development for Science (H-TIDeS) program to develop and test a technique for observing the Sun in ultraviolet light, and for quantifying the polarization of that light. By demonstrating successful measurement and interpretation of the polarization in hydrogen Lyman-alpha and the Mg II h and k spectral lines, the CLASP and CLASP2 missions are vital first steps towards routine quantitative characterization of the local thermal and magnetic conditions in the solar chromosphere. In October of 2021, we re-flew the CLASP2 payload with a modified observing program to further demonstrate the maturity of the UV spectropolarimetry techniques, and readiness for development into a satellite observatory. During the reflight, called CLASP2.1, the spectrograph slit was scanned across an active region plage to acquire a two-dimensional map of Stokes V/I, to demonstrate the ability of UV spectropolarimetry to yield chromospheric magnetic fields over a large area. This presentation will display preliminary results from the flight of CLASP2.1. Title: Polarization of the Lyα Lines of H I and He II as a Tool for Exploring the Solar Corona Authors: Hebbur Dayananda, Supriya; Trujillo Bueno, Javier; de Vicente, Ángel; del Pino Alemán, Tanausú Bibcode: 2021ApJ...920..140H Altcode: 2021arXiv210701015H; 2021ApJ...920..140S The near-Earth space weather is driven by the quick release of magnetic free energy in the solar corona. Probing this extremely hot and rarified region of the extended solar atmosphere requires modeling the polarization of forbidden and permitted coronal lines. To this end, it is important to develop efficient codes to calculate the Stokes profiles that emerge from given three-dimensional (3D) coronal models and this should be done taking into account the symmetry breaking produced by the presence of magnetic fields and non-radial solar wind velocities. We have developed such a tool with the aim of theoretically predicting and interpreting spectropolarimetric observations of the solar corona in permitted and forbidden lines. In this paper, we show the results of a theoretical investigation of the linear polarization signals produced by scattering processes in the H I Lyα line at 1216 Å and in the He II Lyα line at 304 Å using 3D coronal models by Predictive Science Inc. These spectral lines have very different critical magnetic fields for the onset of the Hanle effect (53 G and 850 G, respectively), as well as different sensitivities to the Doppler effect caused by the solar wind velocities. We study under which circumstances simultaneous observations of the scattering polarization in these Lyα lines can facilitate the determination of magnetic fields and macroscopic velocities in the solar corona. Title: Solving the Paradox of the Solar Sodium D1 Line Polarization Authors: Alsina Ballester, Ernest; Belluzzi, Luca; Trujillo Bueno, Javier Bibcode: 2021PhRvL.127h1101A Altcode: 2021arXiv210808334A Twenty-five years ago, enigmatic linear polarization signals were discovered in the core of the sodium D1 line. The only explanation that could be found implied that the solar chromosphere is practically unmagnetized, in contradiction with other evidences. This opened a paradox that has challenged physicists for many years. Here we present its solution, demonstrating that these polarization signals can be properly explained in the presence of magnetic fields in the gauss range. This result opens a novel diagnostic window for exploring the elusive magnetism of the solar chromosphere. Title: PORTA: POlarized Radiative TrAnsfer Authors: Stěpán, Jiří.; Trujillo Bueno, Javier Bibcode: 2021ascl.soft06037S Altcode: PORTA solves three-dimensional non-equilibrium radiative transfer problems with massively parallel computers. The code can be used for modeling the spectral line polarization produced by the scattering of anisotropic radiation and the Hanle and Zeeman effects assuming complete frequency redistribution, either using two-level or multilevel atomic models. The numerical method of solution used to find the self-consistent values of the atomic density matrix at each point of the model's Cartesian grid is based on Jacobi iterative scheme and on a short-characteristics formal solver of the Stokes-vector transfer equation that uses monotonic Bézier interpolation. The code can also be used to compute the linear polarization of the continuum radiation caused by Rayleigh and Thomson scattering in 3D models of stellar atmospheres, and to solve the simpler 3D radiative transfer problem of unpolarized radiation in multilevel systems. PORTA accepts/produces HDF5 input/output and offers an advanced graphical user interface. Title: Mapping of Solar Magnetic Fields from the Photosphere to the Top of the Chromosphere with CLASP2 Authors: McKenzie, D.; Ishikawa, R.; Trujillo Bueno, J.; Auchere, F.; del Pino Aleman, T.; Okamoto, T.; Kano, R.; Song, D.; Yoshida, M.; Rachmeler, L.; Kobayashi, K.; Narukage, N.; Kubo, M.; Ishikawa, S.; Hara, H.; Suematsu, Y.; Sakao, T.; Bethge, C.; De Pontieu, B.; Vigil, G.; Winebarger, A.; Alsina Ballester, E.; Belluzzi, L.; Stepan, J.; Asensio Ramos, A.; Carlsson, M.; Leenaarts, J. Bibcode: 2021AAS...23810603M Altcode: Coronal heating, chromospheric heating, and the heating & acceleration of the solar wind, are well-known problems in solar physics. Additionally, knowledge of the magnetic energy that powers solar flares and coronal mass ejections, important drivers of space weather, is handicapped by imperfect determination of the magnetic field in the sun's atmosphere. Extrapolation of photospheric magnetic measurements into the corona is fraught with difficulties and uncertainties, partly due to the vastly different plasma beta between the photosphere and the corona. Better results in understanding the coronal magnetic field should be derived from measurements of the magnetic field in the chromosphere. To that end, we are pursuing quantitative determination of the magnetic field in the chromosphere, where plasma beta transitions from greater than unity to less than unity, via ultraviolet spectropolarimetry. The CLASP2 mission, flown on a sounding rocket in April 2019, succeeded in measuring all four Stokes polarization parameters in UV spectral lines formed by singly ionized Magnesium and neutral Manganese. Because these ions produce spectral lines under different conditions, CLASP2 thus was able to quantify the magnetic field properties at multiple heights in the chromosphere simultaneously, as shown in the recent paper by Ishikawa et al. In this presentation we will report the findings of CLASP2, demonstrating the variation of magnetic fields along a track on the solar surface and as a function of height in the chromosphere; and we will illustrate what is next for the CLASP missions and the demonstration of UV spectropolarimetry in the solar chromosphere. Title: Long Characteristics versus Short Characteristics in 3D Radiative Transfer Simulations of Polarized Radiation Authors: de Vicente, A.; del Pino Alemán, T.; Trujillo Bueno, J. Bibcode: 2021ApJ...912...63D Altcode: 2021arXiv210304065D We compare maps of scattering polarization signals obtained from three-dimensional (3D) radiation transfer calculations in a magnetoconvection model of the solar atmosphere using formal solvers based on the "short characteristics" (SC) and the "long characteristics" (LC) methods. The SC method requires less computational work, but it is known to introduce spatial blurring in the emergent radiation for inclined lines of sight. For polarized radiation this effect is generally more severe due to it being a signed quantity and to the sensitivity of the scattering polarization to the model's inhomogeneities. We study the differences in the polarization signals of the emergent spectral line radiation calculated with such formal solvers. We take as a case study already published results of the scattering polarization in the Sr I 4607 Å line obtained with the SC method, demonstrating that in high-resolution grids it is accurate enough for that type of study. In general, the LC method is the preferred one for accurate calculations of the emergent radiation, which is the reason why it is now one of the options in the public version of the 3D radiative transfer code PORTA. Title: Evaluating the Reliability of a Simple Method to Map the Magnetic Field Azimuth in the Solar Chromosphere Authors: Jurčák, Jan; Štěpán, Jiří; Trujillo Bueno, Javier Bibcode: 2021ApJ...911...23J Altcode: 2021arXiv210202880J The Zeeman effect is of limited utility for probing the magnetism of the quiet solar chromosphere. The Hanle effect in some spectral lines is sensitive to such magnetism, but the interpretation of the scattering polarization signals requires taking into account that the chromospheric plasma is highly inhomogeneous and dynamic (i.e., that the magnetic field is not the only cause of symmetry breaking). Here we investigate the reliability of a well-known formula for mapping the azimuth of chromospheric magnetic fields directly from the scattering polarization observed in the Ca II 8542 Å line, which is typically in the saturation regime of the Hanle effect. To this end, we use the Stokes profiles of the Ca II 8542 Å line computed with the PORTA radiative transfer code in a three-dimensional (3D) model of the solar chromosphere, degrading them to mimic spectropolarimetric observations for a range of telescope apertures and noise levels. The simulated observations are used to obtain the magnetic field azimuth at each point of the field of view, which we compare with the actual values within the 3D model. We show that, apart from intrinsic ambiguities, the method provides solid results. Their accuracy depends more on the noise level than on the telescope diameter. Large-aperture solar telescopes, like DKIST and EST, are needed to achieve the required polarimetric sensitivity using reasonable exposure times. Title: The Impact of Limited Time Resolution on the Forward-scattering Polarization in the Solar Sr I 4607 Å Line Authors: del Pino Alemán, T.; Trujillo Bueno, J. Bibcode: 2021ApJ...909..180D Altcode: 2021ApJ...909..180P; 2021arXiv210108485D Theoretical investigations predicted that high spatiotemporal resolution observations in the Sr I 4607 Å line must show a conspicuous scattering polarization pattern at the solar disk center, which encodes information on the unresolved magnetism of the intergranular photospheric plasma. Here we present a study of the impact of limited time resolution on the observability of such forward-scattering (disk-center) polarization signals. Our investigation is based on three-dimensional radiative transfer calculations in a time-dependent magnetoconvection model of the quiet solar photosphere, taking into account anisotropic radiation pumping and the Hanle effect. This type of radiative transfer simulation is computationally costly, which is why the time variation had not been investigated before for this spectral line. We compare our theoretical results with recent disk-center filter polarimetric observations in the Sr I 4607 Å line, showing that there is good agreement in the polarization patterns. We also show what we can expect to observe with the Visible Spectro-Polarimeter at the upcoming Daniel K. Inouye Solar Telescope. Title: The Effects of Three-dimensional Radiative Transfer on the Resonance Polarization of the Ca I 4227 Å Line Authors: Jaume Bestard, J.; Trujillo Bueno, J.; Štěpán, J.; del Pino Alemán, T. Bibcode: 2021ApJ...909..183J Altcode: 2021arXiv210104421J The sizable linear polarization signals produced by the scattering of anisotropic radiation in the core of the Ca I 4227 Å line constitute an important observable for probing the inhomogeneous and dynamic plasma of the lower solar chromosphere. Here we show the results of a three-dimensional (3D) radiative transfer complete frequency redistribution investigation of the line's scattering polarization in a magnetohydrodynamical 3D model of the solar atmosphere. We take into account not only the Hanle effect produced by the model's magnetic field but also the symmetry breaking caused by the horizontal inhomogeneities and macroscopic velocity gradients. The spatial gradients of the horizontal components of the macroscopic velocities produce very significant forward scattering polarization signals without the need of magnetic fields, while the Hanle effect tends to depolarize them at the locations where the model's magnetic field is stronger than about 5 G. The standard 1.5D approximation is found to be unsuitable for understanding the line's scattering polarization, but we introduce a novel improvement to this approximation that produces results in qualitative agreement with the full 3D results. The instrumental degradation of the calculated polarization signals is also investigated, showing what we can expect to observe with the Visible Spectro-Polarimeter at the upcoming Daniel K. Inouye Solar Telescope. Title: Magnetic Imaging of the Outer Solar Atmosphere (MImOSA): Unlocking the driver of the dynamics in the upper solar atmosphere Authors: Peter, H.; Alsina Ballester, E.; Andretta, V.; Auchere, F.; Belluzzi, L.; Bemporad, A.; Berghmans, D.; Buchlin, E.; Calcines, A.; Chitta, L. P.; Dalmasse, K.; del Pino Aleman, T.; Feller, A.; Froment, C.; Harrison, R.; Janvier, M.; Matthews, S.; Parenti, S.; Przybylski, D.; Solanki, S. K.; Stepan, J.; Teriaca, L.; Trujillo Bueno, J. Bibcode: 2021arXiv210101566P Altcode: The magnetic activity of the Sun directly impacts the Earth and human life. Likewise, other stars will have an impact on the habitability of planets orbiting these host stars. The lack of information on the magnetic field in the higher atmospheric layers hampers our progress in understanding solar magnetic activity. Overcoming this limitation would allow us to address four paramount long-standing questions: (1) How does the magnetic field couple the different layers of the atmosphere, and how does it transport energy? (2) How does the magnetic field structure, drive and interact with the plasma in the chromosphere and upper atmosphere? (3) How does the magnetic field destabilise the outer solar atmosphere and thus affect the interplanetary environment? (4) How do magnetic processes accelerate particles to high energies? New ground-breaking observations are needed to address these science questions. We suggest a suite of three instruments that far exceed current capabilities in terms of spatial resolution, light-gathering power, and polarimetric performance: (a) A large-aperture UV-to-IR telescope of the 1-3 m class aimed mainly to measure the magnetic field in the chromosphere by combining high spatial resolution and high sensitivity. (b) An extreme-UV-to-IR coronagraph that is designed to measure the large-scale magnetic field in the corona with an aperture of about 40 cm. (c) An extreme-UV imaging polarimeter based on a 30 cm telescope that combines high throughput in the extreme UV with polarimetry to connect the magnetic measurements of the other two instruments. This mission to measure the magnetic field will unlock the driver of the dynamics in the outer solar atmosphere and thereby greatly advance our understanding of the Sun and the heliosphere. Title: Improved near optimal angular quadratures for polarised radiative transfer in 3D MHD models Authors: Jaume Bestard, Jaume; Štěpán, Jiří; Trujillo Bueno, Javier Bibcode: 2021A&A...645A.101J Altcode: 2020arXiv201204981J Accurate angular quadratures are crucial for the numerical solution of three-dimensional (3D) radiative transfer problems, especially when the spectral line polarisation produced by the scattering of anisotropic radiation is included. There are two requirements for obtaining an optimal quadrature and they are difficult to satisfy simultaneously: high accuracy and short computing time. By imposing certain symmetries, we were recently able to derive a set of near optimal angular quadratures. Here, we extend our previous investigation by considering other symmetries. Moreover, we test the performance of our new quadratures by numerically solving a radiative transfer problem of resonance line polarisation in a 3D model of the solar atmosphere resulting from a magneto-hydrodynamical simulation. The new angular quadratures derived here outperform the previous ones in terms of the number of rays needed to achieve any given accuracy.

The tables mentioned in Sect. 4 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/645/A101 Title: Optical design of the Chromospheric LAyer Spectro-Polarimeter (CLASP2) Authors: Tsuzuki, Toshihiro; Ishikawa, Ryohko; Kano, Ryouhei; Narukage, Noriyuki; Song, Donguk; Yoshida, Masaki; Uraguchi, Fumihiro; Okamoto, Takenori J.; McKenzie, David; Kobayashi, Ken; Rachmeler, Laurel; Auchere, Frederic; Trujillo Bueno, Javier Bibcode: 2020SPIE11444E..6WT Altcode: Chromospheric LAyer Spectro-Polarimeter (CLASP2) was a sounding rocket experiment, which is a follow-up mission to the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP1) in 2015. To measure the magnetic fields in the upper solar atmosphere in a highly quantitative manner, CLASP2 changes the target wavelengths from the hydrogen Ly-α line (121.567 nm) to Mg II lines near 280 nm. We reused the main structure and most of the optical components in the CLASP1 instrument, which reduced the turnaround time and cost. We added a magnifying optical system to maintain the wavelength resolution, even at the longer wavelength of CLASP2. Here, we describe the optical design and performance of the CLASP2 instrument. Title: VizieR Online Data Catalog: 3D MHD models angular quadratures (Jaume Bestard+, 2021) Authors: Jaume Bestard, J.; Stepan, J.; Trujillo Bueno, J. Bibcode: 2020yCat..36450101J Altcode: The file name has the following convention: u: quadrature for unpolarised radiation p: quadrature for polarised radiation lX: quadrature for the order L=X nY: quadrature with N=Y rays in total. All the files have the same structure.

(28 data files). Title: The polarization of the Hα line in the quiet solar chromosphere Authors: Jaume Bestard, J.; Trujillo Bueno, J.; Štěpán, J.; Bianda, M.; Ramelli, R. Bibcode: 2020sea..confE.200J Altcode: One-dimensional radiative transfer (RT) calculations suggested that the scattering polarization profiles of the Hα line are very sensitive to the strength and structure of the chromospheric magnetic field. Here we present unprecedented spectropolarimetric observations of the Hα line obtained with ZIMPOL-3 at IRSOL. The linear polarization profiles show a rich variety of shapes and amplitudes, as well as an interesting spatial variability. We confront them with the theoretical scattering polarization profiles we have obtained by solving the complex RT problem of the Hα polarization in a 3D model from MHD simulations, highlighting the impact produced by the model's magnetic and velocity field. This investigation reveals the great interest of the Hα polarization in the present new era of large-aperture solar telescopes. Title: Near optimal angular quadratures for polarised radiative transfer Authors: Štěpán, Jiří; Jaume Bestard, Jaume; Trujillo Bueno, Javier Bibcode: 2020A&A...636A..24S Altcode: 2020arXiv200212736S In three-dimensional (3D) radiative transfer (RT) problems, the tensor product quadratures are generally not optimal in terms of the number of discrete ray directions needed for a given accuracy of the angular integration of the radiation field. In this paper, we derive a new set of angular quadrature rules that are more suitable for solving 3D RT problems with the short- and long-characteristics formal solvers. These quadratures are more suitable than the currently used ones for the numerical calculation of the radiation field tensors that are relevant in the problem of the generation and transfer of polarised radiation without assuming local thermodynamical equilibrium (non-LTE). We show that our new quadratures can save up to about 30% of computing time with respect to the Gaussian-trapezoidal product quadratures with the same accuracy.

The tables mentioned in Appendix A are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/636/A24 Title: The Magnetic Sensitivity of the Resonance and Subordinate Lines of Mg II in the Solar Chromosphere Authors: del Pino Alemán, T.; Trujillo Bueno, J.; Casini, R.; Manso Sainz, R. Bibcode: 2020ApJ...891...91D Altcode: 2020arXiv200409176D We carry out a theoretical study of the polarization of the solar Mg II h-k doublet (including its extended wings) and the subordinate ultraviolet (UV) triplet around 280 nm. These lines are of great diagnostic interest, as they encode information on the physical properties of the solar atmosphere from the upper photosphere to the chromosphere-corona transition region. We base our study on radiative transfer calculations of spectral line polarization in one-dimensional models of quiet and plage regions of the solar atmosphere. Our calculations take into account the combined action of atomic polarization, quantum level interference, frequency redistribution, and magnetic fields of arbitrary strength. In particular, we study the sensitivity of the emergent Stokes profiles to changes in the magnetic field through the Zeeman and Hanle effects. We also study the impact of the chromospheric plasma dynamics on the emergent Stokes profiles, taking into account the angle-dependent frequency redistribution in the h-k resonance transitions. The results presented here are of interest for the interpretation of spectropolarimetric observations in this important region of the solar UV spectrum. Title: VizieR Online Data Catalog: Polarised radiative transfer angular quadratures (Stepan+, 2020) Authors: Stepan, J.; Jaume Bestard, J.; Trujillo Bueno, J. Bibcode: 2020yCat..36360024S Altcode: Quadrature for unpolarised and polarised radiations.

(13 data files). Title: The Chromospheric Layer Spectro-Polarimeter (CLASP2) Sounding Rocket Mission: First Results Authors: McKenzie, D. E.; Ishikawa, R.; Trujillo Bueno, J.; Auchere, F.; Rachmeler, L.; Kano, R.; Song, D.; Okamoto, J.; Kobayashi, K.; Yoshida, M. Bibcode: 2019AGUFMSH44A..06M Altcode: A major challenge for heliophysics is to decipher the magnetic structure of the chromosphere, because of its vital role in the transport of energy into the corona and solar wind. Routine satellite measurements of the chromospheric magnetic field will dramatically improve our understanding of the chromosphere and its connection to the rest of the solar atmosphere. Before such a satellite can be considered for flight, we must refine the measurement techniques by exploring emission lines with a range of magnetic sensitivities. In 2015, CLASP achieved the first measurement of linear polarization produced by scattering processes in a far UV resonance line (hydrogen Lyman­-α), and the first exploration of the magnetic field (via the Hanle effect) and geometrical complexity in quiet regions of the chromosphere/­corona transition region. These measurements are a first step towards routine quantitative characterization of the local thermal and magnetic conditions in this key layer of the solar atmosphere.

Nonetheless, Lyman­-α is only one of the magnetically sensitive spectral lines in the UV spectrum. CLASP2 extends the capability of UV spectropolarimetry by acquiring ground­breaking measurements in the Mg II h and k spectral lines near 280 nm, whose cores form about 100 km below the Lyman-­α core. These lines are sensitive to a larger range of field strengths than Lyman­-α, through both the Hanle and Zeeman effects. CLASP2 captures measurements of linear and circular polarization to enable the first determination of all 4 Stokes parameters in chromospheric UV radiation. Coupled with numerical modeling of the observed spectral line polarization (anisotropic radiation pumping with Hanle, Zeeman and magneto-­optical effects), CLASP2 is a pathfinder for determination of the magnetic field's strength and direction, as well as of the geometry of the plasma in the upper solar chromosphere.

CLASP2 was launched from White Sands Missile Range in April 2019. In this presentation, we will summarize the characteristics of the CLASP2 flight, the performance of the UV telescope and spectropolarimeter, and our preliminary findings. Title: Center-to-Limb Variation of the polarization of Mg II h & k lines as measured by CLASP2 Authors: Rachmeler, L.; McKenzie, D. E.; Ishikawa, R.; Kano, R.; Trujillo Bueno, J.; Kobayashi, K.; Song, D.; Yoshida, M.; Auchere, F.; Okamoto, J. Bibcode: 2019AGUFMSH11D3380R Altcode: The Chromospheric LAyer SpectroPolarimeter 2 (CLASP2) is a sounding rocket that was launched from White Sands Missile Range in April 2019. CLASP2 is a reflight of the CLASP instrument, and has been modified to observe the polarization of the Magnesium II h & k lines in the solar chromosphere. The instrument contains a slit-jaw context imager at Lyman Alpha (~121.6nm) and two spectropolarimetric cameras that capture Mg II h & k near 280nm. A rotating polarization modulation unit allows us to capture the full polarization state of Mg II h & k; the measured polarization signals are sensitive to the Hanle and the Zeeman magnetic effects, and magneto-optical effects. The center-to-limb variations (CLV) of the intensity of these lines has been measured, but the CLV of the polarization signals has only been investigated theoretically. The first flight of CLASP, which measured the linear polarization of the Lyman alpha line, found a surprising lack of CLV in the line core (Kano et al. 2017), which has important implications for the magnetic strength and geometrical complexity of the chromosphere-corona transition region (Trujillo Bueno et al. 2018). We present here initial results on the CLV of the Mg II polarization signals. Title: Science Requirement Document (SRD) for the European Solar Telescope (EST) (2nd edition, December 2019) Authors: Schlichenmaier, R.; Bellot Rubio, L. R.; Collados, M.; Erdelyi, R.; Feller, A.; Fletcher, L.; Jurcak, J.; Khomenko, E.; Leenaarts, J.; Matthews, S.; Belluzzi, L.; Carlsson, M.; Dalmasse, K.; Danilovic, S.; Gömöry, P.; Kuckein, C.; Manso Sainz, R.; Martinez Gonzalez, M.; Mathioudakis, M.; Ortiz, A.; Riethmüller, T. L.; Rouppe van der Voort, L.; Simoes, P. J. A.; Trujillo Bueno, J.; Utz, D.; Zuccarello, F. Bibcode: 2019arXiv191208650S Altcode: The European Solar Telescope (EST) is a research infrastructure for solar physics. It is planned to be an on-axis solar telescope with an aperture of 4 m and equipped with an innovative suite of spectro-polarimetric and imaging post-focus instrumentation. The EST project was initiated and is driven by EAST, the European Association for Solar Telescopes. EAST was founded in 2006 as an association of 14 European countries. Today, as of December 2019, EAST consists of 26 European research institutes from 18 European countries. The Preliminary Design Phase of EST was accomplished between 2008 and 2011. During this phase, in 2010, the first version of the EST Science Requirement Document (SRD) was published. After EST became a project on the ESFRI roadmap 2016, the preparatory phase started. The goal of the preparatory phase is to accomplish a final design for the telescope and the legal governance structure of EST. A major milestone on this path is to revisit and update the Science Requirement Document (SRD). The EST Science Advisory Group (SAG) has been constituted by EAST and the Board of the PRE-EST EU project in November 2017 and has been charged with the task of providing with a final statement on the science requirements for EST. Based on the conceptual design, the SRD update takes into account recent technical and scientific developments, to ensure that EST provides significant advancement beyond the current state-of-the-art. The present update of the EST SRD has been developed and discussed during a series of EST SAG meetings. The SRD develops the top-level science objectives of EST into individual science cases. Identifying critical science requirements is one of its main goals. Those requirements will define the capabilities of EST and the post-focus instrument suite. The technical requirements for the final design of EST will be derived from the SRD. Title: High-frequency Wave Propagation Along a Spicule Observed by CLASP Authors: Yoshida, Masaki; Suematsu, Yoshinori; Ishikawa, Ryohko; Okamoto, Takenori J.; Kubo, Masahito; Kano, Ryouhei; Narukage, Noriyuki; Bando, Takamasa; Winebarger, Amy R.; Kobayashi, Ken; Trujillo Bueno, Javier; Auchère, Frédéric Bibcode: 2019ApJ...887....2Y Altcode: The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) sounding rocket experiment, launched in 2015 September, observed the hydrogen Lyα line (121.6 nm) in an unprecedented high temporal cadence of 0.3 s. CLASP performed sit-and-stare observations of the quiet Sun near the limb for 5 minutes with a slit perpendicular to the limb and successfully captured an off-limb spicule evolving along the slit. The Lyα line is well suited for investigating how spicules affect the corona because it is sensitive to higher temperatures than other chromospheric lines, owing to its large optical thickness. We found high-frequency oscillations of the Doppler velocity with periods of 20-50 s and low-frequency oscillation of periods of ∼240 s on the spicule. From a wavelet analysis of the time sequence data of the Doppler velocity, in the early phase of the spicule evolution, we found that waves with a period of ∼30 s and a velocity amplitude of 2-3 km s-1 propagated upward along the spicule with a phase velocity of ∼470 km s-1. In contrast, in the later phase, possible downward and standing waves with smaller velocity amplitudes were also observed. The high-frequency waves observed in the early phase of the spicule evolution would be related with the dynamics and the formation of the spicules. Our analysis enabled us to identify the upward, downward, and standing waves along the spicule and to obtain the velocity amplitude of each wave directly from the Doppler velocity for the first time. We evaluated the energy flux by the upward-propagating waves along the spicule, and discussed the impact to the coronal heating. Title: PolStar - An Explorer-Class FUV Spectropolarimetry Mission to Map the Environments of Massive Stars Authors: Scowen, Paul; Ignace, Richard; Neiner, Coralie; Wade, Gregg; Beasley, Matt; Bjorkman, Jon; Bouret, Jean-Claude; Casini, Roberto; del Pino Alemán, Tanausu; Edgington, Samantha; Gayley, Ken; Guinan, Ed; Hoffman, Jennifer; Howarth, Ian; Hull, Tony; Manso Sainz, Rafael; Naze, Yael; Nordt, Alison; Owocki, Stan; Petrinec, Steve; Prinja, Raman; Sana, Hugues; Shultz, Matt; Sparks, William; St-Louis, Nicole; Tillier, Clem; Trujillo Bueno, Javier; Vasudevan, Gopal; Woodruff, Bob Bibcode: 2019BAAS...51g.167S Altcode: 2019astro2020U.167S; 2019arXiv191106724S PolStar is an Explorer-class far ultraviolet (FUV) spectropolarimetry mission designed to target massive stars and their environments. PolStar will take advantage of resonance lines only available in the FUV to measure for the first time the magnetic and wind environment around massive stars to constrain models of rotation and mass loss. Title: The diagnostic potential of the weak field approximation for investigating the quiet Sun magnetism: the Si I 10 827 Å line Authors: Shchukina, N. G.; Trujillo Bueno, J. Bibcode: 2019A&A...628A..47S Altcode:
Aims: We aim to investigate the validity of the weak field approximation (WFA) for determining magnetic fields in quiet regions of the solar photosphere using the polarization caused by the Zeeman effect in the Si I 10 827 Å line.
Methods: We solved the NLTE line formation problem by means of multilevel radiative transfer calculations in a three-dimensional (3D) snapshot model taken from a state-of-the-art magneto-convection simulation of the small-scale magnetic activity in the quiet solar photosphere. The 3D model used is characterized by a surface mean magnetic field strength of about 170 G. The calculated Stokes profiles were degraded because of the atmospheric turbulence of Earth and light diffraction by the telescope aperture. We apply the WFA to the Stokes I, Q, U, V profiles calculated for different seeing conditions and for the apertures of the VTT, GREGOR, EST and DKIST telescopes. We compare the inferred longitudinal and transverse components of the magnetic field with the original vertical and horizontal fields of the 3D model.
Results: We find that with a spatial resolution significantly better than 0.5″ the surface maps of the magnetic field inferred from the Stokes profiles of the Si I 10 827 Å line applying the WFA are close to the magnetic field of the model on the corrugated surface, corresponding to line optical depth unity at Δλ ≈ 0.1 Å for a disk-center line of sight. The correlation between them is relatively high, except that the inferred longitudinal and transverse components of the magnetic field turn out to be lower than in the 3D model.
Conclusions: The use of the WFA for interpreting high-spatial-resolution spectropolarimetric observations of the Si I 10 827 Å line obtained with telescopes like GREGOR, EST, and DKIST allows the longitudinal and transverse components of the magnetic field to be retrieved with reasonable precision over the whole quiet solar photosphere, the result being worse for telescopes of lower aperture. Title: Magnetic Sensitivity in the Wing Scattering Polarization Signals of the Hydrogen Lyman-α Line of the Solar Disk Radiation Authors: Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J. Bibcode: 2019ApJ...880...85A Altcode: 2019arXiv190110994A The linear polarization produced by scattering processes in the hydrogen Lyα line of the solar disk radiation is a key observable for probing the chromosphere-corona transition region (TR) and the underlying chromospheric plasma. While the line-center signal encodes information on the magnetic field and the three-dimensional structure of the TR, the sizable scattering polarization signals that the joint action of partial frequency redistribution and J-state interference produce in the Lyα wings have generally been thought to be sensitive only to the thermal structure of the solar atmosphere. Here we show that the wings of the Q/I and U/I scattering polarization profiles of this line are actually sensitive to the presence of chromospheric magnetic fields, with strengths similar to those that produce the Hanle effect in the line core (i.e., between 5 and 100 G, approximately). In spite of the fact that the Zeeman splitting induced by such weak fields is very small compared to the total width of the line, the magneto-optical effects that couple the transfer equations for Stokes Q and U are actually able to produce sizable changes in the Q/I and U/I wings. We find that magnetic fields with longitudinal components larger than 100 G produce an almost complete depolarization of the wings of the Lyα Q/I profiles within a ±5 Å spectral range around the line center, while stronger fields are required for the U/I wing signals to be depolarized to a similar extent. The theoretical results presented here further expand the diagnostic content of the unprecedented spectropolarimetric observations provided by the Chromospheric Lyman-Alpha Spectro-Polarimeter. Title: The Chromospheric Layer Spectro-Polarimeter (CLASP2) Sounding Rocket Mission: First Results Authors: McKenzie, David Eugene; Ishikawa, Ryohko; Kano, Ryouhei; Rachmeler, Laurel; Trujillo Bueno, Javier; Kobayashi, Ken; Song, Donguk; Yoshida, Masaki; Auchere, Frederic; Okamoto, Takenori Bibcode: 2019AAS...23412601M Altcode: A major challenge for heliophysics is to decipher the magnetic structure of the chromosphere, because of its vital role in the transport of energy into the corona and solar wind. Routine satellite measurements of the chromospheric magnetic field will dramatically improve our understanding of the chromosphere and its connection to the rest of the solar atmosphere. Before such a satellite can be considered for flight, we must refine the measurement techniques by exploring emission lines with a range of magnetic sensitivities. In 2015, CLASP achieved the first measurement of linear polarization produced by scattering processes in a far UV resonance line (hydrogen Lyman-α), and the first exploration of the magnetic field (via the Hanle effect) and geometrical complexity in quiet regions of the chromosphere-corona transition region. These measurements are a first step towards routine quantitative characterization of the local thermal and magnetic conditions in this key layer of the solar atmosphere.

Nonetheless, Lyman-α is only one of the magnetically sensitive spectral lines in the UV spectrum. CLASP2 extends the capability of UV spectropolarimetry by acquiring ground-breaking measurements in the Mg II h and k spectral lines near 280 nm, whose cores form about 100 km below the Lyman-α core. These lines are sensitive to a larger range of field strengths than Lyman-α, through both the Hanle and Zeeman effects. CLASP2 will capture measurements of linear and circular polarization to enable the first determination of all 4 Stokes parameters in chromospheric UV radiation. Coupled with numerical modeling of the observed spectral line polarization (anisotropic radiation pumping with Hanle, Zeeman and magneto-optical effects), CLASP2 is a pathfinder for determination of the magnetic field's strength and direction, as well as of the geometry of the plasma in the upper solar chromosphere.

CLASP2 will launch from White Sands Missile Range in April 2019. In this presentation, we will summarize the characteristics of the CLASP2 flight, the performance of the UV telescope and spectropolarimeter, and our preliminary findings. Title: Lyman-α imaging polarimetry with the CLASP2 sounding rocket mission Authors: Kano, Ryouhei; Ishikawa, Ryohko; McKenzie, David Eugene; Trujillo Bueno, Javier; Song, Donguk; Yoshida, Masaki; Okamoto, Takenori; Rachmeler, Laurel; Kobayashi, Ken; Auchere, Frederic Bibcode: 2019AAS...23430216K Altcode: Ultraviolet polarimetry offers a unique opportunity to explore the upper solar chromosphere and the transition region (TR) to the million-degree corona. These outer atmospheric regions play a key role in the transfer of mass and energy from the solar photosphere to the corona. With a sounding rocket experiment called the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP), in September 2015 we succeeded in obtaining the first measurement of the linear polarization produced by scattering processes in the hydrogen Lyman-α line of the solar disk radiation. The analysis and interpretation of such spectro-polarimetric observation allowed us to obtain information on the geometrical complexity of the corrugated surface that delineates the TR, as well as on the magnetic field strength via the Hanle effect. At the same time, the CLASP slit-jaw (SJ) optics system, which is a Lyman-α filter imager characterized by a FWHM= 7 nm, allowed us to obtain broad-band Stokes-I and Q/I images over a large field of view. The obtained broad-band Q/I images are dominated by the scattering polarization signals of the Lyman-α wings, and not by the much weaker line-center signals where the Hanle effect operates. Recently, Alsina Ballester et al. (2019, ApJ, in press) showed that the scattering polarization signals of the Lyman-α wings are sensitive to chromospheric magnetic fields via the magneto-optical effects. Therefore, Lyman-α imaging polarimetry is of scientific interest also for magnetic-field investigations. On April 11, 2019, we performed another sounding rocket experiment, called the Chromospheric LAyer Spectro-Polarimeter (CLASP2). We used the same instrument after significant modifications in order to obtain spectro-polarimetric observations of a plage and a quiet region in the ionized magnesium lines around 280 nm (i.e., the Mg II h & k lines). At the same time, the CLASP2 SJ optics system allowed us to obtain broad-band Q/I and U/I images at the Lyman-α wavelength, in addition to the well-known SJ intensity images. In this presentation, we provide a first overview of the CLASP2 SJ data. Title: Modeling the Scattering Polarization of the Hydrogen Lyα Line Observed by CLASP in a Filament Channel Authors: Štěpán, J.; Trujillo Bueno, J.; Gunár, S.; Heinzel, P.; del Pino Alemán, T.; Kano, R.; Ishikawa, R.; Narukage, N.; Bando, T.; Winebarger, A.; Kobayashi, K.; Auchère, F. Bibcode: 2019ASPC..526..165S Altcode: The 400 arcsec spectrograph slit of CLASP crossed mainly quiet regions of the solar chromosphere, from the limb towards the solar disk center. Interestingly, in the CLASP slit-jaw images and in the SDO images of the He II line at 304 Å, we can identify a filament channel (FC) extending over more than 60 arcsec crossing the slit of the spectrograph. In order to interpret the peculiar spatial variation of the Q/I and U/I signals observed by CLASP in the hydrogen Lyα line (1216 Å), we perform multi-dimensional radiative transfer modeling in given filament models. In this contribution, we show the first results of the two-dimensional calculations we have carried out, with the aim of determining the filament thermal and magnetic structure by comparing the theoretical and the observed polarization signals. Our results suggest that the temperature gradients in the filament observed by CLASP are significantly larger than previously thought. Title: CLASP2: The Chromospheric LAyer Spectro-Polarimeter Authors: McKenzie, D. E.; Ishikawa, R.; Trujillo Bueno, J.; Auchére, F.; Rachmeler, L. A.; Kubo, M.; Kobayashi, K.; Winebarger, A. R.; Bethge, C. W.; Narukage, N.; Kano, R.; Ishikawa, S.; de Pontieu, B.; Carlsson, M.; Yoshida, M.; Belluzzi, L.; Štěpán, J.; del Pino Alemán, T.; Alsina Ballester, E.; Asensio Ramos, A. Bibcode: 2019ASPC..526..361M Altcode: The hydrogen Lyman-α line at 121.6 nm and the Mg k line at 279.5 nm are especially relevant for deciphering the magnetic structure of the chromosphere since their line-center signals are formed in the chromosphere and transition region, with unique sensitivities to magnetic fields. We propose the Chromospheric LAyer Spectro-Polarimeter (CLASP2), to build upon the success of the first CLASP flight, which measured the linear polarization in H I Lyman-α. The existing CLASP instrument will be refitted to measure all four Stokes parameters in the 280 nm range, including variations due to the anisotropic radiation pumping, the Hanle effect, and the Zeeman effect. Title: A Quantitative Comparison of Observed and Theoretical Stokes Profiles of the Ca II 8542 Å Line in the Quiet Sun Authors: Jurčák, J.; Stěpán, J.; Bianda, M.; Trujillo Bueno, J. Bibcode: 2019ASPC..526..235J Altcode: We present an analysis of the Stokes profiles of the Ca II 8542 Å line produced by the joint action of atomic level polarization and the Hanle and Zeeman effects. We compare spectropolarimetric observations of this line in a quiet region at the solar disk centre, taken with the ZIMPOL instrument at IRSOL, with the theoretical Stokes profiles computed with the PORTA radiative transfer code using as solar model atmosphere a 3D snapshot taken from a radiation MHD simulation of an enhanced-network region. Even though the spatial sampling of the ZIMPOL observations is only 1.43 arcsec/pixel, we detect Q/I, U/I and V/I polarization signals of the order of 0.1%. The synthetic line profiles have been obtained by solving the full 3D NLTE radiative transfer problem taking into account the symmetry breaking effects due to the model's horizontal inhomogeneities and macroscopic velocity gradients. After spatial and spectral degradation, in some locations we find similar amplitudes between the observed and calculated linear polarization profiles. However, in general, the observations show stronger polarization signals than the calculated ones, a discrepancy that could be used to refine the numerical models of the quiet solar chromosphere. Title: The Transfer of Resonance Line Polarization with PRD in the General Hanle-Zeeman Regime Authors: Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J. Bibcode: 2019ASPC..526..119A Altcode: We present numerical radiative transfer calculations of the four Stokes parameters of the radiation emerging from one-dimensional model atmospheres. In this investigation we account for the impact of partial frequency redistribution (PRD) in scattering and the joint action of the Hanle and Zeeman effects. Strong resonance lines of interest for chromospheric magnetic field diagnostics have been considered, namely the Ca I line at 4227 Å and the Mg II k line at 2795 Å. The Stokes profiles of these lines have been obtained by considering two-level atomic models, both in the absence and in the presence of magnetic fields. We draw attention to the fact that the magneto-optical terms of the transfer equations for Stokes Q and U are responsible for an interesting and previously unnoticed magnetic sensitivity of their scattering polarization profiles beyond the Doppler core. This important discovery contributes to paint a more detailed picture of the influence of relatively weak magnetic fields on the observable linear polarization signals of strong chromospheric lines, highlighting the importance of a PRD treatment for such lines. Title: The Last Twenty Years. Optically Polarized Atoms in the Solar Atmosphere Authors: Trujillo Bueno, J. Bibcode: 2019ASPC..526...69T Altcode: The 8th Solar Polarization Workshop in honor of Prof. Egidio Landi Degl'Innocenti was a unique opportunity to review the last twenty years of research on the generation and transfer of spectral line polarization produced by optically pumped atoms in the solar atmosphere. Many investigations have been carried out since 1997, when Egidio and I published our first joint paper in The Astrophysical Journal Letters, arguing that lower-level depopulation pumping is a key physical mechanism for understanding some of the enigmatic spectral features of the Second Solar Spectrum. Here I provide an overview of the research carried out since then on optically polarized atoms in the solar photosphere, chromosphere and transition region, with emphasis on applications based on the quantum theory of spectral line polarization to which Egidio contributed so many pioneering and fundamental results. Title: 3D Whole-Prominence Fine Structure Model as a Test Case for Verification and Development of Magnetic Field Inversion Techniques Authors: Gunár, S.; Mackay, D. H.; Štěpán, J.; Heinzel, P.; Trujillo Bueno, J. Bibcode: 2019ASPC..526..159G Altcode: We show the potential of a new 3D whole-prominence fine structure model to serve as a well-controlled yet complex environment for testing inversion techniques for the magnetic field inference. The realistic 3D magnetic field and plasma environment provided by the model can be used for the direct synthesis of spectro-polarimetric data. Such synthetic data can be analyzed by advanced inversion tools and their results compared with the known properties provided by the model. Title: Solar Polarization Workshop 8 Authors: Belluzzi, L.; Casini, R.; Romoli, M.; Trujillo Bueno, J. Bibcode: 2019ASPC..526.....B Altcode: No abstract at ADS Title: Comparison of Scattering Polarization Signals Observed by CLASP: Possible Indication of the Hanle Effect Authors: Ishikawa, R.; Trujillo Bueno, J.; Uitenbroek, H.; Kubo, M.; Tsuneta, S.; Goto, M.; Kano, R.; Narukage, N.; Bando, T.; Katsukawa, Y.; Ishikawa, S.; Giono, G.; Suematsu, Y.; Hara, H.; Shimizu, T.; Sakao, T.; Winebarger, A.; Kobayashi, K.; Cirtain, J.; Champey, P.; Auchère, F.; Štěpán, J.; Belluzzi, L.; Asensio Ramos, A.; Manso Sainz, R.; De Pomtieu, B.; Ichimoto, K.; Carlsson, M.; Casini, R. Bibcode: 2019ASPC..526..305I Altcode: The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP; Kano et al. 2012; Kobayashi et al. 2012; Kubo et al. 2014) observed, for the first time, the linear polarization produced by scattering processes in the hydrogen Lyman-α (121.57 nm) and Si III (120.56 nm) lines of the solar disk radiation. The complexity of the observed scattering polarization (i.e., conspicuous spatial variations in Q/I and U/I at spatial scales of 10″-20″ and the absence of center-to- limb variation at the Lyman-α center; see Kano et al. 2017) motivated us to search for possible hints of the operation of the Hanle effect by comparing: (a) the Lyman-α line center signal, for which the critical field strength (BH) for the onset of the Hanle effect is 53 G, (b) the Lyman-α wing, which is insensitive to the Hanle effect, and (c) the Si III line, whose BH = 290 G. We focus on four regions with different total unsigned photospheric magnetic fluxes (estimated from SDO/HMI observations), and compare the corresponding U/I spatial variations in the Lyman-α wing, Lyman-α center, and Si III line. The U/I signal in the Lyman-α wing shows an antisymmetric spatial distribution, which is caused by the presence of a bright structure in all the selected regions, regardless of the total unsigned photospheric magnetic flux. In an internetwork region, the Lyman-α center shows an antisymmetric spatial variation across the selected bright structure, but it does not show it in other more magnetized regions. In the Si III line, the spatial variation of U/I deviates from the above-mentioned antisymmetric shape as the total unsigned photospheric magnetic flux increases. We argue that a plausible explanation of this differential behavior is the operation of the Hanle effect.

This work, presented in an oral contribution at this Workshop, has been published on The Astrophysical Journal (Ishikawa et al. 2017). Title: Comparison of theoretical and observed Ca II 8542 Stokes profiles in quiet regions at the centre of the solar disc Authors: Jurčák, J.; Štěpán, J.; Trujillo Bueno, J.; Bianda, M. Bibcode: 2018A&A...619A..60J Altcode: 2018arXiv180809470J Context. Interpreting the Stokes profiles observed in quiet regions of the solar chromosphere is a challenging task. The Stokes Q and U profiles are dominated by the scattering polarisation and the Hanle effect, and these processes can only be correctly quantified if 3D radiative transfer effects are taken into account. Forward-modelling of the intensity and polarisation of spectral lines using a 3D model atmosphere is a suitable approach in order to statistically compare the theoretical and observed line profiles.
Aims: Our aim is to present novel observations of the Ca II 8542 Å line profiles in a quiet region at the centre of the solar disc and to quantitatively compare them with the theoretical Stokes profiles obtained by solving the problem of the generation and transfer of polarised radiation in a 3D model atmosphere. We aim at estimating the reliability of the 3D model atmosphere, excluding its known lack of dynamics and/or insufficient density, using not only the line intensity but the full vector of Stokes parameters.
Methods: We used data obtained with the ZIMPOL instrument at the Istituto Ricerche Solari Locarno (IRSOL) and compared the observations with the theoretical profiles computed with the PORTA radiative transfer code, using as solar model atmosphere a 3D snapshot taken from a radiation-magnetohydrodynamics simulation. The synthetic profiles were degraded to match the instrument and observing conditions.
Results: The degraded theoretical profiles of the Ca II 8542 line are qualitatively similar to the observed ones. We confirm that there is a fundamental difference in the widths of all Stokes profiles: the observed lines are wider than the theoretical lines. We find that the amplitudes of the observed profiles are larger than those of the theoretical ones, which suggests that the symmetry breaking effects in the solar chromosphere are stronger than in the model atmosphere. This means that the isosurfaces of temperature, velocity, and magnetic field strength and orientation are more corrugated in the solar chromosphere than in the currently available 3D radiation-magnetohydrodynamics simulation. Title: CLASP Constraints on the Magnetization and Geometrical Complexity of the Chromosphere-Corona Transition Region Authors: Trujillo Bueno, J.; Štěpán, J.; Belluzzi, L.; Asensio Ramos, A.; Manso Sainz, R.; del Pino Alemán, T.; Casini, R.; Ishikawa, R.; Kano, R.; Winebarger, A.; Auchère, F.; Narukage, N.; Kobayashi, K.; Bando, T.; Katsukawa, Y.; Kubo, M.; Ishikawa, S.; Giono, G.; Hara, H.; Suematsu, Y.; Shimizu, T.; Sakao, T.; Tsuneta, S.; Ichimoto, K.; Cirtain, J.; Champey, P.; De Pontieu, B.; Carlsson, M. Bibcode: 2018ApJ...866L..15T Altcode: 2018arXiv180908865T The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a suborbital rocket experiment that on 2015 September 3 measured the linear polarization produced by scattering processes in the hydrogen Lyα line of the solar disk radiation. The line-center photons of this spectral line radiation mostly stem from the chromosphere-corona transition region (TR). These unprecedented spectropolarimetric observations revealed an interesting surprise, namely that there is practically no center-to-limb variation (CLV) in the Q/I line-center signals. Using an analytical model, we first show that the geometric complexity of the corrugated surface that delineates the TR has a crucial impact on the CLV of the Q/I and U/I line-center signals. Second, we introduce a statistical description of the solar atmosphere based on a 3D model derived from a state-of-the-art radiation magnetohydrodynamic simulation. Each realization of the statistical ensemble is a 3D model characterized by a given degree of magnetization and corrugation of the TR, and for each such realization we solve the full 3D radiative transfer problem taking into account the impact of the CLASP instrument degradation on the calculated polarization signals. Finally, we apply the statistical inference method presented in a previous paper to show that the TR of the 3D model that produces the best agreement with the CLASP observations has a relatively weak magnetic field and a relatively high degree of corrugation. We emphasize that a suitable way to validate or refute numerical models of the upper solar chromosphere is by confronting calculations and observations of the scattering polarization in ultraviolet lines sensitive to the Hanle effect. Title: A Statistical Inference Method for Interpreting the CLASP Observations Authors: Štěpán, J.; Trujillo Bueno, J.; Belluzzi, L.; Asensio Ramos, A.; Manso Sainz, R.; del Pino Alemán, T.; Casini, R.; Kano, R.; Winebarger, A.; Auchère, F.; Ishikawa, R.; Narukage, N.; Kobayashi, K.; Bando, T.; Katsukawa, Y.; Kubo, M.; Ishikawa, S.; Giono, G.; Hara, H.; Suematsu, Y.; Shimizu, T.; Sakao, T.; Tsuneta, S.; Ichimoto, K.; Cirtain, J.; Champey, P.; De Pontieu, B.; Carlsson, M. Bibcode: 2018ApJ...865...48S Altcode: 2018arXiv180802725S On 2015 September 3, the Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) successfully measured the linear polarization produced by scattering processes in the hydrogen Lyα line of the solar disk radiation, revealing conspicuous spatial variations in the Q/I and U/I signals. Via the Hanle effect, the line-center Q/I and U/I amplitudes encode information on the magnetic field of the chromosphere-corona transition region, but they are also sensitive to the three-dimensional structure of this corrugated interface region. With the help of a simple line-formation model, here we propose a statistical inference method for interpreting the Lyα line-center polarization observed by CLASP. Title: A Novel Investigation of the Small-scale Magnetic Activity of the Quiet Sun via the Hanle Effect in the Sr I 4607 Å Line Authors: del Pino Alemán, T.; Trujillo Bueno, J.; Štěpán, J.; Shchukina, N. Bibcode: 2018ApJ...863..164D Altcode: 2018arXiv180607293D One of the key research problems in stellar physics is to decipher the small-scale magnetic activity of the quiet solar atmosphere. Recent magneto-convection simulations that account for small-scale dynamo action have provided three-dimensional (3D) models of the solar photosphere characterized by a high degree of small-scale magnetic activity, similar to that found through theoretical interpretation of the scattering polarization observed in the Sr I 4607 Å line. Here we present the results of a novel investigation of the Hanle effect in this resonance line based on 3D radiative transfer calculations in a high-resolution magneto-convection model having most of the convection zone magnetized close to the equipartition and a surface mean field strength < B> ≈ 170 G. The Hanle effect produced by the model’s magnetic field depolarizes the zero-field scattering polarization signals significantly, to the extent that the center-to-limb variation (CLV) of the calculated spatially averaged polarization amplitudes is compatible with the observations. The standard deviation of the horizontal fluctuations of the calculated scattering polarization signals is very sensitive to the model’s magnetic field, and we find that the predicted spatial variations are sufficiently sizable so as to be able to detect them, especially with the next generation of solar telescopes. We find that at all on-disk positions, the theoretical scattering polarization signals are anticorrelated with the continuum intensity. To facilitate reaching new observational breakthroughs, we show how the theoretically predicted polarization signals and spatial variations are modified when deteriorating the signal-to-noise ratio and the spectral and spatial resolutions of the simulated observations. Title: Current State of UV Spectro-Polarimetry and its Future Direction Authors: Ishikawa, Ryohko; Sakao, Taro; Katsukawa, Yukio; Hara, Hirohisa; Ichimoto, Kiyoshi; Shimizu, Toshifumi; Kubo, Masahito; Auchere, Frederic; De Pontieu, Bart; Winebarger, Amy; Kobayashi, . Ken; Kano, Ryouhei; Narukage, Noriyuki; Trujillo Bueno, Javier; Song, Dong-uk; Manso Sainz, Rafael; Asensio Ramos, Andres; Leenaarts, Jorritt; Carlsson, Mats; Bando, Takamasa; Ishikawa, Shin-nosuke; Tsuneta, Saku; Belluzzi, Luca; Suematsu, Yoshinori; Giono, Gabriel; Yoshida, Masaki; Goto, Motoshi; Del Pino Aleman, Tanausu; Stepan, Jiri; Okamoto, Joten; Tsuzuki, Toshihiro; Uraguchi, Fumihiro; Champey, Patrick; Alsina Ballester, Ernest; Casini, Roberto; McKenzie, David; Rachmeler, Laurel; Bethge, Christian Bibcode: 2018cosp...42E1564I Altcode: To obtain quantitative information on the magnetic field in low beta regions (i.e., upper chromosphere and above) has been increasingly important to understand the energetic phenomena of the outer solar atmosphere such as flare, coronal heating, and the solar wind acceleration. In the UV range, there are abundant spectral lines that originate in the upper chromosphere and transition region. However, the Zeeman effect in these spectral lines does not give rise to easily measurable polarization signals because of the weak magnetic field strength and the larger Doppler broadening compared with the Zeeman effect. Instead, the Hanle effect in UV lines is expected to be a suitable diagnostic tool of the magnetic field in the upper atmospheric layers. To investigate the validity of UV spectro-polarimetry and the Hanle effect, the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP), which is a NASA sounding- rocket experiment, was launched at White Sands in US on September 3, 2015. During its 5 minutes ballistic flight, it successfully performed spectro-polarimetric observations of the hydrogen Lyman-alpha line (121.57 nm) with an unprecedentedly high polarization sensitivity of 0.1% in this wavelength range. CLASP observed the linear polarization produced by scattering process in VUV lines for the first time and detected the polarization signals which indicate the operation of the Hanle effect. Following the success of CLASP, we are confident that UV spectro-polarimetry is the way to proceed, and we are planning the second flight of CLASP (CLASP2: Chromospheric LAyer SpectroPolarimeter 2). For this second flight we will carry out spectro-polarimetry in the Mg II h and k lines around 280 nm, with minimum modifications of the CLASP1 instrument. The linear polarization in the Mg II k line is induced by scattering processes and the Hanle effect, being sensitive to magnetic field strengths of 5 to 50 G. In addition, the circular polarizations in the Mg II h and k lines induced by the Zeeman effect can be measurable in at least plage and active regions. The combination of the Hanle and Zeeman effects could help us to more reliably infer the magnetic fields of the upper solar chromosphere. CLASP2 was selected for flight and is being developed for launch in the spring of 2019.Based on these sounding rocket experiments (CLASP1 and 2), we aim at establishing the strategy and refining the instrument concept for future space missions to explore the enigmatic atmospheric layers via UV spectro-polarimetry. Title: Wavefront error measurements and alignment of CLASP2 telescope with a dual-band pass cold mirror coated primary mirror Authors: Yoshida, Masaki; Song, Donguk; Ishikawa, Ryoko; Kano, Ryouhei; Katsukawa, Yukio; Suematsu, Yoshinori; Narukage, Noriyuki; Kubo, Masahito; Shinoda, Kazuya; Okamoto, Takenori J.; McKenzie, David E.; Rachmeler, Laurel A.; Auchère, Frédéric; Trujillo Bueno, Javier Bibcode: 2018SPIE10699E..30Y Altcode: "Chromospheric LAyer Spectro-Polarimeter (CLASP2)" is the next sounding rocket experiment of the "Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP)" that succeeded in observing for the first time the linear polarization spectra in the hydrogen Lyman-α line (121.6 nm) and is scheduled to be launched in 2019. In CLASP2, we will carry out full Stokes-vector spectropolarimetric observations in the Mg ii h and k lines near 280 nm with the spectro-polarimeter (SP), while imaging observations in the Lyman-α line will be conducted with the slitjaw optics (SJ). For the wavelength selection of CLASP2, the primary mirror of the telescope uses a new dual-band pass cold mirror coating targeting both at 121.6 nm and 280 nm. Therefore, we have to perform again the alignment of the telescope after the installation of the recoated primary mirror. Before unmounting the primary mirror from the telescope structure, we measured the wave-front error (WFE) of the telescope. The measured WFE map was consistent with what we had before the CLASP flight, clearly indicating that the telescope alignment has been maintained even after the flight. After the re-coated primary mirror was installed the WFE was measured, and coma aberration was found to be larger. Finally, the secondary mirror shim adjustments were carried out based on the WFE measurements. In CLASP2 telescope, we improved a fitting method of WFE map (applying 8th terms circular Zernike polynomial fitting instead of 37th terms circular Zernike fitting) and the improved method enables to achieve better performance than CLASP telescope. Indeed, WFE map obtained after the final shim adjustment indicated that the required specification (< 5.5 μm RMS spot radius) that is more stringent than CLASP telescope was met. Title: Optical alignment of the high-precision UV spectro-polarimeter (CLASP2) Authors: Song, Donguk; Ishikawa, Ryohko; Kano, Ryouhei; Yoshida, Masaki; Tsuzuki, Toshihiro; Uraguchi, Fumihiro; Shinoda, Kazuya; Hara, Hirohisa; Okamoto, Takenori J.; Auchère, Frédéric; McKenzie, David E.; Rachmeler, Laurel A.; Trujillo Bueno, Javier Bibcode: 2018SPIE10699E..2WS Altcode: Chromospheric LAyer Spectro-Polarimeter (CLASP2) is our next sounding rocket experiment after the success of Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP1). CLASP2 is scheduled to launch in 2019, and aims to achieve high precision measurements (< 0.1 %) of the linear and circular polarizations in the Mg ii h and k lines near the 280 nm, whose line cores originate in the upper solar chromosphere. The CLASP2 spectro-polarimeter follows very successful design concept of the CLASP1 instrument with the minimal modification. A new grating was fabricated with the same radius of curvature as the CLASP1 grating, but with a different ruling density. This allows us to essentially reuse the CLASP1 mechanical structures and layout of the optics. However, because the observing wavelength of CLASP2 is twice longer than that of CLASP1, a magnifier optical system was newly added in front of the cameras to double the focal length of CLASP2 and to maintain the same wavelength resolution as CLASP1 (0.01 nm). Meanwhile, a careful optical alignment of the spectro-polarimeter is required to reach the 0.01 nm wavelength resolution. Therefore, we established an efficient alignment procedure for the CLASP2 spectro-polarimeter based on an experience of CLASP1. Here, we explain in detail the methods for achieving the optical alignment of the CLASP2 spectro-polarimeter and discuss our results by comparing with the performance requirements. Title: Constraining the magnetization and geometrical complexity of the chromosphere-corona transition region via radiative transfer modeling of the CLASP observations Authors: Trujillo Bueno, Javier Bibcode: 2018cosp...42E3433T Altcode: On 3 September 2015 an international team of scientists from Japan, USA and Europe carried out succesfully a challenging measurement using a vacuum ultraviolet telescope and aspectropolarimeter called CLASP, launched by a NASA suborbital rocket (see the talk by R. Ishikawa et al.). For the first time, CLASP measured the linear polarization profiles of the hydrogen Lyman-alpha and Si III 120.6 nm lines produced by the scattering of anisotropic radiation in the enigmatic chromosphere-corona transition region of the Sun. Here we report about the radiative transfer investigations we have carried out for interpreting the Stokes Q/I and U/I profiles observed by CLASP in such ultraviolet resonance lines. We explain why the observed Lyman-alpha polarization does not show a clear center to limb variation. We show also that the observed Stokes profiles encode a rich information on the magnetization and geometrical complexity of the plasma of the upper solar chromosphere.AUTHORS: J. Trujillo Bueno, J. Stepan, T. del Pino Alemán, L. Belluzzi, A. Asensio Ramos, R. Manso Sainz, R. Ishikawa, R. Kano, A. Winebarger, F. Auchere, N. Narukage, K. Kobayashi, T. Bando, Y. Katsukawa, M. Kubo, S. Ishikawa, G. Giono, H. Hara, Y. Suematsu, T. Shimizu, T. Sakao, S. Tsuneta, K. Ichimoto, J. Cirtain, P. Champey, B. De Pontieu, R. Casini, and M. Carlsson Title: Magneto-optical Effects in the Scattering Polarization Wings of the Ca I 4227 Å Resonance Line Authors: Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J. Bibcode: 2018ApJ...854..150A Altcode: 2017arXiv171100372A; 2017arXiv171100372B The linear polarization pattern produced by scattering processes in the Ca I 4227 Å resonance line is a valuable observable for probing the solar atmosphere. Via the Hanle effect, the very significant Q/I and U/I line-center signals are sensitive to the presence of magnetic fields in the lower chromosphere with strengths between 5 and 125 G, approximately. On the other hand, partial frequency redistribution (PRD) produces sizable signals in the wings of the Q/I profile, which have always been thought to be insensitive to the presence of magnetic fields. Interestingly, novel observations of this line revealed a surprising behavior: fully unexpected signals in the wings of the U/I profile and spatial variability in the wings of both Q/I and U/I. We show that the magneto-optical (MO) terms of the Stokes-vector transfer equation produce sizable signals in the wings of U/I and a clear sensitivity of the Q/I and U/I wings to the presence of photospheric magnetic fields with strengths similar to those that produce the Hanle effect in the line core. This radiative transfer investigation on the joint action of scattering processes and the Hanle and Zeeman effects in the Ca I 4227 Å line should facilitate the development of more reliable techniques for exploring the magnetism of stellar atmospheres. To this end, we can now exploit the circular polarization produced by the Zeeman effect, the magnetic sensitivity caused by the above-mentioned MO effects in the Q/I and U/I wings, and the Hanle effect in the line core. Title: The Physics and Diagnostic Potential of Ultraviolet Spectropolarimetry Authors: Trujillo Bueno, Javier; Landi Degl'Innocenti, Egidio; Belluzzi, Luca Bibcode: 2018smf..book..183T Altcode: No abstract at ADS Title: Comparison of Solar Fine Structure Observed Simultaneously in Lyα and Mg II h Authors: Schmit, D.; Sukhorukov, A. V.; De Pontieu, B.; Leenaarts, J.; Bethge, C.; Winebarger, A.; Auchère, F.; Bando, T.; Ishikawa, R.; Kano, R.; Kobayashi, K.; Narukage, N.; Trujillo Bueno, J. Bibcode: 2017ApJ...847..141S Altcode: 2017arXiv170900035S The Chromospheric Lyman Alpha Spectropolarimeter (CLASP) observed the Sun in H I Lyα during a suborbital rocket flight on 2015 September 3. The Interface Region Imaging Telescope (IRIS) coordinated with the CLASP observations and recorded nearly simultaneous and co-spatial observations in the Mg II h and k lines. The Mg II h and Lyα lines are important transitions, energetically and diagnostically, in the chromosphere. The canonical solar atmosphere model predicts that these lines form in close proximity to each other and so we expect that the line profiles will exhibit similar variability. In this analysis, we present these coordinated observations and discuss how the two profiles compare over a region of quiet Sun at viewing angles that approach the limb. In addition to the observations, we synthesize both line profiles using a 3D radiation-MHD simulation. In the observations, we find that the peak width and the peak intensities are well correlated between the lines. For the simulation, we do not find the same relationship. We have attempted to mitigate the instrumental differences between IRIS and CLASP and to reproduce the instrumental factors in the synthetic profiles. The model indicates that formation heights of the lines differ in a somewhat regular fashion related to magnetic geometry. This variation explains to some degree the lack of correlation, observed and synthesized, between Mg II and Lyα. Our analysis will aid in the definition of future observatories that aim to link dynamics in the chromosphere and transition region. Title: The Physics and Diagnostic Potential of Ultraviolet Spectropolarimetry Authors: Trujillo Bueno, Javier; Landi Degl'Innocenti, Egidio; Belluzzi, Luca Bibcode: 2017SSRv..210..183T Altcode: 2017SSRv..tmp...15T The empirical investigation of the magnetic field in the outer solar atmosphere is a very important challenge in astrophysics. To this end, we need to identify, measure and interpret observable quantities sensitive to the magnetism of the upper chromosphere, transition region and corona. This paper provides an overview of the physics and diagnostic potential of spectropolarimetry in permitted spectral lines of the ultraviolet solar spectrum, such as the Mg ii h and k lines around 2800 Å, the hydrogen Lyman-α line at 1216 Å, and the Lyman-α line of He ii at 304 Å. The outer solar atmosphere is an optically pumped vapor and the linear polarization of such spectral lines is dominated by the atomic level polarization produced by the absorption and scattering of anisotropic radiation. Its modification by the action of the Hanle and Zeeman effects in the inhomogeneous and dynamic solar atmosphere needs to be carefully understood because it encodes the magnetic field information. The circular polarization induced by the Zeeman effect in some ultraviolet lines (e.g., Mg ii h & k) is also of diagnostic interest, especially for probing the outer solar atmosphere in plages and more active regions. The few (pioneering) observational attempts carried out so far to measure the ultraviolet spectral line polarization produced by optically pumped atoms in the upper chromosphere, transition region and corona are also discussed. We emphasize that ultraviolet spectropolarimetry is a key gateway to the outer atmosphere of the Sun and of other stars. Title: CLASP/SJ Observations of Rapid Time Variations in the Lyα Emission in a Solar Active Region Authors: Ishikawa, Shin-nosuke; Kubo, Masahito; Katsukawa, Yukio; Kano, Ryouhei; Narukage, Noriyuki; Ishikawa, Ryohko; Bando, Takamasa; Winebarger, Amy; Kobayashi, Ken; Trujillo Bueno, Javier; Auchère, Frédéric Bibcode: 2017ApJ...846..127I Altcode: The Chromospheric Lyα SpectroPolarimeter (CLASP) is a sounding rocket experiment launched on 2015 September 3 to investigate the solar chromosphere and transition region. The slit-jaw (SJ) optical system captured Lyα images with a high time cadence of 0.6 s. From the CLASP/SJ observations, many variations in the solar chromosphere and transition region emission with a timescale of <1 minute were discovered. In this paper, we focus on the active region within the SJ field of view and investigate the relationship between short (<30 s) temporal variations in the Lyα emission and the coronal structures observed by Solar Dynamics Observatory/Atmospheric Imaging Assembly (AIA). We compare the Lyα temporal variations at the coronal loop footpoints observed in the AIA 211 Å (≈2 MK) and AIA 171 Å (≈0.6 MK) channels with those in the regions with bright Lyα features without a clear association with the coronal loop footpoints. We find more short (<30 s) temporal variations in the Lyα intensity in the footpoint regions. Those variations did not depend on the temperature of the coronal loops. Therefore, the temporal variations in the Lyα intensity at this timescale range could be related to the heating of the coronal structures up to temperatures around the sensitivity peak of 171 Å. No signature was found to support the scenario that these Lyα intensity variations were related to the nanoflares. Waves or jets from the lower layers (lower chromosphere or photosphere) are possible causes for this phenomenon. Title: CLASP2: The Chromospheric LAyer Spectro-Polarimeter Authors: Rachmeler, Laurel; E McKenzie, David; Ishikawa, Ryohko; Trujillo Bueno, Javier; Auchère, Frédéric; Kobayashi, Ken; Winebarger, Amy; Bethge, Christian; Kano, Ryouhei; Kubo, Masahito; Song, Donguk; Narukage, Noriyuki; Ishikawa, Shin-nosuke; De Pontieu, Bart; Carlsson, Mats; Yoshida, Masaki; Belluzzi, Luca; Stepan, Jiri; del Pino Alemná, Tanausú; Ballester, Ernest Alsina; Asensio Ramos, Andres Bibcode: 2017SPD....4811010R Altcode: We present the instrument, science case, and timeline of the CLASP2 sounding rocket mission. The successful CLASP (Chromospheric Lyman-Alpha Spectro-Polarimeter) sounding rocket flight in 2015 resulted in the first-ever linear polarization measurements of solar hydrogen Lyman-alpha line, which is sensitive to the Hanle effect and can be used to constrain the magnetic field and geometric complexity of the upper chromosphere. Ly-alpha is one of several upper chromospheric lines that contain magnetic information. In the spring of 2019, we will re-fly the modified CLASP telescope to measure the full Stokes profile of Mg II h & k near 280 nm. This set of lines is sensitive to the upper chromospheric magnetic field via both the Hanle and the Zeeman effects. Title: CLASP2: The Chromospheric LAyer Spectro-Polarimeter Authors: Rachmeler, Laurel A.; McKenzie, D. E.; Ishikawa, R.; Trujillo-Bueno, J.; Auchere, F.; Kobayashi, K.; Winebarger, A.; Bethge, C.; Kano, R.; Kubo, M.; Song, D.; Narukage, N.; Ishikawa, S.; De Pontieu, B.; Carlsson, M.; Yoshida, M.; Belluzzi, L.; Stepan, J.; del Pino Alemán, T.; Alsina Ballester, E.; Asensio Ramos, A. Bibcode: 2017shin.confE..79R Altcode: We present the instrument, science case, and timeline of the CLASP2 sounding rocket mission. The successful CLASP (Chromospheric Lyman-Alpha Spectro-Polarimeter) sounding rocket flight in 2015 resulted in the first-ever linear polarization measurements of solar hydrogen Lyman-alpha line, which is sensitive to the Hanle effect and can be used to constrain the magnetic field and geometric complexity of the upper chromosphere. Ly-alpha is one of several upper chromospheric lines that contain magnetic information. In the spring of 2019, we will re-fly the modified CLASP telescope to measure the full Stokes profile of Mg II h & k near 280 nm. This set of lines is sensitive to the upper chromospheric magnetic field via both the Hanle and the Zeeman effects. Title: A Si I atomic model for NLTE spectropolarimetric diagnostics of the 10 827 Å line Authors: Shchukina, N. G.; Sukhorukov, A. V.; Trujillo Bueno, J. Bibcode: 2017A&A...603A..98S Altcode:
Aims: The Si I 10 827 Å line is commonly used for spectropolarimetric diagnostics of the solar atmosphere. First, we aim at quantifying the sensitivity of the Stokes profiles of this line to non-local thermodynamic equilibrium (NLTE) effects. Second, we aim at facilitating NLTE diagnostics of the Si I 10 827 Å line. To this end, we propose the use of a relatively simple silicon model atom, which allows a fast and accurate computation of Stokes profiles. The NLTE Stokes profiles calculated using this simple model atom are very similar to those obtained via the use of a very comprehensive silicon model atom.
Methods: We investigate the impact of the NLTE effects on the Si I 10 827 Å line by means of multilevel radiative transfer calculations in a three-dimensional (3D) model atmosphere taken from a state-of-the-art magneto-convection simulation with small-scale dynamo action. We calculate the emergent Stokes profiles for this line at the solar disk center and for every vertical column of the 3D snapshot model, neglecting the effects of horizontal radiative transfer.
Results: We find significant departures from LTE in the Si I 10 827 Å line, not only in the intensity but also in the linearly and circularly polarized profiles. At wavelengths around 0.1 Å, where most of the Stokes Q, U, and V peaks of the Si I 10 827 Å line occur, the differences between the NLTE and LTE profiles are comparable with the Stokes amplitudes themselves. The deviations from LTE increase with increasing Stokes Q, U, and V signals. Concerning the Stokes V profiles, the NLTE effects correlate with the magnetic field strength in the layers where such circular polarization signals are formed.
Conclusions: The NLTE effects should be taken into account when diagnosing the emergent Stokes I profiles as well as the Stokes Q, U, and V profiles of the Si I 10 827 Å line. The sixteen-level silicon model atom proposed here, with six radiative bound-bound transitions, is suitable to account for the physics of formation of the Si I 10 827 Å line and for modeling and inverting its Stokes profiles without assuming LTE. Title: Center-to-limb variation of the continuum intensity and linear polarization of stars with transiting exoplanets Authors: Shchukina, N. G.; Trujillo Bueno, J.; Vasilyeva, I. E.; Frantseva, K. V. Bibcode: 2017KPCB...33..166S Altcode: The limb darkening and center-to-limb variation of the continuum polarization is calculated for a grid of one-dimensional stellar model atmospheres and for a wavelength range between 300 and 950 nm. Model parameters match those of the transiting stars taken from the NASA exoplanet archive. The limb darkening of the continuum radiation for these stars is shown to decrease with the rise in their effective temperature. For the λ = 370 nm wavelength, which corresponds to the maximum of the Johnson-Cousins UX filter, the limb darkening values of the planet transiting stars lie in a range between 0.03 and 0.3. The continuum linear polarization depends not only on the effective temperature of the star but also on its gravity and metallicity. Its value decreases for increasing values of these parameters. In the UX band, the maximum linear polarization of stars with transiting planets amounts to 4%, while the minimum value is approximately 0.3%. The continuum limb darkening and the linear polarization decrease rapidly with wavelength. At the R band maximum (λ = 700 nm), the linear polarization close to the limb is in fact two orders of magnitude smaller than in the UX band. The center- to-limb variation of the continuum intensity and the linear polarization of the stars with transiting planets can be approximated, respectively, by polynomials of the fourth and the sixth degree. The coefficients of the polynomials, as well as the IDL procedures for reading them, are available in electronic form. It is shown that there are two classes of stars with high linear polarization at the limb. The first one consists of cold dwarfs. Their typical representatives are HATS-6, Kepler-45, as well as all the stars with similar parameters. The second class of stars includes hotter giants and subgiants. Among them we have CoRoT-28, Kepler-91, and the group of stars with effective temperatures and gravities of approximately 5000 K and 3.5, respectively. Title: Indication of the Hanle Effect by Comparing the Scattering Polarization Observed by CLASP in the Lyα and Si III 120.65 nm Lines Authors: Ishikawa, R.; Trujillo Bueno, J.; Uitenbroek, H.; Kubo, M.; Tsuneta, S.; Goto, M.; Kano, R.; Narukage, N.; Bando, T.; Katsukawa, Y.; Ishikawa, S.; Giono, G.; Suematsu, Y.; Hara, H.; Shimizu, T.; Sakao, T.; Winebarger, A.; Kobayashi, K.; Cirtain, J.; Champey, P.; Auchère, F.; Štěpán, J.; Belluzzi, L.; Asensio Ramos, A.; Manso Sainz, R.; De Pontieu, B.; Ichimoto, K.; Carlsson, M.; Casini, R. Bibcode: 2017ApJ...841...31I Altcode: The Chromospheric Lyman-Alpha Spectro-Polarimeter is a sounding rocket experiment that has provided the first successful measurement of the linear polarization produced by scattering processes in the hydrogen Lyα line (121.57 nm) radiation of the solar disk. In this paper, we report that the Si III line at 120.65 nm also shows scattering polarization and we compare the scattering polarization signals observed in the Lyα and Si III lines in order to search for observational signatures of the Hanle effect. We focus on four selected bright structures and investigate how the U/I spatial variations vary between the Lyα wing, the Lyα core, and the Si III line as a function of the total unsigned photospheric magnetic flux estimated from Solar Dynamics Observatory/Helioseismic and Magnetic Imager observations. In an internetwork region, the Lyα core shows an antisymmetric spatial variation across the selected bright structure, but it does not show it in other more magnetized regions. In the Si III line, the spatial variation of U/I deviates from the above-mentioned antisymmetric shape as the total unsigned photospheric magnetic flux increases. A plausible explanation of this difference is the operation of the Hanle effect. We argue that diagnostic techniques based on the scattering polarization observed simultaneously in two spectral lines with very different sensitivities to the Hanle effect, like Lyα and Si III, are of great potential interest for exploring the magnetism of the upper solar chromosphere and transition region. Title: Polarization Calibration of the Chromospheric Lyman-Alpha SpectroPolarimeter for a 0.1% Polarization Sensitivity in the VUV Range. Part II: In-Flight Calibration Authors: Giono, G.; Ishikawa, R.; Narukage, N.; Kano, R.; Katsukawa, Y.; Kubo, M.; Ishikawa, S.; Bando, T.; Hara, H.; Suematsu, Y.; Winebarger, A.; Kobayashi, K.; Auchère, F.; Trujillo Bueno, J.; Tsuneta, S.; Shimizu, T.; Sakao, T.; Cirtain, J.; Champey, P.; Asensio Ramos, A.; Štěpán, J.; Belluzzi, L.; Manso Sainz, R.; De Pontieu, B.; Ichimoto, K.; Carlsson, M.; Casini, R.; Goto, M. Bibcode: 2017SoPh..292...57G Altcode: The Chromospheric Lyman-Alpha SpectroPolarimeter is a sounding rocket instrument designed to measure for the first time the linear polarization of the hydrogen Lyman-α line (121.6 nm). The instrument was successfully launched on 3 September 2015 and observations were conducted at the solar disc center and close to the limb during the five-minutes flight. In this article, the disc center observations are used to provide an in-flight calibration of the instrument spurious polarization. The derived in-flight spurious polarization is consistent with the spurious polarization levels determined during the pre-flight calibration and a statistical analysis of the polarization fluctuations from solar origin is conducted to ensure a 0.014% precision on the spurious polarization. The combination of the pre-flight and the in-flight polarization calibrations provides a complete picture of the instrument response matrix, and a proper error transfer method is used to confirm the achieved polarization accuracy. As a result, the unprecedented 0.1% polarization accuracy of the instrument in the vacuum ultraviolet is ensured by the polarization calibration. Title: Discovery of Scattering Polarization in the Hydrogen Lyα Line of the Solar Disk Radiation Authors: Kano, R.; Trujillo Bueno, J.; Winebarger, A.; Auchère, F.; Narukage, N.; Ishikawa, R.; Kobayashi, K.; Bando, T.; Katsukawa, Y.; Kubo, M.; Ishikawa, S.; Giono, G.; Hara, H.; Suematsu, Y.; Shimizu, T.; Sakao, T.; Tsuneta, S.; Ichimoto, K.; Goto, M.; Belluzzi, L.; Štěpán, J.; Asensio Ramos, A.; Manso Sainz, R.; Champey, P.; Cirtain, J.; De Pontieu, B.; Casini, R.; Carlsson, M. Bibcode: 2017ApJ...839L..10K Altcode: 2017arXiv170403228K There is a thin transition region (TR) in the solar atmosphere where the temperature rises from 10,000 K in the chromosphere to millions of degrees in the corona. Little is known about the mechanisms that dominate this enigmatic region other than the magnetic field plays a key role. The magnetism of the TR can only be detected by polarimetric measurements of a few ultraviolet (UV) spectral lines, the Lyα line of neutral hydrogen at 121.6 nm (the strongest line of the solar UV spectrum) being of particular interest given its sensitivity to the Hanle effect (the magnetic-field-induced modification of the scattering line polarization). We report the discovery of linear polarization produced by scattering processes in the Lyα line, obtained with the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) rocket experiment. The Stokes profiles observed by CLASP in quiet regions of the solar disk show that the Q/I and U/I linear polarization signals are of the order of 0.1% in the line core and up to a few percent in the nearby wings, and that both have conspicuous spatial variations with scales of ∼10 arcsec. These observations help constrain theoretical models of the chromosphere-corona TR and extrapolations of the magnetic field from photospheric magnetograms. In fact, the observed spatial variation from disk to limb of polarization at the line core and wings already challenge the predictions from three-dimensional magnetohydrodynamical models of the upper solar chromosphere. Title: The Physical Origin and Magnetic Sensitivity of the Scattering Polarization Observed in the O I IR Triplet at 777 nm Authors: del Pino Alemán, T.; Trujillo Bueno, J. Bibcode: 2017ApJ...838..164D Altcode: The linearly polarized solar limb spectrum caused by the absorption and scattering of anisotropic radiation has a very rich diagnostic potential, given its sensitivity to the thermal, dynamic, and magnetic structure of the solar atmosphere. A crucial first step toward its scientific exploitation is understanding the physical origin of the observed spectral line polarization and its magnetic sensitivity via the Hanle and Zeeman effects. Here, we study the linear polarization signals observed in the IR triplet of O I at 777 nm, describing in detail the multilevel radiative transfer calculations that allowed us to decipher their physical origin. We investigate the sensitivity of the calculated scattering polarization signals to various modeling parameters, finding that the observed fractional linear polarization pattern originates mainly in the solar chromosphere, although the intensity profiles of the O I IR triplet come mainly from the lower photosphere. We find that the three lines are sensitive, via the Hanle effect, to magnetic fields with strengths between 0.01 and 30 G, in a extended region of the solar atmosphere. We show this through calculations of the response function to magnetic field perturbations in a semi-empirical model of the quiet Sun atmosphere. The dominant response of the linear polarization signals occurs at heights ∼ 1000 km above the visible model’s surface, which demonstrates that the scattering linear polarization signals of the oxygen IR triplet encode information on the magnetism of the solar chromosphere. Title: The Transfer of Resonance Line Polarization with Partial Frequency Redistribution in the General Hanle-Zeeman Regime Authors: Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J. Bibcode: 2017ApJ...836....6A Altcode: 2016arXiv160905723B; 2016arXiv160905723A; 2017ApJ...836....6B The spectral line polarization encodes a wealth of information about the thermal and magnetic properties of the solar atmosphere. Modeling the Stokes profiles of strong resonance lines is, however, a complex problem both from a theoretical and computational point of view, especially when partial frequency redistribution (PRD) effects need to be taken into account. In this work, we consider a two-level atom in the presence of magnetic fields of arbitrary intensity (Hanle-Zeeman regime) and orientation, both deterministic and micro-structured. Working within the framework of a rigorous PRD theoretical approach, we have developed a numerical code that solves the full non-LTE radiative transfer problem for polarized radiation, in one-dimensional models of the solar atmosphere, accounting for the combined action of the Hanle and Zeeman effects, as well as for PRD phenomena. After briefly discussing the relevant equations, we describe the iterative method of solution of the problem and the numerical tools that we have developed and implemented. We finally present some illustrative applications to two resonance lines that form at different heights in the solar atmosphere, and provide a detailed physical interpretation of the calculated Stokes profiles. We find that magneto-optical effects have a strong impact on the linear polarization signals that PRD effects produce in the wings of strong resonance lines. We also show that the weak-field approximation has to be used with caution when PRD effects are considered. Title: A novel radiative transfer investigation of the magnetic micro-activity of the quiet Sun via the Hanle effect in the Sr I 460.7 nm line Authors: Trujillo Bueno, Javier Bibcode: 2017psio.confE..29T Altcode: No abstract at ADS Title: Polarization Calibration of the Chromospheric Lyman-Alpha SpectroPolarimeter for a 0.1 % Polarization Sensitivity in the VUV Range. Part I: Pre-flight Calibration Authors: Giono, G.; Ishikawa, R.; Narukage, N.; Kano, R.; Katsukawa, Y.; Kubo, M.; Ishikawa, S.; Bando, T.; Hara, H.; Suematsu, Y.; Winebarger, A.; Kobayashi, K.; Auchère, F.; Trujillo Bueno, J. Bibcode: 2016SoPh..291.3831G Altcode: 2016SoPh..tmp..177G The Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) is a sounding rocket experiment designed to measure for the first time the linear polarization of the hydrogen Lyman-α line (121.6 nm) and requires a 0.1 % polarization sensitivity, which is unprecedented for a spectropolarimeter in the vacuum UV (VUV) spectral range. Title: Discovery of Ubiquitous Fast-Propagating Intensity Disturbances by the Chromospheric Lyman Alpha Spectropolarimeter (CLASP) Authors: Kubo, M.; Katsukawa, Y.; Suematsu, Y.; Kano, R.; Bando, T.; Narukage, N.; Ishikawa, R.; Hara, H.; Giono, G.; Tsuneta, S.; Ishikawa, S.; Shimizu, T.; Sakao, T.; Winebarger, A.; Kobayashi, K.; Cirtain, J.; Champey, P.; Auchère, F.; Trujillo Bueno, J.; Asensio Ramos, A.; Štěpán, J.; Belluzzi, L.; Manso Sainz, R.; De Pontieu, B.; Ichimoto, K.; Carlsson, M.; Casini, R.; Goto, M. Bibcode: 2016ApJ...832..141K Altcode: High-cadence observations by the slit-jaw (SJ) optics system of the sounding rocket experiment known as the Chromospheric Lyman Alpha Spectropolarimeter (CLASP) reveal ubiquitous intensity disturbances that recurrently propagate in either the chromosphere or the transition region or both at a speed much higher than the speed of sound. The CLASP/SJ instrument provides a time series of two-dimensional images taken with broadband filters centered on the Lyα line at a 0.6 s cadence. The multiple fast-propagating intensity disturbances appear in the quiet Sun and in an active region, and they are clearly detected in at least 20 areas in a field of view of 527″ × 527″ during the 5 minute observing time. The apparent speeds of the intensity disturbances range from 150 to 350 km s-1, and they are comparable to the local Alfvén speed in the transition region. The intensity disturbances tend to propagate along bright elongated structures away from areas with strong photospheric magnetic fields. This suggests that the observed fast-propagating intensity disturbances are related to the magnetic canopy structures. The maximum distance traveled by the intensity disturbances is about 10″, and the widths are a few arcseconds, which are almost determined by a pixel size of 1.″03. The timescale of each intensity pulse is shorter than 30 s. One possible explanation for the fast-propagating intensity disturbances observed by CLASP is magnetohydrodynamic fast-mode waves. Title: The Magnetic Sensitivity of the Mg II k Line to the Joint Action of Hanle, Zeeman, and Magneto-optical Effects Authors: Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J. Bibcode: 2016ApJ...831L..15A Altcode: 2016arXiv161000649A We highlight the main results of a radiative transfer investigation on the magnetic sensitivity of the solar Mg II k resonance line at 2795.5 Å, accounting for the joint action of the Hanle and Zeeman effects as well as partial frequency redistribution phenomena. We confirm that at the line center, the linear polarization signals produced by scattering processes are measurable, and that they are sensitive, via the Hanle effect, to magnetic fields with strengths between 5 and 50 G, approximately. We also show that the Zeeman effect produces conspicuous circular polarization signals, especially for longitudinal fields stronger than 50 G, which can be used to estimate the magnetization of the solar chromosphere via the familiar magnetograph formula. The most novel result is that magneto-optical effects produce, in the wings of the line, a decrease of the Q/I scattering polarization pattern and the appearance of U/I signals (I.e., a rotation of the plane of linear polarization). This sensitivity of the Q/I and U/I wing signals to both weak (∼5 G) and stronger magnetic fields expands the scientific interest of the Mg II k line for probing the chromosphere in quiet and active regions of the Sun. Title: Optical alignment of the Chromospheric Lyman-Alpha Spectro-Polarimeter using sophisticated methods to minimize activities under vacuum Authors: Giono, G.; Katsukawa, Y.; Ishikawa, R.; Narukage, N.; Kano, R.; Kubo, M.; Ishikawa, S.; Bando, T.; Hara, H.; Suematsu, Y.; Winebarger, A.; Kobayashi, K.; Auchère, F.; Trujillo Bueno, J. Bibcode: 2016SPIE.9905E..3DG Altcode: The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a sounding-rocket instrument developed at the National Astronomical Observatory of Japan (NAOJ) as a part of an international collaboration. The instrument main scientific goal is to achieve polarization measurement of the Lyman-α line at 121.56 nm emitted from the solar upper-chromosphere and transition region with an unprecedented 0.1% accuracy. The optics are composed of a Cassegrain telescope coated with a "cold mirror" coating optimized for UV reflection and a dual-channel spectrograph allowing for simultaneous observation of the two orthogonal states of polarization. Although the polarization sensitivity is the most important aspect of the instrument, the spatial and spectral resolutions of the instrument are also crucial to observe the chromospheric features and resolve the Ly-α profiles. A precise alignment of the optics is required to ensure the resolutions, but experiments under vacuum conditions are needed since Ly-α is absorbed by air, making the alignment experiments difficult. To bypass this issue, we developed methods to align the telescope and the spectrograph separately in visible light. We explain these methods and present the results for the optical alignment of the CLASP telescope and spectrograph. We then discuss the combined performances of both parts to derive the expected resolutions of the instrument, and compare them with the flight observations performed on September 3rd 2015. Title: Chromospheric LAyer SpectroPolarimeter (CLASP2) Authors: Narukage, Noriyuki; McKenzie, David E.; Ishikawa, Ryoko; Trujillo-Bueno, Javier; De Pontieu, Bart; Kubo, Masahito; Ishikawa, Shin-nosuke; Kano, Ryouhei; Suematsu, Yoshinori; Yoshida, Masaki; Rachmeler, Laurel A.; Kobayashi, Ken; Cirtain, Jonathan W.; Winebarger, Amy R.; Asensio Ramos, Andres; del Pino Aleman, Tanausu; Štępán, Jiri; Belluzzi, Luca; Larruquert, Juan Ignacio; Auchère, Frédéric; Leenaarts, Jorrit; Carlsson, Mattias J. L. Bibcode: 2016SPIE.9905E..08N Altcode: The sounding rocket Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) was launched on September 3rd, 2015, and successfully detected (with a polarization accuracy of 0.1 %) the linear polarization signals (Stokes Q and U) that scattering processes were predicted to produce in the hydrogen Lyman-alpha line (Lyα 121.567 nm). Via the Hanle effect, this unique data set may provide novel information about the magnetic structure and energetics in the upper solar chromosphere. The CLASP instrument was safely recovered without any damage and we have recently proposed to dedicate its second flight to observe the four Stokes profiles in the spectral region of the Mg II h and k lines around 280 nm; in these lines the polarization signals result from scattering processes and the Hanle and Zeeman effects. Here we describe the modifications needed to develop this new instrument called the "Chromospheric LAyer SpectroPolarimeter" (CLASP2). Title: The Hanle and Zeeman Polarization Signals of the Solar Ca II 8542 Å Line Authors: Štěpán, Jiří; Trujillo Bueno, Javier Bibcode: 2016ApJ...826L..10S Altcode: 2016arXiv160607741S We highlight the main results of a three-dimensional (3D) multilevel radiative transfer investigation about the solar disk-center polarization of the Ca II 8542 Å line. First, through the use of a 3D model of the solar atmosphere, we investigate the linear polarization that occurs due to the atomic level polarization produced by the absorption and scattering of anisotropic radiation, taking into account the symmetry-breaking effects caused by its thermal, dynamic, and magnetic structure. Second, we study the contribution of the Zeeman effect to the linear and circular polarization. Finally, we show examples of the Stokes profiles produced by the joint action of the atomic level polarization and the Hanle and Zeeman effects. We find that the Zeeman effect tends to dominate the linear polarization signals only in the localized patches of opposite magnetic polarity, where the magnetic field is relatively strong and slightly inclined; outside such very localized patches, the linear polarization is often dominated by the contribution of atomic level polarization. We demonstrate that a correct modeling of this last contribution requires taking into account the symmetry-breaking effects caused by the thermal, dynamic, and magnetic structure of the solar atmosphere, and that in the 3D model used the Hanle effect in forward-scattering geometry (disk-center observation) mainly reduces the polarization corresponding to the zero-field case. We emphasize that, in general, a reliable modeling of the linear polarization in the Ca II 8542 Å line requires taking into account the joint action of atomic level polarization and the Hanle and Zeeman effects. Title: Spectro-polarimetric observation in UV with CLASP to probe the chromosphere and transition region Authors: Kano, Ryouhei; Ishikawa, Ryohko; Winebarger, Amy R.; Auchère, Frédéric; Trujillo Bueno, Javier; Narukage, Noriyuki; Kobayashi, Ken; Bando, Takamasa; Katsukawa, Yukio; Kubo, Masahito; Ishikawa, Shin-Nosuke; Giono, Gabriel; Hara, Hirohisa; Suematsu, Yoshinori; Shimizu, Toshifumi; Sakao, Taro; Tsuneta, Saku; Ichimoto, Kiyoshi; Goto, Motoshi; Cirtain, Jonathan W.; De Pontieu, Bart; Casini, Roberto; Manso Sainz, Rafael; Asensio Ramos, Andres; Stepan, Jiri; Belluzzi, Luca; Carlsson, Mats Bibcode: 2016SPD....4710107K Altcode: The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a NASA sounding-rocket experiment that was performed in White Sands in the US on September 3, 2015. During its 5-minute ballistic flight, CLASP successfully made the first spectro-polarimetric observation in the Lyman-alpha line (121.57 nm) originating in the chromosphere and transition region. Since the Lyman-alpha polarization is sensitive to magnetic field of 10-100 G by the Hanle effect, we aim to infer the magnetic field information in such upper solar atmosphere with this experiment.The obtained CLASP data showed that the Lyman-alpha scattering polarization is about a few percent in the wings and the order of 0.1% in the core near the solar limb, as it had been theoretically predicted, and that both polarization signals have a conspicuous spatio-temporal variability. CLASP also observed another upper-chromospheric line, Si III (120.65 nm), whose critical field strength for the Hanle effect is 290 G, and showed a measurable scattering polarization of a few % in this line. The polarization properties of the Si III line could facilitate the interpretation of the scattering polarization observed in the Lyman-alpha line.In this presentation, we would like to show how the upper chromosphere and transition region are seen in the polarization of these UV lines and discuss the possible source of these complicated polarization signals. Title: Impact of surface dynamo magnetic fields on the solar abundance of the CNO elements Authors: Shchukina, N.; Sukhorukov, A.; Trujillo Bueno, J. Bibcode: 2016A&A...586A.145S Altcode: Most studies of the solar metallicity, based on abundance determinations of the CNO elements, ignore the fact that the quiet solar photosphere is significantly magnetized by a small-scale magnetic field with a mean field strength of ~100 G. Here we quantify how this significant magnetization affects determinations of the abundances of these chemical elements. To this end, we used two three-dimensional models of the solar photosphere taken from a magneto-convection simulation with small-scale dynamo action, one virtually unmagnetized, and the other characterized by a mean field strength of 160 G in the low photosphere. We performed local thermodynamic equilibrium spectral synthesis for a large set of C I, N I, and O I lines to derive abundance corrections. We included the magnetic broadening of the lines (direct effect) and the magnetically induced changes of the photospheric temperature stratification (indirect effect). We find that these small-scale dynamo magnetic fields only negligibly affect the determination of the solar abundances of carbon, nitrogen, and oxygen. Title: Radiative Transfer Modeling of the Enigmatic Scattering Polarization in the Solar Na I D1 Line Authors: Belluzzi, Luca; Trujillo Bueno, Javier; Landi Degl'Innocenti, Egidio Bibcode: 2015ApJ...814..116B Altcode: 2015arXiv151105801B The modeling of the peculiar scattering polarization signals observed in some diagnostically important solar resonance lines requires the consideration of the detailed spectral structure of the incident radiation field as well as the possibility of ground level polarization, along with the atom's hyperfine structure and quantum interference between hyperfine F-levels pertaining either to the same fine structure J-level, or to different J-levels of the same term. Here we present a theoretical and numerical approach suitable for solving this complex non-LTE radiative transfer problem. This approach is based on the density-matrix metalevel theory (where each level is viewed as a continuous distribution of sublevels) and on accurate formal solvers of the transfer equations and efficient iterative methods. We show an application to the D-lines of Na i, with emphasis on the enigmatic D1 line, pointing out the observable signatures of the various physical mechanisms considered. We demonstrate that the linear polarization observed in the core of the D1 line may be explained by the effect that one gets when the detailed spectral structure of the anisotropic radiation responsible for the optical pumping is taken into account. This physical ingredient is capable of introducing significant scattering polarization in the core of the Na i D1 line without the need for ground-level polarization. Title: Atomic Scattering Polarization. Observations, Modeling, Predictions Authors: Trujillo Bueno, J.; Del Pino Alemán, T.; Belluzzi, L. Bibcode: 2015IAUS..305..127T Altcode: 2015IAUS..305..127B This paper highlights very recent advances concerning the identification of new mechanisms that introduce polarization in spectral lines, which turn out to be key for understanding some of the most enigmatic scattering polarization signals of the solar visible spectrum. We also show a radiative transfer prediction on the scattering polarization pattern across the Mg ii h & k lines, whose radiation can only be observed from space. Title: Isotropic Inelastic Collisions in a Multiterm Atom with Hyperfine Structure Authors: Belluzzi, Luca; Landi Degl'Innocenti, Egidio; Trujillo Bueno, Javier Bibcode: 2015ApJ...812...73B Altcode: A correct modeling of the scattering polarization profiles observed in some spectral lines of diagnostic interest, the sodium doublet being one of the most important examples, requires taking hyperfine structure (HFS) and quantum interference between different J-levels into account. An atomic model suitable for taking these physical ingredients into account is the so-called multiterm atom with HFS. In this work, we introduce and study the transfer and relaxation rates due to isotropic inelastic collisions with electrons, which enter the statistical equilibrium equations (SEE) for the atomic density matrix of this atomic model. Under the hypothesis that the electron-atom interaction is described by a dipolar operator, we provide useful relations between the rates describing the transfer and relaxation of quantum interference between different levels (whose numerical values are in most cases unknown) and the usual rates for the atomic level populations, for which experimental data and/or approximate theoretical expressions are generally available. For the particular case of a two-term atom with HFS, we present an analytical solution of the SEE for the spherical statistical tensors of the upper term, including both radiative and collisional processes, and we derive the expression of the emission coefficient in the four Stokes parameters. Finally, an illustrative application to the Na i D1 and D2 lines is presented. Title: The impact of surface dynamo magnetic fields on the chemical abundance determination Authors: Shchukina, Nataliya G.; Sukhorukov, Andrii V.; Trujillo Bueno, Javier Bibcode: 2015IAUS..305..368S Altcode: The solar abundances of Fe and of the CNO elements play an important role in addressing a number of important issues such as the formation, structure, and evolution of the Sun and the solar system, the origin of the chemical elements, and the evolution of stars and galaxies. Despite the large number of papers published on this issue, debates about the solar abundances of these elements continue. The aim of the present investigation is to quantify the impact of photospheric magnetic fields on the determination of the solar chemical abundances. To this end, we used two 3D snapshot models of the quiet solar photosphere with a different magnetization taken from recent magneto-convection simulations with small-scale dynamo action. Using such 3D models we have carried out spectral synthesis for a large set of Fei, Ci, Ni, and Oi lines, in order to derive abundance corrections caused by the magnetic, Zeeman broadening of the intensity profiles and the magnetically induced changes of the photospheric temperature structure. We find that if the magnetism of the quiet solar photosphere is mainly produced by a small-scale dynamo, then its impact on the determination of the abundances of iron, carbon, nitrogen and oxygen is negligible. Title: Formation of polarized spectral lines in atmospheres with horizontal inhomogeneities Authors: Tichý, A.; Štěpán, J.; Trujillo Bueno, J.; Kubát, J. Bibcode: 2015IAUS..305..401T Altcode: We study the problem of the generation and transfer of spectral line intensity and polarization in models of stellar atmospheres with horizontal plasma inhomogeneities. We solve the non-LTE radiative transfer problem in full 3D geometry taking into account resonant scattering polarization and its modification by magnetic fields via the Hanle effect. We show that horizontal fluctuations of the thermodynamical conditions of stellar atmospheres can have a significant impact on the linear polarization of the emergent spectral line radiation and its center-to-limb variation. Title: CLASP: A UV Spectropolarimeter on a Sounding Rocket for Probing theChromosphere-Corona Transition Regio Authors: Ishikawa, Ryohko; Kano, Ryouhei; Winebarger, Amy; Auchere, Frederic; Trujillo Bueno, Javier; Bando, Takamasa; Narukage, Noriyuki; Kobayashi, Ken; Katsukawa, Yukio; Kubo, Masahito; Ishikawa, Shin-nosuke; Giono, Gabriel; Tsuneta, Saku; Hara, Hirohisa; Suematsu, Yoshinori; Shimizu, Toshifumi; Sakao, Taro; Ichimoto, Kiyoshi; Cirtain, Jonathan; De Pontieu, Bart; Casini, Roberto; Manso Sainz, Rafael; Asensio Ramos, Andres; Stepan, Jiri; Belluzzi, Luca Bibcode: 2015IAUGA..2254536I Altcode: The wish to understand the energetic phenomena of the outer solar atmosphere makes it increasingly important to achieve quantitative information on the magnetic field in the chromosphere-corona transition region. To this end, we need to measure and model the linear polarization produced by scattering processes and the Hanle effect in strong UV resonance lines, such as the hydrogen Lyman-alpha line. A team consisting of Japan, USA, Spain, France, and Norway has been developing a sounding rocket experiment called the Chromospheric Lyman-alpha Spectro-Polarimeter (CLASP). The aim is to detect the scattering polarization produced by anisotropic radiation pumping in the hydrogen Lyman-alpha line (121.6 nm), and via the Hanle effect to try to constrain the magnetic field vector in the upper chromosphere and transition region. In this talk, we will present an overview of our CLASP mission, its scientific objectives, ground tests made, and the latest information on the launch planned for the Summer of 2015. Title: The impact of surface dynamo magnetic fields on the solar iron abundance Authors: Shchukina, N.; Trujillo Bueno, J. Bibcode: 2015A&A...579A.112S Altcode: Most chemical abundance determinations ignore that the solar photosphere is significantly magnetized by the ubiquitous presence of a small-scale magnetic field. A recent investigation has suggested that there should be a significant impact on the derived iron abundance, owing to the magnetically induced changes on the photospheric temperature and density structure (indirect effect). The three-dimensional (3D) photospheric models used in that investigation have non-zero net magnetic flux values and stem from magneto-convection simulations without small-scale dynamo action. Here we address the same problem by instead using 3D models of the quiet solar photosphere that result from a state-of-the-art magneto-convection simulation with small-scale dynamo action, where the net magnetic flux is zero. One of these 3D models has negligible magnetization, while the other is characterized by a mean field strength of 160 Gauss in the low photosphere. With such 3D models we carried out spectral synthesis for a large set of Fe i lines to derive abundance corrections, taking the above-mentioned indirect effect and the Zeeman broadening of the intensity profiles (direct effect) into account. We conclude that if the magnetism of the quiet solar photosphere is mainly produced by a small-scale dynamo, then its impact on the determination of the solar iron abundance is negligible.

Table 1 is available in electronic form at http://www.aanda.org Title: Chromospheric Polarization in the Photospheric Solar Oxygen Infrared Triplet Authors: Del Pino Alemán, Tanausú; Trujillo Bueno, Javier Bibcode: 2015ApJ...808L..13D Altcode: 2015arXiv150703356D We present multilevel radiative transfer modeling of the scattering polarization observed in the solar O i infrared triplet around 777 nm. We demonstrate that the scattering polarization pattern observed on the solar disk forms in the chromosphere, far above the photospheric region where the bulk of the emergent intensity profiles originate. We investigate the sensitivity of the polarization pattern to the thermal structure of the solar atmosphere and to the presence of weak magnetic fields (10-2-100 G) through the Hanle effect, showing that the scattering polarization signals of the oxygen infrared triplet encode information on the magnetism of the solar chromosphere. Title: An open-source, massively parallel code for non-LTE synthesis and inversion of spectral lines and Zeeman-induced Stokes profiles Authors: Socas-Navarro, H.; de la Cruz Rodríguez, J.; Asensio Ramos, A.; Trujillo Bueno, J.; Ruiz Cobo, B. Bibcode: 2015A&A...577A...7S Altcode: 2014arXiv1408.6101S With the advent of a new generation of solar telescopes and instrumentation, interpreting chromospheric observations (in particular, spectropolarimetry) requires new, suitable diagnostic tools. This paper describes a new code, NICOLE, that has been designed for Stokes non-LTE radiative transfer, for synthesis and inversion of spectral lines and Zeeman-induced polarization profiles, spanning a wide range of atmospheric heights from the photosphere to the chromosphere. The code features a number of unique features and capabilities and has been built from scratch with a powerful parallelization scheme that makes it suitable for application on massive datasets using large supercomputers. The source code is written entirely in Fortran 90/2003 and complies strictly with the ANSI standards to ensure maximum compatibility and portability. It is being publicly released, with the idea of facilitating future branching by other groups to augment its capabilities.

The source code is currently hosted at the following repository: https://github.com/hsocasnavarro/NICOLE Title: Highlights of Spanish Astrophysics VIII Authors: Cenarro, A. J.; Figueras, F.; Hernández-Monteagudo, C.; Trujillo Bueno, J.; Valdivielso, L. Bibcode: 2015hsa8.conf.....C Altcode: No abstract at ADS Title: Height Variation of the Vector Magnetic Field in Solar Spicules Authors: Orozco Suárez, D.; Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2015ApJ...803L..18O Altcode: 2015arXiv150404637O Proving the magnetic configuration of solar spicules has hitherto been difficult due to the lack of spatial resolution and image stability during off-limb ground-based observations. We report spectropolarimetric observations of spicules taken in the He i 1083 nm spectral region with the Tenerife Infrared Polarimeter II at the German Vacuum Tower Telescope of the Observatorio del Teide (Tenerife, Canary Islands, Spain). The data provide the variation with geometrical height of the Stokes I, Q, U, and V profiles, whose encoded information allows the determination of the magnetic field vector by means of the HAZEL inversion code. The inferred results show that the average magnetic field strength at the base of solar spicules is about 80 gauss, and then it decreases rapidly with height to about 30 gauss at a height of 3000 km above the visible solar surface. Moreover, the magnetic field vector is close to vertical at the base of the chromosphere and has mid-inclinations (about 50°) above 2 Mm height. Title: Three-dimensional Radiative Transfer Simulations of the Scattering Polarization of the Hydrogen Lyα Line in a Magnetohydrodynamic Model of the Chromosphere-Corona Transition Region Authors: Štěpán, J.; Trujillo Bueno, J.; Leenaarts, J.; Carlsson, M. Bibcode: 2015ApJ...803...65S Altcode: 2015arXiv150106382S Probing the magnetism of the upper solar chromosphere requires measuring and modeling the scattering polarization produced by anisotropic radiation pumping in UV spectral lines. Here we apply PORTA (a novel radiative transfer code) to investigate the hydrogen Lyα line in a three-dimensional model of the solar atmosphere resulting from a state of the art magnetohydrodynamic (MHD) simulation. At full spatial resolution the linear polarization signals are very significant all over the solar disk, with a large fraction of the field of view (FOV) showing line-center amplitudes well above the 1% level. Via the Hanle effect the line-center polarization signals are sensitive to the magnetic field of the model's transition region, even when its mean field strength is only 15 G. The breaking of the axial symmetry of the radiation field produces significant forward-scattering polarization in Lyα, without the need of an inclined magnetic field. Interestingly, the Hanle effect tends to decrease such forward-scattering polarization signals in most of the points of the FOV. When the spatial resolution is degraded, the line-center polarization of Lyα drops below the 1% level, reaching values similar to those previously found in one-dimensional (1D) semi-empirical models (i.e., up to about 0.5 %). The center to limb variation (CLV) of the spatially averaged polarization signals is qualitatively similar to that found in 1D models, with the largest line-center amplitudes at μ =cos θ ≈ 0.4 (θ being the heliocentric angle). These results are important, both for designing the needed space-based instrumentation and for a reliable interpretation of future observations of the Lyα polarization. Title: Non Coherent Continuum Scattering as a Polarization Mechanism of the Enigmatic Ba <font size=2>II D1 Line Authors: del Pino Alemán, T.; Trujillo Bueno, J.; Uitenbroek, H. Bibcode: 2014ASPC..489..107D Altcode: Line scattering polarization can be strongly affected by Rayleigh scattering by neutral hydrogen and Thomson scattering by free electrons. The assumption that continuum polarization can be modeled as coherent scattering, an excellent approximation far from the spectral line, yields a continuum depolarization when applied to an intrinsically unpolarizable spectral line. However, the radiation field is not always constant over the spectral line and continuum scattering has to be treated non-coherently. A recent investigation showed that the redistribution of the spectral line radiation due to the non coherence of the continuum scattering can significantly modify the shape of the emergent fractional linear polarization profiles, even yielding emission Q/I features in intrinsically unpolarizable lines. Here we show an application to the enigmatic D1 line of Ba <font size=2>II at 4934 Å, neglecting the hyperfine structure of the 18% of the barium isotopes whose nuclear spin is non-zero. We show that with this assumption Q/I signals above the continuum polarization level can be produced in solar atmospheric models representative of polar faculae. Title: Polarized Radiation Observables for Probing the Magnetism of the Outer Solar Atmosphere Authors: Trujillo Bueno, J. Bibcode: 2014ASPC..489..137T Altcode: The basic idea of optical pumping, for which Alfred Kastler received the 1966 Nobel Prize in Physics, is that the absorption and scattering of anisotropic radiation can produce population imbalances and quantum coherence among the magnetic substates of atomic levels. The degree of this radiatively-induced atomic level polarization, which is very sensitive to the presence of magnetic fields, can be determined by observing the polarization of the scattered or transmitted spectral line radiation. The most important point for solar physics is that the outer solar atmosphere is an optically pumped vapor and that the polarization of the emergent spectral line radiation can be exploited to obtain quantitative information on the strength and/or geometry of magnetic fields within the chromosphere, transition region, and corona. Here we review some recent investigations of the polarization produced by optical pumping in selected IR, FUV, and EUV spectral lines, showing that their magnetic sensitivity is suitable for probing the magnetism of the outer solar atmosphere. Title: A Sounding Rocket Experiment for the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) Authors: Kubo, M.; Kano, R.; Kobayashi, K.; Bando, T.; Narukage, N.; Ishikawa, R.; Tsuneta, S.; Katsukawa, Y.; Ishikawa, S.; Suematsu, Y.; Hara, H.; Shimizu, T.; Sakao, T.; Ichimoto, K.; Goto, M.; Holloway, T.; Winebarger, A.; Cirtain, J.; De Pontieu, B.; Casini, R.; Auchère, F.; Trujillo Bueno, J.; Manso Sainz, R.; Belluzzi, L.; Asensio Ramos, A.; Štěpán, J.; Carlsson, M. Bibcode: 2014ASPC..489..307K Altcode: A sounding-rocket experiment called the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is presently under development to measure the linear polarization profiles in the hydrogen Lyman-alpha (Lyα) line at 121.567 nm. CLASP is a vacuum-UV (VUV) spectropolarimeter to aim for first detection of the linear polarizations caused by scattering processes and the Hanle effect in the Lyα line with high accuracy (0.1%). This is a fist step for exploration of magnetic fields in the upper chromosphere and transition region of the Sun. Accurate measurements of the linear polarization signals caused by scattering processes and the Hanle effect in strong UV lines like Lyα are essential to explore with future solar telescopes the strength and structures of the magnetic field in the upper chromosphere and transition region of the Sun. The CLASP proposal has been accepted by NASA in 2012, and the flight is planned in 2015. Title: The magnetic field configuration of a solar prominence inferred from spectropolarimetric observations in the He i 10 830 Å triplet Authors: Orozco Suárez, D.; Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2014A&A...566A..46O Altcode: 2014arXiv1403.7976O Context. Determining the magnetic field vector in quiescent solar prominences is possible by interpreting the Hanle and Zeeman effects in spectral lines. However, observational measurements are scarce and lack high spatial resolution.
Aims: We determine the magnetic field vector configuration along a quiescent solar prominence by interpreting spectropolarimetric measurements in the He i 1083.0 nm triplet obtained with the Tenerife Infrared Polarimeter installed at the German Vacuum Tower Telescope of the Observatorio del Teide.
Methods: The He i 1083.0 nm triplet Stokes profiles were analyzed with an inversion code that takes the physics responsible for the polarization signals in this triplet into account. The results are put into a solar context with the help of extreme ultraviolet observations taken with the Solar Dynamic Observatory and the Solar Terrestrial Relations Observatory satellites.
Results: For the most probable magnetic field vector configuration, the analysis depicts a mean field strength of 7 gauss. We do not find local variations in the field strength except that the field is, on average, lower in the prominence body than in the prominence feet, where the field strength reaches ~25 gauss. The averaged magnetic field inclination with respect to the local vertical is ~77°. The acute angle of the magnetic field vector with the prominence main axis is 24° for the sinistral chirality case and 58° for the dextral chirality. These inferences are in rough agreement with previous results obtained from the analysis of data acquired with lower spatial resolutions.

A movie is available in electronic form at http://www.aanda.org Title: On the Inversion of the Scattering Polarization and the Hanle Effect Signals in the Hydrogen Lyα Line Authors: Ishikawa, R.; Asensio Ramos, A.; Belluzzi, L.; Manso Sainz, R.; Štěpán, J.; Trujillo Bueno, J.; Goto, M.; Tsuneta, S. Bibcode: 2014ApJ...787..159I Altcode: 2014arXiv1404.0786I Magnetic field measurements in the upper chromosphere and above, where the gas-to-magnetic pressure ratio β is lower than unity, are essential for understanding the thermal structure and dynamical activity of the solar atmosphere. Recent developments in the theory and numerical modeling of polarization in spectral lines have suggested that information on the magnetic field of the chromosphere-corona transition region could be obtained by measuring the linear polarization of the solar disk radiation at the core of the hydrogen Lyα line at 121.6 nm, which is produced by scattering processes and the Hanle effect. The Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) sounding rocket experiment aims to measure the intensity (Stokes I) and the linear polarization profiles (Q/I and U/I) of the hydrogen Lyα line. In this paper, we clarify the information that the Hanle effect can provide by applying a Stokes inversion technique based on a database search. The database contains all theoretical Q/I and U/I profiles calculated in a one-dimensional semi-empirical model of the solar atmosphere for all possible values of the strength, inclination, and azimuth of the magnetic field vector, though this atmospheric region is highly inhomogeneous and dynamic. We focus on understanding the sensitivity of the inversion results to the noise and spectral resolution of the synthetic observations as well as the ambiguities and limitation inherent to the Hanle effect when only the hydrogen Lyα is used. We conclude that spectropolarimetric observations with CLASP can indeed be a suitable diagnostic tool for probing the magnetism of the transition region, especially when complemented with information on the magnetic field azimuth that can be obtained from other instruments. Title: Depolarizing Collisions with Hydrogen: Neutral and Singly Ionized Alkaline Earths Authors: Manso Sainz, Rafael; Roncero, Octavio; Sanz-Sanz, Cristina; Aguado, Alfredo; Asensio Ramos, Andrés; Trujillo Bueno, Javier Bibcode: 2014ApJ...788..118M Altcode: 2014arXiv1404.6339M Depolarizing collisions are elastic or quasielastic collisions that equalize the populations and destroy the coherence between the magnetic sublevels of atomic levels. In astrophysical plasmas, the main depolarizing collider is neutral hydrogen. We consider depolarizing rates on the lowest levels of neutral and singly ionized alkali earths Mg I, Sr I, Ba I, Mg II, Ca II, and Ba II, due to collisions with H°. We compute ab initio potential curves of the atom-H° system and solve the quantum mechanical dynamics. From the scattering amplitudes, we calculate the depolarizing rates for Maxwellian distributions of colliders at temperatures T <= 10,000 K. A comparative analysis of our results and previous calculations in the literature is completed. We discuss the effect of these rates on the formation of scattering polarization patterns of resonant lines of alkali earths in the solar atmosphere, and their effect on Hanle effect diagnostics of solar magnetic fields. Title: The transfer of resonance line polarization with partial frequency redistribution and J-state interference. Theoretical approach and numerical methods Authors: Belluzzi, L.; Trujillo Bueno, J. Bibcode: 2014A&A...564A..16B Altcode: 2014arXiv1403.1701B The linear polarization signals produced by scattering processes in strong resonance lines are rich in information on the magnetic and thermal structure of the chromosphere and transition region of the Sun and of other stars. A correct modeling of these signals requires accounting for partial frequency redistribution effects, as well as for the impact of quantum interference between different fine structure levels (J-state interference). In this paper, we present a theoretical approach suitable for modeling the transfer of resonance line polarization when taking these effects into account, along with an accurate numerical method of solution of the problem's equations. We consider a two-term atom with unpolarized lower term and infinitely sharp lower levels, in the absence of magnetic fields. We show that by making simple formal substitutions on the quantum numbers, the theoretical approach derived here for a two-term atom can also be applied to describe a two-level atom with hyperfine structure. An illustrative application to the Mg ii doublet around 2800 Å is presented. Title: Continuum polarization of stars as a result of occupation by transiting exoplanets Authors: Shchukina, N.; Frantseva, K.; Trujillo Bueno, J. Bibcode: 2014ysc..conf...12S Altcode: No abstract at ADS Title: Time Evolution of Plasma Parameters during the Rise of a Solar Prominence Instability Authors: Orozco Suárez, D.; Díaz, A. J.; Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2014ApJ...785L..10O Altcode: 2014arXiv1403.5640O We present high-spatial resolution spectropolarimetric observations of a quiescent hedgerow prominence taken in the He I 1083.0 nm triplet. The observation consisted of a time series in sit-and-stare mode of ~36 minutes duration. The spectrograph's slit crossed the prominence body and we recorded the time evolution of individual vertical threads. Eventually, we observed the development of a dark Rayleigh-Taylor plume that propagated upward with a velocity, projected onto the plane of the sky, of 17 km s-1. Interestingly, the plume apex collided with the prominence threads pushing them aside. We inferred Doppler shifts, Doppler widths, and magnetic field strength variations by interpreting the He I Stokes profiles with the HAZEL code. The Doppler shifts show that clusters of threads move coherently while individual threads have oscillatory patterns. Regarding the plume we found strong redshifts (~9-12 km s-1) and large Doppler widths (~10 km s-1) at the plume apex when it passed through the prominence body and before it disintegrated. We associate the redshifts with perspective effects while the Doppler widths are more likely due to an increase in the local temperature. No local variations of the magnetic field strength associated with the passage of the plume were found; this leads us to conclude that the plumes are no more magnetized than the surroundings. Finally, we found that some of the threads' oscillations are locally damped, what allowed us to apply prominence seismology techniques to infer additional prominence physical parameters. Title: Non-coherent Continuum Scattering as a Line Polarization Mechanism Authors: del Pino Alemán, T.; Manso Sainz, R.; Trujillo Bueno, J. Bibcode: 2014ApJ...784...46D Altcode: 2014arXiv1401.8094D Line scattering polarization can be strongly affected by Rayleigh scattering at neutral hydrogen and Thomson scattering at free electrons. Often a depolarization of the continuum results, but the Doppler redistribution produced by the continuum scatterers, which are light (hence, fast), induces more complex interactions between the polarization in spectral lines and in the continuum. Here we formulate and solve the radiative transfer problem of scattering line polarization with non-coherent continuum scattering consistently. The problem is formulated within the spherical tensor representation of atomic and light polarization. The numerical method of solution is a generalization of the Accelerated Lambda Iteration that is applied to both the atomic system and the radiation field. We show that the redistribution of the spectral line radiation due to the non-coherence of the continuum scattering may modify the shape of the emergent fractional linear polarization patterns significantly, even yielding polarization signals above the continuum level in intrinsically unpolarizable lines. Title: Chromospheric Lyman-alpha spectro-polarimeter (CLASP) Authors: Kano, Ryouhei; Katsukawa, Yukio; Kubo, Masahito; Auchere, Frederic; Ishikawa, Ryohko; Kobayashi, Ken; Narukage, Noriyuki; Trujillo Bueno, Javier; Bando, Takamasa; Ishikawa, Shin-nosuke Bibcode: 2014cosp...40E1383K Altcode: In the solar chromosphere, magneto-hydrodynamic waves and super-sonic jets ubiquitously happen as revealed by the Japanese solar satellite Hinode. Now, we understand that the solar chromosphere is not a simple intermediate layer smoothly connecting the photosphere and corona, but a site where those dynamics may play an important role in the chromospheric and coronal heating. Such discoveries imply that the next frontier in solar physics lies in simultaneous observations between the dynamics and magnetic structures in the chromosphere and transition region, where the gas-dominant photosphere changes to the magnetic-dominant corona. Therefore, we promote the Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP), which is a NASA's sounding rocket experiment scheduled in 2015 for aiming to infer the magnetic field information in the solar chromosphere and transition region. CLASP makes precise measurement (0.1%) of the polarization profile of the Lyman-alpha line, and aims to make the first ever measurement of the Hanle effect polarization caused by magnetic fields in the upper solar atmosphere. It is also a pathfinder to establish a new measurement tool for chromospheric and transition-region magnetic fields, and to make progress on chromospheric studies in future missions. Title: Polarized Radiation Diagnostics for Exploring the Magnetic Activity of the Chromosphere, Transition Region and Corona Authors: Trujillo Bueno, Javier Bibcode: 2014cosp...40E3395T Altcode: In order to investigate the magnetic activity of the chromosphere, transition region and corona of the Sun we need (1) to identify observables sensitive to the magnetic fields of such outer atmospheric regions, (2) to build the telescopes and instruments needed for measuring the observables, and (3) to develop suitable plasma diagnostic techniques to infer the relevant physical quantities from the observables (e.g., the strength and orientation of the magnetic field). Here I critically review old and recent investigations on the polarization produced by anisotropic radiation pumping and the Hanle and Zeeman effects in selected IR, FUV and EUV spectral lines, showing their sensitivity to the presence of a magnetic field in the chromosphere, transition region and corona. I argue that spectropolarimetry from ground-based and space telescopes is indeed our key gateway for the exploration of the magnetic activity of the outer solar atmosphere, but I also emphasize that we need novel breakthroughs in the development of instruments and polarized radiation diagnostics to achieve such goal. Title: A first look into the magnetic field configuration of prominence threads using spectropolarimetric data Authors: Orozco Suárez, D.; Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2014IAUS..300..112O Altcode: 2014IAUS..300..112S; 2013arXiv1310.0257O We show preliminary results of an ongoing investigation aimed at determining the configuration of the magnetic field vector in the threads of a quiescent hedgerow solar prominence using high-spatial resolution spectropolarimetric observations taken in the He I 1083.0 nm multiplet. The data consist of a two-dimensional map of a quiescent hedgerow prominence showing vertical threads. The observations were obtained with the Tenerife Infrared Polarimeter attached to the German Vacuum Tower Telescope at the Observatorio del Teide (Spain). The He I 1083.0 nm Stokes signals are interpreted with an inversion code, which takes into account the key physical processes that generate and/or modify circular and linear polarization signals in the He I 1083.0 nm triplet: the Zeeman effect, anisotropic radiation pumping, and the Hanle effect. We present initial results of the inversions, i.e, the strength and orientation of the magnetic field vector along the prominence and in prominence threads. Title: A Key Physical Mechanism for Understanding the Enigmatic Linear Polarization of the Solar Ba II and Na I D1 Lines Authors: Belluzzi, Luca; Trujillo Bueno, Javier Bibcode: 2013ApJ...774L..28B Altcode: 2013arXiv1308.5422B The linearly polarized spectrum of the solar limb radiation produced by scattering processes is of great diagnostic potential for exploring the magnetism of the solar atmosphere. This spectrum shows an impressive richness of spectral details and enigmatic Q/I signals, whose physical origin must be clearly understood before they can be exploited for diagnostic purposes. The most enduring enigma is represented by the polarization signals observed in the D1 resonance lines of Na I (5896 Å) and Ba II (4934 Å), which were expected to be intrinsically unpolarizable. The totality of sodium and 18% of barium have hyperfine structure (HFS), and it has been argued that the only way to produce a scattering polarization signal in such lines is through the presence of a substantial amount of atomic polarization in their lower HFS levels. The strong sensitivity of these long-lived levels to depolarizing mechanisms led to the paradoxical conclusion that the observed D1-line polarization is incompatible with the presence in the lower solar chromosphere of inclined magnetic fields sensibly stronger than 0.01 G. Here we show that by properly taking into account the fact that the solar D1-line radiation has a non-negligible spectral structure over the short frequency interval spanned by the HFS transitions, it is possible to produce scattering polarization signals in the D1 lines of Na I and Ba II without the need of ground-level polarization. The resulting linear polarization is not so easily destroyed by elastic collisions and/or magnetic fields. Title: PORTA: A three-dimensional multilevel radiative transfer code for modeling the intensity and polarization of spectral lines with massively parallel computers Authors: Štěpán, Jiří; Trujillo Bueno, Javier Bibcode: 2013A&A...557A.143S Altcode: 2013arXiv1307.4217S The interpretation of the intensity and polarization of the spectral line radiation produced in the atmosphere of the Sun and of other stars requires solving a radiative transfer problem that can be very complex, especially when the main interest lies in modeling the spectral line polarization produced by scattering processes and the Hanle and Zeeman effects. One of the difficulties is that the plasma of a stellar atmosphere can be highly inhomogeneous and dynamic, which implies the need to solve the non-equilibrium problem of the generation and transfer of polarized radiation in realistic three-dimensional (3D) stellar atmospheric models. Here we present PORTA, an efficient multilevel radiative transfer code we have developed for the simulation of the spectral line polarization caused by scattering processes and the Hanle and Zeeman effects in 3D models of stellar atmospheres. The numerical method of solution is based on the non-linear multigrid iterative method and on a novel short-characteristics formal solver of the Stokes-vector transfer equation which uses monotonic Bézier interpolation. Therefore, with PORTA the computing time needed to obtain at each spatial grid point the self-consistent values of the atomic density matrix (which quantifies the excitation state of the atomic system) scales linearly with the total number of grid points. Another crucial feature of PORTA is its parallelization strategy, which allows us to speed up the numerical solution of complicated 3D problems by several orders of magnitude with respect to sequential radiative transfer approaches, given its excellent linear scaling with the number of available processors. The PORTA code can also be conveniently applied to solve the simpler 3D radiative transfer problem of unpolarized radiation in multilevel systems. Title: Chromospheric Lyman Alpha SpectroPolarimeter: CLASP Authors: Kobayashi, Ken; Kano, R.; Trujillo Bueno, J.; Winebarger, A. R.; Cirtain, J. W.; Bando, T.; De Pontieu, B.; Ishikawa, R.; Katsukawa, Y.; Kubo, M.; Narukage, N.; Sakao, T.; Tsuneta, S.; Auchère, F.; Asensio Ramos, A.; Belluzzi, L.; Carlsson, M.; Casini, R.; Hara, H.; Ichimoto, K.; Manso Sainz, R.; Shimizu, T.; Stepan, J.; Suematsu, Y.; Holloway, T. Bibcode: 2013SPD....44..142K Altcode: The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a VUV spectropolarimeter optimized for measuring the linear polarization of the Lyman-alpha line (121.6 nm). The Lyman-alpha line is predicted to show linear polarization caused by atomic scattering in the chromosphere and modified by the magnetic field through the Hanle effect. The Hanle effect is sensitive to weaker magnetic fields than Zeeman effect, and is not canceled by opposing fields, making it sensitive to tangled or unresolved magnetic field structures. These factors make the Hanle effect a valuable tool for probing the magnetic field in the chromosphere above the quiet sun. To meet this goal, CLASP is designed to measure linear polarization with 0.1% polarization sensitivity at 0.01 nm spectral resolution and 10" spatial resolution. CLASP is scheduled to be launched in 2015. Title: Spectropolarimetric diagnostics of unresolved magnetic fields in the quiet solar photosphere Authors: Shchukina, Nataliya G.; Trujillo Bueno, Javier Bibcode: 2013IAUS..294..107S Altcode: 2012arXiv1212.3048S A few years before the Hinode space telescope was launched, an investigation based on the Hanle effect in atomic and molecular lines indicated that the bulk of the quiet solar photosphere is significantly magnetized, due to the ubiquitous presence of an unresolved magnetic field with an average strength <B>, ~ 130 G. It was pointed out also that this ``hidden'' field must be much stronger in the intergranular regions of solar surface convection than in the granular regions, and it was suggested that this unresolved magnetic field could perhaps provide the clue for understanding how the outer solar atmosphere is energized. In fact, the ensuing magnetic energy density is so significant that the energy flux estimated using the typical value of 1 km/s for the convective velocity (thinking in rising magnetic loops) or the Alfvén speed (thinking in Alfvén waves generated by magnetic reconnection) turns out to be substantially larger than that required to balance the chromospheric energy losses. Here we present a brief review of the research that led to such conclusions, with emphasis on a new three-dimensional radiative transfer investigation aimed at determining the magnetization of the quiet Sun photosphere from the Hanle effect in the Sr I 4607 Å line and the Zeeman effect in Fe I lines. Title: Measuring vector magnetic fields in solar prominences Authors: Orozco Suárez, D.; Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2013hsa7.conf..786O Altcode: 2012arXiv1211.2119O We present spectropolarimetric observations in the He I 1083.0 nm multiplet of a quiescent, hedgerow solar prominence. The data were taken with the Tenerife Infrared Polarimeter attached to the German Vacuum Tower Telescope at the Observatorio del Teide (Tenerife; Canary Islands; Spain). The observed He I circular and linear polarization signals are dominated by the Zeeman effect and by atomic level polarization and the Hanle effect, respectively. These observables are sensitive to the strength and orientation of the magnetic field vector at each spatial point of the field of view. We determine the magnetic field vector of the prominence by applying the HAZEL inversion code to the observed Stokes profiles. We briefly discuss the retrieved magnetic field vector configuration. Title: Theoretical formulation of Doppler redistribution in scattering polarization within the framework of the velocity-space density matrix formalism Authors: Belluzzi, L.; Landi Degl'Innocenti, E.; Trujillo Bueno, J. Bibcode: 2013A&A...552A..72B Altcode: 2013arXiv1302.2887B Within the framework of the density matrix theory for the generation and transfer of polarized radiation, velocity density matrix correlations represent an important physical aspect that, however, is often neglected in practical applications when adopting the simplifying approximation of complete redistribution on velocity. In this paper, we present an application of the non-LTE problem for polarized radiation taking such correlations into account through the velocity-space density matrix formalism. We consider a two-level atom with infinitely sharp upper and lower levels, and we derive the corresponding statistical equilibrium equations, neglecting the contribution of velocity-changing collisions. Coupling such equations with the radiative transfer equations for polarized radiation, we derive a set of coupled equations for the velocity-dependent source function. This set of equations is then particularized to the case of a plane-parallel atmosphere. The equations presented in this paper provide a complete and solid description of the physics of pure Doppler redistribution, a phenomenon generally described within the framework of the redistribution matrix formalism. The redistribution matrix corresponding to this problem (generally referred to as RI) is derived starting from the statistical equilibrium equations for the velocity-space density matrix and from the radiative transfer equations for polarized radiation, thus showing the equivalence of the two approaches. Title: Isotropic inelastic and superelastic collisional rates in a multiterm atom Authors: Belluzzi, L.; Landi Degl'Innocenti, E.; Trujillo Bueno, J. Bibcode: 2013A&A...551A..84B Altcode: 2013arXiv1302.0990B The spectral line polarization of the radiation emerging from a magnetized astrophysical plasma depends on the state of the atoms within the medium, whose determination requires considering the interactions between the atoms and the magnetic field, between the atoms and photons (radiative transitions), and between the atoms and other material particles (collisional transitions). In applications within the framework of the multiterm model atom (which accounts for quantum interference between magnetic sublevels pertaining either to the same J-level or to different J-levels within the same term) collisional processes are generally neglected when solving the master equation for the atomic density matrix. This is partly due to the lack of experimental data and/or of approximate theoretical expressions for calculating the collisional transfer and relaxation rates (in particular the rates for interference between sublevels pertaining to different J-levels, and the depolarizing rates due to elastic collisions). In this paper we formally define and investigate the transfer and relaxation rates due to isotropic inelastic and superelastic collisions that enter the statistical equilibrium equations for the atomic density matrix of a multiterm atom. Under the hypothesis that the interaction between the collider and the atom can be described by a dipolar operator, we provide expressions that relate the collisional rates for interference between different J-levels to the usual collisional rates for J-level populations, for which experimental data or approximate theoretical expressions are generally available. We show that the rates for populations and interference within the same J-level reduce to those previously obtained for the multilevel model atom (where quantum interference is assumed to be present only between magnetic sublevels pertaining to any given J-level). Finally, we apply the general equations to the case of a two-term atom with unpolarized lower term, illustrating the impact of inelastic and superelastic collisions on the scattering line polarization through radiative transfer calculations in a slab of stellar atmospheric plasma anisotropically illuminated by the photospheric radiation field. Title: Temporal Evolution of the Scattering Polarization of the Ca II IR Triplet in Hydrodynamical Models of the Solar Chromosphere Authors: Carlin, E. S.; Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2013ApJ...764...40C Altcode: 2012arXiv1210.1525C Velocity gradients in a stellar atmospheric plasma have an effect on the anisotropy of the radiation field that illuminates each point within the medium, and this may in principle influence the scattering line polarization that results from the induced atomic level polarization. Here, we analyze the emergent linear polarization profiles of the Ca II infrared triplet after solving the radiative transfer problem of scattering polarization in time-dependent hydrodynamical models of the solar chromosphere, taking into account the effect of the plasma macroscopic velocity on the atomic level polarization. We discuss the influence that the velocity and temperature shocks in the considered chromospheric models have on the temporal evolution of the scattering polarization signals of the Ca II infrared lines as well as on the temporally averaged profiles. Our results indicate that the increase of the linear polarization amplitudes caused by macroscopic velocity gradients may be significant in realistic situations. We also study the effect of the integration time, the microturbulent velocity, and the photospheric dynamical conditions, and discuss the feasibility of observing with large-aperture telescopes the temporal variation of the scattering polarization profiles. Finally, we explore the possibility of using a Hanle effect line-ratio technique in the IR triplet of Ca II to facilitate magnetic field diagnostics in dynamic situations. Title: Evidence for Rotational Motions in the Feet of a Quiescent Solar Prominence Authors: Orozco Suárez, D.; Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2012ApJ...761L..25O Altcode: 2012arXiv1211.6980O We present observational evidence of apparent plasma rotational motions in the feet of a solar prominence. Our study is based on spectroscopic observations taken in the He I 1083.0 nm multiplet with the Tenerife Infrared Polarimeter attached to the German Vacuum Tower Telescope. We recorded a time sequence of spectra with 34 s cadence placing the slit of the spectrograph almost parallel to the solar limb and crossing two feet of an intermediate size, quiescent hedgerow prominence. The data show opposite Doppler shifts, ±6 km s-1, at the edges of the prominence feet. We argue that these shifts may be interpreted as prominence plasma rotating counterclockwise around the vertical axis to the solar surface as viewed from above. The evolution of the prominence seen in EUV images taken with the Solar Dynamics Observatory provided us with clues to interpret the results as swirling motions. Moreover, time-distance images taken far from the central wavelength show plasma structures moving parallel to the solar limb with velocities of about 10-15 km s-1. Finally, the shapes of the observed intensity profiles suggest the presence of, at least, two components at some locations at the edges of the prominence feet. One of them is typically Doppler shifted (up to ~20 km s-1) with respect to the other, thus suggesting the existence of supersonic counter-streaming flows along the line of sight. Title: Polarized Radiation Diagnostics for Measuring the Magnetic Field of the Outer Solar Atmosphere Authors: Trujillo Bueno, J. Bibcode: 2012IAUSS...6E.302T Altcode: The basic idea of optical pumping, for which Alfred Kastler received the 1966 Nobel Prize in physics, is that the absorption and scattering of light that is near-resonant with an optical transition can produce large population imbalances among the magnetic sublevels of atomic ground states as well as in excited states. The degree of this radiatively-induced atomic level polarization, which is very sensitive to the presence of magnetic fields, can be determined by observing the polarization of the scattered or transmitted spectral line radiation. Probably, the most important point for solar physics is that the outer solar atmosphere is indeed an optically pumped vapor and that the polarization of the emergent spectral line radiation can be exploited for detecting magnetic fields that are too weak and/or too tangled so as to produce measurable Zeeman polarization signals. In this talk we review some recent radiative transfer simulations of the polarization produced by optical pumping in selected IR, FUV and EUV spectral lines, showing that their sensitivity to the Hanle effect is very suitable for magnetic field measurements in the outer solar atmosphere. We argue that solar magnetometry using the spectral lines of optically pumped atoms in the chromosphere, transition region and corona should be a high-priority goal for large aperture solar telescopes, such as ATST, EST and SOLAR-C. Title: LEMUR: Large European module for solar Ultraviolet Research. European contribution to JAXA's Solar-C mission Authors: Teriaca, Luca; Andretta, Vincenzo; Auchère, Frédéric; Brown, Charles M.; Buchlin, Eric; Cauzzi, Gianna; Culhane, J. Len; Curdt, Werner; Davila, Joseph M.; Del Zanna, Giulio; Doschek, George A.; Fineschi, Silvano; Fludra, Andrzej; Gallagher, Peter T.; Green, Lucie; Harra, Louise K.; Imada, Shinsuke; Innes, Davina; Kliem, Bernhard; Korendyke, Clarence; Mariska, John T.; Martínez-Pillet, Valentin; Parenti, Susanna; Patsourakos, Spiros; Peter, Hardi; Poletto, Luca; Rutten, Robert J.; Schühle, Udo; Siemer, Martin; Shimizu, Toshifumi; Socas-Navarro, Hector; Solanki, Sami K.; Spadaro, Daniele; Trujillo-Bueno, Javier; Tsuneta, Saku; Dominguez, Santiago Vargas; Vial, Jean-Claude; Walsh, Robert; Warren, Harry P.; Wiegelmann, Thomas; Winter, Berend; Young, Peter Bibcode: 2012ExA....34..273T Altcode: 2011ExA...tmp..135T; 2011arXiv1109.4301T The solar outer atmosphere is an extremely dynamic environment characterized by the continuous interplay between the plasma and the magnetic field that generates and permeates it. Such interactions play a fundamental role in hugely diverse astrophysical systems, but occur at scales that cannot be studied outside the solar system. Understanding this complex system requires concerted, simultaneous solar observations from the visible to the vacuum ultraviolet (VUV) and soft X-rays, at high spatial resolution (between 0.1'' and 0.3''), at high temporal resolution (on the order of 10 s, i.e., the time scale of chromospheric dynamics), with a wide temperature coverage (0.01 MK to 20 MK, from the chromosphere to the flaring corona), and the capability of measuring magnetic fields through spectropolarimetry at visible and near-infrared wavelengths. Simultaneous spectroscopic measurements sampling the entire temperature range are particularly important. These requirements are fulfilled by the Japanese Solar-C mission (Plan B), composed of a spacecraft in a geosynchronous orbit with a payload providing a significant improvement of imaging and spectropolarimetric capabilities in the UV, visible, and near-infrared with respect to what is available today and foreseen in the near future. The Large European Module for solar Ultraviolet Research (LEMUR), described in this paper, is a large VUV telescope feeding a scientific payload of high-resolution imaging spectrographs and cameras. LEMUR consists of two major components: a VUV solar telescope with a 30 cm diameter mirror and a focal length of 3.6 m, and a focal-plane package composed of VUV spectrometers covering six carefully chosen wavelength ranges between 170 Å and 1270 Å. The LEMUR slit covers 280'' on the Sun with 0.14'' per pixel sampling. In addition, LEMUR is capable of measuring mass flows velocities (line shifts) down to 2 km s - 1 or better. LEMUR has been proposed to ESA as the European contribution to the Solar C mission. Title: The Hanle Effect of Lyα in a Magnetohydrodynamic Model of the Solar Transition Region Authors: Štěpán, J.; Trujillo Bueno, J.; Carlsson, M.; Leenaarts, J. Bibcode: 2012ApJ...758L..43S Altcode: 2012arXiv1208.4929S In order to understand the heating of the solar corona it is crucial to obtain empirical information on the magnetic field in its lower boundary (the transition region). To this end, we need to measure and model the linear polarization produced by scattering processes in strong UV lines, such as the hydrogen Lyα line. The interpretation of the observed Stokes profiles will require taking into account that the outer solar atmosphere is highly structured and dynamic, and that the height of the transition region may well vary from one place in the atmosphere to another. Here, we report on the Lyα scattering polarization signals we have calculated in a realistic model of an enhanced network region, resulting from a state-of-the-art radiation magnetohydrodynamic simulation. This model is characterized by spatially complex variations of the physical quantities at transition region heights. The results of our investigation lead us to emphasize that scattering processes in the upper solar chromosphere should indeed produce measurable linear polarization in Lyα. More importantly, we show that via the Hanle effect the model's magnetic field produces significant changes in the emergent Q/I and U/I profiles. Therefore, we argue that by measuring the polarization signals produced by scattering processes and the Hanle effect in Lyα and contrasting them with those computed in increasingly realistic atmospheric models, we should be able to decipher the magnetic, thermal, and dynamic structure of the upper chromosphere and transition region of the Sun. Title: Chromospheric Lyman-alpha spectro-polarimeter (CLASP) Authors: Kano, Ryouhei; Bando, Takamasa; Narukage, Noriyuki; Ishikawa, Ryoko; Tsuneta, Saku; Katsukawa, Yukio; Kubo, Masahito; Ishikawa, Shin-nosuke; Hara, Hirohisa; Shimizu, Toshifumi; Suematsu, Yoshinori; Ichimoto, Kiyoshi; Sakao, Taro; Goto, Motoshi; Kato, Yoshiaki; Imada, Shinsuke; Kobayashi, Ken; Holloway, Todd; Winebarger, Amy; Cirtain, Jonathan; De Pontieu, Bart; Casini, Roberto; Trujillo Bueno, Javier; Štepán, Jiří; Manso Sainz, Rafael; Belluzzi, Luca; Asensio Ramos, Andres; Auchère, Frédéric; Carlsson, Mats Bibcode: 2012SPIE.8443E..4FK Altcode: One of the biggest challenges in heliophysics is to decipher the magnetic structure of the solar chromosphere. The importance of measuring the chromospheric magnetic field is due to both the key role the chromosphere plays in energizing and structuring the outer solar atmosphere and the inability of extrapolation of photospheric fields to adequately describe this key boundary region. Over the last few years, significant progress has been made in the spectral line formation of UV lines as well as the MHD modeling of the solar atmosphere. It is found that the Hanle effect in the Lyman-alpha line (121.567 nm) is a most promising diagnostic tool for weaker magnetic fields in the chromosphere and transition region. Based on this groundbreaking research, we propose the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) to NASA as a sounding rocket experiment, for making the first measurement of the linear polarization produced by scattering processes and the Hanle effect in the Lyman-alpha line (121.567 nm), and making the first exploration of the magnetic field in the upper chromosphere and transition region of the Sun. The CLASP instrument consists of a Cassegrain telescope, a rotating 1/2-wave plate, a dual-beam spectrograph assembly with a grating working as a beam splitter, and an identical pair of reflective polarization analyzers each equipped with a CCD camera. We propose to launch CLASP in December 2014. Title: The Scattering Polarization of the Lyα Lines of H I and He II Taking into Account Partial Frequency Redistribution and J-state Interference Effects Authors: Belluzzi, Luca; Trujillo Bueno, Javier; Štěpán, Jiří Bibcode: 2012ApJ...755L...2B Altcode: 2012arXiv1207.0415B Recent theoretical investigations have pointed out that the cores of the Lyα lines of H I and He II should show measurable scattering polarization signals when observing the solar disk, and that the magnetic sensitivity, through the Hanle effect, of such linear polarization signals is suitable for exploring the magnetism of the solar transition region. Such investigations were carried out in the limit of complete frequency redistribution (CRD) and neglecting quantum interference between the two upper J-levels of each line. Here we relax both approximations and show that the joint action of partial frequency redistribution and J-state interference produces much more complex fractional linear polarization (Q/I) profiles, with large amplitudes in their wings. Such wing polarization signals turn out to be very sensitive to the temperature structure of the atmospheric model, so that they can be exploited for constraining the thermal properties of the solar chromosphere. Finally, we show that the approximation of CRD without J-state interference is however suitable for estimating the amplitude of the linear polarization signals in the core of the lines, where the Hanle effect operates. Title: Non-LTE Determination of the Silicon Abundance Using a Three-dimensional Hydrodynamical Model of the Solar Photosphere Authors: Shchukina, N.; Sukhorukov, A.; Trujillo Bueno, J. Bibcode: 2012ApJ...755..176S Altcode: Confrontations of spectroscopic observations with local thermodynamic equilibrium (LTE) spectral syntheses in a three-dimensional (3D) hydrodynamical model of the solar photosphere led to a downward revision of the photospheric and meteoritic silicon abundances. Here we derive the photospheric silicon abundance taking into account non-LTE (NLTE) effects in the same 3D model. We show that the above-mentioned downward revision of the silicon abundance is caused by using the LTE approximation in the context of 3D modeling, an experimental scale of oscillator strengths, and a small number of Si I lines. We demonstrate that no revision of the solar silicon abundance is required if NLTE effects are taken into account and one uses a "solar" oscillator strength scale and an extended list of Si I lines. The NLTE abundance value we find by fitting the equivalent widths of 65 Si I lines is A NLTE Si = 7.549 ± 0.016. This value agrees well with the silicon abundance that had been recommended earlier by Grevesse & Sauval and Lodders for the solar photosphere and CI chondrite meteorites. Title: Scattering Polarization in the Ca II Infrared Triplet with Velocity Gradients Authors: Carlin, E. S.; Manso Sainz, R.; Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2012ApJ...751....5C Altcode: 2012arXiv1203.4438C Magnetic field topology, thermal structure, and plasma motions are the three main factors affecting the polarization signals used to understand our star. In this theoretical investigation, we focus on the effect that gradients in the macroscopic vertical velocity field have on the non-magnetic scattering polarization signals, establishing the basis for general cases. We demonstrate that the solar plasma velocity gradients may have a significant effect on the linear polarization produced by scattering in chromospheric spectral lines. In particular, we show the impact of velocity gradients on the anisotropy of the radiation field and on the ensuing fractional alignment of the Ca II levels, and how they can lead to an enhancement of the zero-field linear polarization signals. This investigation remarks on the importance of knowing the dynamical state of the solar atmosphere in order to correctly interpret spectropolarimetric measurements, which is important, among other things, for establishing a suitable zero-field reference case to infer magnetic fields via the Hanle effect. Title: The Chromospheric Lyman-Alpha SpectroPolarimeter: CLASP Authors: Kobayashi, K.; Kano, R.; Trujillo-Bueno, J.; Asensio Ramos, A.; Bando, T.; Belluzzi, L.; Carlsson, M.; De Pontieu, R. C. B.; Hara, H.; Ichimoto, K.; Ishikawa, R.; Katsukawa, Y.; Kubo, M.; Manso Sainz, R.; Narukage, N.; Sakao, T.; Stepan, J.; Suematsu, Y.; Tsuneta, S.; Watanabe, H.; Winebarger, A. Bibcode: 2012ASPC..456..233K Altcode: The magnetic field plays a crucial role in the chromosphere and the transition region, and our poor empirical knowledge of the magnetic field in the upper chromosphere and transition region is a major impediment to advancing the understanding of the solar atmosphere. The Hanle effect promises to be a valuable alternative to Zeeman effect as a method of measuring the magnetic field in the chromosphere and transition region; it is sensitive to weaker magnetic fields, and also sensitive to tangled, unresolved field structures.

CLASP is a sounding rocket experiment that aims to observe the Hanle effect polarization of the Lyman α (1215.67Å) line in the solar chromosphere and transition region, and prove the usefulness of this technique in placing constraints on the magnetic field strength and orientation in the low plasma-β region of the solar atmosphere. The Ly-α line has been chosen because it is a chromospheric/transition-region line, and because the Hanle effect polarization of this line is predicted to be sensitive to 10-250 Gauss, encompassing the range of interest. The CLASP instrument is designed to measure linear polarization in the Ly-α line with a polarization sensitivity of 0.1%. The instrument is currently funded for development. The optical design of the instrument has been finalized, and an extensive series of component-level tests are underway to validate the design. Title: The Polarization of the Solar Mg II h and k Lines Authors: Belluzzi, Luca; Trujillo Bueno, Javier Bibcode: 2012ApJ...750L..11B Altcode: 2012arXiv1203.4351B Although the h and k lines of Mg II are expected to be of great interest for probing the upper solar chromosphere, relatively little is known about their polarization properties which encode the information on the magnetic field. Here we report the first results of an investigation whose main goal is to understand the physical mechanisms that control the scattering polarization across these resonance lines and to achieve a realistic radiative transfer modeling in the presence of arbitrary magnetic fields. We show that the joint action of partial frequency redistribution (PRD) and quantum interference between the upper J-levels of the two lines produces a complex fractional linear polarization (Q/I) pattern with large polarization amplitudes in the blue and red wings, and a negative feature in the spectral region between the two lines. Another remarkable peculiarity of the Q/I profile is a conspicuous antisymmetric signal around the center of the h line, which cannot be obtained unless both PRD and J-state interference effects are taken into account. In the core of the k line, PRD effects alone produce a triplet peak structure in the Q/I profile, the modeling of which can also be achieved via the two-level atom approximation. In addition to the Hanle effect in the core of the k line, we also emphasize the diagnostic potential of the circular polarization produced by the Zeeman effect in the h and k lines, as well as in other Mg II lines located in their wings. Title: The Hanle Effect in the Lyα Lines of H I and He II for Measuring the Magnetic Fields of the Solar Transition Region Authors: Trujillo Bueno, J.; Štepán, J.; Belluzzi, L. Bibcode: 2012ASPC..456..225T Altcode: 2012ASPC..456..225B The Ly α lines of H I and He II are two of the spectral lines of choice for FUV and EUV channels of narrowband imagers on board sounding rockets and space telescopes, which provide spectacular intensity images of the outer solar atmosphere. Since the magnetic field information is encoded in the polarization of the spectral line radiation, it is important to investigate whether the ensuing Ly α radiation from the solar disk can be polarized, along with its magnetic sensitivity. Here we present some theoretical predictions concerning the amplitudes and magnetic sensitivities of the linear polarization signals produced by scattering processes in these strong emission lines of the solar transition region, taking into account radiative transfer and the Hanle effect caused by the presence of organized and random magnetic fields. We find that the line-center amplitudes of the fractional polarization signals vary typically between a fraction of a percent and ∼1%, depending on the Ly α line under consideration, the scattering geometry and the strength and orientation of the magnetic field. Interestingly, while the Ly α line of He II starts to be sensitive to the Hanle effect for magnetic strengths B>̰100 G the hydrogen Lyα line is mainly sensitive to magnetic strengths between 10 and 100 G. These results encourage the development of FUV and EUV polarimeters for sounding rockets and space telescopes with the aim of opening up a diagnostic window for magnetic field measurements in the upper chromosphere and transition region of the Sun. Title: A 3D Radiative Transfer Code for Modeling the Hanle Effect in the Lyman α line Authors: Štepán, J.; Trujillo Bueno, J. Bibcode: 2012ASPC..456...59S Altcode: 2012arXiv1205.2959S In order to obtain empirical information on the magnetism of the solar transition region we need to measure and interpret the linear polarization produced by scattering processes in FUV and EUV spectral lines. Via the Hanle effect such linear polarization signals are sensitive to the magnetic fields expected for the quiet and active regions of the outer solar atmosphere. For example, the Ly$\alpha$ line of H\,{\sc i} at 1216\,Å is mainly sensitive to magnetic strengths between 10 and 100 G. The interpretation of the observed spectral line polarization requires the development of suitable modeling tools. To this end, we have developed a three-dimensional (3D), non-LTE multilevel radiative transfer code for modeling the intensity and linear polarization produced by scattering processes in spectral lines and its modification by the Hanle effect. Title: Solar magnetism eXplorer (SolmeX). Exploring the magnetic field in the upper atmosphere of our closest star Authors: Peter, Hardi; Abbo, L.; Andretta, V.; Auchère, F.; Bemporad, A.; Berrilli, F.; Bommier, V.; Braukhane, A.; Casini, R.; Curdt, W.; Davila, J.; Dittus, H.; Fineschi, S.; Fludra, A.; Gandorfer, A.; Griffin, D.; Inhester, B.; Lagg, A.; Landi Degl'Innocenti, E.; Maiwald, V.; Sainz, R. Manso; Martínez Pillet, V; Matthews, S.; Moses, D.; Parenti, S.; Pietarila, A.; Quantius, D.; Raouafi, N. -E.; Raymond, J.; Rochus, P.; Romberg, O.; Schlotterer, M.; Schühle, U.; Solanki, S.; Spadaro, D.; Teriaca, L.; Tomczyk, S.; Trujillo Bueno, J.; Vial, J. -C. Bibcode: 2012ExA....33..271P Altcode: 2011arXiv1108.5304P; 2011ExA...tmp..134P The magnetic field plays a pivotal role in many fields of Astrophysics. This is especially true for the physics of the solar atmosphere. Measuring the magnetic field in the upper solar atmosphere is crucial to understand the nature of the underlying physical processes that drive the violent dynamics of the solar corona—that can also affect life on Earth. SolmeX, a fully equipped solar space observatory for remote-sensing observations, will provide the first comprehensive measurements of the strength and direction of the magnetic field in the upper solar atmosphere. The mission consists of two spacecraft, one carrying the instruments, and another one in formation flight at a distance of about 200 m carrying the occulter to provide an artificial total solar eclipse. This will ensure high-quality coronagraphic observations above the solar limb. SolmeX integrates two spectro-polarimetric coronagraphs for off-limb observations, one in the EUV and one in the IR, and three instruments for observations on the disk. The latter comprises one imaging polarimeter in the EUV for coronal studies, a spectro-polarimeter in the EUV to investigate the low corona, and an imaging spectro-polarimeter in the UV for chromospheric studies. SOHO and other existing missions have investigated the emission of the upper atmosphere in detail (not considering polarization), and as this will be the case also for missions planned for the near future. Therefore it is timely that SolmeX provides the final piece of the observational quest by measuring the magnetic field in the upper atmosphere through polarimetric observations. Title: Observables for Measuring the Outer-Atmospheric Magnetic Field from Chromosphere to Corona Authors: Trujillo Bueno, J.; et al. Bibcode: 2012decs.confE..35T Altcode: The basic idea of optical pumping, for which Alfred Kastler received the 1966 Nobel Prize in physics, is that the absorption and scattering of light that is near-resonant with an optical transition can produce large population imbalances among the magnetic sublevels of atomic ground states as well as in excited states. The degree of this radiatively-induced atomic level polarization, which is very sensitive to the presence of magnetic fields, can be determined by observing the intensity and polarization of the scattered or transmitted spectral line radiation. Probably, the most important point for solar physics is that the outer solar atmosphere is indeed an optically pumped vapor and that the polarization of the emergent spectral line radiation can be exploited for detecting magnetic fields that are too weak and/or too tangled so as to produce measurable Zeeman polarization signals. Here we present several radiative transfer simulations of the linear polarization produced by optical pumping in selected FUV and EUV lines of the solar atmosphere, showing that their sensitivity to the Hanle effect is very suitable for magnetic field measurements in the upper chromosphere and transition region of the Sun. These results suggest that solar magnetometry using the spectral lines of optically pumped atoms in the chromosphere, transition region and corona should be a high-priority goal for large aperture solar telescopes, such as ATST, EST and SOLAR-C. Title: The Lyα Lines of H I and He II: A Differential Hanle Effect for Exploring the Magnetism of the Solar Transition Region Authors: Trujillo Bueno, Javier; Štěpán, Jiří; Belluzzi, Luca Bibcode: 2012ApJ...746L...9T Altcode: 2011arXiv1112.4746T The Lyα line of He II at 304 Å is one of the spectral lines of choice for EUV channels of narrowband imagers on board space telescopes, which provide spectacular intensity images of the outer solar atmosphere. Since the magnetic field information is encoded in the polarization of the spectral line radiation, it is important to investigate whether the He II line radiation from the solar disk can be polarized, along with its magnetic sensitivity. Here we report some theoretical predictions concerning the linear polarization signals produced by scattering processes in this strong emission line of the solar transition region, taking into account radiative transfer and the Hanle effect caused by the presence of organized and random magnetic fields. We find that the fractional polarization amplitudes are significant (~1%), even when considering the wavelength-integrated signals. Interestingly, the scattering polarization of the Lyα line of He II starts to be sensitive to the Hanle effect for magnetic strengths B >~ 100 G (i.e., for magnetic strengths of the order of and larger than the Hanle saturation field of the hydrogen Lyα line at 1216 Å). We therefore propose simultaneous observations of the scattering polarization in both Lyα lines to facilitate magnetic field measurements in the upper solar chromosphere. Even the development of a narrowband imaging polarimeter for the He II 304 Å line alone would be already of great diagnostic value for probing the solar transition region. Title: Ly-alpha polarimeter design for CLASP rocket experiment Authors: Kubo, M.; Watanabe, H.; Narukage, N.; Ishikawa, R.; Bando, T.; Kano, R.; Tsuneta, S.; Kobayashi, K.; Ichimoto, K.; Trujillo Bueno, J.; Song, D. Bibcode: 2011AGUFM.P11F1627K Altcode: A sounding-rocket program called the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is proposed to be launched in the Summer of 2014. CLASP will observe the upper solar chromosphere in Ly-alpha (121.567 nm), aiming to detect the linear polarization signal produced by scattering processes and the Hanle effect for the first time. The CLASP needs a rotating half-waveplate and a polarization analyzer working at the Ly-alpha wavelength to measure the linear polarization signal. We select Magnesium Fluoride (MgF2) as a material of the optical components because of its birefringent property and high transparency at UV wavelength. We have confirmed that the reflection at the Brewster's Angle of MgF2 plate is a good polarization analyzer for the Ly-alpha line by deriving its ordinary refractive index and extinction coefficient along the ordinary and extraordinary axes. These optical parameters are calculated with a least-square fitting in such a way that the reflectance and transmittance satisfy the Kramers-Kronig relation. The reflectance and transmittance against oblique incident angles for the s-polarized and the p-polarized light are measured using the synchrotron beamline at the Ultraviolet Synchrotron Orbital Radiation Facility (UVSOR). We have also measured a retardation of a zeroth-order waveplate made of MgF2. The thickness difference of the waveplate is 14.57 um.This waveplate works as a half-waveplate at 121.74 nm. From this measurement, we estimate that a waveplate with the thickness difference of 15.71 um will work as a half-waveplate at the Ly-alpha wavelength. We have developed a rotating waveplate - polarization analyzer system called a prototype of CLASP polarimeter, and input the perfect Stokes Q and U signals. The modulation patterns that are consistent with the theoretical prediction are successfully obtained in both cases. Title: The Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP)j Authors: Kobayashi, K.; Tsuneta, S.; Trujillo Bueno, J.; Bando, T.; Belluzzi, L.; Casini, R.; Carlsson, M.; Cirtain, J. W.; De Pontieu, B.; Hara, H.; Ichimoto, K.; Ishikawa, R.; Kano, R.; Katsukawa, Y.; Kim, T.; Kubo, M.; Manso Sainz, R.; Narukage, N.; Asensio Ramos, A.; Robinson, B.; Sakao, T.; Shimizu, T.; Stepan, J.; Suematsu, Y.; Watanabe, H.; West, E.; Winebarger, A. R. Bibcode: 2011AGUFM.P14C..05K Altcode: We present an overview of the Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) program. CLASP is a proposed sounding rocket experiment currently under development as collaboration between Japan, USA and Spain. The aim is to achieve the first measurement of magnetic field in the upper chromosphere and transition region of the Sun through the detection and measurement of Hanle effect polarization of the Lyman alpha line. The Hanle effect (i.e. the magnetic field induced modification of the linear polarization due to scattering processes in spectral lines) is believed to be a powerful tool for measuring the magnetic field in the upper chromosphere, as it is more sensitive to weaker magnetic fields than the Zeeman effect, and also sensitive to magnetic fields tangled at spatial scales too small to be resolved. The Lyman-alpha (121.567 nm) line has been chosen because it is a chromospheric/transition-region line, and because the Hanle effect polarization of the Lyman-alpha line is predicted to be sensitive to 10-250 Gauss, encompassing the range of interest. Hanle effect is predicted to be observable as linear polarization or depolarization, depending on the geometry, with a fractional polarization amplitude varying between 0.1% and 1% depending on the strength and orientation of the magnetic field. This quantification of the chromospheric magnetic field requires a highly sensitive polarization measurement. The CLASP instrument consists of a large aperture (287 mm) Cassegrain telescope mated to a polarizing beamsplitter and a matched pair of grating spectrographs. The polarizing beamsplitter consists of a continuously rotating waveplate and a linear beamsplitter, allowing simultaneous measurement of orthogonal polarizations and in-flight self-calibration. Development of the instrument is underway, and prototypes of all optical components have been tested using a synchrotron beamline. The experiment is proposed for flight in 2014. Title: The Impact of Quantum Interference between Different J-levels on Scattering Polarization in Spectral Lines Authors: Belluzzi, Luca; Trujillo Bueno, Javier Bibcode: 2011ApJ...743....3B Altcode: 2011arXiv1109.0424B The spectral line polarization produced by optically pumped atoms contains a wealth of information on the thermal and magnetic structure of a variety of astrophysical plasmas, including that of the solar atmosphere. A correct decoding of such information from the observed Stokes profiles requires a clear understanding of the effects that radiatively induced quantum interference (or coherence) between pairs of magnetic sublevels produces on these observables, in the absence of and in the presence of magnetic fields of arbitrary strength. Here we present a detailed theoretical investigation of the role of coherence between pairs of sublevels pertaining to different fine-structure J-levels, clarifying when it can be neglected for facilitating the modeling of the linear polarization produced by scattering processes in spectral lines. To this end, we apply the quantum theory of spectral line polarization and calculate the linear polarization patterns of the radiation scattered at 90° by a slab of stellar atmospheric plasma, both taking into account and neglecting the above-mentioned quantum interference. Particular attention is given to the 2 S - 2 P, 5 S - 5 P, and 3 P - 3 S multiplets. We point out the observational signatures of this kind of interference and analyze its sensitivity to the energy separation between the interfering levels, to the amount of emissivity in the background continuum radiation, to lower-level polarization, and to the presence of a magnetic field. Some interesting applications to the following spectral lines are also presented: Ca II H and K, Mg II h and k, Na I D1 and D2, the Ba II 4554 Å and 4934 Å resonance lines, the Cr I triplet at 5207 Å, the O I triplet at 7773 Å, the Mg I b-lines, and the Hα and Lyα lines of H I. Title: Scattering Polarization and Hanle Effect in Stellar Atmospheres with Horizontal Inhomogeneities Authors: Manso Sainz, Rafael; Trujillo Bueno, Javier Bibcode: 2011ApJ...743...12M Altcode: 2011arXiv1108.2958M Scattering of light from an anisotropic source produces linear polarization in spectral lines and in the continuum. In the outer layers of a stellar atmosphere the anisotropy of the radiation field is typically dominated by the radiation escaping away, but local horizontal fluctuations of the physical conditions may also contribute, distorting the illumination and, hence, the polarization pattern. Additionally, a magnetic field may perturb and modify the line scattering polarization signals through the Hanle effect. Here, we study such symmetry-breaking effects. We develop a method to solve the transfer of polarized radiation in a scattering atmosphere with weak horizontal fluctuations of the opacity and source functions. It comprises linearization (small opacity and Planck function fluctuations are assumed), reduction to a quasi-plane-parallel problem through harmonic analysis, and the problem's numerical solution by generalized standard techniques. We apply this method to study scattering polarization in atmospheres with horizontal fluctuations in the Planck function and opacity. We derive several very general results and constraints from considerations on the symmetries and dimensionality of the problem, and we give explicit solutions of a few illustrative problems of special interest. For example, we show (1) how the amplitudes of the fractional linear polarization signals change when considering increasingly smaller horizontal atmospheric inhomogeneities, (2) that in the presence of such inhomogeneities even a vertical magnetic field may modify the scattering line polarization, and (3) that forward scattering polarization may be produced without the need for an inclined magnetic field. These results are important for understanding the physics of the problem and as benchmarks for multidimensional radiative transfer codes. Title: The Hanle Effect from Space for Measuring the Magnetic Fields of the Upper Solar Chromosphere Authors: Trujillo Bueno, J.; Stepan, J.; Belluzzi, L.; Manso Sainz, R. Bibcode: 2011AGUFM.P11F1626T Altcode: We present some theoretical predictions concerning the amplitudes and magnetic sensitivities of the linear polarization signals produced by scattering processes in some UV and FUV spectral lines of the upper chromosphere and transition region, such as Ly-alpha and Mg II k. To this end, we have calculated the atomic level polarization (population imbalances and quantum coherences) induced by anisotropic radiation pumping in semi-empirical and hydrodynamical models of the solar atmosphere, taking into account radiative transfer and the Hanle effect caused by the presence of organized and random magnetic fields. The amplitudes of the emergent linear polarization signals are found to vary typically between a fraction of a percent and a few percent, depending on the scattering geometry and the strength and orientation of the magnetic field. The results shown here encourage the development of UV polarimeters for sounding rockets and space telescopes with the aim of opening up a true diagnostic window for magnetic field measurements in the upper chromosphere and transition region of the Sun. Title: Ly-alpha polarimeter design for CLASP rocket experiment Authors: Watanabe, H.; Narukage, N.; Kubo, M.; Ishikawa, R.; Bando, T.; Kano, R.; Tsuneta, S.; Kobayashi, K.; Ichimoto, K.; Trujillo-Bueno, J. Bibcode: 2011SPIE.8148E..0TW Altcode: 2011SPIE.8148E..25W; 2014arXiv1407.4577W A sounding-rocket program called the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is proposed to be launched in the summer of 2014. CLASP will observe the solar chromosphere in Ly-alpha (121.567 nm), aiming to detect the linear polarization signal produced by scattering processes and the Hanle effect for the first time. The polarimeter of CLASP consists of a rotating half-waveplate, a beam splitter, and a polarization analyzer. Magnesium Fluoride (MgF2) is used for these optical components, because MgF2 exhibits birefringent property and high transparency at ultraviolet wavelength. The development and comprehensive testing program of the optical components of the polarimeter is underway using the synchrotron beamline at the Ultraviolet Synchrotron Orbital Radiation Facility (UVSOR). The first objective is deriving the optical constants of MgF2 by the measurement of the reflectance and transmittance against oblique incident angles for the s-polarized and the p-polarized light. The ordinary refractive index and extinction coefficient along the ordinary and extraordinary axes are derived with a least-square fitting in such a way that the reflectance and transmittance satisfy the Kramers-Krönig relation. The reflection at the Brewster's Angle of MgF2 plate is confirmed to become a good polarization analyzer at Ly-alpha. The second objective is the retardation measurement of a zeroth-order waveplate made of MgF2. The retardation of a waveplate is determined by observing the modulation amplitude that comes out of a waveplate and a polarization analyzer. We tested a waveplate with the thickness difference of 14.57 um. The 14.57 um waveplate worked as a half-waveplate at 121.74 nm. We derived that a waveplate with the thickness difference of 15.71 um will work as a half-waveplate at Ly-alpha wavelength. We developed a prototype of CLASP polarimeter using the MgF2 half-waveplate and polarization analyzers, and succeeded in obtaining the modulation patterns that are consistent with the theoretical prediction. We confirm that the performance of the prototype is optimized for measuring linear polarization signal with the least effect of the crosstalk from the circular polarization. Title: Overview of Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) Authors: Narukage, Noriyuki; Tsuneta, Saku; Bando, Takamasa; Kano, Ryouhei; Kubo, Masahito; Ishikawa, Ryoko; Hara, Hirohisa; Suematsu, Yoshinori; Katsukawa, Yukio; Watanabe, Hiroko; Ichimoto, Kiyoshi; Sakao, Taro; Shimizu, Toshifumi; Kobayashi, Ken; Robinson, Brian; Kim, Tony; Winebarger, Amy; West, Edward; Cirtain, Jonathan; De Pontieu, Bart; Casini, Roberto; Trujillo Bueno, Javier; Stepan, Jiri; Manso Sainz, Rafael; Belluzzi, Luca; Asensio Ramos, Andres; Carlsson, Mats Bibcode: 2011SPIE.8148E..0HN Altcode: 2011SPIE.8148E..16N The solar chromosphere is an important boundary, through which all of the plasma, magnetic fields and energy in the corona and solar wind are supplied. Since the Zeeman splitting is typically smaller than the Doppler line broadening in the chromosphere and transition region, it is not effective to explore weak magnetic fields. However, this is not the case for the Hanle effect, when we have an instrument with high polarization sensitivity (~ 0.1%). "Chromospheric Lyman- Alpha SpectroPolarimeter (CLASP)" is the sounding rocket experiment to detect linear polarization produced by the Hanle effect in Lyman-alpha line (121.567 nm) and to make the first direct measurement of magnetic fields in the upper chromosphere and lower transition region. To achieve the high sensitivity of ~ 0.1% within a rocket flight (5 minutes) in Lyman-alpha line, which is easily absorbed by materials, we design the optical system mainly with reflections. The CLASP consists of a classical Cassegrain telescope, a polarimeter and a spectrometer. The polarimeter consists of a rotating 1/2-wave plate and two reflecting polarization analyzers. One of the analyzer also works as a polarization beam splitter to give us two orthogonal linear polarizations simultaneously. The CLASP is planned to be launched in 2014 summer. Title: The Hanle Effect of the Hydrogen Lyα Line for Probing the Magnetism of the Solar Transition Region Authors: Trujillo Bueno, Javier; Štěpán, Jiří; Casini, Roberto Bibcode: 2011ApJ...738L..11T Altcode: 2011arXiv1107.4787T We present some theoretical predictions concerning the amplitude and magnetic sensitivity of the linear-polarization signals produced by scattering processes in the hydrogen Lyα line of the solar transition region. To this end, we have calculated the atomic-level polarization (population imbalances and quantum coherences) induced by anisotropic radiation pumping in semiempirical and hydrodynamical models of the solar atmosphere, taking into account radiative transfer and the Hanle effect caused by the presence of organized and random magnetic fields. The line-center amplitudes of the emergent linear-polarization signals are found to vary typically between 0.1% and 1%, depending on the scattering geometry and the strength and orientation of the magnetic field. The results shown here encourage the development of UV polarimeters for sounding rockets and space telescopes with the aim of opening up a diagnostic window for magnetic field measurements in the upper chromosphere and transition region of the Sun. Title: HAZEL: HAnle and ZEeman Light Authors: Asensio Ramos, Andrés; Trujillo Bueno, Javier; Landi Degl'Innocenti, E. Bibcode: 2011ascl.soft09004A Altcode: A big challenge in solar and stellar physics in the coming years will be to decipher the magnetism of the solar outer atmosphere (chromosphere and corona) along with its dynamic coupling with the magnetic fields of the underlying photosphere. To this end, it is important to develop rigorous diagnostic tools for the physical interpretation of spectropolarimetric observations in suitably chosen spectral lines. HAZEL is a computer program for the synthesis and inversion of Stokes profiles caused by the joint action of atomic level polarization and the Hanle and Zeeman effects in some spectral lines of diagnostic interest, such as those of the He I 1083.0 nm and 587.6 nm (or D3) multiplets. It is based on the quantum theory of spectral line polarization, which takes into account in a rigorous way all the relevant physical mechanisms and ingredients (optical pumping, atomic level polarization, level crossings and repulsions, Zeeman, Paschen-Back and Hanle effects). The influence of radiative transfer on the emergent spectral line radiation is taken into account through a suitable slab model. The user can either calculate the emergent intensity and polarization for any given magnetic field vector or infer the dynamical and magnetic properties from the observed Stokes profiles via an efficient inversion algorithm based on global optimization methods. Title: Erratum: "Scattering Polarization of the Ca II IR Triplet for Probing the Quiet Solar Chromosphere" (2010, ApJ, 722, 1416) Authors: Manso Sainz, R.; Trujillo Bueno, J. Bibcode: 2011ApJ...737..106M Altcode: No abstract at ADS Title: Scattering Polarization of Hydrogen Lines in Weakly Magnetized Stellar Atmospheres. I. Formulation and Application to Isothermal Models Authors: Štěpán, Jiří; Trujillo Bueno, Javier Bibcode: 2011ApJ...732...80S Altcode: 2011arXiv1102.4012S Although the spectral lines of hydrogen contain valuable information on the physical properties of a variety of astrophysical plasmas, including the upper solar chromosphere, relatively little is known about their scattering polarization signals, whose modification via the Hanle effect may be exploited for magnetic field diagnostics. Here we report on a basic theoretical investigation of the linear polarization produced by scattering processes and the Hanle effect in Lyα, Lyβ, and Hα taking into account multilevel radiative transfer effects in an isothermal stellar atmosphere model, the fine-structure of the hydrogen levels, as well as the impact of collisions with electrons and protons. The main aim of this first paper is to elucidate the key physical mechanisms that control the emergent fractional linear polarization in the three lines, as well as its sensitivity to the perturbers' density and to the strength and structure of microstructured and deterministic magnetic fields. To this end, we apply an efficient radiative transfer code we have developed for performing numerical simulations of the Hanle effect in multilevel systems with overlapping line transitions. For low-density plasmas, such as that of the upper solar chromosphere, collisional depolarization is caused mainly by collisional transitions between the fine-structure levels of the n = 3 level, so that it is virtually insignificant for Lyα but important for Lyβ and Hα. We show the impact of the Hanle effect on the three lines taking into account the radiative transfer coupling between the different hydrogen line transitions. For example, we demonstrate that the linear polarization profile of the Hα line is sensitive to the presence of magnetic field gradients in the line core formation region, and that in solar-like chromospheres selective absorption of polarization components does not play any significant role in the emergent scattering polarization. Title: Exploring the Magnetic Fields of Solar Prominences and Spicules via He I D3 Spectropolarimetry Authors: Ramelli, R.; Trujillo Bueno, J.; Bianda, M.; Asensio Ramos, A. Bibcode: 2011ASPC..437..109R Altcode: Over the last few years a large set of spectropolarimetric observations of prominences and spicules in the He I D3 multiplet have been collected at the observatory of the

Istituto Ricerche Solari Locarno (IRSOL), using increasingly improved versions of the ZIMPOL polarimeter. The novel HAZEL inversion code of Stokes profiles generated by the joint action of the Hanle and Zeeman effects is being applied to this set of data, in order to infer the strength and geometry of the magnetic field present in these structures. This paper presents a brief overview of the most recent observations and inversions. Title: A Sounding Rocket Experiment for Spectropolarimetric Observations with the Lyα Line at 121.6 nm (CLASP) Authors: Ishikawa, R.; Bando, T.; Fujimura, D.; Hara, H.; Kano, R.; Kobiki, T.; Narukage, N.; Tsuneta, S.; Ueda, K.; Wantanabe, H.; Kobayashi, K.; Trujillo Bueno, J.; Manso Sainz, R.; Stepan, J.; de Pontieu, B.; Carlsson, M.; Casini, R. Bibcode: 2011ASPC..437..287I Altcode: A team consisting of Japan, USA, Spain, and Norway is developing a high-throughput Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP), which is proposed to fly with a NASA sounding rocket in 2014. CLASP will explore the magnetism of the upper solar chromosphere and transition region via the Hanle effect of the Lyα line for the first time. This experiment requires spectropolarimetric observations with high polarimetric sensitivity (∼0.1%) and wavelength resolution (0.1 Å). The final spatial resolution (slit width) is being discussed taking into account the required high signal-to-noise ratio. We have demonstrated the performance of the Lyα polarimeter by extensively using the Ultraviolet Synchrotron ORbital Radiation Facility (UVSOR) at the Institute for Molecular Sciences. In this contribution, we report these measurements at UVSOR together with the current status of the CLASP project. Title: Determining the Magnetization of the Quiet Sun Photosphere from the Hanle Effect and Surface Dynamo Simulations Authors: Shchukina, Nataliya; Trujillo Bueno, Javier Bibcode: 2011ApJ...731L..21S Altcode: 2011arXiv1103.5652S The bulk of the quiet solar photosphere is thought to be significantly magnetized, due to the ubiquitous presence of a tangled magnetic field at subresolution scales with an average strength langBrang ~ 100 G. This conclusion was reached through detailed three-dimensional (3D) radiative transfer modeling of the Hanle effect in the Sr I 4607 Å line, using the microturbulent field approximation and assuming that the shape of the probability density function of the magnetic field strength is exponential. Here, we relax both approximations by modeling the observed scattering polarization in terms of the Hanle effect produced by the magnetic field of a 3D photospheric model resulting from a (state-of-the-art) magneto-convection simulation with surface dynamo action. We show that the scattering polarization amplitudes observed in the Sr I 4607 Å line can be explained only after enhancing the magnetic strength of the photospheric model by a sizable scaling factor, F ≈ 10, which implies langBrang ≈ 130 G in the upper photosphere. We also argue that in order to explain both the Hanle depolarization of the Sr I 4607 Å line and the Zeeman signals observed in Fe I lines, we need to introduce a height-dependent scaling factor, such that the ensuing langBrang ≈ 160 G in the low photosphere and langBrang ≈ 130 G in the upper photosphere. Title: Modeling Scattering Polarization for Probing Solar Magnetism Authors: Trujillo Bueno, J. Bibcode: 2011ASPC..437...83T Altcode: 2011arXiv1105.0323T This paper considers the problem of modeling the light polarization that emerges from an astrophysical plasma composed of atoms whose excitation state is significantly influenced by the anisotropy of the incident radiation field. In particular, it highlights how radiative transfer simulations in three-dimensional models of the “quiet” solar atmosphere may help us to probe its thermal and magnetic structure, from the near equilibrium photosphere to the highly non-equilibrium upper chromosphere. The paper finishes with predictions concerning the amplitudes and magnetic sensitivities of the linear polarization signals produced by scattering processes in two transition region lines, which should encourage us to develop UV polarimeters for sounding rockets and space telescopes with the aim of opening up a new diagnostic window in astrophysics. Title: Scattering Polarization and the Hanle Effect in Hα as a Probe of Chromospheric Magnetism: Modeling vs. Observations Authors: Štěpán, J.; Trujillo Bueno, J.; Ramelli, R.; Bianda, M. Bibcode: 2011ASPC..437..117S Altcode: 2011arXiv1102.4941S The Hanle effect in strong spectral lines is the physical mechanism that should be increasingly exploited for quantitative explorations of the magnetism of the quiet solar chromospheric plasma. Here we show, by means of multilevel radiative transfer calculations and new spectropolarimetric observations, that the amplitude and shape of the scattering polarization profiles of the Hα line is very sensitive to the strength and structure of the chromospheric magnetic field. The interpretation of the observations in terms of one-dimensional radiative transfer modeling suggests that there is an abrupt magnetization in the upper chromosphere of the quiet Sun. Title: Solar Polarization Workshop 6 Authors: Kuhn, J. R.; Harrington, D. M.; Lin, H.; Berdyugina, S. V.; Trujillo-Bueno, J.; Keil, S. L.; Rimmele, T. Bibcode: 2011ASPC..437.....K Altcode: No abstract at ADS Title: The Chromospheric Lyman Alpha SpectroPolarimeter (CLASP) Authors: Kobayashi, K.; Tsuneta, S.; Trujillo Bueno, J.; Cirtain, J. W.; Bando, T.; Kano, R.; Hara, H.; Fujimura, D.; Ueda, K.; Ishikawa, R.; Watanabe, H.; Ichimoto, K.; Sakao, T.; de Pontieu, B.; Carlsson, M.; Casini, R. Bibcode: 2010AGUFMSH11B1632K Altcode: Magnetic fields in the solar chromosphere play a key role in the energy transfer and dynamics of the solar atmosphere. Yet a direct observation of the chromospheric magnetic field remains one of the greatest challenges in solar physics. While some advances have been made for observing the Zeeman effect in strong chromospheric lines, the effect is small and difficult to detect outside sunspots. The Hanle effect offers a promising alternative; it is sensitive to weaker magnetic fields (e.g., 5-500 G for Ly-Alpha), and while its magnitude saturates at stronger magnetic fields, the linear polarization signals remain sensitive to the magnetic field orientation. The Hanle effect is not only limited to off-limb observations. Because the chromosphere is illuminated by an anisotropic radiation field, the Ly-Alpha line is predicted to show linear polarization for on-disk, near-limb regions, and magnetic field is predicted to cause a measurable depolarization. At disk center, the Ly-Alpha radiation is predicted to be negligible in the absence of magnetic field, and linearly polarized to an order of 0.3% in the presence of an inclined magnetic field. The proposed CLASP sounding rocket instrument is designed to detect 0.3% linear polarization of the Ly-Alpha line at 1.5 arcsecond spatial resolution (0.7’’ pixel size) and 10 pm spectral resolution. The instrument consists of a 30 cm aperture Cassegrain telescope and a dual-beam spectropolarimeter. The telescope employs a ``cold mirror’’ design that uses multilayer coatings to reflect only the target wavelength range into the spectropolarimeter. The polarization analyzer consists of a rotating waveplate and a polarizing beamsplitter that comprises MgF2 plates placed at Brewster’s Angle. Each output beam of the polarizing beamsplitter, representing two orthogonal linear polarizations, is dispersed and focused using a separate spherical varied-line-space grating, and imaged with a separate 512x512 CCD camera. Prototypes of key optical components have been fabricated and tested. Instrument design is being finalized, and the experiment will be proposed for a 2014 flight aboard a NASA sounding rocket. Title: On the Sensitivity of Partial Redistribution Scattering Polarization Profiles to Various Atmospheric Parameters Authors: Sampoorna, M.; Trujillo Bueno, J.; Landi Degl'Innocenti, E. Bibcode: 2010ApJ...722.1269S Altcode: 2010arXiv1008.2585S This paper presents a detailed study of the scattering polarization profiles formed under partial frequency redistribution (PRD) in two thermal models of the solar atmosphere. Particular attention is given to understanding the influence of several atmospheric parameters on the emergent fractional linear polarization profiles. The shapes of these Q/I profiles are interpreted in terms of the anisotropy of the radiation field, which in turn depends on the source function gradient that sets the angular variation of the specific intensity. We define a suitable frequency integrated anisotropy factor for PRD that can be directly related to the emergent linear polarization. We show that complete frequency redistribution is a good approximation to model weak resonance lines. We also show that the emergent linear polarization profiles can be very sensitive to the thermal structure of the solar atmosphere and, in particular, to spatial variations of the damping parameter. Title: Scattering Polarization of the Ca II IR Triplet for Probing the Quiet Solar Chromosphere Authors: Manso Sainz, R.; Trujillo Bueno, J. Bibcode: 2010ApJ...722.1416M Altcode: 2010arXiv1007.0727M The chromosphere of the quiet Sun is a very important stellar atmospheric region whose thermal and magnetic structure we need to decipher in order to unlock new discoveries in solar and stellar physics. To this end, we need to identify and exploit observables sensitive to weak magnetic fields (B <~ 100 G) and to the presence of cool and hot gas in the bulk of the solar chromosphere. Here, we report on an investigation of the Hanle effect in two semi-empirical models of the quiet solar atmosphere with different chromospheric thermal structures. Our study reveals that the linear polarization profiles produced by scattering in the Ca II IR triplet have thermal and magnetic sensitivities potentially of great diagnostic value. The linear polarization in the 8498 Å line shows a strong sensitivity to inclined magnetic fields with strengths between 0.001 and 10 G, while the emergent linear polarization in the 8542 Å and 8662 Å lines is mainly sensitive to magnetic fields with strengths between 0.001 and 0.1 G. The reason for this is that the scattering polarization of the 8542 Å and 8662 Å lines, unlike the 8498 Å line, is controlled mainly by the Hanle effect in their (metastable) lower levels. Therefore, in regions with magnetic strengths noticeably larger than 1 G, their Stokes Q and U profiles are sensitive only to the orientation of the magnetic field vector. We also find that for given magnetic field configurations the sign of the Q/I and U/I profiles of the 8542 Å and 8662 Å lines is the same in both atmospheric models, while the sign of the linear polarization profile of the 8498 Å line turns out to be very sensitive to the thermal structure of the lower chromosphere. We suggest that spectropolarimetric observations providing information on the relative scattering polarization amplitudes of the Ca II IR triplet will be very useful to improve our empirical understanding of the thermal and magnetic structure of the quiet chromosphere. Title: Gauss-Seidel and Successive Overrelaxation Methods for Radiative Transfer with Partial Frequency Redistribution Authors: Sampoorna, M.; Trujillo Bueno, J. Bibcode: 2010ApJ...712.1331S Altcode: 2010arXiv1002.4179S The linearly polarized solar limb spectrum that is produced by scattering processes contains a wealth of information on the physical conditions and magnetic fields of the solar outer atmosphere, but the modeling of many of its strongest spectral lines requires solving an involved non-local thermodynamic equilibrium radiative transfer problem accounting for partial redistribution (PRD) effects. Fast radiative transfer methods for the numerical solution of PRD problems are also needed for a proper treatment of hydrogen lines when aiming at realistic time-dependent magnetohydrodynamic simulations of the solar chromosphere. Here we show how the two-level atom PRD problem with and without polarization can be solved accurately and efficiently via the application of highly convergent iterative schemes based on the Gauss-Seidel and successive overrelaxation (SOR) radiative transfer methods that had been previously developed for the complete redistribution case. Of particular interest is the Symmetric SOR method, which allows us to reach the fully converged solution with an order of magnitude of improvement in the total computational time with respect to the Jacobi-based local accelerated lambda iteration method. Title: On the Probable Existence of an Abrupt Magnetization in the Upper Chromosphere of the Quiet Sun Authors: Štěpán, Jiří; Trujillo Bueno, Javier Bibcode: 2010ApJ...711L.133S Altcode: 2010arXiv1002.1574S We report on a detailed radiative transfer modeling of the observed scattering polarization in the Hα line, which allows us to infer quantitative information on the magnetization of the quiet solar chromosphere. Our analysis suggests the presence of a magnetic complexity zone with a mean field strength langBrang > 30 G lying just below the sudden transition region to the coronal temperatures. The chromospheric plasma directly underneath is very weakly magnetized, with langBrang ~ 1 G. The possible existence of this abrupt change in the degree of magnetization of the upper chromosphere of the quiet Sun might have large significance for our understanding of chromospheric (and, therefore, coronal) heating. Title: On the 3D structure of the magnetic field in regions of emerging flux . Authors: Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2010MmSAI..81..625A Altcode: 2010arXiv1001.2108A We explore the photospheric and chromospheric magnetic field in an emerging flux region. An image of the equivalent width of the He I 10830 Å red blended component shows the presence of filamentary structures that might be interpreted as magnetic loops. We point out that the magnetic field strength in the chromosphere resembles a smoothed version of that found in the photosphere and that it is not correlated at all with the above-mentioned equivalent width map. Lacking other diagnostics, this suggests that one cannot discard the possibility that the chromospheric field we infer from the observations is tracing the lower chromosphere of the active region instead of tracing the magnetic field along loops. If the He I line is formed within magnetic loops, we point out a potential problem that appears when interpreting observations using only one component along the line-of-sight. Title: On the sensitivity of the Halpha scattering polarization to chromospheric magnetism Authors: Štěpán, Jiří; Trujillo Bueno, Javier Bibcode: 2010MmSAI..81..810S Altcode: 2010arXiv1001.2720S A particularly interesting line for exploring the physical conditions of the quiet solar chromosphere is Halpha , but its intensity profile is magnetically insensitive and the small circular polarization signatures produced by the longitudinal Zeeman effect come mainly from the underlying photosphere. Here we show that the Hanle effect in Halpha provides quantitative information on the magnetism of the quiet chromosphere. To this end, we calculate the response function of the emergent scattering polarization to perturbations in the magnetic field. Title: On the Magnetic Field of Off-limb Spicules Authors: Centeno, Rebecca; Trujillo Bueno, Javier; Asensio Ramos, Andrés Bibcode: 2010ApJ...708.1579C Altcode: 2009arXiv0911.3149C Determining the magnetic field related to solar spicules is vital for developing adequate models of these plasma jets, which are thought to play a key role in the thermal, dynamic, and magnetic structure of the chromosphere. Here we report on the magnetic properties of off-limb spicules in a very quiet region of the solar atmosphere, as inferred from new spectropolarimetric observations in the He I 10830 Å triplet obtained with the Tenerife Infrared Polarimeter. We have used a novel inversion code for Stokes profiles caused by the joint action of atomic level polarization and the Hanle and Zeeman effects to interpret the observations (HAZEL, from HAnle and ZEeman Light). Magnetic fields as strong as ~50 G were detected in a very localized area of the slit, which could represent a possible lower value of the field strength of organized network spicules. Title: The Magnetic Field of Solar Spicules Authors: Centeno, R.; Trujillo Bueno, J.; Asensio Ramos, A. Bibcode: 2010ASSP...19..255C Altcode: 2010mcia.conf..255C; 2009arXiv0903.0002C Determining the magnetic field of solar spicules is vital for developing adequate models of these plasma jets, which are thought to play a key role in the thermal, dynamic, and magnetic structure of the chromosphere. Here we report on magnetic spicule properties in a very quiet region of the off-limb solar atmosphere, as inferred from new spectropolarimetric observations in the He I 10830Å triplet. We have used a novel inversion code for Stokes profiles caused by the joint action of atomic level polarization and the Hanle and Zeeman effects (HAZEL) to interpret the observations. Magnetic fields as strong as 40G were unambiguously detected in a very localized area of the slit, which may represent a possible lower value of the field strength of organized network spicules. Title: Recent Advances in Chromospheric and Coronal Polarization Diagnostics Authors: Trujillo Bueno, J. Bibcode: 2010ASSP...19..118T Altcode: 2010mcia.conf..118T; 2009arXiv0903.4372T; 2010ASSP...19..118B I review some recent advances in methods to diagnose polarized radiation with which we may hope to explore the magnetism of the solar chromosphere and corona. These methods are based on the remarkable signatures that the radiatively induced quantum coherences produce in the emergent spectral line polarization and on the joint action of the Hanle and Zeeman effects. Some applications to spicules, prominences, active region filaments, emerging flux regions, and the quiet chromosphere are discussed. Title: Spectropolarimetric investigations of the magnetization of the quiet-Sun chromosphere . Authors: Trujillo Bueno, J. Bibcode: 2010MmSAI..81..681T Altcode: 2010arXiv1002.2615T This paper reviews some recent advances in the development and application of polarized radiation diagnostics to infer the mean magnetization of the quiet solar atmosphere, from the near equilibrium photosphere to the highly non-equilibrium upper chromosphere. In particular, I show that interpretations of the scattering polarization observed in some spectral lines suggest that while the magnetization of the photosphere and upper chromosphere is very significant, the lower chromosphere seems to be weakly magnetized. Title: Two Suggestions to See the Hidden Magnetism of the Solar Chromosphere Authors: Trujillo Bueno, J. Bibcode: 2009ASPC..415..121T Altcode: 2009arXiv0909.4463T Solar magnetic fields leave their fingerprints in the polarization signatures of the emergent spectral line radiation. This occurs through a variety of rather unfamiliar physical mechanisms, not only via the Zeeman effect. In particular, magnetic fields modify the atomic level polarization (population imbalances and quantum coherences) that anisotropic radiative pumping processes induce in the atoms and molecules of the solar atmosphere. Interestingly, this so-called Hanle effect allows us to ``see'' magnetic fields to which the Zeeman effect is blind within the limitations of the available instrumentation. Here I argue that the Ca II IR triplet and the He I 10830 Å multiplet would be very suitable choices for investigating the magnetism of the solar chromosphere via spectropolarimetric observations from a future space telescope, such as JAXA's SOLAR-C mission. Title: The Physical Origin and the Diagnostic Potential of the Scattering Polarization in the Li I Resonance Doublet at 6708 Å Authors: Belluzzi, Luca; Landi Degl'Innocenti, Egidio; Trujillo Bueno, Javier Bibcode: 2009ApJ...705..218B Altcode: 2009arXiv0909.3748B High-sensitivity measurements of the linearly polarized solar limb spectrum produced by scattering processes in quiet regions of the solar atmosphere showed that the Q/I profile of the lithium doublet at 6708 Å has an amplitude ~10-4 and a curious three-peak structure, qualitatively similar to that found and confirmed by many observers in the Na I D2 line. Given that a precise measurement of the scattering polarization profile of the lithium doublet lies at the limit of the present observational possibilities, it is worthwhile to clarify the physical origin of the observed polarization, its diagnostic potential, and what kind of Q/I shapes can be expected from theory. To this end, we have applied the quantum theory of atomic level polarization taking into account the hyperfine structure of the two stable isotopes of lithium, as well as the Hanle effect of a microturbulent magnetic field of arbitrary strength. We find that quantum interferences between the sublevels pertaining to the upper levels of the D2 and D1 line transitions of lithium do not cause any observable effect on the emergent Q/I profile. Our theoretical calculations show that only two Q/I peaks can be expected, with the strongest one caused by the D2 line of 7Li I and the weakest one due to the D2 line of 6Li I. Interestingly, we find that these two peaks in the theoretical Q/I profile stand out clearly only when the kinetic temperature of the thin atmospheric region that produces the emergent spectral line radiation is lower than 4000 K. The fact that such a thin atmospheric region is located around a height of 200 km in standard semi-empirical models, where the kinetic temperature is about 5000 K, leads us to suggest that the most likely Q/I profile produced by the Sun in the lithium doublet should be slightly asymmetric and dominated by the 7Li I peak. Title: The Second Stellar Spectrum and the non-LTE Problem of the 2nd Kind Authors: Trujillo Bueno, Javier Bibcode: 2009AIPC.1171...27T Altcode: 2009AIPC.1171...27B; 2009arXiv0911.4669T This paper presents an overview of the radiative transfer problem of calculating the spectral line intensity and polarization that emerges from a (generally magnetized) astrophysical plasma composed of atoms and molecules whose excitation state is significantly influenced by radiative transitions produced by an anisotropic radiation field. The numerical solution of this non-LTE problem of the 2nd kind is facilitating the physical understanding of the second solar spectrum and the exploration of the complex magnetism of the extended solar atmosphere, but much more could be learned if high-sensitivity polarimeters were developed also for the present generation of night-time telescopes. Interestingly, I find that the population ratio between the levels of some resonance line transitions can be efficiently modulated by the inclination of a weak magnetic field when the anisotropy of the incident radiation is significant, something that could provide a new diagnostic tool in astrophysics. Title: The Sensitivity of the Second Solar Spectrum of the Li I 6708 Å Doublet to the Lithium Isotopic Abundance and to a Microturbulent Magnetic Field Authors: Belluzzi, L.; Landi Degl'Innocenti, E.; Trujillo Bueno, J. Bibcode: 2009ASPC..405..107B Altcode: The second solar spectrum offers a novel diagnostic window not only to explore the magnetic field of our nearest star, but also to determine the atomic and isotopic abundances in the solar atmosphere. A very interesting example, concerning this second aspect, is the Li I resonance doublet at 6708~Å. Here we present a detailed theoretical investigation, carried out within the framework of the density matrix formalism, on the atomic physics that is involved in the generation of the observed Q/I profile, which is the result of a complex blend of the D_1 and D_2 lines of the two stable isotopes of lithium (^6Li and ^7Li), both showing hyperfine structure. We discuss the diagnostic potential of this linear polarization profile for the determination of the lithium isotopic abundance in the quiet Sun, and we investigate the magnetic sensitivity of this profile to a microturbulent magnetic field. Title: Three-dimensional Radiative Transfer Modeling of the Second Solar Spectrum of Titanium Authors: Shchukina, N. G.; Trujillo Bueno, J. T. Bibcode: 2009ASPC..405..275S Altcode: Here we present multilevel radiative transfer calculations of the scattering polarization of neutral titanium lines in a three-dimensional model of the solar photosphere, which we have obtained from realistic hydrodynamical simulations of solar surface convection. We consider the particularly interesting case of the Ti I multiplet a{^5}F - y{^5}Fo, which is well suited for investigating the Sun's hidden magnetism via the differential Hanle effect. After considering the unmagnetized reference case, we show that the depolarization produced by a volume-filling microturbulent field with a strength such that it produces saturation of the upper-level Hanle effect leads to a much better fit to Gandorfer's (2002) observations. Title: The Magnetic Sensitivity of the Second Solar Spectrum Authors: Trujillo Bueno, J. Bibcode: 2009ASPC..405...65T Altcode: 2008arXiv0812.4400T This paper reviews some of the developments that over the last 10 years have allowed us to go from deciphering the physical origin of several of the enigmatic features of the second solar spectrum to discovering unknown aspects of the Sun's hidden magnetism via sophisticated radiative transfer modeling. The second solar spectrum is the observational signature of radiatively induced quantum coherences in the atoms and molecules of the solar atmosphere. Magnetic fields produce partial decoherence via the Hanle effect, giving rise to fascinating observable effects in the emergent spectral line polarization. Interestingly, these effects allow us to ``see" magnetic fields to which the Zeeman effect is blind within the limitations of the available instrumentation. In the coming years, the physical interpretation of observations of the spectral line polarization resulting from the joint action of the Hanle and Zeeman effects might lead to a new revolution in our empirical understanding of solar magnetic fields. Title: Synthesis and Inversion of He I Stokes Profiles Caused by the Joint Action of the Hanle and Zeeman Effects Authors: Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2009ASPC..405..281A Altcode: 2009ASPC..405..281R We have developed a robust but user-friendly computer program for the synthesis and inversion of Stokes profiles caused by the joint action of the Hanle and Zeeman effects in some spectral lines of diagnostic interest. The influence of radiative transfer on the emergent spectral line radiation is taken into account through a suitable slab model. The dynamical and magnetic properties of the observed plasma structure are inferred from the observed Stokes profiles via an efficient inversion algorithm based on global optimization methods. Here we introduce it briefly to the solar and stellar physics community by showing the first results of an application to the He I 10830 Å multiplet, whose aim is to obtain empirical information on the strength and orientation of the magnetic field vector in an internetwork region of the solar chromosphere. Title: Flux Cancellation in the Solar Photosphere: a near-IR Line of Mn I as a Diagnostic Tool Authors: Asensio Ramos, A.; Martínez González, M. J.; López Ariste, A.; Trujillo Bueno, J.; Collados, M. Bibcode: 2009ASPC..405..215A Altcode: Recently, \cite{asensio_2_asensio_mn07} pointed out that the near-IR line of Mn I at 15262.702 Å provides a new diagnostic window for exploring the magnetism of the quiet Sun. In contrast with previously considered Mn I lines located at visible wavelengths this near-IR line has the advantage that the shape of its intensity profile is very sensitive to the presence of magnetic fields. This enhanced magnetic sensitivity is produced by the coincidence of two favorable facts: the enhanced Zeeman sensitivity of near-IR lines and because this line is subjected to particularly strong Paschen-Back perturbations due to the hyperfine structure of manganese. Of great diagnostic interest is that the intensity profile itself give us information on the unsigned magnetic flux, while the polarization profiles are sensitive to the net flux. An application to spectropolarimetric observations with the Tenerife Infrared Polarimeter has allowed us to obtain the first flux cancellation map in an enhanced network region. Title: A Possible Polarization Mechanism of EUV Coronal Lines Authors: Manso Sainz, R.; Trujillo Bueno, J. Bibcode: 2009ASPC..405..423M Altcode: The alignment generated in the ground level of Fe X by scattering in the coronal red line λ6374, can be efficiently transferred by collisions to upper levels; the ensuing spontaneous emission in allowed EUV transitions (as 174.5~Å and 177~Å) is then linearly polarized. The polarization thus generated would be sensitive to the electronic density (Ne) and to the magnetic field orientation although not to the field strength. At Ne∼ 10^7~cm-3 the linear polarization would be ∼1% just over the solar surface and ∼5% at 0.2R_⊙, rapidly decaying for Ne> 10^8~cm-3. The mechanism works for other spectral lines and ions too. Title: Observations of the Joint Action of the Hanle and Zeeman Effects in the D_2 Line of Ba II Authors: Ramelli, R.; Bianda, M.; Trujillo Bueno, J.; Belluzzi, L.; Landi Degl'Innocenti, E. Bibcode: 2009ASPC..405...41R Altcode: 2009arXiv0906.2320R We show a selection of high-sensitivity spectropolarimetric observations obtained over the last few years in the Ba II D_2-line with the Zürich Imaging Polarimeter (ZIMPOL) attached to the Gregory Coudé Telescope of IRSOL. The measurements were collected close to the solar limb, in several regions with varying degree of magnetic activity. The Stokes profiles we have observed show clear signatures of the joint action of the Hanle and Zeeman effects, in very good qualitative agreement with the theoretical expectations. Polarimetric measurements of this line show to be very well suited for magnetic field diagnostics of the lower solar chromosphere, from regions with field intensities as low as 1 gauss to strongly magnetized ones having kG field strengths. Title: Influence of Coronal EUV Irradiance on the Stokes Profiles of the He I 10830 Å Multiplet Authors: Centeno, R.; Trujillo Bueno, J.; Uitenbroek, H.; Collados, M. Bibcode: 2009ASPC..405..297C Altcode: One of the most useful spectral windows for spectropolarimetric investigations of the solar chromosphere is the one provided by the spectral lines of the He I 10830 Å multiplet, whose polarization signals are sensitive to the Hanle and Zeeman effects. However, in order to be able to carry out reliable diagnostics of the dynamic and magnetic properties of the solar outer atmosphere it is crucial to have a good physical understanding of the sensitivity of the observed spectral line radiation to the various competing triggering mechanisms. Here we report a series of on-disk and off-the-limb non-LTE calculations of the 10830 Å absorption and emission profiles, focusing our investigation on their sensitivity to the EUV coronal irradiation and the model atmosphere used in the calculations. We show in what respects the on-disk case sensitivity of the polarization signals induced by the Zeeman effect to the EUV coronal irradiance, and investigate whether or not inversions based on the Milne-Eddington model are reliable. Concerning the off-the-limb case we demonstrate that the intensity ratio of the blue to the red components of the He I 10830 Å multiplet is a sensitive function of the amount of EUV coronal illumination. Therefore, measurements of this observable as a function of the distance to the limb and its confrontation with radiative transfer modeling might give us valuable information on the physical properties of the solar atmosphere and on the amount of EUV radiation penetrating the chromosphere from above. Title: Three-dimensional Radiative Transfer Modeling of the Polarization of the Sun's Continuous Spectrum Authors: Trujillo Bueno, Javier; Shchukina, Nataliya Bibcode: 2009ApJ...694.1364T Altcode: 2008arXiv0812.3494T; 2009ApJ...694.1364B Polarized light provides the most reliable source of information at our disposal for diagnosing the physical properties of astrophysical plasmas, including the three-dimensional (3D) structure of the solar atmosphere. Here we formulate and solve the 3D radiative transfer problem of the linear polarization of the solar continuous radiation, which is principally produced by Rayleigh and Thomson scattering. Our approach takes into account not only the anisotropy of the solar continuum radiation but also the symmetry-breaking effects caused by the horizontal atmospheric inhomogeneities produced by the solar surface convection. We show that such symmetry-breaking effects do produce observable signatures in Q/I and U/I, even at the very center of the solar disk where we observe the forward scattering case, but their detection would require obtaining very high resolution linear polarization images of the solar surface. Without spatial and/or temporal resolution U/I ≈ 0 and the only observable quantity is Q/I, whose wavelength variation at a solar disk position close to the limb has been recently determined semi-empirically. Interestingly, our 3D radiative transfer modeling of the polarization of the Sun's continuous spectrum in a well-known 3D hydrodynamical model of the solar photosphere shows remarkable agreement with the semi-empirical determination, significantly better than that obtained via the use of one-dimensional (1D) atmospheric models. Although this result confirms that the above-mentioned 3D model was indeed a suitable choice for our Hanle-effect estimation of the substantial amount of "hidden" magnetic energy that is stored in the quiet solar photosphere, we have found however some small discrepancies whose origin may be due to uncertainties in the semi-empirical data and/or in the thermal and density structure of the 3D model. For this reason, we have paid some attention also to other (more familiar) observables, like the center-limb variation of the continuum intensity, which we have calculated taking into account the scattering contribution to the continuum source function. The overall agreement with the observed center-limb variation turns out to be impressive, but we find a hint that the model's temperature gradients in the continuum-forming layers could be slightly too steep, perhaps because all current simulations of solar surface convection and magnetoconvection compute the radiative flux divergence ignoring the fact that the effective polarizability is not completely negligible, especially in the downward-moving intergranular lane plasma. Title: Diagnostic methods based on scattering polarization and the joint action of the Hanle and Zeeman effects Authors: Trujillo Bueno, Javier Bibcode: 2009IAUS..259..623T Altcode: Polarized light provides the most reliable source of information at our disposal for diagnosing the physical properties of astrophysical plasmas, including the magnetic fields of the solar atmosphere. The interaction between radiation and hydrogen plus free electrons through Rayleigh and Thomson scattering gives rise to the polarization of the stellar continuous spectrum, which is very sensitive to the medium's thermal and density structure. Anisotropic radiative pumping processes induce population imbalances and quantum coherences among the sublevels of degenerate energy levels (that is, atomic level polarization), which produce polarization in spectral lines without the need of a magnetic field. The Hanle effect caused by the presence of relatively weak magnetic fields modifies the atomic polarization of the upper and lower levels of the spectral lines under consideration, allowing us to detect magnetic fields to which the Zeeman effect is blind. After discussing the physical origin of the polarized radiation in stellar atmospheres, this paper highlights some recent developments in polarized radiation diagnostic methods and a few examples of their application in solar physics. Title: Wave Propagation and Shock Formation in Different Magnetic Structures Authors: Centeno, R.; Collados, M.; Trujillo Bueno, J. Bibcode: 2009ApJ...692.1211C Altcode: 2008arXiv0810.3613C Velocity oscillations "measured" simultaneously at the photosphere and the chromosphere—from time series of spectropolarimetric data in the 10830 Å region—of different solar magnetic features allow us to study the properties of wave propagation as a function of the magnetic flux of the structure (i.e., two different-sized sunspots, a tiny pore, and a facular region). While photospheric oscillations have similar characteristics everywhere, oscillations measured at chromospheric heights show different amplitudes, frequencies, and stages of shock development depending on the observed magnetic feature. The analysis of the power and the phase spectra, together with simple theoretical modeling, lead to a series of results concerning wave propagation within the range of heights of this study. We find that, while the atmospheric cutoff frequency and the propagation properties of different oscillating modes depend on the magnetic feature, in all the cases the power that reaches the high chromosphere above the atmospheric cutoff comes directly from the photosphere by means of linear vertical wave propagation rather than from nonlinear interaction of modes. Title: Advanced Forward Modeling and Inversion of Stokes Profiles Resulting from the Joint Action of the Hanle and Zeeman Effects Authors: Asensio Ramos, A.; Trujillo Bueno, J.; Landi Degl'Innocenti, E. Bibcode: 2008ApJ...683..542A Altcode: 2008arXiv0804.2695A A big challenge in solar and stellar physics in the coming years will be to decipher the magnetism of the solar outer atmosphere (chromosphere and corona) along with its dynamic coupling with the magnetic fields of the underlying photosphere. To this end, it is important to develop rigorous diagnostic tools for the physical interpretation of spectropolarimetric observations in suitably chosen spectral lines. Here we present a computer program for the synthesis and inversion of Stokes profiles caused by the joint action of atomic level polarization and the Hanle and Zeeman effects in some spectral lines of diagnostic interest, such as those of the He I 10830 Å and 5876 Å (or D3) multiplets. It is based on the quantum theory of spectral line polarization, which takes into account in a rigorous way all the relevant physical mechanisms and ingredients (optical pumping, atomic level polarization, level crossings and repulsions, Zeeman, Paschen-Back, and Hanle effects). The influence of radiative transfer on the emergent spectral line radiation is taken into account through a suitable slab model. The user can either calculate the emergent intensity and polarization for any given magnetic field vector or infer the dynamical and magnetic properties from the observed Stokes profiles via an efficient inversion algorithm based on global optimization methods. The reliability of the forward modeling and inversion code presented here is demonstrated through several applications, which range from the inference of the magnetic field vector in solar active regions to determining whether or not it is canopy-like in quiet chromospheric regions. This user-friendly diagnostic tool called "HAZEL" (from HAnle and ZEeman Light) is offered to the astrophysical community, with the hope that it will facilitate new advances in solar and stellar physics. Title: The Influence of Coronal EUV Irradiance on the Emission in the He I 10830 Å and D3 Multiplets Authors: Centeno, R.; Trujillo Bueno, J.; Uitenbroek, H.; Collados, M. Bibcode: 2008ApJ...677..742C Altcode: 2007arXiv0712.2203C Two of the most attractive spectral windows for spectropolarimetric investigations of the physical properties of the plasma structures in the solar chromosphere and corona are the ones provided by the spectral lines of the He I 10830 and 5876 Å (or D3) multiplets, whose polarization signals are sensitive to the Hanle and Zeeman effects. However, in order to be able to carry out reliable diagnostics, it is crucial to have a good physical understanding of the sensitivity of the observed spectral line radiation to the various competing driving mechanisms. Here we report a series of off-the-limb non-LTE calculations of the He I D3 and 10830 Å emission profiles, focusing our investigation on their sensitivity to the EUV coronal irradiation and the model atmosphere used in the calculations. We show in particular that the intensity ratio of the blue to the red components in the emission profiles of the He I 10830 Å multiplet turns out to be a good candidate as a diagnostic tool for the coronal irradiance. Measurements of this observable as a function of the distance to the limb and its confrontation with radiative transfer modeling might give us valuable information on the physical properties of the solar atmosphere and on the amount of EUV radiation at relevant wavelengths penetrating the chromosphere from above. Title: Channeling 5 Minute Photospheric Oscillations into the Solar Outer Atmosphere through Small-Scale Vertical Magnetic Flux Tubes Authors: Khomenko, E.; Centeno, R.; Collados, M.; Trujillo Bueno, J. Bibcode: 2008ApJ...676L..85K Altcode: 2008arXiv0802.0938K We report two-dimensional MHD simulations which demonstrate that photospheric 5 minute oscillations can leak into the chromosphere inside small-scale vertical magnetic flux tubes. The results of our numerical experiments are compatible with those inferred from simultaneous spectropolarimetric observations of the photosphere and chromosphere obtained with the Tenerife Infrared Polarimeter (TIP) at 10830 Å. We conclude that the efficiency of energy exchange by radiation in the solar photosphere can lead to a significant reduction of the cutoff frequency and may allow for the propagation of the 5 minute waves vertically into the chromosphere. Title: Three-dimensional Radiative Transfer with Multilevel Atoms Authors: Fabiani Bendicho, P.; Trujillo Bueno, J. Bibcode: 2007arXiv0710.5427F Altcode: The efficient numerical solution of Non-LTE multilevel transfer problems requires the combination of highly convergent iterative schemes with fast and accurate formal solution methods of the radiative transfer (RT) equation. This contribution begins presenting a method for the formal solution of the RT equation in three-dimensional (3D) media with horizontal periodic boundary conditions. This formal solver is suitable for both, unpolarized and polarized 3D radiative transfer and it can be easily combined with the iterative schemes for solving non-LTE multilevel transfer problems that we have developed over the last few years. We demonstrate this by showing some schematic 3D multilevel calculations that illustrate the physical effects of horizontal radiative transfer. These Non-LTE calculations have been carried out with our code MUGA 3D, a 3D multilevel Non-LTE code based on the Gauss-Seidel iterative scheme that Trujillo Bueno and Fabiani Bendicho (1995) developed for RT applications. Title: Towards the Modelling of the Second Solar Spectrum Authors: Trujillo Bueno, J. Bibcode: 2007arXiv0710.5421T Altcode: This paper addresses the modelling issue of the linearly-polarized solar limb spectrum, which is due to scattering processes and it offers a rich diagnostic potential for exploring solar magnetic fields via the Hanle effect. However, this so-called second solar spectrum is full of mysterious spectral features, which cannot be understood with simplified polarization transfer theories, thus suggesting that the underlying scattering physics is more complex than previously thought. In this paper we argue that the physical understanding of the second solar spectrum requires the consideration of scattering processes in multilevel atomic models, taking fully into account the transfer of atomic polarization among all the levels involved. The consideration of lower-level atomic polarization leads to non-linear and non-local coupled sets of equations, even for the two-level model atom case considered in this paper. The unknowns of the problem are the irreducible tensor components of the atomic density matrix whose self-consistent values have first to be obtained to be able to calculate the emergent Stokes profiles. To solve numerically this non-LTE problem of the second kind we present some iterative methods that are very suitable for developing a general multilevel scattering polarization code. We demonstrate that there exists metastable-level atomic polarization in the solar chromosphere, which suggests that the solution to some recently-formulated "paradoxes" is to be found by carefully revising our current ideas about the chromospheric magnetic field. Title: Are collisions with neutral hydrogen important for modeling the second solar spectrum of Ti I and Ca II? Authors: Derouich, M.; Trujillo Bueno, J.; Manso Sainz, R. Bibcode: 2007A&A...472..269D Altcode: 2007arXiv0705.1968D Context: The physical interpretation of scattering line polarization offers a novel diagnostic window for exploring the thermal and magnetic structure of the quiet regions of the solar atmosphere.
Aims: We evaluate the impact of isotropic collisions with neutral hydrogen atoms on the scattering polarization signals of the 13 lines of multiplet 42 of Ti i and on those of the K line and of the IR triplet of Ca ii, with emphasis on the collisional transfer rates between nearby J-levels.
Methods: We calculate the linear polarization produced by scattering processes in a plane-parallel layer illuminated by the radiation field from the underlying solar photosphere. We consider realistic multilevel models and solve the statistical equilibrium equations for the multipolar components of the atomic density matrix.
Results: We give suitable formulae for calculating the collisional rates as a function of temperature and hydrogen number density. We confirm that the lower levels of the 13 lines of multiplet 42 of Ti i are completely depolarized by elastic collisions. Upper-level depolarization caused by the collisional transfer rates between nearby J-levels turns out to have an unnoticeable impact on the emergent linear polarization amplitudes, except for the λ4536 and λ4544.7 lines. Concerning the Ca ii lines, we show that the collisional rates play no role in the polarization of the upper level of the K line, while they have a rather small depolarizing effect on the atomic polarization of the metastable lower levels of the Ca ii IR triplet.
Conclusions: Although the collisional transfer rates seem to play a minor role for most of the lines we considered in this paper, except, for example, for the magnetically insensitive λ4536 line of Ti i, they might be important for other atomic or molecular systems with closer J-levels (e.g., hyperfine structured multiplets and/or molecules). Therefore, future research in this direction will be worthwhile.

Appendices A and B are only available in electronic form at http://www.aanda.org Title: The Magnetic Sensitivity of the Ba II D1 and D2 Lines of the Fraunhofer Spectrum Authors: Belluzzi, Luca; Trujillo Bueno, Javier; Landi Degl'Innocenti, Egidio Bibcode: 2007ApJ...666..588B Altcode: 2007arXiv0704.3979B The physical interpretation of the spectral line polarization produced by the joint action of the Hanle and Zeeman effects offers a unique opportunity to obtain empirical information about hidden aspects of solar and stellar magnetism. To this end, it is important to achieve a complete understanding of the sensitivity of the emergent spectral line polarization to the presence of a magnetic field. Here we present a detailed theoretical investigation on the role of resonance scattering and magnetic fields on the polarization signals of the Ba II D1 and D2 lines of the Fraunhofer spectrum at 4934 and 4554 Å, respectively. We adopt a three-level model of Ba II, and we take into account the hyperfine structure that is shown by the 135Ba and 137Ba isotopes. Despite their relatively small abundance (18%), the contribution coming from these two isotopes is indeed fundamental for the interpretation of the polarization signals observed in these lines. We consider an optically thin slab model, through which we can investigate in a rigorous way the essential physical mechanisms involved (resonance polarization, Zeeman, Paschen-Back, and Hanle effects), avoiding complications due to radiative transfer effects. We assume the slab to be illuminated from below by the photospheric solar continuum radiation field, and we investigate the radiation scattered at 90°, both in the absence and in the presence of magnetic fields, deterministic and microturbulent. We show in particular the existence of a differential magnetic sensitivity of the three-peak Q/I profile that is observed in the D2 line in quiet regions close to the solar limb, which is of great interest for magnetic field diagnostics. Title: Spicule emission profiles observed in He i 10 830 Å Authors: Sánchez-Andrade Nuño, B.; Centeno, R.; Puschmann, K. G.; Trujillo Bueno, J.; Blanco Rodríguez, J.; Kneer, F. Bibcode: 2007A&A...472L..51S Altcode: 2007arXiv0707.4421S Aims:Off-the-limb observations with high spatial and spectral resolution will help us understand the physical properties of spicules in the solar chromosphere.
Methods: Spectropolarimetric observations of spicules in the He i 10 830 Å multiplet were obtained with the Tenerife Infrared Polarimeter on the German Vacuum Tower Telescope at the Observatorio del Teide (Tenerife, Spain). The analysis shows the variation of the off-limb emission profiles as a function of the distance to the visible solar limb. The ratio between the intensities of the blue and the red components of this triplet (R=I_blue/I_red) is an observational signature of the optical thickness along the light path, which is related to the intensity of the coronal irradiation.
Results: We present observations of the intensity profiles of spicules above a quiet Sun region. The observable R as a function of the distance to the visible limb is also given. We have compared our observational results to the intensity ratio obtained from detailed radiative transfer calculations in semi-empirical models of the solar atmosphere assuming spherical geometry. The agreement is purely qualitative. We argue that future models of the solar chromosphere and transition region should account for the observational constraints presented here. Title: The Scattering Polarization of the Sr I λ4607 Line at the Diffraction Limit Resolution of a 1 m Telescope Authors: Trujillo Bueno, Javier; Shchukina, Nataliya Bibcode: 2007ApJ...664L.135T Altcode: 2007arXiv0706.2386T One of the greatest challenges in solar and stellar physics in the coming years will be to observe the second solar spectrum with a spatial resolution significantly better than 1". This type of scattering polarization observation would probably allow us to discover hitherto unknown aspects of the Sun's hidden magnetism. Here we report on some theoretical predictions for the photospheric line of Sr I at 4607 Å, which we have obtained by solving the three-dimensional (3D) radiative transfer problem of scattering line polarization in a realistic hydrodynamical model of the solar photosphere. We have taken into account not only the anisotropy of the radiation field in the 3D medium and the Hanle effect of a tangled magnetic field, but also the symmetry-breaking effects caused by the horizontal atmospheric inhomogeneities produced by the solar surface convection. Interestingly, the Q/I and U/I linear polarization signals of the emergent spectral line radiation have sizable values and fluctuations, even at the very center of the solar disk where we observe the forward-scattering case. The ensuing small-scale patterns in Q/I and U/I turn out to be sensitive to the assumed magnetic field model and are of great diagnostic value. We argue that it should be possible to observe them with the help of a 1 m telescope equipped with adaptive optics and a suitable polarimeter. Title: A User-Friendly Code to Diagnose Chromospheric Plasmas Authors: Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2007ASPC..368..163A Altcode: 2007arXiv0704.1723A The physical interpretation of spectropolarimetric observations of lines of neutral helium, such as those of the 10830 Å multiplet, represents an excellent opportunity for investigating the magnetism of plasma structures in the solar chromosphere. Here we present a powerful forward modeling and inversion code that permits either to calculate the emergent intensity and polarization for any given magnetic field vector or to infer the dynamical and magnetic properties from the observed Stokes profiles. This diagnostic tool is based on the quantum theory of spectral line polarization, which self-consistently accounts for the Hanle and Zeeman effects in the most general case of the incomplete Paschen-Back effect regime. We also take into account radiative transfer effects. An efficient numerical scheme based on global optimization methods has been applied. Our Stokes inversion code permits a fast and reliable determination of the global minimum. Title: Scattering Polarization of the Ca II Infrared Triplet as Diagnostic of the Quiet Solar Chromosphere Authors: Manso Sainz, R.; Trujillo Bueno, J. Bibcode: 2007ASPC..368..155M Altcode: In observations close to the edge of the solar disk, the Ca II infrared triplet has a scattering polarization pattern at the level of 10-3 in its two strongest lines and at the 10-4 level in the weakest. This polarimetric sensitivity is now accessible with modern spectropolarimeters and scattering polarization has been observed by several authors in the Ca II infrared triplet. We discuss here the interest of observing these lines for the detection and mapping of magnetic fields in the quiet solar chromosphere. To this end, we investigate theoretically the sensitivity of the polarization pattern to the presence of magnetic fields through the Hanle effect. We find that the Ca II 8542 Å and 8662 Å lines are mostly sensitive to fields between 0.001 and 0.1 G, the former is also weakly sensitive to fields between 1 and 10 G. Otherwise the polarization signals are dependent only on the magnetic field geometry, but not on its intensity. The 8498 Å line is highly sensitive in the whole regime 0.001-10 G, but its polarization signal is much lower. Diagnostic diagrams giving the Q/I vs. U/I values at the line core for several magnetic field configurations and intensities are presented. We also discuss the sensitivity of the scattering polarization signals to the temperature gradient of the atmospheric model. Title: Polarimetric Properties of the D2 Lines of Alkali Atoms Authors: Belluzzi, L.; Landi Degl'Innocenti, E.; Trujillo Bueno, J. Bibcode: 2007ASPC..368..311B Altcode: We present a theoretical investigation on the influence of a deterministic magnetic field on the linear and circular polarization of the Na I, K I, and Ba II D2 lines. We describe the three ions by means of a three-level atomic model, and we take into account the hyperfine structure which is shown by some isotopes. We focus on the ``solar prominence case'', in which an optically thin slab of chromospheric plasma, situated 7000 km above the visible solar ``surface'', and permeated by a magnetic field of given strength and orientation, is illuminated from below by the continuum photospheric radiation field. Title: A Magnetic Map of a Solar Filament Authors: Merenda, L.; Trujillo Bueno, J.; Collados, M. Bibcode: 2007ASPC..368..347M Altcode: One of the most useful diagnostic tools for the detection of inclined magnetic fields in solar chromospheric and coronal structures is that based on the fact that the Hanle effect in forward scattering at the solar disk center creates linear polarization in some spectral lines, such as those of the He I 10830 Å multiplet. Here we show the preliminary results of an ongoing investigation on the magnetic field of a solar filament, based on the inversion of spectropolarimetric observations obtained with the Tenerife Infrared Polarimeter. Title: Radiative transfer modeling of the Hanle effect in convective atmospheres Authors: Trujillo Bueno, Javier Bibcode: 2007IAUS..239...44T Altcode: 2007IAUS..239...44B No abstract at ADS Title: The Magnetic Field of Solar Chromospheric Spicules Authors: Trujillo Bueno, J.; Ramelli, R.; Merenda, L.; Bianda, M. Bibcode: 2007ASPC..368..161T Altcode: A suitable diagnostic tool for investigating the magnetism of the solar chromosphere is the observation and theoretical modeling of the Hanle and Zeeman effects in solar spicules. In our presentation we highlighted the great scientific interest of this new diagnostic window by showing how the magnetic field vector can be inferred from spectropolarimetric observations of solar chromospheric spicules in several spectral lines, such as those of the 10830 Å and 5876 Å multiplets of neutral helium. Our off-limb spectropolarimetric observations of the He I 10830 Å multiplet were obtained with the Tenerife Infrared Polarimeter (TIP) attached to the Vacuum Tower Telescope at the Observatorio del Teide (Tenerife), while those of the 5876 Å multiplet resulted from observations with the Zurich Imaging Polarimeter (ZIMPOL) at the Gregory Coudé Telescope of the Istituto Ricerche Solari Locarno. The application of a Stokes profiles inversion technique led to the following conclusion. In quiet Sun spicules the magnetic field vector at a height of about 2000 km above the solar visible ``surface" has a typical strength of the order to 10 G and is inclined by approximately 35° with respect to the solar local vertical direction. In spicules observed close to active regions the strength of the magnetic field was of the order of 50 G. The two figures below show the off-limb Stokes profiles of the He I 5876 Å multiplet observed in a quiet region (upper figure) and close to an active region (lower figure). Note that in both cases Stokes U is non-zero, which is the observational signature of the Hanle effect of an inclined magnetic field. The change of sign in Stokes U along the spatial direction of the spectrograph's slit can be explained by variations in the azimuth of the magnetic field vector. Interestingly, while the Stokes V profiles corresponding to the observed quiet region are caused by the alignment-to-orientation transfer mechanism (see, e.g., page 607 of Landi Degl'Innocenti & Landolfi 2004), that observed in the spicules close to the active region is dominated by the longitudinal Zeeman effect. The interested reader will find more information in the papers by Trujillo Bueno et al. (2005) and by Ramelli et al. (2006). Title: A Near-Infrared Line of Mn I as a Diagnostic Tool of the Average Magnetic Energy in the Solar Photosphere Authors: Asensio Ramos, A.; Martínez González, M. J.; López Ariste, A.; Trujillo Bueno, J.; Collados, M. Bibcode: 2007ApJ...659..829A Altcode: 2006astro.ph.12389A We report on spectropolarimetric observations of a near-IR line of Mn I located at 15262.702 Å whose intensity and polarization profiles are very sensitive to the presence of hyperfine structure. A theoretical investigation of the magnetic sensitivity of this line uncovers several interesting properties. The most important one is that the presence of strong Paschen-Back perturbations due to the hyperfine structure produces an intensity line profile whose shape changes according to the absolute value of the magnetic field strength. A line ratio technique is developed from the intrinsic variations of the line profile. This line ratio technique is applied to spectropolarimetric observations of the quiet solar photosphere in order to explore the probability distribution function of the magnetic field strength. Particular attention is given to the quietest area of the observed field of view, which was encircled by an enhanced network region. A detailed theoretical investigation shows that the inferred distribution yields information on the average magnetic field strength and on the spatial scale at which the magnetic field is organized. A first estimation gives ~250 G for the mean field strength and a tentative value of ~0.4" for the spatial scale at which the observed magnetic field is horizontally organized. Title: Influence of Atomic Polarization and Horizontal Illumination on the Stokes Profiles of the He I 10830 Å Multiplet Authors: Trujillo Bueno, Javier; Asensio Ramos, Andrés Bibcode: 2007ApJ...655..642T Altcode: 2006astro.ph.10128T The polarization observed in the spectral lines of the He I 10830 Å multiplet carries valuable information on the dynamical and magnetic properties of plasma structures in the solar chromosphere and corona. Therefore, it is crucial to have a good physical understanding of its sensitivity to the various competing physical mechanisms. Here we focus on investigating the influence of atomic-level polarization on the emergent Stokes profiles for a broad range of magnetic field strengths, in both 90° and forward-scattering geometry. We show that, contrary to a widespread belief, the selective emission and absorption processes caused by the presence of atomic-level polarization may have an important influence on the emergent linear polarization, even for magnetic field strengths as large as 1000 G. Consequently, the modeling of the Stokes Q- and U-profiles should not be done by taking into account only the contribution of the transverse Zeeman effect within the framework of the Paschen-Back effect theory, unless the magnetic field intensity of the observed plasma structure is sensibly larger than 1000 G. We also point out that in low-lying optically thick plasma structures, such as those of active region filaments, the (horizontal) radiation field generated by the structure itself may substantially reduce the positive contribution to the anisotropy factor caused by the (vertical) radiation field coming from the underlying solar photosphere, so that the amount of atomic-level polarization may turn out to be negligible. Only under such circumstances can the emergent linear polarization of the He I 10830 Å multiplet in such structures of the solar atmospheric plasma be dominated by the contribution caused by the transverse Zeeman effect. Title: Off-limb spectroscopy of the He I 10830 Å multiplet: observations vs. modelling Authors: Sánchez-Andrade Nuño, B.; Centeno, R.; Puschmann, K. G.; Trujillo Bueno, J.; Kneer, F. Bibcode: 2007msfa.conf..177S Altcode: Spectropolarimetric observations of spicules were carried out with the new optical setup of the Tenerife Infrared Polarimeter (TIP 2) at the VTT, showing the variation of the He I 10830Å multiplet off-limb emission profiles with increasing distance to the solar visible limb. The ratio between the intensities of the blue and the red components of this triplet (R = Iblue/Ired) is an observational signature of the optical thickness along the light path, which, at the same time, is proportional to the population of the lower (metastable) level that takes part in these transitions. Our observational results show a variation of R as a function of the distance to the limb. In agreement with recent theoretical results we conclude that R could be used as a diagnostic tool for downward UV coronal irradiance, which is believed to be responsible for the population of the metastable level of the He I 10830 Å triplet. We have compared our observational results with the ratio obtained from detailed radiative transfer calculations in semi-empirical models of the solar atmosphere (assuming spherical geometry) finding a fairly poor agreement. We argue that future models of the solar chromosphere and transition region should account for the observational constraints presented here. Title: THEMIS observations of the Hanle effect in C_2 lines. Authors: Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2007MmSAI..78...42A Altcode: Analysis of the Hanle effect in solar molecular lines allows us to obtain empirical information on unresolved, tangled magnetic fields at subresolution scales in the (granular) upflowing regions of the `quiet' solar photosphere. Here we show observations of scattering polarization in selected C_2 lines at increasingly closer distances to the solar limb, pointing out that the ratio of polarization amplitudes between suitably chosen line pairs varies in a systematic way. We interpret this variation in terms of a microturbulent magnetic field of the order of a few gauss whose strength decreases with height in the solar atmosphere. Title: Magnetic mappers of the quiet solar atmosphere Authors: Trujillo Bueno, J. Bibcode: 2007msfa.conf..311T Altcode: The magnetic sensitivity of the solar spectrum is caused by the Zeeman effect and by a variety of less familiar physical mechanisms by means of which a magnetic field can create and destroy spectral line polarization. This paper highlights how plasma diagnostic tools based on them could help us to explore the magnetism of the quiet regions of the solar photosphere and chromosphere. A few suggestions for increasing the discovery potential of some telescopes and polarimeters are also given. Title: A New Way for Exploring Solar and Stellar Magnetic Fields Authors: Trujillo Bueno, J. Bibcode: 2007hsa..conf..311T Altcode: No abstract at ADS Title: Wave propagation and shock formation in diverse magnetic structures Authors: Centeno, R.; Collados, M.; Trujillo Bueno, J. Bibcode: 2007msfa.conf..245C Altcode: Velocity oscillations measured simultaneously at the photosphere and the chromosphere of different solar magnetic features (sunspots, pores and facular regions) allow us to study the properties of wave propagation as a function of the magnetic flux of the structure. While photospheric oscillations are similar everywhere, oscillations measured at chromospheric heights show different amplitudes, frequencies and stages of shock development depending on the observed magnetic feature. The analysis via power and phase spectra, together with simple theoretical modeling, lead to a series of results concerning wave propagation within the range of heights of this study. We find that, while the atmospheric cut-off frequency and the propagation properties of the different oscillating modes depend on the magnetic feature, in all the cases the power that reaches the high chromosphere comes directly from the photosphere by means of linear wave propagation rather than from non-linear interaction of modes. Title: Oscillations and Wave Propagation in Different Solar Magnetic Features Authors: Centeno, R.; Collados, M.; Trujillo Bueno, J. Bibcode: 2006ASPC..358..465C Altcode: We present the results of the analysis of temporal series of spectro-polarimetric data measured simultaneously in the photospheric Si I 10827 Å line and the chromospheric He I 10830 Å triplet, on top of two different targets: a facular region and the umbra of a sunspot. The full Stokes inversion of both spectral features gives us the temporal variability of the physical conditions at two different regions in the solar atmosphere, allowing us to compare the LOS velocity oscillations at the photosphere and the chromosphere, and infer the main characteristics of wave propagation in both magnetic structures. Title: The Hanle and Zeeman Effects in the Ba II D2 Line Authors: Belluzzi, L.; Trujillo Bueno, J.; Landi Degl'Innocenti, E. Bibcode: 2006ASPC..358..459B Altcode: We present a theoretical investigation on the influence of the magnetic field on the linear and circular polarization of the Ba II D2 line at 4554 Å, through the Hanle and Zeeman effects. We show in particular the existence of a differential magnetic sensitivity of the three-peak structure of the Q/I profile observed for this line, which is of interest for magnetic field diagnostics. Title: The Hanle Effect in Atomic and Molecular Lines: A New Look at the Sun's Hidden Magnetism Authors: Trujillo Bueno, J.; Asensio Ramos, A.; Shchukina, N. Bibcode: 2006ASPC..358..269T Altcode: 2006astro.ph.12678T This paper reviews some of the most recent advances in the application of the Hanle effect to solar physics, and how these developments are allowing us to explore the magnetism of the photospheric regions that look ``empty'' in solar magnetograms--that is, the Sun's ``hidden'' magnetism. In particular, we show how a joint analysis of the Hanle effect in atomic and molecular lines indicates that there is a vast amount of hidden magnetic energy and unsigned magnetic flux localized in the (intergranular) downflowing regions of the quiet solar photosphere, carried mainly by tangled fields at sub-resolution scales with strengths between the equipartition field values and ∼1 kG.

This article combines in one contribution Trujillo Bueno's invited keynote paper and the contributed papers by Asensio Ramos & Trujillo Bueno and by Shchukina & Trujillo Bueno. Title: Three-Dimensional Radiative Transfer Modeling of the Scattering Polarization in MgH Lines Authors: Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2006ASPC..358..318A Altcode: Analysis of the Hanle effect in solar molecular lines allows us to obtain empirical information on hidden, mixed-polarity magnetic fields at subresolution scales in the (granular) upflowing regions of the `quiet' solar photosphere. Here we report that collisions seem to be very efficient in depolarizing the rotational levels of MgH lines. This has the interesting consequence that in the upflowing regions of the quiet solar photosphere the strength of the hidden magnetic field cannot be sensibly larger than 10 G, assuming the simplest case of a single valued microturbulent field that fills the entire upflowing photospheric volume. Alternatively, an equally good theoretical fit to the observed scattering polarization amplitudes can be achieved by assuming that the rate of depolarizing collisions is an order of magnitude smaller than in the previous collisionally dominated case, but then the required strength of the hidden field in the upflowing regions turns out to be unrealistically high. These constraints reinforce our previously obtained conclusion that there is a vast amount of hidden magnetic energy and unsigned magnetic flux localized in the (intergranular) downflowing regions of the quiet solar photosphere. This work has been presented by tet{a4 AT05}. Title: Spectro-Polarimetry of Solar Prominences Authors: Ramelli, R.; Bianda, M.; Trujillo Bueno, J.; Merenda, L.; Stenflo, J. O. Bibcode: 2006ASPC..358..471R Altcode: 2009arXiv0906.2342R A large set of high-precision, full-Stokes spectro-polarimetric observations of prominences in He I D3, Hα, and Hβ lines has been recorded with the ZIMPOL polarimeter at the Gregory-Coudé Telescope in Locarno. The observational technique allows us to obtain measurements free from seeing-induced spurious effects. The instrumental polarization is well under control and taken into account in the data analysis. We present our observational results for each of the lines mentioned above. Of particular interest is that most of our Hα measurements show

antisymmetric V profiles that are a characteristic signature of the Zeeman effect in the prominence plasma. A Stokes inversion technique based on the quantum theory of the Hanle and Zeeman effects is being applied on observed Stokes profiles in the He I D3 line in order to obtain information on the magnetic field vector that confines the prominence plasma. Title: Spectro-Polarimetric Observations of Filaments in Hα and He I D3 Authors: Bianda, M.; Ramelli, R.; Trujillo Bueno, J.; Stenflo, J. O. Bibcode: 2006ASPC..358..454B Altcode: Recordings of the full Stokes vector of filaments in the Hα and He I D3 lines were obtained at the Gregory-Coudé telescope in Locarno with the polarimeter ZIMPOL. The aim was to perform preliminary studies to explore the presence of forward scattering polarization. The observations show linear polarization signatures, whose interpretation is still being investigated, although it seems likely that forward scattering is indeed involved. Title: Erratum: ``Polynomial Approximants for the Calculation of Polarization Profiles in the He I 10830 Å Multiplet'' (ApJS, 160, 312 [2005]) Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Landi Degl'Innocenti, E. Bibcode: 2006ApJS..166..441S Altcode: In our published paper there is an error in the order of the rows in Tables 1 through 6. We stated in the paper that the components are sorted in the order of increasing wavelengths in the linear Zeeman splitting (LZS) regime. However, in the published tables the values for the σ+ and σ- components appear reversed. As a result, researchers trying to implement our polynomial approximants might apply the incomplete Paschen-Back splitting (IPBS) corrections to the wrong components. To avoid any possible confusion, here we reproduce the tables with the correct sorting (i.e., in the order of increasing wavelengths in the LZS regime). For clarity, we have also replaced the σ+/- notation with σred/blue.

We thank Rebecca Centeno for bringing this error to our attention. Title: Determination of the Magnetic Field Vector via the Hanle and Zeeman Effects in the He I λ10830 Multiplet: Evidence for Nearly Vertical Magnetic Fields in a Polar Crown Prominence Authors: Merenda, L.; Trujillo Bueno, J.; Landi Degl'Innocenti, E.; Collados, M. Bibcode: 2006ApJ...642..554M Altcode: The magnetic field is the key physical quantity responsible for the formation, stability, and evolution of solar prominences (ribbons of cool dense gas embedded in the hot tenuous corona). Therefore, it is important to obtain good empirical knowledge of the three-dimensional structure of prominence magnetic fields. Here we show how the magnetic field vector can be inferred via the physical interpretation of spectropolarimetric observations in the He I λ10830 multiplet. To this end, we have developed an inversion code based on the quantum theory of the Hanle and Zeeman effects and on a few modeling assumptions. We show an application to full Stokes vector observations of a polar crown prominence that, in the slit-jaw Hα image, showed nearly vertical plasma structures. Our results provide evidence for magnetic fields on the order of 30 G inclined by about 25° with respect to the local solar vertical direction. Of additional interest is that the inferred nearly vertical magnetic field vector appears to be slightly rotating around a fixed direction in space as one proceeds along the direction of the spectrograph's slit. While these results provide new light on the three-dimensional geometry of the magnetic fields that confine the plasma of polar crown prominences, they also urge us to develop improved solar prominence models and to pursue new diagnostic investigations. Title: Spectropolarimetric Investigation of the Propagation of Magnetoacoustic Waves and Shock Formation in Sunspot Atmospheres Authors: Centeno, Rebecca; Collados, Manuel; Trujillo Bueno, Javier Bibcode: 2006ApJ...640.1153C Altcode: 2005astro.ph.12096C Velocity oscillations in sunspot umbrae have been measured simultaneously in two spectral lines: the photospheric Si I λ10827 line and the chromospheric He I λ10830 multiplet. From the full Stokes inversion of temporal series of spectropolarimetric observations, we retrieved, among other parameters, the line-of-sight velocity temporal variations at photospheric and chromospheric heights. Chromospheric velocity oscillations show a 3 minute period with a clear sawtooth shape typical of propagating shock wave fronts. Photospheric velocity oscillations have basically a 5 minute period, although the power spectrum also shows a secondary peak in the 3 minute band that has been proven to be a predecessor for its chromospheric counterpart. The derived phase spectra yield a value of the atmospheric cutoff frequency around 4 mHz and give evidence for the upward propagation of higher frequency oscillation modes. The phase spectrum has been reproduced with a simple model of linear vertical propagation of slow magnetoacoustic waves in a stratified magnetized atmosphere that accounts for radiative losses through Newton's cooling law. The model explains the main features in the phase spectrum and allows us to compute the theoretical time delay between the photospheric and chromospheric signals, which happens to have a strong dependence on frequency. We find a very good agreement between this and the time delay obtained directly from the cross-correlation of photospheric and chromospheric velocity maps filtered around the 6 mHz band. This allows us to infer that the 3 minute power observed at chromospheric heights comes directly from the photosphere by means of linear wave propagation, rather than from nonlinear interaction of 5 minute (and/or higher frequency) modes. Title: A qualitative interpretation of the second solar spectrum of Ce ll Authors: Manso Sainz, R.; Landi Degl'Innocenti, E.; Trujillo Bueno, J. Bibcode: 2006A&A...447.1125M Altcode: 2005astro.ph.11011M This is a theoretical investigation on the formation of the linearly polarized line spectrum of ionized cerium in the sun. We calculate the scattering line polarization pattern emergent from a plane-parallel layer of Ce ii atoms illuminated from below by the photospheric radiation field, taking into account the differential pumping induced in the various magnetic sublevels by the anisotropic radiation field. We find that the line polarization pattern calculated with this simple model is in good qualitative agreement with reported observations. Interestingly, the agreement improves when some amount of atomic level depolarization is considered. We find that the best fit to the observations corresponds to the situation where the ground and metastable levels are depolarized to about one fifth of the corresponding value obtained in the absence of any depolarizing mechanism. One possibility to have this situation is that the depolarizing rate value of elastic collisions is exactly D=10^6~s-1, which is rather unlikely. Therefore, we interpret that fact as due to the presence of a turbulent magnetic field in the limit of saturated Hanle effect for the lower-levels. For this turbulent magnetic field we obtain a lower limit of 0.8 Gauss and an upper limit of 200-300 Gauss. Title: Theory and Modeling of the Zeeman and Paschen-Back Effects in Molecular Lines Authors: Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2006ApJ...636..548A Altcode: 2005astro.ph.10262A This paper describes a very general approach to the calculation of the Zeeman splitting effect produced by an external magnetic field on the rotational levels of diatomic molecules. The method is valid for arbitrary values of the total electronic spin and of the magnetic field strength-that is, it holds for molecular electronic states of any multiplicity and for both the Zeeman and incomplete Paschen-Back regimes. It is based on an efficient numerical diagonalization of the effective Zeeman Hamiltonian, which can incorporate easily all the contributions one may eventually be interested in, such as the hyperfine interaction of the external magnetic field with the spin motions of the nuclei. The reliability of the method is demonstrated by comparing our results with previous ones obtained via formulae valid only for doublet states. We also present results for molecular transitions arising between nondoublet electronic states, illustrating that their Zeeman patterns show signatures produced by the Paschen-Back effect. Title: Very Efficient Methods for Multilevel Radiative Transfer in Atomic and Molecular Lines Authors: Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2006EAS....18...25A Altcode: 2005astro.ph.12248A The development of fast numerical methods for multilevel radiative transfer (RT) applications often leads to important breakthroughs in astrophysics, because they allow the investigation of problems that could not be properly tackled using the methods previously available. Probably, the most familiar example is the so-called Multilevel Accelerated Λ-Iteration (MALI) technique of Rybicki & Hummer for the case of a local approximate operator, which is based on Jacobi iteration. However, there are superior operator-splitting methods, based on Gauss-Seidel (GS) and Successive Overrelaxation (SOR) iteration, which provide a dramatic increase in the speed with which non-LTE multilevel transfer problems can be solved in one, two and three-dimensional geometries. Such RT methods, which were introduced by Trujillo Bueno & Fabiani Bendicho ten years ago, are the main subject of the first part of this paper. We show in some detail how they can be applied for solving multilevel RT problems in spherical geometry, for both atomic and molecular line transitions. The second part of the article addresses the issue of the calculation of the molecular number densities when the approximation of instantaneous chemical equilibrium turns out to be inadequate, which happens to be the case whenever the dynamical time scales of the astrophysical plasma under consideration are much shorter than the time needed by the molecules to form. Title: A Gentle Introduction to the Physics of Spectral Line Polarization Authors: Trujillo Bueno, Javier Bibcode: 2006spse.conf...77T Altcode: No abstract at ADS Title: Quantum Spectropolarimetry and the Sun's Hidden Magnetism Authors: Trujillo Bueno, J. Bibcode: 2005ESASP.600E...7T Altcode: 2005ESPM...11....7T; 2005dysu.confE...7T No abstract at ADS Title: Evidence for Fine Structure in the Chromospheric Umbral Oscillation Authors: Centeno, R.; Socas-Navarro, H.; Collados, M.; Trujillo Bueno, J. Bibcode: 2005ApJ...635..670C Altcode: 2005astro.ph.10740C Novel spectropolarimetric observations of the He I multiplet are used to explore the dynamics of the chromospheric oscillation above sunspot umbrae. The results presented here provide strong evidence in support of the two-component model proposed by Socas-Navarro and coauthors. According to this model, the waves propagate only inside channels of subarcsecond width (the ``active'' component), whereas the rest of the umbra remains nearly at rest (the ``quiet'' component). Although the observations support the fundamental elements of that model, there is one particular aspect that is not compatible with our data. We find that, contrary to the scenario as originally proposed, the active component remains through the entire oscillation cycle and harbors both the upflowing and the downflowing phase of the oscillation. Title: Evidence for Collisional Depolarization in the MgH Lines of the Second Solar Spectrum Authors: Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2005ApJ...635L.109A Altcode: 2005astro.ph.10711A The analysis of the Hanle effect in solar molecular lines allows us to obtain empirical information on hidden, mixed-polarity magnetic fields at subresolution scales in the (granular) upflowing regions of the ``quiet'' solar photosphere. Here we report that collisions seem to be very efficient in depolarizing the rotational levels of MgH lines. This has the interesting consequence that in the upflowing regions of the quiet solar photosphere the strength of the hidden magnetic field cannot be much larger than 10 G, assuming the simplest case of a single-valued microturbulent field that fills the entire upflowing photospheric volume. Alternatively, an equally good theoretical fit to the observed scattering polarization amplitudes can be achieved by assuming that the rate of depolarizing collisions is an order of magnitude smaller than in the previous collisionally dominated case, but then the required strength of the hidden field in the upflowing regions turns out to be unrealistically high. These constraints reinforce our previously obtained conclusion that there is a vast amount of hidden magnetic energy and unsigned magnetic flux localized in the (intergranular) downflowing regions of the quiet solar photosphere. Title: The scientific case for spectropolarimetry from space: a novel diagnostic window on cosmic magnetic fields Authors: Trujillo Bueno, J.; Landi Degl'Innocenti, E.; Casini, R.; Martínez Pillet, V. Bibcode: 2005ESASP.588..203T Altcode: 2005tssc.conf..203T No abstract at ADS Title: Determination of the Magnetic Field Vector in a Polar Crown Prominence via the Hanle and Zeeman Effects in the He I 10830 Å Multiplet. Authors: Merenda, L.; Trujillo Bueno, J.; Landi Degl'Innocenti, E.; Collados, M. Bibcode: 2005ESASP.596E..18M Altcode: 2005ccmf.confE..18M No abstract at ADS Title: Spectropolarimetric Observations of Prominences and Spicules, and Magnetic Field Diagnostics Authors: Ramelli, R.; Bianda, M.; Trujillo Bueno, J.; Merenda, L.; Stenflo, J. O. Bibcode: 2005ESASP.596E..82R Altcode: 2005ccmf.confE..82R No abstract at ADS Title: The Scientific Case for Quantum Spectropolarimetry from Space Authors: Trujillo Bueno, J.; Landi Degl'Innocenti, E.; Casini, R.; Martínez Pillet, V. Bibcode: 2005ESASP.596E...4T Altcode: 2005ccmf.confE...4T No abstract at ADS Title: Polynomial Approximants for the Calculation of Polarization Profiles in the He I 10830 Å Multiplet Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Landi Degl'Innocenti, E. Bibcode: 2005ApJS..160..312S Altcode: 2005astro.ph..5321S The He I multiplet at 10830 Å is formed in the incomplete Paschen-Back regime for typical conditions found in solar and stellar atmospheres. The positions and strengths of the various components that form the Zeeman structure of this multiplet in the Paschen-Back regime are approximated here by polynomials. The fitting errors are smaller than ~10-2 mÅ in the component positions and ~10-3 in the relative strengths. The approximant polynomials allow for a very fast implementation of the incomplete Paschen-Back regime in numerical codes for the synthesis and inversion of polarization profiles in this important multiplet. Title: The iron and oxygen abundances in the metal-poor star HD 140283 and in the Sun Authors: Shchukina, N. G.; Vasiljeva, I. E.; Trujillo Bueno, J. Bibcode: 2005KFNTS...5...95S Altcode: We present the results of a theoretical investigation of the impact of NLTE effects and of granulation inhomogeneities on the iron and oxygen abundances in the metal-poor star HD 140283 and in the Sun. Our analysis is based on both the classical one-dimensional (1D) stellar atmosphere models and on a new generation of three-dimensional (3D) hydrodynamical models. We consider the Sun as a reference star. The solar iron and oxygen abundances are redefined. Title: Dichroic Masers Due to Radiation Anisotropy and the Influence of the Hanle Effect on the Circumstellar SiO Polarization Authors: Asensio Ramos, A.; Landi Degl'Innocenti, E.; Trujillo Bueno, J. Bibcode: 2005ApJ...625..985A Altcode: 2005astro.ph..2405A The theory of the generation and transfer of polarized radiation, mainly developed for interpreting solar spectropolarimetric observations, allows us to reconsider, in a more rigorous and elegant way, a physical mechanism that has been suggested some years ago to interpret the high degree of polarization often observed in astronomical masers. This mechanism, for which the name of ``dichroic maser'' is proposed, can operate when a low-density molecular cloud is illuminated by an anisotropic source of radiation (e.g., a nearby star). Here we investigate completely unsaturated masers and show that selective stimulated emission processes are capable of producing highly polarized maser radiation in a nonmagnetic environment. The polarization of the maser radiation is linear and is directed tangentially to a ring equidistant to the central star. We show that the Hanle effect due to the presence of a magnetic field can produce a rotation (from the tangential direction) of the polarization by more that 45° for some selected combinations of the strength, inclination, and azimuth of the magnetic field vector. However, these very same conditions produce a drastic inhibition of the maser effect. The rotations of about 90° observed in SiO masers in the evolved stars TX Cam by Kemball & Diamond and IRC +10011 by Desmurs and coworkers may then be explained by a local modification of the anisotropy of the radiation field, being transformed from mainly radial to mainly tangential. Title: Observation and Modeling of Anomalous CN Polarization Profiles Produced by the Molecular Paschen-Back Effect in Sunspots Authors: Asensio Ramos, A.; Trujillo Bueno, J.; Collados, M. Bibcode: 2005ApJ...623L..57A Altcode: 2005astro.ph..5076A We report novel spectropolarimetric observations of sunspots carried out with the Tenerife Infrared Polarimeter in a near-IR spectral region around 15410 Å, which is known to contain two groups of prominent OH lines that show circular polarization signals of opposite polarity. Surrounding these well-known OH lines, we have discovered the presence of CN lines of the Δv=1 band that show anomalous polarization profiles. Although the Stokes V signals of the OH lines are antisymmetric and of a sizable amplitude, the CN lines show almost negligible circular polarization. On the contrary, the linear polarization signals turn out to be much stronger in the CN lines than in the OH lines. Interestingly, these CN lines present striking antisymmetric linear polarization profiles, which we are able to explain and model via the Paschen-Back effect theory for diatomic molecules. The presence of such peculiar CN lines in the same spectral region of the OH lines may be useful to improve our empirical knowledge of solar magnetic fields via the simultaneous observation and modeling of the transverse and longitudinal Zeeman effects in two different molecular species. Title: Full Stokes Spectropolarimetry of Hα in Prominences Authors: López Ariste, A.; Casini, R.; Paletou, F.; Tomczyk, S.; Lites, B. W.; Semel, M.; Landi Degl'Innocenti, E.; Trujillo Bueno, J.; Balasubramaniam, K. S. Bibcode: 2005ApJ...621L.145L Altcode: We report on spectropolarimetric observations of Hα in prominences made with the Télescope Héliographique pour l'Etude du Magnétisme et des Instabilités Solaires and the High Altitude Observatory/Advanced Stokes Polarimeter. Stokes Q and U show the expected profile shape from resonance scattering polarization and the Hanle effect. In contrast, most of the time, Stokes V does not show the antisymmetric profile shape typical of the Zeeman effect but a profile that indicates the presence of strong atomic orientation in the hydrogen levels, to an extent that cannot be explained by invoking the alignment-to-orientation transfer mechanism induced by the prominence magnetic field. We found that the largest signal amplitudes of Stokes V (comparable to that of Stokes Q and U) could be produced by a process of selective absorption of circularly polarized radiation from the photosphere, which requires that the prominence be in the vicinity of an active region. Although recent observations of active region filaments indicate such a selective absorption mechanism as a plausible explanation of the anomalous signals observed, the particular set of conditions that must be met suggest that a different explanation may be required to explain the almost ubiquitous symmetric V signal observed in Hα prominences. Therefore, we speculate that an alternative mechanism inducing strong atomic orientation at the observed level could be due to the presence of electric fields inducing an electric Hanle effect on Hα. Although we are still working toward a careful modeling of this effect, including both electric and magnetic fields, we present some preliminary considerations that seem to support this possibility. Title: The Hanle and Zeeman Effects in Solar Spicules: A Novel Diagnostic Window on Chromospheric Magnetism Authors: Trujillo Bueno, J.; Merenda, L.; Centeno, R.; Collados, M.; Landi Degl'Innocenti, E. Bibcode: 2005ApJ...619L.191T Altcode: 2005astro.ph..1533T; 2005astro.ph..1533B An attractive diagnostic tool for investigating the magnetism of the solar chromosphere is the observation and theoretical modeling of the Hanle and Zeeman effects in spicules, as shown in this Letter for the first time. Here we report on spectropolarimetric observations of solar chromospheric spicules in the He I λ10830 multiplet and on their theoretical modeling accounting for radiative transfer effects. We find that the magnetic field in the observed (quiet-Sun) spicular material at a height of about 2000 km above the visible solar surface has a strength of the order of 10 G and is inclined by approximately 35deg with respect to the local vertical direction. Our empirical finding based on full Stokes vector spectropolarimetry should be taken into account in future magnetohydrodynamical simulations of spicules. Title: The Impact of Non-LTE Effects and Granulation Inhomogeneities on the Derived Iron and Oxygen Abundances in Metal-Poor Halo Stars Authors: Shchukina, N. G.; Trujillo Bueno, J.; Asplund, M. Bibcode: 2005ApJ...618..939S Altcode: 2004astro.ph.10475S This paper presents the results of a detailed theoretical investigation of the impact of non-LTE (NLTE) effects and of granulation inhomogeneities on the derived iron and oxygen abundances in the metal-poor halo subgiant HD 140283. Our analysis is based on both the ``classical'' one-dimensional stellar atmosphere models and on the new generation of three-dimensional hydrodynamic models. The NLTE calculations presented here have been carried out without inelastic collisions with neutral hydrogen atoms. We find that if NLTE effects are taken into account when synthesizing the Fe I spectrum in both types of atmospheric models, then the derived iron abundance turns out to be very similar in both cases. The emergent spectral line profiles in both models are very much weaker in NLTE than in LTE because the UV overionization mechanism produces a very strong underpopulation of the Fe I levels, in particular in the granular regions of the three-dimensional model. As a result, the NLTE effects on the derived iron abundance are very important, amounting to ~0.9 and to ~0.6 dex in the three- and one-dimensional cases, respectively. On the other hand, we find that NLTE and three-dimensional effects have to be taken into account for a reliable determination of the iron abundance from weak Fe II lines, because the significant overexcitation of their upper levels in the granular regions tend to produce emission features. As a result, such Fe II lines are weaker than in LTE and the abundance correction amounts to ~0.4 dex for the three-dimensional case. We also derive the oxygen-to-iron abundance ratio in the metal-poor star HD 140283 by using the O I triplet at 7772-7775 Å and the forbidden [O I] line at 6300 Å. Our results for the oxygen abundance confirm the values reported in some recent investigations. While the oxygen abundance derived from the O I IR triplet is not very sensitive to the presence of granulation inhomogeneities, such three-dimensional effects amount to ~-0.2 dex for the [O I] line. The NLTE abundance correction for the O I IR triplet turns out to be -0.2 dex, approximately. Interestingly, when both NLTE and three-dimensional effects are taken into account there still remain significant discrepancies in the iron abundances derived from Fe I and Fe II lines, as well as in the oxygen abundances inferred from the O I and [O I] lines. We conclude that the discrepancies could be due to uncertainties in the stellar parameters of this metal-poor star. We argue that adopting Teff~5600 K (instead of Teff~5700 K) and [Fe/H]~-2.0 (instead of [Fe/H]~-2.5) substantially reduces the discrepancies in the abundances of iron and oxygen inferred from several spectral lines. Under such circumstances, we find [O/Fe]~0.5 at [Fe/H]=-2. Obviously, our tentative conclusion that the metallicity of this type of metal-poor star is significantly larger than previously thought may have far-reaching implications in stellar astrophysics. Title: Concerning the Existence of a ``Turbulent'' Magnetic Field in the Quiet Sun Authors: Manso Sainz, Rafael; Landi Degl'Innocenti, Egidio; Trujillo Bueno, Javier Bibcode: 2004ApJ...614L..89M Altcode: 2004astro.ph..8360M We report on the a5F-y5Fo multiplet of Ti I and its interest for the study of ``turbulent'' magnetic fields in the quiet solar photosphere. In particular, we argue that the sizable scattering polarization signal of the 4536 Å line (whose lower and upper levels have Landé factors equal to zero), relative to the rest of the lines in the multiplet, gives direct evidence for the existence of a ubiquitous, unresolved magnetic field. We cannot determine precisely the strength of the magnetic field, but its very existence is evidenced by the differential Hanle effect technique that this Ti I multiplet provides. Title: Signatures of Incomplete Paschen-Back Splitting in the Polarization Profiles of the He I λ10830 Multiplet Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Landi Degl'Innocenti, E. Bibcode: 2004ApJ...612.1175S Altcode: 2004astro.ph..9005S We investigate the formation of polarization profiles induced by a magnetic field in the He I multiplet at 10830 Å. Our analysis considers the Zeeman splitting in the incomplete Paschen-Back regime. The effects turn out to be important and produce measurable signatures on the profiles, even for fields significantly weaker than the level-crossing field (~400 G). When compared to profiles calculated with the usual linear Zeeman effect, the incomplete Paschen-Back profiles exhibit the following conspicuous differences: (1) a non-Doppler blueshift of the Stokes V zero-crossing wavelength of the blue component; (2) area and peak asymmetries, even in the absence of velocity and magnetic gradients; and (3) a ~25% reduction in the amplitude of the red component. These features do not vanish in the weak-field limit. The spectral signatures that we analyze in this paper may be found in previous observations published in the literature. Title: Observation of the Molecular Zeeman Effect in the G Band Authors: Asensio Ramos, A.; Trujillo Bueno, J.; Bianda, M.; Manso Sainz, R.; Uitenbroek, H. Bibcode: 2004ApJ...611L..61A Altcode: 2004astro.ph..7332A; 2004astro.ph..7332R Here we report on the first observational investigation of the Zeeman effect in the G band around 4305 Å. Our spectropolarimetric observations of sunspots with the Zürich Imaging Polarimeter at the Istituto Ricerche Solari Locarno confirm our previous theoretical prediction that the molecular Zeeman effect produces measurable circular polarization signatures in several CH lines that are not overlapped with atomic transitions. We also find both circular and linear polarization signals produced by atomic lines whose wavelengths lie in the G-band spectral region. Together, such molecular and atomic lines are potentially important for empirical investigations of solar and stellar magnetism. For instance, a comparison between observed and calculated Stokes profiles suggests that the thermodynamical and/or magnetic properties of the photospheric regions of sunspot umbrae are horizontally structured with a component that might be associated with umbral dots. Title: A substantial amount of hidden magnetic energy in the quiet Sun Authors: Trujillo Bueno, J.; Shchukina, N.; Asensio Ramos, A. Bibcode: 2004Natur.430..326T Altcode: 2004astro.ph..9004T; 2004astro.ph..9004B Deciphering and understanding the small-scale magnetic activity of the quiet solar photosphere should help to solve many of the key problems of solar and stellar physics, such as the magnetic coupling to the outer atmosphere and the coronal heating. At present, we can see only ~1 per cent of the complex magnetism of the quiet Sun, which highlights the need to develop a reliable way to investigate the remaining 99 per cent. Here we report three-dimensional radiative transfer modelling of scattering polarization in atomic and molecular lines that indicates the presence of hidden, mixed-polarity fields on subresolution scales. Combining this modelling with recent observational data, we find a ubiquitous tangled magnetic field with an average strength of ~130G, which is much stronger in the intergranular regions of solar surface convection than in the granular regions. So the average magnetic energy density in the quiet solar photosphere is at least two orders of magnitude greater than that derived from simplistic one-dimensional investigations, and sufficient to balance radiative energy losses from the solar chromosphere. Title: Dynamics of an enhanced network region observed in Hα Authors: Al, N.; Bendlin, C.; Hirzberger, J.; Kneer, F.; Trujillo Bueno, J. Bibcode: 2004A&A...418.1131A Altcode: This investigation is based on Hα observations of high spatial resolution. They stem from an enhanced network region near disk centre of the sun and consist of broad-band and narrow-band images taken with the two-dimensional ``Göttingen'' Fabry-Perot spectrometer mounted in the Vacuum Tower Telescope at the Observatorio del Teide/Tenerife. The ``lambdameter method'' was applied to derive intensity and velocity maps over the two-dimensional field of view reflecting the behaviour of these parameters in the solar chromosphere. The determination of the source function, the line-of-sight (LOS) velocity, the Doppler width and the optical depth was based on Beckers' cloud model (Beckers \cite{beckers}). From the LOS velocity distribution along the Hα structures we conclude that ballistic motions are unlikely to prevail. Especially the bright Hα features cannot be explained by the cloud model. It is shown how, instead, two-dimensional non-LTE radiative transfer calculations of embedded chromospheric structures lead to reasonable agreement with the observed line profiles from these bright features. It is found that many of the bright fibril-like structures near dark fibrils can be explained by radiation which is blocked by absorbing material at large heights and escapes through less opaque regions. We estimate the number densities and the temperature. With these and with the measured velocities, the kinetic energy flux and the enthalpy flux related to the motions of the fine structures can be calculated. Both fall short by at least an order of magnitude of covering energy losses by radiation of active chromospheric regions. Title: The Zeeman Effect in the G Band Authors: Uitenbroek, H.; Miller-Ricci, E.; Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2004ApJ...604..960U Altcode: 2004astro.ph..1467U We investigate the possibility of measuring magnetic field strength in G-band bright points through the analysis of Zeeman polarization in molecular CH lines. To this end we solve the equations of polarized radiative transfer in the G band through a standard plane-parallel model of the solar atmosphere with an imposed magnetic field and through a more realistic snapshot from a simulation of solar magnetoconvection. This region of the spectrum is crowded with many atomic and molecular lines. Nevertheless, we find several instances of isolated groups of CH lines that are predicted to produce a measurable Stokes V signal in the presence of magnetic fields. In part this is possible because the effective Landé factors of lines in the stronger main branch of the CH A2Δ-X2Π transition tend to zero rather quickly for increasing total angular momentum J, resulting in a Stokes V spectrum of the G band that is less crowded than the corresponding Stokes I spectrum. We indicate that, by contrast, the effective Landé factors of the R and P satellite subbranches of this transition tend to +/-1 for increasing J. However, these lines are in general considerably weaker and do not contribute significantly to the polarization signal. In one wavelength location near 430.4 nm, the overlap of several magnetically sensitive and nonsensitive CH lines is predicted to result in a single-lobed Stokes V profile, raising the possibility of high spatial resolution narrowband polarimetric imaging. In the magnetoconvection snapshot we find circular polarization signals of the order of 1%, prompting us to conclude that measuring magnetic field strength in small-scale elements through the Zeeman effect in CH lines is a realistic prospect. Title: Detection of Polarization from the E4Π-A4Π System of FeH in Sunspot Spectra Authors: Asensio Ramos, A.; Trujillo Bueno, J.; Collados, M. Bibcode: 2004ApJ...603L.125A Altcode: 2004astro.ph..1597A; 2004astro.ph..1597R Here we report the first detection of polarization signals induced by the Zeeman effect in spectral lines of the E4Π-A4Π system of FeH located around 1.6 μm. Motivated by the tentative detection of this band in the intensity spectrum of late-type dwarfs, we have investigated the full Stokes sunspot spectrum and have found circular and linear polarization signatures that we associate with the FeH lines of the E4Π-A4Π band system. We investigate the Zeeman effect in these molecular transitions and point out that in Hund's case (a) coupling, the effective Landé factors are never negative. For this reason, the fact that our spectropolarimetric observations indicate that the Landé factors of pairs of FeH lines have opposite signs prompts us to conclude that the E4Π-A4Π system must be in intermediate angular momentum coupling between Hund's cases (a) and (b). We emphasize that theoretical and/or laboratory investigations of this molecular system are urgently needed for exploiting its promising diagnostic capabilities. Title: The magnetic fields of the quiet solar photosphere Authors: Shchukina, Nataliya G.; Trujillo Bueno, Javier Bibcode: 2004IAUS..223..483S Altcode: 2005IAUS..223..483S We report on a novel investigation of the complex magnetism of the quiet Sun which is based on a realistic three-dimensional modeling of the Hanle effect. Title: Chromospheric Magnetism and the Hanle Effect Authors: Trujillo Bueno, J. Bibcode: 2003csss...12..196T Altcode: This article shows how some recent developments in the observation and theoretical modeling of weak polarization signals in chromospheric spectral lines are facilitating fundamental new advances in our ability to investigate the magnetism of the solar outer atmosphere via spectropolarimetry. Title: Stellar granulation and the NLTE formation Žf the Fe I lines: The metal-poor halo star HD 140283 Authors: Shchukina, N. G.; Trujillo Bueno, J.; Vasil'va, I. Bibcode: 2003KFNT...19..441S Altcode: 2003KFNT...19e.441S The NLTE Fe I line formation problem in a three-dimensional granulation model of the metal-poor halo star HD 140283 is considered. The self-consistent solutions of the kinetic and radiative transfer equations were obtained with a realistic atomic model Fe I + Fe II. The effects of horizontal radiative transfer are neglected. The near-UV overionisation effect is found to lead to sizeable Fe I opacity deficits in the granular atmospheric regions. This NLTE effect tends to produce weaker emergent spectral lines than in LTE because they are formed deeper in the stellar granules where the source functions are close to the Planck function. The NLTE effects in the integranule spectra are small. If NLTE effects are shown to be fully taken into account both in the three-dimensional and one-dimensional models of the star HD 140283 the iron abundance is in close agreement for both cases. Title: Zero-Field Dichroism in the Solar Chromosphere Authors: Manso Sainz, R.; Trujillo Bueno, J. Bibcode: 2003PhRvL..91k1102M Altcode: 2003astro.ph.11455M; 2003astro.ph.11455S; 2003PhRvL..91k1102S We explain the linear polarization of the Ca ii infrared triplet observed close to the edge of the solar disk. In particular, we demonstrate that the physical origin of the enigmatic polarizations of the 866.2 and 854.2nm lines lies in the existence of atomic polarization in their metastable 2D3/2 , 5/2 lower levels, which produces differential absorption of polarization components (dichroism). To this end, we have solved the problem of the generation and transfer of polarized radiation by taking fully into account all the relevant optical pumping mechanisms in multilevel atomic models. We argue that “zero-field” dichroism may be of great diagnostic value in astrophysics. Title: Quiet-Sun inter-network magnetic fields observed in the infrared Authors: Khomenko, E. V.; Collados, M.; Solanki, S. K.; Lagg, A.; Trujillo Bueno, J. Bibcode: 2003A&A...408.1115K Altcode: This paper presents the results of an investigation of the quiet Sun's magnetic field based on high-resolution infrared spectropolarimetric observations obtained with the Tenerife Infrared Polarimeter (TIP) at the German VTT of the Observatorio del Teide. We observed two very quiet regions at disc centre. The seeing was exceptionally good during both observing runs, being excellent during one of them. In both cases the network was intentionally avoided to the extent possible, to focus the analysis on the characteristics of the weak polarization signals of the inter-network regions. We find that the Stokes V profile of Fe I 15648 Å line in almost 50% of the pixels and Stokes Q and/or U in 20% of the pixels have a signal above 10-3 (in units of continuum intensity Ic), which is significantly above the noise level of 2-3 x 10-4. This implies that we detect fluxes as low as 2 x 1015 Mx/px. We find evidence that we have detected most of the net flux that is in principle detectable at 1'' resolution with the Zeeman effect. The observed linear polarization resulting from the transverse Zeeman effect indicates that the magnetic fields have a broad range of inclinations, although most of the pixels show polarization signatures which imply an inclination of about 20o. Nearly 30% of the selected V-profiles have irregular shapes with 3 or more lobes, suggesting mixed polarities with different LOS velocity within the resolution element. The profiles are classified using a single value decomposition approach. The spatial distribution of the magnetic signal shows that profiles of different classes (having different velocities, splitting, asymmetries) are clustered together and form patches, close to the spatial resolution in size. Most of the field is found to be located in intergranular lanes. The statistical properties of the mainly inter-network field sampled by these observations are presented, showing that most of the observed fields are weak with relatively few kG features. The field strength distribution peaks at 350 G and has a FWHM of 300 G. Other parameters, such as profile asymmetries, filling factors and line-of-sight velocities are also determined and discussed.

Based on observations with the German Vacuum Tower Telescope (VTT) operated by the Kiepenheuer-Institut für Sonnenphysik at the Spanish Observatorio del Teide of the Instituto de Astrofísica de Canarias (IAC). Title: Nonequilibrium CO Chemistry in the Solar Atmosphere Authors: Asensio Ramos, A.; Trujillo Bueno, J.; Carlsson, M.; Cernicharo, J. Bibcode: 2003ApJ...588L..61A Altcode: 2003astro.ph..3460R; 2003astro.ph..3460A Investigating the reliability of the assumption of instantaneous chemical equilibrium (ICE) for calculating the CO number density in the solar atmosphere is of crucial importance for the resolution of the long-standing controversy over the existence of ``cool clouds'' in the chromosphere and for determining whether the cool gas owes its existence to CO radiative cooling or to a hydrodynamical process. Here we report the first results of such an investigation in which we have carried out time-dependent gas-phase chemistry calculations in radiation hydrodynamical simulations of solar chromospheric dynamics. We show that while the ICE approximation turns out to be suitable for modeling the observed infrared CO lines at the solar disk center, it may substantially overestimate the ``heights of formation'' of strong CO lines synthesized close to the edge of the solar disk, especially concerning vigorous dynamic cases resulting from relatively strong photospheric disturbances. This happens because during the cool phases of the hydrodynamical simulations, the CO number density in the outer atmospheric regions is smaller than what is stipulated by the ICE approximation, resulting in decreased CO opacity in the solar chromosphere. As a result, the cool CO-bearing gas that produces the observed molecular lines must be located at atmospheric heights not greater than ~700 km. We conclude that taking into account the nonequilibrium chemistry improves the agreement with the available on-disk and off-limb observations but that the hydrodynamical simulation model has to be even cooler than anticipated by the ICE approximation, and this has to be the case at the ``new'' (i.e., deeper) formation regions of the rovibrational CO lines. Title: Towards a Reliable Diagnostics of `Turbulent' Magnetic Fields Via the Hanle Effect in the Sr I λ4607 Å Line Authors: Shchukina, N.; Trujillo Bueno, J. Bibcode: 2003ASPC..307..336S Altcode: No abstract at ADS Title: Radiative Transfer tools for the GTC Authors: Asensio Ramos, A.; Trujillo Bueno, J.; Cernicharo, J. Bibcode: 2003RMxAC..16..162A Altcode: We present very fast radiative transfer tools of interest for the interpretation of future spectroscopic observations taken with the GTC and its post-focus instrumentation. Our radiative transfer codes are based on the iterative methods introduced by Trujillo Bueno & Fabiani Bendicho (1995). Taking into consideration spherical geometry and macroscopic velocity fields, we present results from the application of these fast radiative transfer methods. We show why it is of interest to develop a medium-high resolution spectrograph in order to be able to resolve the rotational structure of the molecular bands. We show some spectropolarimetric observations made with the Tenerife Infrared Polarimeter (TIP), developed at the IAC and the theoretical modeling done with the theoretical tools we have developed. Title: Non-equilibrium Chemistry and Molecular Spectropolarimetry Authors: Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2003ASPC..307..195A Altcode: No abstract at ADS Title: New Diagnostic Windows on the Weak Magnetism of the Solar Atmosphere Authors: Trujillo Bueno, J. Bibcode: 2003ASPC..307..407T Altcode: No abstract at ADS Title: A Multilevel Radiative Transfer Program for Modeling Scattering Line Polarization and the Hanle Effect in Stellar Atmospheres Authors: Manso Sainz, R.; Trujillo Bueno, J. Bibcode: 2003ASPC..307..251M Altcode: No abstract at ADS Title: Radiative Transfer in Molecular Lines Authors: Asensio Ramos, A.; Trujillo Bueno, J. Bibcode: 2003ASPC..288..335A Altcode: 2003sam..conf..335A Molecular lines are generally very good tracers of the physical conditions in cold regions of the Universe (e.g. molecular clouds, cool stars, etc.), but molecular species are also found in not so cold environments (e.g. the magnetized solar atmosphere). For a reliable interpretation of spectroscopic and spectropolarimetric observations of molecular lines it is often necessary to carry out detailed radiative transfer simulations in molecular lines, both in LTE and NLTE. Here we present a multilevel radiative transfer code for the synthesis of molecular lines in stellar atmospheres, showing some illustrations of calculations in different astrophysical contexts and considering molecules like H2O, CO and OH. We will discuss our implementation of highly convergent iterative methods and formal solvers with especial emphasis on spherical geometry. We will also present a chemical evolution code which is currently allowing us to check the approximation of instantaneous chemical equilibrium in the calculation of the abundances of a variety of molecular species. Title: Astrophysical Spectropolarimetry and Magnetic Field Diagnostics Authors: Trujillo Bueno, J. Bibcode: 2003IAUS..210..243T Altcode: No abstract at ADS Title: Solar Polarization Authors: Trujillo-Bueno, Javier; Sanchez Almeida, Jorge Bibcode: 2003ASPC..307.....T Altcode: 2003sopo.conf.....T No abstract at ADS Title: The Generation and Transfer of Polarized Radiation in Stellar Atmospheres Authors: Trujillo Bueno, J. Bibcode: 2003ASPC..288..551T Altcode: 2003sam..conf..551T The standard Non-LTE problem consists in calculating the atomic level populations that are consistent with the intensity of the radiation field generated within any given stellar atmospheric model. In contrast, the Non-LTE problem of the second kind is, indeed, like an ``algebraic Annapurna'': it requires to calculate the diagonal and non-diagonal elements of the atomic density matrix (associated to each level i of total angular momentum Ji) that are consistent with the intensity and polarization of the radiation field generated within the (generally magnetized) stellar atmospheric model under consideration. After arguing why this problem is of real astrophysical interest, I will introduce the relevant equations and the basic anisotropic radiation pumping processes. Finally, I will show how to solve efficiently Non-LTE problems of the second kind via the development and application of fast iterative methods and accurate formal solvers of the Stokes vector transfer equation. Title: Non-LTE Determination of Iron and Oxygen Abundances Using 3D Hydrodynamical Models: the Metal-Poor Star HD140283 Authors: Shchukina, N. G.; Vasiljeva, I. E.; Trujillo Bueno, J.; Asplund, M. Bibcode: 2003IAUS..210P.B10S Altcode: No abstract at ADS Title: Polarización en Astrofísica Authors: Trujillo Bueno, J. Bibcode: 2003cnam.conf...10T Altcode: Polarized light provides key information on the physical conditions and geometry of astrophysical plasmas otherwise unattainable via conventional spectroscopy. In particular, the remote sensing of solar and stellar magnetic fields requires the measurement and rigorous theoretical interpretation of polarization signals in atomic and molecular lines, which are induced by various subtle physical mechanisms. I will begin presenting a brief introduction to ``Astrophysical Spectropolarimetry'', emphasizing the importance of developing diagnostic tools that take proper account of the Zeeman Effect, optical pumping processes and the Hanle Effect. Only in this way may we hope to investigate the strength and topology of stellar magnetic fields in a parameter domain which ranges from at least milligauss to many thousands of gauss. In the second part of the talk I will discuss some interesting applications in solar magnetism, suggesting also what type of collaborations between physicists and chemists could lead to new advances in astrophysics. Title: New Spectropolarimetric Observations of Solar Coronal Filaments in the He I 10830 Å Multiplet Authors: Collados, M.; Trujillo Bueno, J.; Asensio Ramos, A. Bibcode: 2003ASPC..307..468C Altcode: No abstract at ADS Title: Statistical properties of magnetic fields in intranetwork Authors: Khomenko, E. V.; Collados, M.; Lagg, A.; Solanki, S. K.; Trujillo Bueno, J. Bibcode: 2002ESASP.505..445K Altcode: 2002IAUCo.188..445K; 2002solm.conf..445K We report a study of the quiet sun's magnetic field based on high-resolution infrared spectropolarimetric observations (TIP/VTT). We find that in almost 50% of the pixels Stokes V and in 15% the Stokes Q and/or U profiles have a signal above 10-3. The statistical properties of the mainly intranetwork field sampled by these observations are presented, showing that most of the observed fields are weak (the field strength distribution peaks at 350 G and has a FWHM of 300 G) with very few kG features. The magnetized regions occupy a very small fill fractions (about 2%). The field changes properties on granular spatial scales and the size of the patches formed by similar profiles is close to 1". Most of the parameters of the observed polarization profiles show correlations with granulation parameters. Title: Remote sensing of chromospheric magnetic fields via the Hanle and Zeeman effects Authors: Trujillo Bueno, J.; Manso Sainz, R. Bibcode: 2002NCimC..25..783T Altcode: 2007arXiv0711.2584T The only way to obtain reliable empirical information on the intensity and topology of the weak magnetic fields of the quiet solar chromosphere is via the measurement and rigorous physical interpretation of polarization signals in chromospheric spectral lines. The observed Stokes profiles reported here are due to the Hanle and Zeeman effects operating in a weakly magnetized plasma that is in a state far from local thermodynamic equilibrium. The physical origin of their enigmatic linear polarization Q and U components is the existence of atomic polarization in their metastable lower-levels, which permits the action of a dichroism mechanism that has nothing to do with the transverse Zeeman effect. It is also pointed out that the population imbalances and coherences among the Zeeman sublevels of such long-lived atomic levels can survive in the presence of horizontal magnetic fields having intensities in the gauss range, and produce significant polarization signals. Finally, it is shown how the most recent developments in the observation and theoretical modelling of weak polarization signals are facilitating fundamental new advances in our ability to investigate the magnetism of the outer solar atmosphere via spectropolarimetry. Title: On the Atomic Polarization of the Ground Level of Na I Authors: Casini, Roberto; Landi Degl'Innocenti, Egidio; Landolfi, Marco; Trujillo Bueno, Javier Bibcode: 2002ApJ...573..864C Altcode: 2002astro.ph..4341C In a recent Letter, we showed the remarkable result that the atomic alignment of the levels P1/2 and S1/2 of the D1 line of Na I is practically destroyed in the presence of magnetic fields sensibly larger than 10 G, irrespective of the field direction. In this paper, we analytically demonstrate that this property is a consequence of the decoupling of the electronic and nuclear angular momenta J and I in the excited state P3/2, which is achieved when the Zeeman splitting from the local magnetic field becomes much larger than the typical hyperfine separation for that level. Title: The Physical Origin of the Scattering Polarization of the Na I D Lines in the Presence of Weak Magnetic Fields Authors: Trujillo Bueno, Javier; Casini, Roberto; Landolfi, Marco; Landi Degl'Innocenti, Egidio Bibcode: 2002ApJ...566L..53T Altcode: 2002astro.ph..1389T We demonstrate that the atomic alignment of the hyperfine-structure components of the ground-level S1/2 of Na I and of the upper-level P1/2 of the D1 line are practically negligible for magnetic strengths B>10 G and virtually zero for B>~100 G. This occurs independently of the magnetic field inclination on the stellar surface (also, in particular, for vertical fields). Consequently, the characteristic antisymmetric linear polarization signature of the scattered light in the D1 line is practically suppressed in the presence of magnetic fields larger than 10 G, regardless of their inclination. Remarkably, we find that the scattering polarization amplitude of the D2 line increases steadily with the magnetic strength, for vertical fields above 10 G, while the contribution of the alignment to the polarization of the D1 line rapidly decreases. Therefore, we suggest that spectropolarimetric observations of the ``quiet'' solar chromosphere showing significant linear polarization peaks in both D1 and D2 cannot be interpreted in terms of one-component magnetic field models, implying that the magnetic structuring of the solar chromosphere could be substantially more complex than previously thought. Title: Selective absorption processes as the origin of puzzling spectral line polarization from the Sun Authors: Trujillo Bueno, J.; Landi Degl'Innocenti, E.; Collados, M.; Merenda, L.; Manso Sainz, R. Bibcode: 2002Natur.415..403T Altcode: 2002astro.ph..1409T Magnetic fields play a key role in most astrophysical systems, from the Sun to active galactic nuclei. They can be studied through their effects on atomic energy levels, which produce polarized spectral lines. In particular, anisotropic radiation `pumping' processes (which send electrons to higher atomic levels) induce population imbalances that are modified by weak magnetic fields. Here we report peculiarly polarized light in the HeI 10,830-Å multiplet observed in a coronal filament located at the centre of the solar disk. We show that the polarized light arises from selective absorption from the ground level of the triplet system of helium, and that it implies the presence of magnetic fields of the order of a few gauss that are highly inclined with respect to the solar radius vector. This disproves the common belief that population imbalances in long-lived atomic levels are insignificant in the presence of inclined fields of the order of a few gauss, and opens up a new diagnostic window for the investigation of solar magnetic fields. Title: Astrophysical spectropolarimetry Authors: Trujillo-Bueno, J.; Moreno-Insertis, F.; Sánchez, F. Bibcode: 2002apsp.conf.....T Altcode: The polarization of light is the key to obtaining a wealth of essential information that lies encoded in the electromagnetic radiation from cosmic objects. Spectropolarimetry and imaging polarimetry provide powerful diagnostics of the physical conditions in astrophysical plasmas, which cannot be obtained via conventional spectroscopy. Whilst its application to other fields of astrophysics is still at an early stage of development, spectropolarimetry is being used with great success in solar physics. The book contains the lectures delivered at the XII Canary Islands Winter School of Astrophysics. Written by eight prestigious astrophysics researchers, it covers the physics of polarization, polarized radiation diagnostics of solar magnetic fields, stellar magnetic fields, polarization insights for active galactic nuclei, compact objects and accretion disks, astronomical masers and their polarization, infrared-submillimeter spectropolarimetry, and instrumentation for astrophysical spectropolarimetry. This timely volume will provide graduate students and researchers with an unprecedented introduction to the field of astrophysical spectropolarimetry. Title: Preface (Astrophysical spectropolarimetry) Authors: Trujillo-Bueno, Javier; Moreno-Insertis, Fernando Bibcode: 2002apsp.confD..11T Altcode: No abstract at ADS Title: Astrophysical Spectropolarimetry Authors: Trujillo-Bueno, Javier; Moreno-Insertis, Fernando; Sanchez Martinez, Francisco Bibcode: 2002assp.book.....T Altcode: This book contains the lectures delivered at the XII Canary Islands Winter School of Astrophysics on Astrophysical Spectropolarimetry. It highlights how recent developments in theoretical astrophysics and astronomical instrumentation are leading an ever-growing number of astrophysicists to appreciate the enormous diagnostic potential offered by spectropolarimetry. Title: Radiative transfer in molecular lines Authors: Asensio Ramos, A.; Trujillo Bueno, J.; Cernicharo, J. Bibcode: 2001ESASP.460..265A Altcode: 2001astro.ph..2270R; 2001astro.ph..2270A; 2001phso.conf..265A The highly convergent iterative methods developed by Trujillo Bueno and Fabiani Bendicho (1995) for radiative transfer (RT) applications are generalized to spherical symmetry with velocity fields. These RT methods are based on Jacobi, Gauss-Seidel (GS), and SOR iteration and they form the basis of a new NLTE multilevel transfer code for atomic and molecular lines. The benchmark tests carried out so far are presented and discussed. The main aim is to develop a number of powerful RT tools for the theoretical interpretation of molecular spectra. Title: G-Band Spectral Synthesis in Solar Magnetic Concentrations Authors: Sánchez Almeida, J.; Asensio Ramos, A.; Trujillo Bueno, J.; Cernicharo, J. Bibcode: 2001ApJ...555..978S Altcode: 2001astro.ph..3006A; 2001astro.ph..3006S Narrowband imaging in the G band is commonly used to trace the small magnetic field concentrations of the Sun, although the mechanism that makes them bright has remained unclear. We carry out LTE syntheses of the G band in an assorted set of semiempirical model magnetic concentrations. The syntheses include all CH lines as well as the main atomic lines within the bandpass. The model atmospheres produce bright G-band spectra having many properties in common with the observed G-band bright points. In particular, the contrast referring to the quiet Sun is about twice the contrast in continuum wavelengths. The agreement with observations does not depend on the specificities of the model atmosphere; rather, it holds from single flux tubes to microstructured magnetic atmospheres. However, the agreement requires that the real G-band bright points are not spatially resolved, even in the best observations. Since the predicted G-band intensities exceed by far the observed values, we foresee a notable increase of contrast of the G-band images upon improvement of the angular resolution. According to the LTE modeling, the G-band spectrum emerges from the deep photosphere that produces the continuum. Our syntheses also predict solar magnetic concentrations showing up in continuum images but not in the G band. Finally, we have examined the importance of the CH photodissociation in setting the amount of G-band absorption. It turns out to play a minor role. Title: Radiative Transfer for the FIRST ERA Authors: Trujillo Bueno, J. Bibcode: 2001ESASP.460..261T Altcode: 2001astro.ph..2269B; 2001astro.ph..2269T; 2001phso.conf..261T This paper presents a brief overview of some recent advances in numerical radiative transfer, which may help the molecular astrophysics community to achieve new breakthroughs in the interpretation of spectro-(polarimetric) observations. Title: A Time-dependent Semiempirical Model of the Chromospheric Umbral Oscillation Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Ruiz Cobo, B. Bibcode: 2001ApJ...550.1102S Altcode: We present a time-dependent semiempirical model of the chromospheric umbral oscillation in sunspots. This model has been obtained by applying recently developed non-LTE inversion techniques to a time series of spectropolarimetric observations. The model consists of two optically thick unresolved atmospheric components: a ``quiet'' component with downward velocities that covers most of the resolution element and an ``active'' component with upward velocities as high as 10 km s-1 that covers a smaller filling factor and has a higher temperature at the same chromospheric optical depth. This semiempirical model accounts for all the observational signatures of the chromospheric oscillation when the filling factor of the active component oscillates between a few percent and 20% of the resolution element. We discuss a plausible physical scenario in which upward-propagating waves in a downflowing magnetized environment lead to periodic mass ejections from the atmospheric layers where the waves become nonlinear. Based on observations obtained with the Gregory Coudé Telescope, operated on the island of Tenerife by the Observatory of Göttingen University in the Spanish Observatorio del Teide of the Instituto de Astrofísica de Canarias. Title: The Iron Line Formation Problem in Three-dimensional Hydrodynamic Models of Solar-like Photospheres Authors: Shchukina, Nataliya; Trujillo Bueno, Javier Bibcode: 2001ApJ...550..970S Altcode: This paper presents the results of a detailed theoretical investigation of the iron line formation NLTE problem in a three-dimensional model of the solar photosphere, which we have obtained from a very recent radiation hydrodynamics simulation of solar surface convection. In this first paper we have neglected the effects of horizontal radiative transfer on the atomic level populations, but we have considered a realistic atomic model for iron that contains hundreds of radiative transitions from the UV to the IR. The self-consistent solutions of the kinetic and transfer equations have been obtained with a new NLTE code, which is based on very efficient iterative methods. We find that overionization due to the near-UV radiation field does take place but mainly in the granular atmospheric regions. This well-known NLTE mechanism tends to produce underpopulation of all the Fe I levels and a very small overexcitation of the Fe II levels. All over the three-dimensional photospheric model Fe II is the dominant ionization stage. We find significant LTE versus NLTE discrepancies mainly for the low-excitation Fe I lines. This applies to both the vertically emergent profiles from the granular regions and also to the spatially averaged profiles. These discrepancies are due to the line opacity deficits that result from the aforementioned underpopulation of the Fe I levels. The emergent profiles of the low-excitation lines of Fe I are thus weaker in NLTE than in LTE. In particular, the largest errors in the equivalent widths (due to the LTE assumption) are found for the weakest low-excitation lines of Fe I. We also give quantitative estimates of the errors in the temperature structure of semiempirical solar granulation models obtained via the application of LTE inversion techniques to several groups of Fe I lines. For instance, the widely used Fe I 6301 and 6302 Å lines tend to lead to an overestimation of about 100-200 K in the granular regions but to a similar underestimation in the intergranular plasma. The present paper considers also the case of the Sun observed with low spatial resolution, with particular emphasis on the long-standing iron abundance problem. We show that it is possible to obtain a very good fit to the observed spectral line shapes by slightly changing the iron abundance (for both the LTE and NLTE cases). In general, the iron abundance we need for reaching the best NLTE fit to observed equivalent widths is 0.074+/-0.03 dex larger than that needed to obtain the best LTE fit. Our most relevant conclusion with regard to the solar iron abundance issue is the following: if NLTE effects are fully taken into account in the three-dimensional model of the solar photosphere, we obtain the meteoritic iron abundance value (AFe=7.50). However, if the abundance analysis is done assuming LTE, we find AFe=7.43, in close agreement with the recent LTE analysis of Asplund and collaborators. Our results do indicate that NLTE effects are significant but not above the 0.1 dex level in the Sun. We consider our NLTE result for the iron abundance as an additional hint of the realism of such three-dimensional hydrodynamic simulations. We conclude that the success of the LTE fitting approach is no proof that NLTE effects are negligible because the existing NLTE effects are compensated for in the LTE analysis by a change in the derived iron abundance. The paper ends emphasizing the great importance of a full three-dimensional NLTE approach in order to be able to lead to new advances in the field of quantitative stellar spectroscopy and, in particular, for a correct derivation of elemental abundance ratios in the atmospheres of metal-poor stars. Title: Fe I Lines in the Spectra of Solar-like Stars: NLTE Effects, Temperature Diagnostics and the Iron Abundance (CD-ROM Directory: contribs/shchukin) Authors: Shchukina, N. G.; Trujillo Bueno, J. Bibcode: 2001ASPC..223..868S Altcode: 2001csss...11..868S No abstract at ADS Title: Atomic Polarization and the Hanle Effect Authors: Trujillo Bueno, J. Bibcode: 2001ASPC..236..161T Altcode: 2001aspt.conf..161T; 2002astro.ph..2328T This article presents an introduction to optical pumping, atomic polarization and the Hanle effect in weakly magnetized stellar atmospheres. Although the physical processes and the theoretical framework described here are of interest for applications in a variety of astrophysical contexts (e.g. scattering polarization in circumstellar envelopes and polarization in astronomical masers), the article focuses mainly on the quest for understanding the physical origin of the linearly polarized solar limb spectrum. It considers also the development of the Hanle effect as a reliable diagnostic tool for making feasible new advances in solar photospheric and chromospheric magnetism. Particular emphasis is given to a rigorous modeling of polarization phenomena as the essential link between theory and observations. Some of the most recent advances in this field are presented after carefully explaining how the various radiation pumping mechanisms lead to atomic polarization in the absence and in the presence of weak magnetic fields. Title: Full Stokes LPSP Observations of the Na D1 and D2 Lines in Magnetized Regions close to the Solar Limb Authors: Martínez Pillet, V.; Trujillo Bueno, J.; Collados, M. Bibcode: 2001ASPC..236..133M Altcode: 2001aspt.conf..133M No abstract at ADS Title: Metastable-level Atomic Polarization and the Diagnostic Problem of Chromospheric Magnetic Fields Authors: Trujillo Bueno, J.; Manso Sainz, R. Bibcode: 2001ASPC..248...83T Altcode: 2001mfah.conf...83T No abstract at ADS Title: THÉMIS Observations of the Second Solar Spectrum Authors: Trujillo Bueno, J.; Collados, M.; Paletou, F.; Molodij, G. Bibcode: 2001ASPC..236..141T Altcode: 2001aspt.conf..141T No abstract at ADS Title: Modeling the Scattering Line Polarization of the Ca II Infrared Triplet Authors: Manso Sainz, R.; Trujillo Bueno, J. Bibcode: 2001ASPC..236..213M Altcode: 2001aspt.conf..213M No abstract at ADS Title: Scattering Polarization Observations with the Tenerife Gregory Coudé Telescope Authors: Dittmann, O.; Trujillo Bueno, J.; Semel, M.; López Ariste, A. Bibcode: 2001ASPC..236..125D Altcode: 2001aspt.conf..125D No abstract at ADS Title: Anomalous Circular Polarization Profiles in Sunspot Chromospheres Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Ruiz Cobo, B. Bibcode: 2000ApJ...544.1141S Altcode: This paper presents a detailed description, analysis, and interpretation of the spectropolarimetric observations recently reported by Socas-Navarro, Trujillo Bueno, & Ruiz Cobo. These observations consist of time series of Stokes I and V profiles above a sunspot umbra. The spectral lines observed simultaneously are the Ca II chromospheric lines at 8498 and 8542 Å and the photospheric Fe I line at 8497 Å. These spectropolarimetric observations unveil an intriguing time-dependent behavior of the Stokes V profiles in the chromospheric lines. This behavior should be considered as an observational reference for future radiation magnetohydrodynamic simulations of sunspot chromospheres. The analysis of the observed time series shows that a ``normal,'' nearly antisymmetric V profile rapidly evolves toward an ``anomalous,'' completely asymmetric profile, returning later to the normal state. The occurrence of such anomalous circular polarization profiles repeats itself with a periodicity of ~150 s. After giving arguments to discard other scenarios, we are able to interpret the anomalous V profiles as a consequence of the development of a second unresolved atmospheric component. This unresolved component seems to be the same that produces the umbral flashes observed in other sunspots, where it is present with a larger filling factor. Based on observations obtained with the Gregory Coudé Telescope, operated on the island of Tenerife by the Observatory of Göttingen University, in the Spanish Observatorio del Teide of the Instituto de Astrofísica de Canarias. Title: Anomalous Polarization Profiles in Sunspots: Possible Origin of Umbral Flashes Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Ruiz Cobo, B. Bibcode: 2000Sci...288.1396S Altcode: 2000Sci...288.1398S We present time-series spectropolarimetric observations of sunspots in the Ca II infrared triplet lines, which show a periodic occurrence of anomalous, asymmetric, circular polarization profiles in the umbral chromosphere. The profiles may be caused by the periodic development of an unresolved atmospheric component in a downward flowing magnetized environment. This active component with upward directed velocities as high as 10 kilometers per second is connected to the umbral flash (UF) phenomenon. We can explain the observations with a semiempirical model of the chromospheric oscillation and of the sunspot magnetized atmospheric plasma during a UF event. Title: Non-LTE Inversion of Stokes Profiles Induced by the Zeeman Effect Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Ruiz Cobo, B. Bibcode: 2000ApJ...530..977S Altcode: This paper presents a new diagnostic tool for the inference of the thermal, dynamic, and magnetic properties of the solar chromosphere. It consists of a non-LTE inversion code of Stokes profiles induced by the Zeeman effect in magnetized stellar atmospheres. This code is the generalization, to the non-LTE Stokes transfer case, of the inversion code for unpolarized line profiles of Socas-Navarro, Ruiz Cobo, & Trujillo Bueno. It is based upon a full non-LTE multilevel treatment of Zeeman line transfer in which the thermal, magnetic, and dynamic properties of the atmospheric model are adjusted automatically by means of nonlinear least-squares-fitting techniques until a best fit to the observed Stokes profiles is obtained. Our non-LTE inversion approach is based on the concept of response functions, which measure the emergent Stokes profiles' first-order reaction to changes in the atmospheric parameters. We generalize our fixed departure coefficients (FDC) approximation in order to allow fast computation of such response functions in the present non-LTE Zeeman line transfer context. We present several numerical tests showing the reliability of our inversion method for retrieving the information about the thermodynamics and the magnetic field vector that is contained in the polarization state of the chosen spectral lines. We also explore the limitations of the inversion code by applying it to simulated observations where the physical hypotheses on which it is based on are not met. Finally, we apply our non-LTE Stokes inversion code to real spectropolarimetric observations of a sunspot observed in the IR triplet lines of Ca II. As a result, a new mean model of the sunspot chromosphere is provided. Title: Radiative Transfer in Weakly Polarizing Media Authors: Sánchez Almeida, J.; Trujillo Bueno, J. Bibcode: 1999ApJ...526.1013S Altcode: We study radiative transfer through a weakly polarizing medium, i.e., a medium in which the degree of polarization of the absorbed, retarded, and emitted light is always weak. In this case, the general radiative transfer equation for the Stokes parameters yields very simple formal solutions. The intensity does not depend on the polarization, and the other Stokes parameters are uncoupled from each other. It is shown how this simplified radiative transfer equation holds in many realistic cases relevant for solar and stellar magnetometry. It can be applied whenever the weak magnetic field approximation works, i.e., for weakly split lines. In addition, it handles weak spectral lines, structures with complex magnetic topology, chromospheric lines formed under non-LTE conditions, etc. The merits of the approximation, which we call the weakly polarizing medium (WPM) approximation, are illustrated by means of several LTE and non-LTE line syntheses in realistic solar model atmospheres. The WPM approximation should be useful in planning and understanding measurements based on polarization. It simplifies the relationship between the observed polarization and the physical structure that one tries to retrieve. The approximation may also be used in numerical problems requiring extensive polarized radiative transfer (inversion codes, syntheses of stellar spectra, self-consistent multilevel non-LTE Zeeman line transfer with atomic polarization, etc.). Title: Iterative Methods for the Non-LTE Transfer of Polarized Radiation: Resonance Line Polarization in One-dimensional Atmospheres Authors: Trujillo Bueno, Javier; Manso Sainz, Rafael Bibcode: 1999ApJ...516..436T Altcode: This paper shows how to generalize to non-LTE polarization transfer some operator splitting methods that were originally developed for solving unpolarized transfer problems. These are the Jacobi-based accelerated Λ-iteration (ALI) method of Olson, Auer, & Buchler and the iterative schemes based on Gauss-Seidel and successive overrelaxation (SOR) iteration of Trujillo Bueno and Fabiani Bendicho. The theoretical framework chosen for the formulation of polarization transfer problems is the quantum electrodynamics (QED) theory of Landi Degl'Innocenti, which specifies the excitation state of the atoms in terms of the irreducible tensor components of the atomic density matrix. This first paper establishes the grounds of our numerical approach to non-LTE polarization transfer by concentrating on the standard case of scattering line polarization in a gas of two-level atoms, including the Hanle effect due to a weak microturbulent and isotropic magnetic field. We begin demonstrating that the well-known Λ-iteration method leads to the self-consistent solution of this type of problem if one initializes using the ``exact'' solution corresponding to the unpolarized case. We show then how the above-mentioned splitting methods can be easily derived from this simple Λ-iteration scheme. We show that our SOR method is 10 times faster than the Jacobi-based ALI method, while our implementation of the Gauss-Seidel method is 4 times faster. These iterative schemes lead to the self-consistent solution independently of the chosen initialization. The convergence rate of these iterative methods is very high; they do not require either the construction or the inversion of any matrix, and the computing time per iteration is similar to that of the Λ-iteration method. Title: A new diagnostic tool for the solar chromosphere Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Ruiz Cobo, B. Bibcode: 1999ASSL..243..263S Altcode: 1999sopo.conf..263S No abstract at ADS Title: The Hanle effect in 1D, 2D and 3D Authors: Manso Sainz, R.; Trujillo-Bueno, J. Bibcode: 1999ASSL..243..143M Altcode: 2007arXiv0710.5430M; 1999sopo.conf..143M This paper addresses the problem of scattering line polarization and the Hanle effect in one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) media for the case of a two-level model atom without lower-level polarization and assuming complete frequency redistribution. The theoretical framework chosen for its formulation is the QED theory of Landi Degl'Innocenti (1983), which specifies the excitation state of the atoms in terms of the irreducible tensor components of the atomic density matrix. The self-consistent values of these density-matrix elements is to be determined by solving jointly the kinetic and radiative transfer equations for the Stokes parameters. We show how to achieve this by generalizing to Non-LTE polarization transfer the Jacobi-based ALI method of Olson et al. (1986) and the iterative schemes based on Gauss-Seidel iteration of Trujillo Bueno and Fabiani Bendicho (1995). These methods essentially maintain the simplicity of the Lambda-iteration method, but their convergence rate is extremely high. Finally, some 1D and 2D model calculations are presented that illustrate the effect of horizontal atmospheric inhomogeneities on magnetic and non-magnetic resonance line polarization signals. Title: Three-dimensional radiative transfer with multilevel atoms Authors: Fabiani Bendicho, P.; Trujillo Bueno, J. Bibcode: 1999ASSL..243..219F Altcode: 1999sopo.conf..219F No abstract at ADS Title: Towards the modelling of the second solar spectrum Authors: Trujillo Bueno, J. Bibcode: 1999ASSL..243...73T Altcode: 1999sopo.conf...73T No abstract at ADS Title: Non-LTE Inversion of Line Profiles Authors: Socas-Navarro, H.; Ruiz Cobo, B.; Trujillo Bueno, J. Bibcode: 1998ApJ...507..470S Altcode: In this paper we address the problem of the non-LTE (NLTE) inversion of line profiles by means of a nonlinear least-squares minimization procedure combined with very efficient multilevel transfer methods. Our approach is based on the concept of response functions, which measure the first-order response of the emergent profiles to changes in the atmospheric conditions. We introduce the fixed departure coefficients (FDC) approximation in order to compute these response functions in a fast and straightforward manner. The accuracy of this approximation is checked comparing FDC response functions with those obtained from full NLTE computations. An NLTE inversion code based on these response functions has been developed and extensively tested. Reference synthetic profiles, similar to those expected from real observations, are given as input to the inversion algorithm and the recovered models are shown to be compatible with the reference models within the error bars. Our NLTE inversion code thus provides a new tool for the investigation of the chromospheres of the Sun and other stars. Title: Fe I lines in spectra of cool stars: NLTE effects in solar-like atmosphere. Authors: Shchukina, N. G.; Trujillo Bueno, J. Bibcode: 1998KFNT...14..315S Altcode: 1998KNFT...14..315S A detailed NLTE investigation of Fe I lines is carried out for the MACKKL quiet solar atmospheric model using a very realistic iron atomic model. This improved atomic model consists of about 250 multiplets and nearly 500 UV, optical and IR bound-bound and bound-free transitions including the regime near the Fe I continuum. The authors find and discuss some interesting statistical regularities with respect to the errors found for the heights of formation, source functions, equivalent widths and the central depths of the Fe I lines when the LTE approximation is adopted. Title: Fe I lines in the spectra of cool stars: NLTE effects in solar-like atmospheres. Authors: Shchukina, N. G.; Trujillo Bueno, J. Bibcode: 1998KPCB...14..242S Altcode: A detailed NLTE investigation of Fe I lines is carried out within the framework of the MACKKL model of the quiet solar atmosphere. The authors used a realistic iron atom model which allows for the fine term structure and has about 250 levels; it involves nearly 500 UV, optical, and IR bound-bound and bound-free transitions and includes the conditions near the Fe I continuum. Errors that arise in the heights of formation, source functions, equivalent widths, and central depths of the Fe I lines within the LTE approximation are statistically studied. The authors demonstrate that the behavior of the errors can be described by a parametric set of curves depending on height of line formation, the parameter being the excitation potential of the lower level. Title: Diagnostic of the solar atmosphere using iron lines. Authors: Shchukina, N. S.; Kostyk, R. I.; Trujillo Bueno, J. Bibcode: 1998IBUAA..12Q..32S Altcode: No abstract at ADS Title: NLTE effects in iron spectrum of sunspots. Authors: Shchukina, N. G.; Trujillo Bueno, J. Bibcode: 1998IBUAA..12R..32S Altcode: No abstract at ADS Title: Linearization versus Preconditioning: Which Approach Is Best for Solving Multilevel Transfer Problems? Authors: Socas-Navarro, H.; Trujillo Bueno, J. Bibcode: 1997ApJ...490..383S Altcode: We present a critical analysis of linearization and preconditioning, the two most used approaches proposed for achieving the required linearity in the iterative solution of the multilevel transfer problem. By distinguishing from the outset between the response of the radiation field to the source function and opacity perturbations, we are able to demonstrate that if the linearization strategy, on which the local approximate Λ-operator option of the multilevel transfer code MULTI is based, is applied neglecting the terms coming from the response of the radiation field to the opacity perturbations, one then recovers the same equations obtained using the preconditioning technique of Rybicki & Hummer. It is also shown that if this preconditioning technique is applied taking into account the response of the radiation field to both the source function and opacity variations, one then ends up with the same equations found via the linearization method. Thus these two approaches to the numerical solution of the multilevel transfer problem turn out to be essentially the same, because similar equations are obtained if the same information is taken into account. Finally, it is pointed out that, if one wishes to guarantee positivity for the atomic level populations, it is necessary to neglect the terms associated with the response of the radiation field to the opacity perturbations. Neglecting such terms does not deteriorate the convergence rate of multilevel transfer methods that make use of a local approximate operator. Title: Multidimensional radiative transfer with multilevel atoms. II. The non-linear multigrid method. Authors: Fabiani Bendicho, P.; Trujillo Bueno, J.; Auer, L. Bibcode: 1997A&A...324..161F Altcode: A new iterative method for solving non-LTE multilevel radiative transfer (RT) problems in 1D, 2D or 3D geometries is presented. The scheme obtains the self-consistent solution of the kinetic and RT equations at the cost of only a few (<10) formal solutions of the RT equation. It combines, for the first time, non-linear multigrid iteration (Brandt, 1977, Math. Comp. 31, 333; Hackbush, 1985, Multi-Grid Methods and Applications, springer-Verlag, Berlin), an efficient multilevel RT scheme based on Gauss-Seidel iterations (cf. Trujillo Bueno & Fabiani Bendicho, 1995ApJ...455..646T), and accurate short-characteristics formal solution techniques. By combining a valid stopping criterion with a nested-grid strategy a converged solution with the desired true error is automatically guaranteed. Contrary to the current operator splitting methods the very high convergence speed of the new RT method does not deteriorate when the grid spatial resolution is increased. With this non-linear multigrid method non-LTE problems discretized on N grid points are solved in O(N) operations. The nested multigrid RT method presented here is, thus, particularly attractive in complicated multilevel transfer problems where small grid-sizes are required. The properties of the method are analyzed both analytically and with illustrative multilevel calculations for Ca II in 1D and 2D schematic model atmospheres. Title: The Limited Influence of Pressure Gradients on Late-Type Stellar Line Asymmetries Authors: Allende Prieto, Carlos; García López, Ramón J.; Trujillo Bueno, Javier Bibcode: 1997ApJ...483..941A Altcode: 1997astro.ph..1061P; 1997astro.ph..1061A Line asymmetries and shifts are powerful tools for studying velocity fields in the stellar photospheres. Other effects, however, could also generate asymmetries, blurring the information of the velocity patterns. We have studied the shifts and asymmetries induced in the profiles of spectral lines by pressure effects. The best theoretical and experimental data on line broadening and shifts caused by collisions with atomic hydrogen were used to analyze the Na I D and three Ca I lines. Line bisectors of synthetic spectra computed with accurate data for the Na I and Ca I lines are compared with very high resolution, high signal-to-noise ratio solar spectra and indicate that pressure broadening reproduces the wings of the observed lines, but pressure shifts introduce neither asymmetries nor shifts comparable to the observed ones. Title: Linear Polarization Due to Lower Level Depopulation Pumping in Stellar Atmospheres Authors: Trujillo Bueno, Javier; Landi Degl'Innocenti, Egidio Bibcode: 1997ApJ...482L.183T Altcode: The possible generation of linear polarization in spectral lines due to depopulation-pumping processes in stellar atmospheres is investigated within the framework of a quantum electrodynamic theory for the transfer of polarized radiation based on the atomic density-matrix formalism. It is shown that the radiation field's anisotropy in solar-like atmospheres induces population imbalances among the lower level sublevels of optical line transitions. The depolarizing rates that are assumed to model some typical spectral lines are not capable of completely destroying such a lower level atomic alignment, and the corresponding linear polarization signals are found to lie above the sensitivity limit of some recently developed spectropolarimeters. Title: Are NLTE effects important for the inversion of iron lines? Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Ruiz Cobo, B.; Shchukina, N. G. Bibcode: 1997joso.proc...86S Altcode: No abstract at ADS Title: Is LTE a Suitable Approximation for Fe I - based Diagnostics of the Thermal Structure of Sunspots? Authors: Shchukina, N. G.; Trujillo Bueno, J. Bibcode: 1997ASPC..118..207S Altcode: 1997fasp.conf..207S NLTE effects in iron lines are carefully investigated for the sunspot umbral model of Maltby et. al. (1986). Our model atom is realistic: it has hundreds of levels including many high-excited ones among which infrared transitions take place. The self-consistent solution of the kinetic and radiative transfer equations is obtained using recently-developed multilevel transfer methods suitable for efficiently handling hundreds of radiative transitions in detail from the ultraviolet to the infrared. These NLTE multilevel transfer calculations allow us to investigate whether the currently-used LTE approximation is suitable for diagnosing the temperature structure of sunspots via Fe I lines. Title: The Polarization-Free Approximation Applied to Multi-Level Non-LTE Radiative Transfer Authors: Bruls, J. H. M. J.; Trujillo Bueno, J. Bibcode: 1996SoPh..164..155B Altcode: The polarization-free (POF) approximation (Trujillo Bueno and Landi Degl'Innocenti, 1996) is capable of accounting for the approximate influence of the magnetic field on the statistical equilibrium, without actually solving the full Stokes vector radiative transfer equation. The method introduces the Zeeman splitting or broadening of the line absorption profile φI in the scalar radiative transfer equation, but the coupling between Stokes I and the other Stokes parameters is neglected. The expected influence of the magnetic field is largest for strongly-split strong lines and the effect is greatly enhanced by gradients in the magnetic field strength. Formally the interaction with the other Stokes parameters may not be neglected for strongly-split strong lines, but it turns out that the error in Stokes I obtained through the POF approximation to a large extent cancels the neglect of interaction with the other Stokes parameters, so that the resulting line source functions and line opacities are more accurate than those obtained with the field-free approach. Although its merits have so far only been tested for a two-level atom, we apply the POF approximation to multi-level non-LTE radiative transfer problems on the premise that there is no essential difference between these two cases. Final verification of its validity in multi-level cases still awaits the completion of a non-LTE Stokes vector transfer code. Title: The polarization-free approximation Authors: Trujillo Bueno, J.; Landi Degl'Innocenti, E. Bibcode: 1996SoPh..164..135T Altcode: The development of effective iterative methods capable of accurately solving NLTE Stokes transfer problems is of considerable importance for the investigation of solar and stellar magnetic fields. After briefly indicating the iterative approach which is being presently pursued for the exact solution of such problems, the particular regime where polarization signals can only be due to the Zeeman effect is considered in some detail. By means of NLTE Stokes transfer calculations for a two-level atomic model it is first shown that the currently-used field-free approximation (Rees, 1969) cannot be safely applied in the presence of magnetic field gradients. Such gradients lead to changes in the shape and width of the line profiles and they can produce non-negligible effects on the atomic level populations and line source functions. A new approximate method is then proposed, which does not require the actual solution of the Stokes vector transfer equation and is practically as fast as the field-free one. This polarization-free approximation provides a fairly good account of the effects of homogeneous and inhomogeneous magnetic fields on the statistical equilibrium and is very easy to implement in any existing non-magnetic, multi-level transfer code. Title: A Novel Iterative Scheme for the Very Fast and Accurate Solution of Non-LTE Radiative Transfer Problems Authors: Trujillo Bueno, J.; Fabiani Bendicho, P. Bibcode: 1995ApJ...455..646T Altcode: Iterative schemes based on Gauss-Seidel (G-S) and optimal successive over-relaxation (SOR) iteration are shown to provide a dramatic increase in the speed with which non-LTE radiation transfer (RT) problems can be solved. The convergence rates of these new RT methods are identical to those of upper triangular nonlocal approximate operator splitting techniques, but the computing time per iteration and the memory requirements are similar to those of a local operator splitting method. In addition to these properties, both methods are particularly suitable for multidimensional geometry, since they neither require the actual construction of nonlocal approximate operators nor the application of any matrix inversion procedure.

Compared with the currently used Jacobi technique, which is based on the optimal local approximate operator (see Olson, Auer, & Buchler 1986), the G-S method presented here is faster by a factor 2. It gives excellent smoothing of the high-frequency error components, which makes it the iterative scheme of choice for multigrid radiative transfer. This G-S method can also be suitably combined with standard acceleration techniques to achieve even higher performance.

Although the convergence rate of the optimal SOR scheme developed here for solving non-LTE RT problems is much higher than G-S, the computing time per iteration is also minimal, i.e., virtually identical to that of a local operator splitting method. While the conventional optimal local operator scheme provides the converged solution after a total CPU time (measured in arbitrary units) approximately equal to the number n of points per decade of optical depth, the time needed by this new method based on the optimal SOR iterations is only √n/2√2. This method is competitive with those that result from combining the above-mentioned Jacobi and G-S schemes with the best acceleration techniques.

Contrary to what happens with the local operator splitting strategy currently in use, these novel methods remain effective even under extreme non-LTE conditions in very fine grids. Title: Multidimensional radiative transfer with multilevel atoms. I. ALI method with preconditioning of the rate equations. Authors: Auer, L.; Bendicho, P. Fabiani; Trujillo Bueno, J. Bibcode: 1994A&A...292..599A Altcode: We combine a number of powerful mathematical techniques to produce an effective method for treating multidimensional radiative transfer problems in complex atomic models without assuming LTE. The approach is so efficient that multilevel two-dimensional (2D) modeling can now be performed with no more than a workstation. We employ Accelerated Lambda Iteration (ALI) methods: accurate short characteristics for the formal solution of the transfer equation with an efficient new strategy for horizontal periodic boundary conditions, local approximate {LAMBDA}-operators given by the diagonal of the exact operator, methods to accelerate the convergence, and preconditioning of the rate equations. Of particular interest is a simple grid-doubling strategy which both rapidly finds the converged solution in very fine meshes and also estimates the true error of that solution. The properties of the method are described in detail with the help of 2D line-transfer calculations with multilevel model atoms for Ca II and H. These illustrative multilevel calculations in schematic inhomogeneous atmospheres demonstrate the importance of properly including the effects of horizontal radiative transfer and realistic atomic models. Title: On photospheric flows and chromospheric corks Authors: Brandt, P. N.; Rutten, R. J.; Shine, R. A.; Trujillo Bueno, J. Bibcode: 1994ASIC..433..251B Altcode: No abstract at ADS Title: Multi-dimensional radiative transfer with multi-level atoms and the diagnostic problem of small-scale structures Authors: Trujillo Bueno, J.; Fabiani, P.; Auer, L. Bibcode: 1994smf..conf..328T Altcode: No abstract at ADS Title: Atomic orientation in chromospheric lines. Authors: Trujillo Bueno, J.; Martínez Pillet, V.; Sánchez Almeida, J.; Landi Degl'Innocenti, E. Bibcode: 1993ASPC...46..526T Altcode: 1993ASPC...46..526B; 1993mvfs.conf..526T; 1993IAUCo.141..526T Observations of the Stokes I and V profiles of the Ca II H and K lines in solar magnetic regions are presented. Least-squares fits of dI/dλ to V are obtained and the wavelength variation of the residuals, i.e. V-kdI/dλ, calculated. The authors find significant symmetric residuals in umbrae, which are in agreement with the effect on the V profiles due to atomic orientation, i.e. with the existence of an unequal population of the Zeeman sublevels with M > 0 with respect to those with M < 0. Title: On the photospheric temperature in small-scale magnetic flux concentrations Authors: Fabiani Bendicho, P.; Kneer, F.; Trujillo Bueno, J. Bibcode: 1992A&A...264..229F Altcode: Results are presented of 2D radiative transfer calculations performed for geometric configurations that simulate partly evacuated small-scale magnetic flux sheets embedded in the ambient solar atmosphere. Temperature distributions in (gray) radiative equilibrium at low optical depths where radiation transfer dominates the energy budget are obtained. Two-dimensional radiative equilibrium flux sheet models are calculated using a novel method which shows that the temperature enhancement of the upper layers of photospheric magnetic flux concentrations is due to the radiation channeling effect, i.e., that horizontal radiative transfer tends to channel emerging radiation into the lower opacity regions. The walls of the flux sheets are found to radiate energy from subphotospheric surrounding layers, giving rise to a strong heating of the atmosphere of the flux sheets. Radiative energy migrates horizontally from the heated flux sheets towards the ambient medium and there it heats the atmosphere at low optical depths. Title: Dynamics of the Quiet Solar Atmosphere: K2v Cell Grains Versus Magnetic Elements Authors: Brandt, P. N.; Rutten, R. J.; Shine, R. A.; Trujillo Bueno, J. Bibcode: 1992ASPC...26..161B Altcode: 1992csss....7..161B No abstract at ADS Title: On the Influence of Multi-Dimensional Radiative Transfer on the Energetic Contribution of the CAK Line Authors: Fabiani, P.; Trujillo-Bueno, J. Bibcode: 1991ASIC..341..423F Altcode: 1991sabc.conf..423F No abstract at ADS Title: Do We Really Know What the Actual Chromospheric Heating Requirements Are? (With 1 Figure) Authors: Trujillo Bueno, J. Bibcode: 1991mcch.conf...60T Altcode: No abstract at ADS Title: Multidimensional radiative transfer in stratified atmospheres. VI - Radiative instabilities Authors: Trujillo Bueno, J.; Kneer, F. Bibcode: 1990A&A...232..135T Altcode: The possibility that radiative instabilities in stellar atmospheres are driven by transfer of energy processes between stellar gas and the radiation field is considered. Harmonic temperature fluctuations in gray radiative equilibrium atmospheres are introduced, and the linear response of the radiation field to the ensuing Planck-function and opacity fluctuations is investigated. Analytical and numerical calculations are performed, emphasizing the influence of the multidimensional radiative transfer (MRT) effects of opacity fluctuations on the radiative relaxation time as a function of the wavenumber of the perturbations. Quantitative examples are given for stellar atmospheres with solarlike T(eff) and gravitational stratification. It is concluded that, while the MRT effects of B fluctuations generally tend to be stabilizing, the MRT effects of chi fluctuations are optically important for driving radiative instabilities. Title: Radiative transfer problems in the Solar and Sun-like atmospheres Authors: Trujillo Bueno, J. Bibcode: 1990nwus.book..119T Altcode: No abstract at ADS Title: On the Influence of Opacity Fluctuations on the Energy Transfer by Radiation Authors: Trujillo-Bueno, J.; Kneer, F. Bibcode: 1989ASIC..263..441T Altcode: 1989ssg..conf..441T No abstract at ADS Title: On the κ-mechanism and the multi-dimensional Eddington approximation. Authors: Trujillo Bueno, J. Bibcode: 1988ESASP.286...11T Altcode: 1988ssls.rept...11T To consider the possibility of a κ-like overstability mechanism as the driving force of solar modes of oscillation requires a theoretical description of the interaction between the hydrodynamics of the motion and the radiative energy transport. The aim of this paper is to answer the question of whether or not the frequently-used multi-dimensional Eddington approximation is suitable for such purposes. It is found that Eddington's approximation is generally adequate for giving account of the radiative damping effects of Planck function fluctuations, while it is in gross disagreement with the exact results with respect to the radiative transfer effects of opacity fluctuations. Therefore, it is concluded that its use should be avoided, when investigating κ-like excitation mechanisms for nonradial stellar modes of oscillation. Title: Multi-dimensional energy transfer by radiation in the solar atmosphere Authors: Trujillo Bueno, Javier Bibcode: 1988PhDT.........6T Altcode: Both linear and nonlinear analysis are used to gain a better understanding of the effects of multi-dimensional transfer of energy by radiation in spatially structured stellar atmospheres. Information on the radiative response of RE (radiative equilibrium) model atmospheres to temperature fluctuations is obtained. Whether or not such perturbed systems always return to their initial RE configurations is addressed by investigation of the dependence of the radiative relaxation times on the geometry and structural lengths of the perturbations, as well as establishing the conditions under which radiative instabilities can be possible. Title: Multidimensional radiative transfer in stratified atmospheres Authors: Kneer, F.; Trujillo-Bueno, J. Bibcode: 1987A&A...183...91K Altcode: Energy transport by radiation is an important contribution to the energy budget in stellar atmospheric structures. In this paper, radiative relaxation of small-scale structures is investigated. The authors show in a linear analysis: (1) Already at structural lengths of 10 opacity scale heights, horizontal photon exchange is important for the energy budget. (2) In atmospheric layers near continuum optical depth τc = 1 and below, the continuum absorption and emission processes dominate the radiative relaxation. (3) Weak spectral lines or lines with σlɛ ≤ 1 have little influence on the energy exchange. (4) At large heights, transport in few spectral lines with σlɛ very large 1 can compete with continuum processes. Title: Multidimensional radiative transfer in stratified atmospheres. IV - Radiative cooling by LTE and non-LTE spectral lines Authors: Trujillo-Bueno, J.; Kneer, F. Bibcode: 1987A&A...174..183T Altcode: The question of efficiency of radiative energy losses in spectral lines is addressed. In a semi-infinite atmosphere with constant temperature, the total radiative energy loss (integrated over all depths) in a spectral line without continuum is infinite, in both LTE and non-LTE. Thus, only local energy balances may be considered with such models. The authors give radiative cooling functions for various non-LTE parameters and structural lengths of a two-dimensional stratified atmosphere. At the surface, cooling is less efficient in non-LTE than in LTE. At large optical depths, both become equal and are non-negligible. In these layers horizontal transfer effects become important for the energy balance. Title: Radiative Relaxation in Small Scale Structures Authors: Trujillo-Bueno, J.; Kneer, F. Bibcode: 1987rfsm.conf..281T Altcode: The authors discuss the effects of multidimensional radiative transfer on the energy exchange by radiation in small-scale structures. Title: The visible helium spectrum of a white-light flare. Authors: Lites, B. W.; Meidig, D. F.; Trujillo Bueno, J. Bibcode: 1986lasf.conf..101L Altcode: 1986lasf.symp..101L Emission lines of neutral and ionized helium at visible wavelengths are measured in the white-light flare of 24 April 1981. These intensities, along with accompanying profiles, are presented for the purpose of providing an observational basis for future radiative transfer models of white-light flares. Absolute intensities, both peak and integrated, are given for 14 lines of He I, and for the He II line at 4686 Å. The authors compare intensities of these lines in the white-light emitting region to intensities measured in a flare kernel that does not show significant continuum emission. From this, they infer that the white light emission arises from material at chromospheric temperatures, and not from temperatures greater than about 20,000K. A search for Stark-enhanced forbidden neighbors to the allowed He I lines in this disk flare was unsuccessful. Title: Multidimensional Radiative Transfer in Stratified Atmospheres: Radiative Cooling by LTE and non-LTE Spectral Lines Authors: Trujillo-Bueno, J.; Kneer, F. Bibcode: 1986MitAG..67..304T Altcode: A detailed paper has been submitted to Astron. Astrophys.