Author name code: martinez-gonzalez ADS astronomy entries on 2022-09-14 author:"Martinez Gonzalez, Maria J." ------------------------------------------------------------------------ Title: Mapping the Hidden Magnetic Field of the Quiet Sun Authors: Trelles Arjona, J. C.; Martínez González, M. J.; Ruiz Cobo, B. Bibcode: 2021ApJ...915L..20T Altcode: 2021arXiv210610546T The Sun is the only star where we can resolve the intricate magnetism that all convective stars harbor. Yet, more than 99% of its visible surface along the solar cycle (the so-called quiet Sun) is filled with a tangled, unresolved magnetism. These "hidden" fields are thought to store enough magnetic energy to play a role in the heating of the Sun's outer atmosphere, but its field strength is still not constrained. Previous investigations based on the Hanle effect in atomic lines claim a strong magnetization of about 100 G, while the same effect in molecules show a factor of 10 weaker fields. The discrepancy disappears if the magnetic field strength of the hidden field is not homogeneous in the solar surface. In this Letter, we prove using magnetohydrodynamical simulations that it is possible to infer the average field strength of the hidden quiet-Sun magnetic fields using multiline inversions of intensity profiles in the Zeeman regime. Using this technique with 15 spectral lines in the 1.5 μm spectral range, we reveal that the spatial distribution of the hidden field is strongly correlated with convection motions, and that the average magnetization is about 46 G. Reconciling our findings with the Hanle ones is not obvious and will require future work on both sides, since it implies an increase of the field strength with height, something that is physically questionable. Title: Empirical determination of atomic line parameters of the 1.5 μm spectral region Authors: Trelles Arjona, J. C.; Ruiz Cobo, B.; Martínez González, M. J. Bibcode: 2021A&A...648A..68T Altcode: 2021arXiv210304160T Context. Both the quality and amount of astrophysical data are steadily increasing over time owing to the improvement of telescopes and their instruments. This requires corresponding evolution of the techniques used for obtaining and analyzing the resulting data. The infrared spectral range at 1.56 μm usually observed by the GRegor Infrared Spectrograph (GRIS) at the GREGOR solar telescope has a width of around 30 Å and includes at least 15 spectral lines. Normally, only a handful of spectral lines (five at most) are used in studies using GRIS because of the lack of atomic parameters for the others. Including more spectral lines may alleviate some of the known ambiguities between solar atmospheric parameters.
Aims: We used high-precision spectropolarimetric data for the quiet Sun at 1.56 μm observed with GRIS on the GREGOR along with the SIR inversion code in order to obtain accurate atomic parameters for 15 spectral lines in this spectral range.
Methods: We used inversion techniques to infer both solar atmospheric models and the atomic parameters of spectral lines which, under the local thermodynamic equilibrium approximation, reproduce spectropolarimetric observations.
Results: We present accurate atomic parameters for 15 spectral lines within the spectral range from 15 644 to 15 674 Å. This spectral range is commonly used in solar studies because it enables the study of the low photosphere. Moreover, the infrared spectral lines are better tracers of the magnetic fields than the optical ones. Title: Photospheric magnetic topology of a north polar region Authors: Pastor Yabar, A.; Martínez González, M. J.; Collados, M. Bibcode: 2020A&A...635A.210P Altcode: 2020arXiv200304267P
Aims: We aim to characterise the magnetism of a large fraction of the north polar region close to a maximum of activity, when the polar regions are reversing their dominant polarity.
Methods: We make use of full spectropolarimetric data from the CRisp Imaging Spectro-Polarimeter installed at the Swedish Solar Telescope. The data consist of a photospheric spectral line, which is used to infer the various physical parameters of different quiet Sun regions by means of the solution of the radiative transfer equation. We focus our analysis on the properties found for the north polar region and their comparison to the same analysis applied to data taken at disc centre and low-latitude quiet Sun regions for reference. We also analyse the spatial distribution of magnetic structures throughout the north polar region.
Results: We find that the physical properties of the polar region (line-of-sight velocity, magnetic flux, magnetic inclination and magnetic azimuth) are compatible with those found for the quiet Sun at disc centre and are similar to the ones found at low latitudes close to the limb. Specifically, the polar region magnetism presents no specific features. The structures for which the transformation from a line-of-sight to a local reference frame was possible harbour large magnetic fluxes (>1017 Mx) and are in polarity imbalance with a dominant positive polarity, the largest ones (>1019 Mx) being located below 73° latitude. 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-resolution spectroscopy of Boyajian's star during optical dimming events Authors: Martínez González, M. J.; González-Fernández, C.; Asensio Ramos, A.; Socas-Navarro, H.; Westendorp Plaza, C.; Boyajian, T. S.; Wright, J. T.; Collier Cameron, A.; González Hernández, J. I.; Holgado, G.; Kennedy, G. M.; Masseron, T.; Molinari, E.; Saario, J.; Simón-Díaz, S.; Toledo-Padrón, B. Bibcode: 2019MNRAS.486..236M Altcode: 2019MNRAS.tmp..828M; 2018arXiv181206837M Boyajian's star is an apparently normal main-sequence F-type star with a very unusual light curve. The dipping activity of the star, discovered during the Kepler mission, presents deep, asymmetric, and aperiodic events. Here we present high-resolution spectroscopic follow-up during some dimming events recorded post-Kepler observations, from ground-based telescopes. We analyse data from the HERMES, HARPS-N, and FIES spectrographs to characterize the stellar atmosphere and to put some constraints on the hypotheses that have appeared in the literature concerning the occulting elements. The star's magnetism, if existing, is not extreme. The spots on the surface, if present, would occupy 0.02 per cent of the area, at most. The chromosphere, irrespective of the epoch of observation, is hotter than the values expected from radiative equilibrium, meaning that the star has some degree of activity. We find no clear evidence of the interstellar medium or exocoments being responsible for the dimmings of the light curve. However, we detect at 1-2σ level, a decrease of the radial velocity of the star during the first dip recorded after the Kepler observations. We claim the presence of an optically thick object with likely inclined and high impact parameter orbits that produces the observed Rossiter-McLaughlin effect. Title: Spectropolarimetric analysis of an active region filament. II. Evidence of the limitations of a single-component model Authors: Díaz Baso, C. J.; Martínez González, M. J.; Asensio Ramos, A. Bibcode: 2019A&A...625A.129D Altcode: 2019arXiv190410688D
Aims: Our aim is to demonstrate the limitations of using a single-component model to study the magnetic field of an active region filament. To do this, we analyzed the polarimetric signals of the He I 10830 Å multiplet, which were acquired with the infrared spectrograph GRIS of the GREGOR telescope (Tenerife, Spain).
Methods: After a first analysis of the general properties of the filament using HAZEL under the assumption of a single-component model atmosphere, in this second part we focus our attention on the observed Stokes profiles and the signatures that cannot be explained with this model.
Results: We have found an optically thick filament whose blue and red components have the same sign in the linear polarization as an indication of radiative transfer effects. Moreover, the circular polarization signals inside the filament show strong magnetic field gradients. We also show that even a filament with such high absorption still shows signatures of the circular polarization that is generated by the magnetic field below the filament. The reason is that the absorption of the spectral line decays very quickly toward the wings, just where the circular polarization has a larger amplitude. In order to separate the two contributions, we explore the possibility of a two-component model, but the inference becomes impossible to overcome because very many solutions are compatible with the observations. Title: Spectropolarimetric analysis of an active region filament. I. Magnetic and dynamical properties from single component inversions Authors: Díaz Baso, C. J.; Martínez González, M. J.; Asensio Ramos, A. Bibcode: 2019A&A...625A.128D Altcode: 2019arXiv190409593D
Aims: The determination of the magnetic filed vector in solar filaments is made possible by interpreting the Hanle and Zeeman effects in suitable chromospheric spectral lines like those of the He I multiplet at 10 830 Å. We study the vector magnetic field of an active region filament (NOAA 12087).
Methods: Spectropolarimetric data of this active region was acquired with the GRIS instrument at the GREGOR telescope and studied simultaneously in the chromosphere with the He I 10 830 Å multiplet and in the photosphere Si I 10 827 Å line. As has been done in previous studies, only a single-component model was used to infer the magnetic properties of the filament. The results are put into a solar context with the help of the Solar Dynamic Observatory images.
Results: Some results clearly point out that a more complex inversion had to be performed. First, the Stokes V map of He I does not show a clear signature of the presence of the filament. Second, the local azimuth map follows the same pattern as Stokes V; it appears that polarity of Stokes V is conditioning the inference to very different magnetic fields even with similar linear polarization signals. This indication suggests that the Stokes V could be dominated from below by the magnetic field coming from the active region, and not from the filament itself. This evidence, and others, will be analyzed in depth and a more complex inversion will be attempted in the second part of this series. Title: Diagnostic potential of the Ca II 8542 Å line for solar filaments Authors: Díaz Baso, C. J.; Martínez González, M. J.; Asensio Ramos, A.; de la Cruz Rodríguez, J. Bibcode: 2019A&A...623A.178D Altcode: 2019arXiv190206574D
Aims: In this study we explore the diagnostic potential of the chromospheric Ca II line at 8542 Å for studying the magnetic and dynamic properties of solar filaments. We have acquired high spatial resolution spectropolarimetric observations in the Ca II 8542 Å line using the CRISP instrument at the Swedish 1 m Solar Telescope.
Methods: We used the NICOLE inversion code to infer physical properties from observations of a solar filament. We discuss the validity of the results due to the assumption of hydrostatic equilibrium. We have used observations from other telescopes such as CHROTEL and SDO, in order to study large scale dynamics and the long term evolution of the filament.
Results: We show that the Ca II 8542 Å line encodes information of the temperature, line-of-sight velocity and magnetic field vector from the region where the filament is located. The current noise levels only allow us to estimate an upper limit of 260 G for the total magnetic field of the filament. Our study also reveals that if we consider information from the aforementioned spectral line alone, the geometric height, the temperature and the density could be degenerated parameters outside the hydrostatic equilibrium approach. Title: Magnetic topology of the north solar pole Authors: Pastor Yabar, A.; Martínez González, M. J.; Collados, M. Bibcode: 2018A&A...616A..46P Altcode: 2018arXiv180409075P; 2018A&A...616A..46Y The magnetism at the poles is similar to that of the quiet Sun in the sense that no active regions are present there. However, the polar quiet Sun is somewhat different from that at the activity belt as it has a global polarity that is clearly modulated by the solar cycle. We study the polar magnetism near an activity maximum when these regions change their polarity, from which it is expected that its magnetism should be less affected by the global field. To fully characterise the magnetic field vector, we use deep full Stokes polarimetric observations of the 15 648.5 and 15 652.8 Å FeI lines. We observe the north pole as well as a quiet region at disc centre to compare their field distributions. In order to calibrate the projection effects, we observe an additional quiet region at the east limb. We find that the two limb datasets share similar magnetic field vector distributions. This means that close to a maximum, the poles look like typical limb, quiet-Sun regions. However, the magnetic field distributions at the limbs are different from the distribution inferred at disc centre. At the limbs, we infer a new population of magnetic fields with relatively strong intensities ( 600-800 G), inclined by 30° with respect to the line of sight, and with an azimuth aligned with the solar disc radial direction. This line-of-sight orientation interpreted as a single magnetic field gives rise to non-vertical fields in the local reference frame and aligned towards disc centre. This peculiar topology is very unlikely for such strong fields according to theoretical considerations. We propose that this new population at the limbs is due to the observation of unresolved magnetic loops as seen close to the limb. These loops have typical granular sizes as measured in the disc centre. At the limbs, where the spatial resolution decreases, we observe them spatially unresolved, which explains the new population of magnetic fields that is inferred. This is the first (indirect) evidence of small-scale magnetic loops outside the disc centre and would imply that these small-scale structures are ubiquitous on the entire solar surface. This result has profound implications for the energetics not only of the photosphere, but also of the outer layers since these loops have been reported to reach the chromosphere and the low corona. Title: Cosmic Magnetic Fields Authors: Sánchez Almeida, J.; Martínez González, M. J. Bibcode: 2018cmf..book.....S Altcode: Magnetic fields play an important role in many astrophysical processes. They are difficult to detect and characterize since often their properties have to be inferred through interpreting the polarization of the light. Magnetic fields are also challenging to model and understand. Magnetized plasmas behave following highly non-linear differential equations having no general solution, so that every astrophysical problem represents a special case to be studied independently. Hence, magnetic fields are often an inconvenient subject which is overlooked or simply neglected (the elephant in the room, as they are dubbed in poster of the school). Such difficulty burdens the research on magnetic fields, which has evolved to become a very technical subject, with many small disconnected communities studying specific aspects and details. The school tried to amend the situation by providing a unifying view of the subject. The students had a chance to understand the behavior of magnetic fields in all astrophysical contexts, from cosmology to the Sun, and from starbursts to AGNs. The school was planed to present a balanced yet complete review of our knowledge, with excursions into the unknown to point out present and future lines of research. The subject of Cosmic Magnetic Fields was split into seven different topics: cosmic magnetic field essentials, solar magnetic fields, stellar magnetic fields, the role of magnetic fields on AGN feedback, magnetic fields in galaxies, magnetic fields in galaxy clusters and at larger scales, and primordial magnetic fields and magnetic fields in the early Universe. The corresponding lectures were delivered by seven well known and experienced scientists that have played key roles in the major advances of the field during the last years: F. Cattaneo, P. Judge, O. Kochukhov, R. Keppens, R. Beck, K. Dolag, and F. Finelli. Their lectures were recorded and are freely available at the IAC website: http://iactalks.iac.es/talks/serie/19. Title: The First Post-Kepler Brightness Dips of KIC 8462852 Authors: Boyajian, Tabetha. S.; Alonso, Roi; Ammerman, Alex; Armstrong, David; Asensio Ramos, A.; Barkaoui, K.; Beatty, Thomas G.; Benkhaldoun, Z.; Benni, Paul; Bentley, Rory O.; Berdyugin, Andrei; Berdyugina, Svetlana; Bergeron, Serge; Bieryla, Allyson; Blain, Michaela G.; Capetillo Blanco, Alicia; Bodman, Eva H. L.; Boucher, Anne; Bradley, Mark; Brincat, Stephen M.; Brink, Thomas G.; Briol, John; Brown, David J. A.; Budaj, J.; Burdanov, A.; Cale, B.; Aznar Carbo, Miguel; Castillo García, R.; Clark, Wendy J.; Clayton, Geoffrey C.; Clem, James L.; Coker, Phillip H.; Cook, Evan M.; Copperwheat, Chris M.; Curtis, J. L.; Cutri, R. M.; Cseh, B.; Cynamon, C. H.; Daniels, Alex J.; Davenport, James R. A.; Deeg, Hans J.; De Lorenzo, Roberto; de Jaeger, Thomas; Desrosiers, Jean-Bruno; Dolan, John; Dowhos, D. J.; Dubois, Franky; Durkee, R.; Dvorak, Shawn; Easley, Lynn; Edwards, N.; Ellis, Tyler G.; Erdelyi, Emery; Ertel, Steve; Farfán, Rafael. G.; Farihi, J.; Filippenko, Alexei V.; Foxell, Emma; Gandolfi, Davide; Garcia, Faustino; Giddens, F.; Gillon, M.; González-Carballo, Juan-Luis; González-Fernández, C.; González Hernández, J. I.; Graham, Keith A.; Greene, Kenton A.; Gregorio, J.; Hallakoun, Na'ama; Hanyecz, Ottó; Harp, G. R.; Henry, Gregory W.; Herrero, E.; Hildbold, Caleb F.; Hinzel, D.; Holgado, G.; Ignácz, Bernadett; Ilyin, Ilya; Ivanov, Valentin D.; Jehin, E.; Jermak, Helen E.; Johnston, Steve; Kafka, S.; Kalup, Csilla; Kardasis, Emmanuel; Kaspi, Shai; Kennedy, Grant M.; Kiefer, F.; Kielty, C. L.; Kessler, Dennis; Kiiskinen, H.; Killestein, T. L.; King, Ronald A.; Kollar, V.; Korhonen, H.; Kotnik, C.; Könyves-Tóth, Réka; Kriskovics, Levente; Krumm, Nathan; Krushinsky, Vadim; Kundra, E.; Lachapelle, Francois-Rene; LaCourse, D.; Lake, P.; Lam, Kristine; Lamb, Gavin P.; Lane, Dave; Lau, Marie Wingyee; Lewin, Pablo; Lintott, Chris; Lisse, Carey; Logie, Ludwig; Longeard, Nicolas; Lopez Villanueva, M.; Whit Ludington, E.; Mainzer, A.; Malo, Lison; Maloney, Chris; Mann, A.; Mantero, A.; Marengo, Massimo; Marchant, Jon; Martínez González, M. J.; Masiero, Joseph R.; Mauerhan, Jon C.; McCormac, James; McNeely, Aaron; Meng, Huan Y. A.; Miller, Mike; Molnar, Lawrence A.; Morales, J. C.; Morris, Brett M.; Muterspaugh, Matthew W.; Nespral, David; Nugent, C. R.; Nugent, Katherine M.; Odasso, A.; O'Keeffe, Derek; Oksanen, A.; O'Meara, John M.; Ordasi, András; Osborn, Hugh; Ott, John J.; Parks, J. R.; Rodriguez Perez, Diego; Petriew, Vance; Pickard, R.; Pál, András; Plavchan, P.; Pollacco, Don; Pozo Nuñez, F.; Pozuelos, F. J.; Rau, Steve; Redfield, Seth; Relles, Howard; Ribas, Ignasi; Richards, Jon; Saario, Joonas L. O.; Safron, Emily J.; Sallai, J. Martin; Sárneczky, Krisztián; Schaefer, Bradley E.; Schumer, Clea F.; Schwartzendruber, Madison; Siegel, Michael H.; Siemion, Andrew P. V.; Simmons, Brooke D.; Simon, Joshua D.; Simón-Díaz, S.; Sitko, Michael L.; Socas-Navarro, Hector; Sódor, Á.; Starkey, Donn; Steele, Iain A.; Stone, Geoff; Strassmeier, Klaus G.; Street, R. A.; Sullivan, Tricia; Suomela, J.; Swift, J. J.; Szabó, Gyula M.; Szabó, Róbert; Szakáts, Róbert; Szalai, Tamás; Tanner, Angelle M.; Toledo-Padrón, B.; Tordai, Tamás; Triaud, Amaury H. M. J.; Turner, Jake D.; Ulowetz, Joseph H.; Urbanik, Marian; Vanaverbeke, Siegfried; Vanderburg, Andrew; Vida, Krisztián; Vietje, Brad P.; Vinkó, József; von Braun, K.; Waagen, Elizabeth O.; Walsh, Dan; Watson, Christopher A.; Weir, R. C.; Wenzel, Klaus; Westendorp Plaza, C.; Williamson, Michael W.; Wright, Jason T.; Wyatt, M. C.; Zheng, WeiKang; Zsidi, Gabriella Bibcode: 2018ApJ...853L...8B Altcode: 2018arXiv180100732B We present a photometric detection of the first brightness dips of the unique variable star KIC 8462852 since the end of the Kepler space mission in 2013 May. Our regular photometric surveillance started in 2015 October, and a sequence of dipping began in 2017 May continuing on through the end of 2017, when the star was no longer visible from Earth. We distinguish four main 1%-2.5% dips, named “Elsie,” “Celeste,” “Skara Brae,” and “Angkor,” which persist on timescales from several days to weeks. Our main results so far are as follows: (i) there are no apparent changes of the stellar spectrum or polarization during the dips and (ii) the multiband photometry of the dips shows differential reddening favoring non-gray extinction. Therefore, our data are inconsistent with dip models that invoke optically thick material, but rather they are in-line with predictions for an occulter consisting primarily of ordinary dust, where much of the material must be optically thin with a size scale ≪1 μm, and may also be consistent with models invoking variations intrinsic to the stellar photosphere. Notably, our data do not place constraints on the color of the longer-term “secular” dimming, which may be caused by independent processes, or probe different regimes of a single process. Title: Inference of the chromospheric magnetic field orientation in the Ca II 8542 Å line fibrils Authors: Asensio Ramos, A.; de la Cruz Rodríguez, J.; Martínez González, M. J.; Socas-Navarro, H. Bibcode: 2017A&A...599A.133A Altcode: 2016arXiv161206088A Context. Solar chromospheric fibrils, as observed in the core of strong chromospheric spectral lines, extend from photospheric field concentrations suggesting that they trace magnetic field lines. These images have been historically used as proxies of magnetic fields for many purposes.
Aims: Use statistical analysis to test whether the association between fibrils and magnetic field lines is justified.
Methods: We use a Bayesian hierarchical model to analyze several tens of thousands of pixels in spectro-polarimetric chromospheric images of penumbrae and chromospheric fibrils. We compare the alignment between the field azimuth inferred from the linear polarization signals through the transverse Zeeman effect and the direction of the fibrils in the image.
Results: We conclude that, in the analyzed fields of view, fibrils are often well aligned with the magnetic field azimuth. Despite this alignment, the analysis also shows that there is a non-negligible dispersion. In penumbral filaments, we find a dispersion with a standard deviation of 16°, while this dispersion goes up to 34° in less magnetized regions. Title: How to infer the Sun's global magnetic field using the Hanle effect Authors: Vieu, T.; Martínez González, M. J.; Pastor Yabar, A.; Asensio Ramos, A. Bibcode: 2017MNRAS.465.4414V Altcode: We present a different approach to determine the characteristics of the global magnetic field of the Sun based on the study of the Hanle signals. The Hanle effect of a stellar dipole produces a surface asymmetric pattern of linear polarization that depends on the strength and geometry of this global field. Moreover, if the dipole is misaligned with respect to the rotation, the Hanle signals are modulated following the rotational period. We explore the possibility to retrieve those characteristics by comparing the computed theoretical signatures with actual observations. We show that this is possible, in the case of the Sr I line of the Sun, provided that the polarimetric sensitivity is of the order or below 10-5-10-6. The inference can be done either using the maps of resolved signals, in particular the spread of values obtained along different directions on the stellar disc, or using the disc-integrated signals. Title: Synthetic polarimetric spectra from stellar prominences Authors: Felipe, T.; Martínez González, M. J.; Asensio Ramos, A. Bibcode: 2017MNRAS.465.1654F Altcode: 2016arXiv161009282F Stellar prominences detected in rapidly rotating stars serve as probes of the magnetism in the corona of cool stars. We have synthesized the temporal evolution of the Stokes profiles generated in the He I 10 830 and 5876 Å triplets during the rotation of a prominence around a star. The synthesis was performed with the HAZEL code using a cloud model in which the prominence is characterized by a slab located at a fixed latitude and height. It accounts for the scattering polarization and Zeeman and Hanle effects. Several cases with different prominence magnetic field strengths and orientations have been analysed. The results show an emission feature that drifts across the profile while the prominence is out of the stellar disc. When the prominence eclipses the star, the intensity profile shows an absorption. The scattering induced by the prominence generates linear polarization signals in Stokes Q and U profiles, which are modified by the Hanle effect when a magnetic field is present. Due to the Zeeman effect, Stokes V profiles show a signal with very low amplitude when the magnetic field along the line of sight is different from zero. The estimated linear polarization signals could potentially be detected with the future spectropolarimeter Mid-resolution InfRAreD Astronomical Spectrograph, to be attached to Gran Telescopio Canarias telescope. Title: Probing deep photospheric layers of the quiet Sun with high magnetic sensitivity Authors: Lagg, A.; Solanki, S. K.; Doerr, H. -P.; Martínez González, M. J.; Riethmüller, T.; Collados Vera, M.; Schlichenmaier, R.; Orozco Suárez, D.; Franz, M.; Feller, A.; Kuckein, C.; Schmidt, W.; Asensio Ramos, A.; Pastor Yabar, A.; von der Lühe, O.; Denker, C.; Balthasar, H.; Volkmer, R.; Staude, J.; Hofmann, A.; Strassmeier, K.; Kneer, F.; Waldmann, T.; Borrero, J. M.; Sobotka, M.; Verma, M.; Louis, R. E.; Rezaei, R.; Soltau, D.; Berkefeld, T.; Sigwarth, M.; Schmidt, D.; Kiess, C.; Nicklas, H. Bibcode: 2016A&A...596A...6L Altcode: 2016arXiv160506324L Context. Investigations of the magnetism of the quiet Sun are hindered by extremely weak polarization signals in Fraunhofer spectral lines. Photon noise, straylight, and the systematically different sensitivity of the Zeeman effect to longitudinal and transversal magnetic fields result in controversial results in terms of the strength and angular distribution of the magnetic field vector.
Aims: The information content of Stokes measurements close to the diffraction limit of the 1.5 m GREGOR telescope is analyzed. We took the effects of spatial straylight and photon noise into account.
Methods: Highly sensitive full Stokes measurements of a quiet-Sun region at disk center in the deep photospheric Fe I lines in the 1.56 μm region were obtained with the infrared spectropolarimeter GRIS at the GREGOR telescope. Noise statistics and Stokes V asymmetries were analyzed and compared to a similar data set of the Hinode spectropolarimeter (SOT/SP). Simple diagnostics based directly on the shape and strength of the profiles were applied to the GRIS data. We made use of the magnetic line ratio technique, which was tested against realistic magneto-hydrodynamic simulations (MURaM).
Results: About 80% of the GRIS spectra of a very quiet solar region show polarimetric signals above a 3σ level. Area and amplitude asymmetries agree well with small-scale surface dynamo-magneto hydrodynamic simulations. The magnetic line ratio analysis reveals ubiquitous magnetic regions in the ten to hundred Gauss range with some concentrations of kilo-Gauss fields.
Conclusions: The GRIS spectropolarimetric data at a spatial resolution of ≈0.̋4 are so far unique in the combination of high spatial resolution scans and high magnetic field sensitivity. Nevertheless, the unavoidable effect of spatial straylight and the resulting dilution of the weak Stokes profiles means that inversion techniques still bear a high risk of misinterpretating the data. Title: Inference of magnetic fields in the very quiet Sun Authors: Martínez González, M. J.; Pastor Yabar, A.; Lagg, A.; Asensio Ramos, A.; Collados, M.; Solanki, S. K.; Balthasar, H.; Berkefeld, T.; Denker, C.; Doerr, H. P.; Feller, A.; Franz, M.; González Manrique, S. J.; Hofmann, A.; Kneer, F.; Kuckein, C.; Louis, R.; von der Lühe, O.; Nicklas, H.; Orozco, D.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Verma, M.; Waldman, T.; Volkmer, R. Bibcode: 2016A&A...596A...5M Altcode: 2018arXiv180410089M Context. Over the past 20 yr, the quietest areas of the solar surface have revealed a weak but extremely dynamic magnetism occurring at small scales (<500 km), which may provide an important contribution to the dynamics and energetics of the outer layers of the atmosphere. Understanding this magnetism requires the inference of physical quantities from high-sensitivity spectro-polarimetric data with high spatio-temporal resolution.
Aims: We present high-precision spectro-polarimetric data with high spatial resolution (0.4'') of the very quiet Sun at 1.56 μm obtained with the GREGOR telescope to shed some light on this complex magnetism.
Methods: We used inversion techniques in two main approaches. First, we assumed that the observed profiles can be reproduced with a constant magnetic field atmosphere embedded in a field-free medium. Second, we assumed that the resolution element has a substructure with either two constant magnetic atmospheres or a single magnetic atmosphere with gradients of the physical quantities along the optical depth, both coexisting with a global stray-light component.
Results: Half of our observed quiet-Sun region is better explained by magnetic substructure within the resolution element. However, we cannot distinguish whether this substructure comes from gradients of the physical parameters along the line of sight or from horizontal gradients (across the surface). In these pixels, a model with two magnetic components is preferred, and we find two distinct magnetic field populations. The population with the larger filling factor has very weak ( 150 G) horizontal fields similar to those obtained in previous works. We demonstrate that the field vector of this population is not constrained by the observations, given the spatial resolution and polarimetric accuracy of our data. The topology of the other component with the smaller filling factor is constrained by the observations for field strengths above 250 G: we infer hG fields with inclinations and azimuth values compatible with an isotropic distribution. The filling factors are typically below 30%. We also find that the flux of the two polarities is not balanced. From the other half of the observed quiet-Sun area 50% are two-lobed Stokes V profiles, meaning that 23% of the field of view can be adequately explained with a single constant magnetic field embedded in a non-magnetic atmosphere. The magnetic field vector and filling factor are reliable inferred in only 50% based on the regular profiles. Therefore, 12% of the field of view harbour hG fields with filling factors typically below 30%. At our present spatial resolution, 70% of the pixels apparently are non-magnetised. Title: Upholding the unified model for active galactic nuclei: VLT/FORS2 spectropolarimetry of Seyfert 2 galaxies Authors: Ramos Almeida, C.; Martínez González, M. J.; Asensio Ramos, A.; Acosta-Pulido, J. A.; Hönig, S. F.; Alonso-Herrero, A.; Tadhunter, C. N.; González-Martín, O. Bibcode: 2016MNRAS.461.1387R Altcode: 2016arXiv160602204R The origin of the unification model for active galactic nuclei (AGN) was the detection of broad hydrogen recombination lines in the optical polarized spectrum of the Seyfert 2 galaxy (Sy2) NGC 1068. Since then, a search for the hidden broad-line region (HBLR) of nearby Sy2s started, but polarized broad lines have only been detected in ∼30-40 per cent of the nearby Sy2s observed to date. Here we present new VLT/FORS2 optical spectropolarimetry of a sample of 15 Sy2s, including Compton-thin and Compton-thick sources. The sample includes six galaxies without previously published spectropolarimetry, some of them normally treated as non-hidden BLR (NHBLR) objects in the literature, four classified as NHBLR, and five as HBLR based on previous data. We report ≥4σ detections of a HBLR in 11 of these galaxies (73 per cent of the sample) and a tentative detection in NGC 5793, which is Compton-thick according to the analysis of X-ray data performed here. Our results confirm that at least some NHBLRs are misclassified, bringing previous publications reporting differences between HBLR and NHBLR objects into question. We detect broad Hα and Hβ components in polarized light for 10 targets, and just broad Hα for NGC 5793 and NGC 6300, with line widths ranging between 2100 and 9600 km s-1. High bolometric luminosities and low column densities are associated with higher polarization degrees, but not necessarily with the detection of the scattered broad components. Title: Upholding the unified model for AGN: VLT/FORS2 spectropolarimetry of Seyfert 2 galaxies Authors: Ramos Almeida, Cristina; Martínez González, M.; Asensio Ramos, A.; Acosta Pulido, J.; Hönig, S.; Alonso-Herrero, A.; Tadhunter, C.; González-Martín, O. Bibcode: 2016agnw.confE..52R Altcode: The origin of the unification model for AGN was the detection of broad hydrogen recombination lines in the optical polarized spectrum of the Seyfert 2 galaxy (Sy2) NGC 1068. Since then, a search for the hidden broad-line region (HBLR) of nearby Sy2s started, but polarized broad lines have only been detected in ~30-40% of the nearby Sy2s observed to date. Here we present new VLT/FORS2 optical spectropolarimetry of a sample of 15 Sy2s, including Compton-thin and Compton-thick sources. The sample includes six galaxies without previously published spectropolarimetry, some of them normally treated as non-hidden BLR (NHBLR) objects in the literature, and four Sy2s classified as NHBLR based on previous data. We report >4sigma detections of a HBLR in 11 of these galaxies (73% of the sample). Our results confirm that at least some NHBLRs were misclassified, bringing previous publications reporting differences between HBLR and NHBLR objects into question. We detect broad Ha and Hb components in polarized light for 9 targets, and just broad Ha for the other two. We do not find any correlation between the properties of the polarized spectra and the column densities measured from the X-rays or torus inclination, but a larger sample is required to confirm this. Title: On the Magnetism and Dynamics of Prominence Legs Hosting Tornadoes Authors: Martínez González, M. J.; Asensio Ramos, A.; Arregui, I.; Collados, M.; Beck, C.; de la Cruz Rodríguez, J. Bibcode: 2016ApJ...825..119M Altcode: 2016arXiv160501183M Solar tornadoes are dark vertical filamentary structures observed in the extreme ultraviolet associated with prominence legs and filament barbs. Their true nature and relationship to prominences requires an understanding of their magnetic structure and dynamic properties. Recently, a controversy has arisen: is the magnetic field organized forming vertical, helical structures or is it dominantly horizontal? And concerning their dynamics, are tornadoes really rotating or is it just a visual illusion? Here we analyze four consecutive spectro-polarimetric scans of a prominence hosting tornadoes on its legs, which helps us shed some light on their magnetic and dynamical properties. We show that the magnetic field is very smooth in all the prominence, which is probably an intrinsic property of the coronal field. The prominence legs have vertical helical fields that show slow temporal variation that is probably related to the motion of the fibrils. Concerning the dynamics, we argue that (1) if rotation exists, it is intermittent, lasting no more than one hour, and (2) the observed velocity pattern is also consistent with an oscillatory velocity pattern (waves). Title: Active Region Filaments Might Harbor Weak Magnetic Fields Authors: Díaz Baso, C. J.; Martínez González, M. J.; Asensio Ramos, A. Bibcode: 2016ApJ...822...50D Altcode: 2016arXiv160304645D Recent spectropolarimetric observations of active region filaments have revealed polarization profiles with signatures typical of the strong field Zeeman regime. The conspicuous absence in those observations of scattering polarization and Hanle effect signatures was then pointed out by some authors. This was interpreted as either a signature of mixed “turbulent” field components or as a result of optical thickness. In this article, we present a natural scenario to explain these Zeeman-only spectropolarimetric observations of active region (AR) filaments. We propose a two-component model, one on top of the other. Both components have horizontal fields, with the azimuth difference between them being close to 90°. The component that lies lower in the atmosphere is permeated by a strong field of the order of 600 G, while the upper component has much weaker fields, of the order of 10 G. The ensuing scattering polarization signatures of the individual components have opposite signs, so its combination along the line of sight reduces—and even can cancel out—the Hanle signatures, giving rise to an apparent Zeeman-only profile. This model is also applicable to other chromospheric structures seen in absorption above ARs. Title: Inversion of Stokes profiles with systematic effects Authors: Asensio Ramos, A.; de la Cruz Rodríguez, J.; Martínez González, M. J.; Pastor Yabar, A. Bibcode: 2016A&A...590A..87A Altcode: 2016arXiv160405470A Quantitative thermodynamical, dynamical and magnetic properties of the solar and stellar plasmas are obtained by interpreting their emergent non-polarized and polarized spectrum. This inference requires the selection of a set of spectral lines that are particularly sensitive to the physical conditions in the plasma and a suitable parametric model of the solar/stellar atmosphere. Nonlinear inversion codes are then used to fit the model to the observations. However, the presence of systematic effects, like nearby or blended spectral lines, telluric absorption, or incorrect correction of the continuum, among others, can strongly affect the results. We present an extension to current inversion codes that can deal with these effects in a transparent way. The resulting algorithm is very simple and can be applied to any existing inversion code with the addition of a few lines of code as an extra step in each iteration. Title: Where are the solar magnetic poles? Authors: Pastor Yabar, A.; Martinez Gonzalez, M. J.; Collados, M. Bibcode: 2015MNRAS.453L..69P Altcode: 2018arXiv180409748P Regardless of the physical origin of stellar magnetic fields - fossil or dynamo induced - an inclination angle between the magnetic and rotation axes is very often observed. Absence of observational evidence in this direction in the solar case has led to generally assume that its global magnetic field and rotation axes are well aligned. We present the detection of a monthly periodic signal of the photospheric solar magnetic field at all latitudes, and especially near the poles, revealing that the main axis of the Sun's magnetic field is not aligned with the surface rotation axis. This result reinforces the view of our Sun as a common intermediate-mass star. Furthermore, this detection challenges and imposes a strong observational constraint to modern solar dynamo theories. Title: Estimating the magnetic field strength from magnetograms Authors: Asensio Ramos, A.; Martínez González, M. J.; Manso Sainz, R. Bibcode: 2015A&A...577A.125A Altcode: 2015arXiv150307670A A properly calibrated longitudinal magnetograph is an instrument that measures circular polarization and gives an estimation of the magnetic flux density in each observed resolution element. This usually constitutes a lower bound of the field strength in the resolution element, given that it can be made arbitrarily large as long as it occupies a proportionally smaller area of the resolution element and/or becomes more transversal to the observer while still produce the same magnetic signal. However, we know that arbitrarily stronger fields are less likely - hG fields are more probable than kG fields, with fields above several kG virtually absent - and we may even have partial information about their angular distribution. Based on a set of sensible considerations, we derive simple formulae based on a Bayesian analysis to give an improved estimation of the magnetic field strength for magnetographs. Title: Spectro-Polarimetric Imaging Reveals Helical Magnetic Fields in Solar Prominence Feet Authors: Martínez González, M. J.; Manso Sainz, R.; Asensio Ramos, A.; Beck, C.; de la Cruz Rodríguez, J.; Díaz, A. J. Bibcode: 2015ApJ...802....3M Altcode: 2015arXiv150103295M Solar prominences are clouds of cool plasma levitating above the solar surface and insulated from the million-degree corona by magnetic fields. They form in regions of complex magnetic topology, characterized by non-potential fields, which can evolve abruptly, disintegrating the prominence and ejecting magnetized material into the heliosphere. However, their physics is not yet fully understood because mapping such complex magnetic configurations and their evolution is extremely challenging, and must often be guessed by proxy from photometric observations. Using state-of-the-art spectro-polarimetric data, we reconstruct the structure of the magnetic field in a prominence. We find that prominence feet harbor helical magnetic fields connecting the prominence to the solar surface below. Title: Constraining the shaping mechanism of the Red Rectangle through the spectro-polarimetry of its central star Authors: Martínez González, M. J.; Asensio Ramos, A.; Manso Sainz, R.; Corradi, R. L. M.; Leone, F. Bibcode: 2015A&A...574A..16M Altcode: 2014arXiv1410.6365M We carried out high-sensitivity spectro-polarimetric observations of the central star of the Red Rectangle protoplanetary nebula with the aim of constraining the mechanism that gives its biconical shape. The stellar light of the central binary system is linearly polarised since it is scattered on the dust particles of the nebula. Surprisingly, the linear polarisation in the continuum is aligned with one of the spikes of the biconical outflow. Also, the observed Balmer lines, as well as the Ca ii K lines, are polarised. These observational constraints are used to confirm or reject current theoretical models for the shaping mechanism of the Red Rectangle. We propose that the observed polarisation is not very likely to be generated by a uniform biconical stellar wind. Also, the hypothesis of a precessing jet does not completely match observations since it requires a larger aperture jet than for the nebula. Title: Hierarchical analysis of the quiet-Sun magnetism Authors: Asensio Ramos, A.; Martínez González, M. J. Bibcode: 2014A&A...572A..98A Altcode: 2014arXiv1410.5953A Standard statistical analysis of the magnetic properties of the quiet Sun rely on simple histograms of quantities inferred from maximum-likelihood estimations. Because of the inherent degeneracies, either intrinsic or induced by the noise, this approach is not optimal and can lead to highly biased results. We carried out a meta-analysis of the magnetism of the quiet Sun from Hinode observations using a hierarchical probabilistic method. This method allowed us to infer the statistical properties of the magnetic field vector over the observed field-of-view, consistently taking into account the uncertainties in each pixel that are due to noise and degeneracies. Our results imply that the magnetic fields are very weak, below 275 G with 95% probability, with a slight preference for horizontal fields, although the distribution is not far from a quasi-isotropic distribution. Title: Upper Limits to the Magnetic Field in Central Stars of Planetary Nebulae Authors: Asensio Ramos, A.; Martínez González, M. J.; Manso Sainz, R.; Corradi, R. L. M.; Leone, F. Bibcode: 2014ApJ...787..111A Altcode: 2014arXiv1404.2718A More than about 20 central stars of planetary nebulae (CSPNs) have been observed spectropolarimetrically, yet no clear, unambiguous signal of the presence of a magnetic field in these objects has been found. We perform a statistical (Bayesian) analysis of all the available spectropolarimetric observations of CSPN to constrain the magnetic fields in these objects. Assuming that the stellar field is dipolar and that the dipole axis of the objects is oriented randomly (isotropically), we find that the dipole magnetic field strength is smaller than 400 G with 95% probability using all available observations. The analysis introduced allows integration of future observations to further constrain the parameters of the distribution, and it is general, so that it can be easily applied to other classes of magnetic objects. We propose several ways to improve the upper limits found here. Title: A search for magnetic fields on central stars in planetary nebulae Authors: Leone, F.; Corradi, R. L. M.; Martínez González, M. J.; Asensio Ramos, A.; Manso Sainz, R. Bibcode: 2014A&A...563A..43L Altcode: 2014arXiv1401.6282L Context. One of the possible mechanisms responsible for the panoply of shapes in planetary nebulae is the presence of magnetic fields that drive the ejection of ionized material during the proto-planetary nebula phase.
Aims: Therefore, detecting magnetic fields in such objects is of key importance for understanding their dynamics. Still, magnetic fields have not been detected using polarimetry in the central stars of planetary nebulae.
Methods: Circularly polarized light spectra have been obtained with the Focal Reducer and Low Dispersion Spectrograph at the Very Large Telescope of the European Southern Observatory and the Intermediate dispersion Spectrograph and Imaging System at the William Herschel Telescope. Twentythree planetary nebulae that span very different morphology and evolutionary stages have been selected. Most of central stars have been observed at different rotation phases to point out evidence of magnetic variability.
Results: In this paper, we present the result of two observational campaigns aimed to detect and measure the magnetic field in the central stars of planetary nebulae on the basis of low resolution spectropolarimetry. In the limit of the adopted method, we can state that large scale fields of kG order are not hosted on the central star of planetary nebulae. Title: Granular-sized magnetic flux emergence Authors: Martínez González, María Jesús Bibcode: 2014cosp...40E2017M Altcode: It is believed that solar magnetic fields are created in the tachocline, the interface between the convection zone and the radiative interior. Due to buoyancy instabilities, they move upward and emerge into the solar atmosphere in the form of Ω-shaped flux tubes. The largest emerging active regions produce sunspots or pores, lasting from days or weeks to moths, and the ephemeral regions emerge to feed the quiet Sun at supergranular scales (the network), lasting from hours to days. A surprising finding in solar physics in the last few years was the discovery that, despite their stochastic character, the quietest areas of the quiet Sun are formed, at granular scales, by small dipoles that live for several minutes. Are these dipoles just the end tail of the same emergence phenomenon than active regions? or are they the result of a surface phenomenon? In this talk I review all the present knowledge of the smallest scale emergence phenomenon observed in the Sun and I put in in context on the solar global magnetism. Title: Polarisation and magnetic fields in cool stars and the Sun. A session in memory of Meir Semel Authors: Martínez González, M. J.; Marsden, S. C. Bibcode: 2013AN....334..164M Altcode: 2013csss...17..164M Astrophysical plasmas and magnetic fields are inextricably coupled to each other. But magnetic fields are elusive; they cannot be fully characterized with standard (spectroscopic) observational techniques. Fortunately, polarisation, the third fundamental property of light, encodes all the quantitative information on the magnetism of the objects we observe. This splinter session brought together the solar and stellar spectropolarimetric communities to discuss some of the fundamental unsolved questions of solar and stellar magnetism and the future of spectropolarimetric observations. Title: Hanle Effect for Stellar Dipoles and Quadrupoles Authors: Manso Sainz, R.; Martínez González, M. J. Bibcode: 2012ApJ...760....7M Altcode: 2012arXiv1209.6187M We derive exact expressions for the degree of lineal polarization over a resolved or integrated stellar disc due to resonance scattering and the Hanle effect from a dipolar or quadrupolar distribution of magnetic fields. We apply the theory of scattering polarization within the formalism of the spherical tensors representation for the density matrix and radiation field. The distribution of linear polarization over the stellar disk for different configurations of the magnetic field is studied and its topology discussed. For an unresolved dipole, the resulting polarization can be expressed in terms of just three functions (of the inclination angle and effective dipole strength) that are calculated numerically and their behavior discussed. Dipolar and (aligned) quadrupoles are considered in some detail, but the techniques here—in particular, the extensive use of the spherical tensor formalism for polarization—can easily be applied to more general field configurations. Title: Anomalous Circular Polarization Profiles in the He I 1083.0 nm Multiplet from Solar Spicules Authors: Martínez González, M. J.; Asensio Ramos, A.; Manso Sainz, R.; Beck, C.; Belluzzi, L. Bibcode: 2012ApJ...759...16M Altcode: 2012arXiv1209.2589M We report Stokes vector observations of solar spicules and a prominence in the He I 1083 nm multiplet carried out with the Tenerife Infrared Polarimeter. The observations show linear polarization profiles that are produced by scattering processes in the presence of a magnetic field. After a careful data reduction, we demonstrate the existence of extremely asymmetric Stokes V profiles in the spicular material that we are able to model with two magnetic components along the line of sight, and under the presence of atomic orientation in the energy levels that give rise to the multiplet. We discuss some possible scenarios that can generate the atomic orientation in spicules. We stress the importance of spectropolarimetric observations across the limb to distinguish such signals from observational artifacts. Title: Resolving the Internal Magnetic Structure of the Solar Network Authors: Martínez González, M. J.; Bellot Rubio, L. R.; Solanki, S. K.; Martínez Pillet, V.; Del Toro Iniesta, J. C.; Barthol, P.; Schmidt, W. Bibcode: 2012ApJ...758L..40M Altcode: 2012arXiv1209.2584M We analyze the spectral asymmetry of Stokes V (circularly polarized) profiles of an individual network patch in the quiet Sun observed by Sunrise/IMaX. At a spatial resolution of 0farcs15-0farcs18, the network elements contain substructure which is revealed by the spatial distribution of Stokes V asymmetries. The area asymmetry between the red and blue lobes of Stokes V increases from nearly zero at the core of the structure to values close to unity at its edges (single-lobed profiles). Such a distribution of the area asymmetry is consistent with magnetic fields expanding with height, i.e., an expanding magnetic canopy (which is required to fulfill pressure balance and flux conservation in the solar atmosphere). Inversion of the Stokes I and V profiles of the patch confirms this picture, revealing a decreasing field strength and increasing height of the canopy base from the core to the periphery of the network patch. However, the non-roundish shape of the structure and the presence of negative area and amplitude asymmetries reveal that the scenario is more complex than a canonical flux tube expanding with height surrounded by downflows. Title: Analytical Calculation of Stokes Profiles of Rotating Stellar Magnetic Dipole Authors: Martínez González, M. J.; Asensio Ramos, A. Bibcode: 2012ApJ...755...96M Altcode: 2012arXiv1206.2502M The observation of the polarization emerging from a rotating star at different phases opens up the possibility to map the magnetic field in the stellar surface thanks to the well-known Zeeman-Doppler imaging. When the magnetic field is sufficiently weak, the circular and linear polarization profiles locally in each point of the star are proportional to the first and second derivatives of the unperturbed intensity profile, respectively. We show that the weak-field approximation (for weak lines in the case of linear polarization) can be generalized to the case of a rotating star including the Doppler effect and taking into account the integration on the stellar surface. The Stokes profiles are written as a linear combination of wavelength-dependent terms expressed as series expansions in terms of Hermite polynomials. These terms contain the surface-integrated magnetic field and velocity components. The direct numerical evaluation of these quantities is limited to rotation velocities not larger than eight times the Doppler width of the local absorption profiles. Additionally, we demonstrate that in a rotating star, the circular polarization flux depends on the derivative of the intensity flux with respect to the wavelength and also on the profile itself. Likewise, the linear polarization depends on the profile and on its first and second derivatives with respect to the wavelength. We particularize the general expressions to a rotating dipole. Title: Dead Calm Areas in the Very Quiet Sun Authors: Martínez González, M. J.; Manso Sainz, R.; Asensio Ramos, A.; Hijano, E. Bibcode: 2012ApJ...755..175M Altcode: 2012arXiv1206.4545M We analyze two regions of the quiet Sun (35.6 × 35.6 Mm2) observed at high spatial resolution (lsim100 km) in polarized light by the IMaX spectropolarimeter on board the SUNRISE balloon. We identify 497 small-scale (~400 km) magnetic loops, appearing at an effective rate of 0.25 loop h -1 arcsec-2 further, we argue that this number and rate are underestimated by ~30%. However, we find that these small dipoles do not appear uniformly on the solar surface: their spatial distribution is rather filamentary and clumpy, creating dead calm areas, characterized by a very low magnetic signal and a lack of organized loop-like structures at the detection level of our instruments, which cannot be explained as just statistical fluctuations of a Poisson spatial process. We argue that this is an intrinsic characteristic of the mechanism that generates the magnetic fields in the very quiet Sun. The spatio-temporal coherences and the clumpy structure of the phenomenon suggest a recurrent, intermittent mechanism for the generation of magnetic fields in the quietest areas of the Sun. Title: Model Selection for Spectropolarimetric Inversions Authors: Asensio Ramos, A.; Manso Sainz, R.; Martínez González, M. J.; Viticchié, B.; Orozco Suárez, D.; Socas-Navarro, H. Bibcode: 2012ApJ...748...83A Altcode: 2012arXiv1201.5063A Inferring magnetic and thermodynamic information from spectropolarimetric observations relies on the assumption of a parameterized model atmosphere whose parameters are tuned by comparison with observations. Often, the choice of the underlying atmospheric model is based on subjective reasons. In other cases, complex models are chosen based on objective reasons (for instance, the necessity to explain asymmetries in the Stokes profiles) but it is not clear what degree of complexity is needed. The lack of an objective way of comparing models has, sometimes, led to opposing views of the solar magnetism because the inferred physical scenarios are essentially different. We present the first quantitative model comparison based on the computation of the Bayesian evidence ratios for spectropolarimetric observations. Our results show that there is not a single model appropriate for all profiles simultaneously. Data with moderate signal-to-noise ratios (S/Ns) favor models without gradients along the line of sight. If the observations show clear circular and linear polarization signals above the noise level, models with gradients along the line are preferred. As a general rule, observations with large S/Ns favor more complex models. We demonstrate that the evidence ratios correlate well with simple proxies. Therefore, we propose to calculate these proxies when carrying out standard least-squares inversions to allow for model comparison in the future. Title: Influence of phase-diversity image reconstruction techniques on circular polarization asymmetries Authors: Asensio Ramos, A.; Martínez González, M. J.; Khomenko, E.; Martínez Pillet, V. Bibcode: 2012A&A...539A..42A Altcode: 2011arXiv1111.2496A Context. Full Stokes filter-polarimeters are key instruments for investigating the rapid evolution of magnetic structures on the solar surface. To this end, the image quality is routinely improved using a-posteriori image reconstruction methods.
Aims: We analyze the robustness of circular polarization asymmetries to phase-diversity image reconstruction techniques.
Methods: We used snapshots of magneto-hydrodynamical simulations carried out with different initial conditions to synthesize spectra of the magnetically sensitive Fe i line at 5250.2 Å. We degraded the synthetic profiles spatially and spectrally to simulate observations with the IMaX full Stokes filter-polarimeter. We also simulated the focused/defocused pairs of images used by the phase-diversity algorithm for reconstruction and the polarimetric modulation scheme. We assume that standard optimization methods are able to infer the projection of the wavefront on the Zernike polynomials with 10% precision. We also consider the less favorable case of 25% precision. We obtain reconstructed monochromatic modulated images that are later demodulated and compared with the original maps.
Results: Although asymmetries are often difficult to define in the quiet Sun due to the complexity of the Stokes V profiles, we show how asymmetries are degraded with spatial and spectral smearing. The results indicate that, although image reconstruction techniques reduce the spatial smearing, they can modify the asymmetries of the profiles, which are mainly caused by the appearance of spatially-correlated noise. Title: Analytical maximum likelihood estimation of stellar magnetic fields Authors: Martínez González, M. J.; Manso Sainz, R.; Asensio Ramos, A.; Belluzzi, L. Bibcode: 2012MNRAS.419..153M Altcode: 2011MNRAS.tmp.1583M; 2011arXiv1108.4366M The polarized spectrum of stellar radiation encodes valuable information on the conditions of stellar atmospheres and the magnetic fields that permeate them. In this paper, we give explicit expressions to estimate the magnetic field vector and its associated error from the observed Stokes parameters. We study the solar case where specific intensities are observed and then the stellar case, where we receive the polarized flux. In the second case, we concentrate on the explicit expression for the case of a slow rotator with a dipolar magnetic field geometry. Moreover, we also give explicit formulae to retrieve the magnetic field vector from the least-squares deconvolution (LSD) profiles without assuming mean values for the LSD artificial spectral line. The formulae have been obtained assuming that the spectral lines can be described in the weak-field regime and using a maximum likelihood approach. The errors are recovered by means of the Hermitian matrix. The bias of the estimators is analysed in depth. Title: Advection and dispersal of small magnetic elements in the very quiet Sun Authors: Manso Sainz, R.; Martínez González, M. J.; Asensio Ramos, A. Bibcode: 2011A&A...531L...9M Altcode: We track small magnetic structures on very quiet regions (internetwork) of the Sun. We follow the footpoints of small-scale magnetic loops that appear on the photosphere at granular scales using spectropolarimetric and magnetographic data obtained with Hinode. We find two different regimes for their wanderings. Within granules (where they appear), they seem to be passively advected by the plasma - which is justified by their relatively low magnetic flux (~1016 Mx), and magnetic field strength (~200 G). The plasma flow thus traced is roughly laminar with a characteristic mean velocity of 2 km s-1 and very low vorticity. Once the magnetic markers reach intergranular lanes, they remain there and are buffeted by the random flows of neighbouring granules and turbulent intergranules, follow random walks, and disperse across the solar surface with a diffusion constant of 195 km2 s-1. While on their intergranular random walking, they may fall close to whirlpools (on scales ≲400 km) associated with convective downdrafts, similar to the events recently reported in mesogranular and supergranular cell boundaries tracking magnetic bright points, which provides additional evidence that these events are ubiquitous on the solar surface. Title: Non-detection of Magnetic Fields in the Central Stars of the Planetary Nebulae NGC 1360 and LSS 1362 Authors: Leone, Francesco; Martínez González, María J.; Corradi, Romano L. M.; Privitera, Giovanni; Manso Sainz, Rafael Bibcode: 2011ApJ...731L..33L Altcode: 2011arXiv1104.1042L The presence of magnetic fields is an attractive hypothesis for shaping planetary nebulae (PNe). We report on observations of the central star of the two PNe NGC 1360 and LSS 1326. We performed spectroscopy on circularly polarized light with the Focal Reducer and Low Dispersion Spectrograph at the Very Large Telescope of the European Southern Observatory. Contrary to previous reports, we find that the effective magnetic field, which is the average over the visible stellar disk of longitudinal components of the magnetic fields, is null within errors for both stars. We conclude that direct evidence of magnetic fields on the central stars of PNe is still missing—either the magnetic field is much weaker (<600 G) than previously reported, or more complex (thus leading to cancellations), or both. Certainly, indirect evidence (e.g., MASER emission) fully justify further efforts to point out the strength and morphology of such magnetic fields. Title: The Magnetic Fields of the Quiet Sun Authors: Sánchez Almeida, J.; Martínez González, M. Bibcode: 2011ASPC..437..451S Altcode: 2011arXiv1105.0387S This work reviews our understanding of the magnetic fields observed in the quiet Sun. The subject has undergone a major change during the last decade (quiet revolution), and it will remain changing since the techniques of diagnostic employed so far are known to be severely biased. Keeping these caveats in mind, our work covers the main observational properties of the quiet Sun magnetic fields: magnetic field strengths, unsigned magnetic flux densities, magnetic field inclinations, as well as the temporal evolution on short time-scales (loop emergence), and long time-scales (solar cycle). We also summarize the main theoretical ideas put forward to explain the origin of the quiet Sun magnetism. A final prospective section points out various areas of solar physics where the quiet Sun magnetism may have an important physical role to play (chromospheric and coronal structure, solar wind acceleration, and solar elemental abundances). Title: Unnoticed Magnetic Field Oscillations in the Very Quiet Sun Revealed by SUNRISE/IMaX Authors: Martínez González, M. J.; Asensio Ramos, A.; Manso Sainz, R.; Khomenko, E.; Martínez Pillet, V.; Solanki, S. K.; López Ariste, A.; Schmidt, W.; Barthol, P.; Gandorfer, A. Bibcode: 2011ApJ...730L..37M Altcode: 2011arXiv1103.0145M We present observational evidence for oscillations of magnetic flux density in the quiet areas of the Sun. The majority of magnetic fields on the solar surface have strengths of the order of or lower than the equipartition field (300-500 G). This results in a myriad of magnetic fields whose evolution is largely determined by the turbulent plasma motions. When granules evolve they squash the magnetic field lines together or pull them apart. Here, we report on the periodic deformation of the shapes of features in circular polarization observed at high resolution with SUNRISE. In particular, we note that the area of patches with a constant magnetic flux oscillates with time, which implies that the apparent magnetic field intensity oscillates in antiphase. The periods associated with this oscillatory pattern are compatible with the granular lifetime and change abruptly, which suggests that these oscillations might not correspond to characteristic oscillatory modes of magnetic structures, but to the forcing by granular motions. In one particular case, we find three patches around the same granule oscillating in phase, which means that the spatial coherence of these oscillations can reach 1600 km. Interestingly, the same kind of oscillatory phenomenon is also found in the upper photosphere. Title: Small Magnetic Loops Connecting the Quiet Surface and the Hot Outer Atmosphere of the Sun Authors: Martínez González, M. J.; Manso Sainz, R.; Asensio Ramos, A.; Bellot Rubio, L. R. Bibcode: 2010ApJ...714L..94M Altcode: 2010arXiv1003.1255M Sunspots are the most spectacular manifestation of solar magnetism, yet 99% of the solar surface remains "quiet" at any time of the solar cycle. The quiet sun is not void of magnetic fields, though; they are organized at smaller spatial scales and evolve relatively fast, which makes them difficult to detect. Thus, although extensive quiet Sun magnetism would be a natural driver to a uniform, steady heating of the outer solar atmosphere, it is not clear what the physical processes involved would be, due to lack of observational evidence. We report on the topology and dynamics of the magnetic field in very quiet regions of the Sun from spectropolarimetric observations of the Hinode satellite, showing a continuous injection of magnetic flux with a well-organized topology of Ω-loop from below the solar surface into the upper layers. At first stages, when the loop travels across the photosphere, it has a flattened (staple-like) geometry and a mean velocity ascent of ~3 km s-1. When the loop crosses the minimum temperature region, the magnetic fields at the footpoints become almost vertical and the loop topology resembles a potential field. The mean ascent velocity at chromospheric height is ~12 km s-1. The energy input rate of these small-scale loops in the lower boundary of the chromosphere is (at least) of 1.4 × 106-2.2 × 107 erg cm-2 s-1. Our findings provide empirical evidence for solar magnetism as a multi-scale system, in which small-scale low-flux magnetism plays a crucial role, at least as important as active regions, coupling different layers of the solar atmosphere and being an important ingredient for chromospheric and coronal heating models. Title: The stochastic, intermittent nature of quiet Sun magnetism Authors: Martinez González, M. J.; Manso Sainz, R.; López Ariste, A.; Bellot Rubio, L. R.; Bianda, M. Bibcode: 2010iac..talk....1M Altcode: 2010iac..talk..153M No abstract at ADS Title: Statistical Analysis of the very Quiet Sun Magnetism Authors: Martínez González, M. J.; Manso Sainz, R.; Asensio Ramos, A.; López Ariste, A.; Bianda, M. Bibcode: 2010ApJ...711L..57M Altcode: 2010arXiv1001.4551M The behavior of the observed polarization amplitudes with spatial resolution is a strong constraint on the nature and organization of solar magnetic fields below the resolution limit. We study the polarization of the very quiet Sun at different spatial resolutions using ground- and space-based observations. It is shown that 80% of the observed polarization signals do not change with spatial resolution, suggesting that, observationally, the very quiet Sun magnetism remains the same despite the high spatial resolution of space-based observations. Our analysis also reveals a cascade of spatial scales for the magnetic field within the resolution element. It is manifest that the Zeeman effect is sensitive to the microturbulent field usually associated with Hanle diagnostics. This demonstrates that Zeeman and Hanle studies show complementary perspectives of the same magnetism. Title: Spectropolarimetric multi line analysis of stellar magnetic fields Authors: Ramírez Vélez, J. C.; Semel, M.; Stift, M.; Martínez González, M. J.; Petit, P.; Dunstone, N. Bibcode: 2010A&A...512A...6R Altcode: 2010arXiv1001.1003R
Aims: In this paper we study the feasibility of inferring the magnetic field from polarized multi line spectra using two methods: The pseudo line approach and The PCA-ZDI approach.
Methods: We use multi line techniques, meaning that all the lines of a stellar spectrum contribute to obtain a polarization signature. The use of multiple lines dramatically increases the signal-to-noise-ratio of these polarizations signatures. Using one technique, the pseudo line approach, we construct the pseudo line as the mean profile of all the individual lines. The other technique, the PCA-ZDI approach proposed recently by Semel et al. (2006, ASPC, 358, 355) for the detection of polarized signals, combines principle components analysis (PCA) and the Zeeman Doppler imaging technique (ZDI). This new method has a main advantage: the polarized signature is extracted using cross correlations between the stellar spectra and functions containing the polarization properties of each line. is extracted using cross correlations between the stellar spectra and functions containing the polarization properties of each line." These functions are the principal components of a database of synthetic spectra. The synthesis of the spectra of the database are obtained using the radiative transfer equations in LTE. The profiles built with the PCA-ZDI technique are called multi Zeeman signatures.
Results: The construction of the pseudo line as well as the multi Zeeman signatures is a powerful tool in the study of stellar and solar magnetic fields. The information of the physical parameters that governs the line formation is contained in the final polarized profiles. We have shown in particular using inversion codes that the magnetic field vector can be properly inferred with both approaches despite the magnetic field regime. Title: The quiet Sun magnetic field observed with ZIMPOL on THEMIS. I. The probability density function Authors: Bommier, V.; Martínez González, M.; Bianda, M.; Frisch, H.; Asensio Ramos, A.; Gelly, B.; Landi Degl'Innocenti, E. Bibcode: 2009A&A...506.1415B Altcode: Context: The quiet Sun magnetic field probability density function (PDF) remains poorly known. Modeling this field also introduces a magnetic filling factor that is also poorly known. With these two quantities, PDF and filling factor, the statistical description of the quiet Sun magnetic field is complex and needs to be clarified.
Aims: In the present paper, we propose a procedure that combines direct determinations and inversion results to derive the magnetic field vector and filling factor, and their PDFs.
Methods: We used spectro-polarimetric observations taken with the ZIMPOL polarimeter mounted on the THEMIS telescope. The target was a quiet region at disk center. We analyzed the data by means of the UNNOFIT inversion code, with which we inferred the distribution of the mean magnetic field α B, α being the magnetic filling factor. The distribution of α was derived by an independent method, directly from the spectro-polarimetric data. The magnetic field PDF p(B) could then be inferred. By introducing a joint PDF for the filling factor and the magnetic field strength, we have clarified the definition of the PDF of the quiet Sun magnetic field when the latter is assumed not to be volume-filling.
Results: The most frequent local average magnetic field strength is found to be 13 G. We find that the magnetic filling factor is related to the magnetic field strength by the simple law α = B_1/B with B1 = 15 G. This result is compatible with the Hanle weak-field determinations, as well as with the stronger field determinations from the Zeeman effect (kGauss field filling 1-2% of space). From linear fits, we obtain the analytical dependence of the magnetic field PDF. Our analysis has also revealed that the magnetic field in the quiet Sun is isotropically distributed in direction.
Conclusions: We conclude that the quiet Sun is a complex medium where magnetic fields having different field strengths and filling factors coexist. Further observations with a better polarimetric accuracy are, however, needed to confirm the results obtained in the present work.

Based on observations made with the French-Italian telescope THEMIS operated by the CNRS and CNR on the island of Tenerife in the Spanish Observatorio del Teide of the Instituto de Astrofísica de Canarias. Present address: Instituto de Astrofísica de Canarias, vía Láctea s/n, 38205 La Laguna, Tenerife, Spain. Title: Multiline Zeeman signatures through line addition Authors: Semel, M.; Ramírez Vélez, J. C.; Martínez González, M. J.; Asensio Ramos, A.; Stift, M. J.; López Ariste, A.; Leone, F. Bibcode: 2009A&A...504.1003S Altcode: 2009arXiv0906.4226S Context: To obtain a significant Zeeman signature in the polarised spectra of a magnetic star, we usually “add” the contributions of numerous spectral lines; the ultimate goal is to recover the spectropolarimetric prints of the magnetic field in these line additions.
Aims: Here we want to clarify the meaning of these techniques of line addition; in particular, we try to interpret the meaning of the “pseudo-line” formed during this process and to find out why and how its Zeeman signature is still meaningful.
Methods: We create a synthetic case of line addition and apply well tested standard solar methods routinely used in research on magnetism in the Sun.
Results: The results are convincing and the Zeeman signatures well detected; Solar methods are found to be quite efficient for stellar observations. We statistically compare line addition with least-squares deconvolution and demonstrate that they both give very similar results, as a consequence of the special statistical properties of the weights.
Conclusions: The Zeeman signatures are unequivocally detected in this multiline approach. We suggest that magnetic field detection is reliable well beyond the weak-field approximation. Linear polarisation in the spectra of solar type stars can be detected when the spectral resolution is sufficiently high. Title: Emergence of Small-scale Magnetic Loops Through the Quiet Solar Atmosphere Authors: Martínez González, M. J.; Bellot Rubio, L. R. Bibcode: 2009ApJ...700.1391M Altcode: 2009arXiv0905.2691M We investigate the emergence of magnetic flux in the quiet Sun at very small spatial scales, focusing on the magnetic connection between the photosphere and chromosphere. The observational data consist of spectropolarimetric measurements and filtergrams taken with the Hinode satellite and the Dutch Open Telescope. We find that a significant fraction of the magnetic flux present in internetwork regions appears in the form of Ω-shaped loops. The emergence rate is 0.02 loops per hour and arcsec-2, which brings 1.1 × 1012 Mx s-1 arcsec-2 of new flux to the solar surface. Initially, the loops are observed as small patches of linear polarization above a granular cell. Shortly afterward, two footpoints of opposite polarity become visible in circular polarization within or at the edges of the granule and start moving toward the adjacent intergranular space. The orientation of the footpoints does not seem to obey Hale's polarity rules. The loops are continuously buffeted by convective motions, but they always retain a high degree of coherence. Interestingly, 23% of the loops that emerge in the photosphere reach the chromosphere (16 cases out of 69). They are first detected in Fe I 630 nm magnetograms and 5 minutes later in Mg I b 517.3 nm magnetograms. After about 8 minutes, some of them are also observed in Ca II H line-core images, where the footpoints produce small brightness enhancements. Title: Centre to Limb Variation of the Internetwork Magnetism Authors: Martínez González, M. J.; Asensio Ramos, A.; López Ariste, A. Bibcode: 2009ASPC..405..219M Altcode: We analyse 1.5 μm spectropolarimetric observations of quiet regions at different positions on the solar surface. Neither the circular and linear polarization signals nor the area and amplitude asymmetries present a clear trend with the position on the Sun. This fact discards a network-like scenario for the internetwork magnetic fields. An isotropically distributed magnetic field would reproduce the observations, whatever its organisation (myriads of low-lying loops, turbulent field, etc.). Title: Bayesian Inversion of Stokes Profiles Authors: Asensio Ramos, A.; Martínez González, M. J.; Rubiño-Martín, J. A. Bibcode: 2009ASPC..405..315A Altcode: Inversion techniques are the most powerful methods to obtain information about the thermodynamical and magnetic properties of solar and stellar atmospheres. In the last years, we have witnessed the development of highly sophisticated inversion codes that are now widely applied to spectro-polarimetric observations. However, no realistic confidence intervals are obtained for the inferred parameters. Additionally, it is fundamental to apply efficient techniques to assess the ability of models to reproduce the observations and to know to what extent the models have to be refined or can be simplified. We propose the application of Bayesian techniques. The posterior distribution, that takes into account both the a-priori knowledge and the information given by the data, is efficiently sampled using a Markov Chain Monte Carlo method. We demonstrate the capabilities of the code with the aid of an interesting example. 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: Multiline Zeeman signatures as demonstrated through the Pseudo-line Authors: Semel, M.; Ramirez Velez, J. C.; Stift, M. J.; Martinez Gonzalez, M. J.; Lopez Ariste, A.; Leone, F. Bibcode: 2008arXiv0810.3543S Altcode: In order to get a significant Zeeman signature in the polarised spectra of a magnetic star, we usually 'add' the contributions of numerous spectral lines; the ultimate goal is to recover the spectropolarimetric prints of the magnetic field in these line additions. Here we want to clarify the meaning of these techniques of line addition; in particular, we try to interpret the meaning of the 'pseudo-line' formed during this process and to find out why and how its Zeeman signature is still meaningful. We create a synthetic case of lines addition and apply well tested standard solar methods routinely used in the research on magnetism in our nearest star. The results are convincing and the Zeeman signatures well detected; Solar methods are found to be quite efficient also for stellar observations. The Zeeman signatures are unequivocally detected in this multiline approach. We may anticipate the outcome magnetic fields to be reliable well beyond the weak-field approximation. Linear polarisation in the spectra of solar type stars can be detected when the spectral resolution is sufficiently high. Title: PCA detection and denoising of Zeeman signatures in polarised stellar spectra Authors: Martínez González, M. J.; Asensio Ramos, A.; Carroll, T. A.; Kopf, M.; Ramírez Vélez, J. C.; Semel, M. Bibcode: 2008A&A...486..637M Altcode: 2008arXiv0804.4692M Aims: Our main objective is to develop a denoising strategy to increase the signal to noise ratio of individual spectral lines of stellar spectropolarimetric observations.
Methods: We use a multivariate statistics technique called Principal Component Analysis. The cross-product matrix of the observations is diagonalized to obtain the eigenvectors in which the original observations can be developed. This basis is such that the first eigenvectors contain the greatest variance. Assuming that the noise is uncorrelated a denoising is possible by reconstructing the data with a truncated basis. We propose a method to identify the number of eigenvectors for an efficient noise filtering.
Results: Numerical simulations are used to demonstrate that an important increase of the signal to noise ratio per spectral line is possible using PCA denoising techniques. It can be also applied for detection of magnetic fields in stellar atmospheres. We analyze the relation between PCA and commonly used techniques like line addition and least-squares deconvolution. Moreover, PCA is very robust and easy to compute. Title: Multiline Spectropolarimetry of the Quiet Sun at 5250 and 6302 Å Authors: Socas-Navarro, H.; Borrero, J. M.; Asensio Ramos, A.; Collados, M.; Domínguez Cerdeña, I.; Khomenko, E. V.; Martínez González, M. J.; Martínez Pillet, V.; Ruiz Cobo, B.; Sánchez Almeida, J. Bibcode: 2008ApJ...674..596S Altcode: The reliability of quiet-Sun magnetic field diagnostics based on the Fe I lines at 6302 Å has been questioned by recent work. Here we present the results of a thorough study of high-resolution multiline observations taken with the new spectropolarimeter SPINOR, comprising the 5250 and 6302 Å spectral domains. The observations were analyzed using several inversion algorithms, including Milne-Eddington, LTE with 1 and 2 components, and MISMA codes. We find that the line-ratio technique applied to the 5250 Å lines is not sufficiently reliable to provide a direct magnetic diagnostic in the presence of thermal fluctuations and variable line broadening. In general, one needs to resort to inversion algorithms, ideally with realistic magnetohydrodynamic constrains. When this is done, the 5250 Å lines do not seem to provide any significant advantage over those at 6302 Å. In fact, our results point toward a better performance with the latter (in the presence of turbulent line broadening). In any case, for very weak flux concentrations, neither spectral region alone provides sufficient constraints to fully disentangle the intrinsic field strengths. Instead, we advocate for a combined analysis of both spectral ranges, which yields a better determination of the quiet-Sun magnetic properties. Finally, we propose the use of two other Fe I lines (at 4122 and 9000 Å) with identical line opacities that seem to work much better than the others. Title: Near-IR internetwork spectro-polarimetry at different heliocentric angles Authors: Martínez González, M. J.; Asensio Ramos, A.; López Ariste, A.; Manso Sainz, R. Bibcode: 2008A&A...479..229M Altcode: 2007arXiv0710.5219M Aims:The analysis of near infrared spectropolarimetric data at the internetwork in different regions on the solar surface could offer constraints that reject current modeling of these quiet areas.
Methods: We present spectro-polarimetric observations of very quiet regions for different values of the heliocentric angle for the Fe i lines at 1.56 μm, from disc centre to positions close to the limb. The spatial resolution of the data is 0.7{-}1''. We analyse direct observable properties of the Stokes profiles as the amplitude of circular and linear polarisation, as well as the total degree of polarisation. The area and amplitude asymmetries are also studied.
Results: We do not find any significant variation in the properties of the polarimetric signals with the heliocentric angle. This means that the magnetism of the solar internetwork remains the same regardless of the position on the solar disc. This observational fact discards the possibility of modeling the internetwork as a network-like scenario. The magnetic elements of internetwork areas seem to be isotropically distributed when observed at our spatial resolution. Title: Internetwork magnetic field distribution from simultaneous 1.56 μm and 630 nm observations Authors: Martínez González, M. J.; Collados, M.; Ruiz Cobo, B.; Beck, C. Bibcode: 2008A&A...477..953M Altcode: 2007arXiv0711.0267M Aims:We study the contradictory magnetic field strength distributions retrieved from independent analyses of spectropolarimetric observations in the near-infrared (1.56 μm) and in the visible (630 nm) spectral ranges in internetwork regions.
Methods: To solve this apparent controversy, we present simultaneous and co-spatial 1.56 μm and 630 nm observations of an internetwork area. The properties of the circular and linear polarization signals, as well as the Stokes V area and amplitude asymmetries, are discussed. As a complement, we also used inversion techniques to infer the physical parameters of the solar atmosphere. As a first step, the infrared and visible observations are analysed separately to check their compatibility. Finally, the simultaneous inversion of the two data sets is performed.
Results: The magnetic flux densities retrieved from the individual analysis of the infrared and visible data sets are strongly correlated. The polarity of the Stokes V profiles is the same at co-spatial pixels in both wavelength ranges. This indicates that both 1.56 μm and 630 nm observations trace the same magnetic structures on the solar surface. The simultaneous inversion of the two pairs of lines reveals an internetwork full of sub-kG structures that fill only 2% of the resolution element. A correlation is found between the magnetic field strength and the continuum intensity: equipartition fields (B∼ 500 G) tend to be located in dark intergranular lanes, whereas weaker field structures are found inside granules. The most probable unsigned magnetic flux density is 10 Mx/cm^2. The net magnetic flux density in the whole field of view is nearly zero. This means that both polarities cancel out almost exactly in our observed internetwork area. Title: Bayesian inversion of Stokes profiles Authors: Asensio Ramos, A.; Martínez González, M. J.; Rubiño-Martín, J. A. Bibcode: 2007A&A...476..959A Altcode: 2007arXiv0709.0596A Context: Inversion techniques are the most powerful methods to obtain information about the thermodynamical and magnetic properties of solar and stellar atmospheres. In the recent years, we have witnessed the development of highly sophisticated inversion codes that are now widely applied to spectro-polarimetric observations. The majority of these inversion codes are based on the optimization of a complicated non-linear merit function. The experience gained has facilitated the recovery of the model that best fits a given observation. However, and except for the recently developed inversion codes based on database search algorithms together with the application of Principal Component Analysis, no reliable and statistically well-defined confidence intervals can be obtained for the parameters inferred from the inversions.
Aims: A correct estimation of the confidence intervals for all the parameters that describe the model is mandatory. Additionally, it is fundamental to apply efficient techniques to assess the ability of models to reproduce the observations and to determine to what extent the models have to be refined or can be simplified.
Methods: Bayesian techniques are applied to analyze the performance of the model to fit a given observed Stokes vector. The posterior distribution, that takes into account both the information about the priors and the likelihood, is efficiently sampled using a Markov chain Monte Carlo method. For simplicity, we focus on the Milne-Eddington approximate solution of the radiative transfer equation and we only take into account the generation of polarization through the Zeeman effect. However, the method is extremely general and other more complex forward models can be applied, even allowing for the presence of atomic polarization.
Results: We illustrate the method with different problems, from academic to more realistic examples. We show that the information provided by the posterior distribution is fundamental to understand and determine the amount of information available in the Stokes profiles in these particular cases.

Appendix A and B are only available in electronic form at http://www.aanda.org. Title: Multi-Line Quiet Sun Spectro-Polarimetry at 5250 and 6302 Å Authors: Socas-Navarro, H.; Borrero, J.; Asensio Ramos, A.; Collados, M.; Domínguez Cerdeña, I.; Khomenko, E. V.; Martínez González, M. J.; Martínez Pillet, V.; Ruiz Cobo, B.; Sánchez Almeida, J. Bibcode: 2007arXiv0710.1099S Altcode: The reliability of quiet Sun magnetic field diagnostics based on the \ion{Fe}{1} lines at 6302 Åhas been questioned by recent work. We present here the results of a thorough study of high-resolution multi-line observations taken with the new spectro-polarimeter SPINOR, comprising the 5250 and 6302 Åspectral domains. The observations were analyzed using several inversion algorithms, including Milne-Eddington, LTE with 1 and 2 components, and MISMA codes. We find that the line-ratio technique applied to the 5250 Ålines is not sufficiently reliable to provide a direct magnetic diagnostic in the presence of thermal fluctuations and variable line broadening. In general, one needs to resort to inversion algorithms, ideally with realistic magneto-hydrodynamical constrains. When this is done, the 5250 Ålines do not seem to provide any significant advantage over those at 6302 Å. In fact, our results point towards a better performance with the latter (in the presence of turbulent line broadening). In any case, for very weak flux concentrations, neither spectral region alone provides sufficient constraints to fully disentangle the intrinsic field strengths. Instead, we advocate for a combined analysis of both spectral ranges, which yields a better determination of the quiet Sun magnetic properties. Finally, we propose the use of two other \ion{Fe}{1} lines (at 4122 and 9000 Å) with identical line opacities that seem to work much better than the others. Title: Low-lying magnetic loops in the solar internetwork Authors: Martínez González, M. J.; Collados, M.; Ruiz Cobo, B.; Solanki, S. K. Bibcode: 2007A&A...469L..39M Altcode: 2007arXiv0705.1319M Aims:We study the structure of the magnetic field vector in the internetwork and search for the presence of small-scale loops.
Methods: We invert 1.56 μm spectropolarimetric observations of internetwork regions at disc centre by applying the SIR code. This allows us to recover the atmospheric parameters that play a role in the formation of these spectral lines. We are mainly interested in the structure of the magnetic field vector.
Results: We find that many opposite polarity elements of the internetwork are connected by short (2-6´´), low-lying (photospheric) loops. These loops connect at least the 10-20% of the internetwork flux visible in our data. Also we have some evidence that points towards a dynamic scenario that can be produced by the emergence of internetwork magnetic flux. Title: Turbulent fields in the quiet sun from Hanle and Zeeman effects with THEMIS Authors: López Ariste, A.; Malherbe, J. M.; Manso Sainz, R.; Asensio Ramos, A.; Ramírez Vélez, J. C.; Martínez González, M. Bibcode: 2007sf2a.conf..596L Altcode: The measurement of the Sr I line with full polarimetry and spatial resolution with THEMIS has provided the unexpected result of an apparent correlation between the Hanle effect signals and the Zeeman effect signals on this line. Traditionnally, Hanle effect signals in linear polarisation have been interpreted as the signature of unorganized, turbulent fields, while Zeeman effect signals -mostly in circular polarisation- were interpreted as structured fields. The correlation between both observed by THEMIS requires a change of mind respect to the picture of the quiet sun, with fewer structured fields and a turbulent field visible also in deep magnetograms. This picture is supported also by recent results with Mn lines with strong coupling with its hyperfine structure and of center-to-limb histograms of Zeeman amplitudes. Title: The Intrinsic Dimensionality of Spectropolarimetric Data Authors: Asensio Ramos, A.; Socas-Navarro, H.; López Ariste, A.; Martínez González, M. J. Bibcode: 2007ApJ...660.1690A Altcode: 2007astro.ph..1604A The amount of information available in spectropolarimetric data is estimated. To this end, the intrinsic dimensionality of the data is inferred with the aid of a recently derived estimator based on nearest neighbor considerations and obtained applying the principle of maximum likelihood. We show in detail that the estimator correctly captures the intrinsic dimension of artificial data sets with known dimension. The effect of noise in the estimated dimension is analyzed thoroughly, and we conclude that it introduces a positive bias that needs to be accounted for. Real simultaneous spectropolarimetric observations in the visible 630 nm and the near-infrared 1.5 μm spectral regions are also investigated in detail, showing that the near-infrared data set provides more information of the physical conditions in the solar atmosphere than the visible data set. Finally, we demonstrate that the amount of information present in an observed data set is a monotonically increasing function of the number of available spectral lines. 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: Determination of field strengths in the quiet Sun Authors: López Ariste, A.; Martínez González, M. J.; Ramírez Vélez, J. C. Bibcode: 2007A&A...464..351L Altcode: The magnetism in the quiet regions of the solar photosphere carries information on the dynamo processes and its interaction with the convection of the outer layers of the sun. Unfortunately, the scales of the magnetic structures on these regions are mostly unresolved. It is therefore instrumental to tell apart the intrinsic field strengths in those regions from the flux through the resolution element. This disentanglement has been far from obvious, leading to opposing views of the magnetic topology in the unresolved structures of the quiet Sun. Our study contributes to the disentanglement of field strength from flux in the quiet Sun, at least, through the use of new observational constrains in the form of spectropolarimetry of Mn i lines observed in the solar spectrum.The chosen Mn lines present a strong coupling with hyperfine structure resulting in spectral features, present or absent as a function of field strength alone. We observe one of those lines simultaneously and co-spatially with the Fe i lines at 630 nm, at the core of the previous measurements.The inversion of the observed Fe lines results in either strong or weak fields depending on the initializations of the inversion algorithm. All the solutions show nevertheless equally good values for the σ parameter and are therefore equally valuable as solutions. The Mn however selects unambiguously strong or weak fields, sometimes agreeing with the inversions of the Fe lines, but half the time disagreeing with them.The Fe i lines at 630 nm, in the conditions found in the quiet Sun, carry no binding information on field strength. A proper analysis of quiet Sun magnetism should necessarily pass through its simultaneous and co-spatial observation with other lines imposing constraints on field strength, as the Mn i lines here analyzed. Ultimately, the magnetic topology of the quiet Sun shall arise from the coherent analysis of all these lines, sensitive to the Zeeman effect. Title: Internetwork magnetic fields observed in the visible. Authors: Martínez González, M. J. Bibcode: 2007MmSAI..78...59M Altcode: We present internetwork quiet Sun spectropolarimetric observations taken simultaneously at the Fe I lines at 630 and 525 nm. The magnetometry at 630 nm in these low flux regions of the Sun's photosphere has been put in doubt by a recent work. The 525 nm lines are supposed to give reliable results concerning the magnetic field strength in regions where the magnetic structures are not resolved. We study the compatibility of the results obtained in both spectral ranges to confirm or deny the affirmation that the 630 nm lines are not valid for recovering the magnetic field strength at the internetwork quiet Sun. We conclude that both the 630 nm and the 525 nm pair of lines give no trustable results concerning the magnetic field strength. However, the magnetic flux density is correctly recovered from both spectral ranges. Title: Internetwork magnetic fields Authors: Martínez González, M. J.; Collados, M.; Ruiz Cobo, B. Bibcode: 2007msfa.conf..157M Altcode: Spectropolarimetric observations of the internetwork obtained in the 1.56 [mμ]m Fe I doublet are used to calculate the distributions of magnetic field strength, filling factor and magnetic flux density at different positions on the solar surface. We go one step further and describe what the observations show about the three-dimensional structure of the magnetic fields at the internetwork quiet Sun. Title: Internetwork Magnetic Field Distribution from Simultaneous Fe I 1.5 µ and Fe I 630 nm Observations Authors: Martínez González, M. J.; Collados, M.; Ruiz Cobo, B. Bibcode: 2006ASPC..358...36M Altcode: We took simultaneous observations in Fe I 1.5 µ and Fe I 630 nm of an internetwork region. We discuss the compatibility of the data in the two spectral ranges, and present results for the magnetic field distribution. We show that the larger contribution comes from hG field strengths. Title: On the validity of the 630 nm Fe I lines for magnetometry of the internetwork quiet Sun Authors: Martínez González, M. J.; Collados, M.; Ruiz Cobo, B. Bibcode: 2006A&A...456.1159M Altcode: 2006astro.ph..5446M Aims.The purpose of this work is to analyze the reliability of the magnetic field strengths inferred from the 630 nm pair of Fe i lines in internetwork quiet Sun regions.
Methods: .Some numerical experiments have been performed that demonstrate the inability of these lines to recover the magnetic field strength in such low flux solar regions.
Results: .It is shown how different model atmospheres, with magnetic field strengths ranging from a few hundred Gauss to kiloGauss, give rise to Stokes profiles that cannot be distinguished. The reasons for this degeneracy are discussed.
Title: Campos magnéticos en el Sol en calma Title: Campos magnéticos en el Sol en calma Title: Magnetic fields in the quiet sun; Authors: Martínez González, María Jesús Bibcode: 2006PhDT.......335M Altcode: No abstract at ADS Title: Magnetic flux in the internetwork quiet Sun Authors: Khomenko, E. V.; Martínez González, M. J.; Collados, M.; Vögler, A.; Solanki, S. K.; Ruiz Cobo, B.; Beck, C. Bibcode: 2005A&A...436L..27K Altcode: We report a direct comparison of the amplitudes of Stokes spectra of the Fe i 630 nm and 1.56 μm lines produced by realistic MHD simulations with simultaneous observations in the same spectral regions. The Stokes spectra were synthesized in snapshots with a mixed polarity magnetic field having a spatially averaged strength, < B >, between 10 and 30 G. The distribution of Stokes V amplitudes depends sensitively on < B >. A quiet inter-network region was observed at the German VTT simultaneously with TIP (1.56 μm) and POLIS (630 nm). We find that the Stokes V amplitudes of both infrared and visible observations are best reproduced by the simulation snapshot with < B > = 20 G. In observations with 1 resolution, up to 2/3 of the magnetic flux can remain undetected. Title: Modeling the photometric and dynamical behavior of Super-Schmidt meteors in the Earth's atmosphere Authors: Bellot Rubio, L. R.; Martínez González, M. J.; Ruiz Herrera, L.; Licandro, J.; Martínez-Delgado, D.; Rodríguez-Gil, P.; Serra-Ricart, M. Bibcode: 2002A&A...389..680B Altcode: Faint meteors observed with Super-Schmidt cameras are re-examined in order to assess whether their dynamical and photometric behavior can be described by means of the single body theory. Velocities, decelerations and magnitudes are fitted simultaneously to synthetic curves resulting from integration of the appropriate set of differential equations. The parameters determined by this procedure are the ablation coefficient, the shape-density coefficient and the preatmospheric mass of each individual meteoroid. It turns out that 73% of the meteors analyzed here (with magnitudes in the range from +2.5 to -5) are reasonably well described by this theory, suggesting that they did not undergo significant fragmentation during their atmospheric flight. Nevertheless, we identify some systematic differences between observed and theoretical light curves of meteors for which the fit is good. Meteoroid bulk densities are estimated from the retrieved shape-density coefficients. The distributions of individual values are broad, indicating that objects of different densities coexist within the same meteoroid population. The average density is found to be 2400, 1400, and 400 kg m-3 for A-type, B-type and C-type meteoroids, respectively. These results do not confirm the large values determined from quasicontinuous fragmentation models. Title: Modeling the dynamical and photometric behavior of faint meteors in the Earth's atmosphere Authors: Bellot Rubio, Luis R.; Martínez González, María Jesús; Ruiz Herrera, Lola; Licandro, Javier; Martínez Delgado, David; Rodríguez Gil, Pablo; Serra-Ricart, Miquel Bibcode: 2001ESASP.495..525B Altcode: 2001mete.conf..525B Faint meteors observed with Super-Schmidt cameras are re-examined in order to assess whether their dynamical and photometric behavior can be described by means of the single body theory. Velocities, decelerations and magnitudes are fitted simultaneously to synthetic curves resulting from integration of the appropriate set of differential equations. The parameters determined by this procedure are the ablation coefficient, the shape-density coefficient and the preatmospheric mass of the meteoroids. It turns out that 64% of the meteors analyzed here are reasonably well described by this theory, suggesting that they did not undergo significant fragmentation during their atmospheric flight. We identify some systematic differences between observed and theoretical light curves of meteors for which the fit is good. From the retrieved shape-density coefficients we derive meteoroid bulk densities. The average density turns out to be 1.0 g cm-3, with individual values ranging from 0.2 g cm-3 to 1.4 g cm-3. This work represent a first step toward the analysis of low-light level video observations of very small meteoroids.