52 results
Search Results
Now showing 1 - 10 of 52
- ItemPromoting access to and use of seismic data in a large scientific community(Les Ulis : EDP Sciences, 2017) Michel, Eric; Belkacem, Kevin; Samadi, Reza; de Assis Peralta, Raphael; Renié, Christian; Abed, Mahfoudh; Lin, Guangyuan; Christensen-Dalsgaard, Jørgen; Houdek, Günter; Handberg, Rasmus; Gizon, Laurent; Burston, Raymond; Nagashima, Kaori; Pallé, Pere; Poretti, Ennio; Rainer, Monica; Mistò, Angelo; Panzera, Maria Rosa; Roth, Markus; Monteiro, Mário J. P. F. G.; Cunha, Margarida S.; Ferreira, João Miguel T. S.The growing amount of seismic data available from space missions (SOHO, CoRoT, Kepler, SDO,…) but also from ground-based facilities (GONG, BiSON, ground-based large programmes…), stellar modelling and numerical simulations, creates new scientific perspectives such as characterizing stellar populations in our Galaxy or planetary systems by providing model-independent global properties of stars such as mass, radius, and surface gravity within several percent accuracy, as well as constraints on the age. These applications address a broad scientific community beyond the solar and stellar one and require combining indices elaborated with data from different databases (e.g. seismic archives and ground-based spectroscopic surveys). It is thus a basic requirement to develop a simple and effcient access to these various data resources and dedicated tools. In the framework of the European project SpaceInn (FP7), several data sources have been developed or upgraded. The Seismic Plus Portal has been developed, where synthetic descriptions of the most relevant existing data sources can be found, as well as tools allowing to localize existing data for given objects or period and helping the data query. This project has been developed within the Virtual Observatory (VO) framework. In this paper, we give a review of the various facilities and tools developed within this programme. The SpaceInn project (Exploitation of Space Data for Innovative Helio- and Asteroseismology) has been initiated by the European Helio- and Asteroseismology Network (HELAS).
- ItemNewly formed downflow lanes in exploding granules in the solar photosphere(Les Ulis : EDP Sciences, 2021) Ellwarth, M.; Fischer, C.E.; Vitas, N.; Schmiz, S.; Schmidt, W.Context. Exploding granules have drawn renewed interest because of their interaction with the magnetic field (either emerging or already present). Especially the newly forming downflow lanes developing in their centre seem to be eligible candidates for the intensification of magnetic fields. We analyse spectroscopic data from two different instruments in order to study the intricate velocity pattern within the newly forming downflow lanes in detail. Aims. We aim to examine general properties of a number of exploding granules, such as their lifetime and extend. To gain a better understanding of the formation process of the developing intergranular lane in exploding granules, we study the temporal evolution and height dependence of the line-of-sight velocities at their formation location. Additionally, we search for evidence that exploding granules act as acoustic sources. Methods. We investigated the evolution of several exploding granules using data taken with the Interferometric Bidimensional Spectrometer and the Imaging Magnetograph eXperiment. Velocities for different heights of the solar atmosphere were determined by computing bisectors of the Fe I 6173.0 Å and the Fe I 5250.2 Å lines. We performed a wavelet analysis to study the intensity and velocity oscillations within and around exploding granules. We also compared our observational findings with predictions of numerical simulations. Results. Exploding granules have significantly longer lifetimes (10 to 15 min) than regular granules. Exploding granules larger than 3.8″ form an independent intergranular lane during their decay phase, while smaller granules usually fade away or disappear into the intergranular area (we find only one exception of a smaller exploding granule that also forms an intergranular lane). For all exploding granules that form a new intergranular downflow lane, we find a temporal height-dependent shift with respect to the maximum of the downflow velocity. Our suggestion that this results from a complex atmospheric structure within the newly forming downflow lane is supported by the comparison with synthesised profiles inferred from the simulations. We found an enhanced wavelet power with periods between 120 s to 190 s seen in the intensity and velocity oscillations of high photospheric or chromospheric spectral lines in the region of the dark core of an exploding granule. © M. Ellwarth et al. 2021.
- ItemEffects of solar evolution on finite acquisition time of Fabry-Perot interferometers in high resolution solar physics(Les Ulis : EDP Sciences, 2023) Schlichenmaier, R.; Pitters, D.; Borrero, J.M.; Schubert, M.Context. The Visible Tunable Filter (VTF) imaging spectropolarimeter will be operated at the Daniel K. Inouye Solar Telescope (DKIST) in Hawaii. Due to its capability in resolving dynamic fine structure of smaller than 0.05 arcsec, the finite acquisition time of typically 11 s affects the measurement process and potentially causes errors in deduced physical parameters. Aims. We estimate these errors and investigate ways of minimising them. Methods. We mimicked the solar surface using a magnetohydrodynamic simulation with a spatially averaged vertical field strength of 200 G. We simulated the measurement process scanning through successive wavelength points with a temporal cadence of 1 s. We synthesised Fe 1617.3 nm for corresponding snapshots. In addition to the classical composition of the line profile, we introduce a novel method where the intensity in each wavelength point is normalised using the simultaneous continuum intensity, and then multiplied by the temporal mean of the continuum intensity. Milne-Eddington inversions were used to infer the line-of-sight velocity, vlos, and the vertical (longitudinal) component of the magnetic field, Blos. Results. We quantify systematic errors, defining the temporal average of the simulation during the measurement as the truth. We find that with the classical composition of the line profiles, errors exceed the sensitivity for vlos, and in filigree regions also for Blos. The novel method that includes normalisation reduces the measurement errors in all cases. Spatial binning without reducing the acquisition time decreases the measurement error slightly. Conclusions. The evolutionary timescale in inter-granular lanes, in particular in areas with magnetic features (filigree), is shorter than the timescale within granules. Hence, depending on the science objective, fewer accumulations could be used for strong magnetic field in inter-granular lanes and more accumulations could be used for the weak granular magnetic fields. As a key result of this investigation, we suggest including the novel method of normalisation in corresponding data pipelines.
- ItemStellar magnetic activity and variability of oscillation parameters: An investigation of 24 solar-like stars observed by Kepler(Les Ulis : EDP Sciences, 2017) Kiefer, René; Schad, Ariane; Davies, Guy; Roth, MarkusContext. The Sun and solar-like stars undergo activity cycles for which the underlying mechanisms are not well understood. The oscillations of the Sun are known to vary with its activity cycle and these changes provide diagnostics on the conditions below the photosphere. Kepler has detected solar-like oscillations in hundreds of stars but as of yet, no widespread detection of signatures of magnetic activity cycles in the oscillation parameters of these stars have been reported. Aims. We analysed the photometric short cadence Kepler time series of a set of 24 solar-like stars, which were observed for at least 960 d each, with the aim to find signatures of stellar magnetic activity in the oscillation parameters. Methods. We analyse the temporal evolution of oscillation parameters by measuring mode frequency shifts, changes in the height of the p-mode envelope, as well as granulation timescales. Results. For 23 of the 24 investigated stars, we find significant frequency shifts in time. We present evidence for magnetic activity in six of these stars. We find that the amplitude of the frequency shifts decreases with stellar age and rotation period. For KIC 8006161 (the most prominent example), we find that frequency shifts are smallest for the lowest and largest for the highest p-mode frequencies, as they are for the Sun. Conclusions. These findings show that magnetic activity can be routinely observed in the oscillation parameters for solar-like stars, which opens up the possibility of placing the solar activity cycle in the context of other stars by asteroseismology.
- ItemEvolution of the flow field in decaying active regions II. Converging flows at the periphery of naked spots(Les Ulis : EDP Sciences, 2022) Strecker, H.; Bello González, N.Context. In a previous work, we investigated the evolution of the flow field around sunspots during sunspot decay and compared it with the flow field of supergranular cells. The decay of a sunspot proceeds as it interacts with its surroundings. This is manifested by the changes observed in the flow field surrounding the decaying spot. Aims. We now investigate in detail the evolution of the flow field in the direct periphery of the sunspots of the same sample and aim to provide a complete picture of the role of large-scale flows present in sunspot cells. Methods. We analyse the horizontal velocity profiles of sunspots obtained from observations by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). We follow their evolution across the solar disc from their stable phase to their decay and their final disappearance. Results.We find two different scenarios for the evolution of the flow region surrounding a spot in the final stage of its decay: (i) either the flow cell implodes and disappears under the action of the surrounding supergranules or (ii) it outlives the spot. In the later case, an inwards flow towards the remaining naked spot develops in the vicinity closest to the spot followed by an outflow further out. These findings provide observational evidence to theoretical predictions by realistic magnetohydrodynamic (MHD) sunspot and moat region simulations. Conclusions. The Evershed flow and the moat flow, both connected to the presence of fully fledged sunspots in a spot cell, vanish when penumbrae decay. Moat flows decline into supergranular flows. The final fate of a spot cell depends on its interaction with the surrounding supergranular cells. In the case of non-imploding spot cells, the remaining naked spot develops a converging inflow driven by radiative cooling and a geometrical alignment of granules in its periphery which is similar to that observed in pores.
- ItemChemical composition of the RS CVn-type star lambda andromedae(Berlin : de Gruyter, 2010) Tautvaišiené, G.; Barisevičius, G.; Berdyugina, Svetlana V.; Chorniy, Yuriy; Ilyin, Ilya V.Photospheric parameters and chemical composition are determined for the single-lined chromospherically active RS CVn-type star λ And (HD 222107). From the high resolution spectra obtained on the Nordic Optical Telescope, abundances of 22 chemical elements and isotopes, including such key elements as 12C, 13C, N and O, were investigated. The differential line analysis with the MARCS model atmospheres gives Teff = 4830 K, log g = 2.8, [Fe/H] = -0.53, [C/Fe] = 0.09, [N/Fe] = 0.35, [O/Fe] = 0.45, C/N = 2.21, 12C/13C = 14. The 12C/13C ratio for a star of the RS CVn-type is determined for the first time, and its low value gives a hint that extra-mixing processes may start acting in low-mass chromospherically active stars below the bump of the luminosity function of red giants.
- ItemThe Polarimetric and Helioseismic Imager on Solar Orbiter(Les Ulis : EDP Sciences , 2020) Solanki, S.K.; del Toro Iniesta, J.C.; Woch, J.; Gandorfer, A.; Hirzberger, J.; Alvarez-Herrero, A.; Appourchaux, T.; Martínez Pillet, V.; Pérez-Grande, I.; Sanchis Kilders, E.; Schmidt, W.; Garranzo-García, D.; Laguna, H.; Martín, J.A.; Navarro, R.; Villanueva, J.; Núñez Peral, A.; Royo, M.; Sánchez, A.; Silva-López, M.; Fourmond, J.-J.; Berkefeld, Th.; Ruiz de Galarreta, C.; Bouzit, M.; Hervier, V.; Le Clec'h, J.C.; Szwec, N.; Chaigneau, M.; Buttice, V.; Volkmer, R.; Dominguez-Tagle, C.; Philippon, A.; Baumgartner, J.; Boumier, P.; Le Cocguen, R.; Baranjuk, G.; Bell, A.; Heidecke, F.; Maue, T.; Blanco Rodríguez, J.; Nakai, E.; Scheiffelen, T.; Sigwarth, M.; Soltau, D.; Domingo, V.; Fiethe, B.; Ferreres Sabater, A.; Gasent Blesa, J.L.; Rodríguez Martínez, P.; Osorno Caudel, D.; Bosch, J.; Casas, A.; Carmona, M.; Gómez Cama, J.M.; Herms, A.; Roma, D.; Guan, Y.; Alonso, G.; Gómez-Sanjuan, A.; Piqueras, J.; Torralbo, I.; Lange, T.; Michel, H.; Michalik, H.; Bonet, J.A.; Fahmy, S.; Müller, D.; Zouganelis, I.; Deutsch, W.; Busse, D.; Fernandez-Rico, G.; Grauf, B.; Gizon, L.; Heerlein, K.; Kolleck, M.; Lagg, A.; Meller, R.; Müller, R.; Schühle, U.; Staub, J.; Enge, R.; Albert, K.; Alvarez Copano, M.; Beckmann, U.; Bischoff, J.; Frahm, S.; Germerott, D.; Guerrero, L.; Löptien, B.; Meierdierks, T.; Oberdorfer, D.; Papagiannaki, I.; Ramanath, S.; Bellot Rubio, L.R.; Schou, J.; Werner, S.; Yang, D.; Zerr, A.; Bergmann, M.; Bochmann, J.; Heinrichs, J.; Meyer, S.; Monecke, M.; Müller, M.-F.; Cobos Carracosa, J.P.; Sperling, M.; Álvarez García, D.; Aparicio, B.; Balaguer Jiménez, M.; Girela, F.; Hernández Expósito, D.; Herranz, M.; Labrousse, P.; López Jiménez, A.; Orozco Suárez, D.; Ramos, J.L.; Barandiarán, J.; Vera, I.; Bastide, L.; Campuzano, C.; Cebollero, M.; Dávila, B.; Fernández-Medina, A.; García Parejo, P.This paper describes the Polarimetric and Helioseismic Imager on the Solar Orbiter mission (SO/PHI), the first magnetograph and helioseismology instrument to observe the Sun from outside the Sun-Earth line. It is the key instrument meant to address the top-level science question: How does the solar dynamo work and drive connections between the Sun and the heliosphere? SO/PHI will also play an important role in answering the other top-level science questions of Solar Orbiter, as well as hosting the potential of a rich return in further science. SO/PHI measures the Zeeman effect and the Doppler shift in the FeI 617.3nm spectral line. To this end, the instrument carries out narrow-band imaging spectro-polarimetry using a tunable LiNbO_3 Fabry-Perot etalon, while the polarisation modulation is done with liquid crystal variable retarders (LCVRs). The line and the nearby continuum are sampled at six wavelength points and the data are recorded by a 2kx2k CMOS detector. To save valuable telemetry, the raw data are reduced on board, including being inverted under the assumption of a Milne-Eddington atmosphere, although simpler reduction methods are also available on board. SO/PHI is composed of two telescopes; one, the Full Disc Telescope (FDT), covers the full solar disc at all phases of the orbit, while the other, the High Resolution Telescope (HRT), can resolve structures as small as 200km on the Sun at closest perihelion. The high heat load generated through proximity to the Sun is greatly reduced by the multilayer-coated entrance windows to the two telescopes that allow less than 4% of the total sunlight to enter the instrument, most of it in a narrow wavelength band around the chosen spectral line.
- ItemCombining magneto-hydrostatic constraints with Stokes profiles inversions: III. Uncertainty in the inference of electric currents(Les Ulis : EDP Sciences, 2023) Borrero, J.M.; Pastor Yabar, A.Electric currents play an important role in the energy balance of the plasma in the solar atmosphere. They are also indicative of non-potential magnetic fields and magnetic reconnection. Unfortunately, the direct measuring of electric currents has traditionally been riddled with inaccuracies. Aims. We study how accurately we can infer electric currents under different scenarios. Methods. We carry out increasingly complex inversions of the radiative transfer equation for polarized light applied to Stokes profiles synthesized from radiative three-dimensional magnetohydrodynamic (MHD) simulations. The inversion yields the magnetic field vector. B. from which the electric current density, ./, is derived by applying Ampere's law. Results. We find that the retrieval of the electric current density is only slightly affected by photon noise or spectral resolution. However, the retrieval steadily improves as the Stokes inversion becomes increasingly elaborated. In the least complex case (a Milne- Eddington-like inversion applied to a single spectral region), it is possible to determine the individual components of the electric current density (jx, jy, jz) with an accuracy of cr = 0.90 - l.OOdex, whereas the modulus (|[/
- ItemNumerical solutions to linear transfer problems of polarized radiation III. Parallel preconditioned Krylov solver tailored for modeling PRD effects(Les Ulis : EDP Sciences, 2022) Benedusi, Pietro; Janett, Gioele; Riva, Simone; Krause, Rolf; Belluzzi, LucaContext. The polarization signals produced by the scattering of anistropic radiation in strong resonance lines encode important information about the elusive magnetic fields in the outer layers of the solar atmosphere. An accurate modeling of these signals is a very challenging problem from the computational point of view, in particular when partial frequency redistribution (PRD) effects in scattering processes are accounted for with a general angle-dependent treatment. Aims. We aim at solving the radiative transfer problem for polarized radiation in nonlocal thermodynamic equilibrium conditions, taking angle-dependent PRD effects into account. The problem is formulated for a two-level atomic model in the presence of arbitrary magnetic and bulk velocity fields. The polarization produced by scattering processes and the Zeeman effect is considered. Methods. The proposed solution strategy is based on an algebraic formulation of the problem and relies on a convenient physical assumption, which allows its linearization. We applied a nested matrix-free GMRES iterative method. Effective preconditioning is obtained in a multifidelity framework by considering the light-weight description of scattering processes in the limit of complete frequency redistribution (CRD). Results. Numerical experiments for a one-dimensional (1D) atmospheric model show near optimal strong and weak scaling of the proposed CRD-preconditioned GMRES method, which converges in few iterations, independently of the discretization parameters. A suitable parallelization strategy and high-performance computing tools lead to competitive run times, providing accurate solutions in a few minutes. Conclusions. The proposed solution strategy allows the fast systematic modeling of the scattering polarization signals of strong resonance lines, taking angle-dependent PRD effects into account together with the impact of arbitrary magnetic and bulk velocity fields. Almost optimal strong and weak scaling results suggest that this strategy is applicable to realistic 3D models. Moreover, the proposed strategy is general, and applications to more complex atomic models are possible.
- ItemRefPlanets: Search for reflected light from extra-solar planets with SPHERE/ZIMPOL(Les Ulis : EDP Sciences , 2020) Hunziker, S.; Schmid, H.M.; Mouillet, D.; Milli, J.; Zurlo, A.; Delorme, P.; Abe, L.; Avenhaus, H.; Baruffolo, A.; Bazzon, A.; Boccaletti, A.; Baudoz, P.; Beuzit, J.L.; Carbillet, M.; Chauvin, G.; Claudi, R.; Costille, A.; Daban, J.B.; Desidera, S.; Dohlen, K.; Dominik, C.; Downing, M.; Engler, N.; Feldt, M.; Fusco, T.; Ginski, C.; Gisler, D.; Girard, J.H.; Gratton, R.; Henning, Th.; Hubin, N.; Kasper, M.; Keller, C.U.; Langlois, M.; Lagadec, E.; Martinez, P.; Maire, A.L.; Menard, F.; Meyer, M.R.; Pavlov, A.; Pragt, J.; Puget, P.; Quanz, S.P.; Rickman, E.; Roelfsema, R.; Salasnich, B.; Sauvage, J.F.; Siebenmorgen, R.; Sissa, E.; Snik, F.; Suarez, M.; Szulagyi, J.; Thalmann, Ch.; Turatto, M.; Udry, S.; van Holstein, R.G.; Vigan, A.; Wildi, F.RefPlanets is a guaranteed time observation (GTO) programme that uses the Zurich IMaging POLarimeter (ZIMPOL) of SPHERE/VLT for a blind search for exoplanets in wavelengths from 600-900 nm. The goals of this study are the characterization of the unprecedented high polarimetic contrast and polarimetric precision capabilities of ZIMPOL for bright targets, the search for polarized reflected light around some of the closest bright stars to the Sun and potentially the direct detection of an evolved cold exoplanet for the first time. For our observations of Alpha Cen A and B, Sirius A, Altair, Eps Eri and Tau Ceti we used the polarimetric differential imaging (PDI) mode of ZIMPOL which removes the speckle noise down to the photon noise limit for angular separations >0.6". We describe some of the instrumental effects that dominate the noise for smaller separations and explain how to remove these additional noise effects in post-processing. We then combine PDI with angular differential imaging (ADI) as a final layer of post-processing to further improve the contrast limits of our data at these separations. For good observing conditions we achieve polarimetric contrast limits of 15.0-16.3 mag at the effective inner working angle of about 0.13", 16.3-18.3 mag at 0.5" and 18.8-20.4 mag at 1.5". The contrast limits closer in (<0.6") depend significantly on the observing conditions, while in the photon noise dominated regime (>0.6"), the limits mainly depend on the brightness of the star and the total integration time. We compare our results with contrast limits from other surveys and review the exoplanet detection limits obtained with different detection methods. For all our targets we achieve unprecedented contrast limits. Despite the high polarimetric contrasts we are not able to find any additional companions or extended polarized light sources in the data that has been taken so far.