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- 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.
- 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.
- ItemInnovative and automated method for vortex identification(Les Ulis : EDP Sciences, 2022) Canivete Cuissa, J. R.; Steiner, O.Context. As a universally accepted definition of a vortex has not yet been established, the community lacks an unambiguous and rigorous method for identifying vortices in fluid flows. Such a method would be useful for conducting robust statistical studies on vortices in highly dynamical and turbulent systems such as the solar atmosphere. Aims. We aim to develop an innovative and robust automated methodology for the identification of vortices based on local and global characteristics of the flow, while avoiding the use of a threshold that could potentially prevent the detection of weak vortices in the process. Methods. We present a new method that combines the rigor of mathematical criteria with the global perspective of morphological techniques. The core of the method consists of an estimation of the center of rotation for every point of the flow that presents some degree of curvature in its neighborhood. For this purpose, we employed the Rortex criterion and combined it with morphological considerations of the velocity field. We then identified coherent vortical structures based on clusters of estimated centers of rotation. Results. We demonstrate that the Rortex is a more reliable criterion than the swirling strength and the vorticity for the extraction of physical information from vortical flows, because it measures the rigid-body rotational part of the flow alone and is not biased by the presence of pure or intrinsic shears. We show that the method performs well in the context of a simplistic test case composed of two Lamb-Oseen vortices. We combined the proposed method with a state-of-the-art clustering algorithm to build an automated vortex identification algorithm. The algorithm was applied to an artificial flow composed of multiple Lamb- Oseen vortices, with a random noisy background, and to the turbulent flow of a simulated magneto-hydrodynamical Orszag-Tang vortex test. The results demonstrate the reliability and accuracy of the method. Conclusions. The present automated vortex identification method can be considered a new tool for the detection and study of vortices in dynamical and turbulent (magneto)hydrodynamical flows. By applying the implemented algorithm to numerical simulations and observational data, as well as comparing it to existing detection methods, we seek to successively improve the reliability of the detections and, ultimately, our knowledge on swirling motions in the solar, stellar, and planetary atmospheres.
- ItemDiscovery of magnetic fields in five DC white dwarfs(Les Ulis : EDP Sciences, 2023) Berdyugin, Andrei V.; Piirola, Vilppu; Bagnulo, Stefano; Landstreet, John D.; Berdyugina, Svetlana V.About half of white dwarfs (WDs) evolve to the DC state as they cool; the others become DQ or (temporarily?) DZ WDs. The recent magnetic survey of the local 20 pc volume has established a high frequency of magnetic fields among WDs older than 2-3 Gyr, demonstrating that in low-and average-mass WDs, the effects of magnetism become more common as they age, and the fields on average become stronger. However, the available statistics of WDs older than about 5 Gyr do not clearly establish how fields evolve beyond this age. We are carrying out a survey to clarify the occurrence of magnetism in DC-type WDs in order to better understand this late evolution. We use broadband filter polarimetry, arguably the most efficient way to detect magnetic fields in featureless WDs via continuum circular polarization. Here we report the discovery of a magnetic field in five DC WDs (of 23 observed), almost doubling the total sample of known magnetic WDs belonging to the DC spectral class.
- ItemVerification of the helioseismic Fourier-Legendre analysis for meridional flow measurements(Les Ulis : EDP Sciences, 2016) Roth, M.; Doerr, H.-P.; Hartlep, T.Context. Measuring the Sun’s internal meridional flow is one of the key issues of helioseismology. Using the Fourier-Legendre analysis is a technique for addressing this problem. Aims. We validate this technique with the help of artificial helioseismic data. Methods. The analysed data set was obtained by numerically simulating the effect of the meridional flow on the seismic wave field in the full volume of the Sun. In this way, a 51.2-h long time series was generated. The resulting surface velocity field is then analyzed in various settings: Two 360° × 90° halfspheres, two 120° × 60° patches on the front and farside of the Sun (North and South, respectively) and two 120° × 60° patches on the northern and southern frontside only. We compare two possible measurement setups: observations from Earth and from an additional spacecraft on the solar farside, and observations from Earth only, in which case the full information of the global solar oscillation wave field was available. Results. We find that, with decreasing observing area, the accessible depth range decreases: the 360° × 90° view allows us to probe the meridional flow almost to the bottom of the convection zone, while the 120° × 60° view means only the outer layers can be probed. Conclusions. These results confirm the validity of the Fourier-Legendre analysis technique for helioseismology of the meridional flow. Furthermore these flows are of special interest for missions like Solar Orbiter that promises to complement standard helioseismic measurements from the solar nearside with farside observations.