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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).
- 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.
- ItemThe Direct Effect of Toroidal Magnetic Fields on Stellar Oscillations: An Analytical Expression for the General Matrix Element(London : Institute of Physics Publ., 2017) Kiefer, René; Schad, Ariane; Roth, MarkusWhere is the solar dynamo located and what is its modus operandi? These are still open questions in solar physics. Helio- and asteroseismology can help answer them by enabling us to study solar and stellar internal structures through global oscillations. The properties of solar and stellar acoustic modes are changing with the level of magnetic activity. However, until now, the inference on subsurface magnetic fields with seismic measures has been very limited. The aim of this paper is to develop a formalism to calculate the effect of large-scale toroidal magnetic fields on solar and stellar global oscillation eigenfunctions and eigenfrequencies. If the Lorentz force is added to the equilibrium equation of motion, stellar eigenmodes can couple. In quasi-degenerate perturbation theory, this coupling, also known as the direct effect, can be quantified by the general matrix element. We present the analytical expression of the matrix element for a superposition of subsurface zonal toroidal magnetic field configurations. The matrix element is important for forward calculations of perturbed solar and stellar eigenfunctions and frequency perturbations. The results presented here will help to ascertain solar and stellar large-scale subsurface magnetic fields, and their geometric configuration, strength, and change over the course of activity cycles.
- ItemThe Effect of Toroidal Magnetic Fields on Solar Oscillation Frequencies(London : Institute of Physics Publ., 2018) Kiefer, René; Roth, MarkusSolar oscillation frequencies change with the level of magnetic activity. Localizing subsurface magnetic field concentrations in the Sun with helioseismology will help us to understand the solar dynamo. Because the magnetic fields are not considered in standard solar models, adding them to the basic equations of stellar structure changes the eigenfunctions and eigenfrequencies. We use quasi-degenerate perturbation theory to calculate the effect of toroidal magnetic fields on solar oscillation mean multiplet frequencies for six field configurations. In our calculations, we consider both the direct effect of the magnetic field, which describes the coupling of modes, and the indirect effect, which accounts for changes in stellar structure due to the magnetic field. We limit our calculations to self-coupling of modes. We find that the magnetic field affects the multiplet frequencies in a way that depends on the location and the geometry of the field inside the Sun. Comparing our theoretical results with observed shifts, we find that strong tachocline fields cannot be responsible for the observed frequency shifts of p modes over the solar cycle. We also find that part of the surface effect in helioseismic oscillation frequencies might be attributed to magnetic fields in the outer layers of the Sun. The theory presented here is also applicable to models of solar-like stars and their oscillation frequencies.
- ItemGONG p-Mode Parameters Through Two Solar Cycles(Dordrecht [u.a.] : Springer Science + Business Media B.V, 2018) Kiefer, René; Komm, Rudi; Hill, Frank; Broomhall, Anne-Marie; Roth, MarkusWe investigate the parameters of global solar p-mode oscillations, namely damping width Γ, amplitude A, mean squared velocity ⟨v2⟩, energy E, and energy supply rate dE/dt, derived from two solar cycles’ worth (1996 – 2018) of Global Oscillation Network Group (GONG) time series for harmonic degrees l=0--150. We correct for the effect of fill factor, apparent solar radius, and spurious jumps in the mode amplitudes. We find that the amplitude of the activity-related changes of Γ and A depends on both frequency and harmonic degree of the modes, with the largest variations of Γ for modes with 2400 μHz≤ν≤3300 μHz and 31≤l≤60 with a minimum-to-maximum variation of 26.6±0.3% and of A for modes with 2400 μHz≤ν≤3300 μHz and 61≤l≤100 with a minimum-to-maximum variation of 27.4±0.4%. The level of correlation between the solar radio flux F10.7 and mode parameters also depends on mode frequency and harmonic degree. As a function of mode frequency, the mode amplitudes are found to follow an asymmetric Voigt profile with νmax=3073.59±0.18 μHz. From the mode parameters, we calculate physical mode quantities and average them over specific mode frequency ranges. In this way, we find that the mean squared velocities ⟨v2⟩ and energies E of p modes are anticorrelated with the level of activity, varying by 14.7±0.3% and 18.4±0.3%, respectively, and that the mode energy supply rates show no significant correlation with activity. With this study we expand previously published results on the temporal variation of solar p-mode parameters. Our results will be helpful to future studies of the excitation and damping of p modes, i.e., the interplay between convection, magnetic field, and resonant acoustic oscillations.