Fournier, Damien

[["dc.bibliographiccitation.firstpage","1469"],["dc.bibliographiccitation.issue","6498"],["dc.bibliographiccitation.journal","Science"],["dc.bibliographiccitation.lastpage","1472"],["dc.bibliographiccitation.volume","368"],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Cameron, Robert H."],["dc.contributor.author","Pourabdian, Majid"],["dc.contributor.author","Liang, Zhi-Chao"],["dc.contributor.author","Fournier, Damien"],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Hanson, Chris S."],["dc.date.accessioned","2021-03-05T08:59:01Z"],["dc.date.available","2021-03-05T08:59:01Z"],["dc.date.issued","2020"],["dc.identifier.doi","10.1126/science.aaz7119"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/80330"],["dc.language.iso","en"],["dc.notes.intern","DOI Import GROB-393"],["dc.relation.eissn","1095-9203"],["dc.relation.issn","0036-8075"],["dc.title","Meridional flow in the Sun’s convection zone is a single cell in each hemisphere"],["dc.type","journal_article"],["dc.type.internalPublication","unknown"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","L10"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","640"],["dc.contributor.author","Goddard, C. R."],["dc.contributor.author","Birch, A. C."],["dc.contributor.author","Fournier, Damien"],["dc.contributor.author","Gizon, Laurent"],["dc.date.accessioned","2021-03-05T08:58:40Z"],["dc.date.available","2021-03-05T08:58:40Z"],["dc.date.issued","2020"],["dc.identifier.doi","10.1051/0004-6361/202038539"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/80203"],["dc.notes.intern","DOI Import GROB-393"],["dc.relation.eissn","1432-0746"],["dc.relation.issn","0004-6361"],["dc.title","Predicting frequency changes of global-scale solar Rossby modes due to solar cycle changes in internal rotation"],["dc.type","journal_article"],["dc.type.internalPublication","unknown"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","L6"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","652"],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Cameron, Robert H."],["dc.contributor.author","Bekki, Yuto"],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Bogart, Richard S."],["dc.contributor.author","Sacha Brun, Allan"],["dc.contributor.author","Damiani, Cilia"],["dc.contributor.author","Fournier, Damien"],["dc.contributor.author","Hyest, Laura"],["dc.contributor.author","Jain, Kiran"],["dc.contributor.author","Proxauf, Bastian"],["dc.date.accessioned","2021-09-01T06:42:17Z"],["dc.date.available","2021-09-01T06:42:17Z"],["dc.date.issued","2021"],["dc.description.abstract","The oscillations of a slowly rotating star have long been classified into spheroidal and toroidal modes. The spheroidal modes include the well-known 5-min acoustic modes used in helioseismology. Here we report observations of the Sun’s toroidal modes, for which the restoring force is the Coriolis force and whose periods are on the order of the solar rotation period. By comparing the observations with the normal modes of a differentially rotating spherical shell, we are able to identify many of the observed modes. These are the high-latitude inertial modes, the critical-latitude inertial modes, and the equatorial Rossby modes. In the model, the high-latitude and critical-latitude modes have maximum kinetic energy density at the base of the convection zone, and the high-latitude modes are baroclinically unstable due to the latitudinal entropy gradient. As a first application of inertial-mode helioseismology, we constrain the superadiabaticity and the turbulent viscosity in the deep convection zone."],["dc.description.abstract","The oscillations of a slowly rotating star have long been classified into spheroidal and toroidal modes. The spheroidal modes include the well-known 5-min acoustic modes used in helioseismology. Here we report observations of the Sun’s toroidal modes, for which the restoring force is the Coriolis force and whose periods are on the order of the solar rotation period. By comparing the observations with the normal modes of a differentially rotating spherical shell, we are able to identify many of the observed modes. These are the high-latitude inertial modes, the critical-latitude inertial modes, and the equatorial Rossby modes. In the model, the high-latitude and critical-latitude modes have maximum kinetic energy density at the base of the convection zone, and the high-latitude modes are baroclinically unstable due to the latitudinal entropy gradient. As a first application of inertial-mode helioseismology, we constrain the superadiabaticity and the turbulent viscosity in the deep convection zone."],["dc.identifier.doi","10.1051/0004-6361/202141462"],["dc.identifier.pii","aa41462-21"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/89024"],["dc.notes.intern","DOI-Import GROB-455"],["dc.relation.eissn","1432-0746"],["dc.relation.issn","0004-6361"],["dc.title","Solar inertial modes: Observations, identification, and diagnostic promise"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","A136"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","620"],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Fournier, Damien"],["dc.contributor.author","Yang, Dan"],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Barucq, Hélène"],["dc.date.accessioned","2020-12-10T18:11:43Z"],["dc.date.available","2020-12-10T18:11:43Z"],["dc.date.issued","2018"],["dc.identifier.doi","10.1051/0004-6361/201833825"],["dc.identifier.eissn","1432-0746"],["dc.identifier.issn","0004-6361"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/74121"],["dc.notes.intern","DOI Import GROB-354"],["dc.title","Signal and noise in helioseismic holography"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","A168"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","643"],["dc.contributor.author","Poulier, P.-L."],["dc.contributor.author","Fournier, Damien"],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Duvall, T. L."],["dc.date.accessioned","2021-03-05T08:58:40Z"],["dc.date.available","2021-03-05T08:58:40Z"],["dc.date.issued","2020"],["dc.identifier.doi","10.1051/0004-6361/202039201"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/80206"],["dc.notes.intern","DOI Import GROB-393"],["dc.relation.eissn","1432-0746"],["dc.relation.issn","0004-6361"],["dc.title","Acoustic wave propagation through solar granulation: Validity of effective-medium theories, coda waves"],["dc.type","journal_article"],["dc.type.internalPublication","unknown"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","A178"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","642"],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Fournier, Damien"],["dc.contributor.author","Albekioni, M."],["dc.date.accessioned","2021-03-05T08:58:40Z"],["dc.date.available","2021-03-05T08:58:40Z"],["dc.date.issued","2020"],["dc.identifier.doi","10.1051/0004-6361/202038525"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/80202"],["dc.notes.intern","DOI Import GROB-393"],["dc.relation.eissn","1432-0746"],["dc.relation.issn","0004-6361"],["dc.title","Effect of latitudinal differential rotation on solar Rossby waves: Critical layers, eigenfunctions, and momentum fluxes in the equatorial β plane"],["dc.type","journal_article"],["dc.type.internalPublication","unknown"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","A189"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.volume","664"],["dc.contributor.author","Poulier, P.-L."],["dc.contributor.author","Liang, Z.-C."],["dc.contributor.author","Fournier, D."],["dc.contributor.author","Gizon, L."],["dc.date.accessioned","2022-10-04T10:22:20Z"],["dc.date.available","2022-10-04T10:22:20Z"],["dc.date.issued","2022"],["dc.description.abstract","Context.\n In local helioseismology, the travel times of acoustic waves propagating in opposite directions along the same meridian inform us about horizontal flows in the north-south direction. The longitudinal averages of the north-south helioseismic travel-time shifts vary with the sunspot cycle.\n \n \n Aims.\n We aim to study the contribution of inflows into solar active regions to this solar-cycle variation.\n \n \n Methods.\n To do so, we identified the local flows around active regions in the horizontal flow maps obtained from correlation tracking of granulation in continuum images of the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory. We computed the forward-modeled travel-time perturbations caused by these inflows using 3D sensitivity kernels. In order to compare with the observations, we averaged these forward-modeled travel-time perturbations over longitude and time in the same way as the measured travel times.\n \n \n Results.\n The forward-modeling approach shows that the inflows associated with active regions may account for only a fraction of the solar-cycle variations in the north-south travel-time measurements.\n \n \n Conclusions.\n The travel-time perturbations caused by the large-scale inflows surrounding the active regions do not explain in full the solar-cycle variations seen in the helioseismic measurements of the meridional circulation."],["dc.identifier.doi","10.1051/0004-6361/202243476"],["dc.identifier.pii","aa43476-22"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/114645"],["dc.notes.intern","DOI-Import GROB-600"],["dc.relation.eissn","1432-0746"],["dc.relation.issn","0004-6361"],["dc.title","Contribution of flows around active regions to the north-south helioseismic travel-time measurements"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","A6"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.volume","664"],["dc.contributor.author","Fournier, Damien"],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Hyest, Laura"],["dc.date.accessioned","2022-09-01T09:50:15Z"],["dc.date.available","2022-09-01T09:50:15Z"],["dc.date.issued","2022"],["dc.description.abstract","Context.\n In a previous paper, we studied the effect of latitudinal rotation on solar equatorial Rossby modes in the\n β\n -plane approximation. Since then, a rich spectrum of inertial modes has been observed on the Sun, which is not limited to the equatorial Rossby modes and includes high-latitude modes.\n \n \n Aims.\n Here we extend the computation of toroidal modes in 2D to spherical geometry using realistic solar differential rotation and including viscous damping. The aim is to compare the computed mode spectra with the observations and to study mode stability.\n \n \n Methods.\n At a fixed radius, we solved the eigenvalue problem numerically using a spherical harmonics decomposition of the velocity stream function.\n \n \n Results.\n Due to the presence of viscous critical layers, the spectrum consists of four different families: Rossby modes, high-latitude modes, critical-latitude modes, and strongly damped modes. For each longitudinal wavenumber\n m\n  ≤ 3, up to three Rossby-like modes are present on the sphere, in contrast to the equatorial\n β\n plane where only the equatorial Rossby mode is present. The least damped modes in the model have eigenfrequencies and eigenfunctions that resemble the observed modes; the comparison improves when the radius is taken in the lower half of the convection zone. For radii above 0.75 \n R\n ⊙\n and Ekman numbers\n E\n  < 10\n −4\n , at least one mode is unstable. For either\n m\n  = 1 or\n m\n  = 2, up to two Rossby modes (one symmetric and one antisymmetric) are unstable when the radial dependence of the Ekman number follows a quenched diffusivity model (\n E\n  ≈ 2 × 10\n −5\n at the base of the convection zone). For\n m\n  = 3, up to two Rossby modes can be unstable, including the equatorial Rossby mode.\n \n \n Conclusions.\n Although the 2D model discussed here is highly simplified, the spectrum of toroidal modes appears to include many of the observed solar inertial modes. The self-excited modes in the model have frequencies close to those of the observed modes with the largest amplitudes."],["dc.identifier.doi","10.1051/0004-6361/202243473"],["dc.identifier.pii","aa43473-22"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/113660"],["dc.notes.intern","DOI-Import GROB-597"],["dc.relation.eissn","1432-0746"],["dc.relation.issn","0004-6361"],["dc.title","Viscous inertial modes on a differentially rotating sphere: Comparison with solar observations"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A111"],["dc.bibliographiccitation.firstpage","1"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.lastpage","9"],["dc.bibliographiccitation.volume","599"],["dc.contributor.author","Nagashima, Kaori"],["dc.contributor.author","Fournier, Damien"],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Gizon, Laurent"],["dc.date.accessioned","2017-09-07T11:48:41Z"],["dc.date.available","2017-09-07T11:48:41Z"],["dc.date.issued","2017"],["dc.description.abstract","Context. In time-distance helioseismology, wave travel times are measured from the two-point cross-covariance function of solar oscillations and are used to image the solar convection zone in three dimensions. There is, however, also information in the amplitude of the cross-covariance function, for example, about seismic wave attenuation. Aims. We develop a convenient procedure to measure the amplitude of the cross-covariance function of solar oscillations. Methods. In this procedure, the amplitude of the cross-covariance function is linearly related to the cross-covariance function and can be measured even for high levels of noise. Results. As an example application, we measure the amplitude perturbations of the seismic waves that propagate through the sunspot in active region NOAA 9787. We can recover the amplitude variations due to the scattering and attenuation of the waves by the sunspot and associated finite-wavelength effects. Conclusions. The proposed definition of cross-covariance amplitude is robust to noise, can be used to relate measured amplitudes to 3D perturbations in the solar interior under the Born approximation, and provides independent information from the travel times."],["dc.identifier.doi","10.1051/0004-6361/201629846"],["dc.identifier.gro","3147023"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4762"],["dc.language.iso","en"],["dc.notes.status","final"],["dc.notes.submitter","chake"],["dc.relation.issn","0004-6361"],["dc.title","The amplitude of the cross-covariance function of solar oscillations as a diagnostic tool for wave attenuation and geometrical spreading"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","no"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","1"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.lastpage","23"],["dc.bibliographiccitation.volume","600"],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Barucq, Hélène"],["dc.contributor.author","Duruflé, Marc"],["dc.contributor.author","Hanson, Chris"],["dc.contributor.author","Leguèbe, Michael"],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Chabassier, Juliette"],["dc.contributor.author","Fournier, Damien"],["dc.contributor.author","Hohage, Thorsten"],["dc.contributor.author","Papini, Emanuele"],["dc.date.accessioned","2017-09-07T11:52:31Z"],["dc.date.available","2017-09-07T11:52:31Z"],["dc.date.issued","2017"],["dc.description.abstract","Context. Local helioseismology has so far relied on semi-analytical methods to compute the spatial sensitivity of wave travel times to perturbations in the solar interior. These methods are cumbersome and lack flexibility. Aims. Here we propose a convenient framework for numerically solving the forward problem of time-distance helioseismology in the frequency domain. The fundamental quantity to be computed is the cross-covariance of the seismic wavefield. Methods. We choose sources of wave excitation that enable us to relate the cross-covariance of the oscillations to the Green’s function in a straightforward manner. We illustrate the method by considering the 3D acoustic wave equation in an axisymmetric reference solar model, ignoring the effects of gravity on the waves. The symmetry of the background model around the rotation axis implies that the Green’s function can be written as a sum of longitudinal Fourier modes, leading to a set of independent 2D problems. We use a high-order finite-element method to solve the 2D wave equation in frequency space. The computation is embarrassingly parallel, with each frequency and each azimuthal order solved independently on a computer cluster. Results. We compute travel-time sensitivity kernels in spherical geometry for flows, sound speed, and density perturbations under the first Born approximation. Convergence tests show that travel times can be computed with a numerical precision better than one millisecond, as required by the most precise travel-time measurements. Conclusions. The method presented here is computationally efficient and will be used to interpret travel-time measurements in order to infer, e.g., the large-scale meridional flow in the solar convection zone. It allows the implementation of (full-waveform) iterative inversions, whereby the axisymmetric background model is updated at each iteration."],["dc.identifier.doi","10.1051/0004-6361/201629470"],["dc.identifier.gro","3146350"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/15002"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4118"],["dc.language.iso","en"],["dc.notes.intern","In goescholar not merged with http://resolver.sub.uni-goettingen.de/purl?gs-1/15003 but duplicate"],["dc.notes.status","final"],["dc.relation","info:eu-repo/grantAgreement/EC/FP7/312844/EU//SPACEINN"],["dc.relation.orgunit","Institut für Numerische und Angewandte Mathematik"],["dc.rights","Goescholar"],["dc.rights.uri","https://goedoc.uni-goettingen.de/licenses"],["dc.title","Computational helioseismology in the frequency domain: acoustic waves in axisymmetric solar models with flows"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","no"],["dspace.entity.type","Publication"]]