Hohage, Thorsten

[["dc.bibliographiccitation.artnumber","137"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.volume","567"],["dc.contributor.author","Fournier, Damien"],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Hohage, Thorsten"],["dc.contributor.author","Birch, Aaron C."],["dc.date.accessioned","2017-09-07T11:52:31Z"],["dc.date.available","2017-09-07T11:52:31Z"],["dc.date.issued","2014"],["dc.description.abstract","Context. In time-distance helioseismology, information about the solar interior is encoded in measurements of travel times between pairs of points on the solar surface. Travel times are deduced from the cross-covariance of the random wave field. Here, we consider travel times and also products of travel times as observables. They contain information about the statistical properties of convection in the Sun. Aims. We derive analytic formulae for the noise covariance matrix of travel times and products of travel times. Methods. The basic assumption of the model is that noise is the result of the stochastic excitation of solar waves, a random process that is stationary and Gaussian. We generalize the existing noise model by dropping the assumption of horizontal spatial homogeneity. Using a recurrence relation, we calculate the noise covariance matrices for the moments of order 4, 6, and 8 of the observed wave field, for the moments of order 2, 3 and 4 of the cross-covariance, and for the moments of order 2, 3 and 4 of the travel times. Results. All noise covariance matrices depend only on the expectation value of the cross-covariance of the observed wave field. For products of travel times, the noise covariance matrix consists of three terms proportional to 1 /T, 1 /T2, and 1 /T3, where T is the duration of the observations. For typical observation times of a few hours, the term proportional to 1 /T2 dominates and Cov [ τ1τ2,τ3τ4 ] ≈ Cov [ τ1,τ3 ] Cov [ τ2,τ4 ] + Cov [ τ1,τ4 ] Cov [ τ2,τ3 ], where the τi are arbitrary travel times. This result is confirmed for p1 travel times by Monte Carlo simulations and comparisons with SDO/HMI observations. Conclusions. General and accurate formulae have been derived to model the noise covariance matrix of helioseismic travel times and products of travel times. These results could easily be generalized to other methods of local helioseismology, such as helioseismic holography and ring diagram analysis."],["dc.identifier.doi","10.1051/0004-6361/201423580"],["dc.identifier.fs","609657"],["dc.identifier.gro","3146347"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/10928"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4115"],["dc.language.iso","en"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","final"],["dc.notes.submitter","chake"],["dc.relation.orgunit","Institut für Numerische und Angewandte Mathematik"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Generalization of the noise model for time-distance helioseismology"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","no"],["dc.type.version","published_version"],["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"]]

[["dc.bibliographiccitation.firstpage","19"],["dc.bibliographiccitation.journal","Solar Phys"],["dc.bibliographiccitation.lastpage","33"],["dc.bibliographiccitation.volume","276"],["dc.contributor.author","Jackiewicz, J."],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Hanasoge, S."],["dc.contributor.author","Hohage, Thorsten"],["dc.contributor.author","Ruffio, J.B."],["dc.contributor.author","Švanda, Michal"],["dc.date.accessioned","2017-09-07T11:52:57Z"],["dc.date.available","2017-09-07T11:52:57Z"],["dc.date.issued","2012"],["dc.identifier.doi","10.1007/s11207-011-9873-8"],["dc.identifier.fs","596690"],["dc.identifier.gro","3146386"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/7596"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4155"],["dc.notes","This study was supported by the European Research Council under the European\r\nCommunity’s Seventh Framework Programme (FP7/2007–2013)/ERC grant agreement #210949, “Seismic\r\nImaging of the Solar Interior,” to PI L. Gizon (progress toward Milestone #5). Additional support from the\r\nGerman Aerospace Center (DLR) through project “German Data Center for SDO” is acknowledged. ACB\r\nacknowledges support from NASA contracts NNH09CF68C, NNH07CD25C, and NNH09CE41C. MŠ acknowledges\r\npartial support from the Grant Agency of the Academy of Sciences of the Czech Republic under\r\ngrant IAA30030808."],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","chake"],["dc.relation","info:eu-repo/grantAgreement/EC/FP7/210949/EU//SISI"],["dc.relation.orgunit","Institut für Numerische und Angewandte Mathematik"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Multichannel Three-dimensional OLA Inversion for Local Helioseismology Solar Physics"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","no"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","1"],["dc.bibliographiccitation.issue","10"],["dc.bibliographiccitation.journal","Inverse Problems"],["dc.bibliographiccitation.lastpage","27"],["dc.bibliographiccitation.volume","32"],["dc.contributor.author","Fournier, Damien"],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Holzke, Martin"],["dc.contributor.author","Hohage, Thorsten"],["dc.date.accessioned","2017-09-07T11:52:31Z"],["dc.date.available","2017-09-07T11:52:31Z"],["dc.date.issued","2016"],["dc.identifier.doi","10.1088/0266-5611/32/10/105002"],["dc.identifier.gro","3146348"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4116"],["dc.notes.status","public"],["dc.notes.submitter","chake"],["dc.relation.orgunit","Institut für Numerische und Angewandte Mathematik"],["dc.title","Pinsker estimators for local helioseismology: inversion of travel times for mass-conserving flows"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","no"],["dspace.entity.type","Publication"]]