Schou, Jesper

[["dc.bibliographiccitation.artnumber","A79"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.volume","586"],["dc.contributor.author","Schunker, H."],["dc.contributor.author","Schou, J."],["dc.contributor.author","Ball, Warrick H."],["dc.contributor.author","Nielsen, M. B."],["dc.contributor.author","Gizon, Laurent"],["dc.date.accessioned","2017-09-07T11:48:42Z"],["dc.date.available","2017-09-07T11:48:42Z"],["dc.date.issued","2015"],["dc.description.abstract","Context. Radial differential rotation is an important parameter for stellar dynamo theory and for understanding angular momentum transport.Aims. We investigate the potential of using a large number of similar stars simultaneously to constrain their average radial differential rotation gradient: we call this “ensemble fitting”.Methods. We use a range of stellar models along the main sequence, each with a synthetic rotation profile. The rotation profiles are step functions with a step of ΔΩ = −0.35 μHz, which is located at the base of the convection zone. These models are used to compute the rotational splittings of the p modes and to model their uncertainties. We then fit an ensemble of stars to infer the average ΔΩ.Results. All the uncertainties on the inferred ΔΩ for individual stars are of the order 1 μHz. Using 15 stellar models in an ensemble fit, we show that the uncertainty on the average ΔΩ is reduced to less than the input ΔΩ, which allows us to constrain the sign of the radial differential rotation. We show that a solar-like ΔΩ ≈ 30 nHz can be constrained by an ensemble fit of thousands of main-sequence stars. Observing the number of stars required to successfully exploit the ensemble fitting method will be possible with future asteroseismology missions, such as PLATO. We demonstrate the potential of ensemble fitting by showing that any systematic differences in the average ΔΩ between F, G, and K-type stars larger than 100 nHz could be detected."],["dc.identifier.doi","10.1051/0004-6361/201527485"],["dc.identifier.gro","3147043"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/13440"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4775"],["dc.language.iso","en"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","public"],["dc.notes.submitter","chake"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Asteroseismic inversions for radial differential rotation of Sun-like stars: ensemble fits"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","no"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A9"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.volume","587"],["dc.contributor.author","Löptien, Björn"],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Duvall, Thomas L."],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Schou, J."],["dc.date.accessioned","2017-09-07T11:49:58Z"],["dc.date.available","2017-09-07T11:49:58Z"],["dc.date.issued","2016"],["dc.description.abstract","Context. Several upcoming and proposed space missions, such as Solar Orbiter, will be limited in telemetry and thus require data compression.Aims. We test the impact of data compression on local correlation tracking (LCT) of time series of continuum intensity images. We evaluate the effect of several lossy compression methods (quantization, JPEG compression, and a reduced number of continuum images) on measurements of solar differential rotation with LCT.Methods. We applied the different compression methods to tracked and remapped continuum intensity maps obtained by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory. We derived 2D vector velocities using the local correlation tracking code Fourier Local Correlation Tracking (FLCT) and determined the additional bias and noise introduced by compression to differential rotation.Results. We find that probing differential rotation with LCT is very robust to lossy data compression when using quantization. Our results are severely affected by systematic errors of the LCT method and the HMI instrument. The sensitivity of LCT to systematic errors is a concern for Solar Orbiter."],["dc.identifier.doi","10.1051/0004-6361/201526805"],["dc.identifier.gro","3147451"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/13430"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/5015"],["dc.language.iso","en"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","public"],["dc.notes.submitter","chake"],["dc.relation","info:eu-repo/grantAgreement/EC/FP7/ 312844"],["dc.relation","info:eu-repo/grantAgreement/EC/FP7/ 312495"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Data compression for local correlation tracking of solar granulation"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","no"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A8"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.volume","595"],["dc.contributor.author","Barekat, A."],["dc.contributor.author","Schou, J."],["dc.contributor.author","Gizon, Laurent"],["dc.date.accessioned","2017-09-07T11:49:43Z"],["dc.date.available","2017-09-07T11:49:43Z"],["dc.date.issued","2016"],["dc.identifier.doi","10.1051/0004-6361/201628673"],["dc.identifier.gro","3147399"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/14280"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4992"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","public"],["dc.notes.submitter","chake"],["dc.publisher","EDP Sciences"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Solar-cycle variation of the rotational shear near the solar surface"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","no"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A42"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.volume","571"],["dc.contributor.author","Löptien, Björn"],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Schou, J."],["dc.date.accessioned","2017-09-07T11:48:41Z"],["dc.date.available","2017-09-07T11:48:41Z"],["dc.date.issued","2014"],["dc.description.abstract","Context. Several upcoming helioseismology space missions are very limited in telemetry and will have to perform extensive data compression. This requires the development of new methods of data compression.Aims. We give an overview of the influence of lossy data compression on local helioseismology. We investigate the effects of several lossy compression methods (quantization, JPEG compression, and smoothing and subsampling) on power spectra and time-distance measurements of supergranulation flows at disk center.Methods. We applied different compression methods to tracked and remapped Dopplergrams obtained by the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory. We determined the signal-to-noise ratio of the travel times computed from the compressed data as a function of the compression efficiency.Results. The basic helioseismic measurements that we consider are very robust to lossy data compression. Even if only the sign of the velocity is used, time-distance helioseismology is still possible. We achieve the best results by applying JPEG compression on spatially subsampled data. However, our conclusions are only valid for supergranulation flows at disk center and may not be valid for all helioseismology applications."],["dc.identifier.doi","10.1051/0004-6361/201424315"],["dc.identifier.fs","609658"],["dc.identifier.gro","3147017"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/11404"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4759"],["dc.language.iso","en"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","public"],["dc.notes.submitter","chake"],["dc.relation","info:eu-repo/grantAgreement/EC/FP7/312844/EU//SPACEINN"],["dc.relation","info:eu-repo/grantAgreement/EC/FP7/312495/EU//SOLARNET"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Image compression in local helioseismology"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","no"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","L12"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.volume","568"],["dc.contributor.author","Nielsen, M. B."],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Schunker, H."],["dc.contributor.author","Schou, J."],["dc.date.accessioned","2017-09-07T11:48:42Z"],["dc.date.available","2017-09-07T11:48:42Z"],["dc.date.issued","2014"],["dc.identifier.doi","10.1051/0004-6361/201424525"],["dc.identifier.fs","609660"],["dc.identifier.gro","3147028"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/10930"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4765"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","public"],["dc.notes.submitter","chake"],["dc.publisher","EDP Sciences"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Rotational splitting as a function of mode frequency for six Sun-like stars"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","no"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","L12"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.volume","570"],["dc.contributor.author","Barekat, A."],["dc.contributor.author","Schou, J."],["dc.contributor.author","Gizon, Laurent"],["dc.date.accessioned","2017-09-07T11:49:43Z"],["dc.date.available","2017-09-07T11:49:43Z"],["dc.date.issued","2014"],["dc.identifier.doi","10.1051/0004-6361/201424839"],["dc.identifier.fs","609653"],["dc.identifier.gro","3147398"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/11398"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4991"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","public"],["dc.notes.submitter","chake"],["dc.publisher","EDP Sciences"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","The radial gradient of the near-surface shear layer of the Sun"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","no"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A130"],["dc.bibliographiccitation.firstpage","1"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.lastpage","8"],["dc.bibliographiccitation.volume","590"],["dc.contributor.author","Löptien, B."],["dc.contributor.author","Birch, A. C."],["dc.contributor.author","Duvall Jr., T. L."],["dc.contributor.author","Gizon, L."],["dc.contributor.author","Schou, J."],["dc.date.accessioned","2017-09-07T11:48:41Z"],["dc.date.available","2017-09-07T11:48:41Z"],["dc.date.issued","2016"],["dc.description.abstract","Context. Local correlation tracking of granulation (LCT) is an important method for measuring horizontal flows in the photosphere. This method exhibits a systematic error that looks like a flow converging toward disk center, which is also known as the shrinking-Sun effect. Aims. We aim to study the nature of the shrinking-Sun effect for continuum intensity data and to derive a simple model that can explain its origin. Methods. We derived LCT flow maps by running the LCT code Fourier Local Correlation Tracking (FLCT) on tracked and remapped continuum intensity maps provided by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). We also computed flow maps from synthetic continuum images generated from STAGGER code simulations of solar surface convection. We investigated the origin of the shrinking-Sun effect by generating an average granule from synthetic data from the simulations. Results. The LCT flow maps derived from the HMI data and the simulations exhibit a shrinking-Sun effect of comparable magnitude. The origin of this effect is related to the apparent asymmetry of granulation originating from radiative transfer effects when observing with a viewing angle inclined from vertical. This causes, in combination with the expansion of the granules, an apparent motion toward disk center."],["dc.identifier.doi","10.1051/0004-6361/201628112"],["dc.identifier.gro","3147015"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/14286"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4758"],["dc.language.iso","en"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","final"],["dc.notes.submitter","chake"],["dc.relation","info:eu-repo/grantAgreement/EC/FP7/312844/EU/Exploitation of Space Data for Innovative Helio- and Asteroseismology/SPACEINN"],["dc.relation","info:eu-repo/grantAgreement/EC/FP7/312495/EU/High-Resolution Solar Physics Network/SOLARNET"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","The shrinking Sun: A systematic error in local correlation tracking of solar granulation"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","no"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","A28"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","606"],["dc.contributor.author","Löptien, Björn"],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Duvall, Thomas L."],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Proxauf, B."],["dc.contributor.author","Schou, Jesper"],["dc.date.accessioned","2020-12-10T18:11:35Z"],["dc.date.available","2020-12-10T18:11:35Z"],["dc.date.issued","2017"],["dc.identifier.doi","10.1051/0004-6361/201731064"],["dc.identifier.eissn","1432-0746"],["dc.identifier.issn","0004-6361"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/16810"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/74071"],["dc.notes.intern","DOI Import GROB-354"],["dc.notes.intern","Merged from goescholar"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Measuring solar active region inflows with local correlation tracking of granulation"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A24"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","586"],["dc.contributor.author","Schunker, H."],["dc.contributor.author","Schou, Jesper"],["dc.contributor.author","Ball, Warrick H."],["dc.date.accessioned","2018-11-07T10:18:36Z"],["dc.date.available","2018-11-07T10:18:36Z"],["dc.date.issued","2016"],["dc.description.abstract","Aims. We quantify the effect of observational spectroscopic and asteroseismic uncertainties on regularised least squares (RLS) inversions for the radial differential rotation of Sun-like and subgiant stars. Methods. We first solved the forward problem to model rotational splittings plus the observed uncertainties for models of a Sun-like star, HD 52265, and a subgiant star, KIC 7341231. We randomly perturbed the parameters of the stellar models within the uncertainties of the spectroscopic and asteroseismic constraints and used these perturbed stellar models to compute rotational splittings. We experimented with three rotation profiles: solid body rotation, a step function, and a smooth rotation profile decreasing with radius. We then solved the inverse problem to infer the radial differential rotation profile using a RLS inversion and kernels from the best-fit stellar model. We also compared RLS, optimally localised average (OLA) and direct functional fitting inversion techniques. Results. We found that the inversions for Sun-like stars with solar-like radial differential rotation profiles are insensitive to the uncertainties in the stellar models. The uncertainties in the splittings dominate the uncertainties in the inversions and solid body rotation is not excluded. We found that when the rotation rate below the convection zone is increased to six times that of the surface rotation rate the inferred rotation profile excluded solid body rotation. We showed that when we reduced the uncertainties in the splittings by a factor of about 100, the inversion is sensitive to the uncertainties in the stellar model. With the current observational uncertainties, we found that inversions of subgiant stars are sensitive to the uncertainties in the stellar model. Conclusions. Our findings suggest that inversions for the radial differential rotation of subgiant stars would benefit from more tightly constrained stellar models. We conclude that current observational uncertainties make it difficult to infer radially resolved features of the rotation profile in a Sun-like star using inversions with regularisation. In Sun-like stars, the insensitivity of the inversions to stellar model uncertainties suggests that it may be possible to perform ensemble inversions for the average radial differential rotation of many stars with a range of stellar types to better constrain the inversions."],["dc.description.sponsorship","Deutsche Forschungsgemeinschaft (DFG) [SFB 963/1]"],["dc.identifier.doi","10.1051/0004-6361/201525937"],["dc.identifier.isi","000369715900035"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/13439"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/41482"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.relation.issn","1432-0746"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Asteroseismic inversions for radial differential rotation of Sun-like stars: Sensitivity to uncertainties"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A27"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.volume","564"],["dc.contributor.author","Deheuvels, S."],["dc.contributor.author","Doğan, G."],["dc.contributor.author","Goupil, M. J."],["dc.contributor.author","Appourchaux, Thierry"],["dc.contributor.author","Benomar, O."],["dc.contributor.author","Bruntt, H."],["dc.contributor.author","Campante, T. L."],["dc.contributor.author","Casagrande, L."],["dc.contributor.author","Ceillier, T."],["dc.contributor.author","Davies, G. R."],["dc.contributor.author","Cat, P. de"],["dc.contributor.author","Fu, J. N."],["dc.contributor.author","García, R. A."],["dc.contributor.author","Lobel, A."],["dc.contributor.author","Mosser, B."],["dc.contributor.author","Reese, D. R."],["dc.contributor.author","Regulo, C."],["dc.contributor.author","Schou, J."],["dc.contributor.author","Stahn, Thorsten"],["dc.contributor.author","Thygesen, A. O."],["dc.contributor.author","Yang, X. H."],["dc.contributor.author","Chaplin, W. J."],["dc.contributor.author","Christensen-Dalsgaard, J."],["dc.contributor.author","Eggenberger, P."],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Mathur, Saurabh"],["dc.contributor.author","Molenda-Żakowicz, J."],["dc.contributor.author","Pinsonneault, M."],["dc.date.accessioned","2017-09-07T11:49:44Z"],["dc.date.available","2017-09-07T11:49:44Z"],["dc.date.issued","2014"],["dc.identifier.doi","10.1051/0004-6361/201322779"],["dc.identifier.fs","609656"],["dc.identifier.gro","3147416"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/10916"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/5002"],["dc.language.iso","en"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","final"],["dc.notes.submitter","chake"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Seismic constraints on the radial dependence of the internal rotation profiles of sixKeplersubgiants and young red giants"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","no"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]