Schou, Jesper

[["dc.bibliographiccitation.firstpage","A44"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","634"],["dc.contributor.author","Proxauf, B."],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Löptien, Björn"],["dc.contributor.author","Schou, Jesper"],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Bogart, R. S."],["dc.date.accessioned","2020-12-10T18:11:54Z"],["dc.date.available","2020-12-10T18:11:54Z"],["dc.date.issued","2020"],["dc.identifier.doi","10.1051/0004-6361/201937007"],["dc.identifier.eissn","1432-0746"],["dc.identifier.issn","0004-6361"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/74179"],["dc.notes.intern","DOI Import GROB-354"],["dc.title","Exploring the latitude and depth dependence of solar Rossby waves using ring-diagram analysis"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["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","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.firstpage","A181"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","635"],["dc.contributor.author","Böning, Vincent G. A."],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Duvall, Thomas L."],["dc.contributor.author","Schou, Jesper"],["dc.date.accessioned","2020-12-10T18:11:55Z"],["dc.date.available","2020-12-10T18:11:55Z"],["dc.date.issued","2020"],["dc.identifier.doi","10.1051/0004-6361/201937331"],["dc.identifier.eissn","1432-0746"],["dc.identifier.issn","0004-6361"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/74186"],["dc.notes.intern","DOI Import GROB-354"],["dc.title","Characterizing the spatial pattern of solar supergranulation using the bispectrum"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","568"],["dc.bibliographiccitation.issue","7"],["dc.bibliographiccitation.journal","Nature Astronomy"],["dc.bibliographiccitation.lastpage","573"],["dc.bibliographiccitation.volume","2"],["dc.contributor.author","Löptien, Björn"],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Schou, Jesper"],["dc.contributor.author","Proxauf, Bastian"],["dc.contributor.author","Duvall, Thomas L."],["dc.contributor.author","Bogart, Richard S."],["dc.contributor.author","Christensen, Ulrich R."],["dc.date.accessioned","2020-05-18T14:29:12Z"],["dc.date.available","2020-05-18T14:29:12Z"],["dc.date.issued","2018"],["dc.identifier.doi","10.1038/s41550-018-0460-x"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/65540"],["dc.relation.issn","2397-3366"],["dc.title","Global-scale equatorial Rossby waves as an essential component of solar internal dynamics"],["dc.type","journal_article"],["dc.type.internalPublication","unknown"],["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.firstpage","A109"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","633"],["dc.contributor.author","Nagashima, Kaori"],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Schou, Jesper"],["dc.contributor.author","Hindman, Bradley W."],["dc.contributor.author","Gizon, Laurent"],["dc.date.accessioned","2020-12-10T18:11:53Z"],["dc.date.available","2020-12-10T18:11:53Z"],["dc.date.issued","2020"],["dc.identifier.doi","10.1051/0004-6361/201936662"],["dc.identifier.eissn","1432-0746"],["dc.identifier.issn","0004-6361"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/74174"],["dc.notes.intern","DOI Import GROB-354"],["dc.title","An improved multi-ridge fitting method for ring-diagram helioseismic analysis"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","251"],["dc.bibliographiccitation.issue","1"],["dc.bibliographiccitation.journal","Space Science Reviews"],["dc.bibliographiccitation.lastpage","283"],["dc.bibliographiccitation.volume","196"],["dc.contributor.author","Löptien, Björn"],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Schou, Jesper"],["dc.contributor.author","Appourchaux, Thierry"],["dc.contributor.author","Blanco Rodríguez, Julián"],["dc.contributor.author","Cally, Paul S."],["dc.contributor.author","Dominguez-Tagle, Carlos"],["dc.contributor.author","Gandorfer, Achim"],["dc.contributor.author","Hill, Frank"],["dc.contributor.author","Hirzberger, Johann"],["dc.contributor.author","Scherrer, Philip H."],["dc.contributor.author","Solanki, Sami K."],["dc.date.accessioned","2017-09-07T11:48:41Z"],["dc.date.available","2017-09-07T11:48:41Z"],["dc.date.issued","2014"],["dc.description.abstract","The Solar Orbiter mission, to be launched in July 2017, will carry a suite of remote sensing and in-situ instruments, including the Polarimetric and Helioseismic Imager (PHI). PHI will deliver high-cadence images of the Sun in intensity and Doppler velocity suitable for carrying out novel helioseismic studies. The orbit of the Solar Orbiter spacecraft will reach a solar latitude of up to 21∘ (up to 34∘ by the end of the extended mission) and thus will enable the first local helioseismology studies of the polar regions. Here we consider an array of science objectives to be addressed by helioseismology within the baseline telemetry allocation (51 Gbit per orbit, current baseline) and within the science observing windows (baseline 3×10 days per orbit). A particularly important objective is the measurement of large-scale flows at high latitudes (rotation and meridional flow), which are largely unknown but play an important role in flux transport dynamos. For both helioseismology and feature tracking methods convection is a source of noise in the measurement of longitudinally averaged large-scale flows, which decreases as T−1/2 where T is the total duration of the observations. Therefore, the detection of small amplitude signals (e.g., meridional circulation, flows in the deep solar interior) requires long observation times. As an example, one hundred days of observations at lower spatial resolution would provide a noise level of about three m/s on the meridional flow at 80∘ latitude. Longer time-series are also needed to study temporal variations with the solar cycle. The full range of Earth-Sun-spacecraft angles provided by the orbit will enable helioseismology from two vantage points by combining PHI with another instrument: stereoscopic helioseismology will allow the study of the deep solar interior and a better understanding of the physics of solar oscillations in both quiet Sun and sunspots. We have used a model of the PHI instrument to study its performance for helioseismology applications. As input we used a 6 hr time-series of realistic solar magneto-convection simulation (Stagger code) and the SPINOR radiative transfer code to synthesize the observables. The simulated power spectra of solar oscillations show that the instrument is suitable for helioseismology. In particular, the specified point spread function, image jitter, and photon noise are no obstacle to a successful mission."],["dc.identifier.doi","10.1007/s11214-014-0065-3"],["dc.identifier.gro","3147019"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4760"],["dc.language.iso","en"],["dc.notes.status","public"],["dc.notes.submitter","chake"],["dc.relation.issn","0038-6308"],["dc.title","Helioseismology with Solar Orbiter"],["dc.type","journal_article"],["dc.type.internalPublication","unknown"],["dc.type.peerReviewed","no"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A46"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","601"],["dc.contributor.author","Liang, Zhi-Chao"],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Duvall, Thomas L."],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Schou, Jesper"],["dc.date.accessioned","2018-11-07T10:24:04Z"],["dc.date.available","2018-11-07T10:24:04Z"],["dc.date.issued","2017"],["dc.description.abstract","Context. Time-distance helioseismology is one of the primary tools for studying the solar meridional circulation, especially in the lower convection zone. However, travel-time measurements of the subsurface meridional flow suffer from a variety of systematic errors, such as a center-to-limb variation and an off set due to the position angle (P-angle) uncertainty of solar images. It has been suggested that the center-to-limb variation can be removed by subtracting east-west from south-north travel-time measurements. This ad hoc method for the removal of the center-to-limb effect has been adopted widely but not tested for travel distances corresponding to the lower convection zone. Aims. We explore the effects of two major sources of the systematic errors, the P-angle error arising from the instrumental misalignment and the center-to-limb variation, on the acoustic travel-time measurements in the south-north direction. Methods. We apply the time-distance technique to contemporaneous medium-degree Dopplergrams produced by SOHO/MDI and SDO/HMI to obtain the travel-time difference caused by meridional circulation throughout the solar convection zone. The P-angle off set in MDI images is measured by cross-correlating MDI and HMI images. The travel-time measurements in the south-north and east-west directions are averaged over the same observation period (May 2010 to Apr. 2011) for the two data sets and then compared to examine the consistency of MDI and HMI travel times after applying the above-mentioned corrections. Results. The off sets in the south-north travel-time difference from MDI data induced by the P-angle error gradually diminish with increasing travel distance. However, these off sets become noisy for travel distances corresponding to waves that reach the base of the convection zone. This suggests that a careful treatment of the P-angle problem is required when studying a deep meridional flow. After correcting the P-angle and the removal of the center-to-limb effect, the travel-time measurements from MDI and HMI are consistent within the error bars for meridional circulation covering the entire convection zone. The fluctuations observed in both data sets are highly correlated and thus indicate their solar origin rather than an instrumental origin. Although our results demonstrate that the ad hoc correction is capable of reducing the wide discrepancy in the travel-time measurements from MDI and HMI, we cannot exclude the possibility that there exist other systematic effects acting on the two data sets in the same way."],["dc.identifier.doi","10.1051/0004-6361/201730416"],["dc.identifier.isi","000402313500046"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/42589"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","PUB_WoS_Import"],["dc.relation.issn","1432-0746"],["dc.title","Comparison of acoustic travel-time measurements of solar meridional circulation from SDO/HMI and SOHO/MDI"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dspace.entity.type","Publication"]]