Schunker, Hannah

[["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.firstpage","A183"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.volume","664"],["dc.contributor.author","Baumgartner, C."],["dc.contributor.author","Birch, A. C."],["dc.contributor.author","Schunker, H."],["dc.contributor.author","Cameron, R. H."],["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 The twist of the magnetic field above a sunspot is an important quantity in solar physics. For example, magnetic twist plays a role in the initiation of flares and coronal mass ejections (CMEs). Various proxies for the twist above the photosphere have been found using models of uniformly twisted flux tubes, and are routinely computed from single photospheric vector magnetograms. One class of proxies is based on\n α\n \n z\n \n , the ratio of the vertical current to the vertical magnetic field. Another class of proxies is based on the so-called twist density,\n q\n , which depends on the ratio of the azimuthal field to the vertical field. However, the sensitivity of these proxies to temporal fluctuations of the magnetic field has not yet been well characterized.\n \n \n Aims.\n We aim to determine the sensitivity of twist proxies to temporal fluctuations in the magnetic field as estimated from time-series of SDO/HMI vector magnetic field maps.\n \n \n Methods.\n To this end, we introduce a model of a sunspot with a peak vertical field of 2370 Gauss at the photosphere and a uniform twist density\n q\n  = −0.024 Mm\n −1\n . We add realizations of the temporal fluctuations of the magnetic field that are consistent with SDO/HMI observations, including the spatial correlations. Using a Monte-Carlo approach, we determine the robustness of the different proxies to the temporal fluctuations.\n \n \n Results.\n The temporal fluctuations of the three components of the magnetic field are correlated for spatial separations up to 1.4 Mm (more than expected from the point spread function alone). The Monte-Carlo approach enables us to demonstrate that several proxies for the twist of the magnetic field are not biased in each of the individual magnetograms. The associated random errors on the proxies have standard deviations in the range between 0.002 and 0.006 Mm\n −1\n , which is smaller by approximately one order of magnitude than the mean value of\n q\n ."],["dc.identifier.doi","10.1051/0004-6361/202243357"],["dc.identifier.pii","aa43357-22"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/114646"],["dc.notes.intern","DOI-Import GROB-600"],["dc.relation.eissn","1432-0746"],["dc.relation.issn","0004-6361"],["dc.title","Impact of spatially correlated fluctuations in sunspots on metrics related to magnetic twist"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","93"],["dc.bibliographiccitation.journal","Communications in Asteroseismology"],["dc.bibliographiccitation.lastpage","106"],["dc.bibliographiccitation.volume","156"],["dc.contributor.author","Schunker, Hannah"],["dc.contributor.author","Gizon, Laurent"],["dc.date.accessioned","2021-03-10T10:32:26Z"],["dc.date.available","2021-03-10T10:32:26Z"],["dc.date.issued","2009"],["dc.identifier.doi","10.1553/cia156s93"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/80503"],["dc.relation.issn","1021-2043"],["dc.title","HELAS Local Helioseismology Activities"],["dc.type","journal_article"],["dc.type.internalPublication","unknown"],["dc.type.subtype","original_ja"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","1"],["dc.bibliographiccitation.issue","1-2"],["dc.bibliographiccitation.journal","Solar Physics"],["dc.bibliographiccitation.lastpage","26"],["dc.bibliographiccitation.volume","271"],["dc.contributor.author","Schunker, H."],["dc.contributor.author","Cameron, R. H."],["dc.contributor.author","Gizon, L."],["dc.contributor.author","Moradi, H."],["dc.date.accessioned","2017-09-07T11:48:42Z"],["dc.date.available","2017-09-07T11:48:42Z"],["dc.date.issued","2011"],["dc.description.abstract","In local helioseismology, numerical simulations of wave propagation are useful to model the interaction of solar waves with perturbations to a background solar model. However, the solution to the linearised equations of motion include convective modes that can swamp the helioseismic waves that we are interested in. In this article, we construct background solar models that are stable against convection, by modifying the vertical pressure gradient of Model S (Christensen-Dalsgaard et al., 1996, Science 272, 1286) relinquishing hydrostatic equilibrium. However, the stabilisation affects the eigenmodes that we wish to remain as close to Model S as possible. In a bid to recover the Model S eigenmodes, we choose to make additional corrections to the sound speed of Model S before stabilisation. No stabilised model can be perfectly solar-like, so we present three stabilised models with slightly different eigenmodes. The models are appropriate to study the f and p 1 to p 4 modes with spherical harmonic degrees in the range from 400 to 900. Background model CSM has a modified pressure gradient for stabilisation and has eigenfrequencies within 2% of Model S. Model CSM_A has an additional 10% increase in sound speed in the top 1 Mm resulting in eigenfrequencies within 2% of Model S and eigenfunctions that are, in comparison with CSM, closest to those of Model S. Model CSM_B has a 3% decrease in sound speed in the top 5 Mm resulting in eigenfrequencies within 1% of Model S and eigenfunctions that are only marginally adversely affected. These models are useful to study the interaction of solar waves with embedded three-dimensional heterogeneities, such as convective flows and model sunspots. We have also calculated the response of the stabilised models to excitation by random near-surface sources, using simulations of the propagation of linear waves. We find that the simulated power spectra of wave motion are in good agreement with an observed SOHO/MDI power spectrum. Overall, our convectively stabilised background models provide a good basis for quantitative numerical local helioseismology. The models are available for download from http://www.mps.mpg.de/projects/seismo/NA4/ ."],["dc.identifier.doi","10.1007/s11207-011-9790-x"],["dc.identifier.gro","3147041"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/7173"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4773"],["dc.language.iso","en"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","final"],["dc.notes.submitter","chake"],["dc.relation.issn","0038-0938"],["dc.relation.orgunit","Wirtschaftswissenschaftliche Fakultät"],["dc.rights","Goescholar"],["dc.rights.uri","https://goescholar.uni-goettingen.de/licenses"],["dc.title","Constructing and Characterising Solar Structure Models for Computational 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","A129"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.volume","558"],["dc.contributor.author","Liang, Z.-C."],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Schunker, H."],["dc.contributor.author","Philippe, T."],["dc.date.accessioned","2017-09-07T11:48:42Z"],["dc.date.available","2017-09-07T11:48:42Z"],["dc.date.issued","2013"],["dc.description.abstract","We observe and characterize the scattering of acoustic wave packets by a sunspot in a regime where the wavelength is comparable to the size of the sunspot. Spatial maps of wave travel times and amplitudes are measured from the cross-covariance function of the random wave field observed by SOHO/MDI around the sunspot in active region NOAO 9787. We consider separately incoming plane wave packets consisting of f modes and p modes with radial orders up to four. Observations show that the travel-time perturbations diminish with distance far away from the sunspot – a finite-wavelength phenomenon known as wavefront healing in scattering theory. Observations also show a reduction of the amplitude of the waves after their passage through the sunspot. We suggest that a significant fraction of this amplitude reduction is due to the defocusing of wave energy by the fast wave-speed perturbation introduced by the sunspot. This “geometrical attenuation” will contribute to the wave amplitude reduction in addition to the physical absorption of waves by sunspots. We also observe an enhancement of wave amplitude away from the central path: diffracted rays intersect with unperturbed rays (caustics) and wavefronts fold and triplicate. Wave amplitude measurements in time-distance helioseismology provide independent information that can be used in concert with travel-time measurements."],["dc.identifier.doi","10.1051/0004-6361/201321483"],["dc.identifier.fs","602756"],["dc.identifier.gro","3147013"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/10880"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4757"],["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/FP7/210949/EU//SISI"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Helioseismology of sunspots: defocusing, folding, and healing of wavefronts"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","no"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.issue","7"],["dc.bibliographiccitation.journal","Science Advances"],["dc.bibliographiccitation.volume","2"],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Schunker, H."],["dc.contributor.author","Braun, D. C."],["dc.contributor.author","Cameron, R. H."],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Löptien, Björn"],["dc.contributor.author","Rempel, M."],["dc.date.accessioned","2017-09-07T11:49:43Z"],["dc.date.available","2017-09-07T11:49:43Z"],["dc.date.issued","2016"],["dc.description.abstract","Magnetic field emerges at the surface of the Sun as sunspots and active regions. This process generates a poloidal magnetic field from a rising toroidal flux tube; it is a crucial but poorly understood aspect of the solar dynamo. The emergence of magnetic field is also important because it is a key driver of solar activity. We show that measurements of horizontal flows at the solar surface around emerging active regions, in combination with numerical simulations of solar magnetoconvection, can constrain the subsurface rise speed of emerging magnetic flux. The observed flows imply that the rise speed of the magnetic field is no larger than 150 m/s at a depth of 20 Mm, that is, well below the prediction of the (standard) thin flux tube model but in the range expected for convective velocities at this depth. We conclude that convective flows control the dynamics of rising flux tubes in the upper layers of the Sun and cannot be neglected in models of flux emergence."],["dc.identifier.doi","10.1126/sciadv.1600557"],["dc.identifier.gro","3147404"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4994"],["dc.language.iso","en"],["dc.notes.status","final"],["dc.notes.submitter","chake"],["dc.relation.issn","2375-2548"],["dc.title","A low upper limit on the subsurface rise speed of solar active regions"],["dc.type","journal_article"],["dc.type.internalPublication","unknown"],["dc.type.peerReviewed","no"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A107"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.volume","595"],["dc.contributor.author","Schunker, H."],["dc.contributor.author","Braun, D. C."],["dc.contributor.author","Birch, A. C."],["dc.contributor.author","Burston, R. B."],["dc.contributor.author","Gizon, L."],["dc.date.accessioned","2017-09-07T11:49:59Z"],["dc.date.available","2017-09-07T11:49:59Z"],["dc.date.issued","2016"],["dc.description.abstract","Context. Observations from the Solar Dynamics Observatory (SDO) have the potential for allowing the helioseismic study of the formation of hundreds of active regions, which would enable us to perform statistical analyses. Aims. Our goal is to collate a uniform data set of emerging active regions observed by the SDO/HMI instrument suitable for helioseismic analysis, where each active region is centred on a 60° × 60° area and can be observed up to seven days before emergence. Methods. We restricted the sample to active regions that were visible in the continuum and emerged into quiet Sun largely avoiding pre-existing magnetic regions. As a reference data set we paired a control region (CR), with the same latitude and distance from central meridian, with each emerging active region (EAR). The control regions do not have any strong emerging flux within 10° of the centre of the map. Each region was tracked at the Carrington rotation rate as it crossed the solar disk, within approximately 65° from the central meridian and up to seven days before, and seven days after, emergence. The mapped and tracked data, consisting of line-of-sight velocity, line-of-sight magnetic field, and intensity as observed by SDO/HMI, are stored in datacubes that are 410 min in duration and spaced 320 min apart. We call this data set, which is currently comprised of 105 emerging active regions observed between May 2010 and November 2012, the SDO Helioseismic Emerging Active Region (SDO/HEAR) survey. Results. To demonstrate the utility of a data set of a large number of emerging active regions, we measure the relative east-west velocity of the leading and trailing polarities from the line-of-sight magnetogram maps during the first day after emergence. The latitudinally averaged line-of-sight magnetic field of all the EARs shows that, on average, the leading (trailing) polarity moves in a prograde (retrograde) direction with a speed of 121 ± 22 m s-1 (−70 ± 13 m s-1) relative to the Carrington rotation rate in the first day. However, relative to the differential rotation of the surface plasma, the east-west velocity is symmetric, with a mean of 95 ± 13 m s-1. Conclusions. The SDO/HEAR data set will not only be useful for helioseismic studies, but will also be useful to study other features such as the surface magnetic field evolution of a large sample of EARs. We intend to extend this survey forwards in time to include more EARs observed by SDO/HMI."],["dc.identifier.doi","10.1051/0004-6361/201628388"],["dc.identifier.gro","3147477"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/14279"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/5026"],["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","SDO/HMI survey of emerging active regions for 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","A116"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","640"],["dc.contributor.author","Schunker, Hannah"],["dc.contributor.author","Baumgartner, C."],["dc.contributor.author","Birch, A. C."],["dc.contributor.author","Cameron, R. H."],["dc.contributor.author","Braun, D. C."],["dc.contributor.author","Gizon, Laurent"],["dc.date.accessioned","2021-03-05T08:58:37Z"],["dc.date.available","2021-03-05T08:58:37Z"],["dc.date.issued","2020"],["dc.identifier.doi","10.1051/0004-6361/201937322"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/80198"],["dc.notes.intern","DOI Import GROB-393"],["dc.relation.eissn","1432-0746"],["dc.relation.issn","0004-6361"],["dc.title","Average motion of emerging solar active region polarities"],["dc.title.alternative","II. Joy’s law"],["dc.type","journal_article"],["dc.type.internalPublication","unknown"],["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","012087"],["dc.bibliographiccitation.journal","Journal of Physics. Conference Series"],["dc.bibliographiccitation.volume","118"],["dc.contributor.author","Schunker, Hannah"],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Roth, Markus"],["dc.date.accessioned","2021-03-10T10:32:30Z"],["dc.date.available","2021-03-10T10:32:30Z"],["dc.date.issued","2008"],["dc.identifier.doi","10.1088/1742-6596/118/1/012087"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/80504"],["dc.relation.issn","1742-6596"],["dc.title","HELAS: local helioseismology data website"],["dc.type","journal_article"],["dc.type.internalPublication","unknown"],["dc.type.subtype","original_ja"],["dspace.entity.type","Publication"]]