Schunker, Hannah

[["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","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.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.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.firstpage","293"],["dc.bibliographiccitation.issue","2"],["dc.bibliographiccitation.journal","Solar Physics"],["dc.bibliographiccitation.lastpage","308"],["dc.bibliographiccitation.volume","268"],["dc.contributor.author","Cameron, R. H."],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Schunker, H."],["dc.contributor.author","Pietarila, A."],["dc.date.accessioned","2017-09-07T11:48:39Z"],["dc.date.available","2017-09-07T11:48:39Z"],["dc.date.issued","2011"],["dc.description.abstract","One goal of helioseismology is to determine the subsurface structure of sunspots. In order to do so, it is important to understand first the near-surface effects of sunspots on solar waves, which are dominant. Here we construct simplified, cylindrically-symmetric sunspot models that are designed to capture the magnetic and thermodynamics effects coming from about 500 km below the quiet-Sun τ 5000=1 level to the lower chromosphere. We use a combination of existing semi-empirical models of sunspot thermodynamic structure (density, temperature, pressure): the umbral model of Maltby et al. (1986, Astrophys. J. 306, 284) and the penumbral model of Ding and Fang (1989, Astron. Astrophys. 225, 204). The OPAL equation-of-state tables are used to derive the sound-speed profile. We smoothly merge the near-surface properties to the quiet-Sun values about 1 Mm below the surface. The umbral and penumbral radii are free parameters. The magnetic field is added to the thermodynamic structure, without requiring magnetostatic equilibrium. The vertical component of the magnetic field is assumed to have a Gaussian horizontal profile, with a maximum surface field strength fixed by surface observations. The full magnetic-field vector is solenoidal and determined by the on-axis vertical field, which, at the surface, is chosen such that the field inclination is 45° at the umbral – penumbral boundary. We construct a particular sunspot model based on SOHO/MDI observations of the sunspot in active region NOAA 9787. The helioseismic signature of the model sunspot is studied using numerical simulations of the propagation of f, p 1, and p 2 wave packets. These simulations are compared against cross-covariances of the observed wave field. We find that the sunspot model gives a helioseismic signature that is similar to the observations."],["dc.identifier.doi","10.1007/s11207-010-9631-3"],["dc.identifier.gro","3146973"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4734"],["dc.language.iso","en"],["dc.notes.status","final"],["dc.notes.submitter","chake"],["dc.relation.issn","0038-0938"],["dc.title","Constructing Semi-Empirical Sunspot Models for Helioseismology"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","no"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","A53"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","625"],["dc.contributor.author","Schunker, Hannah"],["dc.contributor.author","Birch, A. C."],["dc.contributor.author","Cameron, R. H."],["dc.contributor.author","Braun, D. C."],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Burston, R. B."],["dc.date.accessioned","2020-12-10T18:11:46Z"],["dc.date.available","2020-12-10T18:11:46Z"],["dc.date.issued","2019"],["dc.identifier.doi","10.1051/0004-6361/201834627"],["dc.identifier.eissn","1432-0746"],["dc.identifier.issn","0004-6361"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/74133"],["dc.notes.intern","DOI Import GROB-354"],["dc.title","Average motion of emerging solar active region polarities"],["dc.title.alternative","I. Two phases of emergence"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","257"],["dc.bibliographiccitation.issue","1-4"],["dc.bibliographiccitation.journal","Space Science Reviews"],["dc.bibliographiccitation.lastpage","258"],["dc.bibliographiccitation.volume","156"],["dc.contributor.author","Gizon, L."],["dc.contributor.author","Schunker, H."],["dc.contributor.author","Baldner, C. S."],["dc.contributor.author","Basu, S."],["dc.contributor.author","Birch, A. C."],["dc.contributor.author","Bogart, R. S."],["dc.contributor.author","Braun, D. C."],["dc.contributor.author","Cameron, R. H."],["dc.contributor.author","Duvall Jr., T. L."],["dc.contributor.author","Hanasoge, S. M."],["dc.contributor.author","Jackiewicz, J."],["dc.contributor.author","Roth, M."],["dc.contributor.author","Stahn, T."],["dc.contributor.author","Thompson, M. J."],["dc.contributor.author","Zharkov, S."],["dc.date.accessioned","2017-09-07T11:48:39Z"],["dc.date.available","2017-09-07T11:48:39Z"],["dc.date.issued","2010"],["dc.identifier.doi","10.1007/s11214-010-9688-1"],["dc.identifier.gro","3146992"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4741"],["dc.language.iso","en"],["dc.notes.status","final"],["dc.notes.submitter","chake"],["dc.relation.eissn","1572-9672"],["dc.relation.iserratumof","/handle/2/4740"],["dc.relation.issn","0038-6308"],["dc.title","Erratum to: Helioseismology of Sunspots: A Case Study of NOAA Region 9787"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","no"],["dc.type.subtype","erratum_ja"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","249"],["dc.bibliographiccitation.issue","1"],["dc.bibliographiccitation.journal","Space Science Reviews"],["dc.bibliographiccitation.lastpage","273"],["dc.bibliographiccitation.volume","144"],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Schunker, H."],["dc.contributor.author","Baldner, C. S."],["dc.contributor.author","Basu, S."],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Bogart, R. S."],["dc.contributor.author","Braun, D. C."],["dc.contributor.author","Cameron, R. H."],["dc.contributor.author","Duvall, Thomas L."],["dc.contributor.author","Hanasoge, Shravan M."],["dc.contributor.author","Jackiewicz, J."],["dc.contributor.author","Roth, M."],["dc.contributor.author","Stahn, Thorsten"],["dc.contributor.author","Thompson, M. J."],["dc.contributor.author","Zharkov, S."],["dc.date.accessioned","2017-09-07T11:48:39Z"],["dc.date.available","2017-09-07T11:48:39Z"],["dc.date.issued","2008"],["dc.identifier.doi","10.1007/s11214-008-9466-5"],["dc.identifier.gro","3146991"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4740"],["dc.notes.status","public"],["dc.notes.submitter","chake"],["dc.publisher","Springer Nature"],["dc.relation.haserratum","/handle/2/4741"],["dc.relation.issn","0038-6308"],["dc.title","Helioseismology of Sunspots: A Case Study of NOAA Region 9787"],["dc.type","journal_article"],["dc.type.internalPublication","unknown"],["dc.type.peerReviewed","no"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","1"],["dc.bibliographiccitation.issue","1"],["dc.bibliographiccitation.journal","Solar Physics"],["dc.bibliographiccitation.lastpage","62"],["dc.bibliographiccitation.volume","267"],["dc.contributor.author","Moradi, H."],["dc.contributor.author","Baldner, C. S."],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Braun, D. C."],["dc.contributor.author","Cameron, R. H."],["dc.contributor.author","Duvall, Thomas L."],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Haber, D. A."],["dc.contributor.author","Hanasoge, Shravan M."],["dc.contributor.author","Hindman, B. W."],["dc.contributor.author","Jackiewicz, J."],["dc.contributor.author","Khomenko, E."],["dc.contributor.author","Komm, R."],["dc.contributor.author","Rajaguru, P."],["dc.contributor.author","Rempel, M."],["dc.contributor.author","Roth, M."],["dc.contributor.author","Schlichenmaier, R."],["dc.contributor.author","Schunker, H."],["dc.contributor.author","Spruit, Henk C."],["dc.contributor.author","Strassmeier, K. G."],["dc.contributor.author","Thompson, M. J."],["dc.contributor.author","Zharkov, S."],["dc.date.accessioned","2017-09-07T11:49:58Z"],["dc.date.available","2017-09-07T11:49:58Z"],["dc.date.issued","2010"],["dc.identifier.doi","10.1007/s11207-010-9630-4"],["dc.identifier.gro","3147459"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/5017"],["dc.notes.status","public"],["dc.notes.submitter","chake"],["dc.publisher","Springer Nature"],["dc.relation.issn","0038-0938"],["dc.title","Modeling the Subsurface Structure of Sunspots"],["dc.type","journal_article"],["dc.type.internalPublication","unknown"],["dc.type.peerReviewed","no"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A130"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.volume","558"],["dc.contributor.author","Schunker, H."],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Cameron, R. H."],["dc.contributor.author","Birch, Aaron C."],["dc.date.accessioned","2017-09-07T11:48:42Z"],["dc.date.available","2017-09-07T11:48:42Z"],["dc.date.issued","2013"],["dc.description.abstract","To assess the ability of helioseismology to probe the subsurface structure and magnetic field of sunspots, we need to determine how helioseismic travel times depend on perturbations to sunspot models. Here we numerically simulate the propagation of f, p1, and p2 wave packets through magnetic sunspot models. Among the models we considered, a ±50 km change in the height of the Wilson depression and a change in the subsurface magnetic field geometry can both be detected above the observational noise level. We also find that the travel-time shifts due to changes in a sunspot model must be modeled by computing the effects of changing the reference sunspot model, and not by computing the effects of changing the subsurface structure in the quiet-Sun model. For p1 modes, the latter is wrong by a factor of four. In conclusion, numerical modeling of MHD wave propagation is an essential tool for interpreting the effects of sunspots on seismic waveforms."],["dc.identifier.doi","10.1051/0004-6361/201321485"],["dc.identifier.gro","3147042"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/4774"],["dc.language.iso","en"],["dc.notes.status","final"],["dc.notes.submitter","chake"],["dc.relation.issn","0004-6361"],["dc.title","Helioseismology of sunspots: how sensitive are travel times to the Wilson depression and to the subsurface magnetic field?"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","no"],["dspace.entity.type","Publication"]]