Cameron, Robert H.

[["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.artnumber","A73"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","586"],["dc.contributor.author","Martin-Belda, D."],["dc.contributor.author","Cameron, R. 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 aim to determine the effect of converging flows on the evolution of a bipolar magnetic region (BMR), and to investigate the role of these inflows in the generation of poloidal flux. We also discuss whether the flux dispersal due to turbulent flows can be described as a diffusion process. Methods. We developed a simple surface flux transport model based on point-like magnetic concentrations. We tracked the tilt angle, the magnetic flux and the axial dipole moment of a BMR in simulations with and without inflows and compared the results. To test the diffusion approximation, simulations of random walk dispersal of magnetic features were compared against the predictions of the diffusion treatment. Results. We confirm the validity of the diffusion approximation to describe flux dispersal on large scales. We find that the inflows enhance flux cancellation, but at the same time affect the latitudinal separation of the polarities of the bipolar region. In most cases the latitudinal separation is limited by the inflows, resulting in a reduction of the axial dipole moment of the BMR. However, when the initial tilt angle of the BMR is small, the inflows produce an increase in latitudinal separation that leads to an increase in the axial dipole moment in spite of the enhanced flux destruction. This can give rise to a tilt of the BMR even when the BMR was originally aligned parallel to the equator."],["dc.description.sponsorship","Deutsche Forschungsgemeinschaft [SFB 963]"],["dc.identifier.doi","10.1051/0004-6361/201527213"],["dc.identifier.isi","000369715900084"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/13438"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/41481"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","1432-0746"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Surface flux transport simulations: Effect of inflows toward active regions and random velocities on the evolution of the Sun's large-scale magnetic field"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","8"],["dc.bibliographiccitation.issue","1"],["dc.bibliographiccitation.journal","The Astrophysical Journal Supplement Series"],["dc.bibliographiccitation.volume","229"],["dc.contributor.author","Jafarzadeh, S."],["dc.contributor.author","Solanki, Parth K."],["dc.contributor.author","Cameron, Robert H."],["dc.contributor.author","Barthol, P."],["dc.contributor.author","Blanco Rodriguez, J."],["dc.contributor.author","del Toro Iniesta, J. C."],["dc.contributor.author","Gandorfer, A."],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Hirzberger, J."],["dc.contributor.author","Knoelker, M."],["dc.contributor.author","Pillet, V. Martinez"],["dc.contributor.author","Orozco Suarez, D."],["dc.contributor.author","Riethmueller, T. L."],["dc.contributor.author","Schmidt, W."],["dc.contributor.author","van Noort, M."],["dc.date.accessioned","2018-11-07T10:26:55Z"],["dc.date.available","2018-11-07T10:26:55Z"],["dc.date.issued","2017"],["dc.description.abstract","Convective flows are known as the prime means of transporting magnetic fields on the solar surface. Thus, small magnetic structures are good tracers of turbulent flows. We study the migration and dispersal of magnetic bright features (MBFs) in intergranular areas observed at high spatial resolution with SUNRISE/IMaX. We describe the flux dispersal of individual MBFs as a diffusion process whose parameters are computed for various areas in the quiet-Sun and the vicinity of active regions from seeing-free data. We find that magnetic concentrations are best described as random walkers close to network areas (diffusion index, gamma = 1.0), travelers with constant speeds over a supergranule (gamma = 1.9-2.0), and decelerating movers in the vicinity of flux emergence and/or within active regions (gamma = 1.4-1.5). The three types of regions host MBFs with mean diffusion coefficients of 130 km(2) s(-1), 80-90 km(2) s(-1), and 25-70 km(2) s(-1), respectively. The MBFs in these three types of regions are found to display a distinct kinematic behavior at a confidence level in excess of 95%."],["dc.identifier.doi","10.3847/1538-4365/229/1/8"],["dc.identifier.isi","000397557300008"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/43141"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","PUB_WoS_Import"],["dc.publisher","Iop Publishing Ltd"],["dc.relation.issn","1538-4365"],["dc.relation.issn","0067-0049"],["dc.title","Kinematics of Magnetic Bright Features in the Solar Photosphere"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","L23"],["dc.bibliographiccitation.issue","2"],["dc.bibliographiccitation.journal","The Astrophysical Journal Letters"],["dc.bibliographiccitation.volume","934"],["dc.contributor.affiliation","Nèmec, N.-E.;"],["dc.contributor.affiliation","Shapiro, A. I.;"],["dc.contributor.affiliation","Işık, E.;"],["dc.contributor.affiliation","Sowmya, K.;"],["dc.contributor.affiliation","Solanki, S. K.;"],["dc.contributor.affiliation","Krivova, N. A.;"],["dc.contributor.affiliation","Cameron, R. H.;"],["dc.contributor.affiliation","Gizon, L.;"],["dc.contributor.author","Nèmec, N.-E."],["dc.contributor.author","Shapiro, A. I."],["dc.contributor.author","Işık, E."],["dc.contributor.author","Sowmya, K."],["dc.contributor.author","Solanki, S. K."],["dc.contributor.author","Krivova, N. A."],["dc.contributor.author","Cameron, R. H."],["dc.contributor.author","Gizon, L."],["dc.date.accessioned","2022-08-01T07:35:55Z"],["dc.date.available","2022-08-01T07:35:55Z"],["dc.date.issued","2022-07-28"],["dc.date.updated","2022-07-30T02:55:06Z"],["dc.description.abstract","Surfaces of the Sun and other cool stars are filled with magnetic fields, which are either seen as dark compact spots or more diffuse bright structures like faculae. Both hamper detection and characterization of exoplanets, affecting stellar brightness and spectra, as well as transmission spectra. However, the expected facular and spot signals in stellar data are quite different, for instance, they have distinct temporal and spectral profiles. Consequently, corrections of stellar data for magnetic activity can greatly benefit from the insight on whether the stellar signal is dominated by spots or faculae. Here, we utilize a surface flux transport model to show that more effective cancellation of diffuse magnetic flux associated with faculae leads to spot area coverages increasing faster with stellar magnetic activity than that by faculae. Our calculations explain the observed dependence between solar spot and facular area coverages and allow its extension to stars that are more active than the Sun. This extension enables anticipating the properties of stellar signal and its more reliable mitigation, leading to a more accurate characterization of exoplanets and their atmospheres."],["dc.description.sponsorship","A. I. Shapiro"],["dc.identifier.doi","10.3847/2041-8213/ac8155"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/112556"],["dc.language.iso","en"],["dc.relation.eissn","2041-8213"],["dc.relation.issn","2041-8205"],["dc.rights","CC BY 4.0"],["dc.rights.uri","http://creativecommons.org/licenses/by/4.0/"],["dc.title","Faculae Cancel out on the Surfaces of Active Suns"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.subtype","original_ja"],["dc.type.version","published_version"],["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.firstpage","1469"],["dc.bibliographiccitation.issue","6498"],["dc.bibliographiccitation.journal","Science"],["dc.bibliographiccitation.lastpage","1472"],["dc.bibliographiccitation.volume","368"],["dc.contributor.author","Gizon, Laurent"],["dc.contributor.author","Cameron, Robert H."],["dc.contributor.author","Pourabdian, Majid"],["dc.contributor.author","Liang, Zhi-Chao"],["dc.contributor.author","Fournier, Damien"],["dc.contributor.author","Birch, Aaron C."],["dc.contributor.author","Hanson, Chris S."],["dc.date.accessioned","2021-03-05T08:59:01Z"],["dc.date.available","2021-03-05T08:59:01Z"],["dc.date.issued","2020"],["dc.identifier.doi","10.1126/science.aaz7119"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/80330"],["dc.language.iso","en"],["dc.notes.intern","DOI Import GROB-393"],["dc.relation.eissn","1095-9203"],["dc.relation.issn","0036-8075"],["dc.title","Meridional flow in the Sun’s convection zone is a single cell in each hemisphere"],["dc.type","journal_article"],["dc.type.internalPublication","unknown"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A42"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","581"],["dc.contributor.author","Beeck, Benjamin"],["dc.contributor.author","Schuessler, M."],["dc.contributor.author","Cameron, R. H."],["dc.contributor.author","Reiners, Ansgar"],["dc.date.accessioned","2018-11-07T09:52:14Z"],["dc.date.available","2018-11-07T09:52:14Z"],["dc.date.issued","2015"],["dc.description.abstract","Context. The convective envelopes of cool main-sequence stars harbour magnetic fields with a complex global and local structure. These fields affect the near-surface convection and the outer stellar atmospheres in many ways and are responsible for the observable magnetic activity of stars. Aims. Our aim is to understand the local structure in unipolar regions with moderate average magnetic flux density. These correspond to plage regions covering a substantial fraction of the surface of the Sun (and likely also the surface of other Sun-like stars) during periods of high magnetic activity. Methods. We analyse the results of 18 local-box magnetohydrodynamics simulations covering the upper layers of the convection zones and the photospheres of cool main-sequence stars of spectral types F to early M. The average vertical field in these simulations ranges from 20 to 500 G. Results. We find a substantial variation of the properties of the surface magnetoconvection between main-sequence stars of different spectral types. As a consequence of a reduced efficiency of the convective collapse of flux tubes, M dwarfs lack bright magnetic structures in unipolar regions of moderate field strength. The spatial correlation between velocity and the magnetic field as well as the lifetime of magnetic structures and their sizes relative to the granules vary significantly along the model sequence of stellar types."],["dc.description.sponsorship","Deutsche Forschungsgemeinschaft (DFG) [SFB 963/1]; DFG [DFG 1664/9-2]"],["dc.identifier.doi","10.1051/0004-6361/201525788"],["dc.identifier.isi","000361803900042"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/12438"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/36077"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","1432-0746"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Three-dimensional simulations of near-surface convection in main-sequence stars III. The structure of small-scale magnetic flux concentrations"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","309"],["dc.bibliographiccitation.issue","2"],["dc.bibliographiccitation.journal","Solar Physics"],["dc.bibliographiccitation.lastpage","320"],["dc.bibliographiccitation.volume","268"],["dc.contributor.author","Daiffallah, K."],["dc.contributor.author","Abdelatif, T."],["dc.contributor.author","Bendib, A."],["dc.contributor.author","Cameron, R."],["dc.contributor.author","Gizon, Laurent"],["dc.date.accessioned","2021-03-05T09:05:23Z"],["dc.date.available","2021-03-05T09:05:23Z"],["dc.date.issued","2010"],["dc.identifier.doi","10.1007/s11207-010-9666-5"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/80456"],["dc.language.iso","en"],["dc.notes.intern","DOI Import GROB-393"],["dc.relation.eissn","1573-093X"],["dc.relation.issn","0038-0938"],["dc.title","3D Numerical Simulations of f-Mode Propagation Through Magnetic Flux Tubes"],["dc.type","journal_article"],["dc.type.internalPublication","unknown"],["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","A21"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","597"],["dc.contributor.author","Martin-Belda, D."],["dc.contributor.author","Cameron, R. H."],["dc.date.accessioned","2018-11-07T10:29:13Z"],["dc.date.available","2018-11-07T10:29:13Z"],["dc.date.issued","2017"],["dc.description.abstract","Aims. We aim to investigate how converging flows towards active regions affect the surface transport of magnetic flux, as well as their impact on the generation of the Sun's poloidal field. The inflows constitute a potential non-linear mechanism for the saturation of the global dynamo and may contribute to the modulation of the solar cycle in the Babcock-Leighton framework. Methods. We build a surface flux transport code incorporating a parametrized model of the inflows and run simulations spanning several cycles. We carry out a parameter study to assess how the strength and extension of the inflows affect the build-up of the global dipole field. We also perform simulations with different levels of activity to investigate the potential role of the inflows in the saturation of the global dynamo. Results. We find that the interaction of neighbouring active regions can lead to the occasional formation of single-polarity magnetic flux clumps that are inconsistent with observations. We propose the darkening caused by pores in areas of high magnetic field strength as a possible mechanism preventing this flux-clumping. We find that inflows decrease the amplitude of the axial dipole moment by similar to 30%, relative to a no-inflows scenario. Stronger (weaker) inflows lead to larger (smaller) reductions of the axial dipole moment. The relative amplitude of the generated axial dipole is about 9% larger after very weak cycles than after very strong cycles. This supports the idea that the inflows are a non-linear mechanism that is capable of saturating the global dynamo and contributing to the modulation of the solar cycle within the Babcock-Leighton framework."],["dc.identifier.doi","10.1051/0004-6361/201629061"],["dc.identifier.isi","000392392900080"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/14292"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/43591"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","PUB_WoS_Import"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","1432-0746"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Inflows towards active regions and the modulation of the solar cycle: A parameter study"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]