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  • 1
    Online Resource
    Online Resource
    Annual Reviews ; 2010
    In:  Annual Review of Astronomy and Astrophysics Vol. 48, No. 1 ( 2010-08-01), p. 289-338
    In: Annual Review of Astronomy and Astrophysics, Annual Reviews, Vol. 48, No. 1 ( 2010-08-01), p. 289-338
    Abstract: The Sun supports a rich spectrum of internal waves that are continuously excited by turbulent convection. The Global Oscillation Network Group (GONG) network and the SOHO/MDI (Solar and Heliospheric Observatory/Michelson Doppler Imager) space instrument provide an exceptional database of spatially resolved observations of solar oscillations, covering more than an entire sunspot cycle (11 years). Local helioseismology is a set of tools for probing the solar interior in three dimensions using measurements of wave travel times and local mode frequencies. Local helioseismology has discovered (a) near-surface vector flows associated with convection, (b) 250 m s −1 subsurface horizontal outflows around sunspots, (c) ∼50 m s −1 extended horizontal flows around active regions (converging near the surface and diverging below), (d) the effect of the Coriolis force on convective flows and active region flows, (e) the subsurface signature of the 15 m s −1 poleward meridional flow, (f) a ±5 m s −1 time-varying depth-dependent component of the meridional circulation around the mean latitude of activity, and (g) magnetic activity on the farside of the Sun.
    Type of Medium: Online Resource
    ISSN: 0066-4146 , 1545-4282
    URL: Issue
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    Language: English
    Publisher: Annual Reviews
    Publication Date: 2010
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    SSG: 16,12
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  • 2
    Online Resource
    Online Resource
    American Astronomical Society ; 2009
    In:  The Astrophysical Journal Vol. 695, No. 1 ( 2009-04-10), p. 325-335
    In: The Astrophysical Journal, American Astronomical Society, Vol. 695, No. 1 ( 2009-04-10), p. 325-335
    Type of Medium: Online Resource
    ISSN: 0004-637X , 1538-4357
    RVK:
    Language: Unknown
    Publisher: American Astronomical Society
    Publication Date: 2009
    detail.hit.zdb_id: 2207648-7
    detail.hit.zdb_id: 1473835-1
    SSG: 16,12
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  • 3
    Online Resource
    Online Resource
    American Association for the Advancement of Science (AAAS) ; 2016
    In:  Science Advances Vol. 2, No. 7 ( 2016-07)
    In: Science Advances, American Association for the Advancement of Science (AAAS), Vol. 2, No. 7 ( 2016-07)
    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.
    Type of Medium: Online Resource
    ISSN: 2375-2548
    Language: English
    Publisher: American Association for the Advancement of Science (AAAS)
    Publication Date: 2016
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  • 4
    Online Resource
    Online Resource
    EDP Sciences ; 2020
    In:  Astronomy & Astrophysics Vol. 635 ( 2020-03), p. A181-
    In: Astronomy & Astrophysics, EDP Sciences, Vol. 635 ( 2020-03), p. A181-
    Abstract: Context. The spatial power spectrum of supergranulation does not fully characterize the underlying physics of turbulent convection. For example, it does not describe the non-Gaussianity in the horizontal flow divergence. Aims. Our aim is to statistically characterize the spatial pattern of solar supergranulation beyond the power spectrum. The next-order statistic is the bispectrum. It measures correlations of three Fourier components and is related to the nonlinearities in the underlying physics. It also characterizes how a skewness in the dataset is generated by the coupling of three Fourier components. Methods. We estimated the bispectrum of supergranular horizontal surface divergence maps that were obtained using local correlation tracking (LCT) and time-distance helioseismology (TD) from one year of data from the helioseismic and magnetic imager on-board the solar dynamics observatory starting in May 2010. Results. We find significantly nonzero and consistent estimates for the bispectrum using LCT and TD. The strongest nonlinearity is present when the three coupling wave vectors are at the supergranular scale. These are the same wave vectors that are present in regular hexagons, which have been used in analytical studies of solar convection. At these Fourier components, the bispectrum is positive, consistent with the positive skewness in the data and consistent with supergranules preferentially consisting of outflows surrounded by a network of inflows. We use the bispectral estimates to generate synthetic divergence maps that are very similar to the data. This is done by a model that consists of a Gaussian term and a weaker quadratic nonlinear component. Using this method, we estimate the fraction of the variance in the divergence maps from the nonlinear component to be of the order of 4–6%. Conclusions. We propose that bispectral analysis is useful for understanding the dynamics of solar turbulent convection, for example for comparing observations and numerical models of supergranular flows. This analysis may also be useful to generate synthetic flow fields.
    Type of Medium: Online Resource
    ISSN: 0004-6361 , 1432-0746
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    RVK:
    Language: English
    Publisher: EDP Sciences
    Publication Date: 2020
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  • 5
    In: Astronomy & Astrophysics, EDP Sciences, Vol. 619 ( 2018-11), p. A99-
    Abstract: Context . The solar meridional flow is an essential ingredient in flux-transport dynamo models. However, no consensus on its subsurface structure has been reached. Aims . We merge the data sets from SOHO/MDI and SDO/HMI with the aim of achieving a greater precision on helioseismic measurements of the subsurface meridional flow. Methods . The south-north travel-time differences are measured by applying time-distance helioseismology to the MDI and HMI medium-degree Dopplergrams covering May 1996–April 2017. Our data analysis corrects for several sources of systematic effects: P -angle error, surface magnetic field effects, and center-to-limb variations. For HMI data, we used the P -angle correction provided by the HMI team based on the Venus and Mercury transits. For MDI data, we used a P -angle correction estimated from the correlation of MDI and HMI data during the period of overlap. The center-to-limb effect is estimated from the east-west travel-time differences and is different for MDI and HMI observations. An interpretation of the travel-time measurements is obtained using a forward-modeling approach in the ray approximation. Results . In the latitude range 20°–35°, the travel-time differences are similar in the southern hemisphere for cycles 23 and 24. However, they differ in the northern hemisphere between cycles 23 and 24. Except for cycle 24’s northern hemisphere, the measurements favor a single-cell meridional circulation model where the poleward flows persist down to ∼0.8 R ⊙ , accompanied by local inflows toward the activity belts in the near-surface layers. Cycle 24’s northern hemisphere is anomalous: travel-time differences are significantly smaller when travel distances are greater than 20°. This asymmetry between northern and southern hemispheres during cycle 24 was not present in previous measurements, which assumed a different P -angle error correction where south-north travel-time differences are shifted to zero at the equator for all travel distances. In our measurements, the travel-time differences at the equator are zero for travel distances less than ∼30°, but they do not vanish for larger travel distances. This equatorial offset for large travel distances need not be interpreted as a deep cross-equator flow; it could be due to the presence of asymmetrical local flows at the surface near the end points of the acoustic ray paths. Conclusions . The combined MDI and HMI helioseismic measurements presented here contain a wealth of information about the subsurface structure and the temporal evolution of the meridional circulation over 21 years. To infer the deep meridional flow, it will be necessary to model the contribution from the complex time-varying flows in the near-surface layers.
    Type of Medium: Online Resource
    ISSN: 0004-6361 , 1432-0746
    RVK:
    RVK:
    Language: English
    Publisher: EDP Sciences
    Publication Date: 2018
    detail.hit.zdb_id: 1458466-9
    SSG: 16,12
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  • 6
    Online Resource
    Online Resource
    American Association for the Advancement of Science (AAAS) ; 2020
    In:  Science Vol. 368, No. 6498 ( 2020-06-26), p. 1469-1472
    In: Science, American Association for the Advancement of Science (AAAS), Vol. 368, No. 6498 ( 2020-06-26), p. 1469-1472
    Abstract: The Sun’s magnetic field is generated by subsurface motions of the convecting plasma. The latitude at which the magnetic field emerges through the solar surface (as sunspots) drifts toward the equator over the course of the 11-year solar cycle. We use helioseismology to infer the meridional flow (in the latitudinal and radial directions) over two solar cycles covering 1996–2019. Two data sources are used, which agree during their overlap period of 2001–2011. The time-averaged meridional flow is shown to be a single cell in each hemisphere, carrying plasma toward the equator at the base of the convection zone with a speed of ~4 meters per second at 45° latitude. Our results support the flux-transport dynamo model, which explains the drift of sunspot-emergence latitudes through the meridional flow.
    Type of Medium: Online Resource
    ISSN: 0036-8075 , 1095-9203
    RVK:
    RVK:
    Language: English
    Publisher: American Association for the Advancement of Science (AAAS)
    Publication Date: 2020
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    detail.hit.zdb_id: 2066996-3
    detail.hit.zdb_id: 2060783-0
    SSG: 11
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  • 7
    Online Resource
    Online Resource
    EDP Sciences ; 2016
    In:  Astronomy & Astrophysics Vol. 593 ( 2016-9), p. A41-
    In: Astronomy & Astrophysics, EDP Sciences, Vol. 593 ( 2016-9), p. A41-
    Type of Medium: Online Resource
    ISSN: 0004-6361 , 1432-0746
    RVK:
    RVK:
    Language: English
    Publisher: EDP Sciences
    Publication Date: 2016
    detail.hit.zdb_id: 1458466-9
    SSG: 16,12
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  • 8
    In: Experimental Astronomy, Springer Science and Business Media LLC, Vol. 54, No. 2-3 ( 2022-12), p. 157-183
    Abstract: A mission to view the solar poles from high helio-latitudes (above 60°) will build on the experience of Solar Orbiter as well as a long heritage of successful solar missions and instrumentation (e.g. SOHO Domingo et al. (Solar Phys. 162 (1-2), 1–37 1995), STEREO Howard et al. (Space Sci. Rev. 136 (1-4), 67–115 2008), Hinode Kosugi et al. (Solar Phys. 243 (1), 3–17 2007), Pesnell et al. Solar Phys. 275 (1–2), 3–15 2012), but will focus for the first time on the solar poles, enabling scientific investigations that cannot be done by any other mission. One of the major mysteries of the Sun is the solar cycle. The activity cycle of the Sun drives the structure and behaviour of the heliosphere and of course, the driver of space weather. In addition, solar activity and variability provides fluctuating input into the Earth climate models, and these same physical processes are applicable to stellar systems hosting exoplanets. One of the main obstructions to understanding the solar cycle, and hence all solar activity, is our current lack of understanding of the polar regions. In this White Paper, submitted to the European Space Agency in response to the Voyage 2050 call, we describe a mission concept that aims to address this fundamental issue. In parallel, we recognise that viewing the Sun from above the polar regions enables further scientific advantages, beyond those related to the solar cycle, such as unique and powerful studies of coronal mass ejection processes, from a global perspective, and studies of coronal structure and activity in polar regions. Not only will these provide important scientific advances for fundamental stellar physics research, they will feed into our understanding of impacts on the Earth and other planets’ space environment.
    Type of Medium: Online Resource
    ISSN: 0922-6435 , 1572-9508
    Language: English
    Publisher: Springer Science and Business Media LLC
    Publication Date: 2022
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  • 9
    Online Resource
    Online Resource
    Springer Science and Business Media LLC ; 2009
    In:  Solar Physics Vol. 257, No. 2 ( 2009-7), p. 217-235
    In: Solar Physics, Springer Science and Business Media LLC, Vol. 257, No. 2 ( 2009-7), p. 217-235
    Type of Medium: Online Resource
    ISSN: 0038-0938 , 1573-093X
    Language: English
    Publisher: Springer Science and Business Media LLC
    Publication Date: 2009
    detail.hit.zdb_id: 2211848-2
    detail.hit.zdb_id: 1473830-2
    SSG: 16,12
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  • 10
    Online Resource
    Online Resource
    American Association for the Advancement of Science (AAAS) ; 2016
    In:  Science Advances Vol. 2, No. 11 ( 2016-11-04)
    In: Science Advances, American Association for the Advancement of Science (AAAS), Vol. 2, No. 11 ( 2016-11-04)
    Abstract: Stars are not perfectly spherically symmetric. They are deformed by rotation and magnetic fields. Until now, the study of stellar shapes has only been possible with optical interferometry for a few of the fastest-rotating nearby stars. We report an asteroseismic measurement, with much better precision than interferometry, of the asphericity of an A-type star with a rotation period of 100 days. Using the fact that different modes of oscillation probe different stellar latitudes, we infer a tiny but significant flattening of the star’s shape of Δ R / R = (1.8 ± 0.6) × 10 −6 . For a stellar radius R that is 2.24 times the solar radius, the difference in radius between the equator and the poles is Δ R = 3 ± 1 km. Because the observed Δ R / R is only one-third of the expected rotational oblateness, we conjecture the presence of a weak magnetic field on a star that does not have an extended convective envelope. This calls to question the origin of the magnetic field.
    Type of Medium: Online Resource
    ISSN: 2375-2548
    Language: English
    Publisher: American Association for the Advancement of Science (AAAS)
    Publication Date: 2016
    detail.hit.zdb_id: 2810933-8
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