Downflows under sunspots detected by helioseismic tomography

Duvall, T. L.; D'Silva, Sydney; Jefferies, S. M.; Harvey, J. W.; Schou, J.

doi:10.1038/379235a0

Cited by 181 publications

(131 citation statements)

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“…At layers shallower than about 4 Mm, the flows might start to change from downflows to upflows, when flux emerges, and then back to downflows after the active regions are established. The flow response to emerging flux agrees with numerical simulations of emerging flux tubes (Fan, 2001;Schüssler and Rempel, 2005) where upflows indicate the beginning of flux emergence and surface cooling due to adiabatic expansion leads to downflows along the emerged loops. The kinetic-helicity density at 127.5°-135.0°longitude as a function of latitude and depth.…”

Section: Large-scale Flows Around Active Regionssupporting

confidence: 74%

“…Quite clear, however, is the picture of flux emergence: the process by which a loop of flux rises from a horizontal layer of magnetic flux at the base of the convection zone to form an active region, as discussed briefly in Section 2.1.1. Simplified 1D calculations in the "thin tube" approximation (D'Silva and Choudhuri, 1993;Fan, Fisher, and McClymont, 1994;Caligari, Moreno-Insertis, and Schüssler, 1995) point to a field strength of about 10 5 G at the base of the convection zone. At this strength, agreement is reached with three independent key properties of active regions: i) the time scale of emergence (a few days), ii) the heliographic-latitude range of emergence, and iii) the tilt of active-region axes (e.g.…”

Section: The Issue Of Flux Emergencementioning

confidence: 97%

“…It is also questionable if the proposed flow would actually be sufficient to keep the flux bundle together, in view of the (interchange) instabilities to be expected in this picture (Schüssler, 1984). What has kept Parker's proposal alive, however, is the helioseismic inference (Duvall et al, 1996) of a huge downflow of 2 km s −1 of the sign proposed by Parker. It is a puzzling observation, which, if confirmed, would require new theoretical ideas.…”

Section: The Anchoring Problemmentioning

confidence: 99%

“…Fan, Braun, and Chou, 1995;Kosovichev, 1996;Jensen, Jacobsen, and Christensen-Dalsgaard, 1998;Chou, Sun, and Chang, 2000;Kosovichev, Duvall, and Scherrer, 2000;Zhao and Kosovichev, 2003;Couvidat et al, 2004;Basu, Antia, and Bogart, 2004;Crouch et al, 2005;Bogart et al, 2008). Despite the long history of work on this topic, there is no general agreement on the subsurface structure of active regions.…”

Section: Diagnostics Potential Of Helioseismologymentioning

confidence: 99%

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Modeling the Subsurface Structure of Sunspots

et al. 2010

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While sunspots are easily observed at the solar surface, determining their subsurface structure is not trivial. There are two main hypotheses for the subsurface structure of sunspots: the monolithic model and the cluster model. Local helioseismology is the only means by which we can investigate subphotospheric structure. However, as current linear inversion techniques do not yet allow helioseismology to probe the internal structure with sufficient confidence to distinguish between the monolith and cluster models, the development of physically realistic sunspot models are a priority for helioseismologists. This is because they are not only important indicators of the variety of physical effects that may influence helioseismic inferences in active regions, but they also enable detailed assessments of the validity of helioseismic interpretations through numerical forward modeling. In this article, we provide a critical review of the existing sunspot models and an overview of numerical methods employed to model wave propagation through model sunspots. We then carry out a helioseismic analysis of the sunspot in Active Region 9787 and address the serious inconsistencies uncovered by Gizon et al. (2009aGizon et al. ( , 2009b. We find that this sunspot is most probably associated with a shallow, positive wave-speed perturbation (unlike the traditional two-layer model) and that travel-time measurements are consistent with a horizontal outflow in the surrounding moat.

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Section: Large-scale Flows Around Active Regionssupporting

confidence: 74%

Section: The Issue Of Flux Emergencementioning

confidence: 97%

Section: The Anchoring Problemmentioning

confidence: 99%

Section: Diagnostics Potential Of Helioseismologymentioning

confidence: 99%

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Modeling the Subsurface Structure of Sunspots

et al. 2010

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“…After they reflect off the Alfvén wave speed gradient, the fast waves may re-enter the solar interior wave field. This could be problematic for helioseismology, since any phase changes produced by the "fast-to-Alfvén" mode conversion process would seriously compromise any inferences derived from helioseismic inversions of phase travel times (e.g., Duvall et al 1996;Kosovichev, Duvall & Scherrer 2000;Couvidat et al 2005), which would normally, but inaccurately, interpret such phase changes as "travel-time shifts" due to subsurface inhomogeneities alone.…”

Section: Introductionmentioning

confidence: 99%

Directional time–distance probing of model sunspot atmospheres

Moradi

Cally

Przybylski

et al. 2015

Monthly Notices of the Royal Astronomical Society

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A crucial feature not widely accounted for in local helioseismology is that surface magnetic regions actually open a window from the interior into the solar atmosphere, and that the seismic waves leak through this window, reflect high in the atmosphere, and then re-enter the interior to rejoin the seismic wave field normally confined there. In a series of recent numerical studies using translation invariant atmospheres, we utilised a "directional time-distance helioseismology" measurement scheme to study the implications of the returning fast and Alfvén waves higher up in the solar atmosphere on the seismology at the photosphere Moradi & Cally 2014). In this study, we extend our directional time-distance analysis to more realistic sunspot-like atmospheres to better understand the direct effects of the magnetic field on helioseismic travel-time measurements in sunspots. In line with our previous findings, we uncover a distinct frequency-dependant directional behaviour in the travel-time measurements, consistent with the signatures of MHD mode conversion. We found this to be the case regardless of the sunspot field strength or depth of its Wilson depression. We also isolated and analysed the direct contribution from purely thermal perturbations to the measured travel times, finding that waves propagating in the umbra are much more sensitive to the underlying thermal effects of the sunspot.

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Seismic imaging of the Sun's far hemisphere and its applications in space weather forecasting

Lindsey

Braun

2017

Space Weather

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The interior of the Sun is filled acoustic waves with periods of about 5 min. These waves, called “ p modes,” are understood to be excited by convection in a thin layer beneath the Sun's surface. The p modes cause seismic ripples, which we call “the solar oscillations.” Helioseismic observatories use Doppler observations to map these oscillations, both spatially and temporally. The p modes propagate freely throughout the solar interior, reverberating between the near and far hemispheres. They also interact strongly with active regions at the surfaces of both hemispheres, carrying the signatures of said interactions with them. Computational analysis of the solar oscillations mapped in the Sun's near hemisphere, applying basic principles of wave optics to model the implied p modes propagating through the solar interior, gives us seismic maps of large active regions in the Sun's far hemisphere. These seismic maps are useful for space weather forecasting. For the past decade, NASA's twin STEREO spacecraft have given us full coverage of the Sun's far hemisphere in electromagnetic (EUV) radiation from the far side of Earth's orbit about the Sun. We are now approaching a decade during which the STEREO spacecraft will lose their farside vantage. There will occur significant periods from thence during which electromagnetic coverage of the Sun's far hemisphere will be incomplete or nil. Solar seismology will make it possible to continue our monitor of large active regions in the Sun's far hemisphere for the needs of space weather forecasters during these otherwise blind periods.

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Downflows under sunspots detected by helioseismic tomography

Cited by 181 publications

References 6 publications

Modeling the Subsurface Structure of Sunspots

Modeling the Subsurface Structure of Sunspots

Directional time–distance probing of model sunspot atmospheres

Seismic imaging of the Sun's far hemisphere and its applications in space weather forecasting

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