arXiv subjects

T. L. Duvall Jr.

Publications and source records attributed to T. L. Duvall Jr..

9 recordsLinked to original sources

arXiv2018

Probing sunspots with two-skip time-distance helioseismology

Previous helioseismology of sunspots has been sensitive to both the structural and magnetic aspects of sunspot structure. We aim to develop a technique that is insensitive to the magnetic component so the two aspects can be more readily separated. We study waves reflected almost vertically from the underside of a sunspot. Time-distance helioseismology was used to measure travel times for the waves. Ray theory and a detailed sunspot model were used to calculate travel times for comparison. It is shown that these large distance waves are insensitive to the magnetic field in the sunspot. The largest travel time differences for any solar phenomena are observed. With sufficient modeling effort, these should lead to better understanding of sunspot structure.

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arXiv2017

Measuring solar active region inflows with local correlation tracking of granulation

Context. Local helioseismology has detected spatially extended converging surface flows into solar active regions. These play an important role in flux-transport models of the solar dynamo. Aims. We aim to validate the existence of the inflows by deriving horizontal flow velocities around active regions with local correlation tracking of granulation. Methods. We generate a six-year long-time series of full-disk maps of the horizontal velocity at the solar surface by tracking granules in continuum intensity images provided by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). Results. On average, active regions are surrounded by inflows extending up to 10 deg from the center of the active region of magnitudes of 20-30 m/s, reaching locally up to 40 m/s, which is in agreement with results from local helioseismology. By computing an ensemble average consisting of 243 individual active regions, we show that the inflows are not azimuthally symmetric but converge predominantly towards the trailing polarity of the active region with respect to the longitudinally and temporally averaged flow field.

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arXiv2016

The shrinking Sun: a systematic error in local correlation tracking of solar granulation

Context. Local correlation tracking of granulation (LCT) is an important method for measuring horizontal flows in the photosphere. This method exhibits a systematic error that looks like a flow converging towards disk center, also known as the shrinking-Sun effect. Aims. We aim at studying the nature of the shrinking-Sun effect for continuum intensity data and at deriving a simple model that can explain its origin. Methods. We derived LCT flow maps by running the local correlation tracking code FLCT on tracked and remapped continuum intensity maps provided by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory. We also computed flow maps from synthetic continuum images generated from STAGGER code simulations of solar surface convection. We investigated the origin of the shrinking-Sun effect by generating an average granule from synthetic data from the simulations. Results. The LCT flow maps derived from HMI and from the simulations exhibit a shrinking-Sun effect of comparable magnitude. The origin of this effect is related to the apparent asymmetry of granulation originating from radiative transfer effects when observing with a viewing angle inclined from vertical. This causes, in combination with the expansion of the granules, an apparent motion towards disk center.

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arXiv2014

Additional Evidence Supporting a Model of Shallow, High-Speed Supergranulation

Recently, Duvall and Hanasoge ({\it Solar Phys.} {\bf 287}, 71-83, 2013) found that large distance $[Δ]$ separation travel-time differences from a center to an annulus $[δt_{\rm{oi}}]$ implied a model of the average supergranular cell that has a peak upflow of $240\rm{ms^{-1}}$ at a depth of $2.3\rm{Mm}$ and a corresponding peak outward horizontal flow of $700\rm{ms^{-1}}$ at a depth of $1.6\rm{Mm}$. In the present work, this effect is further studied by measuring and modeling center-to-quadrant travel-time differences $[δt_{\rm{qu}}]$, which roughly agree with this model. Simulations are analyzed that show that such a model flow would lead to the expected travel-time differences. As a check for possible systematic errors, the center-to-annulus travel-time differences $[δt_{\rm{oi}}]$ are found not to vary with heliocentric angle. A consistency check finds an increase of $δt_{\rm{oi}}$ with the temporal frequency $[ν]$ by a factor of two, which is not predicted by the ray theory.

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arXiv2010

Modeling the Subsurface Structure of Sunspots

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 paper, 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 an helioseismic analysis of the sunspot in Active Region 9787 and address the serious inconsistencies uncovered by \citeauthor{gizonetal2009}~(\citeyear{gizonetal2009,gizonetal2009a}). 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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arXiv2010

Helioseismology of Sunspots: A Case Study of NOAA Region 9787

Various methods of helioseismology are used to study the subsurface properties of the sunspot in NOAA Active Region 9787. This sunspot was chosen because it is axisymmetric, shows little evolution during 20-28 January 2002, and was observed continuously by the MDI/SOHO instrument. (...) Wave travel times and mode frequencies are affected by the sunspot. In most cases, wave packets that propagate through the sunspot have reduced travel times. At short travel distances, however, the sign of the travel-time shifts appears to depend sensitively on how the data are processed and, in particular, on filtering in frequency-wavenumber space. We carry out two linear inversions for wave speed: one using travel-times and phase-speed filters and the other one using mode frequencies from ring analysis. These two inversions give subsurface wave-speed profiles with opposite signs and different amplitudes. (...) From this study of AR9787, we conclude that we are currently unable to provide a unified description of the subsurface structure and dynamics of the sunspot.

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arXiv2010

Seismic Constraints on Interior Solar Convection

We constrain the velocity spectral distribution of global-scale solar convective cells at depth using techniques of local helioseismology. We calibrate the sensitivity of helioseismic waves to large-scale convective cells in the interior by analyzing simulations of waves propagating through a velocity snapshot of global solar convection via methods of time-distance helioseismology. Applying identical analysis techniques to observations of the Sun, we are able to bound from above the magnitudes of solar convective cells as a function of spatial convective scale. We find that convection at a depth of $r/R_\odot = 0.95$ with spatial extent $\ell <20$, where $\ell$ is the spherical harmonic degree, comprise weak flow systems, on the order of 15 m/s or less. Convective features deeper than $r/R_\odot = 0.95$ are more difficult to image due to the rapidly decreasing sensitivity of helioseismic waves.

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arXiv2006

Sensitivity of time-distance helioseismic measurements to spatial variation of oscillation amplitudes I. Observations and a numerical model

It is well known that the observed amplitude of solar oscillations is lower in sunspots than in quiet regions of the Sun. We show that this local reduction in oscillation amplitudes combined with the phase-speed filtering procedure in time-distance helioseismic analyses could be a source of systematic errors in the range of 5 - 40% in the measured travel-time anomalies of acoustic waves around sunspots. Removing these travel time artifacts is important for correctly inferring the subsurface structure of sunspots. We suggest an empirical correction procedure and illustrate its usage for a small sunspot. This work utilizes data from MDI/SOHO.

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arXiv2002

Solar supergranulation as a wavelike phenomenon

Supergranulation on the surface of the Sun is an organized cellular flow pattern with a characteristic scale of 30 Mm. It is superficially similar to the well understood granulation that operates at the 1.5 Mm natural scale of convection, which has led to the conventional view that supergranulation has its origin in the convective motion of cells of gas, though this does not explain the observation that the supergranulation pattern appears to move faster around the Sun than the bulk of its surface. A wave origin has been proposed for supergranulation that may explain the superrotation, but it has never had much support. Here we report that the supergranulation pattern has oscillatory components with periods of 5-10 days, for which the best explanation is a spectrum of traveling waves. We show that there is excess power in the prograde and equatorward directions, which explains the observation of superrotation.

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