Author name code: duvall ADS astronomy entries on 2022-09-14 author:"Duvall, Thomas L." OR author:"Duvall, Tom" ------------------------------------------------------------------------ Title: Helioseismological determination of the subsurface spatial spectrum of solar convection: Demonstration using numerical simulations Authors: Böning, Vincent G. A.; Birch, Aaron C.; Gizon, Laurent; Duvall, Thomas L. Bibcode: 2021A&A...649A..59B Altcode: 2021arXiv210208603B Context. Understanding convection is important in stellar physics, for example, when it is an input in stellar evolution models. Helioseismic estimates of convective flow amplitudes in deeper regions of the solar interior disagree by orders of magnitude among themselves and with simulations.
Aims: We aim to assess the validity of an existing upper limit of solar convective flow amplitudes at a depth of 0.96 solar radii obtained using time-distance helioseismology and several simplifying assumptions.
Methods: We generated synthetic observations for convective flow fields from a magnetohydrodynamic simulation (MURaM) using travel-time sensitivity functions and a noise model. We compared the estimates of the flow amplitude with the actual value of the flow.
Results: For the scales of interest (ℓ < 100), we find that the current procedure for obtaining an upper limit gives the correct order of magnitude of the flow for the given flow fields. We also show that this estimate is not an upper limit in a strict sense because it underestimates the flow amplitude at the largest scales by a factor of about two because the scale dependence of the signal-to-noise ratio has to be taken into account. After correcting for this and after taking the dependence of the measurements on direction in Fourier space into account, we show that the obtained estimate is indeed an upper limit.
Conclusions: We conclude that time-distance helioseismology is able to correctly estimate the order of magnitude (or an upper limit) of solar convective flows in the deeper interior when the vertical correlation function of the different flow components is known and the scale dependence of the signal-to-noise ratio is taken into account. We suggest that future work should include information from different target depths to better separate the effect of near-surface flows from those at greater depths. In addition, the measurements are sensitive to all three flow directions, which should be taken into account. Title: Solar east-west flow correlations that persist for months at low latitudes are dominated by active region inflows Authors: Hanson, Chris S.; Duvall, Thomas L.; Birch, Aaron C.; Gizon, Laurent; Sreenivasan, Katepalli R. Bibcode: 2020A&A...644A.103H Altcode: 2020arXiv201013052H Context. Giant-cell convection is believed to be an important component of solar dynamics. For example, it is expected to play a crucial role in maintaining the Sun's differential rotation.
Aims: We reexamine early reports of giant convective cells detected using a correlation analysis of Dopplergrams. We extend this analysis using 19 years of space- and ground-based observations of near-surface horizontal flows.
Methods: Flow maps are derived through the local correlation tracking of granules and helioseismic ring-diagram analysis. We compute temporal auto-correlation functions of the east-west flows at fixed latitude.
Results: Correlations in the east-west velocity can be clearly seen up to five rotation periods. The signal consists of features with longitudinal wavenumbers up to m = 9 at low latitudes. Comparison with magnetic images indicates that these flow features are associated with magnetic activity. The signal is not seen above the noise level during solar minimum.
Conclusions: Our results show that the long-term correlations in east-west flows at low latitudes are predominantly due to inflows into active regions and not to giant convective cells. Title: Acoustic wave propagation through solar granulation: Validity of effective-medium theories, coda waves Authors: Poulier, P. -L.; Fournier, D.; Gizon, L.; Duvall, T. L. Bibcode: 2020A&A...643A.168P Altcode: 2020arXiv201001174P Context. The frequencies, lifetimes, and eigenfunctions of solar acoustic waves are affected by turbulent convection, which is random in space and in time. Since the correlation time of solar granulation and the periods of acoustic waves (∼5 min) are similar, the medium in which the waves propagate cannot a priori be assumed to be time independent.
Aims: We compare various effective-medium solutions with numerical solutions in order to identify the approximations that can be used in helioseismology. For the sake of simplicity, the medium is one dimensional.
Methods: We consider the Keller approximation, the second-order Born approximation, and spatial homogenization to obtain theoretical values for the effective wave speed and attenuation (averaged over the realizations of the medium). Numerically, we computed the first and second statistical moments of the wave field over many thousands of realizations of the medium (finite-amplitude sound-speed perturbations are limited to a 30 Mm band and have a zero mean).
Results: The effective wave speed is reduced for both the theories and the simulations. The attenuation of the coherent wave field and the wave speed are best described by the Keller theory. The numerical simulations reveal the presence of coda waves, trailing the ballistic wave packet. These late arrival waves are due to multiple scattering and are easily seen in the second moment of the wave field.
Conclusions: We find that the effective wave speed can be calculated, numerically and theoretically, using a single snapshot of the random medium (frozen medium); however, the attenuation is underestimated in the frozen medium compared to the time-dependent medium. Multiple scattering cannot be ignored when modeling acoustic wave propagation through solar granulation.

Movies associated to Figs. 3 and 9 are available at https://www.aanda.org Title: Characterizing the spatial pattern of solar supergranulation using the bispectrum Authors: Böning, Vincent G. A.; Birch, Aaron C.; Gizon, Laurent; Duvall, Thomas L.; Schou, Jesper Bibcode: 2020A&A...635A.181B Altcode: 2020arXiv200208262B 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. Title: Time-distance helioseismology of solar Rossby waves Authors: Liang, Zhi-Chao; Gizon, Laurent; Birch, Aaron C.; Duvall, Thomas L. Bibcode: 2019A&A...626A...3L Altcode: 2018arXiv181207413L Context. Solar Rossby waves (r modes) have recently been discovered in the near-surface horizontal flow field using the techniques of granulation-tracking and ring-diagram analysis applied to six years of SDO/HMI data.
Aims: Here we apply time-distance helioseismology to the combined SOHO/MDI and SDO/HMI data sets, which cover 21 years of observations from May 1996 to April 2017. The goal of this study is to provide an independent confirmation over two solar cycles and in deeper layers of the Sun.
Methods: We have measured south-north helioseismic travel times along the equator, which are sensitive to subsurface north-south flows. To reduce noise, the travel times were averaged over travel distances from 6° to 30°; the mean distance corresponds to a p-mode lower turning point of 0.91 R⊙. The 21-year time series of travel-time measurements was split into three seven-year subsets and transformed to obtain power spectra in a corotating frame.
Results: The power spectra all show peaks near the frequencies of the classical sectoral Rossby waves for azimuthal wavenumbers in the range 3 ≤ m ≤ 15. The mode frequencies and linewidths of the modes with m ≤ 9 are consistent with a previous study whereas modes with m ≥ 10 are shifted toward less negative frequencies by 10-20 nHz. While most of these modes have e-folding lifetimes on the order of a few months, the longest lived mode, m = 3, has an e-folding lifetime of more than one year. For each mode, the rms velocity at the equator is in the range of 1-3 m s-1, with the largest values for m ∼ 10. No evidence for the m = 2 sectoral mode is found in the power spectrum, implying that the rms velocity of this mode is below ∼0.5 m s-1.
Conclusions: This work confirms the existence of equatorial global Rossby waves in the solar interior over the past two solar cycles and shows that time-distance helioseismology is a promising technique to study them deep in the convection zone.

The movie associated to Fig. 1 is available at https://www.aanda.org Title: Twenty-one-year helioseismic measurement of solar meridional circulation from SOHO/MDI and SDO/HMI: Anomalous northern hemisphere during cycle 24 Authors: Liang, Zhi-Chao; Gizon, Laurent; Birch, Aaron C.; Duvall, Thomas L., Jr.; Rajaguru, S. P. Bibcode: 2018csc..confE..59L Altcode: We apply time-distance helioseismology to MDI and HMI medium-degree Dopplergrams covering May 1996-April 2017, i.e., 12-yr of cycle 23 and 9-yr of cycle 24. Our data analysis takes several systematic effects into account, including the P-angle error, surface magnetic field effects, and the center-to-limb variations. For comparison, forward-modeled travel-time differences are computed in the ray approximation for representative meridional flow models. The measured 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 about 0.8 solar radii, accompanied by local inflows toward the activity belts in the near-surface layers. Cycle 24's northern hemisphere is found to be anomalous: travel-time differences are significantly smaller when travel distances are greater than 20 deg. This asymmetry between northern and southern hemispheres during cycle 24 was not present in previous measurements (e.g., Rajaguru & Antia 2015), 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 about 30 deg, but they do not vanish for larger travel distances. Rather than a P-angle error, this equatorial offset for large travel distances might be caused by the asymmetrical near-surface flows around the end points of the acoustic ray paths. Title: Rossby waves in the solar convection zone measured by deep-focus time-distance helioseismology Authors: Duvall, T. L., Jr.; Birch, A. C.; Liang, Z. -C.; Gizon, L. Bibcode: 2018csc..confE..57D Altcode: Recent work by Loeptien et al. has shown spectral signatures of equatorial Rossby waves in the solar photosphere (via correlation tracking of granulation) and in the outer 20 Mm of the convection zone (via helioseismic ring diagrams). This result is potentially extremely important for understanding convection zone dynamics and as such should be studied by all available techniques. To this end we have searched for these Rossby waves using deep-focus time-distance helioseismology in 8 years of HMI medium resolution (medium l) Dopplergrams. We also see the signatures of equatorial Rossby waves for focus depths of 0 Mm (photosphere) down to 70 Mm below the surface. At 105 Mm (mid convection zone) and 210 Mm (bottom of convection zone) no such signatures are seen, although whether this is a s/n issue is not determined. We will hopefully be able to determine the radial eigenfunctions of the Rossby waves from this type of measurement. Title: HMI Data Corrected for Scattered Light Compared to Hinode SOT-SP Data Authors: Norton, A. A.; Duvall, T. L., Jr.; Schou, J.; Cheung, M. C. M.; Scherrer, P. H.; Chu, K. C.; Sommers, J. Bibcode: 2018csc..confE.101N Altcode: In March 2018, the Helioseismic Magnetic Imager (HMI) team began providing full-disk data to the public on a daily basis that were corrected for scattered light. In addition to the intensity and magnetogram data, the improved vector magnetic field maps are also provided. The process uses a Richardson-Lucy algorithm and a known PSF. The deconvolution results in a few percent decrease in umbral intensity corresponding to a 200 K decrease in temperature, a doubling of the intensity contrast of granulation from 3.6 to 7.2%, an increase in total field strength values (not only line-of-sight B) in plage by 1.4, faculae brightening and network darkening, and a partial correction for the convective blue-shift. The new data series can be found in JSOC with names similar to the original but with the qualifying term '_dcon' or '_dconS' appended (denoting whether the deconvolution was applied to the filtergrams or Stokes images). Comparisons to near-simultaneous Hinode SOT-SP data demonstrate that the correction brings the two instruments into much better agreement, including the inverted magnetic field parameters. We compare our results to similar efforts in the literature such as work by Diaz Baso and Asensio Ramos (2018) in which HMI intensity and magnetogram data was enhanced using neural networks and super-resolution. Title: Solar meridional circulation from twenty-one years of SOHO/MDI and SDO/HMI observations. Helioseismic travel times and forward modeling in the ray approximation Authors: Liang, Zhi-Chao; Gizon, Laurent; Birch, Aaron C.; Duvall, Thomas L.; Rajaguru, S. P. Bibcode: 2018A&A...619A..99L Altcode: 2018arXiv180808874L 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.

The data are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/619/A99 Title: Revisiting helioseismic constraints on subsurface convection Authors: Birch, Aaron; Duvall, Tom; Gizon, Laurent; Hanasoge, Shravan; Hindman, Bradley; Nagashima, Kaori; Sreenivasan, Katepalli Bibcode: 2018csc..confE..42B Altcode: There is disagreement by orders of magnitude between different helioseismic measurements of the the amplitude of subsurface convective flows. In addition, there are enormous differences between some measurements and simulations of subsurface convection. Further observational and theoretical work on the topic of solar subsurface convection is crucial. Motivated by the need to establish a clear baseline for future work, we present a uniform view of the existing results by expressing upper limits and flow estimates as root-mean-square velocity per multiplet for all cases. The disagreements between the upper limit of Hanasoge, Duvall, and Sreenivasan (2012), the ASH simulations of Miesch et al. (2008), and the helioseismic analysis of Greer et al. (2015) remain, but are reduced in amplitude. Reconciling the helioseismic masurements may involve reconsidering the assumptions about the vertical correlations of the flow field and the methods for separating signal and noise. Title: VizieR Online Data Catalog: Helioseismic measurements of solar meridional flow (Liang+, 2018) Authors: Liang, Z. -C.; Gizon, L.; Birch, A. C.; Duvall, T. L. Jr; Rajaguru, S. P. Bibcode: 2018yCat..36190099L Altcode: The measured travel-time shifts are averaged over three periods, cycle 23 (May 1996 to April 2008; 3051 days used), cycle 24 (May 2008 to April 2017; 2833 days used), and both the cycles 23 and 24 (May 1996 to April 2017; 5884 days used), as a function of latitude and travel distance. Also provided are the standard errors of the temporal means over the three periods. They are all in units of seconds. The coordinates of these maps can be obtained from the WCS keywords in the headers; that is, latitude = (i - CRPIX1)*CDELT1 + CRVAL1 [deg], and distance = (j - CRPIX2)*CDELT2 + CRVAL2 [deg], where i=1..200 and j=1..61.

(7 data files). Title: Probing sunspots with two-skip time-distance helioseismology Authors: Duvall, Thomas L., Jr.; Cally, Paul S.; Przybylski, Damien; Nagashima, Kaori; Gizon, Laurent Bibcode: 2018A&A...613A..73D Altcode: 2018arXiv180601032D Context. Previous helioseismology of sunspots has been sensitive to both the structural and magnetic aspects of sunspot structure.
Aims: We aim to develop a technique that is insensitive to the magnetic component so the two aspects can be more readily separated.
Methods: 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.
Results: 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.
Conclusions: With sufficient modeling effort, these should lead to better understanding of sunspot structure. Title: Helioseismic Constraints on the Subsurface Flows of the Averaged Supergranule Authors: Braun, Douglas C.; Duvall, Thomas L., Jr.; Felipe, Tobias; DeGrave, Kyle Bibcode: 2018tess.conf11506B Altcode: We report progress on constraining the subsurface flow properties of supergranulation from helioseismic holography applied to HMI/SDO observations of over 63,000 individual supergranules. First, using surface-focused measurements, we confirm the advantages of broader phase-speed filters in reducing diffraction effects, noted by Duvall and collaborators in prior time-distance analyses. Second, we expand the type of measurements to include deep-focusing geometries. Third, we compare all measurements with predictions made using numerical wave-propagation simulations performed with the 3D MANCHA code using a number of prescribed flow patterns. These model flows include those inferred from prior time-distance analyses as well as a model based on results of recent fully-convective MURaM-based computations. While the model predictions are in general qualitative agreement with the ensemble averaged measurements, no single model is fully consistent with the complete set of measurements. This suggests there is room for improvement in constraining the subsurface flows.

This work is supported by NASA grant 80NSSC18K0068 awarded to NWRA, as well as by the NASA High-End Computing program at Ames Research Center. Title: Global-scale equatorial Rossby waves as an essential component of solar internal dynamics Authors: Löptien, Björn; Gizon, Laurent; Birch, Aaron C.; Schou, Jesper; Proxauf, Bastian; Duvall, Thomas L.; Bogart, Richard S.; Christensen, Ulrich R. Bibcode: 2018NatAs...2..568L Altcode: 2018NatAs.tmp...54L; 2018arXiv180507244L The Sun's complex dynamics is controlled by buoyancy and rotation in the convection zone. Large-scale flows are dominated by vortical motions1 and appear to be weaker than expected in the solar interior2. One possibility is that waves of vorticity due to the Coriolis force, known as Rossby waves3 or r modes4, remove energy from convection at the largest scales5. However, the presence of these waves in the Sun is still debated. Here, we unambiguously discover and characterize retrograde-propagating vorticity waves in the shallow subsurface layers of the Sun at azimuthal wavenumbers below 15, with the dispersion relation of textbook sectoral Rossby waves. The waves have lifetimes of several months, well-defined mode frequencies below twice the solar rotational frequency, and eigenfunctions of vorticity that peak at the equator. Rossby waves have nearly as much vorticity as the convection at the same scales, thus they are an essential component of solar dynamics. We observe a transition from turbulence-like to wave-like dynamics around the Rhines scale6 of angular wavenumber of approximately 20. This transition might provide an explanation for the puzzling deficit of kinetic energy at the largest spatial scales. Title: Observing and modeling the poloidal and toroidal fields of the solar dynamo Authors: Cameron, R. H.; Duvall, T. L.; Schüssler, M.; Schunker, H. Bibcode: 2018A&A...609A..56C Altcode: 2017arXiv171007126C Context. The solar dynamo consists of a process that converts poloidal magnetic field to toroidal magnetic field followed by a process that creates new poloidal field from the toroidal field.
Aims: Our aim is to observe the poloidal and toroidal fields relevant to the global solar dynamo and to see if their evolution is captured by a Babcock-Leighton dynamo.
Methods: We used synoptic maps of the surface radial field from the KPNSO/VT and SOLIS observatories, to construct the poloidal field as a function of time and latitude; we also used full disk images from Wilcox Solar Observatory and SOHO/MDI to infer the longitudinally averaged surface azimuthal field. We show that the latter is consistent with an estimate of the longitudinally averaged surface azimuthal field due to flux emergence and therefore is closely related to the subsurface toroidal field.
Results: We present maps of the poloidal and toroidal magnetic fields of the global solar dynamo. The longitude-averaged azimuthal field observed at the surface results from flux emergence. At high latitudes this component follows the radial component of the polar fields with a short time lag of between 1-3 years. The lag increases at lower latitudes. The observed evolution of the poloidal and toroidal magnetic fields is described by the (updated) Babcock-Leighton dynamo model. Title: Measuring solar active region inflows with local correlation tracking of granulation Authors: Löptien, B.; Birch, A. C.; Duvall, T. L.; Gizon, L.; Proxauf, B.; Schou, J. Bibcode: 2017A&A...606A..28L Altcode: 2017arXiv170508833L Context. Sixteen years ago local helioseismology detected spatially extended converging surface flows into solar active regions. These flows 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 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) on board the Solar Dynamics Observatory (SDO).
Results: On average, active regions are surrounded by inflows extending up to 10° 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. Title: Observing and modelling the poloidal and toroidal magnetic fields of the global dynamo Authors: Cameron, Robert; Duvall, Thomas; Schüssler, Manfred; Schunker, Hannah Bibcode: 2017SPD....4830601C Altcode: The large scale solar dynamo is a cycle where poloidal flux is generated from toroidal flux, and toroidal flux is generated from poloidal flux. The toroidal and poloidal fields can be inferred from observations, and the Babcock-Leighton model shows how differential rotation and flux emergence explain the observed evolution of the fields. Title: Stray Light Correction of HMI Data Authors: Norton, Aimee Ann; Duvall, Thomas; Schou, Jesper; Cheung, Mark; Scherrer, Philip H. Bibcode: 2017SPD....4820705N Altcode: The point spread function (PSF) for HMI is an Airy function convolved with a Lorentzian. The parameters are bound by ground-based testing before launch, then post-launch off-limb light curves, lunar eclipse and Venus transit data. The PSF correction is programmed in C and runs within the HMI data processing pipeline environment. A single full-disk intensity image can be processed in less than one second. Deconvolution of the PSF on the Stokes profile data (a linear combination of original filtergrms) is less computationally expensive and is shown to be equivalent to deconvolution applied at the original filtergram level. Results include a decrease in umbral darkness of a few percent (~200 K cooler), a doubling of the granulation contrast in intensity from 3.6 to 7.2%, an increase in plage field strengths by a factor of 1.5, and a partial correction of the convective blueshift in Doppler velocities. Requests for data corrected for stray light are welcome and will be processed by the HMI team. Title: Comparison of acoustic travel-time measurement of solar meridional circulation from SDO/HMI and SOHO/MDI Authors: Duvall, Thomas L.; Liang, Zhi-Chao; Birch, Aaron; Gizon, Laurent; Schou, Jesper Bibcode: 2017SPD....4840103D Altcode: Time-distance helioseismology is one of the primary tools for studying the solar meridional circulation. However, travel-time measurements of the subsurface meridional flow suffer from a variety of systematic errors, such as a center-to-limb variation and an offset due to the P-angle uncertainty of solar images. Here we apply the time-distance technique to contemporaneous medium-degree Dopplergrams produced by SOHO/MDI and SDO/HMI to obtain the travel-time difference caused by meridional circulation throughout the solar convection zone. The P-angle offset in MDI images is measured by cross-correlating MDI and HMI images. The travel-time measurements in the south-north and east-west directions are averaged over the same observation period for the two data sets and then compared to examine the consistency of MDI and HMI travel times after correcting the systematic errors.The offsets in the south-north travel-time difference from MDI data induced by the P-angle error gradually diminish with increasing travel distance. However, these offsets become noisy for travel distances corresponding to waves that reach the base of the convection zone. This suggests that a careful treatment of the P-angle problem is required when studying a deep meridional flow. After correcting the P-angle and the removal of the center-to-limb effect, the travel-time measurements from MDI and HMI are consistent within the error bars for meridional circulation covering the entire convection zone. The fluctuations observed in both data sets are highly correlated and thus indicate their solar origin rather than an instrumental origin. Although our results demonstrate that the ad hoc correction is capable of reducing the wide discrepancy in the travel-time measurements from MDI and HMI, we cannot exclude the possibility that there exist other systematic effects acting on the two data sets in the same way. Title: Comparison of acoustic travel-time measurements of solar meridional circulation from SDO/HMI and SOHO/MDI Authors: Liang, Zhi-Chao; Birch, Aaron C.; Duvall, Thomas L., Jr.; Gizon, Laurent; Schou, Jesper Bibcode: 2017A&A...601A..46L Altcode: 2017arXiv170400475L Context. Time-distance helioseismology is one of the primary tools for studying the solar meridional circulation, especially in the lower convection zone. However, travel-time measurements of the subsurface meridional flow suffer from a variety of systematic errors, such as a center-to-limb variation and an offset due to the position angle (P-angle) uncertainty of solar images. It has been suggested that the center-to-limb variation can be removed by subtracting east-west from south-north travel-time measurements. This ad hoc method for the removal of the center-to-limb effect has been adopted widely but not tested for travel distances corresponding to the lower convection zone.
Aims: We explore the effects of two major sources of the systematic errors, the P-angle error arising from the instrumental misalignment and the center-to-limb variation, on the acoustic travel-time measurements in the south-north direction.
Methods: We apply the time-distance technique to contemporaneous medium-degree Dopplergrams produced by SOHO/MDI and SDO/HMI to obtain the travel-time difference caused by meridional circulation throughout the solar convection zone. The P-angle offset in MDI images is measured by cross-correlating MDI and HMI images. The travel-time measurements in the south-north and east-west directions are averaged over the same observation period (May 2010 to Apr. 2011) for the two data sets and then compared to examine the consistency of MDI and HMI travel times after applying the above-mentioned corrections.
Results: The offsets in the south-north travel-time difference from MDI data induced by the P-angle error gradually diminish with increasing travel distance. However, these offsets become noisy for travel distances corresponding to waves that reach the base of the convection zone. This suggests that a careful treatment of the P-angle problem is required when studying a deep meridional flow. After correcting the P-angle and the removal of the center-to-limb effect, the travel-time measurements from MDI and HMI are consistent within the error bars for meridional circulation covering the entire convection zone. The fluctuations observed in both data sets are highly correlated and thus indicate their solar origin rather than an instrumental origin. Although our results demonstrate that the ad hoc correction is capable of reducing the wide discrepancy in the travel-time measurements from MDI and HMI, we cannot exclude the possibility that there exist other systematic effects acting on the two data sets in the same way. Title: Recent Developments in Helioseismic Analysis Methods and Solar Data Assimilation Authors: Schad, A.; Jouve, L.; Duvall, T. L., Jr.; Roth, M.; Vorontsov, S. Bibcode: 2017hdsi.book..227S Altcode: No abstract at ADS Title: HMI Data Corrected for Stray Light Now Available Authors: Norton, A. A.; Duvall, T. L.; Schou, J.; Cheung, M. C. M.; Scherrer, P. H. Bibcode: 2016usc..confE..95N Altcode: The form of the point spread function (PSF) derived for HMI is an Airy function convolved with a Lorentzian. The parameters are bound by observational ground-based testing of the instrument conducted prior to launch (Wachter et al., 2012), by full-disk data used to evaluate the off-limb behavior of the scattered light, as well as by data obtained during the Venus transit. The PSF correction has been programmed in both C and cuda C and runs within the JSOC environment using either a CPU or GPU. A single full-disk intensity image can be deconvolved in less than one second. The PSF is described in more detail in Couvidat et al. (2016) and has already been used by Hathaway et al. (2015) to forward-model solar-convection spectra, by Krucker et al. (2015) to investigate footpoints of off-limb solar flares and by Whitney, Criscuoli and Norton (2016) to examine the relations between intensity contrast and magnetic field strengths. In this presentation, we highlight the changes to umbral darkness, granulation contrast and plage field strengths that result from stray light correction. A twenty-four hour period of scattered-light corrected HMI data from 2010.08.03, including the isolated sunspot NOAA 11092, is currently available for anyone. Requests for additional time periods of interest are welcome and will be processed by the HMI team. Title: Observables Processing for the Helioseismic and Magnetic Imager Instrument on the Solar Dynamics Observatory Authors: Couvidat, S.; Schou, J.; Hoeksema, J. T.; Bogart, R. S.; Bush, R. I.; Duvall, T. L.; Liu, Y.; Norton, A. A.; Scherrer, P. H. Bibcode: 2016SoPh..291.1887C Altcode: 2016SoPh..tmp..120C; 2016arXiv160602368C NASA's Solar Dynamics Observatory (SDO) spacecraft was launched 11 February 2010 with three instruments onboard, including the Helioseismic and Magnetic Imager (HMI). After commissioning, HMI began normal operations on 1 May 2010 and has subsequently observed the Sun's entire visible disk almost continuously. HMI collects sequences of polarized filtergrams taken at a fixed cadence with two 4096 ×4096 cameras, from which are computed arcsecond-resolution maps of photospheric observables that include line-of-sight velocity and magnetic field, continuum intensity, line width, line depth, and the Stokes polarization parameters [I ,Q ,U ,V ]. Two processing pipelines have been implemented at the SDO Joint Science Operations Center (JSOC) at Stanford University to compute these observables from calibrated Level-1 filtergrams, one that computes line-of-sight quantities every 45 seconds and the other, primarily for the vector magnetic field, that computes averages on a 720-second cadence. Corrections are made for static and temporally changing CCD characteristics, bad pixels, image alignment and distortion, polarization irregularities, filter-element uncertainty and nonuniformity, as well as Sun-spacecraft velocity. We detail the functioning of these two pipelines, explain known issues affecting the measurements of the resulting physical quantities, and describe how regular updates to the instrument calibration impact them. We also describe how the scheme for computing the observables is optimized for actual HMI observations. Initial calibration of HMI was performed on the ground using a variety of light sources and calibration sequences. During the five years of the SDO prime mission, regular calibration sequences have been taken on orbit to improve and regularly update the instrument calibration, and to monitor changes in the HMI instrument. This has resulted in several changes in the observables processing that are detailed here. The instrument more than satisfies all of the original specifications for data quality and continuity. The procedures described here still have significant room for improvement. The most significant remaining systematic errors are associated with the spacecraft orbital velocity. Title: Statistical Differences in Time-Distance Helioseismology Results Authors: Hess Webber, Shea A.; Pesnell, William D.; Duvall, Thomas; Cameron, Robert; Birch, A. C. Bibcode: 2016SPD....4720301H Altcode: Time-distance helioseismology studies phase correlations in solar wave modes. We use these techniques to investigate the phase differences in f-mode wave propagation within a coronal hole feature and without. We isolate the coronal hole boundary location using edge detection techniques on SDO AIA data. We then use this location information to inform the analysis of the corresponding HMI time-distance velocity tracked data product, provided by Stanford's JSOC archive. We look at time-distance results inside the coronal hole, outside the coronal hole, the coronal hole data as a whole, and an independent quiet sun region. We use Student's t-Test to evaluate the significance of the differences between the various regions. Title: The shrinking Sun: A systematic error in local correlation tracking of solar granulation Authors: Löptien, B.; Birch, A. C.; Duvall, T. L.; Gizon, L.; Schou, J. Bibcode: 2016A&A...590A.130L Altcode: 2016arXiv160404469L 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 toward disk center, which is also known as the shrinking-Sun effect.
Aims: We aim to study the nature of the shrinking-Sun effect for continuum intensity data and to derive a simple model that can explain its origin.
Methods: We derived LCT flow maps by running the LCT code Fourier Local Correlation Tracking (FLCT) on tracked and remapped continuum intensity maps provided by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). 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 the HMI data and 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 toward disk center. Title: On HMI's Mod-L Sequence: Test and Evaluation Authors: Liu, Yang; Baldner, Charles; Bogart, R. S.; Bush, R.; Couvidat, S.; Duvall, Thomas L.; Hoeksema, Jon Todd; Norton, Aimee Ann; Scherrer, Philip H.; Schou, Jesper Bibcode: 2016SPD....47.0810L Altcode: HMI Mod-L sequence can produce full Stokes parameters at a cadence of 90 seconds by combining filtergrams from both cameras, the front camera and the side camera. Within the 90-second, the front camera takes two sets of Left and Right Circular Polarizations (LCP and RCP) at 6 wavelengths; the side camera takes one set of Linear Polarizations (I+/-Q and I+/-U) at 6 wavelengths. By combining two cameras, one can obtain full Stokes parameters of [I,Q,U,V] at 6 wavelengths in 90 seconds. In norminal Mod-C sequence that HMI currently uses, the front camera takes LCP and RCP at a cadence of 45 seconds, while the side camera takes observation of the full Stokes at a cadence of 135 seconds. Mod-L should be better than Mod-C for providing vector magnetic field data because (1) Mod-L increases cadence of full Stokes observation, which leads to higher temporal resolution of vector magnetic field measurement; (2) decreases noise in vector magnetic field data because it uses more filtergrams to produce [I,Q,U,V]. There are two potential issues in Mod-L that need to be addressed: (1) scaling intensity of the two cameras’ filtergrams; and (2) if current polarization calibration model, which is built for each camera separately, works for the combined data from both cameras. This presentation will address these questions, and further place a discussion here. Title: The Processing of Observables Made by the HMI Instrument on SDO Authors: Hoeksema, Jon Todd; Schou, Jesper; Couvidat, Sebastien; Bogart, Richard S.; Bush, Rock; Duvall, Thomas L.; Liu, Yang; Norton, Aimee Ann; Scherrer, Philip H. Bibcode: 2016SPD....47.0808H Altcode: The Helioseismic and Magnetic Imager (HMI) acquires sequences of polarized filtergrams of the Sun from which observable quantities are computed. The observables include five line-of-sight quantities - magnetic field, velocity, continuum intensity, line depth, and line width - as well as Stokes polarization parameters. The process of turning a set of filtergrams into calibrated measurements is quite involved. Since May 2010 the streams of data from HMI’s two cameras have been treated separately. The frame list for the Doppler camera repeats every 45 seconds and the images are combined to determine the line-of-sight observables. The Vector camera sequence measures additional polarizations and so requires 135s; images from ten sequences are combined every 720s to determine the four Stokes polarization parameters at each of six wavelengths, as well as the LoS observables. A variety of calibration corrections are made to the Level-1 filtergrams to account for distortion, image motion and alignment, polarization, wavelength and intensity irregularities, camera issues, solar rotation, and other effects. Residual random variations in the final observables are consistent with photon noise levels, but systematic errors remain that have not been fully corrected. Of particular concern are those associated with the velocity of the instrument relative to the Sun due to the geosynchronous orbit of the Solar Dynamics Observatory (SDO) spacecraft. This presentation describes the creation of the observables, characterizes the residual errors, and indicates plans for future improvements - including correction for the instrument point spread function. All HMI data are available at http://jsoc.stanford.edu. Title: Systematic and S/N improvements in Time-Distance Helioseismology from Azimuthal Filtering Authors: Duvall, Thomas L. Bibcode: 2016SPD....4720302D Altcode: Travel times are normally measured between pairs of points in the solar photosphere by making temporal cross correlations of a signal (e.g. Doppler line shift) and detecting the systematic signatures in the correlation. Normally the solar wave field is directionally isotropic and the pair of points is most sensitive to waves propagating between the two points. However, we have found that the oppositely propagating waves, normally cleanly separated in the correlation, contribute inordinately to the noise. In situations such as sunspots or possibly supergranulation, where there are anisotropies of the wave field, there can also be associated systematic errors. By doing directional filtering some situations can be improved. Title: Data compression for local correlation tracking of solar granulation Authors: Löptien, B.; Birch, A. C.; Duvall, T. L.; Gizon, L.; Schou, J. Bibcode: 2016A&A...587A...9L Altcode: 2015arXiv151203243L Context. Several upcoming and proposed space missions, such as Solar Orbiter, will be limited in telemetry and thus require data compression.
Aims: We test the impact of data compression on local correlation tracking (LCT) of time series of continuum intensity images. We evaluate the effect of several lossy compression methods (quantization, JPEG compression, and a reduced number of continuum images) on measurements of solar differential rotation with LCT.
Methods: We applied the different compression methods to tracked and remapped continuum intensity maps obtained by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory. We derived 2D vector velocities using the local correlation tracking code Fourier Local Correlation Tracking (FLCT) and determined the additional bias and noise introduced by compression to differential rotation.
Results: We find that probing differential rotation with LCT is very robust to lossy data compression when using quantization. Our results are severely affected by systematic errors of the LCT method and the HMI instrument. The sensitivity of LCT to systematic errors is a concern for Solar Orbiter. Title: Recent Developments in Helioseismic Analysis Methods and Solar Data Assimilation Authors: Schad, A.; Jouve, L.; Duvall, T. L.; Roth, M.; Vorontsov, S. Bibcode: 2015SSRv..196..221S Altcode: 2015SSRv..tmp...84S; 2016arXiv160304742S We review recent advances and results in enhancing and developing helioseismic analysis methods and in solar data assimilation. In the first part of this paper we will focus on selected developments in time-distance and global helioseismology. In the second part, we review the application of data assimilation methods on solar data. Relating solar surface observations as well as helioseismic proxies with solar dynamo models by means of the techniques from data assimilation is a promising new approach to explore and to predict the magnetic activity cycle of the Sun. Title: Measurement and Interpretation of Travel-Time Shifts in the context of Time-Distance Helioseismic Detection of Meridional Flows in the Solar Convection Zone Authors: Chakraborty, S.; Duvall, T. L., Jr.; Hanasoge, S.; Hartlep, T.; Larson, T. P.; Kholikov, S. Bibcode: 2014AGUFMSH41B4141C Altcode: The role of meridional flow in maintaining the solar dynamo and differential rotation in the solar convection zone is not well understood and is currently under scrutiny. The traditional flux-transport dynamo models have posited the well known single-cell meridional flow with poleward flow at the photosphere and equatorward flow near the base of the convection zone. However, recent investigations seem to be revealing a different picture of meridional flow which is double celled in the radial direction with poleward flow at the photosphere and equatorward flow at a much shallower level in the convection zone. In this work time-distance helioseismology is used to probe the solar convection zone to accurately determine the structure of meridional circulation. Helioseismology uses the photospherically visible aspect of (acoustic, surface-gravity) waves, that propagate and interfere throughout the Sun to form standing oscillation modes, as probes to make inferences about the structure and flows on the solar surface and interior. Time-distance helioseismology is based on measuring the travel-times of wave-packets moving between distinct points on the solar surface. Travel-time shifts obtained by calculating the difference in the travel-times of counter-propagating waves between the same points on the solar surface yield information about flows throughout the solar convection zone. In this work time-distance techniques are applied on artificial and solar Doppler velocity images to detect travel-time shifts due to meridional flow. Modifications are suggested to enhance the signal-to-noise ratio of travel-time shift measurements. The artificial data is constructed by embedding various meridional flow models in 3D acoustic simulators, which is then used to discuss the interpretation of travel-time shifts, so that in the future an inversion procedure may be designed to calculate meridional flow velocities with greater accuracy. The solar data is obtained from the Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO) spacecraft and is used to measure travel-time shifts due to meridional flow. The issue of a systematic error dubbed as the 'center-to-limb effect' that contaminates the travel-time shift measurements of solar meridional flow is also addressed in this work. Title: Additional Evidence Supporting a Model of Shallow, High-Speed Supergranulation Authors: Duvall, T. L.; Hanasoge, S. M.; Chakraborty, S. Bibcode: 2014SoPh..289.3421D Altcode: 2014arXiv1404.2533D; 2014SoPh..tmp...76D Recently, Duvall and Hanasoge (Solar Phys.287, 71, 2013) found that large-distance separation [Δ] travel-time differences from a center to an annulus [δtoi] implied a model of the average supergranular cell that has a peak upflow of 240 m s−1 at a depth of 2.3 Mm and a corresponding peak outward horizontal flow of 700 m s−1 at a depth of 1.6 Mm. In the present work, this effect is further studied by measuring and modeling center-to-quadrant travel-time differences [δtqu], 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 [δtoi] are found not to vary with heliocentric angle. A consistency check finds an increase of δtoi with the temporal frequency [ν] by a factor of two, which is not predicted by the ray theory. Title: Waves Excited by Noise: Applications to Helioseismology and Beyond Authors: Duvall, Thomas L. Bibcode: 2014AAS...22411701D Altcode: The vigorous granular convection just beneath the solar photosphere excites acoustic waves. The resultant normal modes of the whole Sun are analogous to the ringing of a bell in a sandstorm. In classical helioseismology, the normal modes are used to study global solar properties, including the sound speed versus radius throughout the Sun and the rotation rate versus depth and latitude in the outer half of the Sun. But solar astronomers wished to better understand the subphotospheric 3-d structure of smaller scale features observed in the photosphere and were hence not satisfied studying only these global properties. One would like to have something akin to seismology, in which the waves from an earthquake traveling from a source through the interior to a distant receiver depend only on the properties along the path. However, the random nature of the solar convective wave sources generally prevents such a simple analogy. It was discovered that the temporal cross correlation of the solar oscillation signal between two locations averaged over many wave periods does mostly contain information about the properties along the path connecting the two locations. This discovery is the basis for time-distance helioseismology, in which travel times are extracted from the temporal cross correlations and mapped for different pairs of locations on the solar surface. A subsequent 3-d tomography aims to map the solar interior. This technique has been used to study the depth variation of a variety of solar photospheric features from small spatial scales (granulation) to larger scales (sunspots and supergranulation) to the largest scales (meridional circulation and rotation). The technique of temporal cross correlation of noise signals from two locations has subsequently been adopted successfully in a number of other fields including seismology, ultrasound, infrasound, ocean acoustics, structural engineering, lunar seismology, and medical diagnostics. Title: Subsurface Supergranular Vertical Flows as Measured Using Large Distance Separations in Time-Distance Helioseismology Authors: Duvall, T. L.; Hanasoge, S. M. Bibcode: 2013SoPh..287...71D Altcode: 2012SoPh..tmp..136D; 2012arXiv1207.6075D As large-distance rays (say, 10 - 24°) approach the solar surface approximately vertically, travel times measured from surface pairs for these large separations are mostly sensitive to vertical flows, at least for shallow flows within a few Mm of the solar surface. All previous analyses of supergranulation have used smaller separations and have been hampered by the difficulty of separating the horizontal and vertical flow components. We find that the large-separation travel times associated with supergranulation cannot be studied using the standard phase-speed filters of time-distance helioseismology. These filters, whose use is based upon a refractive model of the perturbations, reduce the resultant travel-time signal by at least an order of magnitude at some distances. More effective filters are derived. Modeling suggests that the center-annulus travel-time difference [δtoi] in the separation range Δ=10 - 24∘ is insensitive to the horizontally diverging flow from the centers of the supergranules and should lead to a constant signal from the vertical flow. Our measurement of this quantity, 5.1±0.1 seconds, is constant over the distance range. This magnitude of the signal cannot be caused by the level of upflow at cell centers seen at the photosphere of 10 m s−1 extended in depth. It requires the vertical flow to increase with depth. A simple Gaussian model of the increase with depth implies a peak upward flow of 240 m s−1 at a depth of 2.3 Mm and a peak horizontal flow of 700 m s−1 at a depth of 1.6 Mm. Title: Detection of Equatorward Meridional Flow and Evidence of Double-cell Meridional Circulation inside the Sun Authors: Zhao, Junwei; Bogart, R. S.; Kosovichev, A. G.; Duvall, T. L., Jr.; Hartlep, Thomas Bibcode: 2013ApJ...774L..29Z Altcode: 2013arXiv1307.8422Z Meridional flow in the solar interior plays an important role in redistributing angular momentum and transporting magnetic flux inside the Sun. Although it has long been recognized that the meridional flow is predominantly poleward at the Sun's surface and in its shallow interior, the location of the equatorward return flow and the meridional flow profile in the deeper interior remain unclear. Using the first 2 yr of continuous helioseismology observations from the Solar Dynamics Observatory/Helioseismic Magnetic Imager, we analyze travel times of acoustic waves that propagate through different depths of the solar interior carrying information about the solar interior dynamics. After removing a systematic center-to-limb effect in the helioseismic measurements and performing inversions for flow speed, we find that the poleward meridional flow of a speed of 15 m s-1 extends in depth from the photosphere to about 0.91 R ⊙. An equatorward flow of a speed of 10 m s-1 is found between 0.82 and 0.91 R ⊙ in the middle of the convection zone. Our analysis also shows evidence of that the meridional flow turns poleward again below 0.82 R ⊙, indicating an existence of a second meridional circulation cell below the shallower one. This double-cell meridional circulation profile with an equatorward flow shallower than previously thought suggests a rethinking of how magnetic field is generated and redistributed inside the Sun. Title: Solar Interior Meridional Flow from SDO/HMI Authors: Zhao, Junwei; Bogart, R. S.; Kosovichev, A. G.; Duvall, T. L.; Hartlep, T. Bibcode: 2013SPD....4420402Z Altcode: Since the launch of Solar Dynamics Observatory, the Helioseismic and Magnetic Imager has accumulated 3 years of continuous observations. Using time-distance helioseismology, we have obtained new results on both global and local scales. By analyzing the first two years' observations, we were able to detect the equatorward meridional flow at a depth of around 65 Mm, and detect the existence of a second meridional circulation cell below about 120 Mm. This new profile of interior meridional flow will pose challenges to the solar dynamo models. At the shallower depths, we studied the temporal evolution of the zonal and meridional flows. We found that both quantities showed strong hemispherical asymmetries. Using global wavefield simulations that have pre-set meridional flow profiles, we also assess the capability of our analysis technique in recovering week flows in the deep interior. Title: Recent Local Helioseismology Results from SDO/HMI Authors: Zhao, Junwei; Bogart, R. S.; Kosovichev, A. G.; Duvall, T. L., Jr.; Hartlep, Thomas Bibcode: 2013enss.confE.118Z Altcode: Since the launch of Solar Dynamics Observatory, the Helioseismic and Magnetic Imager has accumulated 2.5 years of continuous observations. Using time-distance helioseismology, we have obtained new results on both global and local scales. By analyzing the first two years' observations, we were able to detect the equator-ward meridional flow at a depth of around 70 Mm, and detect the existence of a second meridional circulation below about 120 Mm. This new profile of interior meridional flow will pose challenges to the current dynamo models. At the shallower depth, we studied the temporal evolution of zonal flows, as well as the residual meridional flow, which was obtained by subtracting an averaged meridional flow profile. We found that both quantities showed strong hemispherical asymmetries. On local scales, we studied subsurface flows inside active regions and supergranules, as well as the potential links between solar flares and these subsurface dynamics. Title: Stray Light Correction for HMI Data Authors: Norton, A. A.; Duvall, T.; Schou, J.; Cheung, M. Bibcode: 2013enss.confE..95N Altcode: Our goal is to find a deconvolution routine that can remove scattered light in sunspot umbrae without introducing extraneous power in high spatial frequencies in helioseismology analysis of the same data. Using ground-based calibration data, a third-order polynomial fit was obtained for the instrumental modulation transfer function (MTF). Images of the solar limb and the limb and disk of Venus during its transit were used to model stray light. An Airy function and a Lorentzian are used in combination to model the instrumental point spread function (PSF) for HMI which is made to be positive definite everywhere and zero above the ideal optical Nyquist frequency. Deconvolution was carried out using a Lucy-Richardson algorithm on a graphics processing unit. The deconvolved image is then compared to the original to determine the extent of introduced Gibb's phenomenon (ringing) and how the power changes as a function of spatial frequency. Title: Approach to Integrate Global-Sun Models of Magnetic Flux Emergence and Transport for Space Weather Studies Authors: Mansour, Nagi Nicolas; Wray, A.; Mehrotra, P.; Henney, C.; arge, N.; Manchester, C.; Godinez, H.; Koller, J.; Kosovichev, A.; Scherrer, P.; Zhao, J.; Stein, R.; Duvall, T.; Fan, Y. Bibcode: 2013enss.confE.125M Altcode: The Sun lies at the center of space weather and is the source of its variability. The primary input to coronal and solar wind models is the activity of the magnetic field in the solar photosphere. Recent advancements in solar observations and numerical simulations provide a basis for developing physics-based models for the dynamics of the magnetic field from the deep convection zone of the Sun to the corona with the goal of providing robust near real-time boundary conditions at the base of space weather forecast models. The goal is to develop new strategic capabilities that enable characterization and prediction of the magnetic field structure and flow dynamics of the Sun by assimilating data from helioseismology and magnetic field observations into physics-based realistic magnetohydrodynamics (MHD) simulations. The integration of first-principle modeling of solar magnetism and flow dynamics with real-time observational data via advanced data assimilation methods is a new, transformative step in space weather research and prediction. This approach will substantially enhance an existing model of magnetic flux distribution and transport developed by the Air Force Research Lab. The development plan is to use the Space Weather Modeling Framework (SWMF) to develop Coupled Models for Emerging flux Simulations (CMES) that couples three existing models: (1) an MHD formulation with the anelastic approximation to simulate the deep convection zone (FSAM code), (2) an MHD formulation with full compressible Navier-Stokes equations and a detailed description of radiative transfer and thermodynamics to simulate near-surface convection and the photosphere (Stagger code), and (3) an MHD formulation with full, compressible Navier-Stokes equations and an approximate description of radiative transfer and heating to simulate the corona (Module in BATS-R-US). CMES will enable simulations of the emergence of magnetic structures from the deep convection zone to the corona. Finally, a plan will be summarized on the development of a Flux Emergence Prediction Tool (FEPT) in which helioseismology-derived data and vector magnetic maps are assimilated into CMES that couples the dynamics of magnetic flux from the deep interior to the corona. Title: Understanding the Dynamics of Supergranulation Using Helioseismology and Linear Acoustic Simulations Authors: Duvall, T. L., Jr.; Hanasoge, S. M. Bibcode: 2013enss.confE..92D Altcode: Some recent work of Duvall and Hanasoge has suggested that supergranulation, on average, is shallow, with peak flows 2 Mm below the photosphere, and is quite vigorous, with peak flows (vertical 200 m/s; horizontal 700 m/s) considerably larger than the photospheric values (vertical 10 m/s; horizontal 300 m/s). This result is based on measurements of center-annulus travel time differences measured in time-distance helioseismology with larger separations (up to 24 heliocentric degrees) than previously considered and on ray theory forward modeling. This result is somewhat at odds with earlier work based on smaller separations (less than 5 heliocentric degrees). To test whether the vigorous, shallow supergranulation is viable, linear acoustic simulations were done with flow perturbations consistent with the above result. The depth of the model is 70 Mm, enabling the use of separations up to 18 deg heliocentric. The simulation has high enough horizontal resolution to study small separations ( 1 deg) and also to examine the f mode. With this simulation then, we should be able to examine then both short and large separations and both p and f modes. By computing rays through the flow model, we can also then test the applicability of ray theory for this particular case. Title: Detection of Equator-ward Meridional Flows in the Deep Solar Interior Authors: Zhao, J.; Bogart, R. S.; Kosovichev, A. G.; Duvall, T. L. Bibcode: 2012AGUFMSH13C2266Z Altcode: The meridional flow observed on the solar surface is a slow plasma motion from the equator to the poles. Flux-transport dynamo models of the solar cycle assume that this flow transports magnetic field of decaying active regions and causes polar field reversals. At what depth the meridional flow turns to equator-ward and how fast is the return flow are important questions for a better understanding of the dynamo process, and are also long-time puzzles of helioseismology. A recent finding of a systematic center-to-limb variation in the time-distance helioseismology measurements allows us to develop an empirical correction procedure for acoustic travel times, and improve the accuracy of helioseismic inferences. Using the helioseismic data of two entire years of SDO/HMI continuous observations and removing the systematic effect, we have detected the equator-ward meridional flows. Inversion of the travel times shows that the near-surface pole-ward meridional flow starts turning equator-ward at approximately 0.92 R_sun at low latitudes, and that the depth of the flow turning point increases with latitude. The equator-ward flow has a speed of 10 m/s or so, and extends from the surface to about 0.82 R_sun. Our analysis also shows evidences for a second meridional circulation cell starting at about 0.82 R_sun and extending deep to near the tachocline area (0.7 R_sun). Title: Comparison of Line-of-Sight Magnetograms Taken by the Solar Dynamics Observatory/Helioseismic and Magnetic Imager and Solar and Heliospheric Observatory/Michelson Doppler Imager Authors: Liu, Y.; Hoeksema, J. T.; Scherrer, P. H.; Schou, J.; Couvidat, S.; Bush, R. I.; Duvall, T. L.; Hayashi, K.; Sun, X.; Zhao, X. Bibcode: 2012SoPh..279..295L Altcode: 2012SoPh..tmp...75L We compare line-of-sight magnetograms from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) and the Michelson Doppler Imager (MDI) onboard the Solar and Heliospheric Observatory (SOHO). The line-of-sight magnetic signal inferred from the calibrated MDI data is greater than that derived from the HMI data by a factor of 1.40. This factor varies somewhat with center-to-limb distance. An upper bound to the random noise for the 1'' resolution HMI 720-second magnetograms is 6.3 Mx cm−2, and 10.2 Mx cm−2 for the 45-second magnetograms. Virtually no p-mode leakage is seen in the HMI magnetograms, but it is significant in the MDI magnetograms. 12-hour and 24-hour periodicities are detected in strong fields in the HMI magnetograms. The newly calibrated MDI full-disk magnetograms have been corrected for the zero-point offset and underestimation of the flux density. The noise is 26.4 Mx cm−2 for the MDI one-minute full-disk magnetograms and 16.2 Mx cm−2 for the five-minute full-disk magnetograms observed with four-arcsecond resolution. The variation of the noise over the Sun's disk found in MDI magnetograms is likely due to the different optical distortions in the left- and right-circular analyzers, which allows the granulation and p-mode to leak in as noise. Saturation sometimes seen in sunspot umbrae in MDI magnetograms is caused by the low intensity and the limitation of the onboard computation. The noise in the HMI and MDI line-of-sight magnetic-field synoptic charts appears to be fairly uniform over the entire map. The noise is 2.3 Mx cm−2 for HMI charts and 5.0 Mx cm−2 for MDI charts. No evident periodicity is found in the HMI synoptic charts. Title: Anomalously weak solar convection Authors: Hanasoge, S. M.; Duvall, T. L.; Sreenivasan, K. R. Bibcode: 2012PNAS..10911928H Altcode: 2012arXiv1206.3173H Convection in the solar interior is thought to comprise structures on a spectrum of scales. This conclusion emerges from phenomenological studies and numerical simulations, though neither covers the proper range of dynamical parameters of solar convection. Here, we analyze observations of the wavefield in the solar photosphere using techniques of time-distance helioseismology to image flows in the solar interior. We downsample and synthesize 900 billion wavefield observations to produce 3 billion cross-correlations, which we average and fit, measuring 5 million wave travel times. Using these travel times, we deduce the underlying flow systems and study their statistics to bound convective velocity magnitudes in the solar interior, as a function of depth and spherical-harmonic degree $\ell$. Within the wavenumber band $\ell<60$, Convective velocities are 20-100 times weaker than current theoretical estimates. This suggests the prevalence of a different paradigm of turbulence from that predicted by existing models, prompting the question: what mechanism transports the heat flux of a solar luminosity outwards? Advection is dominated by Coriolis forces for wavenumbers $\ell<60$, with Rossby numbers smaller than $\sim10^{-2}$ at $r/R_\odot=0.96$, suggesting that the Sun may be a much faster rotator than previously thought, and that large-scale convection may be quasi-geostrophic. The fact that iso-rotation contours in the Sun are not co-aligned with the axis of rotation suggests the presence of a latitudinal entropy gradient. Title: Searching For Equator-ward Meridional Flows In The Solar Interior Authors: Zhao, Junwei; Bogart, R. S.; Kosovichev, A. G.; Duvall, T. L., Jr. Bibcode: 2012AAS...22010905Z Altcode: At what depth the equator-ward meridional flow exists and what is its speed are important questions for a better understanding of solar dynamo and a better prediction of how active a solar cycle is. However, the depth and the speed of the equator-ward flow are still not determined from helioseismology after studies of more than one decade. The new high-resolution observations from HMI has offered us a new chance to tackle these problems. Moreover, the systematic center-to-limb variation that was recently found in time-distance helioseismology analysis probably implies that the equator-ward flows exist in areas much shallower than the tachocline. Utilizing two years' HMI observations, we examine how well we can determine the depth and speed of the returning meridional flow. Title: Subsurface Supergranular Vertical Flows as Measured Using Large Distance Separations in Time-Distance Helioseismology Authors: Duvall, Thomas L., Jr.; Hanasoge, S. M. Bibcode: 2012AAS...22010901D Altcode: As large-distance rays (say, 10-24 deg) approach the solar surface approximately vertically, travel times measured from surface pairs for these large separations are mostly sensitive to vertical flows, at least for shallow flows within a few Mm of the solar surface. All previous analyses of supergranulation have used smaller separations and have been hampered by the difficulty of separating the horizontal and vertical flow components. We find that the large separation travel times associated with supergranulation cannot be studied using the standard phase-speed filters of time-distance helioseismology. These filters, whose use is based upon a refractive model of the perturbations, reduce the resultant travel time signal by at least an order of magnitude at some distances. More effective filters are derived. Modeling suggests that the center-annulus travel time difference in the separation range 10-24 deg is insensitive to the horizontally diverging flow from the centers of the supergranules and should lead to a constant signal from the vertical flow. Our measurement of this quantity for the average supergranule, 5.1 s, is constant over the distance range. This magnitude of signal cannot be caused by the level of upflow at cell centers seen at the photosphere of 10 m/s extended in depth. It requires the vertical flow to increase with depth. A simple Gaussian model of the increase with depth implies a peak upward flow of 240 m/s at a depth of 2.3 Mm and a peak horizontal flow of 700 m/s at a depth of 1.6 Mm. Title: The Effect Of Phase-speed Filtering On Time-distance Analysis Of Meridional Flow In The Solar Convection Zone. Authors: Chakraborty, Sudeepto; Duvall, T. L., Jr.; Hartlep, T. Bibcode: 2012AAS...22052114C Altcode: Meridional circulation in the solar convection zone is a key ingredient in flux-transport type models of the solar magnetic cycle. Time-distance helioseismology is a method that can be used to infer the meridional flow profile in the subsurface regions of the Sun from observations of the solar surface. In this work we use deep-focus time-distance techniques on simulation data to measure travel time differences due to meridional flows deep within the convection zone. In particular, we explore the effect of phase-speed filtering in potentially enhancing our ability to measure travel-time differences due to meridional flows throughout the convection zone. Title: Effects of Spectral Line Formation Height in Time-Distance Helioseismology Authors: Nagashima, K.; Parchevsky, K. V.; Zhao, J.; Duvall, T. L., Jr.; Kosovichev, A. G.; Sekii, T. Bibcode: 2012ASPC..456...57N Altcode: To understand the effect of the formation-height difference in time-distance helioseismology analyses, we consider the wave behavior above the surface. We show that by using the numerically-simulated wavefields at two different heights this difference may cause travel-time shifts due to the non-stationary character of waves excited by near-surface acoustic sources. This needs to be taken into account in multi-wavelength helioseismology and measurements close to the solar limb. Title: A First Look at Magnetic Field Data Products from SDO/HMI Authors: Liu, Y.; Scherrer, P. H.; Hoeksema, J. T.; Schou, J.; Bai, T.; Beck, J. G.; Bobra, M.; Bogart, R. S.; Bush, R. I.; Couvidat, S.; Hayashi, K.; Kosovichev, A. G.; Larson, T. P.; Rabello-Soares, C.; Sun, X.; Wachter, R.; Zhao, J.; Zhao, X. P.; Duvall, T. L., Jr.; DeRosa, M. L.; Schrijver, C. J.; Title, A. M.; Centeno, R.; Tomczyk, S.; Borrero, J. M.; Norton, A. A.; Barnes, G.; Crouch, A. D.; Leka, K. D.; Abbett, W. P.; Fisher, G. H.; Welsch, B. T.; Muglach, K.; Schuck, P. W.; Wiegelmann, T.; Turmon, M.; Linker, J. A.; Mikić, Z.; Riley, P.; Wu, S. T. Bibcode: 2012ASPC..455..337L Altcode: The Helioseismic and Magnetic Imager (HMI; Scherrer & Schou 2011) is one of the three instruments aboard the Solar Dynamics Observatory (SDO) that was launched on February 11, 2010 from Cape Canaveral, Florida. The instrument began to acquire science data on March 24. The regular operations started on May 1. HMI measures the Doppler velocity and line-of-sight magnetic field in the photosphere at a cadence of 45 seconds, and the vector magnetic field at a 135-second cadence, with a 4096× 4096 pixels full disk coverage. The vector magnetic field data is usually averaged over 720 seconds to suppress the p-modes and increase the signal-to-noise ratio. The spatial sampling is about 0".5 per pixel. HMI observes the Fe i 6173 Å absorption line, which has a Landé factor of 2.5. These data are further used to produce higher level data products through the pipeline at the HMI-AIA Joint Science Operations Center (JSOC) - Science Data Processing (Scherrer et al. 2011) at Stanford University. In this paper, we briefly describe the data products, and demonstrate the performance of the HMI instrument. We conclude that the HMI is working extremely well. Title: Systematic Center-to-limb Variation in Measured Helioseismic Travel Times and its Effect on Inferences of Solar Interior Meridional Flows Authors: Zhao, Junwei; Nagashima, Kaori; Bogart, R. S.; Kosovichev, A. G.; Duvall, T. L., Jr. Bibcode: 2012ApJ...749L...5Z Altcode: 2012arXiv1203.1904Z We report on a systematic center-to-limb variation in measured helioseismic travel times, which must be taken into account for an accurate determination of solar interior meridional flows. The systematic variation, found in time-distance helioseismology analysis using SDO/HMI and SDO/AIA observations, is different in both travel-time magnitude and variation trend for different observables. It is not clear what causes this systematic effect. Subtracting the longitude-dependent east-west travel times, obtained along the equatorial area, from the latitude-dependent north-south travel times, obtained along the central meridian area, gives remarkably similar results for different observables. We suggest this as an effective procedure for removing the systematic center-to-limb variation. The subsurface meridional flows obtained from inversion of the corrected travel times are approximately 10 m s-1 slower than those obtained without removing the systematic effect. The detected center-to-limb variation may have important implications in the derivation of meridional flows in the deep interior and needs to be better understood. Title: Implementation and Comparison of Acoustic Travel-Time Measurement Procedures for the Solar Dynamics Observatory/Helioseismic and Magnetic Imager Time - Distance Helioseismology Pipeline Authors: Couvidat, S.; Zhao, J.; Birch, A. C.; Kosovichev, A. G.; Duvall, T. L.; Parchevsky, K.; Scherrer, P. H. Bibcode: 2012SoPh..275..357C Altcode: The Helioseismic and Magnetic Imager (HMI) instrument onboard the Solar Dynamics Observatory (SDO) satellite is designed to produce high-resolution Doppler-velocity maps of oscillations at the solar surface with high temporal cadence. To take advantage of these high-quality oscillation data, a time - distance helioseismology pipeline (Zhao et al., Solar Phys. submitted, 2010) has been implemented at the Joint Science Operations Center (JSOC) at Stanford University. The aim of this pipeline is to generate maps of acoustic travel times from oscillations on the solar surface, and to infer subsurface 3D flow velocities and sound-speed perturbations. The wave travel times are measured from cross-covariances of the observed solar oscillation signals. For implementation into the pipeline we have investigated three different travel-time definitions developed in time - distance helioseismology: a Gabor-wavelet fitting (Kosovichev and Duvall, SCORE'96: Solar Convection and Oscillations and Their Relationship, ASSL, Dordrecht, 241, 1997), a minimization relative to a reference cross-covariance function (Gizon and Birch, Astrophys. J.571, 966, 2002), and a linearized version of the minimization method (Gizon and Birch, Astrophys. J.614, 472, 2004). Using Doppler-velocity data from the Michelson Doppler Imager (MDI) instrument onboard SOHO, we tested and compared these definitions for the mean and difference travel-time perturbations measured from reciprocal signals. Although all three procedures return similar travel times in a quiet-Sun region, the method of Gizon and Birch (Astrophys. J.614, 472, 2004) gives travel times that are significantly different from the others in a magnetic (active) region. Thus, for the pipeline implementation we chose the procedures of Kosovichev and Duvall (SCORE'96: Solar Convection and Oscillations and Their Relationship, ASSL, Dordrecht, 241, 1997) and Gizon and Birch (Astrophys. J.571, 966, 2002). We investigated the relationships among these three travel-time definitions, their sensitivities to fitting parameters, and estimated the random errors that they produce. Title: Time-Distance Helioseismology Data-Analysis Pipeline for Helioseismic and Magnetic Imager Onboard Solar Dynamics Observatory (SDO/HMI) and Its Initial Results Authors: Zhao, J.; Couvidat, S.; Bogart, R. S.; Parchevsky, K. V.; Birch, A. C.; Duvall, T. L.; Beck, J. G.; Kosovichev, A. G.; Scherrer, P. H. Bibcode: 2012SoPh..275..375Z Altcode: 2011SoPh..tmp...86Z; 2011SoPh..tmp..163Z; 2011arXiv1103.4646Z; 2011SoPh..tmp..232Z The Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory (SDO/HMI) provides continuous full-disk observations of solar oscillations. We develop a data-analysis pipeline based on the time-distance helioseismology method to measure acoustic travel times using HMI Doppler-shift observations, and infer solar interior properties by inverting these measurements. The pipeline is used for routine production of near-real-time full-disk maps of subsurface wave-speed perturbations and horizontal flow velocities for depths ranging from 0 to 20 Mm, every eight hours. In addition, Carrington synoptic maps for the subsurface properties are made from these full-disk maps. The pipeline can also be used for selected target areas and time periods. We explain details of the pipeline organization and procedures, including processing of the HMI Doppler observations, measurements of the travel times, inversions, and constructions of the full-disk and synoptic maps. Some initial results from the pipeline, including full-disk flow maps, sunspot subsurface flow fields, and the interior rotation and meridional flow speeds, are presented. Title: Image Quality of the Helioseismic and Magnetic Imager (HMI) Onboard the Solar Dynamics Observatory (SDO) Authors: Wachter, R.; Schou, J.; Rabello-Soares, M. C.; Miles, J. W.; Duvall, T. L.; Bush, R. I. Bibcode: 2012SoPh..275..261W Altcode: 2011SoPh..tmp..100W; 2011SoPh..tmp...19W; 2011SoPh..tmp..148W; 2011SoPh..tmp..217W We describe the imaging quality of the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) as measured during the ground calibration of the instrument. We describe the calibration techniques and report our results for the final configuration of HMI. We present the distortion, modulation transfer function, stray light, image shifts introduced by moving parts of the instrument, best focus, field curvature, and the relative alignment of the two cameras. We investigate the gain and linearity of the cameras, and present the measured flat field. Title: Helioseismology of a Realistic Magnetoconvective Sunspot Simulation Authors: Braun, D. C.; Birch, A. C.; Rempel, M.; Duvall, T. L. Bibcode: 2012ApJ...744...77B Altcode: We compare helioseismic travel-time shifts measured from a realistic magnetoconvective sunspot simulation using both helioseismic holography and time-distance helioseismology, and measured from real sunspots observed with the Helioseismic and Magnetic Imager instrument on board the Solar Dynamics Observatory and the Michelson Doppler Imager instrument on board the Solar and Heliospheric Observatory. We find remarkable similarities in the travel-time shifts measured between the methodologies applied and between the simulated and real sunspots. Forward modeling of the travel-time shifts using either Born or ray approximation kernels and the sound-speed perturbations present in the simulation indicates major disagreements with the measured travel-time shifts. These findings do not substantially change with the application of a correction for the reduction of wave amplitudes in the simulated and real sunspots. Overall, our findings demonstrate the need for new methods for inferring the subsurface structure of sunspots through helioseismic inversions. Title: Design and Ground Calibration of the Helioseismic and Magnetic Imager (HMI) Instrument on the Solar Dynamics Observatory (SDO) Authors: Schou, J.; Scherrer, P. H.; Bush, R. I.; Wachter, R.; Couvidat, S.; Rabello-Soares, M. C.; Bogart, R. S.; Hoeksema, J. T.; Liu, Y.; Duvall, T. L.; Akin, D. J.; Allard, B. A.; Miles, J. W.; Rairden, R.; Shine, R. A.; Tarbell, T. D.; Title, A. M.; Wolfson, C. J.; Elmore, D. F.; Norton, A. A.; Tomczyk, S. Bibcode: 2012SoPh..275..229S Altcode: The Helioseismic and Magnetic Imager (HMI) investigation (Solar Phys. doi:10.1007/s11207-011-9834-2, 2011) will study the solar interior using helioseismic techniques as well as the magnetic field near the solar surface. The HMI instrument is part of the Solar Dynamics Observatory (SDO) that was launched on 11 February 2010. The instrument is designed to measure the Doppler shift, intensity, and vector magnetic field at the solar photosphere using the 6173 Å Fe I absorption line. The instrument consists of a front-window filter, a telescope, a set of waveplates for polarimetry, an image-stabilization system, a blocking filter, a five-stage Lyot filter with one tunable element, two wide-field tunable Michelson interferometers, a pair of 40962 pixel cameras with independent shutters, and associated electronics. Each camera takes a full-disk image roughly every 3.75 seconds giving an overall cadence of 45 seconds for the Doppler, intensity, and line-of-sight magnetic-field measurements and a slower cadence for the full vector magnetic field. This article describes the design of the HMI instrument and provides an overview of the pre-launch calibration efforts. Overviews of the investigation, details of the calibrations, data handling, and the science analysis are provided in accompanying articles. Title: The Helioseismic and Magnetic Imager (HMI) Investigation for the Solar Dynamics Observatory (SDO) Authors: Scherrer, P. H.; Schou, J.; Bush, R. I.; Kosovichev, A. G.; Bogart, R. S.; Hoeksema, J. T.; Liu, Y.; Duvall, T. L.; Zhao, J.; Title, A. M.; Schrijver, C. J.; Tarbell, T. D.; Tomczyk, S. Bibcode: 2012SoPh..275..207S Altcode: The Helioseismic and Magnetic Imager (HMI) instrument and investigation as a part of the NASA Solar Dynamics Observatory (SDO) is designed to study convection-zone dynamics and the solar dynamo, the origin and evolution of sunspots, active regions, and complexes of activity, the sources and drivers of solar magnetic activity and disturbances, links between the internal processes and dynamics of the corona and heliosphere, and precursors of solar disturbances for space-weather forecasts. A brief overview of the instrument, investigation objectives, and standard data products is presented. Title: Measuring Acoustic Travel Times in Higher-Latitude Regions of the Sun using Hinode and SDO Data Authors: Nagashima, K.; Duvall, T.; Zhao, J.; Kosovichev, A. G.; Parchevsky, K.; Sekii, T. Bibcode: 2011AGUFMSH51B2016N Altcode: The interior structure and dynamics of the Sun can be probed by measuring and inverting travel times of acoustic waves, widely known as time-distance helioseismology. Recent high-resolution observations of solar oscillations with Hinode/SOT and SDO/HMI provide us with an opportunity to investigate the flow dynamics in higher-latitude regions of the Sun. Of particular interest is the meridional circulation flow, which is crucial for understanding the solar dynamo mechanism and predicting the solar activity cycles. We investigate systematic uncertainties of the travel times due to the center-to-limb variations, which may significantly affect the helioseismic inferences of the meridional flows. We present the results of analysis of the cross-correlations in the Doppler velocity, line core and intensity observations, and the corresponding travel-time fits for various positions on the solar disk. We discuss the origin of the center-to-limb variations, including the foreshortening effect, the difference in the line formation height, and other effects. For better understanding of the relative role of these effects we use 3D numerical simulations of solar oscillations in a realistic model of the Sun. Title: Meridional Circulation in the Solar Convection Zone: Deep or Shallow? Authors: Chakraborty, Sudeepto; Duvall, Thomas L., Jr.; Hartlep, Thomas Bibcode: 2011sdmi.confE..82C Altcode: Meridional circulation in the solar convection zone is a key ingredient in flux-transport models of the solar dynamo formulated to explain the 22-year solar magnetic activity cycle. Furthermore, poleward meridional flow has been observed on the surface of the Sun using various techniques, including Doppler velocity measurements and local helioseismology. An equatorward return flow is inferred to exist from a consideration of mass conservation, and is invoked in flux-transport dynamo models to explain the equatorward migration of active region formation during the course of a solar cycle. The depth of this return flow is, however, a point of contention due to lack of any conclusive observational evidence. Theory, simulations and local helioseismic inversions seem to suggest that a reverse flow should exist somewhere in the deep convection zone ( 0.80R), perhaps extending to even below the base of convection zone ( 0.60R), i.e., meridional circulation is deep. But recently, based on inferences made from analysis of latitudinal advection of large supergranules using a cross-correlation tracking technique, Hathaway (2011) has proposed that meridional circulation is shallow with flow reversal occuring at 0.95R. In this preliminary work we begin a systematic investigation of this disputed issue using time-distance helioseismology. We analyze and compare the meridional travel-time differences of various flow models and simulated data, thus laying the groundwork for consistent interpretation (deep or shallow?) of future work involving time-distance analysis of real solar data obtained from, e.g., the Helioseismic Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO). Title: Subsurface Supergranular Vertical Flows as Measured from Time-Distance Helioseismology Authors: Duvall, T. L., Jr.; Hanasoge, S. M. Bibcode: 2011sdmi.confE..43D Altcode: As large-distance rays approach the solar surface approximately vertically, travel times for large separations are mostly sensitive to vertical flows. Large distances have not been used much to measure supergranular flows, at least in part because of the increased noise for large separations. By measuring only the mean flow over a large number of supergranules, it is possible to circumvent the deleterious effect of the noise with the downside being the sacrifice of measuring the flows of individual cells. We find in the present work that the travel time difference for point-annulus combinations is about 4-5 [s] for the distance range of 5-24 [deg]. This signal is much larger than expected and would imply vertical flow of hundreds of meters per sec. Modeling and simulations are used to study this result. The results are that supergranular subsurface vertical flows are much larger than the measured photospheric value of about 10 [m/s]. Title: Investigation of a sunspot complex by time-distance helioseismology Authors: Kosovichev, A. G.; Duvall, T. L. Bibcode: 2011IAUS..273..320K Altcode: 2011arXiv1102.3961K Sunspot regions often form complexes of activity that may live for several solar rotations, and represent a major component of the Sun's magnetic activity. It had been suggested that the close appearance of active regions in space and time might be related to common subsurface roots, or ``nests'' of activity. EUV images show that the active regions are magnetically connected in the corona, but subsurface connections have not been established. We investigate the subsurface structure and dynamics of a large complex of activity, NOAA 10987-10989, observed during the SOHO/MDI Dynamics run in March-April 2008, which was a part of the Whole Heliospheric Interval (WHI) campaign. The active regions in this complex appeared in a narrow latitudinal range, probably representing a subsurface toroidal flux tube. We use the MDI full-disk Dopplergrams to measure perturbations of travel times of acoustic waves traveling to various depths by using time-distance helioseismology, and obtain sound-speed and flow maps by inversion of the travel times. The subsurface flow maps show an interesting dynamics of decaying active regions with persistent shearing flows, which may be important for driving the flaring and CME activity, observed during the WHI campaign. Our analyses, including the seismic sound-speed inversion results and the distribution of deep-focus travel-time anomalies, gave indications of diverging roots of the magnetic structures, as could be expected from Ω-loop structures. However, no clear connection in the depth range of 0-48 Mm among the three active regions in this complex of activity was detected. Title: Local Helioseismology of Small-Scale Magnetic Elements Authors: Crouch, Ashley D.; Braun, D. C.; Felipe, T.; Birch, A. C.; Duvall, T. L. Bibcode: 2011SPD....42.1604C Altcode: 2011BAAS..43S.1604C We will discuss recent progress in the measurement and modeling of the interaction of helioseismic waves with small-scale magnetic elements. We will present measurements of the Hankel analysis phase shifts and absorption coefficients associated with an average small-scale magnetic element, measured using ensemble-averaging techniques. We will show results from theoretical calculations and the numerical simulations of wave interactions with thin magnetic flux tubes. We will compare the Hankel analysis measurements with the predictions from these theoretical models, and discuss how these results pertain to the local helioseismology of magnetic flux concentrations. This work is supported by NASA contract NNH09CE43C. Title: On Measuring Deep Meridional Flows with Time-Distance and Ring-Diagrams Authors: Chakraborty, Sudeepto; Bogart, R. S.; Duvall, T. L., Jr.; Rabello-Soares, M. C. Bibcode: 2011SPD....42.1616C Altcode: 2011BAAS..43S.1616C With the launch of the Solar Dynamics Observatory (SDO) we now have access to images of the Sun taken continuously, combining unprecedented spatial and temporal resolution. Meridional circulation, an important component in flux-transport solar-dynamo models, is one of the keys to understanding the solar cycle. In this work we concentrate on preliminary attempts at evaluating the constraints of time-distance helioseismology in measuring meridional flows in the convection zone of the Sun by analyzing data from the Helioseismic and Magnetic Imager (HMI) aboard SDO. We estimate the maximum depth into the convection zone that can be achieved using deep-focus time-distance analysis and compare results with current ring-diagram techniques. Title: Local Helioseismology of Magnetoconvective Sunspot Simulations and the Reliability of Standard Inversion Methods Authors: Braun, Douglas; Birch, A.; Rempel, M.; Duvall, T.; J. Bibcode: 2011SPD....42.1607B Altcode: 2011BAAS..43S.1607B Controversy exists in the interpretation and modeling of helioseismic signals in and around magnetic regions like sunspots. We show the results of applying local helioseismic inversions to travel-time shift measurements from realistic magnetoconvective sunspot simulations. We compare travel-time maps made from several simulations, using different measurements (helioseismic holography and center-annulus time distance helioseismology), and made on real sunspots observed with the HMI instrument onboard the Solar Dynamics Observatory. We find remarkable similarities in the travel-time perturbations measured between: 1) simulations extending both 8 and 16 Mm deep, 2) the methodology (holography or time-distance) applied, and 3) the simulated and real sunspots. The application of RLS inversions, using Born approximation kernels, to narrow frequency-band travel-time shifts from the simulations demonstrates that standard methods fail to reliably reproduce the true wave speed structure. These findings emphasize the need for new methods for inferring the subsurface structure of active regions. Artificial Dopplergrams from our simulations are available to the community at www.hao.ucar.edu under "Data" and "Sunspot Models." This work is supported by NASA under the SDO Science Center project (contract NNH09CE41C). Title: Earth-Affecting Solar Causes Observatory (EASCO): A potential International Living with a Star Mission from Sun-Earth L5 Authors: Gopalswamy, N.; Davila, J. M.; St. Cyr, O. C.; Sittler, E. C.; Auchère, F.; Duvall, T. L.; Hoeksema, J. T.; Maksimovic, M.; MacDowall, R. J.; Szabo, A.; Collier, M. R. Bibcode: 2011JASTP..73..658G Altcode: This paper describes the scientific rationale for an L5 mission and a partial list of key scientific instruments the mission should carry. The L5 vantage point provides an unprecedented view of the solar disturbances and their solar sources that can greatly advance the science behind space weather. A coronagraph and a heliospheric imager at L5 will be able to view CMEs broadsided, so space speed of the Earth-directed CMEs can be measured accurately and their radial structure discerned. In addition, an inner coronal imager and a magnetograph from L5 can give advance information on active regions and coronal holes that will soon rotate on to the solar disk. Radio remote sensing at low frequencies can provide information on shock-driving CMEs, the most dangerous of all CMEs. Coordinated helioseismic measurements from the Sun-Earth line and L5 provide information on the physical conditions at the base of the convection zone, where solar magnetism originates. Finally, in situ measurements at L5 can provide information on the large-scale solar wind structures (corotating interaction regions (CIRs)) heading towards Earth that potentially result in adverse space weather. Title: HMI time-distance pipeline: An overview and data products Authors: Zhao, J.; Couvidat, S.; Bogart, R. S.; Duvall, T. L., Jr.; Kosovichev, A. G.; Beck, J. G.; Birch, A. C. Bibcode: 2011JPhCS.271a2063Z Altcode: The Helioseismic and Magnetic Imager onboard Solar Dynamics Observatory provides uninterrupted high-resolution observations of solar oscillations over the entire disk. This gives a unique opportunity for mapping subsurface flows and wave-speed structures and investigating their role in the Sun's dynamics and magnetic activity on various scales by methods of local helioseismology. A data analysis pipeline for the time-distance helioseismology analysis has been developed and implemented at the SDO Joint Science Operation Center (JSOC) at Stanford. It provides near-real time processing of the helioseismology data. We provide an overview of this pipeline, including the data flow procedures, measurement and inversion codes, and our data products. Title: Local helioseismology of sunspot regions: Comparison of ring-diagram and time-distance results Authors: Kosovichev, A. G.; Basu, S.; Bogart, R.; Duvall, T. L., Jr.; Gonzalez-Hernandez, I.; Haber, D.; Hartlep, T.; Howe, R.; Komm, R.; Kholikov, S.; Parchevsky, K. V.; Tripathy, S.; Zhao, J. Bibcode: 2011JPhCS.271a2005K Altcode: 2010arXiv1011.0799K Local helioseismology provides unique information about the subsurface structure and dynamics of sunspots and active regions. However, because of complexity of sunspot regions local helioseismology diagnostics require careful analysis of systematic uncertainties and physical interpretation of the inversion results. We present new results of comparison of the ring-diagram analysis and time-distance helioseismology for active region NOAA 9787, for which a previous comparison showed significant differences in the subsurface sound-speed structure, and discuss systematic uncertainties of the measurements and inversions. Our results show that both the ring-diagram and time-distance techniques give qualitatively similar results, revealing a characteristic two-layer seismic sound-speed structure consistent with the results for other active regions. However, a quantitative comparison of the inversion results is not straightforward. It must take into account differences in the sensitivity, spatial resolution and the averaging kernels. In particular, because of the acoustic power suppression, the contribution of the sunspot seismic structure to the ring-diagram signal can be substantially reduced. We show that taking into account this effect reduces the difference in the depth of transition between the negative and positive sound-speed variations inferred by these methods. Further detailed analysis of the sensitivity, resolution and averaging properties of the local helioseismology methods is necessary for consolidation of the inversion results. It seems to be important that both methods indicate that the seismic structure of sunspots is rather deep and extends to at least 20 Mm below the surface, putting constraints on theoretical models of sunspots. Title: The Vertical Component of the Supergranular Motion Authors: Duvall, T. L., Jr.; Birch, A. C. Bibcode: 2010ApJ...725L..47D Altcode: Supergranules are observed at the solar photosphere as a cellular horizontal flow pattern with flow diverging from cell centers and converging on cell boundaries. Clark & Johnson calculated that mass conservation leads to an expected vertical flow of only 10 m s-1, which has been difficult to observe. In the present work, Doppler images near the disk center from Michelson Doppler Imager are averaged about locations of cell centers to obtain the necessary signal-to-noise ratio to see the vertical flow. It is found that, for an average over 1100 cell centers, there is a 10 m s-1 upflow at cell center and a 5 m s-1 downflow at the cell boundaries, confirming the previous estimate. The rms vertical flow is 4 m s-1, smaller than Giovanelli's upper limit of 10 m s-1. Title: Initial Results from SDO/HMI Time-Distance Helioseismology Data Analysis Pipeline Authors: Zhao, J.; Bogart, R. S.; Couvidat, S. P.; Duvall, T. L.; Birch, A. C.; Parchevsky, K.; Kosovichev, A. G.; Beck, J. G. Bibcode: 2010AGUFMSH14A..08Z Altcode: The Helioseismic and Magnetic Imager on Solar Dynamics Observatory provides uninterrupted high-resolution observations of solar oscillations over the entire disk. Time-distance helioseismology data analysis pipeline was developed to perform a near real-time analysis of these observations, and provide full-disk subsurface flow fields and wave-speed perturbation maps every 8 hours. These routine productions give us substantial information of the solar interior, and are very useful to study the solar interior dynamics, connections between subsurface dynamics and photospheric activities, and solar large-scale and global-scale flows and structures. We present our initial results in these respects. Title: Investigation of Formation and Subsurface Dynamics of Active Regions by Local Helioseismology from SDO Authors: Kosovichev, A. G.; Duvall, T. L.; Zhao, J. Bibcode: 2010AGUFMSH11A1604K Altcode: Despite a long history of observations of sunspots and active regions the mechanisms of their formation and stability are still a puzzle. These mechanisms are controlled by a complex interaction of magnetic fields and turbulent convection below the solar surface. The helioseismology observations on SOHO/MDI have provided snapshots of the subsurface dynamics during formation and evolution of several active regions and sunspots. They showed substantial changes in the structure and flow patterns at various stages of the evolution and flaring activity. However, the MDI data were too fragmented for systematic studies. The Helioseismic and Magnetic Imager (HMI) on SDO gives us a unique opportunity for detailed high-resolution investigations of the subsurface structures and mass flows associated with the formation and life cycle of active regions. We present the results of the initial analysis of magnetic active regions by time-distance helioseismology of the HMI Doppler-shift data. These include some interesting events, previously not investigated by helioseismology, such as splitting of a sunspot into two separate spot and formation of penumbra. We discuss also the capabilities of SDO for time-distance helioseismology, current uncertainties, and potentials based on the initial experience. Title: Modeling the Subsurface Structure of Sunspots Authors: Moradi, H.; Baldner, C.; Birch, A. C.; Braun, D. C.; Cameron, R. H.; Duvall, T. L.; Gizon, L.; Haber, D.; Hanasoge, S. M.; Hindman, B. W.; Jackiewicz, J.; Khomenko, E.; Komm, R.; Rajaguru, P.; Rempel, M.; Roth, M.; Schlichenmaier, R.; Schunker, H.; Spruit, H. C.; Strassmeier, K. G.; Thompson, M. J.; Zharkov, S. Bibcode: 2010SoPh..267....1M Altcode: 2009arXiv0912.4982M; 2010SoPh..tmp..171M 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. (2009a, 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. Title: Erratum: Erratum to: Helioseismology of Sunspots: A Case Study of NOAA Region 9787 Authors: Gizon, L.; Schunker, H.; Baldner, C. S.; Basu, S.; Birch, A. C.; Bogart, R. S.; Braun, D. C.; Cameron, R.; Duvall, T. L.; Hanasoge, S. M.; Jackiewicz, J.; Roth, M.; Stahn, T.; Thompson, M. J.; Zharkov, S. Bibcode: 2010SSRv..156..257G Altcode: 2010SSRv..tmp...99G No abstract at ADS Title: Initial Time-Distance Helioseismology Results from SDO/HMI. I. Authors: Zhao, Junwei; Couvidat, S.; Bogart, R.; Parchevsky, K. V.; Duvall, T. L., Jr.; Kosovichev, A. G.; Beck, J. G.; Birch, A. C. Bibcode: 2010AAS...21640234Z Altcode: The Helioseismic and Magnetic Imager on Solar Dynamics Observatory provides uninterrupted high-resolution observations of solar oscillations over the entire disk. This gives a unique opportunity for mapping subsurface flows and wave-speed structures and investigating their role in the Sun's dynamics and magnetic activity on various scales by methods of local helioseismology. A data analysis pipeline for the time-distance helioseismology analysis has been developed and implemented at the SDO Joint Science Operation Center (JSOC) at Stanford. It provides near-real time processing of the helioseismology data. We present the basic characteristics and capabilities of the pipeline, initial time-distance measurement results, and compare these with the simultaneous SOHO/MDI measurements. Title: Initial Time-distance Helioseismology Results from SDO/HMI. II. Authors: Zhao, Junwei; Couvidat, S.; Bogart, R.; Parchevsky, K. V.; Duvall, T. L., Jr.; Kosovichev, A. G.; Beck, J. G.; Birch, A. C. Bibcode: 2010AAS...21640235Z Altcode: Investigations of the interaction between large-scale subsurface flows and magnetic fields are very important for understanding and predicting the processes of solar dynamo and evolution of active regions. The time-distance helioseismology data analysis pipeline (presented in our poster I) is designed to provide global-Sun maps of subsurface flows from the SDO/HMI observations every 8 hours. We present an initial subsurface flow map, and compare this with the magnetic field distribution also obtained from the HMI instrument. Title: Investigation of Subsurface Connections in Complexes of Activity by Local Helioseismology Authors: Kosovichev, Alexander G.; Duvall, T. L., Jr. Bibcode: 2010AAS...21631903K Altcode: 2010BAAS...41..909K Solar active regions often emerge close to each other forming complexes of activity. The complexes may live for several solar rotations, and represent a major component of the Sun's magnetic activity. It had been suggested that the close appearance of active regions in space and time might be related to common subsurface roots, "nests" of activity. The EUV images show that the active regions are magnetically connected in the corona, but subsurface connections have not been established. We investigate the subsurface structure and dynamics of two large complexes of activity, observed during the SOHO/MDI Dynamics campaigns: AR 10484-10488 in October-November 2003, and AR 10987-10989 in March-April 2008 (this complex is a part of the Whole Heliospheric Interval campaign). The former was organized across the equator in a range of longitudes, while the later appeared in a narrow latitudinal range, probably representing a subsurface toroidal flux tube. We use the MDI full-disk Dopplergrams to measure perturbations of travel times of acoustic waves traveling to various depths by applying the surface- and deep-focusing time-distance helioseismology techniques, and obtain the sound-speed and flow maps by inversion. We compare the evolution of the large-scale subsurface sound-speed structures and dynamics of these complexes, and discuss the evidence of subsurface connections. Title: Seismic Constraints on Interior Solar Convection Authors: Hanasoge, Shravan M.; Duvall, Thomas L., Jr.; DeRosa, Marc L. Bibcode: 2010ApJ...712L..98H Altcode: 2010arXiv1001.4508H 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 sun = 0.95 with spatial extent ell < 20, where ell is the spherical harmonic degree, comprises weak flow systems, on the order of 15 m s-1 or less. Convective features deeper than r/R sun = 0.95 are more difficult to image due to the rapidly decreasing sensitivity of helioseismic waves. Title: Transport of Supergranules and their Vertical Coherence Authors: Švanda, M.; Kosovichev, A. G.; Klvaňa, M.; Sobotka, M.; Duvall, T. L., Jr. Bibcode: 2009ASPC..416..547S Altcode: In recent papers, we have introduced a method for measuring the photospheric flow field that is based on the tracking of supergranular structures. Here, in combination with helioseismic data, we are able to estimate the depth in the solar convection envelope to which the detected large-scale flow field is coherent. We show that the upper 10 Mm in the convection zone depicts similar features in horizontal velocity. Our interpretation of this observation is that the supergranulation is a coherent structure 10 Mm deep and is subject to large-scale transport by the underlying velocity field. Title: Measuring Meridional Circulation in the Sun Authors: Duvall, T. L., Jr.; Hanasoge, S. M. Bibcode: 2009ASPC..416..103D Altcode: 2009arXiv0905.3132D Measuring the depth variation of the meridional flows is important for understanding the solar cycle, at least according to a number of dynamo models. While attempting to extend the early observations of Giles (2000) of time-distance measurements of flow, we have stumbled upon some systematic errors that can affect these measurements: 1) the additional distance traveled by radiation coming from points away from disk center causes an apparent "shrinking" Sun, that is an apparent flow towards the disk center and 2) in measurements away from the central longitude, the rotation signal can leak into meridional flow signals. Attempts to understand and overcome these systematic problems will be presented. Forward models based on ray theory have been applied in order to test the sensitivity of travel times to various models. Title: Travel-Time Errors Due To Data Gaps Authors: Beck, J. G.; Duvall, T. L., Jr.; Zhao, J. Bibcode: 2009ASPC..416..135B Altcode: Time-distance measurements of wave packet travel times can be used to determine various properties of the plasma through which the p modes propagate, however, missing data can introduce significant errors in the travel time measurements. We find that the magnitude of the error depends on the amount of missing data, the temporal position of the data gaps, and the travel distance, but not on the continuity of data gaps. Further, apodizing the data gaps or linearly filling the gaps fails to reduce the errors. Title: Time-Distance Helioseismology Data Analysis Pipeline for SDO/HMI Authors: Zhao, J.; Couvidat, S. P.; Parchevsky, K.; Duvall, T. L.; Beck, J. G.; Birch, A. C.; Kosovichev, A. G. Bibcode: 2009AGUFMSH13A1507Z Altcode: Solar Dynamics Observatory (SDO) will be launched shortly, and high-resolution helioseismic data from Heliosesimic and Magnetic Imager (HMI) will soon be available. We have developed a data analysis procedure based on the time-distance helioseismology method, to automatically process 2 TB/day of solar oscillation data from the HMI instrument. The pipeline will provide the following data to the solar physics community: nearly whole disk maps of acoustic travel times, subsurface flow fields and sound-speed perturbation maps every eight hours. The data products include also real-time updated synoptic maps for subsurface flows and sound-speed perturbations. For the helioseismology community, the pipeline will provide intermediate data products such as cross-covariances of solar oscillations. This poster explains how we process the observational data, how we perform the travel-time measurements, how we derive the sensitivity kernels for inversions, and how we perform and test the inversion results. Title: Vertical Flow in Supergranules Authors: Duvall, Thomas L., Jr.; Birch, A. Bibcode: 2009SPD....40.0931D Altcode: Measuring the vertical flow in supergranules is a long-standing and difficult problem. We have tried a new strategy that has met with some success, using Doppler maps from the Michelson Doppler Imager (MDI) on SOHO. From maps of the horizontal divergence signal measured from the time-distance helioseismology of f modes, cells are identified by local peaks in the divergence signal. The surface Doppler signal is then averaged about these cell locations, to obtain a mean Doppler signal. Averaging over many cells yields a statistically stable result. From simultaneous magnetograms, systematic effects associated with magnetic elements have been examined. An upward vertical flow in the center half of cells and a downward flow in the outer half are found, much as if the supergranules are simple convective structures. Title: Large-scale horizontal flows in the solar photosphere IV. On the vertical structure of large-scale horizontal flows Authors: Švanda, M.; Klvaňa, M.; Sobotka, M.; Kosovichev, A. G.; Duvall, T. L. Bibcode: 2009NewA...14..429S Altcode: 2008arXiv0812.1971S In the recent papers, we introduced a method utilised to measure the flow field. The method is based on the tracking of supergranular structures. We did not precisely know, whether its results represent the flow field in the photosphere or in some subphotospheric layers. In this paper, in combination with helioseismic data, we are able to estimate the depths in the solar convection envelope, where the detected large-scale flow field is well represented by the surface measurements. We got a clear answer to question what kind of structures we track in full-disc Dopplergrams. It seems that in the quiet Sun regions the supergranular structures are tracked, while in the regions with the magnetic field the structures of the magnetic field are dominant. This observation seems obvious, because the nature of Doppler structures is different in the magnetic regions and in the quiet Sun. We show that the large-scale flow detected by our method represents the motion of plasma in layers down to ∼10 Mm. The supergranules may therefore be treated as the objects carried by the underlying large-scale velocity field. Title: Helioseismology of Sunspots: A Case Study of NOAA Region 9787 Authors: Gizon, L.; Schunker, H.; Baldner, C. S.; Basu, S.; Birch, A. C.; Bogart, R. S.; Braun, D. C.; Cameron, R.; Duvall, T. L.; Hanasoge, S. M.; Jackiewicz, J.; Roth, M.; Stahn, T.; Thompson, M. J.; Zharkov, S. Bibcode: 2009SSRv..144..249G Altcode: 2008SSRv..tmp..188G; 2010arXiv1002.2369G 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. AR 9787 is visible on helioseismic maps of the farside of the Sun from 15 January, i.e. days before it crossed the East limb. Oscillations have reduced amplitudes in the sunspot at all frequencies, whereas a region of enhanced acoustic power above 5.5 mHz (above the quiet-Sun acoustic cutoff) is seen outside the sunspot and the plage region. This enhanced acoustic power has been suggested to be caused by the conversion of acoustic waves into magneto-acoustic waves that are refracted back into the interior and re-emerge as acoustic waves in the quiet Sun. Observations show that the sunspot absorbs a significant fraction of the incoming p and f modes around 3 mHz. A numerical simulation of MHD wave propagation through a simple model of AR 9787 confirmed that wave absorption is likely to be due to the partial conversion of incoming waves into magneto-acoustic waves that propagate down the sunspot. 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. The travel-time measurements also imply different subsurface flow patterns in the surface layer depending on the filtering procedure that is used. Current sensitivity kernels are unable to reconcile these measurements, perhaps because they rely on imperfect models of the power spectrum of solar oscillations. We present a linear inversion for flows of ridge-filtered travel times. This inversion shows a horizontal outflow in the upper 4 Mm that is consistent with the moat flow deduced from the surface motion of moving magnetic features. From this study of AR 9787, we conclude that we are currently unable to provide a unified description of the subsurface structure and dynamics of the sunspot. Title: Subwavelength Resolution Imaging of the Solar Deep Interior Authors: Hanasoge, Shravan M.; Duvall, Thomas L., Jr. Bibcode: 2009ApJ...693.1678H Altcode: 2008arXiv0812.0119H We derive expectations for signatures in the measured travel times of waves that interact with thermal anomalies and jets. A series of numerical experiments that involve the dynamic linear evolution of an acoustic wave field in a solarlike stratified spherical shell in the presence of fully three-dimensional time-stationary perturbations are performed. The imprints of these interactions are observed as shifts in wave travel times, which are extracted from these data through methods of time-distance helioseismology (Duvall et al.). In situations where at least one of the spatial dimensions of the scatterer was smaller than a wavelength, oscillatory time shift signals were recovered from the analyses, pointing directly to a means of resolving subwavelength features. As evidence for this claim, we present analyses of simulations with spatially localized jets and sound-speed perturbations. We analyze one year's worth solar observations to estimate the noise level associated with the time differences. Based on theoretical estimates, Fresnel zone time shifts associated with the (possible) sharp rotation gradient at the base of the convection zone are on the order of 0.01-0.1 s, well below the noise level that could be reached with the currently available amount of data (~0.15 - 0.2 s with 10 yr of data). Title: Helioseismology of Sunspots: A Case Study of NOAA Region 9787 Authors: Gizon, L.; Schunker, H.; Baldner, C. S.; Basu, S.; Birch, A. C.; Bogart, R. S.; Braun, D. C.; Cameron, R.; Duvall, T. L.; Hanasoge, S. M.; Jackiewicz, J.; Roth, M.; Stahn, T.; Thompson, M. J.; Zharkov, S. Bibcode: 2009odsm.book..249G Altcode: 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. AR 9787 is visible on helioseismic maps of the farside of the Sun from 15 January, i.e. days before it crossed the East limb. Title: Helioseismology of Sunspots: Confronting Observations with Three-Dimensional MHD Simulations of Wave Propagation Authors: Cameron, R.; Gizon, L.; Duvall, T. L., Jr. Bibcode: 2008SoPh..251..291C Altcode: 2008arXiv0802.1603C; 2008SoPh..tmp...51C The propagation of solar waves through the sunspot of AR 9787 is observed by using temporal cross-correlations of SOHO/MDI Dopplergrams. We then use three-dimensional MHD numerical simulations to compute the propagation of wave packets through self-similar magnetohydrostatic sunspot models. The simulations are set up in such a way as to allow a comparison with observed cross-covariances (except in the immediate vicinity of the sunspot). We find that the simulation and the f-mode observations are in good agreement when the model sunspot has a peak field strength of 3 kG at the photosphere and less so for lower field strengths. Constraining the sunspot model with helioseismology is only possible because the direct effect of the magnetic field on the waves has been fully taken into account. Our work shows that the full-waveform modeling of sunspots is feasible. Title: Structure and Evolution of Supergranulation from Local Helioseismology Authors: Hirzberger, Johann; Gizon, Laurent; Solanki, Sami K.; Duvall, Thomas L. Bibcode: 2008SoPh..251..417H Altcode: 2008SoPh..tmp..106H Supergranulation is visible at the solar surface as a cellular pattern of horizontal outflows. Although it does not show a distinct intensity pattern, it manifests itself indirectly in, for example, the chromospheric network. Previous studies have reported significant differences in the inferred basic parameters of the supergranulation phenomenon. Here we study the structure and temporal evolution of a large sample of supergranules, measured by using local helioseismology and SOHO/MDI data from the year 2000 at solar activity minimum. Local helioseismology with f modes provides maps of the horizontal divergence of the flow velocity at a depth of about 1 Mm. From these divergence maps supergranular cells were identified by using Fourier segmentation procedures in two dimensions and in three dimensions (two spatial dimensions plus time). The maps that we analyzed contain more than 105 supergranular cells and more than 103 lifetime histories, which makes possible a detailed analysis with high statistical significance. We find that the supergranular cells have a mean diameter of 27.1 Mm. The mean lifetime is estimated to be 1.6 days from the measured distribution of lifetimes (three-dimensional segmentation), with a clear tendency for larger cells to live longer than smaller ones. The pair and mark correlation functions do not show pronounced features on scales larger than the typical cell size, which suggests purely random cell positions. The temporal histories of supergranular cells indicate a smooth evolution from their emergence and growth in the first half of their lives to their decay in the second half of their lives (unlike exploding granules, which reach their maximum size just before they fragment). Title: Development of Time-Distance Helioseismology Data Analysis Pipeline for SDO/HMI Authors: Duvall, T. L.; Zhao, J.; Couvidat, S.; Parchevsky, K. V.; Beck, J.; Kosovichev, A. G.; Scherrer, P. H. Bibcode: 2008AGUSMSP51B..15D Altcode: The Helioseismic and Magnetic Imager of SDO will provide uninterrupted 4kx4k-pixel Doppler-shift images of the Sun with ~45 sec cadence. These data will have a unique potential for advancing local helioseismic diagnostics of the Sun's interior structure and dynamics. They will help to understanding the basics mechanisms of solar activity and developing predictive capabilities for the NASA's Living with a Star Program. Because of the tremendous amount of data the HMI team is developing a data analysis pipeline, which will provide maps of subsurface flows and sound-speed distributions inferred from the Doppler data by the time-distance technique. We discuss the development plan, methods and algorithms, and present the status of the pipeline, testing results and examples of the data products. Title: Local Helioseismology and Magnetic Flux Emergence Authors: Kosovichev, A. G.; Duvall, T. L., Jr. Bibcode: 2008ASPC..383...59K Altcode: Investigations of emerging magnetic flux are important for understanding the basic properties of solar magnetism (such as the depth of the solar dynamo processes and ``nests'' of solar activity, formation of sunspots and active regions, organization of solar activity on various spatial and temporal scales), and also for forecasting solar activity and space weather. Local helioseismology is capable of detecting emerging magnetic flux in the solar interior, and determining variations of the sound speed and large-scale flows caused by the emerging flux. The initial results obtained by time-distance helioseismology for large emerging active regions reveal unexpected properties of the flux emergence and challenge the current theories and models. In this paper, we present results for AR 10488, which was observed from SOHO/MDI in October 2003. In particular, it is found that the magnetic flux propagates very rapidly in the upper convective zone. The active regions are a result of multiple flux emergence events, occurring in the region of the Sun during a period at least several days long. The emergence is accompanied by strong localized shearing outflows. However, no large-scale diverging flow pattern or significant upflows are detected prior to the emergence. The initial analysis shows that it is necessary to develop special local helioseismology methodology and theoretical models for studying fast dynamical processes associated with magnetic flux emergence. Title: Time-Distance Helioseismology: Sensitivity of f-mode Travel Times to Flows Authors: Jackiewicz, J.; Gizon, L.; Birch, A. C.; Duvall, T. L., Jr. Bibcode: 2007ApJ...671.1051J Altcode: 2007arXiv0708.3554J Time-distance helioseismology has shown that f-mode travel times contain information about horizontal flows in the Sun. The purpose of this study is to provide a simple interpretation of these travel times. We study the interaction of surface gravity waves with horizontal flows in an incompressible, plane-parallel solar atmosphere. We show that for uniform flows less than roughly 250 m s-1, the travel-time shifts are linear in the flow amplitude. For stronger flows, perturbation theory up to third order is needed to model waveforms. The case of small-amplitude spatially varying flows is treated using the first-order Born approximation. We derive two-dimensional Fréchet kernels that give the sensitivity of travel-time shifts to local flows. We show that the effect of flows on travel times depends on wave damping and on the direction from which the observations are made. The main physical effect is the advection of the waves by the flow rather than the advection of wave sources or the effect of flows on wave damping. We compare the two-dimensional sensitivity kernels with simplified three-dimensional kernels that only account for wave advection and assume a vertical line of sight. We find that the three-dimensional f-mode kernels approximately separate in the horizontal and vertical coordinates, with the horizontal variations given by the simplified two-dimensional kernels. This consistency between quite different models gives us confidence in the usefulness of these kernels for interpreting quiet-Sun observations. Title: Helioseismic Observations of Active Regions Below the Solar Surface from SOHO/MDI Authors: Kosovichev, A. G.; Duvall, T. L. Bibcode: 2007AGUFMSH23A1168K Altcode: We apply the time-distance helioseismology technique to obtain 3D tomographic images of sound-speed variations and mass flow velocity maps below the visible surface of the Sun, for emerging and evolving magnetic active regions. In particular, using uninterrupted helioseismology observations from the MDI instrument on the SOHO spacecraft we investigate the development of the large complex of activity NOAA 10484-10488, which produced a series of giant proton flares in October, 2003. The flow maps reveal new interesting properties, such as strong divergent and shearing flows associated with the magnetic flux emergence and flaring activity. Using the sound-speed image we attempt to find the common roots and links of these remarkable active regions. Title: Validation of Helioseismology through Forward Modeling: Realization Noise Subtraction and Kernels Authors: Hanasoge, S. M.; Duvall, T. L., Jr.; Couvidat, S. Bibcode: 2007ApJ...664.1234H Altcode: Through a series of numerical simulations of the near-surface acoustic wavefield of the Sun, we show the utility of the forward approach in local helioseismology. We demonstrate and apply a method to subtract a large fraction of the realization noise from the simulated data. The ability to attain high signal-to-noise ratios from brief forward calculations implies that computational resources are less of a bottleneck, making this alternate method for investigations of the solar interior very feasible. We put this method to use by deriving sensitivity kernels for sound-speed perturbations and source suppression for the background state in our computations using techniques of time-distance helioseismology, all from merely 48 hr of artificial data. Title: Can we detect convection in the Sun? Authors: Hanasoge, Shravan M.; Duvall, T. L.; De Rosa, M. L.; Miesch, M. S. Bibcode: 2007IAUS..239..364H Altcode: No abstract at ADS Title: Investigation of Emerging Active Regions by Time-Distance Helioseismology Authors: Kosovichev, Alexander G.; Duvall, T. L., Jr. Bibcode: 2007AAS...210.4602K Altcode: 2007BAAS...39R.160K Prediction of emerging active regions and their evolution is one of the central problems of local helioseismology. Previous investigations showed that magnetic flux emerges very rapidly, so that it very difficult to obtain tomographic images of the associated sound-speed perturbations and flows in the interior before the first magnetic field elements appear on the surface. We continue investigating this problems by reducing the time intervals between the sound-speed images and flow maps to 2 hours. The helioseismology results are compared with the photospheric magnetograms and white-light images. In particular, we present results of the investigation of the emergence and development of large active region NOAA 10488 observed during the SOHO/MDI Dynamics campaign in October 2003, compare the results with helioseismic observations of other emerging active regions, and discuss the potential of local helioseismology for forecasting emergence and evolution of active regions. Title: Local Helioseismology of Supergranulation Authors: Birch, Aaron; Duvall, T. L.; Hanasoge, S. Bibcode: 2007AAS...210.4507B Altcode: 2007BAAS...39..160B Time-distance helioseismology, a particular method of local helioseismology, is based on measuring times for waves to travel between different points on the solar surface. We use surface-gravity wave travel times obtained using MDI/SOHO data to measure near-surface convective flows. We use acoustic (p-mode) travel times, again from MDI/SOHO data, to measure subsurface convective flows. We show results for average supergranulation flows. Title: Forward Modeling In Helioseismology: Sensitivities, Realization Noise Subtraction And Kernels Authors: Hanasoge, Shravan; Duvall, T. L., Jr.; Couvidat, S. Bibcode: 2007AAS...210.4601H Altcode: 2007BAAS...39..160H Simulations of the acoustic wavefield in the Sun are useful in the context of validating results of local helioseismology and to place bounds on the detectability of various perturbations. Through a series of numerical calculations in cartesian and spherical geometry, we determine the sensitivities of acoustic waves to various flow and thermal perturbations in the deep interior and shallow layers of the Sun. The concept of realization noise subtraction is applied to increase the signal to noise ratios in the simulations. Using techniques of time-distance helioseismology, kernels for sound-speed perturbations, damping and source anomalies are extracted from the artificial data as well. These results will be presented. Title: The solar acoustic simulator: % applications and results Authors: Hanasoge, S. M.; Duvall, T. L., Jr. Bibcode: 2007AN....328..319H Altcode: It is important to understand the limits and accuracy of helioseismic techniques in their ability to probe the solar interior. The availability of a method that is able to compute the solar acoustic wave field in the presence of thermal or flow perturbations affords us a means to place bounds on detectability and accuracy of inferences of interior perturbations. We describe the technique used to simulate wave propagation within a spherical shell that extends from a desired depth (not including the center) into the solar atmosphere and which possesses a solar like stratification. Title: Active Region Dynamics Authors: Kosovichev, A. G.; Duvall, T. L., Jr. Bibcode: 2007sdeh.book....1K Altcode: No abstract at ADS Title: Structure and evolution of supergranulation from local helioseismology Authors: Hirzberger, J.; Gizon, L.; Solanki, S. K.; Duvall, T. L. Bibcode: 2007msfa.conf..103H Altcode: Maps of the horizontal divergence of the near-surface velocity field have been calculated using local helioseismology and SOHO/MDI full-disk Dopplergrams. These maps provide a continuous coverage for two to three months each year with a cadence of 12 hours. Geometrical and evolutional properties of individual supergranular cells have been studied. Supergranular cells have sizes in a range around 650Mm2 (circular diameter of 28.77 Mm) with lifetimes of up to 4.5 days. We also observe a clear trend for larger cells to have stronger divergence values and larger lifetimes than smaller ones. Title: Solar acoustic simulator Authors: Hanasoge, S. M.; Duvall, T. L. Bibcode: 2006ESASP.624E..40H Altcode: 2006soho...18E..40H No abstract at ADS Title: Computational Acoustics in Spherical Geometry: Steps toward Validating Helioseismology Authors: Hanasoge, S. M.; Larsen, R. M.; Duvall, T. L., Jr.; De Rosa, M. L.; Hurlburt, N. E.; Schou, J.; Roth, M.; Christensen-Dalsgaard, J.; Lele, S. K. Bibcode: 2006ApJ...648.1268H Altcode: Throughout the past decade, detailed helioseismic analyses of observations of solar surface oscillations have led to advances in our knowledge of the structure and dynamics of the solar interior. Such analyses involve the decomposition of time series of the observed surface oscillation pattern into its constituent wave modes, followed by inversion procedures that yield inferences of properties of the solar interior. While this inverse problem has been a major focus in recent years, the corresponding forward problem has received much less attention. We aim to rectify this situation by taking the first steps toward validating and determining the efficacy of the helioseismic measurement procedure. The goal of this effort is to design a means to perform differential studies of various effects such as flows and thermal perturbations on helioseismic observables such as resonant frequencies, travel-time shifts, etc. Here we describe our first efforts to simulate wave propagation within a spherical shell, which extends from 0.2 to about 1.0004 Rsolar (where Rsolar is the radius of the Sun) and which possesses a solar-like stratification. We consider a model containing no flows that will serve as a reference model for later studies. We discuss the computational procedure, some difficulties encountered in a simulation of this kind, and the means to overcome them. We also present techniques used to validate the simulation. Title: Analysis of a Relation between Subphotospheric Plasma Flows and Photospheric Current Kernels Authors: Kulinová, A.; Dzifčáková, E.; Kosovichev, A. G.; Duvall, T. L. Bibcode: 2006ESASP.617E..69K Altcode: 2006soho...17E..69K No abstract at ADS Title: Direct Measurement of Travel-Time Kernels for Helioseismology Authors: Duvall, T. L., Jr.; Birch, A. C.; Gizon, L. Bibcode: 2006ApJ...646..553D Altcode: Solar f-modes are surface gravity waves that propagate horizontally in a thin layer near the photosphere with a dispersion relation approximately that of deep water waves. At the power maximum near frequency ω/2π=3 mHz, the wavelength of 5 Mm is large enough for various wave scattering properties to be observable. Gizon & Birch have calculated spatial kernels for scattering in the Born approximation. In this paper, using isolated small magnetic features as approximate point scatterers, a linear-response kernel has been measured. In addition, the kernel has been estimated by deconvolving the magnetograms from the travel-time maps. The observed kernel is similar to the theoretical kernel for wave damping computed by Gizon & Birch: it includes elliptical and hyperbolic features. This is the first observational evidence to suggest that it is appropriate to use the Born approximation to compute kernels (as opposed to the ray approximation). Furthermore, the observed hyperbolic features confirm that it is important to take into account scattering of the waves coming from distant source locations (as opposed to the single-source approximation). The observed kernel is due to a superposition of the direct and indirect effects of the magnetic field. A simple model that includes both monopole and dipole scattering compares favorably with the data. This new technique appears to be promising to study how seismic waves interact with magnetic flux tubes. Title: Sensitivity of Time-Distance Helioseismic Measurements to Spatial Variation of Oscillation Amplitudes. I. Observations and a Numerical Model Authors: Rajaguru, S. P.; Birch, A. C.; Duvall, T. L., Jr.; Thompson, M. J.; Zhao, J. Bibcode: 2006ApJ...646..543R Altcode: 2006astro.ph..4048R 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 uses data from SOHO MDI. Title: Active Region Dynamics Authors: Kosovichev, A. G.; Duvall, T. L. Bibcode: 2006SSRv..124....1K Altcode: 2007SSRv..tmp...56K New methods of local helioseismology and uninterrupted time series of solar oscillation data from the Solar and Heliospheric Observatory (SOHO) have led to a major advance in our understanding of the structure and dynamics of active regions in the subsurface layers. The initial results show that large active regions are formed by repeated magnetic flux emergence from the deep interior, and that their roots are at least 50 Mm deep. The active regions change the temperature structure and flow dynamics of the upper convection zone, forming large circulation cells of converging flows. The helioseismic observations also indicate that the processes of magnetic energy release, flares and coronal mass ejections, might be associated with strong (1 2 km/s) shearing flows, 4 6 Mm below the surface. Title: Helioseismology of the "Average" Supergranule Authors: Birch, Aaron; Duvall, T. L.; Gizon, L.; Jackiewicz, J. Bibcode: 2006SPD....37.0505B Altcode: 2006BAAS...38..224B We show time-distance travel times averaged over roughly ten thousand supergranules. The statistical (realization) noise in these measurements is substantially smaller than the noise associated with a single supergranule. By both forward modeling and inversions we determine the range of subsurface flows that are compatible with these travel times. Title: Helioseismic observations of magnetic flux emergence and flare effects Authors: Kosovichev, A. G.; Duvall, T. L. Bibcode: 2006IAUS..233..365K Altcode: Time-distance helioseismology and data from SOHO/MDI are used for obtaining 3D images of subsurface sound-speed perturbations and maps of plasma flows, associated with emerging magnetic flux and flaring activity of large active regions in October 2003. The results reveal extremely complicated dynamical processes in the upper convection zone and indicate that subsurface shear flows may play an important role in magnetic energy release in solar flares. Strong X-class flares generated impulsive seismic waves (“sunquakes”), traveling through surrounding sunspots, thus providing new insight into the interaction of seismic waves with magnetic fields. Title: Simulations Of Acoustic-Flow Interaction In Spherical Geometry: Steps Toward Validating Helioseismology Authors: Hanasoge, S. M.; Duvall, T. L.; De Rosa, M. L.; Hurlburt, N. E. Bibcode: 2005AGUSMSP11B..11H Altcode: We simulate acoustic wave interaction with flows in spherical geometry with the specific intent of using them as artificial data for validation of helioseismology. The numerical procedure is pseudo-spectral; we employ a spherical harmonic representation of the spherical surface, compact finite differences in the radial direction and a fourth order Runge-Kutta time stepping scheme. We also excite surface gravity modes, modeling all waves as linear perturbations to the background state so as to gain further insight into wave-flow interaction. Towards validation, we apply techniques of helioseismology to the artificial data to determine the efficacy of the helioseismic inversion procedure. In other words, we are attempting the forward problem. Title: Direct Measurement of Wave Kernels in Time-Distance Helioseismology Authors: Duvall, T. L.; Birch, A. C.; Gizon, L. Bibcode: 2005AGUSMSP23C..02D Altcode: Solar f-mode waves are surface-gravity waves which propagate horizontally in a thin layer near the photosphere with a dispersion relation approximately that of deep water waves. At the power maximum near 3 mHz, the wavelength of 5 Mm is large enough for various wave scattering properties to be observable. Gizon and Birch (2002,ApJ,571,966) have calculated kernels, in the Born approximation, for the sensitivity of wave travel times to local changes in damping rate and source strength. In this work, using isolated small magnetic features as approximate point source scatterers, such a kernel has been measured. The observed kernel contains similar features to a theoretical damping kernel but not for a source kernel. A full understanding of the effect of small magnetic features on the waves will require more detailed modeling. Title: A Search for the Relationship Between Subphotospheric Dynamics of Active Regions and Flaring Activity Authors: Kosovichev, A. G.; Duvall, T. L. Bibcode: 2005AGUSMSP51C..04K Altcode: MHD models of solar flares and CMEs suggest the magnetic energy for these events can be accumulated and released in magnetic structures sheared and twisted by plasma motions. We use time-distance helioseismology for investigating subphotospheric structures and dynamics of active regions that might be related to their flaring activity. In particular, we present a detailed study of active region NOAA 10486, which produced a series of X-class flares, during its passage on the solar disk for 8 days, Oct.25-Nov.1, 2003. The maps of subsurface flows and sound-speed perturbations are obtained from the SOHO/MDI data every 2-hours during this period, with 8-hour resolution and for the depth range of 0-12 Mm, and compared with the MDI magnetograms and X-ray data from RHESSI and GOES. The results reveal interesting dynamics at the depth of 4-6 Mm, which is compared with the restructuring, emergence and cancellation the magnetic field in this region. Title: Sensitivity of time-distance helioseismic measurements to modulation of oscillation amplitudes Authors: Rajaguru, P.; Zhao, J.; Duvall, T. L. Bibcode: 2005AGUSMSP11B..05R Altcode: Spatial modulation of oscillation amplitudes in solar active regions arise from several causes, most notably due to the absorption of acoustic waves by sunspots and due to incorrect measurements induced by the changes in the spectral line profiles used in the observations. We show that these modulations may introduce significant changes in the travel times of acoustic waves in addition to those arising directly from the real physical causes. These changes are caused by the convolution of the wave-number spectrum of the modulating function with the power spectra of the oscillations. We develop an empirical 'deconvolution' strategy based on the modulation of oscillation power in each pixel of Dopplergrams and test its effectiveness. These corrections are important in inferring correctly the sub-surface structure and dynamics of localized strong perturbations such as sunspots. Title: Diagnostics of Subphotospheric Sources of Solar Variability Authors: Kosovichev, A. G.; Duvall, T. L.; Zhao, J. Bibcode: 2004AGUFMSH13A1144K Altcode: Local helioseismology provides new tools for studying subphotospheric processes that are related to solar variability of various spatial and temporal scales. Large-scale flow patterns beneath active regions and sunspots affect the solar energy transport in the upper convection zone. Smaller-scale shearing and twisting flows may trigger instabilities of magnetic configurations that lead to flares and CMEs. We discuss the recent progress in the local helioseismology diagnostic tools and in our understanding of the subphotospheric dynamics and sources of variability. Title: Comparison of Solar Subsurface Flows Assessed by Ring and Time-Distance Analyses Authors: Hindman, Bradley W.; Gizon, Laurent; Duvall, Thomas L., Jr.; Haber, Deborah A.; Toomre, Juri Bibcode: 2004ApJ...613.1253H Altcode: The solar near-surface shear layer exhibits a rich medley of flows that are now being measured by a variety of local helioseismic techniques. We present comparisons of the horizontal flows obtained with two of these techniques, ring and time-distance analyses, applied to Michelson Doppler Imager (MDI) Dynamics Program data from the years 1998 and 1999. The ring analyses use the frequencies of both f and p modes in inversions to obtain flows within the near-surface shear layer as a function of depth. The f-mode time-distance analyses make velocity inferences just beneath the photosphere. After degrading the spatial resolution of the time-distance analyses to match the coarser resolution of the ring analyses, we find that the flows deduced with the two methods are remarkably similar, with common inflow and outflow sites as well as agreement in flow direction. The flows from ring and time-distance analyses are highly correlated with each other (correlation coefficients ~0.8) direct correspondence of features in the flows is largely realized in both the quiet-Sun and magnetic active regions. Title: a New Method to Search for Solar Gravity Mode Oscillations Authors: Duvall, T. L., Jr. Bibcode: 2004ESASP.559..412D Altcode: 2004soho...14..412D No abstract at ADS Title: Sensitivity of Acoustic Wave Travel Times to Sound-Speed Perturbations in the Solar Interior Authors: Birch, A. C.; Kosovichev, A. G.; Duvall, T. L., Jr. Bibcode: 2004ApJ...608..580B Altcode: For time-distance helioseismology, it is important to establish the relationship between the travel times of acoustic waves propagating between different points on the solar surface through the solar interior and local perturbations to the sound speed in the propagation region. We use the Born approximation to derive a general expression for the linear sensitivity of travel times to local sound-speed perturbations in plane-parallel solar models with stochastic wave sources. The results show that the sensitivity of time-distance measurements to perturbations in sound speed depends on the details of the measurement procedure, such as the phase-speed filter used in typical time-distance data analysis. As a result, the details of the measurement procedure should be taken into account in the inversion of time-distance data. Otherwise, the inferred depths of perturbations may be incorrect. Title: On the Relationship between the Rotational Velocity and the Field Strength of Solar Magnetic Elements Authors: Zhao, Junwei; Kosovichev, Alexander G.; Duvall, Thomas L., Jr. Bibcode: 2004ApJ...607L.135Z Altcode: By tracking various solar surface tracers, previous studies have found that magnetic structures, such as plages and sunspots, rotate faster than the quiet solar regions. We investigate how the rotational speed of these magnetic features is related to their magnetic field strength. By use of near-surface horizontal velocities inferred from time-distance helioseismology, we have studied a few Carrington rotations covering the years 1997-2002, from near the solar minimum to the maximum. It is found that the residual rotational velocity of magnetic elements nearly linearly depends on their magnetic field strength: the stronger the magnetic field strength, the faster the magnetic elements rotate relative to the quiet solar regions. It is also found that the magnetic elements rotate faster in the solar maximum years than the elements of the same magnetic strength but in years with moderate solar activity. For all Carrington rotations studied, magnetic elements of the following polarity rotate faster than the leading polarity elements of the same magnetic strength. Possible interpretations of the observed relations are discussed. Prograde supergranular advection may cause the faster rotation of the following polarity elements. Title: Subphotospheric Dynamics During the Period of Massive Solar Flares Authors: Kosovichev, A. G.; Duvall, T. L., Jr. Bibcode: 2004AAS...204.4703K Altcode: 2004BAAS...36Q.737K Unstable coronal structures that lead to impulsive energy release in solar flares are created by magnetic flux emergence and by photospheric and subphotospheric motions shearing and twisting magnetic field lines. We present results of investigation of subsurface plasma flows and emerging structures associated with AR 10484, 10486, and 10488, which produced the majority of massive flares of October-November 2003. The results representing 3-D maps of sound-speed perturbations and flow velocities are obtained from SOHO/MDI full-disk Doppler data by time-distance helioseismology with the horizontal resolution of 3 Mm, vertical resolution of 0.7-4 Mm (in the depth range of 0-40 Mm), and temporal resolution of 8 hours. These maps are compared with the corresponding MDI magnetograms, and also with TRACE and RHESSI images to determine the role of subphotospheric dynamics in the development of the active regions and their flaring activity. Title: Structure Properties of Supergranulation and Granulation Authors: Berrilli, F.; Del Moro, D.; Consolini, G.; Pietropaolo, E.; Duvall, T. L., Jr.; Kosovichev, A. G. Bibcode: 2004SoPh..221...33B Altcode: We investigate spatial dislocation ordering of the solar structures associated with supergranulation and granulation scales. The supergranular and granular structures are automatically segmented from time-distance divergence maps and from broad-band images, respectively. The spatial dislocation ordering analysis is accomplished by applying the statistical method of Pair Correlation Function, g2(r), to segmented features in the solar fields. We compare the computed g2(r) functions obtained from both single and persistent, i.e., time-averaged, fields associated with supergranulation and granulation. We conclude that supergranulation and granulation patterns present a different topological order both in single and persistent fields. The analysis carried out on single fields suggests that the granulation behaves as an essentially random distribution of soft plasma features with a very broad distribution in size, while supergranulation behaves as a random distribution of close packed, coherent stiff features with a rather defined mean size. Title: Dynamics and Structure of Supergranulation Authors: Del Moro, D.; Berrilli, F.; Duvall, T. L., Jr.; Kosovichev, A. G. Bibcode: 2004SoPh..221...23D Altcode: In this paper we investigate the temporal evolution and geometric properties of solar supergranular features. For this purpose we apply an automatic feature-tracking algorithm to a 6-day time series of 18 near-surface flowmaps containing 548 target objects. Lifetimes are calculated by measuring the time elapsing between the birth and death of each target. Using an exponential fit on the lifetime distribution of single supergranules we derived a mean lifetime of 22 hours. Based on the application of segmentation numerical procedures, we estimated characteristic geometric parameters such as area distributions of supergranular cells. We also derive the relationship between measured lifetime and the area of the supergranules. Title: Solar-cycle variations in the spectrum of supergranulation Authors: Gizon, Laurent; Duvall, Thomas L. Bibcode: 2004IAUS..223...41G Altcode: 2005IAUS..223...41G Using local helioseismology, we construct maps of the horizontal divergence of the velocity field near the solar surface and study the spectrum of solar supergranulation during the period from 1996 to 2002. Supergranulation oscillates and propagates like a wave interference pattern. The variations of the oscillation frequency with latitude and time are less than 5%. We find significant solar-cycle variations in the lifetime and the anisotropic distribution of power. We also measure the time-dependent zonal and meridional flows that advect supergranules. Title: Evolution of Active Regions in the Solar Interior Authors: Kosovichev, A. G.; Duvall, T. L. Bibcode: 2003AGUFMSH22A0186K Altcode: Using data from SOHO and TRACE, we investigate the emergence, evolution and dissipation of magnetic active regions in the solar interior and atmosphere by comparing 3D maps of subsurface structures and plasma flows, obtained by acoustic tomography, with the corresponding photospheric magnetograms and coronal EUV images. We find that the growth of active regions is characterized by multiple emergence of magnetic flux structures propagating very rapidly in the upper convection zone and by the formation of large-scale converging flows. During the decay, we have observed mostly diverging flows, and have attempted to detect submergence of magnetic flux. We look at some details of the dynamics of active regions, and discuss initial results of a search for the relationship between subphotospheric shearing flows, and changes in magnetic topology and flaring activity in the corona. Title: Analysis of relationship between flaring activity and subphotospheric flows in NOAA 9393 Authors: Kulinová, A.; Dzifcáková, E.; Duvall, T. L., Jr.; Kosovichev, A. G. Bibcode: 2003ESASP.535..125K Altcode: 2003iscs.symp..125K The relationship between the subphotospheric flows and flaring activity is not well understood. It is believed that subphotospheric shearing flows play important role in creating unstable magnetic topology that leads to initiation of flares and CMEs. In this paper, we study subphotospheric flows and their relationship with two flares observed in active region NOAA 9393. One of the flares is connected with halo CME. SOHO/MDI and helioseismology data are used for determining the changes in morphology and are compared with changes of the topology as observed by TRACE. We find evidence of some connections between subphotospheric flows within 12 Mm below the photosphere and changes of photospheric magnetic fields and also the flaring activity. Title: Imaging of the solar interior: possibilities and limitations Authors: Kosovichev, Alexander G.; Duvall, Thomas L., Jr. Bibcode: 2003SPIE.4853..327K Altcode: Helioseismic tomography is a promising new method for probing 3-D structures and flows beneath the solar surface. It is based on observation of solar acoustic waves, and provides great possibilities for studying the birth of active regions in the Sun's interior and for understanding the relation between the internal dynamics of active regions and chromospheric and coronal activity. We discuss observational requirements, challenges and limitations of this technique for investigating physical processes in the solar interior on their intrinsic spatial and temporal scales. Title: Nonaxisymmetric variations deep in the convection zone Authors: Duvall, Thomas L., Jr. Bibcode: 2003ESASP.517..259D Altcode: 2003soho...12..259D Using a deep-focusing time-distance technique and the MDI medium-1 data, a preliminary study on nonaxisymmetric variability deep in the convection zone has been performed. The purpose of the present study is to see what signals might be present in raw travel times indicating variation, and what are the noise levels. Correlations with point separations in the range 40-50 deg have been measured for the entire 6+ year dataset over a significant fraction of the solar disk. Both flows and mean-time variations have been examined. Travel time maps are correlated from one day to the next, indicating real solar signals. Title: Supergranulation supports waves Authors: Gizon, L.; Duvall, T. L., Jr. Bibcode: 2003ESASP.517...43G Altcode: 2003soho...12...43G Supergranulation on the surface of the Sun is a pattern of horizontal outflows with a distinct scale of 30 Mm and an apparent lifetime of 1 day, outlined by a network of small magnetic features. The dynamics of the supergranulation is poorly understood and there is as yet no explanation for the observation that the supergranular pattern appears to rotate faster than the magnetic features. In this paper we show that supergranulation undergoes oscillations and supports waves with periods of 6-9 days. The nature of supergranulation appears to be travelling-wave convection. Waves are predominantly prograde, which explains the apparent superrotation of the pattern. We also show that supergranular flows have a net kinetic helicity, which is negative in the northern hemisphere. Title: Test of helioseismic time-distance inversion using 3-D finite-difference wavefield modeling Authors: Jensen, Jesper Munk; Olsen, Kim Bak; Duvall, Thomas L., Jr.; Jacobsen, Bo Holm Bibcode: 2003ESASP.517..319J Altcode: 2003soho...12..319J Here we present the first validation test of helioseismic time-distance inversion which includes a stochastic waveform computation in a 3-D solar model. For a given velocity structure a stochastic shallow source is propagating by finite-difference acoustic computation to generate random 3-D acoustic vibrations for which synthetic Dopplergrams are computed. The preliminary results indicate that state-of-the-art processing and inversion may recover only a noisy picture of interior sound-speed perturbations even for essentially noise-free data. Title: Noise propagation in inversion of helioseismic time-distance data Authors: Jensen, Jesper Munk; Duvall, Thomas L., Jr.; Jacobsen, Bo Holm Bibcode: 2003ESASP.517..315J Altcode: 2003soho...12..315J We present an analysis of noise propagation in time-distance inversion. The data covariance is estimated from a quiet region. We obtain estimates for the correlation and uncertainties of the inversion result both from theoretical propagation of the covariance matrix and from inversion of realizations of the noise model. Inversion of data containing a sunspot is shown along with the estimates of the uncertainties. Title: erratum: Wave-like properties of solar supergranulation Authors: Gizon, L.; Duvall, T. L.; Schou, J. Bibcode: 2003Natur.421..764G Altcode: No abstract at ADS Title: Wave-like properties of solar supergranulation Authors: Gizon, L.; Duvall, T. L.; Schou, J. Bibcode: 2003Natur.421...43G Altcode: 2002astro.ph..8343G Supergranulation on the surface of the Sun is a pattern of horizontal outflows, outlined by a network of small magnetic features, with a distinct scale of 30 million metres and an apparent lifetime of one day. It is generally believed that supergranulation corresponds to a preferred `cellular' scale of thermal convection; rising magnetic fields are dragged by the outflows and concentrated into `ropes' at the `cell' boundaries. But as the convection zone is highly turbulent and stratified, numerical modelling has proved to be difficult and the dynamics remain poorly understood. Moreover, there is as yet no explanation for the observation that the pattern appears to rotate faster around the Sun than the magnetic features. Here we report observations showing that supergranulation undergoes oscillations and supports waves with periods of 6-9 days. The waves are predominantly prograde, which explains the apparent super-rotation of the pattern. The rotation of the plasma through which the pattern propagates is consistent with the motion of the magnetic network. Title: Acoustic Tomography of the Sun's Interior with SDO: Possibilities and Limitations Authors: Kosovichev, A. G.; Duvall, T. L.; Birch, A. C.; Gizon, L.; Zhao, J.; Sekii, T.; Shibahashi, H. Bibcode: 2002AGUFMSH21C..06K Altcode: Helioseismic and Magnetic Imager on board SDO will significantly expand the possibilities for imaging plasma flows and structures in the solar interior. It will provide for the first time high-resolution data for continuous monitoring of emerging flux and developing active regions in the upper convection zone. It will also allow us to look for localized structures and flows in the tachocline region and deeper interior, and also investigate the near-polar regions. The expected results may give important clues of how the solar dynamo works and active regions develop. HMI will observe the entire spectrum of the solar acoustic and surface gravity waves, and provide the most comprehensive data for global and local helioseismology. The high-frequency part of the oscillation spectrum will be used for studying seismic properties of the solar atmosphere in the quiet Sun and active regions. The method of acoustic tomography or time-distance helioseismology is one of the primary tools of the HMI investigation. It is based on measurements and inversions of travel-time delays of solar waves, caused by plasma flows and variations of temperature and magnetic field. The data analysis requires deep understanding of the physics of wave propagation in the Sun and substantial computer resources. One of the important goals is to provide the flow and sound-speed maps of the upper convection zone in near-real time for space weather applications. We present the current status of the field, and discuss plans and challenges for the HMI data analyses and interpretation. Title: Helioseismic observations of subphotospheric dynamics of sunspots and developing active regions Authors: Kosovichev, A. G.; Duvall, T. L., Jr.; Zhao, Junwei Bibcode: 2002ESASP.505...79K Altcode: 2002IAUCo.188...79K; 2002solm.conf...79K New methods of time-distance helioseismology provide us unique information about the structure and dynamics of sunspots and active regions in the upper convection zone. We present three-dimensional maps of the sound-speed perturbations and flow velocities obtained from the SOHO/MDI data for sunspots, emerging flux events and evolving active regions. The results reveal complex dynamics of magnetic structures below the solar surface, and shed light on the mechanisms of sunspots and active regions, and magnetic field dynamics. One interesting case that includes a fast spinning sunspot accompanied with subphotospheric vortex motions and twisting coronal loops represents an intriguing example of magnetic coupling between the subphotospheric processes and the atmospheric activity. The evolution of a large active region, NOAA 9393, has been studied for almost 3 solar rotations in March-April 2001, including the periods of emergence, maximum activity and decay. It is concluded that this active region was formed by fragmented magnetic flux tubes emerging during an extended period of time rather than by a single large Ω-loop broken into smaller flux tubes near the surface. Title: A New Component of Solar Dynamics: North-South Diverging Flows Migrating toward the Equator with an 11 Year Period Authors: Beck, J. G.; Gizon, L.; Duvall, T. L., Jr. Bibcode: 2002ApJ...575L..47B Altcode: Time-distance helioseismology analysis of Dopplergrams provides maps of torsional oscillations and meridional flows. Meridional flow maps show a time-varying component that has a banded structure that matches the torsional oscillations with an equatorward migration over the solar cycle. The time-varying component of meridional flow consists of a flow diverging from the dominant latitude of magnetic activity. These maps are compared with other torsional oscillation maps and with magnetic flux maps, showing a strong correlation with active latitudes. These results demonstrate a strong link between the time-varying component of the meridional flow and the torsional oscillations. Title: Advances in Time-Distance Helioseismology Authors: Duvall, T. L., Jr.; Beck, J. G.; Gizon, L.; Kosovichev, A. G. Bibcode: 2002AAS...200.7902D Altcode: 2002BAAS...34..780D Time-distance helioseismology is a way to measure travel times between surface locations for waves traversing the solar interior. Coupling the travel time measurements with an extensive modeling effort has proven to be a powerful tool for measuring flows and other wave speed inhomogeneities in the solar interior. Problems receiving current attention include studying the time variation of the meridional circulation and torsional oscillation and active region emergence and evolution. Current results on these topics will be presented. Title: Local-area helioseismology as a diagnostic tool for solar variability Authors: Kosovichev, A. G.; Duvall, T. L.; Birch, A. C.; Gizon, L.; Scherrer, P. H.; Zhao, Junwei Bibcode: 2002AdSpR..29.1899K Altcode: Dynamical and thermal variations of the internal structure of the Sun can affect the energy flow and result in variations in irradiance at the surface. Studying variations in the interior is crucial for understanding the mechanisms of the irradiance variations. "Global" helioseismology based on analysis of normal mode frequencies, has helped to reveal radial and latitudinal variations of the solar structure and dynamics associated with the solar cycle in the deep interior. A new technique, - "local-area" helioseismology or heliotomography, offers additional potentially important diagnostics by providing three-dimensional maps of the sound speed and flows in the upper convection zone. These diagnostics are based on inversion of travel times of acoustic waves which propagate between different points on the solar surface through the interior. The most significant variations in the thermodynamic structure found by this method are associated with sunspots and complexes of solar activity. The inversion results provide evidence for areas of higher sound speed beneath sunspot regions located at depths of 4-20 Mm, which may be due to accumulated heat or magnetic field concentrations. However, the physics of these structures is not yet understood. Heliotomography also provides information about large-scale stable longitudinal structures in the solar interior, which can be used in irradiance models. This new diagnostic tool for solar variability is currently under development. It will require both a substantial theoretical and modeling effort and high-resolution data to develop new capabilities for understanding mechanisms of solar variability. Title: The Largest Active Region of the Solar Cycle Authors: Kosovichev, A. G.; Bush, R. I.; Duvall, T. L.; Scherrer, P. H. Bibcode: 2001AGUFMSH11C0730K Altcode: The largest and most active sunspot region of the current solar cycle (known as AR 9393) was observed by the MDI instrument on SOHO continuously during three solar rotations in March-May 2001. On April 2 this active region produced the largest solar flare of the last 25 years. By using time-distance helioseismology we have investigated the development of the active region in the solar interior during that period starting from the processes of emergence. We present tomographic images of the sound-speed structures associated with this active region up to 100 Mm below the solar surface, and discuss their relation to the evolution of the surface magnetic field. Title: Time-Distance Studies of Large Scale Flows on the Sun Authors: Beck, J. G.; Duvall, T. L. Bibcode: 2001AGUFMSH11B0707B Altcode: Time distance helioseismology is a valuable tool for examining flows in the convection zone. It can produce vector maps of flows from the travel time of waves traversing subsurface ray paths. This technique has been proven useful for studying solar phenomena ranging in size from supergranules to global flows. (1999) Giles et al has demonstrated the efficacy of using the time-distance technique on meridional and zonal flows. We extend this work and show resulting measurements. Title: Time-distance Helioseismology Study Over a Rotating Sunspot Authors: Zhao, J.; Kosovichev, A. G.; Duvall, T. L. Bibcode: 2001AGUFMSH11B0708Z Altcode: Time-distance helioseismology has provided a unique tool in studying interior structures of the Sun. The structure of sound speed variations and flow fields beneath the sunspot surface have been obtained by use of inversion technique in some previous studies. In this study we have applied the time-distance measurements from SOHO/MDI and the inversion technique to investigate a sunspot which showed unusually fast rotation around its center for a couple of days from Aug 7 to Aug 8, 2000. The sound speed structure which is related to the magnetic field structures beneath the surface and associated temperature variations was obtained. The results revealed some twists in the sound-speed internal structure of the spot relative to the surface magnetic structure. This kind of subsurface twist was not seen 2 days after the start of rotation. This is consistent with the surface observation showing a reduction of transverse magnetic field twists after the surface rotation stopped. It could be explained as the magnetic field lines were twisted beneath the surface and the untwisting of field lines caused the surface rotation. Flow fields beneath the sunspot surface were also obtained. A strong vortex was found near the surface and a few megameters below the surface. Whether the subsurface vortical flows caused the magnetic field twists or the untwisting of field lines caused the subsurface vortical flows will be discussed. Title: Observational Constraints on Solar Dynamo Models: Helioseismic Inferences and Magnetic Properties Authors: Duvall, T. L. Bibcode: 2001AGUFMGP22B..03D Altcode: For many years the solar dynamo could only be constrained by observations at and above the visible layers. Some observations thought to be important include the variation of the number of sunspots with the 11-year cycle, the magnetic polarity of the sunspots, the systematic variation in latitude of the spots during the cycle, the faster rotation at the equator, the poleward flow in both hemispheres (meridional circulation), the apparent diffusion of magnetic fields as sunspot regions age, and the variation of the polar magnetic field over the cycle. With the advent of helioseismology, it has become possible to significantly add to these constraining observations. Using the splitting of global mode frequencies, rotation has been measured reliably over the outer half of the solar interior. Two regions of rotational shear have been revealed, one encompassing the base of the convection zone at r/R=0.7 and the other in the outer 5% of the radius. It is commonly thought that the lower shear region is the site of the main dynamo generation. In addition to rotation, the meridional circulation has also been measured below the surface. Near the minimum of the 11-year cycle, the poleward flow down to r/R=0.9 appears similar to the surface flow. There are indications that the meridional circulation may be changing over the cycle. In addition to these observations whose significance are thought to be understood, there are large-scale flows whose ultimate connection to the dynamo is unknown. In this group are the zonal rotation bands, weak flows first observed at the surface and now below that migrate towards the equator during the cycle and are periodic with the cycle period. Also, the recently observed time variations of rotation near the convection zone bottom could also be important. Title: Probing Deep Structure of the Sun by Time-Distance Helioseismology Authors: Birch, A. C.; Duvall, T. L.; Kosovichev, A. G. Bibcode: 2001AGUFMSH11B0710B Altcode: Time-distance helioseismology is a method for inferring sound-speed perturbations and flow velocities by measuring the travel times for acoustic wave packets as they move between points on the solar surface through the solar interior. It has been successfully applied to infer structures and flows in the upper convection zone. However, probing the deep convection zone and, in particular, the tachocline region at the bottom of the convection zone where the solar dynamo is believed to be operating is quite challenging. Using the solar oscillation data from SOHO/MDI we have attempted to detect deep structures in a low-latitude band of the convection zone. For inversion of the travel-time measurements we used the theoretical sensitivity, in the first Born approximation, of travel times to sound speed inhomogeneities in the solar convection zone. We have obtained synoptic sound-speed maps for two solar rotations in 2000. The results show resolved structures in the lower convection zone. We compare the sound-speed maps with surface magnetic field synoptic maps and discuss possible relations between the deep structures and the surface field. Title: Time-distance helioseismology and the Solar Orbiter mission Authors: Gizon, L.; Birch, A. C.; Bush, R. I.; Duvall, T. L., Jr.; Kosovichev, A. G.; Scherrer, P. H.; Zhao, Junwei Bibcode: 2001ESASP.493..227G Altcode: 2001sefs.work..227G No abstract at ADS Title: Investigation of Mass Flows beneath a Sunspot by Time-Distance Helioseismology Authors: Zhao, Junwei; Kosovichev, Alexander G.; Duvall, Thomas L., Jr. Bibcode: 2001ApJ...557..384Z Altcode: A time-distance helioseismic technique is employed to analyze a set of high-resolution Dopplergram observations of a large sunspot by SOHO/MDI on 1998 June 18. A regularized, damped least-squares inversion is applied to the measurements of travel times to infer mass flows around the sunspot below the solar surface. Powerful converging and downward directed flows are detected at depths of 1.5-5 Mm, which may provide observational evidence for the downdrafts and vortex flows that were suggested by Parker for a cluster model of sunspots. Strong outflows extending more than 30 Mm are found below the downward and converging flows. It is suggested that the sunspot might be a relatively shallow phenomenon, with a depth of 5-6 Mm, as defined by its thermal and hydrodynamic properties. A strong mass flow across the sunspot is found at depths of 9-12 Mm, which may provide more evidence in support of the cluster model, as opposed to the monolithic sunspot model. We suggest that a new magnetic emergence that was found 5 hr after our analysis period is related to this mass flow. Title: Imaging an Emerging Active Region with Helioseismic Tomography Authors: Jensen, Jesper Munk; Duvall, Thomas L., Jr.; Jacobsen, Bo Holm; Christensen-Dalsgaard, Jørgen Bibcode: 2001ApJ...553L.193J Altcode: The wave-speed structure beneath an emerging active region is derived from helioseismic time-distance data using inversion techniques developed in geophysics. We use Fresnel zone-based sensitivity kernels along with a Fourier domain-based regularized least-squares inversion technique. The results show that wave-speed anomalies extend down to 20 Mm below active regions. We also see some evidence of deeper anomalies that appear around 16-20 hr after the emergence of an active region and then disappear within 8 hr. These anomalies could be the signature of a change in the subsurface structure of the active region. Title: Deep Focusing in Time-Distance Helioseismology Authors: Duvall, T. L.; Jensen, J. M.; Kosovichev, A. G.; Birch, A. C. Bibcode: 2001AGUSM..SP22A03D Altcode: Much progress has been made by measuring the travel times of solar acoustic waves from a central surface location to points at equal arc distance away. Depth information is obtained from the range of arc distances examined, with the larger distances revealing the deeper layers. This method we will call surface-focusing, as the common point, or focus, is at the surface. To obtain a clearer picture of the subsurface region, it would, no doubt, be better to focus on points below the surface. Our first attempt to do this used the ray theory to pick surface location pairs that would focus on a particular subsurface point. This is not the ideal procedure, as Born approximation kernels suggest that this focus should have zero sensitivity to sound speed inhomogeneities. However, the sensitivity is concentrated below the surface in a much better way than the old surface-focusing method, and so we expect the deep-focusing method to be more sensitive. A large sunspot group was studied by both methods. Inversions based on both methods will be compared. Title: Inversion of Time-Distance Data Using Non-ray-Theoretical Sensitivity Kernels Authors: Jensen, J. M.; Duvall, T. L.; Jacobsen, B. H.; Pijpers, F. P.; Christensen-Dalsgaard, J. Bibcode: 2001AGUSM..SP31A23J Altcode: Inversions of Helioseismic Time-Distance data present unique views of the three-dimensional wave-speed variations in the solar interior. It has e.g. become possible to image the subsurface wave-speed structures beneath active regions and thus explore the magnetic activity of the sun in a new way. Since the solar oscillations have long wavelengths compared to the spatial extent of the features we wish to investigate, the ray approximation is inadequate in such imaging. We present inversions of Helioseismic Time-Distance data using non-ray-theoretical sensitivity kernels. The kernels are obtained using the Rytov approximation and have the so-called banana-doughnut shape. They are zero along the raypath and most of the sensitivity is located in an area corresponding to the first Fresnel zone around the ray. For the inversion we use a Fourier-domain based regularized least-squares method. We present results obtained using this inversion procedure, along with averaging kernels and error estimates of the results. We also present first results obtained using a SOLA inversion algorithm. Title: Mass Flows Beneath the Sunspot from Inversion of Time-distance Helioseismology Authors: Zhao, J.; Kosovichev, A. G.; Duvall, T. L. Bibcode: 2001AGUSM..SP22A04Z Altcode: Time-distance helioseismic technique has provided a useful tool to study the interior structure of the Sun. The inversion of time-distance measurements can help us reveal mass flows and sound speed perturbation beneath the solar surface. We have applied time-distance measurements to a set of high resolution Dopplergram observations of a sunspot by SOHO/MDI, and a regularized damped least-squares inversion was used to infer the mass flows beneath this sunspot. Powerful converging and downward flows are detected at a depth of 1.5 to 5 Mm, which may provide observational evidence for the cluster sunspot model. Strong outflows which extend more than 30Mm outside the center of the sunspot are found below 5Mm. A full disk observation of an interesting event in August 2000, an apparent spin of a sunspot, was also analyzed by the same approach but with lower resolution. For this event a vortex flow has been detected in subsurface layers. This may provide an insight into the study of helicity below the photosphere of the Sun. Title: Using a Wave-Theory Approach to Time-Distance Helioseismology Authors: Birch, A. C.; Duvall, T. L.; Kosovichev, A. G. Bibcode: 2001AGUSM..SP31A21B Altcode: Time-distance helioseismology is a method for measuring the travel times for acoustic wave packets as they move between points on the solar surface through the solar interior. In order to interpret travel times we derive, employing the Born approximation to the wave equation, a linear relationship between travel time variations and perturbations to a solar model; the results are essentially the "banana-doughnut" kernels familiar from geophysics. We show preliminary inversion results for large-scale structure inside the sun using these sensitivity kernels. Title: Probing Magnetic Structures in the Solar Interior by Helioseismic Tomography Authors: Kosovichev, A. G.; Duvall, T. L., Jr.; Scherrer, P. H. Bibcode: 2001ASPC..248..169K Altcode: 2001mfah.conf..169K No abstract at ADS Title: Heliotomography of the outer layers of the Sun Authors: Kosovichev, A. G.; Duvall, T. L., Jr.; Birch, A. C.; Gizon, L.; Scherrer, P. H.; Zhao, Junwei Bibcode: 2001ESASP.464..701K Altcode: 2001soho...10..701K Heliotomography offers important diagnostics of the solar interior by providing three-dimensional maps of the sound speed and flows in the upper convection zone. These diagnostics are based on inversion of travel times of acoustic waves which propagate between different points on the solar surface through the interior. The most significant variations in the thermodynamic structure found by this method are associated with sunspots and complexes of solar activity. The inversion results provide evidence for areas of higher sound speed beneath sunspot regions located at depths of 4 - 20 Mm, which may be due to accumulated heat or magnetic field concentrations. The results reveal structures and flows associated with active regions and sunspots at various stages of their evolution, and provide important constraints for theories of solar dynamics and activity. Title: Local-area helioseismology by SOT on-board Solar-B Authors: Sekii, T.; Shibahashi, H.; Kosovichev, A. G.; Duvall, T. L., Jr.; Berger, T. E.; Bush, R.; Scherrer, P. H. Bibcode: 2001ESASP.464..327S Altcode: 2001soho...10..327S Solar-B satellite, a successor to Yohkoh, will be launched in 2005. Placed in a sun-synchronous orbit, it will carry out multi-wavelength observation in optical, EUV and X-ray ranges. One of the instruments on Solar-B, Solar Optical Telescope (SOT), a Japan/US collaboration, aims at measuring the magnetic field and the Doppler velocity field in the solar photosphere. Although it is not specifically designed for helioseismic observations, the high-resolution Dopplergram produced by SOT is potentially a very powerful tool for detailed seismic investigation of subsurface magnetic and thermal structures and associated mass flows. If successful, these measurements will be an important contribution to the main goal of the Solar-B project: understanding the origin and dynamics of the basic magnetic structures and their effects on the solar corona. We discuss the prospect and challenges of local-area helioseismology by SOT. Title: Probing Surface Flows and Magnetic Activity with Time-Distance Helioseismology Authors: Gizon, L.; Duvall, T. L., Jr.; Larsen, R. M. Bibcode: 2001IAUS..203..189G Altcode: Time-distance helioseismology, applied to surface gravity waves, has been shown to be a useful tool to study horizontal flows near the solar surface, and supergranulation in particular (Duvall & Gizon, 2000). Here, we present maps of horizontal flows and horizontal magnetic fields, in both quiet and active regions. Travel-time sensitivity kernels based on wave theory, as opposed to ray theory, are used in the inversions. Title: Time-distance study of supergranulation Authors: Beck, J. G.; Duvall, T. L., Jr. Bibcode: 2001ESASP.464..577B Altcode: 2001soho...10..577B Time-distance helioseismology is a valuable tool for examining near surface flows: accurate maps of supergranulation flows are produced from the travel times of surface gravity waves. The lifetimes of individual supergranules are measured from time-distance maps and found to be inconsistent with measurements obtained from autocorrelation techniques. The spatial cross-correlation shows a East-West shift over time, suggesting that supergranulation has two components which do not co-rotate. Title: New Developments in Local Area Helioseismology Authors: Duvall, T. L., Jr.; Kosovichev, A. G. Bibcode: 2001IAUS..203..159D Altcode: Several techniques are used to study local areas in helioseismology, including time-distance helioseismology, acoustic imaging/holography, and ring diagram analysis. These techniques can be used to study flows, magnetic fields, and temperature inhomogeneities. The "local" area studied can be as small as a supergranule, or as large as the entire convection zone in the case of meridional circulation as studied by Giles and colleagues. Active regions have been studied with some interesting results, with complicated flow patterns below sunspots and detectable sound speed inhomogeneitities in the 10 Mm below the spots. Another interesting result is the detection of sunspots on the back side of the Sun by Lindsey and Braun using the holography technique. A confirmation of their result using the time-distance technique will be discussed. Title: Solar Interior: Local Helioseismology Authors: Duvall, T. Bibcode: 2000eaa..bookE2250D Altcode: This article describes how we use acoustic waves to study the subsurface properties of particular areas of the Sun. Local helioseismology is not as well developed as the more mature area of study, global HELIOSEISMOLOGY. New techniques are still being suggested and developed.This article will discuss some of these techniques and the most solid of the inferences about the SOLAR INTERIOR derived fr... Title: Seismic Tomography of the Near Solar Surface Authors: Gizon, L.; Duvall, T. L., Jr.; Larsen, R. M. Bibcode: 2000JApA...21..339G Altcode: No abstract at ADS Title: Near-surface Horizontal Flows in Sunspots and Supergranules Authors: Gizon, L.; Duvall, T. L., Jr.; Larsen, R. M. Bibcode: 2000SPD....31.0108G Altcode: 2000BAAS...32..802G Surface gravity waves have been used to probe flows in the two megameters beneath the photosphere using the techniques of time-distance helioseismology. Realistic spatial kernels were calculated using the Born approximation and used in an iterative deconvolution to obtain an estimate of the subsurface horizontal flows. We detect an outward flow outside sunspots: the moat flow. Penumbral outward flows are also present, but appear to be smaller than the Evershed flow observed at the surface. The effect of the Coriolis force on supergranular motion has been measured. Title: Diagnostics of Solar Magnetic Fields by Time-Distance Helioseismology Authors: Zhao, J.; Duvall, T. L., Jr.; Kosovichev, A. G. Bibcode: 2000SPD....31.0120Z Altcode: 2000BAAS...32..804Z Sunspot seismology has been developed in recent years, and the time-distance analysis plays an important role in it. Most of the current inferences for interior structures were made by measuring perturbations of the acoustic wave speed which is due to both temperature and magnetic field variations. An important problem of the time-distance seismology is to disentangle the effects of temperature and magnetic field. The standard technique for the travel time measurements is to divide annuli for given wave travel distances into four sectors to get both the wave speed and flow velocity under the surface. Here, we consider the inhomogeneity caused by the magnetic field of sunspots. By dividing the annuli into eight sectors and analyzing the travel time of each octants, we can obtain the direction and the magnitude of the magnetic field in sunspot regions. Scattering and absorption of incoming waves in each different direction may also play an important role in these measurements. Title: Heliotomography: what happens just below the surface? Authors: Kosovichev, A. G.; Duvall, T. L., Jr. Bibcode: 2000SPD....31.0601K Altcode: 2000BAAS...32..838K Heliotomography (or time-distance helioseismology) is a relatively new tool for diagnostics of internal structures and dynamics of the Sun. It is based on inversion of travel times of acoustic wave packets propagating through the solar interior and bouncing back to the surface. The travel times provide information about the variations of temperature, magnetic fields and flow velocities along the wave paths. These properties of the solar interior are inferred from the travel times by tomographic inversions. Heliotomography has provided a three-dimensional view of the interior, not accessible by traditional helioseismology based on mode frequencies. This method has been applied to study both large-scale flows (meridional circulation, North-South asymmetry of solar rotation) and small-scale phenomena (supergranulation, sunspots, emerging magnetic flux). The results reveal very dynamical and complicated structures below the surface, associated with convection and magnetic fields, and shed new light on the formation and evolution of active regions and sunspots. We discuss the current limits for the temporal and spatial resolution and recent achievements. Most inversion results provide the results to a depth of 20 Mm. It has been demonstrated that with this method we can measure the solar flows to the base of the convection zone which is 200 Mm deep. However, resolving deep and small-scale features is very challenging, and requires concentrated efforts for developing both the measurement techniques and theoretical interpretations. We review the recent progress in developing a wave-theory approach to heliotomographic inversions, and perspectives for the diagnostics of the physical processes below the Sun's surface. Title: Sunspots: Frontside and Backside Measurements with Time-Distance Helioseismology Authors: Duvall, T. L., Jr.; Kosovichev, A. G.; Scherrer, P. H. Bibcode: 2000SPD....31.0505D Altcode: 2000BAAS...32..837D In time-distance helioseismology, travel times measured between different surface locations are used to infer subsurface flows, temperature inhomogeneities and magnetic fields. It has been suggested that most of the travel time reduction near sunspots may be due to the lowered reflection layer associated with the Wilson depression. This will be examined by looking at rays that travel below the sunspot but do not begin or end in the spot. A time-distance method of imaging sunspots on the backside will be compared with that of Lindsey and Braun. Title: Time-Distance Studies of Supergranule Evolution Authors: Beck, J. G.; Duvall, T. L., Jr. Bibcode: 2000SPD....31.0105B Altcode: 2000BAAS...32Q.802B Time distance helioseismology is a valuable tool for examining near surface flow. It can produce vector maps of flows from the travel time of waves traversing subsurface ray paths. It has been demonstrated that time-distance helioseismology of the surface gravity waves can produce accurate surface maps of supergranulation. These maps average over the two megameters immediately below the surface (Duvall and Gizon, Solar Physics, 2000, in press). The temporal resolution of time-distance flow maps is large compared with the lifetime of small scale convection, but small compared with supergranule lifetimes. Therefore, it is possible to produce a time-series of time-distance flow maps which reveal patterns of supergranulation evolution. We have observed certain modes of supergranulation `birth' and `death' using time-distance helioseismology and have compared these with results from line-of-sight doppler velocities. This work was supported by the SOI-MDI NASA grant at Stanford and by the Solar Physics Branch of the Space Science Division of NASA. Title: Sunspots: frontside and backside measurements with time-distance helioseismology. Authors: Duvall, T. L., Jr.; Kosovichev, A. G.; Scherrer, P. H. Bibcode: 2000BAAS...32Q.837D Altcode: No abstract at ADS Title: Heliotomography: what happens just below the surface? Authors: Kosovichev, A. G.; Duvall, T. L., Jr. Bibcode: 2000BAAS...32..837K Altcode: No abstract at ADS Title: Phase Time and Envelope Time in Time-Distance Analysis and Acoustic Imaging Authors: Chou, Dean-Yi; Duvall, Thomas L., Jr. Bibcode: 2000ApJ...533..568C Altcode: Time-distance analysis and acoustic imaging are two related techniques for probing the local properties of the solar interior. In this study, we discuss the relation of phase time and envelope time between the two techniques. The location of the envelope peak of the cross-correlation function in time-distance analysis is identified as the travel time of the wave packet formed by modes with the same horizontal phase velocity. The phase time of the cross-correlation function provides information on the phase change accumulated along the wave path, including the phase change at the boundaries of the mode cavity. The acoustic signals constructed with the technique of acoustic imaging contain both phase and intensity information. The phase of constructed signals can be studied by computing the cross-correlation function between time series constructed with ingoing and outgoing waves. We use a simple theory of wave packets to obtain two predictions about the cross-correlation function of constructed ingoing and outgoing time series. First, if the envelope time measured in time-distance analysis is used to construct signals in acoustic imaging, the envelope time of the cross-correlation is zero. Second, the phase time of the cross-correlation is twice the difference between the phase time and envelope time measured in time-distance analysis. In this study, we use data taken with the Taiwan Oscillation Network (TON) instrument and the Michelson Doppler Imager (MDI) instrument. The analysis is carried out for the quiet Sun. We use the relation of envelope time versus distance measured in time-distance analysis to construct the acoustic signals in acoustic imaging analysis. The phase time of the cross-correlation function of constructed ingoing and outgoing time series is twice the difference between phase time and envelope time in time-distance analysis, as predicted. The envelope peak of the cross-correlation function between constructed ingoing and outgoing time series is located at zero time, as predicted for one-bounce results at 3 mHz for all four data sets and two-bounce results at 3 mHz for two TON data sets, but it is different from zero for other cases. The deviation of the envelope peak from zero has the same sign for all these cases. The cause is not known. Title: Helioseismic diagnostics of solar convection and activity. Part 1, 2. Proceedings. SOHO-9 Workshop, Stanford, CA (USA), 12 - 15 Jul 1999. Authors: Švestka, Z.; Harvey, J. W.; Kosovichev, A. G.; Duvall, T. L., Jr. Bibcode: 2000SoPh..192....1S Altcode: The following topics were dealt with: theories of solar convection, rotation and activity, helioseismic tomography, acoustic imaging and holography, ring-diagram analysis, magnetic fields and oscillations, solar cycle variations of the internal structure and rotation, solar convective structures and oscillations. Title: Near-Surface Flow Fields Deduced Using Correlation Tracking and Time-Distance Analyses Authors: De Rosa, Marc; Duvall, T. L., Jr.; Toomre, Juri Bibcode: 2000SoPh..192..351D Altcode: Near-photospheric flow fields on the Sun are deduced using two independent methods applied to the same time series of velocity images observed by SOI-MDI on SOHO. Differences in travel times between f modes entering and leaving each pixel measured using time-distance helioseismology are used to determine sites of supergranular outflows. Alternatively, correlation tracking analysis of mesogranular scales of motion applied to the same time series is used to deduce the near-surface flow field. These two approaches provide the means to assess the patterns and evolution of horizontal flows on supergranular scales even near disk center, which is not feasible with direct line-of-sight Doppler measurements. We find that the locations of the supergranular outflows seen in flow fields generated from correlation tracking coincide well with the locations of the outflows determined from the time-distance analysis, with a mean correlation coefficient after smoothing of s=0.890. Near-surface velocity field measurements can be used to study the evolution of the supergranular network, as merging and splitting events are observed to occur in these images. The data consist of one 2048-min time series of high-resolution (0.6'' pixels) line-of-sight velocity images taken by MDI on 1997 January 16 -18 at a cadence of one minute. Title: Time-Distance Helioseismology with f Modes as a Method for Measurement of Near-Surface Flows Authors: Duvall, T. L., Jr.; Gizon, L. Bibcode: 2000SoPh..192..177D Altcode: Travel times measured for the f mode have been used to study flows near the solar surface in conjunction with simultaneous measurements of the magnetic field. Previous flow measurements of Doppler surface rotation, small magnetic feature rotation, supergranular pattern rotation, and surface meridional circulation have been confirmed. In addition, the flow in supergranules due to Coriolis forces has been measured. The spatial and temporal power spectra for a six-day observing sequence have been measured. Title: Time-Distance Inversion Methods and Results - (Invited Review) Authors: Kosovichev, A. G.; Duvall, T. L. _Jr., Jr.; Scherrer, P. H. Bibcode: 2000SoPh..192..159K Altcode: The current interpretations of the travel-time measurements in quiet and active regions on the Sun are discussed. These interpretations are based on various approximations to the 3-D wave equation such as the Fermat principle for acoustic rays and the Born approximation. The ray approximation and its modifications have provided the first view of the 3-D structures and flows in the solar interior. However, more accurate and computationally efficient approximations describing the relation between the wave travel times and the internal properties are required to study the structures and flows in detail. Inversion of the large three-dimensional datasets is efficiently carried out by regularized iterative methods. Some results of time-distance inversions for emerging active regions, sunspots, meridional flows and supergranulation are presented. An active region which emerged on the solar disk in January 1998, was studied from SOHO/MDI for eight days, both before and after its emergence at the surface. The results show a complicated structure of the emerging region in the interior, and suggest that the emerging flux ropes travel very quickly through the depth range of our observations. The estimated speed of emergence is about 1.3 km s−1. Tomographic images of a large sunspot reveal sunspot `fingers' - long narrow structures at a depth of about 4 Mm, which connect the sunspot with surrounding pores of the same polarity. Title: Commission 12: Solar Radiation and Structure (Radiation et Structure Solaires) Authors: Foukal, Peter; Solanki, Sami; Mariska, J.; Baliunas, S.; Dravins, D.; Duvall, T.; Fang, C.; Gaizauskas, V.; Heinzel, P.; Kononovich, E.; Koutchmy, S.; Melrose, D.; Stix, M.; Suematsu, Y.; Deubner, F. Bibcode: 2000IAUTA..24...73F Altcode: No abstract at ADS Title: Solar tomography Authors: Kosovichev, A. G.; Duvall, T. L., Jr. Bibcode: 1999CSci...77.1467K Altcode: No abstract at ADS Title: Imaging of Emerging Magnetic Flux by Time-Distance Helioseismology Authors: Kosovichev, A. G.; Duvall, T. L., Jr.; Scherrer, P. H. Bibcode: 1999AAS...194.5901K Altcode: 1999BAAS...31..917K We have used measurements of acoustic travel time in the convection zone to infer local perturbations of the sound speed and 3D flow velocities associated with emerging active regions in July 1996 and January 1998. Both regions were observed with the MDI instrument on SOHO before and after emergence continuously for 9 days. The first active region emerged in a long-lived complex of activity and produced a strong X-class flare. The second active region was a high-latitude region of the new solar cycle. The time-distance inversion results show complicated dynamics of the magnetic flux in the convection zone, and indicate that the emerging flux travels faster in the convection zone than predicted by theory. We discuss the differences in the dynamics of these active regions. Title: The SOI-MDI Dynamics Program: Observing the Solar Cycle Authors: Bush, R. I.; Beck, J. G.; Bogart, R. S.; Hoeksema, J. T.; Kosovichev, A. G.; Scherrer, P. H.; Schou, J.; Sommers, J.; Duvall, T. L. Bibcode: 1999AAS...194.9205B Altcode: 1999BAAS...31..987B The Michelson Doppler Imager instrument on the SOHO spacecraft has been observing the Sun over the last three years. The MDI Dynamics Program provides nearly continuous full disk Doppler measurements of the solar photosphere with 4 arc-second resolution for periods of 60 to 90 days each year. Three of these Dynamics periods have been completed: 23 May to 24 July 1996, 13 April to 14 July 1997, 9 January to 10 April 1998. A fourth Dynamics observing period began on 13 March and is scheduled to continue through mid July. These observations provide a unique view of the evolution of the Sun in the early part of the solar cycle, both from interior flows deduced by helioseimic analysis and changes in large scale surface motion. Details of the Dynamics programs will be presented along with an overview of current results. This research is supported by the SOI-MDI NASA grant NAG5-3077 at Stanford University. Title: SONAR - Solar Near-surface Active Region Rendering Authors: Scherrer, P. H.; Hoeksema, J. T.; Kosovichev, A. G.; Duvall, T. L.; Schrijver, K. J.; Title, A. M. Bibcode: 1999AAS...194.7606S Altcode: 1999BAAS...31Q.957S The processes in the top 20,000-km of the Sun's convection zone govern the growth and decay of active regions and provide the magnetic flux and energy for the active phenomena of the upper solar atmosphere. The MDI experiment on SOHO has demonstrated that this region is now accessible to study by means of local helioseismology. However, SOHO provides neither the temporal nor spatial resolution and coverage necessary to exploit these techniques to study the eruption and evolution of active region magnetic structures. The SONAR mission with moderate resolution full disk Doppler and vector magnetic field observations, and atmospheric magnetic connectivity observations via EUV imaging can provide the necessary data. The science motivation and general instrumentation requirements for the mission are presented. Title: New Views of Active Regions Authors: Duvall, T. L., Jr.; Kosovichev, A. G.; Scherrer, P. H. Bibcode: 1999AAS...194.4203D Altcode: 1999BAAS...31..882D 3-d acoustic tomography of the region below the photosphere is providing qualitatively new insights about solar active regions. This tomography is based on the measurement of travel times between different surface locations and is sensitive to subsurface flows and to wave-speed inhomogeneities caused by temperature and magnetic field variations. A flow cell has been seen below sunspots similar to the model of Parker (Ap.J. 230,905-913,1979) in which there is a horizontal inflow near the top of the convection zone, a downflow directly below the sunspot and a horizontal outflow below. This flow may be what stabilizes the sunspot. A wave-speed reduction is seen in the 2 Mm below the surface sunspot and a wave-speed enhancement is seen below to at least 10 Mm depth for a reasonably large spot. At 10 Mm depth, a wave-speed increase of 3% could be caused by a 10 kG magnetic field or a temperature excess of 6%. At present we cannot distinguish between temperature and magnetic field effects on the wave speed, but we will present a comparison between the wave speed as predicted from a sunspot model and that measured with the tomography. This research is supported by NASA contract NAG5-3077 at Stanford University. Title: Large-Scale Solar Flows From Time-Distance Helioseismology Authors: Giles, P. M.; Duvall, T. L., Jr.; Scherrer, P. H. Bibcode: 1999AAS...194.2102G Altcode: 1999BAAS...31..858G Over the past thirty years, helioseismology has proven to be an extremely useful tool for probing the solar interior. Using global mode frequencies, the structure and the rotation of the Sun have been determined with unprecedented accuracy. More recently, there has been rapid evolution of so-called "local" methods in helioseismology. These techniques are able to examine aspects of the Sun's structure and dynamics which are otherwise inaccessible. One of the most successful of these techniques is time-distance helioseismology, which relies on the determination of wave travel times to infer properties of the subsurface region. This approach has been particularly successful in measuring flows in the solar convection zone which were previously observable only at the surface. In this paper we present our most recent results in these investigations, including our latest determination of the meridional circulation as a function of depth, and our search for large-scale nonaxisymmetric velocity structures. Data for this research was obtained by the MDI instrument on SOHO. This research is supported by NASA contract NAG5-3077 at Stanford University. Title: Subsurface Observations of Sunspots and Solar Supergranulation Authors: Duvall, T. L., Jr.; Kosovichev, A. G.; Scherrer, P. H. Bibcode: 1999AAS...194.5606D Altcode: 1999BAAS...31..912D 3-d acoustic tomography of the region below the photosphere is providing new insights into sunspots and the apparently convective flow observed at the surface called supergranulation. The tomography is based on the measurement of travel times between different surface locations and is sensitive to subsurface flows and to wave-speed inhomogeneities caused by temperature and magnetic field variations. This study uses dopplergrams from the MDI instrument on the SOHO spacecraft. A flow cell has been seen below sunspots similar to the model of Parker (Ap.J. 230,905-913,1979) in which there is a horizontal inflow near the top of the convection zone, a downflow directly below the sunspot and a horizontal outflow below. The depth of the supergranulation flow will be discussed. This research is supported by NASA contract NAG5-3077 at Stanford University. Title: Comparison Between Near-Surface Flow Fields Deduced from Correlation Tracking and Time-Distance Helioseismology Methods Authors: De Rosa, M. L.; Toomre, J.; Duvall, T. L., Jr. Bibcode: 1999AAS...194.5608D Altcode: 1999BAAS...31..913D Near-photospheric flow fields deduced using two independent methods applied to the same SOI-MDI time series of images from SOHO are compared. Differences in travel times between incoming and outgoing f modes measured using time-distance helioseismology are used to determine the sites of supergranule outflows. Alternatively, correlation tracking analysis is applied to granular and mesogranular structures seen in time series of Doppler and intensity images. We find that the locations of the supergranular outflows seen in flow fields generated from correlation tracking coincide well with the locations of the outflows determined from the time-distance analysis. The near-surface flow fields provide us with insight in understanding the dyanmics of the turbulent convection occurring below the photosphere. The data consist of four 512-minute time series of high-resolution (0.6'' pixels) Doppler images and continuum intensity images taken by MDI on 17--18 January 1997 at a cadence of one minute. Title: Time-distance Measurements of Meridional Circulation Deep in the Convection Zone Authors: Giles, P. M.; Duvall, T. L., Jr.; Scherrer, P. H. Bibcode: 1999soho....9E..23G Altcode: Explaining the solar cycle is one of the central goals of solar physics. Some of the most successful models of the cycle fall under the broad category of Babcock-Leighton dynamo theories. Babcock and Leighton developed this model in the 1960s, making use of the most recent observations of the Sun's magnetic field and surface motions. The model reproduces the large-scale properties of the cycle by invoking both differential rotation and supergranular diffusion of magnetic elements. Although the original work predates the birth of helioseismology, it still underlies much of our current understanding of the solar cycle. The development of helioseismology has, however, necessitated some evolution of the theory. For example, dynamo theorists now must match their models to the observed differential rotation profile in the solar interior. Prodded by more sophisticated surface measurements, several groups have also proposed models including a meridional circulation. Until recently, theorists were free to speculate on the characteristics of this flow below the surface. In the past few years, however, several helioseismic techniques have been used to successfully measure the meridional circulation in the solar interior. In this paper, the authors present their latest measurements of the meridional flow using the time-distance technique on MDI data. These measurements now reach far enough into the convection zone that they might be a useful constraint on solar dynamo theories. This research is supported by NASA contract NAG5-3077 at Stanford University. Title: Comparison Between Near-Surface Flow Fields Deduced from Correlation Tracking and Time-Distance Helioseismology Methods Authors: de Rosa, Marc; Toomre, Juri; Duvall, T. L., Jr. Bibcode: 1999soho....9E..51D Altcode: Near-photospheric flow fields deduced using two independent methods applied to the same SOI-MDI time series of images from SOHO are compared. Differences in travel times between incoming and outgoing f modes measured using time-distance helioseismology are used to determine the sites of supergranule outflows. Alternatively, correlation tracking analysis is applied to granular and mesogranular structures seen in time series of Doppler and intensity images. We find that the locations of the supergranular outflows seen in flow fields generated from correlation tracking coincide well with the locations of the outflows determined from the time-distance analysis. The near-surface flow fields provide us with insight in understanding the dynamics of the turbulent convection occurring below the photosphere. The data consist of four 512-minute time series of high-resolution (0.6 arc-second pixels) Doppler images and continuum intensity images taken by MDI on 17-18 January 1997 at a cadence of one minute. Title: Time-Distance Diagnostics Authors: Duvall, T. L., Jr. Bibcode: 1999soho....9E..19D Altcode: Time-distance helioseismology is being used to study a variety of solar phenomena, including meridional circulation, solar rotation, sunspots, plage, supergranulation and giant velocity cells. The technique is based on the measurement of the cross-covariance function of the signal at different surface locations and the subsequent interpretation of travel times from this function. The travel times are sensitive to flows and to wave-speed inhomogeneities caused by temperature and magnetic field variations. In the limit of short wavelengths, the travel times are sensitive to these quantities only along the ray path connecting the surface points. To extract quantitative information, ray theory has generally been used to analyze the results, e.g. the 3-d tomography of Kosovichev. The current status of the various investigations will be presented. This research is supported by NASA contract NAG5-3077 at Stanford University. Title: Giant Cells: Convection or Wave? Authors: Beck, J. G.; Duvall, T. L., Jr. Bibcode: 1999soho....9E..39B Altcode: Giant velocity cells have been unambiguously detected at the solar surface using different techniques and different data. Whereas there is some agreement regarding the size, amplitude and shape of these cells, the physical process which produces them remains in doubt. The possibility of giant cells inertial waves (such as r-modes) is considered here. This research is supported by NASA grant NAG5-3077 at Stanford University. Title: Helioseismic Diagnostics of Solar Convection and Activity Authors: Duvall, T. L., Jr.; Kosovichev, A. G. Bibcode: 1999soho....9E....D Altcode: No abstract at ADS Title: Time-distance helioseismology Authors: Kosovichev, A. G.; Duvall, T. L.; Scherrer, P. H. Bibcode: 1999AdSpR..24..163K Altcode: The time-distance helioseismology (or helioseismic tomography) is a new promising method for probing 3-D structures and flows beneath the solar surface, which is potentially important for studying the birth of active regions in the sun's interior and for understanding the relation between the internal dynamics of active regions and chromospheric and coronal activity. In this method, the time for waves to travel along subsurface ray paths is determined from the temporal cross correlation of signals at two separated surface points. By measuring the times for many pairs of points from Dopplergrams covering the visible hemisphere, a tremendous quantity of information about the state of the solar interior is derived. As an example, we present the results for supergranular flows and for an active region which emerged near the center of the solar disk in July 1996, and was studied from SOHO/MDI for nine days, both before and after its emergence at the surface. Initial results show a complicated structure of the emerging region in the interior, and suggest that the emerging flux ropes travel very quickly through the depth range of our observations. Title: Surface Wave Time-Distance Helioseismology Authors: Gizon, L.; Duvall, T. L., Jr. Bibcode: 1999soho....9E..24G Altcode: The propagation of solar surface gravity waves (or f modes) is affected by essentially two types of perturbations: the horizontal components of the flows and of the magnetic field. We probe the first few megameters below the photosphere using the time-distance technique, combined with MDI/SOHO observations of surface waves. Two-dimensional theoretical sensitivity kernels are computed in the Born approximation, to provide a connection between the Sun's structure and the observed travel-time anomalies. This research is supported by NASA contract NAG5-3077 at Stanford University. Title: Travel Time and Phase Time in Time-Distance Analysis and Acoustic Imaging Authors: Chou, Dean-Yi; Duvall, Thomas L., Jr. Bibcode: 1999soho....9E..49C Altcode: The time-distance analysis and acoustic imaging are two related techniques to probe the local properties of solar interior. In this study, we discuss the relation of travel time and phase time between the two techniques. The location of the envelope peak of the cross correlation function in time-distance analyses is identified as the travel time of the wave packet formed by modes with the same wl. The phase time of the cross correlation function provides information of the phase change along the wave path, including the phase change at the boundaries of the mode cavity. The acoustic signals constructed with the technique of acoustic imaging contain both phase and intensity information. The phase of constructed signals can be studied by computing the cross correlation function between time series constructed with ingoing and outgoing waves. In this study, we use the data taken with the Taiwan Oscillation Network (TON) instrument and the Michelson Doppler Imager (MDI) instrument. The analysis is carried out for the quiet Sun. The results from TON data and MDI data are consistent. If the relation of travel time versus distance measured in time-distance analyses is used to construct the acoustic signals in acoustic imaging analyses, the envelope peak of the cross correlation function between constructed ingoing and outgoing time series is located at zero time. The phase time of the cross correlation function is twice the difference between the phase time and envelope time in time-distance analyses. This phase time doubles if the signals are constructed with the two-bounce time-distance relation. This relation of travel time and phase time between the two methods helps understand acoustic imaging and interpret its results. Title: Random Damping and Frequency Reduction of the Solar F Mode Authors: Duvall, T. L., Jr.; Kosovichev, A. G.; Murawski, K. Bibcode: 1998ApJ...505L..55D Altcode: We present observations showing that the frequency of the high-degree f-mode is significantly lower than the frequency given by the simple dispersion relation, ω2=gk , and that the line width grows with the wavenumber k. We attempt to explain that this behavior is the result of the interaction with granulation, which we model as a random flow. Because the f-mode spends more time propagating against the flow than with the flow, its effective speed and, consequently, frequency are reduced. Additionally, an eddy viscosity introduces the negative imaginary part of frequency. This negative imaginary part represents the damping of the coherent field due to scattering. The line width is proportional to the magnitude of the imaginary part of the frequency. We apply an analytical perturbation technique and numerical methods to estimate the line width and the frequency shift, and we show that the results are consistent with the properties of the f-mode obtained from the high-resolution Michelson Doppler Imager data from the Solar and Heliospheric Observatory. Title: Long-lived giant cells detected at the surface of the Sun Authors: Beck, J. G.; Duvall, T. L.; Scherrer, P. H. Bibcode: 1998Natur.394..653B Altcode: Giant convective cells have been predicted to exist in the Sun. Such cells should span the entire zone unstable to convective motions - now known to cover the outer 29 per cent of the Sun's radius - and could be dredging up the magnetic flux that is thought to be the source of solar activity (sunspots). Several studies have failed to detect these giant cells, although there have been hints of their existence. We have detected long-lived velocity cells, which we identify as the elusive giant convective cells, extending over 40-50 degrees of longitude but less than 10 degrees of latitude. The large aspect ratio (>4) is surprising (although predicted by one model) and may be a consequence of the Sun's differential rotation, whereby features with a larger extent in latitude are broken up by rotational shear. Title: Time-Distance Measurements of Subsurface Rotation and Meridional Flow Authors: Giles, P. M.; Duvall, T. L., Jr.; Scherrer, P. H. Bibcode: 1998ESASP.418..775G Altcode: 1998soho....6..775G One of the most promising applications of time-distance helioseismology is the measurement of flows beneath the solar surface. In some cases the time-distance approach can yield information which cannot be obtained with more traditional helioseismology. Recently this technique has been used to measure the global meridional circulation (Giles et al., 1997) and differential rotation (Giles & Duvall, 1997). So far, these meaurements have only been able to penetrate a short distance into the solar interior --- up to a few percent of the solar radius. We will present the results of work which extends these measurements to greater depths, with a special focus on the meridional circulation. Probing this deeper region, while simple in principle, offers significant practical challenges. We will present a brief analysis of these difficulties along with our new results. Title: Helio-Atmospheric Links Explorer (HALE): A MIDEX Experiment for Exploring the Emergence of Magnetic Flux from Below the Solar Photosphere through the Corona Authors: Scherrer, P. H.; Title, A. M.; Bush, R. I.; Duvall, T. L., Jr.; Gurman, J. B.; Kosovichev, J. T.; Hoeksema, A. G.; Poland, A. I.; Tarbell, T. D. Bibcode: 1998ESASP.417..285S Altcode: 1998cesh.conf..285S No abstract at ADS Title: Damping and Frequency Shift of the Solar f-mode Due to the Interaction with Turbulent Convection Authors: Murawski, K.; Duvall, T. L., Jr.; Kosovichev, A. G. Bibcode: 1998ESASP.418..825M Altcode: 1998soho....6..825M Observations indicate that the frequency of the high-degree f-mode is substantially smaller that the frequency given by the simple dispersion relation, ω2 = gk, and that the line width grows with the wavenumber k. We attempt to explain this behaviour as a result of interaction with granulation which we model as a random flow. Because of buffeting from the random flow the f-mode wave speed and consequently frequency are reduced. Additionally, a random flow makes an appearance of the negative imaginary part of frequency, ωi. This negative complex part represents the damping of the mean field, i. e. the generation of random field at the expense of the mean field energy. The line width is proportional to the magnitude of the imaginary part of the frequency. We apply an analytical perturbation technique and numerical methods to estimate the imaginary part and the frequency shift, and show that the results are consistent with the properties of the f-mode obtained from the high-resolution MDI data. Title: The Detection of Giant Velocity Cells on the Sun Authors: Beck, J. G.; Duvall, T. L., Jr.; Scherrer, P. H.; Hocksema, J. T. Bibcode: 1998ESASP.418..725B Altcode: 1998soho....6..725B No abstract at ADS Title: Recent Results and Theoretical Advances in Local Helioseismology Authors: Duvall, T. L., Jr. Bibcode: 1998ESASP.418..581D Altcode: 1998soho....6..581D Local helioseismology works! Or at least it shows great promise. A variety of solar phenomena is being studied, including sunspots, plage, supergranulation, active region evolution, meridional circulation, giant cells, and solar rotation. Exciting new insights into these phenomena have been derived recently. The phenomena are being studied with a zoo of techniques, including time-distance helioseismology, ring diagrams, Fourier-Hankel modal analysis, acoustic imaging, helioseismic holography, back-side imaging, and Hilbert transforms. In coming years, this list of techniques will no doubt grow longer as we are in a very early stage. The different techniques sometimes agree, for example three methods have seen poleward meridional flow near the surface with comparable magnitudes. An example of apparent disagreement is in the wave speed variation below sunspots. Some time-distance results suggest a rather shallow region of 10 Mm depth in which wave-speed variations are present. But the acoustic imaging results suggest a depth of 40 Mm in which acoustic absorption is occurring. It may be that the two techniques are measuring different quantities, although both techniques need to be developed further in order to understand such differences. Local helioseismology can give us important insights into the sun, and should be pursued vigorously. Title: Solar rotation and large-scale flows determined by time-distance helioseismology MDI Authors: Giles, P. M.; Duvall, T. L., Jr.; Kosovichev, A. G. Bibcode: 1998IAUS..185..149G Altcode: We use differences of reciprocal travel time of acoustic waves propagating along parallels and meridians to determine variations of solar rotation and large-scale flows beneath the Sun's surface. This new technique can provide an effective measure of the antisymmetric component of the differential rotation and meridional flows that are not determined by the traditional helioseismic techniques based on inversion of frequency splitting of normal modes. We investigate resolution and precision of the time-distance measurements using the Michelson Doppler Imager (MDI) Medium-l data. The first results show a clear picture of the latitudinal differential rotation and its variation with depth. We discuss preliminary results of the meridional flow measurements. Title: Acoustic tomography of solar convective flows and structures Authors: Kosovichev, A. G.; Duvall, T. L., Jr. Bibcode: 1997ASSL..225..241K Altcode: 1997scor.proc..241K; 2018arXiv180603273K We present a new method for helioseismic diagnostics of the three-dimensional structure of sound speed, magnetic fields and flow velocities in the convection zone by inversion of acoustic travel-time data. The data are measurements of the time for acoustic waves to travel between points on the solar surface and surrounding annuli obtained from continuous observations at the South Pole in 1991 and from high-resolution observations from the Solar and Heliospheric Observatory (SOHO) in 1996. The travel time of the waves depends primarily on the sound speed perturbations and the velocity of flow along the ray paths. The effects of the sound speed perturbations and flows can be separated by measuring the travel time of waves propagating in opposite directions along the same ray paths. Magnetic fields result in anisotropy of the wave speed. A 3D inversion method based on Fermat's Principle and a regularized least-squares technique have been applied to infer the properties of convection in the quiet Sun and in active regions. Title: A subsurface flow of material from the Sun's equator to its poles Authors: Giles, P. M.; Duvall, T. L.; Scherrer, P. H.; Bogart, R. S. Bibcode: 1997Natur.390...52G Altcode: Gas on the Sun's surface has been observed to flow away from the equator towards both poles. If the same flow persists to great depths, it could play an important dynamical role in the eleven-year sunspot cycle, by carrying the magnetic remnants of the sunspots to high latitudes. An even deeper counterflow, which would be required to maintain mass balance, could explain why new sunspots form at lower latitudes as the cycle progresses. These deep flows would also redistribute angular momentum within the Sun, and therefore help to maintain the faster rotation of the equator relative to the poles. Here we report the detection, using helioseismic tomography, of the longitude-averaged subsurface flow in the outer 4% of the Sun. We find that the subsurface flow is approximately constant in this depth range, and that the speed is similar to that seen on the surface. This demonstrates that the surface flow penetrates deeply, so that it is likely to be an important factor in solar dynamics. Title: Sounding the Sun's Chromosphere Authors: Jefferies, S. M.; Osaki, Y.; Shibahashi, H.; Harvey, J. W.; D'Silva, S.; Duvall, T. L., Jr. Bibcode: 1997ApJ...485L..49J Altcode: Time-distance analysis of solar acoustic waves with frequencies above the nominal atmospheric acoustic cutoff frequency (~5.3 mHz) shows partial reflection of the waves at both the Sun's photosphere and a layer located higher in the atmosphere. This result supports recent reports of chromospheric modes. Title: Solar Meridional Circulation and Rotation Determined by Time-Distance Helioseismology using MDI Data From SOHO Authors: Giles, P. M.; Duvall, T. L., Jr.; Kosovichev, A. G.; Scherrer, P. H. Bibcode: 1997SPD....28.1002G Altcode: 1997BAAS...29..914G Using the technique of time-distance helioseismology, acoustic wave travel times can be measured between pairs of points on the solar surface. The travel time of the waves depends primarily on the wave group velocity and on the component of flow velocity which is parallel to the direction of wave propagation. By choosing pairs of points which share a common longitude, it is possible to use these waves to probe the meridional flow beneath the surface. Any flows present will cause a difference between the northward and southward travel times along the meridian. Varying the distance between points allows isolation of waves which propagate to different depths beneath the surface, and thus the flow velocity can be measured as a function of latitude and depth. Similarly, by choosing pairs of points which share a common latitude it is possible to measure the effects of solar rotation using an analogous procedure. This technique could provide high resolution in latitude and allows study of the northern and southern hemispheres independently. Using velocity images taken by the Michelson Doppler Imager during June 1996, we have detected meridional flows in the uppermost layers of the sun. Measurements of this flow velocity and of the rotation rate as functions of latitude and depth will be presented. This research is supported by NASA contract NAG5-3077 at Stanford University. Title: Progress in Local Helioseismology Authors: Duvall, T. L., Jr. Bibcode: 1997SPD....28.1001D Altcode: 1997BAAS...29..914D Much has been learned about the solar interior in the last two decades with the techniques of global helioseismology. By comparison, the efforts in local helioseismology, defined as the study of subsurface nonaxisymmetric phenomena, have been relatively modest. Part of the difficulty has been conceptual - it was not obvious how to attack the problem. The first attempts at local helioseismology were really scaled-down versions of the global techniques, in which mode measurements were made over a restricted area. These included the ring diagrams of Hill et al. and the Hankel function decompositions of Braun et al. Braun's methods have been extended to include other properties of standard scattering theory, including phase shift and S-matrix measurements. Both of these approaches are important components of our still rather limited arsenal of local helioseismic techniques. Recently more radical approaches to local helioseismology have been initiated, including the time-distance method of Duvall et al. and the knife-edge approach of Lindsey et al. In the time-distance method, the time for waves to travel along subsurface ray paths is determined from the temporal crosscorrelation of signals at two separated surface points. By measuring the times for many pairs of points from our dopplergrams covering the visible hemisphere, a tremendous quantity of information about the state of the solar interior is derived. The correct interpretation of all this information is considerably challenging. These methods show promise as a way of developing 3-d tomography of the solar interior. The current state of the different methods of local helioseismology will be reviewed. Title: Sensitivity of Time-Distance Helioseismology to Unwanted Instrumental and Solar Effects Authors: Harvey, J.; Jefferies, S.; Duvall, T. L., Jr. Bibcode: 1997SPD....28.1003H Altcode: 1997BAAS...29..914H Correlation of the wave motions at one point on the surface of the sun with other points at different times and distances is the basis of time-distance helioseismology. This technique is especially promising for studies of local sub-surface structure and dynamics. Like any observational technique, the results depend on proper correction of instrumental effects and limitations, and also on an allowance for other signals and effects arising from the object that interfere with the desired signal. The former effects arise mainly from temporal and spatial sampling restrictions, data processing methods, geometrical distortion, and signal-to-noise limitations. The solar effects include the solar background noise, and surface and atmospheric excitation, propagation, and damping characteristics that tend to mask the small signals that represent internal conditions. In this study, we concentrate on instrumental effects. Based on data obtained at the geographic South Pole for 18 days in 1994, we find that time-distance correlations are not systematically affected by most instrumental characteristics with one exception. This exception is the spatial frequency response of the instrument. There is a substantial difference between correlations computed with and without allowance for spatial frequency response. This factor should be carefully considered when doing time-distance helioseismology analyses. Title: Performance of the Michelson Doppler Imager Instrument on SOHO Authors: Scherrer, P.; Bogart, R.; Bush, R.; Duvall, T.; Hoeksema, J. T.; Kosovichev, A.; Schou, J.; Morrison, M.; Tarbell, T.; Title, A. Bibcode: 1997SPD....28.0207S Altcode: 1997BAAS...29..894S Launched on SOHO in December 1995, the MDI instrument took its 10 millionth filtergram in early April, 1997. The instrument and spacecraft have performed admirably since commissioning, providing over a year of virtually uninterrupted time series of velocity and intensity measurements at moderate resolution, a continuous 60-day time series of full disk 4" velocity and line depth maps, monthly 72+ hour time series in various observables, a host of daily 8-hour campaigns, and full-disk magnetograms every 96 minutes. Another uninterrupted 90-day interval of nearly full data recovery is scheduled to be completed in mid July. Various scientific results using MDI data are being presented at this meeting. About a dozen terabytes of data sets have been created and archived and normal pipeline processing is now completed soon after retrieving the data, typically less than a month after the observations are made. Most of the data products are generally available on the WWW, see http://soi.stanford.edu. Selected data are available in near real time. The SOI team welcomes collaborations. Routine and extraordinary calibrations along with analysis of scientific data sets allow us to make good estimates of the noise and understand many of the sources of systematic errors in the instrument. In almost every respect the instrument performs as well or better than expected before launch, the primary limitations being photon noise on the short term and fixed or slowly varying offsets on the long term. We have found that the Michelsons are somewhat more sensitive to operational temperature variations than was expected, adding some additional constraints on our observing sequences. Title: Restoration of Long-Exposure Full-Disk Solar Intensity Images Authors: Toner, C. G.; Jefferies, S. M.; Duvall, T. L. Bibcode: 1997ApJ...478..817T Altcode: We describe an algorithm for restoring full-disk solar intensity images blurred by a smooth, quasi-stationary point-spread function (PSF). For Earth-based observations, this type of data can be obtained by using exposure times that are much longer than the redistribution time of the atmosphere. Using simulated data for a wide range of observing conditions, we show that the algorithm restores data in such a way that the RMS difference between an original, unblurred image and the restored image is typically less than 1.0%. Thus, we substantially improve the photometric precision. The simulations also show that under ``reasonable'' seeing conditions (<~4"), exposure times of 5-10 s are adequate to produce smooth calibratable PSFs if the observing instrument uses a centroid-shifting tip/tilt wavefront correction. The algorithm determines the PSF for each observation directly from the recorded image and does not require separate measurements of point sources. Title: Structure and Rotation of the Solar Interior: Initial Results from the MDI Medium-L Program Authors: Kosovichev, A. G.; Schou, J.; Scherrer, P. H.; Bogart, R. S.; Bush, R. I.; Hoeksema, J. T.; Aloise, J.; Bacon, L.; Burnette, A.; de Forest, C.; Giles, P. M.; Leibrand, K.; Nigam, R.; Rubin, M.; Scott, K.; Williams, S. D.; Basu, Sarbani; Christensen-Dalsgaard, J.; Dappen, W.; Rhodes, E. J., Jr.; Duvall, T. L., Jr.; Howe, R.; Thompson, M. J.; Gough, D. O.; Sekii, T.; Toomre, J.; Tarbell, T. D.; Title, A. M.; Mathur, D.; Morrison, M.; Saba, J. L. R.; Wolfson, C. J.; Zayer, I.; Milford, P. N. Bibcode: 1997SoPh..170...43K Altcode: The medium-l program of the Michelson Doppler Imager instrument on board SOHO provides continuous observations of oscillation modes of angular degree, l, from 0 to ∽ 300. The data for the program are partly processed on board because only about 3% of MDI observations can be transmitted continuously to the ground. The on-board data processing, the main component of which is Gaussian-weighted binning, has been optimized to reduce the negative influence of spatial aliasing of the high-degree oscillation modes. The data processing is completed in a data analysis pipeline at the SOI Stanford Support Center to determine the mean multiplet frequencies and splitting coefficients. The initial results show that the noise in the medium-l oscillation power spectrum is substantially lower than in ground-based measurements. This enables us to detect lower amplitude modes and, thus, to extend the range of measured mode frequencies. This is important for inferring the Sun's internal structure and rotation. The MDI observations also reveal the asymmetry of oscillation spectral lines. The line asymmetries agree with the theory of mode excitation by acoustic sources localized in the upper convective boundary layer. The sound-speed profile inferred from the mean frequencies gives evidence for a sharp variation at the edge of the energy-generating core. The results also confirm the previous finding by the GONG (Gough et al., 1996) that, in a thin layer just beneath the convection zone, helium appears to be less abundant than predicted by theory. Inverting the multiplet frequency splittings from MDI, we detect significant rotational shear in this thin layer. This layer is likely to be the place where the solar dynamo operates. In order to understand how the Sun works, it is extremely important to observe the evolution of this transition layer throughout the 11-year activity cycle. Title: Internal structure and rotation of the Sun: First results from MDI data Authors: Kosovichev, A. G.; Schou, J.; Scherrer, P. H.; Bogart, R. S.; Bush, R. I.; Hoeksema, J. T.; Aloise, J.; Bacon, L.; Burnette, A.; De Forest, C.; Giles, P. M.; Leibrand, K.; Nigam, R.; Rubin, M.; Scott, K.; Williams, S. D.; Basu, Sarbani; Christensen-Dalsgaard, J.; Däppen, W.; Rhodes, E. J., Jr.; Duvall, T. L., Jr.; Howe, R.; Thompson, M. J.; Gough, D. O.; Sekii, T.; Toomre, J.; Tarbell, T. D.; Title, A. M.; Mathur, D.; Morrison, M.; Saba, J. L. R.; Wolfson, C. J.; Zayer, I.; Milford, P. N. Bibcode: 1997IAUS..181..203K Altcode: No abstract at ADS Title: Time-Distance Helioseismology with the MDI Instrument: Initial Results Authors: Duvall, T. L., Jr.; Kosovichev, A. G.; Scherrer, P. H.; Bogart, R. S.; Bush, R. I.; de Forest, C.; Hoeksema, J. T.; Schou, J.; Saba, J. L. R.; Tarbell, T. D.; Title, A. M.; Wolfson, C. J.; Milford, P. N. Bibcode: 1997SoPh..170...63D Altcode: In time-distance helioseismology, the travel time of acoustic waves is measured between various points on the solar surface. To some approximation, the waves can be considered to follow ray paths that depend only on a mean solar model, with the curvature of the ray paths being caused by the increasing sound speed with depth below the surface. The travel time is affected by various inhomogeneities along the ray path, including flows, temperature inhomogeneities, and magnetic fields. By measuring a large number of times between different locations and using an inversion method, it is possible to construct 3-dimensional maps of the subsurface inhomogeneities. The SOI/MDI experiment on SOHO has several unique capabilities for time-distance helioseismology. The great stability of the images observed without benefit of an intervening atmosphere is quite striking. It has made it possible for us to detect the travel time for separations of points as small as 2.4 Mm in the high-resolution mode of MDI (0.6 arc sec pixel-1). This has enabled the detection of the supergranulation flow. Coupled with the inversion technique, we can now study the 3-dimensional evolution of the flows near the solar surface. Title: Observation of solar convection with the MDI instrument on SOHO. Authors: Kosovichev, A. G.; Scherrer, P. H.; Duvall, T. L., Jr. Bibcode: 1996BAAS...28.1298K Altcode: No abstract at ADS Title: New Views of the Sun's Interior from the SOHO/MDI Space Experiment Authors: Scherrer, P. H.; Bogart, R. S.; Bush, R. I.; Hoeksema, J. T.; Kosovichev, A. G.; Nigam, R.; Schou, J.; Duvall, T. L., Jr. Bibcode: 1996AAS...189.1803S Altcode: 1996BAAS...28.1298S The strking stability of solar Dopplergrams measured by the Michelson Doppler Imager (MDI) instrument on the SOHO spacecraft, without an intervening atmosphere, substantially decreases the noise in the solar oscillations power spectrum compared with groundbased observations. This permits detection of lower amplitude oscillations, extending the range of measured normal mode frequencies. This is important for improving resolution and precision of helioseismic inferences about the Sun's internal structure and dynamics. The MDI observations also reveal the asymmetries of oscillation spectral lines that until now have been largely hidden in noise. The line asymmetries agree with a theory of excitation of solar oscillations by acoustic sources localized in the upper convective boundary layer. High-resolution MDI images make it possible to measure the travel time of acoustic waves propagating inside the Sun by comparing points on the surface as close as 2.4 Mm. This is sufficient to detect supergranulation flows beneath the surface. Coupled with tomographic inversion techniques, we can now study the 3-dimensional evolution of the flows near the photosphere. The sound-speed profile inferred from normal modes frequencies shows a sharp variation at the edge of the energy-generating core, something not accounted for by the standard evolution theory. The analysis also confirms recent GONG results suggesting that helium is less abundant than theory predicts in a thin layer just beneath the convection zone. Inversion of the multiplet frequency splittings shows significant rotational shear in this thin layer. This shear flow probably generates turbulence that mixes the plasma in the upper radiative zone. This layer is likely to be the place where the solar dynamo operates. Continuous observation of the evolution of this transition layer during the entire 11-year activity cycle will be extremely important for understanding the mechanisms of solar activity. Title: Helioseismic Tomography Authors: D'Silva, Sydney; Duvall, Thomas L., Jr.; Jefferies, Stuart M.; Harvey, John W. Bibcode: 1996ApJ...471.1030D Altcode: "Helioseismic tomography" is a method using observations to construct slices of the Sun's internal structure. It is based on a reduction of observations to time-distance surfaces and hypersurfaces. We present a procedure for measuring time-distance surfaces and hypersurfaces, and thereby a method of studying localized inhomogeneities in the interior of the Sun, such as abnormalities in the sound speed (e.g., a thermal shadow, Parker 1987a), or local subsurface flows, or magnetic fields. We also present a simulation of measuring time-distance surfaces and illustrate how to measure the size of an inhomogeneity, its location in depth, and the deviation of its sound speed compared to its local surroundings. Title: Dynamics of the Chromospheric Network: Mobility, Dispersal, and Diffusion Coefficients Authors: Schrijver, Carolus J.; Shine, Richard A.; Hagenaar, Hermance J.; Hurlburt, Neal E.; Title, Alan M.; Strous, Louis H.; Jefferies, Stuart M.; Jones, Andrew R.; Harvey, John W.; Duvall, Thomas L., Jr. Bibcode: 1996ApJ...468..921S Altcode: Understanding the physics behind the dispersal of photo spheric magnetic flux is crucial to studies of magnetoconvection, dynamos, and stellar atmospheric activity. The rate of flux dispersal is often quantified by a diffusion coefficient, D. Published values of D differ by more than a factor of 2, which is more than the uncertainties allow. We propose that the discrepancies between the published values for D are the result of a correlation between the mobility and flux content of concentrations of magnetic flux. This conclusion is based on measurements of displacement velocities of Ca II K mottles using an uninterrupted 2 day sequence of filtergrams obtained at the South Pole near cycle minimum. We transform the Ca II K intensity to an equivalent magnetic flux density through a power-law relationship defined by a comparison with a nearly simultaneously observed magnetogram. One result is that, wherever the network is clearly defined in the filtergrams, the displacement vectors of the mottles are preferentially aligned with the network, suggesting that network-aligned motions are more important to field dispersal than deformation of the network pattern by cell evolution. The rms value of the inferred velocities, R = <|v|2>½, decreases with increasing flux, Φ, contained in the mottles, from R ≍ 240 m s-1 down to 140 s-1. The value of R(Φ) appears to be independent of the flux surrounding the concentration, to the extreme that it does not matter whether the concentration is in a plage or in the network. The determination of a proper effective diffusion coefficient requires that the function R(Φ) be weighted by the number density n(Φ) of mottles that contain a total flux. We find that n(Φ) decreases exponentially with Φ and propose a model of continual random splitting and merging of concentrations of flux to explain this dependence. Traditional methods used to measure D tend to be biased toward the larger, more sluggish flux concentrations. Such methods neglect or underestimate the significant effects of the relatively large number of the more mobile, smaller concentrations. We argue that the effective diffusion coefficient for the dispersal of photo spheric magnetic flux is ∼600 km2 s-1. Title: Detection of Subsurface Supergranulation Structure and Flows from MDI High-Resolution Data using Time-Distance Techniques Authors: Duvall, T. L., Jr.; Kosovichev, A. G.; Scherrer, P. H.; Milford, P. N. Bibcode: 1996AAS...188.4908D Altcode: 1996BAAS...28Q.898D The supergranulation is seen at the surface of the sun in the doppler shift of spectrum lines as an apparent cellular convection pattern with a scale of about 4% of the solar radius. This scale is about 30 times larger than the granulation, seen in white light. Why these distinct scales would be present (and possibly a third intermediate scale mesogranulation) is somewhat of a mystery. Also unknown is the depth structure of the convection. We have used acoustic wave measurements from the MDI experiment on SOHO to address these questions. By crosscorrelating the signal at one location with that on annuli centered on the location, it is possible to measure times for waves to travel over known subsurface ray paths. With some variations on this theme, it is possible to measure horizontal and vertical flows and sound speed variations. Of course, the resulting measurements refer to quantities integrated along these ray paths. An inversion technique based on Fermat's principle has been developed and used to map the flow velocities and sound speed variations as a function of horizontal position and depth. The MDI experiment on SOHO makes doppler shift maps with 1Kx1K points in two choices of image scale, 2 and 0.6 arcsec/pixel. For the present study, we have used the higher resolution mode to observe 8.5 hours of doppler maps sampled once per minute. In order to average enough crosscorrelations to see time-distance effects, the resultant time-distance maps are reduced in resolution by a factor of 10 from the initial data. This still yields about 7 samples across a single supergranulation cell, or 49 over the area of a square cell. Our initial inversions based on the ray theory suggest that the supergranulation flow extends at least to 0.5% of the solar radius below the surface. This research is supported by the SOI-MDI NASA contract NAG5-3077 at Stanford University. Title: Preliminary Ring-Diagram Analysis of Doppler Velocity Fields Observed with MDI on SOHO Authors: Haber, D. A.; Bogart, R. S.; Sa, L. A. D.; Hill, F.; Toomre, J.; Duvall, T. L., Jr. Bibcode: 1996AAS...188.3710H Altcode: 1996BAAS...28Q.879H We analyze properties of high-degree acoustic wave fields over small patches of the sun using high-resolution Doppler velocity observations with the Michelson Doppler Imager (MDI) on the Solar Heliospheric Observatory (SOHO). By studying asymmetric frequency shifts in the acoustic waves that propagate in different horizontal directions, we can make inferences about the underlying large-scale flows which contribute to these shifts. We here analyze two different sets of data obtained from early observations with MDI. One is a continuous 80-hour sequence of full-disk Doppler images with a 60 s cadence and 4'' resolution, the other is an 8-hour sequence of high-resolution images that have 1.2'' resolution. Both sets have 1024 x 1024 pixels but the second set only covers about 36deg on the sun and is centered on the central meridian and somewhat above disk center. In both cases we remap a number of smaller areas of the data and compute three-dimensional Fourier transforms (two in space, one in time) over each patch. The resulting power diagrams have cross-sections in frequency that exhibit power distributed along rings. The detailed shapes and displacements of the rings depend upon the averaged velocities and their gradients, which can be estimated by theory. We measure the displacements of the rings using two different analysis techniques, thereby determining the frequency splittings which are then used in inversion procedures to deduce the underlying smoothed flow fields in each region. The results from the various patches provide preliminary estimates of the flow structures present in the upper convection zone. Title: The Solar Acoustic Spectrum and Eigenmode Parameters Authors: Hill, F.; Stark, P. B.; Stebbins, R. T.; Anderson, E. R.; Antia, H. M.; Brown, T. M.; Duvall, T. L., Jr.; Haber, D. A.; Harvey, J. W.; Hathaway, D. H.; Howe, R.; Hubbard, R. P.; Jones, H. P.; Kennedy, J. R.; Korzennik, S. G.; Kosovichev, A. G.; Leibacher, J. W.; Libbrecht, K. G.; Pintar, J. A.; Rhodes, E. J., Jr.; Schou, J.; Thompson, M. J.; Tomczyk, S.; Toner, C. G.; Toussaint, R.; Williams, W. E. Bibcode: 1996Sci...272.1292H Altcode: The Global Oscillation Network Group (GONG) project estimates the frequencies, amplitudes, and linewidths of more than 250,000 acoustic resonances of the sun from data sets lasting 36 days. The frequency resolution of a single data set is 0.321 microhertz. For frequencies averaged over the azimuthal order m, the median formal error is 0.044 microhertz, and the associated median fractional error is 1.6 x 10-5. For a 3-year data set, the fractional error is expected to be 3 x 10-6. The GONG m-averaged frequency measurements differ from other helioseismic data sets by 0.03 to 0.08 microhertz. The differences arise from a combination of systematic errors, random errors, and possible changes in solar structure. Title: Calibration and Performance of the Michelson Doppler Imager on SOHO. Authors: Zayer, I.; Morrison, M.; Tarbell, T. D.; Title, A.; Wolfson, C. J.; MDI Engineering Team; Bogart, R. S.; Bush, R. I.; Hoeksema, J. T.; Duvall, T.; Sa, L. A. D.; Scherrer, P. H.; Schou, J. Bibcode: 1996AAS...188.3712Z Altcode: 1996BAAS...28..879Z The Michelson Doppler Imager (MDI) instrument probes the interior of the Sun by measuring the photospheric manifestations of solar oscillations. MDI was launched in December, 1995, on the Solar and Heliospheric Observatory (SOHO) and has been successfully observing the Sun since then. The instrument images the Sun on a 1024 x 1024 pixel CCD camera through a series of increasingly narrow spectral filters. The final elements, a pair of tunable Michelson interferometers, enable MDI to record filtergrams with FWHM bandwidth of 94 m Angstroms with a resolution of 4 arcseconds over the whole disk. Images can also be collected in MDI's higher resolution (1.25 arcsecond) field centered about 160 arcseconds north of the equator. An extensive calibration program has verified the end-to-end performance of the instrument in flight. MDI is working very well; we present the most important calibration results and a sample of early science observations. The Image Stabilization System (ISS) maintains overall pointing to better than ca. 0.01 arcsec, while the ISS' diagnostic mode allows us to measure spectrally narrow pointing jitter down to less than 1 mili-arcsec. We have confirmed the linearity of each CCD pixel to lie within 0.5%\ (the FWHM of the distribution is 0.2% ), and have to date not detected any contamination on the detector, which is cooled to -72 C. The noise in a single Dopplergram is of the order of 20 m/s, and initial measurements of transverse velocities are reliable to 100 m/s. The sensitivity of magnetograms reach 5G in a 10 minute average (15G in a single magnetogram). MDI's primary observable, the p-modes from full-disk medium-l data, are of very high quality out to l=300 as seen in the initial l-nu diagram. The SOI-MDI program is supported by NASA contract NAG5-3077. Title: Diagnostics of Shallow Convective Structures by Time-Distance Helioseismology Authors: Kosovichev, A. G.; Duvall, T. L., Jr.; Scherrer, P. H. Bibcode: 1996AAS...188.3709K Altcode: 1996BAAS...28R.878K We present a new method of 3D helioseismic diagnostics to study subphotospheric flow and thermal and magnetic structure associated with turbulent convection. The main difference from the previous studies by Duvall et al. (1996, Nature, 379, 235) and by Kosovichev (1996, ApJL, 461, L55) is that the new method can be applied for measuring solar properties in the shallow layer just beneath the surface. The shallow layer of superadiabatic convection, which is only few thousand kilometers deep, is the region of the greatest uncertainty in our knowledge of the Sun's interior. Recent numerical simulations have demonstrated substantial deviations of the structure of this layer from the mixing-length theory commonly used in modeling stellar structure and evolution. The uncertainty in the physics of turbulent convection also affects helioseismic inferences about the deep interior. Our method of 3D diagnostics is based on measuring and inverting anomalies of the sound-wave travel time between two areas on the solar surface. Because of the stochastic nature of solar waves, these two areas must be sufficiently large to provide a good signal-to-noise ratio. In practice, the travel time can be measured from the cross-correlation function averaged over several thousand cross-correlations between individual points on the surface. Therefore, it is essential to have stable high-resolution series of Doppler images. Such data have been obtained from the Michelson Doppler Imager instrument on SOHO. In this paper, we present some details of the cross-correlation time-distance analysis, and the technique to invert the travel-time measurements using the optical ray approximation. The travel time of the waves depends primarily on the wave group velocity and on the velocity of flow along the ray paths. The effects of the wave speed structure and of flows are separated by measuring the travel time of waves propagating in opposite directions along the same ray paths. The effects of magnetic fields are measured through anisotropy of the wave speed. We discuss the limits for observing small-scale features beneath the surface. This research is supported by the SOI-MDI NASA contract NAG5-3077 at Stanford University. Title: Downflows under sunspots detected by helioseismic tomography Authors: Duvall, T. L.; D'Silva, S.; Jefferies, S. M.; Harvey, J. W.; Schou, J. Bibcode: 1996Natur.379..235D Altcode: SUNSPOTS are areas of cooler gas and stronger magnetic fields in the Sun's photosphere (its 'surface'), but just how they form and are maintained has long been a puzzle. It has been proposed1 that small vertical magnetic flux tubes, generated deep within the Sun, develop downflows around them when they emerge at the surface. The downflows bring together a large number of flux tubes in a cluster to form a sunspot, which behaves as a single flux bundle as long as the downflows bind the flux tubes together. Until now, however, it has not been possible to test this model with subsurface observations. Here we use the recently developed technique of travel-time helioseismology2 to detect the presence of strong downflows beneath both sunspots and the bright features known as plages. The flows have a velocity of ~2 kms-1, and they persist to a depth of about 2,000 km. The data suggest, however, that the vertical magnetic field can be a coherent flux bundle only to a depth of ~600 km; below this depth it is possible that the downflows hold together a loose collection of flux tubes to maintain the sunspots that we see. Title: Plane-Wave Analysis of Solar Acoustic-Gravity Waves: a (slightly) New Approach Authors: Bogart, R. S.; Sá, L. A. D.; Duvall, T. L.; Haber, D. A.; Toomre, J.; Hill, F. Bibcode: 1995ESASP.376b.147B Altcode: 1995soho....2..147B; 1995help.confP.147B Plane-wave decomposition of acoustic-gravity wave effects observed in the photosphere provides a computationally efficient technique for probing the structure of the upper convective zone and boundary, where the flat-Sun approximation is reasonably accurate. The authors describe the technique to be used for systematic plane-wave analysis of MDI data as part of the SOI data analysis pipeline, and the SOI analysis plan. The authors present estimates of sensitivity and discuss the effects of using different planar mappings. The technique is compared with previous approaches to the 3-dimensional plane-wave problem. Title: Measurement of High-Degree Solar Oscillation Frequencies Authors: Bachmann, K. T.; Duvall, T. L., Jr.; Harvey, J. W.; Hill, F. Bibcode: 1995ApJ...443..837B Altcode: We present m-averaged solar p- and f-mode oscillation frequencies over the frequency range nu greater than 1.8 and less than 5.0 mHz and the spherical harmonic degree range l greater than or equal to 100 and less than or equal to 1200 from full-disk, 1000 x 1024 pixel, Ca II intensity images collected 1993 June 22-25 with a temporal cadence of 60 s. We itemize the sources and magnitudes of statistical and systematic uncertainties and of small frequency corrections, and we show that our frequencies represent an improvement in accuracy and coverage over previous measurements. Our frequencies agree at the 2 micro Hz level with Mount Wilson frequencies determined for l less than or equal to 600 from full-disk images, and we find systematic offsets of 10-20 micro Hz with respect to frequencies measured from Big Bear and La Palma observations. We give evidence that these latter offsets are indicative of spatial scaling uncertainties associated with the analysis of partial-disk images. In comparison with theory, our p-mode frequencies agree within 10 micro Hz of frequencies predicted by the Los Alamos model but are as much as 100 micro Hz smaller than frequencies predicted by the Denmark and Yale models at degrees near 1000. We also find systematic differences between our n = 0 frequencies and the frequencies closely agreed upon by all three models. Title: Mapping Wave Speed in the Outer Convection Zone Authors: Duvall, T. L., Jr.; Jefferies, S. M.; Harvey, J. W. Bibcode: 1995SPD....26..105D Altcode: 1995BAAS...27..950D No abstract at ADS Title: Frequencies of High Degree Solar Oscillations Authors: Bachmann, K. T.; Duvall, T. L., Jr.; Harvey, J. W.; Hill, F. Bibcode: 1995ASPC...76..156B Altcode: 1995gong.conf..156B No abstract at ADS Title: Other Groundbased Observations Authors: Duvall, T. L., Jr. Bibcode: 1995ESASP.376a.107D Altcode: 1995heli.conf..107D; 1995soho....1..107D There are still roles to be played for modest ground-based observational programs in these days of big space experiments and ground networks (SOI/MDI, GOLF, VIRGO, GONG). SOI/MDI will have excellent capabilities to observe short wavelength oscillations. However, it may not be observing long enough to give information about the solar cycle, leaving an opening for programs making synoptic observations of high-degree modes. For similar reasons, long term observations of low-degree modes can also make important contributions. Another area that the big projects may miss is at high temporal frequencies. Another role that modest ground-based programs will play is as a testbed for new techniques and instrumentation. If twenty-four hour coverage is required, the South Pole is a good possibility for a testing site. Some of the programs have been run for many years and the systematic errors are well understood. It will be advantageous to compare the results of the new experiments with the older ones to look for systematic errors. Title: Time-Distance Helioseismology in the Vicinity of Sunspots Authors: D'Silva, Sydney; Duvall, Thomas L., Jr. Bibcode: 1995ApJ...438..454D Altcode: We use the ray description of acoustic-gravity modes to calculate time-distance diagrams for the quiet Sun and for regions in the vicinity of a sunspot with a monolithic flux-tube structure. Time-distance curves for the quiet Sun match the observations of Duvall et al. In the vicinity of a sunspot these quiet Sun curves split into a family of closely spaced curves. The structure of this bandlike feature is found to be sensitive to the sunspot model and can be a diagnostic of the subsurface geometry of the sunspot flux tube. Title: Time-Distance Helioseismology: an Update Authors: Duvall, T. L., Jr. Bibcode: 1995ASPC...76..465D Altcode: 1995gong.conf..465D No abstract at ADS Title: Measurements of High-Degree Solar Oscillation Parameters Authors: Bachmann, K. T.; Duvall, T. L., Jr.; Harvey, J. W.; Hill, F. Bibcode: 1994AAS...185.4405B Altcode: 1994BAAS...26R1377B We present results obtained from full-disk, 1000times 1024 pixel, Ca II intensity images of the Sun collected with the High-L Helioseismometer (HLH). Our measurement of p- and f-mode oscillation frequencies over the frequency range 1.8<=nu <=5.0 mHz and the spherical harmonic degree range 100<=l<=1200 from 22-25 June 1993 data represents an improvement over previous measurements. We are able to differentiate among the predictions of several solar models, thus constraining physical models of the solar convection zone. We also include recent splitting and frequency results from data collected during the entire month of June 1994. The purpose of the HLH research program is to measure high-degree solar oscillation parameters for the remainder of this decade in support of the Solar Oscillations Investigation - Michelson Doppler Imager collaboration, which is part of the Solar and Heliospheric Observatory, a joint ESA-NASA satellite mission. Title: Use of Acoustic Wave Travel-Time Measurements to Probe the Near-Surface Layers of the Sun Authors: Jefferies, S. M.; Osaki, Y.; Shibahashi, H.; Duvall, T. L., Jr.; Harvey, J. W.; Pomerantz, M. A. Bibcode: 1994ApJ...434..795J Altcode: The variation of solar p-mode travel times with cyclic frequency nu is shown to provide information on both the radial variation of the acoustic potential and the depth of the effective source of the oscillations. Observed travel-time data for waves with frequency lower than the acoustic cutoff frequency for the solar atmosphere (approximately equals 5.5 mHz) are inverted to yield the local acoustic cutoff frequency nuc as a function of depth in the outer convection zone and lower atmosphere of the Sun. The data for waves with nu greater than 5.5 mHz are used to show that the source of the p-mode oscillations lies approximately 100 km beneath the base of the photosphere. This depth is deeper than that determined using a standard mixing-length calculation. Title: Limits on Coronal Reflection Using High-Frequency Solar Oscillations Authors: Kumar, P.; Fardal, M. A.; Jefferies, S. M.; Duvall, T. L., Jr.; Harvey, J. W.; Pomerantz, M. A. Bibcode: 1994ApJ...422L..29K Altcode: Acoustic waves in the Sun with frequencies above about 5.3 mHz can propagate in the chromosphere. We examine imaged solar intensity data for evidence of reflection of these waves in the upper chromosphere, where the temperature increases by a large factor over a short distance. Our method is to compare the observed and theoretically derived frequency spacings between peaks in the power spectrum. We find that our theoretical frequencies provide the best fit to the data when the reflection in the upper atmosphere is eliminated. In particular, the model of Kumar (1993b), which includes the source depth, and radiative damping, in the calculation of power spectra but ignores chromospheric reflection, gives peak frequencies that are in good agreement with the observations. For acoustic waves of frequency greater than 6 mHz we put an upper limit to the reflectivity of chromosphere and corona, using our method, of about 10%. At a given spherical harmonic degree, the frequency spacing between peaks in the data generally decreases with increasing frequency, because the lower turning point of the waves is moving inward. However, between 5 and 5.5 mHz the frequency spacing increases slightly. This feature is probably associated with the acoustic cutoff frequency in the solar atmosphere, i.e., it indicates a transition from trapped waves to propagating waves. We are able to reproduce the observed behavior by a crude modeling of the solar atmosphere. Further study of these peaks should provide an independent way of exploring the mean structure of the solar atmosphere, particularly around the temperature minimum region. Title: Internal Rotation of the Sun Authors: Duvall, T. L., Jr.; Dziembowski, W.; Goode, P. R.; Gough, D. O.; Harvey, J. W.; Leibacher, J. W. Bibcode: 1994snft.book..414D Altcode: No abstract at ADS Title: Time-Distance Helioseismology Authors: Duvall, T. L., Jr.; Jefferies, S. M.; Harvey, J. W.; Pomerantz, M. A. Bibcode: 1993BAAS...25.1220D Altcode: No abstract at ADS Title: Asymmetries of Solar Oscillation Line Profiles Authors: Duvall, T. L., Jr.; Jefferies, S. M.; Harvey, J. W.; Osaki, Y.; Pomerantz, M. A. Bibcode: 1993ApJ...410..829D Altcode: Asymmetries of the power spectral line profiles of solar global p-modes are detected in full-disk intensity observations of the Ca II K Fraunhofer line. The asymmetry is a strong function of temporal frequency being strongest at the lowest frequencies observed and vanishing near the peak of the power distribution. The variation with spherical harmonic degree is small. The asymmetry is interpreted in terms of a model in which the solar oscillation cavity is compared to a Fabry-Perot interferometer with the source slightly outside the cavity. A phase difference between an outward direct wave and a corresponding inward wave that passes through the cavity gives rise to the asymmetry. The asymmetry is different in velocity and intensity observations. Neglecting the asymmetry when modeling the power spectrum can lead to systematic errors in the measurement of mode frequencies of as much as 10 exp -4 of the mode frequency. The present observations and interpretation locate the source of the oscillations to be approximately 60 km beneath the photosphere, the shallowest position suggested to date. Title: Time-distance helioseismology Authors: Duvall, T. L., Jr.; Jefferies, S. M.; Harvey, J. W.; Pomerantz, M. A. Bibcode: 1993Natur.362..430D Altcode: THE application of seismology to the study of the solar interior1, 2(helioseismology) has advanced almost solely by the prediction and measurement of the Sun's frequencies of free oscillation, or normal modes. Direct measurement of the travel times and distances of individual acoustic waves-the predominant approach in terrestrial seismology3-would appear to be more difficult in view of the number and stochastic nature of solar seismic sources. Here, however, we show that it is possible to extract time-distance information from temporal cross-correlations of the intensity fluctuations on the solar surface. This approach opens the way for seismic studies of local solar phenomena, such as subsurface in homogeneities near sunspots, and should help to refine global models of the internal velocity stratification in the Sun. Title: The P-Mode Scattering Properties of a Sunspot Authors: Braun, D. C.; Labonte, B. J.; Duvall, T. L., Jr.; Jefferies, S. M.; Harvey, M. A.; Pomerantz, J. W. Bibcode: 1993ASPC...42...77B Altcode: 1993gong.conf...77B No abstract at ADS Title: Chromospheric Oscillations and the Background Spectrum Authors: Harvey, J. W.; Duvall, T. L., Jr.; Jefferies, S. M.; Pomerantz, M. A. Bibcode: 1993ASPC...42..111H Altcode: 1993gong.conf..111H No abstract at ADS Title: Artificial Data for Testing Helioseismology Algorithms Authors: Bogart, R. S.; Hill, F.; Toussaint, R.; Hathaway, D. H.; Duvall, T. L., Jr. Bibcode: 1993ASPC...42..429B Altcode: 1993gong.conf..429B No abstract at ADS Title: The NASA/NSO Spectromagnetograph Authors: Jones, Harrison P.; Duvall, Thomas L., Jr.; Harvey, John W.; Mahaffey, Charles T.; Schwitters, Jan D.; Simmons, Jorge E. Bibcode: 1992SoPh..139..211J Altcode: The NASA/NSO Spectromagnetograph is a new focal plane instrument for the National Solar Observatory/Kitt Peak Vacuum Telescope which features real-time digital analysis of long-slit spectra formed on a two-dimensional CCD detector. The instrument is placed at an exit port of a Littrow spectrograph and uses an existing modulator of circular polarization. The new instrument replaces the 512-channel Diode Array Magnetograph first used in 1973. Commercial video processing boards are used to digitize the spectral images at video rates and to separate, accumulate, and buffer the spectra in the two polarization states. An attached processor removes fixed-pattern bias and gain from the spectra in cadence with spatial scanning of the image across the entrance slit. The data control computer performs position and width analysis of the line profiles as they are acquired and records line-of-sight magnetic field, Doppler shift, and other computed parameters. The observer controls the instrument through windowed processes on a data control console using a keyboard and mouse. Early observations made with the spectromagnetograph are presented and plans for future development are discussed. Title: Scattering of p-Modes by a Sunspot Authors: Braun, D. C.; Duvall, T. L., Jr.; Labonte, B. J.; Jefferies, S. M.; Harvey, J. W.; Pomerantz, M. A. Bibcode: 1992ApJ...391L.113B Altcode: The acoustic scattering properties of a large sunspot are determined from a Fourier-Hankel decomposition of p-mode amplitudes as measured from a 68-hr subset of a larger set of observations made at the South Pole in 1988. It is shown that significant improvement in the measurement of p-mode scattering amplitudes results from the increased temporal frequency resolution provided by these data. Scattering phase shifts are unambiguously determined for the first time, and the dependence of the p-mode phase shift and absorption with wavenumber and frequency is presented. Title: Scattering of p-Modes by a Sunspot Authors: Braun, D. C.; Duvall, T. L., Jr.; Labonte, B. J.; Jefferies, S. M.; Harvey, J. W.; Pomerantz, M. A. Bibcode: 1992AAS...180.0604B Altcode: 1992BAAS...24..737B For the first time the scattering phase shifts of solar p-modes from a sunspot have been unambiguously determined. This is made possible by the recent availability of long duration, high duty cycle observations. The results presented here are determined from a Fourier-Hankel decomposition of p-mode amplitudes as measured from a 68 hr subset of a larger set of observations made at the South Pole in 1988. In addition to the detection of the phase shifts, the quality of the data allows the dependence of the p-mode scattering and absorption with azimuthal order, spatial wavenumber and temporal frequency to be independently determined. Thus, unlike previous observations, our measurements of absorption and phase shifts do not represent averages over a range of p-modes. With this information we have for the first time a complete description of the acoustic scattering amplitudes from a large sunspot. Interpretation of these observations requires a suitable theory of the interaction of p-modes and sunspots. However, with the complete scattering amplitudes now available one may apply inverse scattering algorithms, based on a few simplifying assumptions, to deduce a 3-dimensional map of the scattering strength of the active region. This offers the hope that general information about subsurface morphology of active regions might be gained even without a detailed understanding of the physical scattering processes involved. DCB is supported by Air Force URI grant AFOSR-90-0116. The South Pole program is supported in part by National Science Foundation grants DPP87-15791 and 89-17626, and by the Solar Physics Branch of the Space Physics Division of NASA. Title: Helioseismology from South Pole: Surprises from Near the Solar Surface Authors: Jefferies, S. M.; Pomerantz, M. A.; Duvall, T. L.; Harvey, J. W. Bibcode: 1992AnJUS..27..322J Altcode: No abstract at ADS Title: Characteristics of Intermediate-Degree Solar p-Mode Line Widths Authors: Jefferies, S. M.; Duvall, T. L., Jr.; Harvey, J. W.; Osaki, Y.; Pomerantz, M. A. Bibcode: 1991ApJ...377..330J Altcode: Measurements of the p-mode linewidths over the frequency range v = 1.87-4.93 mHz and degree range 1 = 1-150 are presented. The linewidth is observed to vary with mode frequency and degree. The variation with frequency is consistent with the observations of Libbrecht although the measurements are systematically narrower. The frequency variation has been explained in terms of radiative and convective damping of the modes. The observed variation with degree resolves previous contradictory results and is shown to exceed the 1/S variation that is expected in theoretical grounds. Here S is the travel time of a mode from its lower turning point in the solar interior, to its reflection at the solar surface. The deviations from a 1/S variation suggest that there are two possible damping mechanisms, in addition to radiative and convective damping, that affect the modes. Title: Measurements of High-Frequency Solar Oscillation Modes Authors: Duvall, T. L., Jr.; Harvey, J. W.; Jefferies, S. M.; Pomerantz, M. A. Bibcode: 1991ApJ...373..308D Altcode: The spatial-temporal spectrum of solar oscillations exhibits modelike structure at frequencies above the nominal photospheric acoustic cutoff of about 5.3 mHz. The linewidth and frequency of these features are measured as functions of degree from high-quality spectra obtained from observations made at the geographic South Pole. From 5.3 to 6.5 mHz the linewidths are relatively constant with a value of about 70 microHz, approximately one-half the frequency difference between modes of the same degree but successive values of radial order number. This width is larger than can be accounted for by simple considerations of the leakage of trapped acoustic waves. The frequencies of the high-frequency modes adhere to a simple dispersion law if one uses a substantially larger effective phase shift that applies at lower frequencies. The frequency variation of this phase shift changes markedly above the acoustic cutoff frequency. Title: Power Spectra of Solar Convection Authors: Chou, D. -Y.; Labonte, B. J.; Braun, D. C.; Duvall, T. L., Jr. Bibcode: 1991ApJ...372..314C Altcode: The properties of convective motions on the sun are studied using Kitt Peak Doppler images and power spectra of convection. The power peaks at a scale of about 29,000 km and drops off smoothly with wavenumber. There is no evidence of apparent energy excess at the scale of the mesogranulation proposed by other authors. The vertical and horizontal power for each wavenumber are obtained and used to calculate the vertical and horizontal velocities of the supergranulation. The amplitude of vertical and horizontal velocities of the supergranulation are 0.034 (+ or - 0.002) km/s and 0.38 (+ or - 0.01) km/s, respectively. The corresponding rms values are 0.024 (+ or - 0.002) km/s and 0.27 (+ or - 0.01) km/s. Title: A Simple Method for Correcting Spatially Resolved Solar Intensity Oscillation Observations for Variations in Scattered Light Authors: Jefferies, S. M.; Duvall, T. L., Jr. Bibcode: 1991SoPh..132..215J Altcode: A measurement of the intensity distribution in an image of the solar disk will be corrupted by a spatial redistribution of the light that is caused by the Earth's atmosphere and the observing instrument. If the precise form of the spatial point spread function is known and can be modeled, then the observed image can be corrected for its effects. However, accurate modeling of the spatial point spread function, which can be considered as composed of a `blurring' component and a `scattering' component (Zwaan, 1965), is difficult and the correction for its effects is computationally expensive. Title: The Solar Equatorial Internal Rotation Rate Estimated from Combined South Pole and NSO/Sac Peak Helioseismic Data Sets Authors: Hill, F.; Jefferies, S. M.; Pomerantz, M. A.; Duvall, T. L., Jr.; Harvey, J. W. Bibcode: 1991BAAS...23.1050H Altcode: No abstract at ADS Title: Wavelength Variation of p-Mode Intensity Fluctuations Authors: Ronan, R. S.; Harvey, J. W.; Duvall, T. L., Jr. Bibcode: 1991ApJ...369..549R Altcode: The oscillatory signal in the solar p-mode band has been measured as a function of optical wavelength using a grating spectrometer and Fourier transform spectrometer. The relative intensity fluctuations are found to increase with height in the solar photosphere, while the absolute level of intensity fluctuations in the p-mode band is reduced by about 50 percent in the cores and wings of Ca II H and K, H-delta, and H-gamma compared to the neighboring spectral regions. Thus, these spectral regions of diminished absolute p-mode signal could be exploited as signal references by spectrophotometers while attempting to observe nonradial p-mode oscillations in stars from the ground. High spectral and temporal resolution observations of several unblended lines in the red portion of the visible spectrum show an asymmetry in the relative and absolute p-mode intensity oscillations across the line profiles. The peak in intensity oscillations lies in the blue wing of the lines. Title: Characteristics of Intermediate-Degree Solar p-mode Line Widths Authors: Duvall, T. L., Jr.; Jefferies, S. M.; Harvey, J. W.; Osaki, Y.; Pomerantz, M. A. Bibcode: 1991BAAS...23.1032D Altcode: No abstract at ADS Title: Helioseismology from South Pole: 1990 High Resolution Campaign Authors: Jefferies, S. M.; Pomerantz, M. A.; Duvall, T. L.; Harvey, J. W. Bibcode: 1991AnJUS..26..285J Altcode: No abstract at ADS Title: Modeling of Solar Oscillation Power Spectra Authors: Anderson, Edwin R.; Duvall, Thomas L., Jr.; Jefferies, Stuart M. Bibcode: 1990ApJ...364..699A Altcode: To produce accurate estimates of the line-profile parameters of a model used to represent the spectral features in a solar oscillation power spectrum, it is necessary to (1) select the appropriate probability density function when deriving the maximum-likelihood function to be employed for the parameter estimation and (2) allow for the redistribution of spectral power caused by gaps in the data string. This paper describes a maximum-likelihood method for estimating the model parameters (based on the observed power spectrum statistics) that accounts for redistribution of spectral power caused by gaps in the data string, by convolving the model with the power spectrum of the observed window function. The accuracy and reliability of the method were tested using both artificial and authentic solar oscillation power spectrum data. A comparison of this method with various least-squares techniques is also presented. Title: p-mode absorption in the giant active region of 10 March, 1989 Authors: Braun, D. C.; Duvall, T. L., Jr. Bibcode: 1990SoPh..129...83B Altcode: A time series of velocity oscillations is observed in the vicinity of NOAA region 5395 with the Kitt Peak vacuum telescope for 6.8 hours on 1989 March 10 as part of a program to study the interaction of solar p-mode oscillations with solar active regions. The data is transformed in a cylindrical coordinate system centered on the visible sunspot, then Hankel- and Fourier-transformed to produce the power spectra of in- and outgoing acoustic waves. It is observed that a maximum of nearly 70% of the power of incident high-degree modes is absorbed by this unusually large sunspot group. The absorptive properties of this active region are compared with those of more typical regions studied previously. Title: The Spatial Distribution of p-Mode Absorption in Active Regions Authors: Braun, D. C.; Labonte, B. J.; Duvall, T. L., Jr. Bibcode: 1990ApJ...354..372B Altcode: The interaction of solar p-mode waves and active regions has been the subject of recent observational and theoretical investigations. Observations show that up to one-half of the power of incident high-degree acoustic may be absorbed in and around sunspots. In this paper the horizontal spatial distribution of high-degree p-mode absorption in solar active regions is explored. An appropriate Fourier-Hankel transform can be used to detect the mean absorption of waves passing through any given point on the solar surface. By repeating the analysis at multiple positions a map of the absorption can be constructed. A technique for optimal computation of absorption maps is developed and applied to observations of several active regions and an area of quiet sun near disk center. By comparing the distribution of p-mode absorption with magnetograms and line-wing intensity images, it is directly observed that the absorption is not limited to the location of the visible sunspots but is also associated with magnetic fields in the surrounding plage. It is estimated that the absorption efficiency scales roughly with the magnetic flux density, although the absorption appears to saturate inside the strongest fields. Title: First Magnetograms from the NASA/NSO Spectromagnetograph Authors: Jones, H. P.; Duvall, T. L., Jr.; Mahaffey, C. T. Bibcode: 1990BAAS...22..809J Altcode: No abstract at ADS Title: Observed Characteristics of High-Frequency Solar Global Oscillations Authors: Harvey, J. W.; Duvall, T. L., Jr.; Jefferies, S. M.; Pomerantz, M. A. Bibcode: 1990BAAS...22..896H Altcode: No abstract at ADS Title: A Review of Observational Helioseismology Authors: Duvall, Thomas L., Jr. Bibcode: 1990ASSL..159..253D Altcode: 1990IAUCo.121..253D; 1990insu.conf..253D No abstract at ADS Title: What are the Observed High-Frequency Solar Acoustic Modes? Authors: Kumar, P.; Duvall, T. L., Jr.; Harvey, J. W.; Jefferies, S. M.; Pomerantz, M. A.; Thompson, M. J. Bibcode: 1990LNP...367...87K Altcode: 1990psss.conf...87K Jefferies et al. (1988) observe discrete peaks up to 7mHz in the power spectra of their intermediate degree solar intensity oscillation data obtained at South Pole. This is perhaps surprising since waves with frequency greater than the acoustic cut-off frequency at the temperature minimum ( 5.5mHz), unlike their lower frequency counterparts, are not trapped in the solar interior. We propose that the observed peaks are associated with what are principally progressive waves emanating from a broad-band acoustic source. The geometrical effect of projecting observations of these progressive waves onto spherical harmonics then gives rise to peaks in the power spectra. The frequencies and amplitudes of the peaks will depend on the spatial characteristics of the source. Partial reflections in the solar atmosphere modify the power spectra, but in this picture they are not the primary reason for the appearance of the peaks. We estimate the frequency and power which would be expected from this model and compare it with the observations. We argue that these high frequency mock-modes are not overstable, and that they are excited by acoustic emission from turbulent convection. Title: Observations of p-Mode Absorption in Active Regions Authors: Braun, D. C.; Duvall, T. L., Jr.; Jefferies, S. M. Bibcode: 1990LNP...367..181B Altcode: 1990psss.conf..181B We present here a summary of results on the interaction of p-modes with solar active regions based on observations made at the Kitt Peak Solar Vacuum Telescope and the geographic South Pole. A travelling wave decomposition of p-modes is performed in a cylindrical coordinate system centered on the active regions. Significant absorption of p-mode wave power is observed to occur in all of the regions and is a function of horizontal wavenumber () - increasing linearly with k up to some maximum value and remaining constant for higher wavenumbers. The maximum fractional absorption of incident power is about 0.2 for small pores and 0.4 for typical isolated sunspots (radius = 15 Mm). A maximum of 70% absorption is seen in the large sunspot group of March 1989 (radius = 60 Mm). No convincing variation of the absorption with temporal frequency (i.e. radial order) is seen, although not entirely ruled out considering the relative errors involved with the power measurements. No significant difference in the amount of p-mode absorption is detected between equal 3-hour time intervals before and after a class X4 flare in the March 1989 region. No excess of outgoing waves following the time of the flare is detected. These observations do not support the suggestion that large flares may excite observable acoustic waves in the photosphere. Title: Helioseismology from South Pole: Solar Cycle Connection Authors: Jefferies, S. M.; Pomerantz, M. A.; Duvall, T. L.; Harvey, J. W. Bibcode: 1990AnJUS..25..271J Altcode: No abstract at ADS Title: Helioseismology from the South Pole: Results from the 1987 Campaign Authors: Jefferies, S. M.; Duvall, T. L., Jr.; Harvey, J. W.; Pomerantz, M. A. Bibcode: 1990LNP...367..135J Altcode: 1990psss.conf..135J This paper presents some results on the frequencies and line widths of features in solar p-mode spectra obtained from 460 hours of observations made at South Pole in 1987. To investigate the possibility of temporal variations in these quantities, a comparison is made with measurements obtained from data taken in 1981. The differences between the frequencies measured from the 1981 and 1987 data sets appear to be independent of both frequency (2.4 v 4.8 mHz) and degree (3 98). The mean difference (v 1981 - v 1987) averaged. over v and is found to be 224 ± 19 nl1z. The line width measurements display the same variation with v as that previously reported (Libbrecht 1988a), an increase with ℓ (Duvall et ad. 1988) and with solar activity. Measurement of the rotational splittings of sectoral modes (m = ±ℓ) in the range (3 ≤ ℓ ≤ 15), shows no indication of a dependence on the depth of the lower turning points of these modes. Title: Digitizing Video Spectra at the NASA/NSO Spectromagnetograph Authors: Jones, H. P.; Brodzik, D.; Duvall, T. L., Jr. Bibcode: 1989BAAS...21..852J Altcode: No abstract at ADS Title: Rotational Splitting of the Low-Degree Solar P-Modes Authors: Jefferies, S. M.; Duvall, T. L., Jr.; Harvey, J. W.; Pomerantz, M. A. Bibcode: 1989BAAS...21..831J Altcode: No abstract at ADS Title: A Test of a Modified Algorithm for Computing Spherical Harmonic Coefficients Using an FFT Authors: Elowitz, Mark; Hill, Frank; Duvall, Thomas L., Jr. Bibcode: 1989JCoPh..80..506E Altcode: No abstract at ADS Title: Helioseismology from the South Pole: 1987 Results and 1988 Campaign Authors: Jefferies, S. M.; Pomerantz, M. A.; Duvall, T. L.; Harvey, J. W. Bibcode: 1989AnJUS..24..244J Altcode: No abstract at ADS Title: Helioseismology from the South Pole: comparison of 1987 and 1981 results. Authors: Jefferies, S. M.; Pomerantz, M. A.; Duvall, T. L., Jr.; Harvey, J. W.; Jaksha, D. B. Bibcode: 1988ESASP.286..279J Altcode: 1988ssls.rept..279J Full disk images with ≡10 arc sec pixels and filtered to a ≡7 Å passband centered on the Ca II K line were obtained from the geographic South Pole in 1981 and 1987. In 1981, about 50 hours of essentially uninterrupted data were obtained. In 1987, three such runs were obtained over a period of 325 hours for a duty cycle of about 47%. The 1987 observations are characterized by a much lower level of solar activity than 1981, a much improved CCD camera, considerably better image stability and a varying amount of instrumental scatter. The 1987 data have a substantially better signal-to-noise ratio than the 1981 data so that oscillations with degrees from 0 to 150 and frequencies from 2 to 7 mHz are well observed. The observations were reduced to spectra in l, m and ν. This paper presents a comparison of p-mode frequencies measured in 1981 and 1987 and coefficients of Legendre polynomial expansions of frequency shifts caused by solar rotation. The authors also study the time behavior of systematic frequency shifts which depend upon m but which do not arise from rotation. Title: The GONG instrument. Authors: Harvey, J.; Abdel-Gawad, K.; Ball, W.; Boxum, B.; Bull, F.; Cole, J.; Cole, L.; Colley, S.; Dowdney, K.; Drake, R.; Dunn, R.; Duvall, T.; Farris, D.; Green, A.; Hartlmeier, R.; Harvey, J.; Hubbard, R.; Jackson, P.; Kucera, D.; Miller, C.; Miller, D.; Petri, A.; Poczulp, G.; Schwitters, J.; Simmons, J.; Smartt, R.; Streander, G.; Vaughn, F.; Wiborg, P.; GONG Instrument Development Team Bibcode: 1988ESASP.286..203H Altcode: 1988ssls.rept..203H An instrument is being developed to provide high-quality Doppler oscillation measurements for the Global Oscillation Network Group (GONG) project. This instrument uses the Fourier tachometer principle of sweeping a squared-cosine transmission function across a limited region of the solar spectrum centered on the Ni I line at 676.8 nm. Doppler shift is detected as a phase shift between the modulated solar signal and a simultaneously modulated signal from a stabilized laser. The solar signal is measured with an array of about 250 by 250 pixels covering the full disk. Design goals include a short-term noise level at a single pixel of about 10 m/s per integration interval of 60 s and a long-term stability of better than one m/s. A "breadboard" model has been in operation since early 1988 and a prototype is under construction. Most of the observations as of August 1988 have been of the Doppler shift of the entire sun imaged onto a single detector. These observations easily show the global p-mode oscillations with good short-term signal-to-noise ratio but have revealed a number of interesting problems. Imaging measurements have started with promising results. Title: The Absorption of High-Degree p-Mode Oscillations in and around Sunspots Authors: Braun, D. C.; Duvall, T. L., Jr.; Labonte, B. J. Bibcode: 1988ApJ...335.1015B Altcode: The direct interaction of p-modes and sunspots is investigated in four observed active regions using a partial wave analysis in a cylindrical coordinate system centered on the spots. Up to half the power of incident p-modes is absorbed by the sunspots. By measuring the magnitude of absorption as a function of horizontal wavenumber and azimuthal order the authors have determined that the absorption is not only from the umbrae of the spots but is also significant within the penumbrae, and in some cases it appears to be associated with the presence of extended magnetic fields surrounding the sunspots. The amount of p-mode energy removed by magnetic fields is estimated for each of the regions observed. Title: Tomography of Solar Active Regions Authors: Braun, D. C.; Labonte, B. J.; Duvall, T. L., Jr. Bibcode: 1988BAAS...20..701B Altcode: No abstract at ADS Title: Frequencies of Solar p-Mode Oscillations Authors: Duvall, T. L., Jr.; Harvey, J. W.; Libbrecht, K. G.; Popp, B. D.; Pomerantz, M. A. Bibcode: 1988ApJ...324.1158D Altcode: Acoustic oscillations of the Sun were observed by measuring Doppler shifts at Big Bear Solar Observatory in 1985 and by measuring intensity fluctuations at the geographic South Pole in 1981. These data are reduced to spectra in frequency and spherical harmonic degree, l, by averaging over azimuthal order after removing frequency shifts caused by rotation. Distinct spectral features are identified and fitted with models to produce estimates of multiplet frequencies and errors. The authors present a table of measured frequencies for 4 ≤ l ≤ 99, with measurement uncertainties of the order of one part in 104. Tables of published frequency measurements for l ≤ 5 are also included. Title: Intermediate Degree Solar Oscillations Authors: Duvall, T. L., Jr.; Harvey, J. W.; Pomerantz, M. A. Bibcode: 1988IAUS..123...37D Altcode: Spectra of solar intensity oscillations in the degree range l = 20 - 98 were obtained with a 92% duty cycle over a 50 hour period from the geographic South Pole. After correction for solar rotation, the spectra have been averaged over azimuthal order m and fit with Lorentzian functions to provide values of background noise, amplitude, frequency and line width for 636 oscillation modes. Title: Helioseismology from the South Pole: 1987 Campaign Authors: Jefferies, S. M.; Pomerantz, M. A.; Duvall, T. L.; Harvey, J. W.; Jaksha, D. Bibcode: 1988AnJUS..23..191J Altcode: No abstract at ADS Title: Acoustic Absorption by Sunspots Authors: Braun, D. C.; Duvall, T. L., Jr.; Labonte, B. J. Bibcode: 1987ApJ...319L..27B Altcode: The paper presents the initial results of a series of observations designed to probe the nature of sunspots by detecting their influence on high-degree p-mode oscillations in the surrounding photosphere. The analysis decomposes the observed oscillations into radially propagating waves described by Hankel functions in a cylindrical coordinate system centered on the sunspot. From measurements of the differences in power between waves traveling outward and inward, it is demonstrated that sunspots appear to absorb as much as 50 percent of the incoming acoustic waves. It is found that for all three sunspots observed, the amount of absorption increases linearly with horizontal wavenumber. The effect is present in p-mode oscillations with wavelengths both significantly larger and smaller than the diameter of the sunspot umbrae. Actual absorption of acoustic energy of the magnitude observed may produce measurable decreases in the power and lifetimes of high-degree p-mode oscillations during periods of high solar activity. Title: The Solar P-Mode Signal as a Function of Optical Wavelength; 3800 through 4400 A. and 6500 through 6900 A. Authors: Ronan, R. S.; Harvey, J. W.; Duvall, T. L.; Noyes, R. W. Bibcode: 1987BAAS...19..936R Altcode: No abstract at ADS Title: The Absorption of 5-Minute Oscillations by Sunspots Authors: Braun, D. C.; Labonte, B. J.; Duvall, T. L., Jr. Bibcode: 1987BAAS...19..936B Altcode: No abstract at ADS Title: Sunspots as Sinks of P-Mode Wave Energy Authors: Duvall, T. L., Jr.; Harvey, J. W.; Braun, D. C.; Labonte, B. J.; Pomerantz, M. A. Bibcode: 1987BAAS...19R.934D Altcode: No abstract at ADS Title: Latitude and Depth Variation of Solar Rotation Authors: Duvall, T. L., Jr.; Harvey, J. W.; Pomerantz, M. A. Bibcode: 1987ASSL..137...19D Altcode: 1987isav.symp...19D Spectra of solar intensity oscillations in the degree range l = 20 - 98 were obtained with a 92% duty cycle over a 50 hour period from the geographic south pole. The spectra have been analyzed for frequency shifts caused by solar internal rotation as functions of latitude and depth. Some of the intermediate steps in producing the results are illustrated. Title: Helioseismology Results from South Pole Observations Authors: Harvey, J. W.; Duvall, T. L., Jr.; Pomerantz, M. A. Bibcode: 1986BAAS...18R1011H Altcode: No abstract at ADS Title: Latitude and depth variation of solar rotation Authors: Duvall, T. L., Jr.; Harvey, J. W.; Pomerantz, M. A. Bibcode: 1986Natur.321..500D Altcode: Measurements of the frequencies of various modes of trapped acoustic waves provide information about rotation and structure within the Sun. Previous work dealt with observations of wave modes confined near the solar equator, which provided some information about the depth variation of rotation without resolving a possible latitude variation1,2. Recent work extended measurements to modes covering various latitude ranges from which the variation with latitude of solar rotation can be studied3-5. Since these measurements were restricted to modes with spherical harmonic degrees less than 50, they provide averages of rotation over great depth ranges that do not resolve the convective envelope. We now present new results for degrees up to 98 which allow the convective envelope to be isolated. For degrees between 20 and 98 we find no evidence that internal rotation differs significantly with depth or latitude from the rotation of surface magnetic field patterns. Modes covering a wide latitude range have systematically lower frequencies than those confined near the equator, indicating the existence of a structural asymmetry within the Sun. Title: Solar Doppler shifts: sources of continuous spectra. Authors: Duvall, T. L., Jr.; Harvey, J. W. Bibcode: 1986ASIC..169..105D Altcode: 1986ssds.proc..105D Oscillation observations can be used to study non-oscillatory solar phenomena that exhibit Doppler shifts. The authors discuss several effects of these phenomena and their associated temporal and spatial power spectra: (1) They limit the signal-to-noise ratio and sometimes detectability of oscillation modes. (2) There is the potential for better understanding and/or detection of solar phenomena: surface rotation, supergranulation, granulation, active regions, giant cells, and mesogranulation. (3) Large-scale convection may spatially modulate oscillation modes, leading to a continuous background spectrum. (4) In regions of the spectrum where the resolution to separate modes is lacked, it is possible to determine upper limits for the integrated effects of modes. Title: Rotation of the Solar Interior Authors: Duvall, T. L.; Harvey, J. W.; Pomerantz, M. A. Bibcode: 1986AnJUS..21..280D Altcode: No abstract at ADS Title: Speed of sound in the solar interior Authors: Christensen-Dalsgaard, J.; Duvall, T. L., Jr.; Gough, D. O.; Harvey, J. W.; Rhodes, E. J., Jr. Bibcode: 1985Natur.315..378C Altcode: Frequencies of solar 5-min oscillations can be used to determine directly the sound speed of the solar interior. The determination described here does not depend on a solar model, but relies only on a simple asymptotic description of the oscillations in terms of trapped acoustic waves. Title: Amplitude Ratio of Solar p-Mode Intensity and Doppler Oscillations Authors: Duvall, T. L., Jr.; Harvey, J. W.; Pomerantz, M. A. Bibcode: 1985BAAS...17..643D Altcode: No abstract at ADS Title: Advances in Solar Seismology at the South Pole Authors: Pomerantz, M. A.; Fossat, E.; Gelly, B.; Grec, C.; Harvey, J. W.; Duvall, T. L. Bibcode: 1985AnJUS..20..221P Altcode: No abstract at ADS Title: Solar Oscillations Authors: Duvall, T. L. Bibcode: 1985IAUTA..19...97D Altcode: 1985IAUT...19...97D No abstract at ADS Title: Observations of intermediate-degree solar oscillations. Authors: Harvey, J. W.; Duvall, T. L., Jr. Bibcode: 1984sses.nasa..165H Altcode: 1984sss..conf..165H A progress report on observations of intermediate degree oscillations is presented. The authors list frequencies of zonal p-mode oscillations with amplitudes in excess of ≡2 cm s-1. These frequencies show systematic disagreement with recent theoretical calculations. The frequencies are compared with asymptotic formula estimates. Small scatter is obtained for low degree modes but large scatter at large degree. A first look at sectoral harmonic observations shows that magnetic active regions provide a major signal at low frequencies. Title: Internal rotation of the Sun Authors: Duvall, T. L., Jr.; Dziembowski, W. A.; Goode, P. R.; Gough, D. O.; Harvey, J. W.; Leibacher, J. W. Bibcode: 1984Natur.310...22D Altcode: The frequency difference between prograde and retrograde sectoral solar oscillations is analysed to determine the rotation rate of the solar interior, assuming no latitudinal dependence. Much of the solar interior rotates slightly less rapidly than the surface, while the innermost part apparently rotates more rapidly. The resulting solar gravitational quadrupole moment is J2 = (1.7+/-0.4) × 10-7 and provides a negligible contribution to current planetary tests of Einstein's theory of general relativity. Title: Rotational frequency splitting of solar oscillations Authors: Duvall, T. L., Jr.; Harvey, J. W. Bibcode: 1984Natur.310...19D Altcode: Prograde and retrograde sectoral oscillations of the Sun have been observed so as to determine frequency differences produced by rotation. Oscillations in the frequency range 2.1 - 3.7 mHz and with spherical harmonic degrees from 1 to 100 have been identified. Average frequency shifts due to rotation in a sidereal reference frame are found to range from a high of ≡660 nHz at degree 1 to a low of ≡423 nHz at degree 6, rising to ≡471 nHz at degree 100. These results indicate that most of the Sun's volume rotates at a rate close to that of the surface, but also that the energy-generating core may rotate more rapidly than the surface. Title: Rotational Frequency Splitting of Solar Oscillations Authors: Harvey, J. W.; Duvall, T. L., Jr. Bibcode: 1984BAAS...16Q.451H Altcode: No abstract at ADS Title: Frequencies of Solar P-Modes Oscillations Authors: Harvey, J. W.; Duvall, T. L., Jr. Bibcode: 1984LIACo..25..209H Altcode: 1984trss.conf..209H; 1984tpss.conf..209H No abstract at ADS Title: Solar oscillations with 13-day period Authors: Duvall, T. L., Jr.; Jones, H. P.; Harvey, J. W. Bibcode: 1983Natur.304..517D Altcode: Reference is made to the solar observations made by Claverie et al. (1982) over a three-month period in the summer of 1981 which show oscillatory velocity with a period of 13.1 days and amplitude of 6.6 m/s. These investigators reject the possibility that they see the Doppler shift from a radial oscillation, because the amplitude is implausibly large. They also do not believe that their signal was induced by solar magnetic fields, since typical mean solar fields are too small. Photo-electric drift-scan measurements of the solar diameter and full-disk magnetograms taken at Kitt Peak National Observatory are examined here for evidence of variations corresponding to the velocity oscillations of the 13.1-day period. An upper limit on radius variations is reported which is a factor of six below the amplitude needed to explain the velocity observations as a radial oscillation. Attention is also given to the possible role of the rotation of large-scale surface magnetic features. Title: Observations of solar oscillations of low and intermediate degree Authors: Duvall, T. L., Jr.; Harvey, J. W. Bibcode: 1983Natur.302...24D Altcode: Measurements are presented of solar velocity oscillations with spherical harmonic degree 1-139 and angular order ~0. With an amplitude sensitivity of ~2 cm s -1, trapped acoustic wave modes of radial orders 2-26 are observed at frequencies between 1.7 and 5.5 mHz. The radial order identifications of low-degree modes previously inferred from theory are confirmed. Only marginal evidence of long-period, gravity-mode oscillations is found Title: Intermediate Degree Solar Oscillations Authors: Harvey, J.; Duvall, T. Bibcode: 1983BAAS...15..705H Altcode: No abstract at ADS Title: Recent Observations of High-Degree Solar P-Mode Oscillations at the Kitt-Peak National Observatory Authors: Rhodes, Edward J.; Harvey, John W.; Duvall, Thomas L. Bibcode: 1983SoPh...82..111R Altcode: 1983IAUCo..66..111R A brief summary is given of a program which is currently being carried out with the McMath telescope of the Kitt Peak National Observatory in order to study high-degree (l ≳ 150) solar p-mode oscillations. This program uses a 244 × 248 pixel CID camera and the main spectrograph of the McMath telescope to obtain velocity-time maps of the oscillations which can be converted into two-dimensional (kh - ω) power spectra of the oscillations. Several different regions of the solar spectrum have been used in order to study the oscillations at different elevations in the solar atmosphere. The program concentrates on eastward- and westward-propagating sectoral harmonic waves so that measurements can be made of the absolute rotational velocities of the solar photospheric and shallow sub-photospheric layers. Some preliminary results from this program are now available. First, we have been unable to confirm the existence of a radial gradient in the equatorial rotational velocity as was previously suggested. Second, we have indeed been able to confirm the presence of p-mode waves in the solar chromosphere as was first suggested by Rhodes et al. (1977). Third, we have been able to demonstrate differences in photospheric and chromospheric power spectra. Title: Observations of Solar p-Mode Oscillations of Intermediate Degree Authors: Duvall, T. L. Bibcode: 1983EOSTr..64..304D Altcode: No abstract at ADS Title: A dispersion law for solar oscillations Authors: Duvall, T. L., Jr. Bibcode: 1982Natur.300..242D Altcode: The pressure or p-modes are acoustic vibrations trapped in a resonant cavity below the solar surface. The waves are most easily observed as primarily vertical velocities at the solar surface by Doppler shift techniques. The vertical velocities vary harmonically in time and in space across the solar surface. These harmonic variations lead to a natural description of the oscillation modes in terms of their positions in a diagram of temporal frequency (ω) versus horizontal spatial frequency (kh). Observationally, the positions in the kh-ω diagram are determined from a two-dimensional power spectrum of a series of Doppler shifts of a solar spectrum line measured equidistant in space and time. An example of the observed positions is shown in Fig. 1 (see ref. 1). Theoretically, the mode positions are determined by an analysis of the solar cavity, including an uncertain model of the solar interior. Most theoretical work has consisted of numerical solutions of the equations of motion leading to predicted positions in the kh-ω diagram (see, for instance, ref. 2). This approach has been reasonably successful, yielding inferences about the depth of the convection zone3. Here we compare a simple model of the oscillations discussed by Leibacher and Stein4 with observations. Title: The Equatorial Photospheric Rotation Rate Authors: Duvall, T. L., Jr. Bibcode: 1982SoPh...76..137D Altcode: The equatorial photospheric rotation rate has been observed on 14 days in 1978-1980. The resulting rotation rate, ω = 14.14±0.04°/day, is 2% slower than the rate as observed for long-lived sunspots. Title: Large Scale Motions and the Structure of the Sun Authors: Pomerantz, M. A.; Harvey, J. W.; Duvall, T. L. Bibcode: 1982AnJUS..17..232P Altcode: No abstract at ADS Title: Solar radius measurements Authors: Duvall, T. L., Jr.; Jones, H. P. Bibcode: 1981NASCP2191..129D Altcode: 1981vsc..conf..129D Preliminary results of measurements made during 1979-1980 are discussed. Variability in the radius measurements of 0.4 pi is found, of unknown origin. Title: Solar Radius Measurements Authors: Duvall, T.; Jones, H. Bibcode: 1981siwn.conf..366D Altcode: No abstract at ADS Title: The Five-Minute Oscillations: What's Left to BE done Authors: Rhodes, E.; Ulrich, R.; Harvey, J.; Duvall, T. Bibcode: 1981siwn.conf...37R Altcode: No abstract at ADS Title: Measurement of the Equatorial Solar Rotation Rate Authors: Duvall, Thomas L., Jr. Bibcode: 1980BAAS...12..896D Altcode: No abstract at ADS Title: Support of SMM with the Kitt Peak Vacuum Telescope; A Survey of Active Regions Authors: Jones, H. P.; Duvall, T. L., Jr.; Recely, F. J. Bibcode: 1980BAAS...12..906J Altcode: No abstract at ADS Title: Comment on `average photospheric poloidal and toroidal magnetic field components near solar minimum' by Duvall et al. Authors: Foukal, P.; Duvall, T. L., Jr. Bibcode: 1980SoPh...67....9F Altcode: 1980STIA...8047634F We discuss the dynamical interpretation of evidence for an azimuthal tilt of the global magnetic field from the radial direction at the photosphere. We point out that the Reynolds stresses of supergranular convective motions might produce the required small tilt of intense flux tubes, without implying an unacceptably large momentum flux across the photospheric surface into the solar wind. Our calculations lead us to conclude that there is little reason, at present, to infer (Duvall et al., 1979) a separate low intensity constituent of the global magnetic field, from the observational evidence for an azimuthal tilt. More precise measurements of the vertical component of supergranular motions would be useful in determining the actual torque exerted by the Reynolds stresses on the magnetic field. Title: The Equatorial Rotation Rate of the Supergranulation Cells Authors: Duvall, T. L., Jr. Bibcode: 1980SoPh...66..213D Altcode: The equatorial rotation rate of the supergranulation cells has been observed to be 14.72±0.07°/day. Velocity patterns observed at different times are cross-correlated to derive the rotation rate. The observed rate is 3% faster than recent observations of the surface rotation rate by Doppler shifts. The difference between the cell rate and surface rate is consistent with a model of the supergranular convection in which angular momentum per unit mass is conserved in the radial flow (Foukal, 1977). Title: A New Method for Measureing the Solar Radius Authors: Duvall, T. L., Jr.; Jones, H. P. Bibcode: 1980BAAS...12..474D Altcode: No abstract at ADS Title: The five-minute oscillations: What's left to be done Authors: Rhodes, E. J., Jr.; Ulrich, R. K.; Harvey, J. W.; Duvall, T. L., Jr. Bibcode: 1980STIN...8115929R Altcode: Current observational methods for studying these oscillations at large horizontal wavenumbers are discussed in detail and several two dimensional power spectra obtained with a CID camera on the main spectrograph of the McMath telescope at Kitt Peak National Observatory are described. The best-resolved observations of the p-mode obtained at chromospheric elevations are also presented. Recent progress in studies of the p-modes at low wavenumbers with full-disk velocity detection schemes is summarized. These full-disk observations of radial and low-degree non-radial modes were shown to place severe constraints on the theoretical calculation of solar interior structure. Progress in making fully-consistent solar models which fit both the high- and low-wave number observations is described. Finally, the observational and theoretical improvements that are necessary for further progress in solar seismology are summarized. Title: Observations with high temporal resolution of the solar Ca/+/ K line Authors: Duvall, T.; Livingston, W.; Mahaffey, C. Bibcode: 1980LNP...125..237D Altcode: 1980nnsp.work..237D High time resolution photometric scans of chromospheric Ca(+) K are examined for evidence of propagating waves. The scans refer to a quiet area (1 x 7 arcsec) near disk center. Diagnostics include line profile movies, time sequence spectrograms and power spectra. Both upward and downward (reflected) disturbances having lifetimes of approximately 1-2 min are seen. Title: Large-scale solar velocity fields. Authors: Duvall, T. L., Jr. Bibcode: 1979SoPh...63....3D Altcode: Daily observations of Doppler line shifts made with very low spatial resolution (3') with the Stanford magnetograph have been used to study the equatorial rotation rate, limb effect on the disk, and the mean meridonial circulation. The equatorial rotation rate was found to be approximately constant over the interval May 1976-January 1977 and to have the value 2.82 μrad s−1 (1.96 km s−1). This average compares favorably with the results of Howard (1977) of 2.83 μrad s−1 for the same time period. The RMS deviation of the daily measurements about the mean value was 1% of the rate (20 m s−1), much smaller than the fluctuations reported by Howard and Harvey (1970) of several per cent. These 1% fluctuations are uncorrelated from day-to-day and may be due to instrumental problems. The limb effect on the disk was studied in equatorial scans (after suppressing solar rotation). A redshift at the center of the disk relative to a position 0.60R⊙ from the center of 30 m s−1 was found for the line Fe I λ5250 Å. Central meridian scans were used (after correcting for the limb effect defined in the equatorial scans) to search for the component of mean meridonial circulation symmetric across the equator. A signal is found consistent with a polewards flow of 20 m s−1 approximately constant over the latitude range 10-50°. Models of the solar differential rotation driven by an axisymmetric meridonial circulation and an anisotropic eddy viscosity (Kippenhahn, 1963; Cocke, 1967; Köhler, 1970) predict an equatorwards flow at the surface. However, giant cell convection models (Gilman, 1972, 1976, 1977) predict a mean polewards flow (at the surface). The poleward-directed meridonial flow is created as a by-product of the giant cell convection and tends to limit the differential rotation. The observation of a poleward-directed meridonial circulation lends strong support to the giant cell models over the anisotropic eddy viscosity models. Title: Average photospheric poloidal and toroidal magnetic field components near solar minimum. Authors: Duvall, T. L., Jr.; Scherrer, P. H.; Svalgaard, L.; Wilcox, J. M. Bibcode: 1979SoPh...61..233D Altcode: Average (over longitude and time) photospheric magnetic field components are derived from 3' Stanford magnetograms made near the solar minimum of cycle 21. The average magnetograph signal is found to behave as the projection of a vector for measurements made across the disk. The poloidal field exhibits the familiar dipolar structure near the poles, with a measured signal in the line Fe I λ 5250 Å of ≈ 1 G. At low latitudes the poloidal field has the polarity of the poles, but is of reduced magnitude (≈ 0.1 G). A net photospheric toroidal field with a broad latitudinal extent is found. The polarity of the toroidal field is opposite in the nothern and southern hemispheres and has the same sense as subsurface flux tubes giving rise to active regions of solar cycle 21. Title: Solar rotation, 1966 1978 Authors: Livingston, W.; Duvall, T. L., Jr. Bibcode: 1979SoPh...61..219L Altcode: Photospheric and chromospheric spectroscopic Doppler rotation rates for the full solar disk are analyzed for the period July, 1966 to July, 1978. An approximately linear secular increase of the equatorial rate of 3.7% for these 12 years is found (in confirmation of Howard, 1976). The high latitude rates above 65 ° appear to vary with a peak-to-peak amplitude of 8%, or more, phased to the sunspot cycle such that the most rapid rotation occurs at, or following, solar maximum. The chromosphere, as indicated by Hα, has continued to rotate on the average 3% faster than the photosphere agreeing with past observations. Sources of error are discussed and evaluated. Title: The strength of the Sun's polar fields. Authors: Svalgaard, L.; Duvall, T. L., Jr.; Scherrer, P. H. Bibcode: 1978SoPh...58..225S Altcode: The magnetic field strength within the polar caps of the Sun is an important parameter for both the solar activity cycle and for our understanding of the interplanetary magnetic field. Measurements of the line-of-sight component of the magnetic field generally yield 0.1 to 0.2 mT near times of sunspot minimum. In this paper we report measurements of the polar fields made at the Stanford Solar Observatory using the Fe I line λ 525.02 nm. We find that the average flux density poleward of 55° latitude is about 0.6 mT peaking to more than 1 mT at the pole and decreasing to 0.2 mT at the polar cap boundary. The total open flux through either polar cap thus becomes about 3 × 1014 Wb. We also show that observed magnetic field strengths vary as the line-of-sight component of nearly radial fields. Title: On the supposed anticorrelation of solar polar and equatorial rotation rates. Authors: Duvall, T. L., Jr.; Svalgaard, L. Bibcode: 1978SoPh...56..463D Altcode: Howard and Harvey (1970) analyzed Mt. Wilson Doppler shifts to obtain a daily measure of the Sun's differential rotation. The data were fitted to give an angular velocity of the form ω = a + b sin2B + c sin4B (B = heliographic latitude). Changes in a, b, c were found to be correlated (Howard and Harvey, 1970). Yoshimura (1972) used the anticorrelation of the b and c parameters to infer the existence of large-scale convection. Wolff (1975) used the b-c anticorrelation and a weak correlation between a and b to infer that variations of the Sun's polar and equatorial rotation rates are anticorrelated. In this paper, the anticorrelation of b and c is shown to be due to numerical coupling. Title: A Study of Large-Scale Solar Magnetic and Velocity Fields. Authors: Duvall, T. L., Jr. Bibcode: 1978PhDT.........4D Altcode: Full-disk maps of the solar magnetic field were made daily over a one year period (weather permitting) and used to derive the average poloidal and toroidal components of the magnetospheric field near solar minimum as a function of latitude. The toroidal field was then used to derive the electric current crossing the photosphere. Line shift measurements were obtained and used to study the solar rotation, the limb effect on the disk, and the possibility of a mean meridional circulation in the photosphere. The equatorial rotation rate is found to have a value of 2.81 micron rad/sec. An analysis of the equatorial velocity residuals shows that the line profile is redshifted at the center of the disk relative to a position 0.6 solar radii from the center by 30 m/s. By comparing the equatorial velocity residuals with meridional scans a signal is found consistent with a meridional flow towards the poles of 20 m/s between latitudes 10 - 50 deg. Title: A study of large-scale solar magnetic and velocity fields Authors: Duvall, Thomas Lee, Jr. Bibcode: 1978PhDT.......149D Altcode: No abstract at ADS Title: The strength of the sun's polar fields Authors: Svalgaard, L.; Duvall, T. L., Jr.; Scherrer, P. H. Bibcode: 1978STIN...7829029S Altcode: Observations at Stanford Solar Observatory of solar magnetic fields in the FeI line lambda 525.02 nm shows that a radial magnetic field measured at a point where the radius makes an angle rho with the line of sight is observed to be decreased by a factor cos rho. For field elements of 150 mT, magnetograph saturation causes the measured field to be too low by a factor of 1.8. The average field magnitude poleward 55 deg latitude is measured to be near 100 micron T. Variation of the apparent field over a 3 arc min aperture grazing the limb at central meridian amounts to a factor of two over the year; the field being strongest when the pole is tipped the most (7 1/4 deg) towards the observer. Combination of all the above results leads to the following picture of the magnetic field within the polar caps. The field is nearly radial, varying as Bp cos to the 8th power theta where the field strength Bp at the pole (theta = 0 deg) is 1.15 mT, and falling off to below 0.2 mT at the polar cap boundary (theta = 35 deg). Within coronal holes outside of the polar cap the magnetic field strength at sunspot minimum is rather small (0.15 mT). Title: A study of large-scale solar magnetic and velocity fields Authors: Duvall, T. L., Jr. Bibcode: 1977STIN...7829027D Altcode: Magnetograms are used to derive the average poloidal and toroidal components of the photospheric magnetic field near solar minimum as a function of the poloidal field near the poles is found to have the dipolar structure characteristic of solar minimum with a positive field in the north and negative field in the south, both with a measured strength of approximately 1 gauss. At lower latitudes the poloidal field has the same polarity as the pole in that hemisphere but the field strength has the reduced magnitude of approximately 0.1 gauss. A toroidal field of broad latitude extent is found to have opposite sense in the two solar hemispheres and a magnitude of approximately 0.1 gauss. The sense of the toroidal field is the same as that of the assumed subsurface toroidal field giving rise to active regions solar cycle 21. The toroidal field is used to derive the electric current crossing the photosphere in conjunction with the magnetogram observations, an accurate determination of the relative wavelength shift of the spectral line is made for each position on the solar disk. The solar rotation, the limb effect on the disk and the possibility of a mean meridional circulation in the photosphere are studied. Title: Comparison of Hα synoptic charts with the large-scale solar magnetic field as observed at Stanford Authors: Duvall, T. L., Jr.; Wilcox, J. M.; Svalgaard, L.; Scherrer, P. H.; McIntosh, P. S. Bibcode: 1977SoPh...55...63D Altcode: Two methods of observing the neutral line of the large-scale photospheric magnetic field are compared: (1) neutral line positions inferred from Hα photographs (McIntosh, 1972a, 1975; McIntosh and Nolte, 1975) and (2) observations of the photospheric magnetic field made with low spatial resolution (3') and high sensitivity using the Stanford magnetograph. The comparison is found to be very favorable. Title: The mean magnetic field of the Sun: observations at Stanford. Authors: Scherrer, P. H.; Wilcox, J. M.; Svalgaard, L.; Duvall, T. L., Jr.; Dittmer, P. H.; Gustafson, E. K. Bibcode: 1977SoPh...54..353S Altcode: A solar telescope has been built at Stanford University to study the organization and evolution of large-scale solar magnetic fields and velocities. The observations are made using a Babcock-type magnetograph which is connected to a 22.9 m vertical Littrow spectrograph. Sun-as-a-star integrated light measurements of the mean solar magnetic field have been made daily since May 1975. The typical mean field magnitude has been about 0.15 G with typical measurement error less than 0.05 G. The mean field polarity pattern is essentially identical to the interplanetary magnetic field sector structure (see near the Earth with a 4 day lag). The differences in the observed structures can be understood in terms of a `warped current sheet' model. Title: Comparison of H alpha synoptic charts with the large-scale solar magnetic field as observed at Stanford Authors: Duvall, T. L., Jr.; Wilcox, J. M.; Svalgaard, L.; Scherrer, P. H.; McIntosh, P. S. Bibcode: 1977STIN...7729049D Altcode: Two methods of observing the neutral line of the large-scale photospheric magnetic field are compared: (1) neutral line positions inferred from H alpha photographs and (2) observations of the photospheric magnetic field made with low spatial resolution (3 arc min.) and high sensitivity using the Stanford magnetograph. The comparison is found to be very favorable. Title: On the supposed anticorrelation of solar polar and equatorial rotation rates Authors: Duvall, T. L., Jr.; Svalgaard, L. Bibcode: 1977STIN...7822997D Altcode: This report discusses a result that was thought to be caused by correlated variations of the sun's rotation at different latitudes is known to be the result of crosstalk between two of the parameters used to fit the differential rotation. Title: The mean magnetic field of the sun: Observations at Stanford Authors: Scherrer, P. H.; Wilcox, J. M.; Svalgaard, L.; Duvall, T. L., Jr.; Dittmer, P. H.; Gustafson, E. K. Bibcode: 1977STIN...7726055S Altcode: A solar telescope was built at Stanford University to study the organization and evolution of large-scale solar magnetic fields and velocities. The observations are made using a Babcock-type magnetograph which is connected to a 22.9 m vertical Littrow spectrograph. Sun-as-a-star integrated light measurements of the mean solar magnetic field were made daily since May 1975. The typical mean field magnitude is about 0.15 gauss with typical measurement error less than 0.05 gauss. The mean field polarity pattern is essentially identical to the interplanetary magnetic field sector structure (seen near the earth with a 4 day lag). The differences in the observed structures can be understood in terms of a warped current sheet model. Title: A Study of Large-Scale Solar Velocity Fields. Authors: Duvall, T. L., Jr. Bibcode: 1977BAAS....9Q.336D Altcode: No abstract at ADS Title: Stanford Solar Observatory - The First Year Authors: Scherrer, P. H.; Duvall, T. L., Jr.; Dittmer, P. H.; Gustafson, E. K.; Wilcox, J. M. Bibcode: 1976BAAS....8Q.370S Altcode: No abstract at ADS Title: Measurements of Large-Scale Solar Velocity Fields Authors: Dittmer, P. H.; Scherrer, P. H.; Wilcox, J. M.; Duvall, T. L., Jr.; Gustafson, E. K. Bibcode: 1976BAAS....8..311D Altcode: No abstract at ADS Title: Measurements of the Photospheric Magnetic Field with 3' Resolution Authors: Duvall, T. L., Jr.; Scherrer, P. H.; Wilcox, J. M.; Dittmer, P. H.; Gustafson, E. K. Bibcode: 1976BAAS....8..344D Altcode: No abstract at ADS Title: Relation of the Observed Far Ultraviolet Solar Irradiance to the Solar Magnetic Sector Structure Authors: Heath, D. F.; Wilcox, J. M.; Svalgaard, L.; Duvall, T. L. Bibcode: 1975SoPh...45...79H Altcode: Comparison of the observed solar far ultraviolet irradiance and the observed solar sector structure during 1969 through 1972 shows a tendency for EUV maxima to be located near sector boundaries. Title: Relation of the observed far ultraviolet solar irradiance to the solar magnetic sector structure Authors: Heath, D. F.; Wilcox, J. M.; Svalgaard, L.; Duvall, T. L. Bibcode: 1975STIN...7624118H Altcode: Comparison of the observed solar far ultraviolet irradiance and the observed solar sector structure during 1969 through 1972 shows a tendency for EUV maxima to be located near sector boundaries. Title: The Stanford Solar Observatory Authors: Scherrer, P. H.; Wilcox, J. M.; Svalgaard, L.; Dittmer, P. H.; Duvall, T. L. Bibcode: 1975BAAS....7..350S Altcode: No abstract at ADS Title: Solar cycle variation of large-scale coronal structures Authors: Antonucci, E.; Duvall, T. L. Bibcode: 1974SoPh...38..439A Altcode: A green line intensity variation is associated with the interplanetary and photospheric magnetic sector structure. This effect depends on the solar cycle and occurs with the same amplitude in the latitude range 60° N-60° S. Extended longitudinal coronal structures are suggested, which indicate the existence of closed magnetic field lines over the neutral line, separating adjacent regions of opposite polarities on the photospheric surface. Title: A Model Combining the Polar and the Sector Structured Solar Magnetic Fields Authors: Svalgaard, Leif; Wilcox, John M.; Duvall, Thomas L. Bibcode: 1974SoPh...37..157S Altcode: A phenomenological model of the interplay between the polar magnetic fields of the Sun and the solar sector structure is discussed. Current sheets separate regions of opposite polarity and mark the sector boundaries in the corona. The sheets are visible as helmet streamers. The solar sector boundary is tilted with respect to central meridian, and boundaries with opposite polarity change are oppositely tilted. The tilt of a given type of boundary [(+, −) or (−, +)] changes systematically during the sunspot cycle as the polarity of the polar fields reverses. Similar reversals of the position of the streamers at the limbs takes place.