duvall

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Author name code: duvall
ADS astronomy entries on 2022-09-14
author:"Duvall, Thomas L." OR author:"Duvall, Tom"

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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.
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
&lt; 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.

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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.
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.

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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.
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 <A
href="https://www.aanda.org/10.1051/0004-6361/202039201/olm">https://www.aanda.org</A>

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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
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.

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Title: Time-distance helioseismology of solar Rossby waves
Authors: Liang, Zhi-Chao; Gizon, Laurent; Birch, Aaron C.; Duvall,
Thomas L.
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 <A
href="https://www.aanda.org/10.1051/0004-6361/201834849/olm">https://www.aanda.org</A>

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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.
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 &amp; 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.

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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.
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.

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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.
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.

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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.
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 <A
href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/619/A99">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/619/A99</A>

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Title: Revisiting helioseismic constraints on subsurface convection
Authors: Birch, Aaron; Duvall, Tom; Gizon, Laurent; Hanasoge, Shravan;
Hindman, Bradley; Nagashima, Kaori; Sreenivasan, Katepalli
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.

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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.
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).

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Title: Probing sunspots with two-skip time-distance helioseismology
Authors: Duvall, Thomas L., Jr.; Cally, Paul S.; Przybylski, Damien;
Nagashima, Kaori; Gizon, Laurent
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.

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Title: Helioseismic Constraints on the Subsurface Flows of the
Averaged Supergranule
Authors: Braun, Douglas C.; Duvall, Thomas L., Jr.; Felipe, Tobias;
DeGrave, Kyle
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.

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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.
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.

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Title: Observing and modeling the poloidal and toroidal fields of
the solar dynamo
Authors: Cameron, R. H.; Duvall, T. L.; Schüssler, M.; Schunker, H.
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.

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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.
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.

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Title: Observing and modelling the poloidal and toroidal magnetic
fields of the global dynamo
Authors: Cameron, Robert; Duvall, Thomas; Schüssler, Manfred;
Schunker, Hannah
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.

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Title: Stray Light Correction of HMI Data
Authors: Norton, Aimee Ann; Duvall, Thomas; Schou, Jesper; Cheung,
Mark; Scherrer, Philip H.
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.

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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
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
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: HMI Data Corrected for Stray Light Now Available
Authors: Norton, A. A.; Duvall, T. L.; Schou, J.; Cheung, M. C. M.;
Scherrer, P. H.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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
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.
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.
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.
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.
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.
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.
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 $\ell&lt;60$, 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 $\ell&lt;60$, 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.
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.
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.
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.
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.
2012ASPC..455..337L Altcode:
The Helioseismic and Magnetic Imager (HMI; Scherrer &amp; 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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.

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Title: Local Helioseismology of Magnetoconvective Sunspot Simulations
and the Reliability of Standard Inversion Methods
Authors: Braun, Douglas; Birch, A.; Rempel, M.; Duvall, T.; J.
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).

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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.
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.

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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.
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.

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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.
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.

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Title: The Vertical Component of the Supergranular Motion
Authors: Duvall, T. L., Jr.; Birch, A. C.
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 &amp; 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.

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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.
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.
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.

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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.
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.
2010SSRv..156..257G Altcode: 2010SSRv..tmp...99G
No abstract at ADS

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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.
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.
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.
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.

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Title: Seismic Constraints on Interior Solar Convection
Authors: Hanasoge, Shravan M.; Duvall, Thomas L., Jr.; DeRosa, Marc L.
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 &lt; 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.

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Title: Transport of Supergranules and their Vertical Coherence
Authors: Švanda, M.; Kosovichev, A. G.; Klvaňa, M.; Sobotka, M.;
Duvall, T. L., Jr.
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.
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.
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.
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.
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.
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.

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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.
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.
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: Confronting Observations with
Three-Dimensional MHD Simulations of Wave Propagation
Authors: Cameron, R.; Gizon, L.; Duvall, T. L., Jr.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 &amp; 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 &amp; 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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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
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.
2003Natur.421..764G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Wave-like properties of solar supergranulation
Authors: Gizon, L.; Duvall, T. L.; Schou, J.
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.
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
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.
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.
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
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.
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.
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.
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.
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.
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
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.
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
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.
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.
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.
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.
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.
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
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.
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.
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 &amp; 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
2000BAAS...32Q.837D 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.
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.
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
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.
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.
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.
2000IAUTA..24...73F Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar tomography
Authors: Kosovichev, A. G.; Duvall, T. L., Jr.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
1999soho....9E....D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Time-distance helioseismology
Authors: Kosovichev, A. G.; Duvall, T. L.; Scherrer, P. H.
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.
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.
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.
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.
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 (&gt;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.
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 &amp; 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 (&lt;~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.
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.
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.
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.
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.
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.
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.
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 = &lt;|v|2&gt;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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
1995ASPC...76..156B Altcode: 1995gong.conf..156B
No abstract at ADS

---------------------------------------------------------
Title: Other Groundbased Observations
Authors: Duvall, T. L., Jr.
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.
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.
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.
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&lt;=nu &lt;=5.0 mHz and the spherical harmonic
degree range 100&lt;=l&lt;=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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
1990BAAS...22..896H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A Review of Observational Helioseismology
Authors: Duvall, Thomas L., Jr.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
1988AnJUS..23..191J Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Acoustic Absorption by Sunspots
Authors: Braun, D. C.; Duvall, T. L., Jr.; Labonte, B. J.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
1985AnJUS..20..221P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar Oscillations
Authors: Duvall, T. L.
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.
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.
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.
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.
1984BAAS...16Q.451H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Frequencies of Solar P-Modes Oscillations
Authors: Harvey, J. W.; Duvall, T. L., Jr.
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.
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.
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.
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.
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.
1983EOSTr..64..304D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A dispersion law for solar oscillations
Authors: Duvall, T. L., Jr.
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.
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.
1982AnJUS..17..232P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar radius measurements
Authors: Duvall, T. L., Jr.; Jones, H. P.
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.
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.
1981siwn.conf...37R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Measurement of the Equatorial Solar Rotation Rate
Authors: Duvall, Thomas L., Jr.
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.
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.
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.
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.
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.
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.
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.

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Title: Large-scale solar velocity fields.
Authors: Duvall, T. L., Jr.
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.

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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.
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.

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Title: Solar rotation, 1966 1978
Authors: Livingston, W.; Duvall, T. L., Jr.
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.

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Title: The strength of the Sun's polar fields.
Authors: Svalgaard, L.; Duvall, T. L., Jr.; Scherrer, P. H.
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.

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Title: On the supposed anticorrelation of solar polar and equatorial
rotation rates.
Authors: Duvall, T. L., Jr.; Svalgaard, L.
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.

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Title: A Study of Large-Scale Solar Magnetic and Velocity Fields.
Authors: Duvall, T. L., Jr.
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.

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Title: A study of large-scale solar magnetic and velocity fields
Authors: Duvall, Thomas Lee, Jr.
1978PhDT.......149D Altcode:
No abstract at ADS

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Title: The strength of the sun's polar fields
Authors: Svalgaard, L.; Duvall, T. L., Jr.; Scherrer, P. H.
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).

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Title: A study of large-scale solar magnetic and velocity fields
Authors: Duvall, T. L., Jr.
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.

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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.
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.

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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.
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.

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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.
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.

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Title: On the supposed anticorrelation of solar polar and equatorial
rotation rates
Authors: Duvall, T. L., Jr.; Svalgaard, L.
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.

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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.
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.

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Title: A Study of Large-Scale Solar Velocity Fields.
Authors: Duvall, T. L., Jr.
1977BAAS....9Q.336D Altcode:
No abstract at ADS

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Title: Stanford Solar Observatory - The First Year
Authors: Scherrer, P. H.; Duvall, T. L., Jr.; Dittmer, P. H.;
Gustafson, E. K.; Wilcox, J. M.
1976BAAS....8Q.370S Altcode:
No abstract at ADS

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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.
1976BAAS....8..311D Altcode:
No abstract at ADS

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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.
1976BAAS....8..344D Altcode:
No abstract at ADS

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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.
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.

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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.
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.
1975BAAS....7..350S Altcode:
No abstract at ADS

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Title: Solar cycle variation of large-scale coronal structures
Authors: Antonucci, E.; Duvall, T. L.
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.

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Title: A Model Combining the Polar and the Sector Structured Solar
Magnetic Fields
Authors: Svalgaard, Leif; Wilcox, John M.; Duvall, Thomas L.
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.