schou

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Author name code: schou
ADS astronomy entries on 2022-09-14
author:"Schou, Jesper"

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Title: The on-ground data reduction and calibration pipeline for
SO/PHI-HRT
Authors: Sinjan, J.; Calchetti, D.; Hirzberger, J.; Orozco Suárez,
D.; Albert, K.; Albelo Jorge, N.; Appourchaux, T.; Alvarez-Herrero,
A.; Blanco Rodríguez, J.; Gandorfer, A.; Germerott, D.; Guerrero,
L.; Gutierrez Marquez, P.; Kahil, F.; Kolleck, M.; Solanki, S. K.; del
Toro Iniesta, J. C.; Volkmer, R.; Woch, J.; Fiethe, B.; Gómez Cama,
J. M.; Pérez-Grande, I.; Sanchis Kilders, E.; Balaguer Jiménez,
M.; Bellot Rubio, L. R.; Carmona, M.; Deutsch, W.; Fernandez-Rico,
G.; Fernández-Medina, A.; García Parejo, P.; Gasent Blesa, J. L.;
Gizon, L.; Grauf, B.; Heerlein, K.; Korpi-Lagg, A.; Lange, T.; López
Jiménez, A.; Maue, T.; Meller, R.; Michalik, H.; Moreno Vacas, A.;
Müller, R.; Nakai, E.; Schmidt, W.; Schou, J.; Schühle, U.; Staub,
J.; Strecker, H.; Torralbo, I.; Valori, G.
2022arXiv220814904S Altcode:
The ESA/NASA Solar Orbiter space mission has been successfully launched
in February 2020. Onboard is the Polarimetric and Helioseismic Imager
(SO/PHI), which has two telescopes, a High Resolution Telescope
(HRT) and the Full Disc Telescope (FDT). The instrument is designed
to infer the photospheric magnetic field and line-of-sight velocity
through differential imaging of the polarised light emitted by the
Sun. It calculates the full Stokes vector at 6 wavelength positions
at the Fe I 617.3 nm absorption line. Due to telemetry constraints,
the instrument nominally processes these Stokes profiles onboard,
however when telemetry is available, the raw images are downlinked and
reduced on ground. Here the architecture of the on-ground pipeline
for HRT is presented, which also offers additional corrections not
currently available on board the instrument. The pipeline can reduce
raw images to the full Stokes vector with a polarimetric sensitivity
of $10^{-3}\cdot I_{c}$ or better.

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Title: Using birefringent elements and imaging Michelsons for the
calibration of high-precision planet-finding spectrographs
Authors: Schou, J.
2022A&A...662A.119S Altcode: 2022arXiv220213421S
Context. One of the main methods used for finding extrasolar planets
is the radial velocity technique, in which the Doppler shift of a star
due to an orbiting planet is measured. These measurements are typically
performed using cross-dispersed echelle spectrographs. Unfortunately,
such spectrographs are large and expensive, and their accurate
calibration continues to be challenging. Aims: The aim is
to develop a different way to provide a calibration signal. 
Methods: A commonly used way to introduce a calibration signal is
to insert an iodine cell in the beam. Disadvantages of this include
that the lines are narrow, do not cover the entire spectrum, and
light is absorbed. Here I show that inserting a birefringent element
or an imaging Michelson, combined with Wollaston prisms, eliminates
these three shortcomings while maintaining most of the benefits of
the iodine approach. Results: The proposed designs can be
made very compact, thereby providing a convenient way of calibrating
a spectrograph. Similar to the iodine cell approach, the calibration
signal travels with the stellar signal, thereby reducing the sensitivity
to spectrograph stability. The imposed signal covers the entire visible
range, and any temperature drifts will be consistent and describable
by a single number. Based on experience with similar devices that were
used in a different configuration by the Helioseismic and Magnetic
Imager, it is shown that the calibration device can be made stable
at the 0.1 m/s level over a significant wavelength range on short to
medium timescales. Conclusions: While the design is promising,
many details still need to be worked out. In particular, a number of
laboratory measurements are required in order to finalize a design
and estimate actual performance, and it would be desirable to make a
proof of concept.

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Title: The magnetic drivers of campfires seen by the Polarimetric
and Helioseismic Imager (PHI) on Solar Orbiter
Authors: Kahil, F.; Hirzberger, J.; Solanki, S. K.; Chitta, L. P.;
Peter, H.; Auchère, F.; Sinjan, J.; Orozco Suárez, D.; Albert,
K.; Albelo Jorge, N.; Appourchaux, T.; Alvarez-Herrero, A.; Blanco
Rodríguez, J.; Gandorfer, A.; Germerott, D.; Guerrero, L.; Gutiérrez
Márquez, P.; Kolleck, M.; del Toro Iniesta, J. C.; Volkmer, R.;
Woch, J.; Fiethe, B.; Gómez Cama, J. M.; Pérez-Grande, I.; Sanchis
Kilders, E.; Balaguer Jiménez, M.; Bellot Rubio, L. R.; Calchetti,
D.; Carmona, M.; Deutsch, W.; Fernández-Rico, G.; Fernández-Medina,
A.; García Parejo, P.; Gasent-Blesa, J. L.; Gizon, L.; Grauf, B.;
Heerlein, K.; Lagg, A.; Lange, T.; López Jiménez, A.; Maue, T.;
Meller, R.; Michalik, H.; Moreno Vacas, A.; Müller, R.; Nakai,
E.; Schmidt, W.; Schou, J.; Schühle, U.; Staub, J.; Strecker, H.;
Torralbo, I.; Valori, G.; Aznar Cuadrado, R.; Teriaca, L.; Berghmans,
D.; Verbeeck, C.; Kraaikamp, E.; Gissot, S.
2022A&A...660A.143K Altcode: 2022arXiv220213859K
Context. The Extreme Ultraviolet Imager (EUI) on board the Solar Orbiter
(SO) spacecraft observed small extreme ultraviolet (EUV) bursts,
termed campfires, that have been proposed to be brightenings near the
apexes of low-lying loops in the quiet-Sun atmosphere. The underlying
magnetic processes driving these campfires are not understood. Aims: During the cruise phase of SO and at a distance of 0.523
AU from the Sun, the Polarimetric and Helioseismic Imager on Solar
Orbiter (SO/PHI) observed a quiet-Sun region jointly with SO/EUI,
offering the possibility to investigate the surface magnetic field
dynamics underlying campfires at a spatial resolution of about 380
km. Methods: We used co-spatial and co-temporal data of the
quiet-Sun network at disc centre acquired with the High Resolution
Imager of SO/EUI at 17.4 nm (HRIEUV, cadence 2 s) and the
High Resolution Telescope of SO/PHI at 617.3 nm (HRT, cadence 2.5
min). Campfires that are within the SO/PHI−SO/EUI common field
of view were isolated and categorised according to the underlying
magnetic activity. Results: In 71% of the 38 isolated events,
campfires are confined between bipolar magnetic features, which seem to
exhibit signatures of magnetic flux cancellation. The flux cancellation
occurs either between the two main footpoints, or between one of the
footpoints of the loop housing the campfire and a nearby opposite
polarity patch. In one particularly clear-cut case, we detected the
emergence of a small-scale magnetic loop in the internetwork followed
soon afterwards by a campfire brightening adjacent to the location
of the linear polarisation signal in the photosphere, that is to
say near where the apex of the emerging loop lays. The rest of the
events were observed over small scattered magnetic features, which
could not be identified as magnetic footpoints of the campfire hosting
loops. Conclusions: The majority of campfires could be driven
by magnetic reconnection triggered at the footpoints, similar to the
physical processes occurring in the burst-like EUV events discussed
in the literature. About a quarter of all analysed campfires, however,
are not associated to such magnetic activity in the photosphere, which
implies that other heating mechanisms are energising these small-scale
EUV brightenings.

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Title: Multi-view magnetic synoptic maps with SO/PHI and SDO/HMI
Authors: Löschl, P.; Hirzberger, J.; Schou, J.; Solanki, S. K.
2020AGUFMSH0360028L Altcode:
With the recent launch of Solar Orbiter (SO) and the first data slowly
becoming available, it will soon be possible to simultaneously observe
the Sun from additional vantage points off the Earth-Sun line. One
of its instruments, the Polarimetric and Heliospheric Imager (PHI),
is the first spectro-polarimeter to operate outside of this line of
sight. This opens the possibility for joint observational campaigns
with similar instruments, such as the Heliospheric and Magnetic Imager
(HMI) on-board the Solar Dynamics Observatory (SDO). We utilise these
new observational possibilities to produce combined magnetic synoptic
maps from magnetograms of the PHI and HMI instruments. Building on the
existing software infrastructure for HMI synoptic maps, we extended
its current functionality to include PHI data and correct for the
different and varying relative orbital characteristics of the two
spacecraft. The result are joint magnetic synoptic maps, that can
be produced significantly faster than the approximately 27 days of
one solar rotation and therefore are less likely to suffer from the
evolution of the magnetic field over the observation period. Once Solar
Orbiter leaves the ecliptic plane, we will also be able to include
observations of the polar magnetic field into our synoptic maps,
which will give an unprecedented insight into the magnetic field of
the Sun. This work presents our preparatory modelling efforts and
gives an outlook for the future capabilities of this novel data product.

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Title: First results from SO/PHI's on-board data reduction
Authors: Albert, K.; Hirzberger, J.; Kolleck, M.; Albelo Jorge,
N.; Busse, D.; Blanco Rodriguez, J.; Cobos Carrascosa, J. P.;
Fiethe, B.; Gandorfer, A.; Germerott, D.; Guan, Y.; Guerrero, L.;
Gutierrez-Marques, P.; Hernández Expósito, D.; Lange, T.; Michalik,
H.; Orozco Suárez, D.; Schou, J.; Solanki, S. K.; Woch, J. G.
2020AGUFMSH038..05A Altcode:
The Polarimetric and Helioseismic Imager (PHI), on-board Solar
Orbiter (SO), is a spectropolarimeter imaging the solar photosphere
at the wavelengths of the Fe I 617.3 nm Zeeman sensitive absorption
line. SO/PHI's aim is to provide data about the magnetic structures and
the line-of-sight (LOS) velocity in the solar atmosphere. For this, it
takes time series of data sets consisting of 2048 x 2048 pixel images of
the Sun at 6 wavelengths, each in 4 different polarisation states. With
the minimum necessary 17 bits pixel depth, one data set amounts to
approx. 0.2 GB. The guaranteed data telemetry for PHI, in contrast,
is only 50 GiB/orbit which would also need to contain any calibration
data obtained on-board, i.e. our flat and dark fields. To cope with
this discrepancy, SO/PHI is performing full data reduction on-board,
including the inversion of the radiative transfer equation. The
downloaded results are science ready data, containing 5 final images: a
total intensity image from nearby the spectral line, the magnetic field
strength, azimuth and inclination (describing the magnetic vector) and
the LOS velocity. This process maximises the science return by reducing
the number of necessary images in a data set, as well as rendering the
download of calibration data unessential. In the commissioning phase
of SO/PHI we used the on-board data reduction system successfully
for the first time. We have calibrated the instrument to its optimal
operational parameters (calculation of exposure time, focus, etc.),
acquired and processed calibration data (dark and flat fields),
removed the most significant instrumental artefacts from the data
(dark field, flat field, polarimetric modulation and polarimetric
cross-talk), and performed the inversion of the radiative transfer
equation. The data have then been compressed to further maximise the
use of our telemetry. This contribution presents and discusses the
final results from this process.

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Title: The Solar Orbiter Science Activity Plan. Translating solar
and heliospheric physics questions into action
Authors: Zouganelis, I.; De Groof, A.; Walsh, A. P.; Williams, D. R.;
Müller, D.; St Cyr, O. C.; Auchère, F.; Berghmans, D.; Fludra,
A.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.;
Owen, C. J.; Rodríguez-Pacheco, J.; Romoli, M.; Solanki, S. K.;
Watson, C.; Sanchez, L.; Lefort, J.; Osuna, P.; Gilbert, H. R.;
Nieves-Chinchilla, T.; Abbo, L.; Alexandrova, O.; Anastasiadis, A.;
Andretta, V.; Antonucci, E.; Appourchaux, T.; Aran, A.; Arge, C. N.;
Aulanier, G.; Baker, D.; Bale, S. D.; Battaglia, M.; Bellot Rubio,
L.; Bemporad, A.; Berthomier, M.; Bocchialini, K.; Bonnin, X.; Brun,
A. S.; Bruno, R.; Buchlin, E.; Büchner, J.; Bucik, R.; Carcaboso,
F.; Carr, R.; Carrasco-Blázquez, I.; Cecconi, B.; Cernuda Cangas, I.;
Chen, C. H. K.; Chitta, L. P.; Chust, T.; Dalmasse, K.; D'Amicis, R.;
Da Deppo, V.; De Marco, R.; Dolei, S.; Dolla, L.; Dudok de Wit, T.;
van Driel-Gesztelyi, L.; Eastwood, J. P.; Espinosa Lara, F.; Etesi,
L.; Fedorov, A.; Félix-Redondo, F.; Fineschi, S.; Fleck, B.; Fontaine,
D.; Fox, N. J.; Gandorfer, A.; Génot, V.; Georgoulis, M. K.; Gissot,
S.; Giunta, A.; Gizon, L.; Gómez-Herrero, R.; Gontikakis, C.; Graham,
G.; Green, L.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler,
D. M.; Hirzberger, J.; Ho, G. C.; Hurford, G.; Innes, D.; Issautier,
K.; James, A. W.; Janitzek, N.; Janvier, M.; Jeffrey, N.; Jenkins,
J.; Khotyaintsev, Y.; Klein, K. -L.; Kontar, E. P.; Kontogiannis,
I.; Krafft, C.; Krasnoselskikh, V.; Kretzschmar, M.; Labrosse, N.;
Lagg, A.; Landini, F.; Lavraud, B.; Leon, I.; Lepri, S. T.; Lewis,
G. R.; Liewer, P.; Linker, J.; Livi, S.; Long, D. M.; Louarn, P.;
Malandraki, O.; Maloney, S.; Martinez-Pillet, V.; Martinovic, M.;
Masson, A.; Matthews, S.; Matteini, L.; Meyer-Vernet, N.; Moraitis,
K.; Morton, R. J.; Musset, S.; Nicolaou, G.; Nindos, A.; O'Brien,
H.; Orozco Suarez, D.; Owens, M.; Pancrazzi, M.; Papaioannou, A.;
Parenti, S.; Pariat, E.; Patsourakos, S.; Perrone, D.; Peter, H.;
Pinto, R. F.; Plainaki, C.; Plettemeier, D.; Plunkett, S. P.; Raines,
J. M.; Raouafi, N.; Reid, H.; Retino, A.; Rezeau, L.; Rochus, P.;
Rodriguez, L.; Rodriguez-Garcia, L.; Roth, M.; Rouillard, A. P.;
Sahraoui, F.; Sasso, C.; Schou, J.; Schühle, U.; Sorriso-Valvo, L.;
Soucek, J.; Spadaro, D.; Stangalini, M.; Stansby, D.; Steller, M.;
Strugarek, A.; Štverák, Š.; Susino, R.; Telloni, D.; Terasa, C.;
Teriaca, L.; Toledo-Redondo, S.; del Toro Iniesta, J. C.; Tsiropoula,
G.; Tsounis, A.; Tziotziou, K.; Valentini, F.; Vaivads, A.; Vecchio,
A.; Velli, M.; Verbeeck, C.; Verdini, A.; Verscharen, D.; Vilmer, N.;
Vourlidas, A.; Wicks, R.; Wimmer-Schweingruber, R. F.; Wiegelmann,
T.; Young, P. R.; Zhukov, A. N.
2020A&A...642A...3Z Altcode: 2020arXiv200910772Z
Solar Orbiter is the first space mission observing the solar plasma
both in situ and remotely, from a close distance, in and out of the
ecliptic. The ultimate goal is to understand how the Sun produces
and controls the heliosphere, filling the Solar System and driving
the planetary environments. With six remote-sensing and four in-situ
instrument suites, the coordination and planning of the operations are
essential to address the following four top-level science questions:
(1) What drives the solar wind and where does the coronal magnetic field
originate?; (2) How do solar transients drive heliospheric variability?;
(3) How do solar eruptions produce energetic particle radiation that
fills the heliosphere?; (4) How does the solar dynamo work and drive
connections between the Sun and the heliosphere? Maximising the
mission's science return requires considering the characteristics
of each orbit, including the relative position of the spacecraft
to Earth (affecting downlink rates), trajectory events (such
as gravitational assist manoeuvres), and the phase of the solar
activity cycle. Furthermore, since each orbit's science telemetry
will be downloaded over the course of the following orbit, science
operations must be planned at mission level, rather than at the level
of individual orbits. It is important to explore the way in which those
science questions are translated into an actual plan of observations
that fits into the mission, thus ensuring that no opportunities are
missed. First, the overarching goals are broken down into specific,
answerable questions along with the required observations and the
so-called Science Activity Plan (SAP) is developed to achieve this. The
SAP groups objectives that require similar observations into Solar
Orbiter Observing Plans, resulting in a strategic, top-level view of
the optimal opportunities for science observations during the mission
lifetime. This allows for all four mission goals to be addressed. In
this paper, we introduce Solar Orbiter's SAP through a series of
examples and the strategy being followed.

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Title: Autonomous on-board data processing and instrument calibration
software for the Polarimetric and Helioseismic Imager on-board the
Solar Orbiter mission
Authors: Albert, Kinga; Hirzberger, Johann; Kolleck, Martin; Jorge,
Nestor Albelo; Busse, Dennis; Rodríguez, Julian Blanco; Carrascosa,
Juan Pedro Cobos; Fiethe, Björn; Gandorfer, Achim; Germerott, Dietmar;
Guan, Yejun; Guerrero, Lucas; Gutierrez-Marques, Pablo; Expósito,
David Hernández; Lange, Tobias; Michalik, Harald; Suárez, David
Orozco; Schou, Jesper; Solanki, Sami K.; del Toro Iniesta, José
Carlos; Woch, Joachim
2020JATIS...6d8004A Altcode:
A frequent problem arising for deep space missions is the discrepancy
between the amount of data desired to be transmitted to the ground
and the available telemetry bandwidth. A part of these data consists
of scientific observations, being complemented by calibration data
to help remove instrumental effects. We present our solution for this
discrepancy, implemented for the Polarimetric and Helioseismic Imager
on-board the Solar Orbiter mission, the first solar spectropolarimeter
in deep space. We implemented an on-board data reduction system that
processes calibration data, applies them to the raw science observables,
and derives science-ready physical parameters. This process reduces
the raw data for a single measurement from 24 images to five, thus
reducing the amount of downlinked data, and in addition, renders the
transmission of the calibration data unnecessary. Both these on-board
actions are completed autonomously.

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Title: The Polarimetric and Helioseismic Imager on Solar Orbiter
Authors: Solanki, S. K.; del Toro Iniesta, J. C.; Woch, J.; Gandorfer,
A.; Hirzberger, J.; Alvarez-Herrero, A.; Appourchaux, T.; Martínez
Pillet, V.; Pérez-Grande, I.; Sanchis Kilders, E.; Schmidt, W.;
Gómez Cama, J. M.; Michalik, H.; Deutsch, W.; Fernandez-Rico, G.;
Grauf, B.; Gizon, L.; Heerlein, K.; Kolleck, M.; Lagg, A.; Meller, R.;
Müller, R.; Schühle, U.; Staub, J.; Albert, K.; Alvarez Copano, M.;
Beckmann, U.; Bischoff, J.; Busse, D.; Enge, R.; Frahm, S.; Germerott,
D.; Guerrero, L.; Löptien, B.; Meierdierks, T.; Oberdorfer, D.;
Papagiannaki, I.; Ramanath, S.; Schou, J.; Werner, S.; Yang, D.;
Zerr, A.; Bergmann, M.; Bochmann, J.; Heinrichs, J.; Meyer, S.;
Monecke, M.; Müller, M. -F.; Sperling, M.; Álvarez García, D.;
Aparicio, B.; Balaguer Jiménez, M.; Bellot Rubio, L. R.; Cobos
Carracosa, J. P.; Girela, F.; Hernández Expósito, D.; Herranz, M.;
Labrousse, P.; López Jiménez, A.; Orozco Suárez, D.; Ramos, J. L.;
Barandiarán, J.; Bastide, L.; Campuzano, C.; Cebollero, M.; Dávila,
B.; Fernández-Medina, A.; García Parejo, P.; Garranzo-García, D.;
Laguna, H.; Martín, J. A.; Navarro, R.; Núñez Peral, A.; Royo, M.;
Sánchez, A.; Silva-López, M.; Vera, I.; Villanueva, J.; Fourmond,
J. -J.; de Galarreta, C. Ruiz; Bouzit, M.; Hervier, V.; Le Clec'h,
J. C.; Szwec, N.; Chaigneau, M.; Buttice, V.; Dominguez-Tagle, C.;
Philippon, A.; Boumier, P.; Le Cocguen, R.; Baranjuk, G.; Bell,
A.; Berkefeld, Th.; Baumgartner, J.; Heidecke, F.; Maue, T.; Nakai,
E.; Scheiffelen, T.; Sigwarth, M.; Soltau, D.; Volkmer, R.; Blanco
Rodríguez, J.; Domingo, V.; Ferreres Sabater, A.; Gasent Blesa,
J. L.; Rodríguez Martínez, P.; Osorno Caudel, D.; Bosch, J.; Casas,
A.; Carmona, M.; Herms, A.; Roma, D.; Alonso, G.; Gómez-Sanjuan, A.;
Piqueras, J.; Torralbo, I.; Fiethe, B.; Guan, Y.; Lange, T.; Michel,
H.; Bonet, J. A.; Fahmy, S.; Müller, D.; Zouganelis, I.
2020A&A...642A..11S Altcode: 2019arXiv190311061S
 Aims: This paper describes the Polarimetric and Helioseismic
Imager on the Solar Orbiter mission (SO/PHI), the first magnetograph and
helioseismology instrument to observe the Sun from outside the Sun-Earth
line. It is the key instrument meant to address the top-level science
question: How does the solar dynamo work and drive connections between
the Sun and the heliosphere? SO/PHI will also play an important role
in answering the other top-level science questions of Solar Orbiter,
while hosting the potential of a rich return in further science. Methods: SO/PHI measures the Zeeman effect and the Doppler shift
in the Fe I 617.3 nm spectral line. To this end, the instrument
carries out narrow-band imaging spectro-polarimetry using a tunable
LiNbO3 Fabry-Perot etalon, while the polarisation modulation
is done with liquid crystal variable retarders. The line and the nearby
continuum are sampled at six wavelength points and the data are recorded
by a 2k × 2k CMOS detector. To save valuable telemetry, the raw data
are reduced on board, including being inverted under the assumption of
a Milne-Eddington atmosphere, although simpler reduction methods are
also available on board. SO/PHI is composed of two telescopes; one,
the Full Disc Telescope, covers the full solar disc at all phases of
the orbit, while the other, the High Resolution Telescope, can resolve
structures as small as 200 km on the Sun at closest perihelion. The high
heat load generated through proximity to the Sun is greatly reduced by
the multilayer-coated entrance windows to the two telescopes that allow
less than 4% of the total sunlight to enter the instrument, most of
it in a narrow wavelength band around the chosen spectral line. 
Results: SO/PHI was designed and built by a consortium having partners
in Germany, Spain, and France. The flight model was delivered to
Airbus Defence and Space, Stevenage, and successfully integrated into
the Solar Orbiter spacecraft. A number of innovations were introduced
compared with earlier space-based spectropolarimeters, thus allowing
SO/PHI to fit into the tight mass, volume, power and telemetry budgets
provided by the Solar Orbiter spacecraft and to meet the (e.g. thermal)
challenges posed by the mission's highly elliptical orbit.

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Title: PMI: The Photospheric Magnetic Field Imager
Authors: Staub, Jan; Fernandez-Rico, German; Gandorfer, Achim; Gizon,
Laurent; Hirzberger, Johann; Kraft, Stefan; Lagg, Andreas; Schou,
Jesper; Solanki, Sami K.; del Toro Iniesta, Jose Carlos; Wiegelmann,
Thomas; Woch, Joachim
2020JSWSC..10...54S Altcode:
We describe the design and the capabilities of the Photospheric Magnetic
field Imager (PMI), a compact and lightweight vector magnetograph,
which is being developed for ESA's Lagrange mission to the Lagrange
L5 point. After listing the design requirements and give a scientific
justification for them, we describe the technical implementation and
the design solution capable of fulfilling these requirements. This is
followed by a description of the hardware architecture as well as the
operations principle. An outlook on the expected performance concludes
the paper.

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Title: Estimating the nonstructural component of the helioseismic
surface term using hydrodynamic simulations
Authors: Schou, J.; Birch, A. C.
2020A&A...638A..51S Altcode: 2020arXiv200413548S
Context. As the amount of asteroseismic data available continues
to grow, the inability to accurately model observed oscillation
frequencies is becoming a critical problem for interpreting these
frequencies. A major component of this problem is the modeling of
the near-surface layers. Aims: Our aim is to develop a method
to estimate the effect of the near-surface layers on oscillation
frequencies. Methods: In the proposed method we numerically
estimate eigenfunctions in 3D hydrodynamic simulations. We match those
to the eigenfunctions calculated from the classic equations applied
to the horizontal averages of the structure variables. We use this
procedure to calculate the frequency perturbation resulting from the
dynamical part of the interaction of the oscillations with near-surface
convection. As the last step we scale the numbers to the Sun. To provide
a qualitative test of our method we performed a series of simulations,
calculated the perturbations using our procedure, and compared them
to previously reported residuals relative to solar models. 
Results: We find that we can largely reproduce the observed frequency
residuals without resorting to poorly justified theoretical models. We
find that, while the calculations of Houdek et al. (2017, MNRAS, 464,
L124) produce similar frequency perturbations, the density-pressure
phase differences computed here do not match those of that work.

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Title: The Solaris Solar Polar Mission
Authors: Hassler, Donald M.; Newmark, Jeff; Gibson, Sarah; Harra,
Louise; Appourchaux, Thierry; Auchere, Frederic; Berghmans, David;
Colaninno, Robin; Fineschi, Silvano; Gizon, Laurent; Gosain, Sanjay;
Hoeksema, Todd; Kintziger, Christian; Linker, John; Rochus, Pierre;
Schou, Jesper; Viall, Nicholeen; West, Matt; Woods, Tom; Wuelser,
Jean-Pierre
2020EGUGA..2217703H Altcode:
The solar poles are one of the last unexplored regions of the solar
system. Although Ulysses flew over the poles in the 1990s, it did
not have remote sensing instruments onboard to probe the Sun's polar
magnetic field or surface/sub-surface flows.We will discuss Solaris,
a proposed Solar Polar MIDEX mission to revolutionize our understanding
of the Sun by addressing fundamental questions that can only be answered
from a polar vantage point. Solaris uses a Jupiter gravity assist to
escape the ecliptic plane and fly over both poles of the Sun to &gt;75
deg. inclination, obtaining the first high-latitude, multi-month-long,
continuous remote-sensing solar observations. Solaris will address key
outstanding, breakthrough problems in solar physics and fill holes in
our scientific understanding that will not be addressed by current
missions.With focused science and a simple, elegant mission design,
Solaris will also provide enabling observations for space weather
research (e.g. polar view of CMEs), and stimulate future research
through new unanticipated discoveries.

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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.

---------------------------------------------------------
Title: Exploring the latitude and depth dependence of solar Rossby
waves using ring-diagram analysis
Authors: Proxauf, B.; Gizon, L.; Löptien, B.; Schou, J.; Birch,
A. C.; Bogart, R. S.
2020A&A...634A..44P Altcode: 2019arXiv191202056P
Context. Global-scale equatorial Rossby waves have recently been
unambiguously identified on the Sun. Like solar acoustic modes, Rossby
waves are probes of the solar interior. Aims: We study the
latitude and depth dependence of the Rossby wave eigenfunctions. 
Methods: By applying helioseismic ring-diagram analysis and granulation
tracking to observations by HMI aboard SDO, we computed maps of
the radial vorticity of flows in the upper solar convection zone
(down to depths of more than 16 Mm). The horizontal sampling of the
ring-diagram maps is approximately 90 Mm (∼7.5°) and the temporal
sampling is roughly 27 hr. We used a Fourier transform in longitude
to separate the different azimuthal orders m in the range 3 ≤ m ≤
15. At each m we obtained the phase and amplitude of the Rossby waves
as functions of depth using the helioseismic data. At each m we also
measured the latitude dependence of the eigenfunctions by calculating
the covariance between the equator and other latitudes. Results:
We conducted a study of the horizontal and radial dependences of the
radial vorticity eigenfunctions. The horizontal eigenfunctions are
complex. As observed previously, the real part peaks at the equator
and switches sign near ±30°, thus the eigenfunctions show significant
non-sectoral contributions. The imaginary part is smaller than the real
part. The phase of the radial eigenfunctions varies by only ±5° over
the top 15 Mm. The amplitude of the radial eigenfunctions decreases
by about 10% from the surface down to 8 Mm (the region in which
ring-diagram analysis is most reliable, as seen by comparing with the
rotation rate measured by global-mode seismology). Conclusions:
The radial dependence of the radial vorticity eigenfunctions deduced
from ring-diagram analysis is consistent with a power law down to 8 Mm
and is unreliable at larger depths. However, the observations provide
only weak constraints on the power-law exponents. For the real part,
the latitude dependence of the eigenfunctions is consistent with
previous work (using granulation tracking). The imaginary part is
smaller than the real part but significantly nonzero.

---------------------------------------------------------
Title: An improved multi-ridge fitting method for ring-diagram
helioseismic analysis
Authors: Nagashima, Kaori; Birch, Aaron C.; Schou, Jesper; Hindman,
Bradley W.; Gizon, Laurent
2020A&A...633A.109N Altcode: 2019arXiv191107772N
Context. There is a wide discrepancy in current estimates of
the strength of convection flows in the solar interior obtained
using different helioseismic methods applied to observations from
the Helioseismic and Magnetic Imager onboard the Solar Dynamics
Observatory. The cause for these disparities is not known. 
Aims: As one step in the effort to resolve this discrepancy, we aim to
characterize the multi-ridge fitting code for ring-diagram helioseismic
analysis that is used to obtain flow estimates from local power spectra
of solar oscillations. Methods: We updated the multi-ridge
fitting code developed by Greer et al. (2014, Sol. Phys., 289, 2823)
to solve several problems we identified through our inspection of the
code. In particular, we changed the (1) merit function to account for
the smoothing of the power spectra, (2) model for the power spectrum,
and (3) noise estimates. We used Monte Carlo simulations to generate
synthetic data and to characterize the noise and bias of the updated
code by fitting these synthetic data. Results: The bias in
the output fit parameters, apart from the parameter describing the
amplitude of the p-mode resonances in the power spectrum, is below
what can be measured from the Monte-Carlo simulations. The amplitude
parameters are underestimated; this is a consequence of choosing to
fit the logarithm of the averaged power. We defer fixing this problem
as it is well understood and not significant for measuring flows in the
solar interior. The scatter in the fit parameters from the Monte-Carlo
simulations is well-modeled by the formal error estimates from the
code. Conclusions: We document and demonstrate a reliable
multi-ridge fitting method for ring-diagram analysis. The differences
between the updated fitting results and the original results are less
than one order of magnitude and therefore we suspect that the changes
will not eliminate the aforementioned orders-of-magnitude discrepancy
in the amplitude of convective flows in the solar interior.

---------------------------------------------------------
Title: On the latitude dependence of Rossby waves in the Sun
Authors: Proxauf, Bastian Severin Niklas; Gizon, Laurent; Löptien,
Björn; Schou, Jesper; Birch, Aaron C.; Bogart, Richard S.
2019AAS...23431801P Altcode:
We study the latitude and depth dependence of solar Rossby waves. We
use horizontal flows from local helioseismology (ring-diagram analysis)
at different depths in the solar interior. From these we compute maps
of the radial vorticity. We confirm the existence of solar Rossby waves
in the sectoral (m = l) power spectra at all depths down to 17 Mm below
the surface. The depth dependence of the eigenfunctions is consistent
with rm, although this is a weak constraint due to the noise
level. The latitudinal eigenfunctions are observed to be more narrow
than |sin(θ)|m, likely indicating that the modes sense the
latitudinal differential rotation. Furthermore, we detect a non-zero
imaginary component of the latitudinal eigenfunctions, possibly related
to viscous dissipation. These new observations provide additional
constraints on the physics of large-scale Rossby waves in the Sun.

---------------------------------------------------------
Title: Towards improved multi-ridge fitting method for ring-diagram
analysis
Authors: Nagashima, Kaori; Birch, Aaron C.; Schou, Jesper; Hindman,
Bradley; Gizon, Laurent
2018csc..confE..50N Altcode:
Ring-diagram analysis is one of the important methods of local
helioseismology for probing subsurface flows. In ring-diagram analysis
the Doppler shifts of oscillation mode frequencies due to flows
are measured by fitting a model function to the local oscillation
power spectra. Here we propose alteration of the multi-ridge fitting
method developed by Greer et al. (2014). It is well known that the
solar oscillation power is chi-square distributed (with two degrees
of freedom), and the fitting in the existing multi-ridge fitting is
done with the maximum likelihood method based on this probability
distribution function. However, the power is in practice remapped
from Cartesian to polar coordinates and/or smoothed in azimuth
of the wavevector. The smoothed power is approximately normally
distributed. We demonstrate that the probability distribution function
of the logarithm of the normally-distributed power is approximated
by a normal distribution with a variance that is independent of the
expectation value of the power. Therefore, we alter the fitting method
using the logarithm of the power with a least-square method. In this
presentation we report the bias and noise levels in the updated fitting
results as well as the crosstalk between the parameters using a Monte
Carlo simulation of the power spectra.

---------------------------------------------------------
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.

---------------------------------------------------------
Title: On the depth dependence of solar equatorial Rossby waves
Authors: Proxauf, Bastian; Gizon, Laurent; Löptien, Björn; Birch,
Aaron C.; Schou, Jesper; Bogart, Richard S.
2018csc..confE..43P Altcode:
Here we use local helioseismology and local correlation tracking of
granulation to infer horizontal flows on the solar surface and in the
interior. From these flows, we compute maps of the radial vorticity
at different depths in order to study Rossby waves. We show that
the frequencies of these waves agree well with a simple theoretical
dispersion relation. Also, we show that Rossby waves have significant
amplitudes in the first 20 Mm below the surface and investigate
the dependence of the Rossby waves on depth. We find an unexpected,
presumably spurious dip in the wave power and a depth-independent
phase and we conclude that further studies are needed.

---------------------------------------------------------
Title: Interactions of waves with solar convection
Authors: Schou, Jesper
2018csc..confE..47S Altcode:
Some of the most significant problems in our understanding of solar and
stellar oscillations are believed to be related to their interaction
with the near surface convection. One such problem is the center
to limb effect seen in many helioseismic measurements. Another the
so-called surface term. Here I will briefly describe these problems
and some preliminary results of trying to address them using large
scale hydrocode simulations.

---------------------------------------------------------
Title: Modeling and use of stellar oscillation visibilities
Authors: Schou, J.
2018A&A...617A.111S Altcode: 2018arXiv180601055S
Context. Recently our ability to study stars using asteroseismic
techniques has increased dramatically, largely through the use of
space based photometric observations. Work has also been performed
using ground based spectroscopic observations and more is expected in
the near future from the SONG network. Unfortunately, the intensity
observations have an inferior signal-to-noise ratio and details of
the observations do not agree with theory, while the data analysis
used in the spectroscopic method has often been based on overly
simple models of the spectra. Aims: The aim is to improve the
reliability of measurements of the parameters of stellar oscillations
using spectroscopic observations and to enable the optimal use of the
observations. Methods: While previous investigations have used
1D models, I argue that realistic magnetohydrodynamic simulations,
combined with radiative transfer calculations, should be used to model
the effects of the oscillations on the spectra. I then demonstrate how
to calculate the visibility of the oscillation modes for a variety of
stellar parameters and fitting methods. In addition to the methods used
in previous investigations, I introduce a singular value decomposition
based technique. This new technique enables the determination of the
information content available from spectral perturbations and allows
this content to be expressed most compactly. Finally I describe how
the time series obtained may be analyzed. Results: It is shown
that it is important to model the visibilities carefully and that the
results deviate substantially from previous models, especially in the
presence of rotation. Detailed spectral modeling may be exploited to
measure the properties of a larger number of modes than possible via
the commonly used cross-correlation method. With moderate rotation,
there is as much information in the line shape changes as in the
Doppler shift and an outline of how to extract this is given.

---------------------------------------------------------
Title: Autonomous on-board data processing and instrument calibration
software for the SO/PHI
Authors: Albert, K.; Hirzberger, J.; Busse, D.; Lange, T.; Kolleck, M.;
Fiethe, B.; Orozco Suárez, D.; Woch, J.; Schou, J.; Blanco Rodriguez,
J.; Gandorfer, A.; Guan, Y.; Cobos Carrascosa, J. P.; Hernández
Expósito, D.; del Toro Iniesta, J. C.; Solanki, S. K.; Michalik, H.
2018SPIE10707E..0OA Altcode: 2018arXiv181003493A
The extension of on-board data processing capabilities is an
attractive option to reduce telemetry for scientific instruments
on deep space missions. The challenges that this presents, however,
require a comprehensive software system, which operates on the limited
resources a data processing unit in space allows. We implemented such
a system for the Polarimetric and Helioseismic Imager (PHI) on-board
the Solar Orbiter (SO) spacecraft. It ensures autonomous operation
to handle long command-response times, easy changing of the processes
after new lessons have been learned and meticulous book-keeping of all
operations to ensure scientific accuracy. This contribution presents
the requirements and main aspects of the software implementation,
followed by an example of a task implemented in the software frame,
and results from running it on SO/PHI. The presented example shows
that the different parts of the software framework work well together,
and that the system processes data as we expect. The flexibility of
the framework makes it possible to use it as a baseline for future
applications with similar needs and limitations as SO/PHI.

---------------------------------------------------------
Title: Signatures of Solar Cycle 25 in Subsurface Zonal Flows
Authors: Howe, R.; Hill, F.; Komm, R.; Chaplin, W. J.; Elsworth, Y.;
Davies, G. R.; Schou, J.; Thompson, M. J.
2018ApJ...862L...5H Altcode: 2018arXiv180702398H
The pattern of migrating zonal flow bands associated with the solar
cycle, known as the torsional oscillation, has been monitored with
continuous global helioseismic observations by the Global Oscillations
Network Group (GONG), together with those made by the Michelson
Doppler Imager (MDI) on board the Solar and Heliospheric Observatory
(SOHO) and its successor, the Helioseismic and Magnetic Imager (HMI)
on board the Solar Dynamics Observatory (SDO), since 1995, giving us
nearly two full solar cycles of observations. We report that the flows
now show traces of the mid-latitude acceleration that is expected to
become the main equatorward-moving branch of the zonal flow pattern for
Cycle 25. Based on the current position of this branch, we speculate
that the onset of widespread activity for Cycle 25 is unlikely to be
earlier than the middle of 2019.

---------------------------------------------------------
Title: Fragile Detection of Solar g -Modes by Fossat et al.
Authors: Schunker, Hannah; Schou, Jesper; Gaulme, Patrick; Gizon,
Laurent
2018SoPh..293...95S Altcode: 2018arXiv180404407S
The internal gravity modes of the Sun are notoriously difficult to
detect, and the claimed detection of gravity modes presented by Fossat
et al. (Astron. Astrophys.604, A40, 2017) is thus very exciting. Given
the importance of these modes for understanding solar structure and
dynamics, the results must be robust. While Fossat et al. described
their method and parameter choices in detail, the sensitivity of their
results to several parameters was not presented. Therefore, we test the
sensitivity of the results to a selection of the parameters. The most
concerning result is that the detection vanishes when we adjust the
start time of the 16.5-year velocity time-series by a few hours. We
conclude that this reported detection of gravity modes is extremely
fragile and should be treated with utmost caution.

---------------------------------------------------------
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.

---------------------------------------------------------
Title: On-Orbit Performance of the Helioseismic and Magnetic Imager
Instrument onboard the Solar Dynamics Observatory
Authors: Hoeksema, J. T.; Baldner, C. S.; Bush, R. I.; Schou, J.;
Scherrer, P. H.
2018SoPh..293...45H Altcode: 2018arXiv180201731H
The Helioseismic and Magnetic Imager (HMI) instrument is a major
component of NASA's Solar Dynamics Observatory (SDO) spacecraft. Since
commencement of full regular science operations on 1 May 2010, HMI
has operated with remarkable continuity, e.g. during the more than
five years of the SDO prime mission that ended 30 September 2015, HMI
collected 98.4% of all possible 45-second velocity maps; minimizing gaps
in these full-disk Dopplergrams is crucial for helioseismology. HMI
velocity, intensity, and magnetic-field measurements are used in
numerous investigations, so understanding the quality of the data is
important. This article describes the calibration measurements used
to track the performance of the HMI instrument, and it details trends
in important instrument parameters during the prime mission. Regular
calibration sequences provide information used to improve and update the
calibration of HMI data. The set-point temperature of the instrument
front window and optical bench is adjusted regularly to maintain
instrument focus, and changes in the temperature-control scheme have
been made to improve stability in the observable quantities. The
exposure time has been changed to compensate for a 20% decrease in
instrument throughput. Measurements of the performance of the shutter
and tuning mechanisms show that they are aging as expected and continue
to perform according to specification. Parameters of the tunable
optical-filter elements are regularly adjusted to account for drifts
in the central wavelength. Frequent measurements of changing CCD-camera
characteristics, such as gain and flat field, are used to calibrate the
observations. Infrequent expected events such as eclipses, transits,
and spacecraft off-points interrupt regular instrument operations and
provide the opportunity to perform additional calibration. Onboard
instrument anomalies are rare and seem to occur quite uniformly in
time. The instrument continues to perform very well.

---------------------------------------------------------
Title: Global-Mode Analysis of Full-Disk Data from the Michelson
Doppler Imager and the Helioseismic and Magnetic Imager
Authors: Larson, Timothy P.; Schou, Jesper
2018SoPh..293...29L Altcode:
Building upon our previous work, in which we analyzed smoothed and
subsampled velocity data from the Michelson Doppler Imager (MDI),
we extend our analysis to unsmoothed, full-resolution MDI data. We
also present results from the Helioseismic and Magnetic Imager
(HMI), in both full resolution and processed to be a proxy for the
low-resolution MDI data. We find that the systematic errors that we saw
previously, namely peaks in both the high-latitude rotation rate and
the normalized residuals of odd a -coefficients, are almost entirely
absent in the two full-resolution analyses. Furthermore, we find that
both systematic errors seem to depend almost entirely on how the input
images are apodized, rather than on resolution or smoothing. Using the
full-resolution HMI data, we confirm our previous findings regarding the
effect of using asymmetric profiles on mode parameters, and also find
that they occasionally result in more stable fits. We also confirm
our previous findings regarding discrepancies between 360-day and
72-day analyses. We further investigate a six-month period previously
seen in f -mode frequency shifts using the low-resolution datasets,
this time accounting for solar-cycle dependence using magnetic-field
data. Both HMI and MDI saw prominent six-month signals in the frequency
shifts, but we were surprised to discover that the strongest signal
at that frequency occurred in the mode coverage for the low-resolution
proxy. Finally, a comparison of mode parameters from HMI and MDI shows
that the frequencies and a -coefficients agree closely, encouraging
the concatenation of the two datasets.

---------------------------------------------------------
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.

---------------------------------------------------------
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.

---------------------------------------------------------
Title: Interactions of Oscillations with Near Surface Convection
Authors: Schou, Jesper
2017SPD....4840101S Altcode:
There are a number of large but unexplained effects in helioseismic
observations. One is the so-called surface effect, which manifests
itself as a difference between the theoretical and observed frequencies,
and which appears to originate close to the surface. Another is a large
apparent phase shift in the oscillations depending on the center-to-limb
distance and the observable used.Both of these effects are likely due
to interactions of the waves with the near surface convection.Here
I will discuss one of the directly observable consequences of these
interactions, namely the change in the properties of the modes depending
on where in the granulation they are observed. To that end I will use
both observations from HMI and results from hydrodynamic simulations.

---------------------------------------------------------
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: Limits on radial differential rotation in Sun-like stars from
parametric fits to oscillation power spectra
Authors: Nielsen, M. B.; Schunker, H.; Gizon, L.; Schou, J.; Ball,
W. H.
2017A&A...603A...6N Altcode: 2017arXiv170510517N
Context. Rotational shear in Sun-like stars is thought to be
an important ingredient in models of stellar dynamos. Thanks to
helioseismology, rotation in the Sun is characterized well, but the
interior rotation profiles of other Sun-like stars are not so well
constrained. Until recently, measurements of rotation in Sun-like stars
have focused on the mean rotation, but little progress has been made on
measuring or even placing limits on differential rotation. Aims:
Using asteroseismic measurements of rotation we aim to constrain the
radial shear in five Sun-like stars observed by the NASA Kepler mission:
<ASTROBJ>KIC 004914923</ASTROBJ>, <ASTROBJ>KIC 005184732</ASTROBJ>,
<ASTROBJ>KIC 006116048</ASTROBJ>, <ASTROBJ>KIC 006933899</ASTROBJ>,
and <ASTROBJ>KIC 010963065</ASTROBJ>. Methods: We used stellar
structure models for these five stars from previous works. These models
provide the mass density, mode eigenfunctions, and the convection
zone depth, which we used to compute the sensitivity kernels for the
rotational frequency splitting of the modes. We used these kernels as
weights in a parametric model of the stellar rotation profile of each
star, where we allowed different rotation rates for the radiative
interior and the convective envelope. This parametric model was
incorporated into a fit to the oscillation power spectrum of each
of the five Kepler stars. This fit included a prior on the rotation
of the envelope, estimated from the rotation of surface magnetic
activity measured from the photometric variability. Results:
The asteroseismic measurements without the application of priors are
unable to place meaningful limits on the radial shear. Using a prior
on the envelope rotation enables us to constrain the interior rotation
rate and thus the radial shear. In the five cases that we studied,
the interior rotation rate does not differ from the envelope by more
than approximately ± 30%. Uncertainties in the rotational splittings
are too large to unambiguously determine the sign of the radial shear.

---------------------------------------------------------
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.

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Title: Far side Helioseismology with Solar Orbiter
Authors: Appourchaux, T.; Birch, A.; Gizon, L. C.; Löptien, B.;
Schou, J.; Solanki, S. K.; del Toro Iniesta, J. C.; Gandorfer, A.;
Hirzberger, J.; Alvarez-Herrero, A.; Woch, J. G.; Schmidt, W.
2016AGUFMSH43A2554A Altcode:
The Solar Orbiter mission, to be launched in October 2018, will
carry a suite of remote sensing and in-situ instruments, including
the Polarimetric and Helioseismic Imager (PHI). PHI will deliver
high-cadence images of the Sun in intensity and Doppler velocity
suitable for carrying out novel helioseismic studies. The orbit
of the Solar Orbiter spacecraft will reach a solar latitude up to
34 degrees by the end of the extended mission and thus will enable
the first local helioseismology studies of the polar regions. The
full range of Earth-Sun-spacecraft angles provided by the orbit will
enable helioseismology from two vantage points by combining PHI with
another instrument: stereoscopic helioseismology will allow the study
of the deep solar interior and a better understanding of the physics
of solar oscillations in both quiet Sun and sunspots. In this paper
we will review the helioseismic objectives achievable with PHI, and
will also give a short status report of the development of the Flight
Model of PHI.

---------------------------------------------------------
Title: The Efffect of Image Apodization on Global Mode Parameters
and Rotational Inversions
Authors: Larson, Tim; Schou, Jesper
2016usc..confE.127L Altcode:
It has long been known that certain systematic errors in the global mode
analysis of data from both MDI and HMI depend on how the input images
were apodized. Recently it has come to light, while investigating a
six-month period in f-mode frequencies, that mode coverage is highest
when B0 is maximal. Recalling that the leakage matrix is calculated in
the approximation that B0=0, it comes as a surprise that more modes are
fitted when the leakage matrix is most incorrect. It is now believed
that the six-month oscillation has primarily to do with what portion
of the solar surface is visible. Other systematic errors that depend
on the part of the disk used include high-latitude anomalies in the
rotation rate and a prominent feature in the normalized residuals
of odd a-coefficients. Although the most likely cause of all these
errors is errors in the leakage matrix, extensive recalculation
of the leaks has not made any difference. Thus we conjecture that
another effect may be at play, such as errors in the noise model or
one that has to do with the alignment of the apodization with the
spherical harmonics. In this poster we explore how differently shaped
apodizations affect the results of inversions for internal rotation,
for both maximal and minimal absolute values of B0.

---------------------------------------------------------
Title: Solar-cycle variation of the rotational shear near the
solar surface
Authors: Barekat, A.; Schou, J.; Gizon, L.
2016A&A...595A...8B Altcode: 2016arXiv160807101B
Context. Helioseismology has revealed that the angular velocity of the
Sun increases with depth in the outermost 35 Mm of the Sun. Recently,
we have shown that the logarithmic radial gradient (dlnΩ/dlnr) in the
upper 10 Mm is close to -1 from the equator to 60° latitude. 
Aims: We aim to measure the temporal variation of the rotational shear
over solar cycle 23 and the rising phase of cycle 24 (1996-2015). Methods: We used f mode frequency splitting data spanning 1996 to
2011 from the Michelson Doppler Imager (MDI) and 2010 to 2015 from
the Helioseismic Magnetic Imager (HMI). In a first for such studies,
the f mode frequency splitting data were obtained from 360-day time
series. We used the same method as in our previous work for measuring
dlnΩ/dlnr from the equator to 80° latitude in the outer 13 Mm of
the Sun. Then, we calculated the variation of the gradient at annual
cadence relative to the average over 1996 to 2015. Results:
We found the rotational shear at low latitudes (0° to 30°) to vary
in-phase with the solar activity, varying by ~± 10% over the period
1996 to 2015. At high latitudes (60° to 80°), we found rotational
shear to vary in anti-phase with the solar activity. By comparing
the radial gradient obtained from the splittings of the 360-day and
the corresponding 72-day time series of HMI and MDI data, we suggest
that the splittings obtained from the 72-day HMI time series suffer
from systematic errors. Conclusions: We provide a quantitative
measurement of the temporal variation of the outer part of the near
surface shear layer which may provide useful constraints on dynamo
models and differential rotation theory.

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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.

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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.

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Title: Nominal Values for Selected Solar and Planetary Quantities:
IAU 2015 Resolution B3
Authors: Prša, Andrej; Harmanec, Petr; Torres, Guillermo; Mamajek,
Eric; Asplund, Martin; Capitaine, Nicole; Christensen-Dalsgaard,
Jørgen; Depagne, Éric; Haberreiter, Margit; Hekker, Saskia; Hilton,
James; Kopp, Greg; Kostov, Veselin; Kurtz, Donald W.; Laskar, Jacques;
Mason, Brian D.; Milone, Eugene F.; Montgomery, Michele; Richards,
Mercedes; Schmutz, Werner; Schou, Jesper; Stewart, Susan G.
2016AJ....152...41P Altcode: 2016arXiv160509788P
In this brief communication we provide the rationale for and the
outcome of the International Astronomical Union (IAU) resolution
vote at the XXIXth General Assembly in Honolulu, Hawaii, in 2015,
on recommended nominal conversion constants for selected solar and
planetary properties. The problem addressed by the resolution is a lack
of established conversion constants between solar and planetary values
and SI units: a missing standard has caused a proliferation of solar
values (e.g., solar radius, solar irradiance, solar luminosity, solar
effective temperature, and solar mass parameter) in the literature,
with cited solar values typically based on best estimates at the time
of paper writing. As precision of observations increases, a set of
consistent values becomes increasingly important. To address this, an
IAU Working Group on Nominal Units for Stellar and Planetary Astronomy
formed in 2011, uniting experts from the solar, stellar, planetary,
exoplanetary, and fundamental astronomy, as well as from general
standards fields to converge on optimal values for nominal conversion
constants. The effort resulted in the IAU 2015 Resolution B3, passed at
the IAU General Assembly by a large majority. The resolution recommends
the use of nominal solar and planetary values, which are by definition
exact and are expressed in SI units. These nominal values should be
understood as conversion factors only, not as the true solar/planetary
properties or current best estimates. Authors and journal editors are
urged to join in using the standard values set forth by this resolution
in future work and publications to help minimize further confusion.

---------------------------------------------------------
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.

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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: 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.

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Title: Asteroseismic inversions for radial differential rotation of
Sun-like stars: ensemble fits
Authors: Schunker, H.; Schou, J.; Ball, W. H.; Nielsen, M. B.;
Gizon, L.
2016A&A...586A..79S Altcode: 2015arXiv151207169S
Context. Radial differential rotation is an important parameter
for stellar dynamo theory and for understanding angular momentum
transport. Aims: We investigate the potential of using a large
number of similar stars simultaneously to constrain their average radial
differential rotation gradient: we call this "ensemble fitting". Methods: We use a range of stellar models along the main sequence,
each with a synthetic rotation profile. The rotation profiles are step
functions with a step of ΔΩ = -0.35 μHz, which is located at the base
of the convection zone. These models are used to compute the rotational
splittings of the p modes and to model their uncertainties. We then
fit an ensemble of stars to infer the average ΔΩ. Results: All
the uncertainties on the inferred ΔΩ for individual stars are of the
order 1 μHz. Using 15 stellar models in an ensemble fit, we show that
the uncertainty on the average ΔΩ is reduced to less than the input
ΔΩ, which allows us to constrain the sign of the radial differential
rotation. We show that a solar-like ΔΩ ≈ 30 nHz can be constrained
by an ensemble fit of thousands of main-sequence stars. Observing the
number of stars required to successfully exploit the ensemble fitting
method will be possible with future asteroseismology missions, such
as PLATO. We demonstrate the potential of ensemble fitting by showing
that any systematic differences in the average ΔΩ between F, G,
and K-type stars larger than 100 nHz could be detected.

---------------------------------------------------------
Title: Asteroseismic inversions for radial differential rotation of
Sun-like stars: Sensitivity to uncertainties
Authors: Schunker, H.; Schou, J.; Ball, W. H.
2016A&A...586A..24S Altcode: 2015arXiv151108365S
 Aims: We quantify the effect of observational spectroscopic
and asteroseismic uncertainties on regularised least squares (RLS)
inversions for the radial differential rotation of Sun-like and subgiant
stars. Methods: We first solved the forward problem to model
rotational splittings plus the observed uncertainties for models
of a Sun-like star, HD 52265, and a subgiant star, KIC 7341231. We
randomly perturbed the parameters of the stellar models within the
uncertainties of the spectroscopic and asteroseismic constraints and
used these perturbed stellar models to compute rotational splittings. We
experimented with three rotation profiles: solid body rotation, a step
function, and a smooth rotation profile decreasing with radius. We
then solved the inverse problem to infer the radial differential
rotation profile using a RLS inversion and kernels from the best-fit
stellar model. We also compared RLS, optimally localised average (OLA)
and direct functional fitting inversion techniques. Results:
We found that the inversions for Sun-like stars with solar-like radial
differential rotation profiles are insensitive to the uncertainties
in the stellar models. The uncertainties in the splittings dominate
the uncertainties in the inversions and solid body rotation is not
excluded. We found that when the rotation rate below the convection
zone is increased to six times that of the surface rotation rate the
inferred rotation profile excluded solid body rotation. We showed
that when we reduced the uncertainties in the splittings by a factor
of about 100, the inversion is sensitive to the uncertainties in the
stellar model. With the current observational uncertainties, we found
that inversions of subgiant stars are sensitive to the uncertainties
in the stellar model. Conclusions: Our findings suggest that
inversions for the radial differential rotation of subgiant stars would
benefit from more tightly constrained stellar models. We conclude that
current observational uncertainties make it difficult to infer radially
resolved features of the rotation profile in a Sun-like star using
inversions with regularisation. In Sun-like stars, the insensitivity
of the inversions to stellar model uncertainties suggests that it
may be possible to perform ensemble inversions for the average radial
differential rotation of many stars with a range of stellar types to
better constrain the inversions.

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Title: Solar Dynamics, Rotation, Convection and Overshoot
Authors: Hanasoge, S.; Miesch, M. S.; Roth, M.; Schou, J.; Schüssler,
M.; Thompson, M. J.
2015SSRv..196...79H Altcode: 2015SSRv..tmp...24H; 2015arXiv150308539H
We discuss recent observational, theoretical and modeling progress
made in understanding the Sun's internal dynamics, including its
rotation, meridional flow, convection and overshoot. Over the past
few decades, substantial theoretical and observational effort has
gone into appreciating these aspects of solar dynamics. A review of
these observations, related helioseismic methodology and inference and
computational results in relation to these problems is undertaken here.

---------------------------------------------------------
Title: Helioseismology with Solar Orbiter
Authors: Löptien, Björn; Birch, Aaron C.; Gizon, Laurent; Schou,
Jesper; Appourchaux, Thierry; Blanco Rodríguez, Julián; Cally,
Paul S.; Dominguez-Tagle, Carlos; Gandorfer, Achim; Hill, Frank;
Hirzberger, Johann; Scherrer, Philip H.; Solanki, Sami K.
2015SSRv..196..251L Altcode: 2014arXiv1406.5435L; 2014SSRv..tmp...31L
The Solar Orbiter mission, to be launched in July 2017, will
carry a suite of remote sensing and in-situ instruments, including
the Polarimetric and Helioseismic Imager (PHI). PHI will deliver
high-cadence images of the Sun in intensity and Doppler velocity
suitable for carrying out novel helioseismic studies. The orbit of
the Solar Orbiter spacecraft will reach a solar latitude of up to
21∘ (up to 34∘ by the end of the extended
mission) and thus will enable the first local helioseismology studies of
the polar regions. Here we consider an array of science objectives to be
addressed by helioseismology within the baseline telemetry allocation
(51 Gbit per orbit, current baseline) and within the science observing
windows (baseline 3×10 days per orbit). A particularly important
objective is the measurement of large-scale flows at high latitudes
(rotation and meridional flow), which are largely unknown but play an
important role in flux transport dynamos. For both helioseismology
and feature tracking methods convection is a source of noise in
the measurement of longitudinally averaged large-scale flows, which
decreases as T -1/2 where T is the total duration of the
observations. Therefore, the detection of small amplitude signals (e.g.,
meridional circulation, flows in the deep solar interior) requires long
observation times. As an example, one hundred days of observations at
lower spatial resolution would provide a noise level of about three m/s
on the meridional flow at 80∘ latitude. Longer time-series
are also needed to study temporal variations with the solar cycle. The
full range of Earth-Sun-spacecraft angles provided by the orbit will
enable helioseismology from two vantage points by combining PHI with
another instrument: stereoscopic helioseismology will allow the study
of the deep solar interior and a better understanding of the physics
of solar oscillations in both quiet Sun and sunspots. We have used a
model of the PHI instrument to study its performance for helioseismology
applications. As input we used a 6 hr time-series of realistic solar
magneto-convection simulation (Stagger code) and the SPINOR radiative
transfer code to synthesize the observables. The simulated power
spectra of solar oscillations show that the instrument is suitable for
helioseismology. In particular, the specified point spread function,
image jitter, and photon noise are no obstacle to a successful mission.

---------------------------------------------------------
Title: Improved Helioseismic Analysis of Medium-ℓ Data from the
Michelson Doppler Imager
Authors: Larson, Timothy P.; Schou, Jesper
2015SoPh..290.3221L Altcode: 2015SoPh..tmp..175L; 2015arXiv151105217L
We present a comprehensive study of one method for measuring various
parameters of global modes of oscillation of the Sun. Using velocity
data taken by the Michelson Doppler Imager (MDI), we analyze spherical
harmonic degrees ℓ ≤300 . Both current and historical methodologies
are explained, and the various differences between the two are
investigated to determine their effects on global-mode parameters
and systematic errors in the analysis. These differences include
a number of geometric corrections made during spherical harmonic
decomposition; updated routines for generating window functions,
detrending time series, and filling gaps; and consideration of physical
effects such as mode-profile asymmetry, horizontal displacement at
the solar surface, and distortion of eigenfunctions by differential
rotation. We apply these changes one by one to three years of data, and
then reanalyze the entire MDI mission applying all of them, using both
the original 72-day long time series and 360-day long time series. We
find significant changes in mode parameters, both as a result of the
various changes to the processing, as well as between the 72-day and
360-day analyses. We find reduced residuals of inversions for internal
rotation, but seeming artifacts remain, such as the peak in the rotation
rate near the surface at high latitudes. An annual periodicity in the
f -mode frequencies is also investigated.

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Title: IAU 2015 Resolution B2 on Recommended Zero Points for the
Absolute and Apparent Bolometric Magnitude Scales
Authors: Mamajek, E. E.; Torres, G.; Prsa, A.; Harmanec, P.;
Asplund, M.; Bennett, P. D.; Capitaine, N.; Christensen-Dalsgaard,
J.; Depagne, E.; Folkner, W. M.; Haberreiter, M.; Hekker, S.; Hilton,
J. L.; Kostov, V.; Kurtz, D. W.; Laskar, J.; Mason, B. D.; Milone,
E. F.; Montgomery, M. M.; Richards, M. T.; Schou, J.; Stewart, S. G.
2015arXiv151006262M Altcode:
The XXIXth IAU General Assembly in Honolulu adopted IAU 2015 Resolution
B2 on recommended zero points for the absolute and apparent bolometric
magnitude scales. The resolution was proposed by the IAU Inter-Division
A-G Working Group on Nominal Units for Stellar and Planetary
Astronomy after consulting with a broad spectrum of researchers from
the astronomical community. Resolution B2 resolves the long-standing
absence of an internationally-adopted zero point for the absolute and
apparent bolometric magnitude scales. Resolution B2 defines the zero
point of the absolute bolometric magnitude scale such that a radiation
source with $M_{\rm Bol}$ = 0 has luminosity L$_{\circ}$ = 3.0128e28
W. The zero point of the apparent bolometric magnitude scale ($m_{\rm
Bol}$ = 0) corresponds to irradiance $f_{\circ}$ = 2.518021002e-8
W/m$^2$. The zero points were chosen so that the nominal solar
luminosity (3.828e26 W) adopted by IAU 2015 Resolution B3 corresponds
approximately to $M_{\rm Bol}$(Sun) = 4.74, the value most commonly
adopted in recent literature. The nominal total solar irradiance (1361
W/m$^2$) adopted in IAU 2015 Resolution B3 corresponds approximately to
apparent bolometric magnitude $m_{\rm bol}$(Sun) = -26.832. Implicit
in the IAU 2015 Resolution B2 definition of the apparent bolometric
magnitude scale is an exact definition for the parsec (648000/$\pi$ au)
based on the IAU 2012 Resolution B2 definition of the astronomical unit.

---------------------------------------------------------
Title: IAU 2015 Resolution B3 on Recommended Nominal Conversion
Constants for Selected Solar and Planetary Properties
Authors: Mamajek, E. E.; Prsa, A.; Torres, G.; Harmanec, P.;
Asplund, M.; Bennett, P. D.; Capitaine, N.; Christensen-Dalsgaard,
J.; Depagne, E.; Folkner, W. M.; Haberreiter, M.; Hekker, S.; Hilton,
J. L.; Kostov, V.; Kurtz, D. W.; Laskar, J.; Mason, B. D.; Milone,
E. F.; Montgomery, M. M.; Richards, M. T.; Schou, J.; Stewart, S. G.
2015arXiv151007674M Altcode:
Astronomers commonly quote the properties of celestial objects in
units of parameters for the Sun, Jupiter, or the Earth. The resolution
presented here was proposed by the IAU Inter-Division Working Group
on Nominal Units for Stellar and Planetary Astronomy and passed by the
XXIXth IAU General Assembly in Honolulu. IAU 2015 Resolution B3 adopts
a set of nominal solar, terrestrial, and jovian conversion constants
for stellar and (exo)planetary astronomy which are defined to be
exact SI values. While the nominal constants are based on current best
estimates (CBEs; which have uncertainties, are not secularly constant,
and are updated regularly using new observations), they should be
interpreted as standard values and not as CBEs. IAU 2015 Resolution
B3 adopts five solar conversion constants (nominal solar radius,
nominal total solar irradiance, nominal solar luminosity, nominal
solar effective temperature, and nominal solar mass parameter) and six
planetary conversion constants (nominal terrestrial equatorial radius,
nominal terrestrial polar radius, nominal jovian equatorial radius,
nominal jovian polar radius, nominal terrestrial mass parameter,
and nominal jovian mass parameter).

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Title: Effects of granulation on the visibility of solar oscillations
Authors: Schou, J.
2015A&A...580L..11S Altcode: 2015arXiv150708856S
Context. The interaction of solar oscillations with near surface
convection is poorly understood. These interactions are likely the
cause of several problems in helio- and astero-seismology, including the
so-called surface effect and apparently unphysical travel time shifts
as a function of center to limb distance. There is thus a clear need
for further theoretical understanding and observational tests. Aims: The aim is to determine how the observed modes are affected
by the convection. Methods: I used HMI velocity and intensity
images to construct k-ω diagrams showing how the oscillation amplitude
and phase depend on the local granulation intensity. Results:
There is a clear and significant dependence of the observed properties
of the oscillations on the local convection state.

---------------------------------------------------------
Title: A Method for the Estimation of p-Mode Parameters from Averaged
Solar Oscillation Power Spectra
Authors: Reiter, J.; Rhodes, E. J., Jr.; Kosovichev, A. G.; Schou,
J.; Scherrer, P. H.; Larson, T. P.
2015ApJ...803...92R Altcode: 2015arXiv150407493R
A new fitting methodology is presented that is equally well suited for
the estimation of low-, medium-, and high-degree mode parameters from
m-averaged solar oscillation power spectra of widely differing spectral
resolution. This method, which we call the “Windowed, MuLTiple-Peak,
averaged-spectrum” or WMLTP Method, constructs a theoretical profile
by convolving the weighted sum of the profiles of the modes appearing
in the fitting box with the power spectrum of the window function of
the observing run, using weights from a leakage matrix that takes into
account observational and physical effects, such as the distortion of
modes by solar latitudinal differential rotation. We demonstrate that
the WMLTP Method makes substantial improvements in the inferences of
the properties of the solar oscillations in comparison with a previous
method, which employed a single profile to represent each spectral
peak. We also present an inversion for the internal solar structure,
which is based upon 6366 modes that we computed using the WMLTP method
on the 66 day 2010 Solar and Heliospheric Observatory/MDI Dynamics
Run. To improve both the numerical stability and reliability of the
inversion, we developed a new procedure for the identification and
correction of outliers in a frequency dataset. We present evidence
for a pronounced departure of the sound speed in the outer half of the
solar convection zone and in the subsurface shear layer from the radial
sound speed profile contained in Model S of Christensen-Dalsgaard and
his collaborators that existed in the rising phase of Solar Cycle 24
during mid-2010.

---------------------------------------------------------
Title: Temporal evolution of the solar torsional oscillation and
implications for cycle 25
Authors: Hill, Frank; Howe, Rachel; Komm, Rudolf; Schou, Jesper;
Thompson, Michael; Larson, Timothy
2015TESS....110502H Altcode:
The zonal flow known as the torsional oscillation has been observed
on the Sun’s surface since 1980 and in its interior since 1995. It
has two branches that migrate during the solar cycle, with one moving
towards the equator and the other towards the poles. The rate at which
these branches migrate in latitude is tightly correlated with the
timing of the solar cycle, as seen during the long minimum between
cycles 23 and 24. The poleward branch generally becomes visible 10
to 12 years before the appearance of the magnetic activity associated
with the corresponding sunspot cycle as it did for the current cycle
24. However, the poleward flow for cycle 25, which was expected to
appear in 2008-2010, was not observed. Subsequent analysis showed
that it is a very weak flow, and is masked by an apparent change
in the background solar differential rotation rate. We will present
the latest observations of the zonal flow as determined from global
helioseismology, and will discuss the implications for the strength
and timing of cycle 25.

---------------------------------------------------------
Title: Image compression in local helioseismology
Authors: Löptien, B.; Birch, A. C.; Gizon, L.; Schou, J.
2014A&A...571A..42L Altcode: 2014arXiv1409.4176L
Context. Several upcoming helioseismology space missions are
very limited in telemetry and will have to perform extensive data
compression. This requires the development of new methods of data
compression. Aims: We give an overview of the influence of lossy
data compression on local helioseismology. We investigate the effects
of several lossy compression methods (quantization, JPEG compression,
and smoothing and subsampling) on power spectra and time-distance
measurements of supergranulation flows at disk center. Methods:
We applied different compression methods to tracked and remapped
Dopplergrams obtained by the Helioseismic and Magnetic Imager onboard
the Solar Dynamics Observatory. We determined the signal-to-noise
ratio of the travel times computed from the compressed data as a
function of the compression efficiency. Results: The basic
helioseismic measurements that we consider are very robust to lossy
data compression. Even if only the sign of the velocity is used,
time-distance helioseismology is still possible. We achieve the
best results by applying JPEG compression on spatially subsampled
data. However, our conclusions are only valid for supergranulation
flows at disk center and may not be valid for all helioseismology
applications.

---------------------------------------------------------
Title: The PLATO 2.0 mission
Authors: Rauer, H.; Catala, C.; Aerts, C.; Appourchaux, T.; Benz,
W.; Brandeker, A.; Christensen-Dalsgaard, J.; Deleuil, M.; Gizon,
L.; Goupil, M. -J.; Güdel, M.; Janot-Pacheco, E.; Mas-Hesse,
M.; Pagano, I.; Piotto, G.; Pollacco, D.; Santos, Ċ.; Smith, A.;
Suárez, J. -C.; Szabó, R.; Udry, S.; Adibekyan, V.; Alibert, Y.;
Almenara, J. -M.; Amaro-Seoane, P.; Eiff, M. Ammler-von; Asplund, M.;
Antonello, E.; Barnes, S.; Baudin, F.; Belkacem, K.; Bergemann, M.;
Bihain, G.; Birch, A. C.; Bonfils, X.; Boisse, I.; Bonomo, A. S.;
Borsa, F.; Brandão, I. M.; Brocato, E.; Brun, S.; Burleigh, M.;
Burston, R.; Cabrera, J.; Cassisi, S.; Chaplin, W.; Charpinet, S.;
Chiappini, C.; Church, R. P.; Csizmadia, Sz.; Cunha, M.; Damasso, M.;
Davies, M. B.; Deeg, H. J.; Díaz, R. F.; Dreizler, S.; Dreyer, C.;
Eggenberger, P.; Ehrenreich, D.; Eigmüller, P.; Erikson, A.; Farmer,
R.; Feltzing, S.; de Oliveira Fialho, F.; Figueira, P.; Forveille,
T.; Fridlund, M.; García, R. A.; Giommi, P.; Giuffrida, G.; Godolt,
M.; Gomes da Silva, J.; Granzer, T.; Grenfell, J. L.; Grotsch-Noels,
A.; Günther, E.; Haswell, C. A.; Hatzes, A. P.; Hébrard, G.; Hekker,
S.; Helled, R.; Heng, K.; Jenkins, J. M.; Johansen, A.; Khodachenko,
M. L.; Kislyakova, K. G.; Kley, W.; Kolb, U.; Krivova, N.; Kupka, F.;
Lammer, H.; Lanza, A. F.; Lebreton, Y.; Magrin, D.; Marcos-Arenal,
P.; Marrese, P. M.; Marques, J. P.; Martins, J.; Mathis, S.; Mathur,
S.; Messina, S.; Miglio, A.; Montalban, J.; Montalto, M.; Monteiro,
M. J. P. F. G.; Moradi, H.; Moravveji, E.; Mordasini, C.; Morel, T.;
Mortier, A.; Nascimbeni, V.; Nelson, R. P.; Nielsen, M. B.; Noack,
L.; Norton, A. J.; Ofir, A.; Oshagh, M.; Ouazzani, R. -M.; Pápics,
P.; Parro, V. C.; Petit, P.; Plez, B.; Poretti, E.; Quirrenbach, A.;
Ragazzoni, R.; Raimondo, G.; Rainer, M.; Reese, D. R.; Redmer, R.;
Reffert, S.; Rojas-Ayala, B.; Roxburgh, I. W.; Salmon, S.; Santerne,
A.; Schneider, J.; Schou, J.; Schuh, S.; Schunker, H.; Silva-Valio,
A.; Silvotti, R.; Skillen, I.; Snellen, I.; Sohl, F.; Sousa, S. G.;
Sozzetti, A.; Stello, D.; Strassmeier, K. G.; Švanda, M.; Szabó,
Gy. M.; Tkachenko, A.; Valencia, D.; Van Grootel, V.; Vauclair,
S. D.; Ventura, P.; Wagner, F. W.; Walton, N. A.; Weingrill, J.;
Werner, S. C.; Wheatley, P. J.; Zwintz, K.
2014ExA....38..249R Altcode: 2014ExA...tmp...41R; 2013arXiv1310.0696R
PLATO 2.0 has recently been selected for ESA's M3 launch opportunity
(2022/24). Providing accurate key planet parameters (radius, mass,
density and age) in statistical numbers, it addresses fundamental
questions such as: How do planetary systems form and evolve? Are there
other systems with planets like ours, including potentially habitable
planets? The PLATO 2.0 instrument consists of 34 small aperture
telescopes (32 with 25 s readout cadence and 2 with 2.5 s candence)
providing a wide field-of-view (2232 deg 2) and a large
photometric magnitude range (4-16 mag). It focusses on bright (4-11
mag) stars in wide fields to detect and characterize planets down to
Earth-size by photometric transits, whose masses can then be determined
by ground-based radial-velocity follow-up measurements. Asteroseismology
will be performed for these bright stars to obtain highly accurate
stellar parameters, including masses and ages. The combination of
bright targets and asteroseismology results in high accuracy for
the bulk planet parameters: 2 %, 4-10 % and 10 % for planet radii,
masses and ages, respectively. The planned baseline observing strategy
includes two long pointings (2-3 years) to detect and bulk characterize
planets reaching into the habitable zone (HZ) of solar-like stars
and an additional step-and-stare phase to cover in total about 50 %
of the sky. PLATO 2.0 will observe up to 1,000,000 stars and detect
and characterize hundreds of small planets, and thousands of planets
in the Neptune to gas giant regime out to the HZ. It will therefore
provide the first large-scale catalogue of bulk characterized planets
with accurate radii, masses, mean densities and ages. This catalogue
will include terrestrial planets at intermediate orbital distances,
where surface temperatures are moderate. Coverage of this parameter
range with statistical numbers of bulk characterized planets is unique
to PLATO 2.0. The PLATO 2.0 catalogue allows us to e.g.: - complete
our knowledge of planet diversity for low-mass objects, - correlate the
planet mean density-orbital distance distribution with predictions from
planet formation theories,- constrain the influence of planet migration
and scattering on the architecture of multiple systems, and - specify
how planet and system parameters change with host star characteristics,
such as type, metallicity and age. The catalogue will allow us to study
planets and planetary systems at different evolutionary phases. It
will further provide a census for small, low-mass planets. This will
serve to identify objects which retained their primordial hydrogen
atmosphere and in general the typical characteristics of planets
in such low-mass, low-density range. Planets detected by PLATO 2.0
will orbit bright stars and many of them will be targets for future
atmosphere spectroscopy exploring their atmosphere. Furthermore,
the mission has the potential to detect exomoons, planetary rings,
binary and Trojan planets. The planetary science possible with PLATO
2.0 is complemented by its impact on stellar and galactic science via
asteroseismology as well as light curves of all kinds of variable stars,
together with observations of stellar clusters of different ages. This
will allow us to improve stellar models and study stellar activity. A
large number of well-known ages from red giant stars will probe the
structure and evolution of our Galaxy. Asteroseismic ages of bright
stars for different phases of stellar evolution allow calibrating
stellar age-rotation relationships. Together with the results of ESA's
Gaia mission, the results of PLATO 2.0 will provide a huge legacy to
planetary, stellar and galactic science.

---------------------------------------------------------
Title: The radial gradient of the near-surface shear layer of the Sun
Authors: Barekat, A.; Schou, J.; Gizon, L.
2014A&A...570L..12B Altcode: 2014arXiv1410.3162B
Context. Helioseismology has provided unprecedented information about
the internal rotation of the Sun. One of the important achievements was
the discovery of two radial shear layers: one near the bottom of the
convection zone (the tachocline) and one near the surface. These shear
layers may be important ingredients for explaining the magnetic cycle of
the Sun. Aims: We measure the logarithmic radial gradient of the
rotation rate (dlnΩ/dlnr) near the surface of the Sun using 15 years of
f mode rotational frequency splittings from the Michelson Doppler Imager
(MDI) and four years of data from the Helioseismic and Magnetic Imager
(HMI). Methods: We model the angular velocity of the Sun in the
upper ~10 Mm as changing linearly with depth and use a multiplicative
optimally localized averaging inversion to infer the gradient of the
rotation rate as a function of latitude. Results: Both the MDI
and HMI data show that dlnΩ/dlnr is close to -1 from the equator
to 60° latitude and stays negative up to 75° latitude. However,
the value of the gradient is different for MDI and HMI for latitudes
above 60°. Additionally, there is a significant difference between
the value of dlnΩ/dlnr using an older and recently reprocessed MDI
data for latitudes above 30°. Conclusions: We could reliably
infer the value of dlnΩ/dlnr up to 60°, but not above this latitude,
which will hopefully constrain theories of the near-surface shear
layer and dynamo. Furthermore, the recently reprocessed MDI splitting
data are more reliable than the older versions which contained clear
systematic errors in the high degree f modes.

---------------------------------------------------------
Title: The Helioseismic and Magnetic Imager (HMI) Vector Magnetic
Field Pipeline: Overview and Performance
Authors: Hoeksema, J. Todd; Liu, Yang; Hayashi, Keiji; Sun, Xudong;
Schou, Jesper; Couvidat, Sebastien; Norton, Aimee; Bobra, Monica;
Centeno, Rebecca; Leka, K. D.; Barnes, Graham; Turmon, Michael
2014SoPh..289.3483H Altcode: 2014SoPh..tmp...57H; 2014arXiv1404.1881H
The Helioseismic and Magnetic Imager (HMI) began near-continuous
full-disk solar measurements on 1 May 2010 from the Solar Dynamics
Observatory (SDO). An automated processing pipeline keeps pace
with observations to produce observable quantities, including the
photospheric vector magnetic field, from sequences of filtergrams. The
basic vector-field frame list cadence is 135 seconds, but to reduce
noise the filtergrams are combined to derive data products every 720
seconds. The primary 720 s observables were released in mid-2010,
including Stokes polarization parameters measured at six wavelengths,
as well as intensity, Doppler velocity, and the line-of-sight magnetic
field. More advanced products, including the full vector magnetic field,
are now available. Automatically identified HMI Active Region Patches
(HARPs) track the location and shape of magnetic regions throughout
their lifetime.

---------------------------------------------------------
Title: The Helioseismic and Magnetic Imager (HMI) Vector Magnetic
Field Pipeline: Optimization of the Spectral Line Inversion Code
Authors: Centeno, R.; Schou, J.; Hayashi, K.; Norton, A.; Hoeksema,
J. T.; Liu, Y.; Leka, K. D.; Barnes, G.
2014SoPh..289.3531C Altcode: 2014SoPh..tmp...44C; 2014arXiv1403.3677C
The Very Fast Inversion of the Stokes Vector (VFISV) is a
Milne-Eddington spectral line inversion code used to determine the
magnetic and thermodynamic parameters of the solar photosphere from
observations of the Stokes vector in the 6173 Å Fe I line by the
Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO). We report on the modifications made to the original
VFISV inversion code in order to optimize its operation within
the HMI data pipeline and provide the smoothest solution in active
regions. The changes either sped up the computation or reduced the
frequency with which the algorithm failed to converge to a satisfactory
solution. Additionally, coding bugs which were detected and fixed in
the original VFISV release are reported here.

---------------------------------------------------------
Title: Rotational splitting as a function of mode frequency for six
Sun-like stars
Authors: Nielsen, M. B.; Gizon, L.; Schunker, H.; Schou, J.
2014A&A...568L..12N Altcode: 2014arXiv1408.4307N
Asteroseismology offers the prospect of constraining differential
rotation in Sun-like stars. Here we have identified six high
signal-to-noise main-sequence Sun-like stars in the Kepler field,
which all have visible signs of rotational splitting of their p-mode
frequencies. For each star, we extract the rotational frequency
splitting and inclination angle from separate mode sets (adjacent modes
with l = 2, 0, and 1) spanning the p-mode envelope. We use a Markov
chain Monte Carlo method to obtain the best fit and errors associated
with each parameter. We are able to make independent measurements of
rotational splittings of ~8 radial orders for each star. For all six
stars, the measured splittings are consistent with uniform rotation,
allowing us to exclude large radial differential rotation. This work
opens the possibility of constraining internal rotation of Sun-like
stars. Table 2 is 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/568/L12">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/568/L12</A>

---------------------------------------------------------
Title: VizieR Online Data Catalog: Rotational frequency splitting
in Sun-like stars (Nielsen+, 2014)
Authors: Nielsen, M. B.; Gizon, L.; Schunker, H.; Schou, J.
2014yCat..35689012N Altcode:
We used short-cadence (~58s) white light observations from the NASA
Kepler mission from March 2009 until the end of the mission in early
2013. The data were obtained from the Mikulski Archive for Space
Telescopes. We fit the power spectrum with a model consisting
of a constant noise level, two frequency-dependent Harvey-like noise
terms (see Eq. (1) in Aigrain et al. 2004A&amp;A...414.1139A), in
addition to the individual oscillation modes. We model these as a
sum of Lorentzian profiles as per Eq. (10) in Handberg &amp; Campante
(2011A&amp;A...527A..56H), each consisting of mode power, frequency,
and linewidth. (2 data files).

---------------------------------------------------------
Title: Helioseismic and Magnetic Imager Observations of Linear
Polarization from a Loop Prominence System
Authors: Saint-Hilaire, Pascal; Schou, Jesper; Martinez Oliveros, Juan
Carlos; Hudson, Hugh S.; Krucker, Sam; Bain, Hazel; Couvidat, Sebastien
2014AAS...22412311S Altcode:
White-light observations by the Solar Dynamics Observatory's
Helioseismic and Magnetic Imager of a loop-prominence system occurring
in the aftermath of an X-class flare on 2013 May 13 near the eastern
solar limb show a linearly polarized component, reaching up to 20%
at an altitude of 33 Mm, about the maximal amount expected if the
emission were due solely to Thomson scattering of photospheric light
by the coronal material. The mass associated with the polarized
component was 8.2x10^14 g. At 15 Mm altitude, the brightest part of
the loop was 3(+/-0.5)% linearly polarized, only about 20% of that
expected from pure Thomson scattering, indicating the presence of an
additional unpolarized component at wavelengths near Fe I (617.33 nm),
probably thermal emission. We estimated the free electron density of
the white-light loop system to possibly be as high as 1.8x10^12 cm^-3.

---------------------------------------------------------
Title: Observations of Linear Polarization in a Solar Coronal Loop
Prominence System Observed near 6173 Å
Authors: Saint-Hilaire, Pascal; Schou, Jesper; Martínez Oliveros,
Juan-Carlos; Hudson, Hugh S.; Krucker, Säm; Bain, Hazel; Couvidat,
Sébastien
2014ApJ...786L..19S Altcode: 2014arXiv1402.7016S
White-light observations by the Solar Dynamics Observatory's
Helioseismic and Magnetic Imager of a loop-prominence system occurring
in the aftermath of an X-class flare on 2013 May 13 near the eastern
solar limb show a linearly polarized component, reaching up to ~20%
at an altitude of ~33 Mm, about the maximum amount expected if the
emission were due solely to Thomson scattering of photospheric light by
the coronal material. The mass associated with the polarized component
was 8.2 × 1014 g. At 15 Mm altitude, the brightest part
of the loop was 3(±0.5)% linearly polarized, only about 20% of that
expected from pure Thomson scattering, indicating the presence of an
additional unpolarized component at wavelengths near Fe I (617.33
nm). We estimate the free electron density of the white-light loop
system to possibly be as high as 1.8 × 1012 cm-3.

---------------------------------------------------------
Title: Seismic constraints on the radial dependence of the internal
rotation profiles of six Kepler subgiants and young red giants
Authors: Deheuvels, S.; Doğan, G.; Goupil, M. J.; Appourchaux, T.;
Benomar, O.; Bruntt, H.; Campante, T. L.; Casagrande, L.; Ceillier,
T.; Davies, G. R.; De Cat, P.; Fu, J. N.; García, R. A.; Lobel,
A.; Mosser, B.; Reese, D. R.; Regulo, C.; Schou, J.; Stahn, T.;
Thygesen, A. O.; Yang, X. H.; Chaplin, W. J.; Christensen-Dalsgaard,
J.; Eggenberger, P.; Gizon, L.; Mathis, S.; Molenda-Żakowicz, J.;
Pinsonneault, M.
2014A&A...564A..27D Altcode: 2014arXiv1401.3096D
Context. We still do not understand which physical mechanisms are
responsible for the transport of angular momentum inside stars. The
recent detection of mixed modes that contain the clear signature of
rotation in the spectra of Kepler subgiants and red giants gives us
the opportunity to make progress on this question. Aims: Our
aim is to probe the radial dependence of the rotation profiles for a
sample of Kepler targets. For this purpose, subgiants and early red
giants are particularly interesting targets because their rotational
splittings are more sensitive to the rotation outside the deeper core
than is the case for their more evolved counterparts. Methods:
We first extracted the rotational splittings and frequencies of the
modes for six young Kepler red giants. We then performed a seismic
modeling of these stars using the evolutionary codes Cesam2k and
astec. By using the observed splittings and the rotational kernels
of the optimal models, we inverted the internal rotation profiles
of the six stars. Results: We obtain estimates of the core
rotation rates for these stars, and upper limits to the rotation in
their convective envelope. We show that the rotation contrast between
the core and the envelope increases during the subgiant branch. Our
results also suggest that the core of subgiants spins up with time,
while their envelope spins down. For two of the stars, we show that a
discontinuous rotation profile with a deep discontinuity reproduces
the observed splittings significantly better than a smooth rotation
profile. Interestingly, the depths that are found to be most probable
for the discontinuities roughly coincide with the location of the
H-burning shell, which separates the layers that contract from those
that expand. Conclusions: We characterized the differential
rotation pattern of six young giants with a range of metallicities, and
with both radiative and convective cores on the main sequence. This will
bring observational constraints to the scenarios of angular momentum
transport in stars. Moreover, if the existence of sharp gradients in
the rotation profiles of young red giants is confirmed, it is expected
to help in distinguishing between the physical processes that could
transport angular momentum in the subgiant and red giant branches. Appendices and Tables 3-9 are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201322779/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: On the information content of stellar spectra
Authors: Schou, Jesper
2014IAUS..301..481S Altcode: 2013arXiv1309.7871S
With the increasing quality of asteroseismic observations it is
important to minimize the random and systematic errors in mode
parameter estimates. To this end it is important to understand how
the oscillations relate to the directly observed quantities, such as
intensities and spectra, and to derived quantities, such as Doppler
velocity. Here I list some of the effects we need to take into account
and show an example of the impact of some of them.

---------------------------------------------------------
Title: Precise and accurate interpolated stellar oscillation
frequencies on the main sequence
Authors: Ball, Warrick H.; Schou, Jesper; Gizon, Laurent; Marques,
João P. C.
2014IAUS..301..379B Altcode: 2013arXiv1309.3044B
High-quality data from space-based observatories present an opportunity
to fit stellar models to observations of individually-identified
oscillation frequencies, not just the large and small frequency
separations. But such fits require the evaluation of a large number of
accurate stellar models, which remains expensive. Here, we show that
global-mode oscillation frequencies interpolated in a grid of stellar
models are precise and accurate, at least in the neighbourhood of a
solar model.

---------------------------------------------------------
Title: Chromospheric and Coronal Observations of Solar Flares with
the Helioseismic and Magnetic Imager
Authors: Martínez Oliveros, Juan-Carlos; Krucker, Säm; Hudson, Hugh
S.; Saint-Hilaire, Pascal; Bain, Hazel; Lindsey, Charles; Bogart,
Rick; Couvidat, Sebastien; Scherrer, Phil; Schou, Jesper
2014ApJ...780L..28M Altcode: 2013arXiv1311.7412M
We report observations of white-light ejecta in the low corona, for
two X-class flares on 2013 May 13, using data from the Helioseismic
and Magnetic Imager (HMI) of the Solar Dynamics Observatory. At least
two distinct kinds of sources appeared (chromospheric and coronal),
in the early and later phases of flare development, in addition to
the white-light footpoint sources commonly observed in the lower
atmosphere. The gradual emissions have a clear identification
with the classical loop-prominence system, but are brighter than
expected and possibly seen here in the continuum rather than line
emission. We find the HMI flux exceeds the radio/X-ray interpolation
of the bremsstrahlung produced in the flare soft X-ray sources by at
least one order of magnitude. This implies the participation of cooler
sources that can produce free-bound continua and possibly line emission
detectable by HMI. One of the early sources dynamically resembles
"coronal rain", appearing at a maximum apparent height and moving
toward the photosphere at an apparent constant projected speed of 134
± 8 km s-1. Not much literature exists on the detection of
optical continuum sources above the limb of the Sun by non-coronagraphic
instruments and these observations have potential implications for our
basic understanding of flare development, since visible observations
can in principle provide high spatial and temporal resolution.

---------------------------------------------------------
Title: Characterization of High-Degree Modes using MDI, HMI and
GONG Data
Authors: Korzennik, S. G.; Eff-Darwich, A.; Larson, T. P.;
Rabello-Soares, M. C.; Schou, J.
2013ASPC..478..173K Altcode:
We present the first characterization of high-degree modes (i.e.,
ℓ up to 900 or 1000), using three instruments and three epochs
corresponding to the 2001, 2002 and 2010 MDI Dynamics runs. For 2001,
we analyzed MDI full-disk Dopplergrams, while for 2002, we analyzed
MDI and GONG full-disk Dopplergrams, and for 2012 we analyzed
MDI, GONG and HMI full-disk Dopplergrams. These Dopplergrams were
spatially decomposed up to ℓ = 900 or 1000, and power spectra for
all degrees and all azimuthal orders were computed using a high-order
multi-taper, power spectrum estimator. These spectra were then fitted
for all degrees and all azimuthal orders, above ℓ = 100, and for all
orders with substantial amplitude. Fitting at high degrees generates
ridge characteristics, characteristics that do not correspond to
the underlying mode characteristics. We used a sophisticated forward
modeling to recover the best possible estimate of the underlying mode
characteristics (mode frequencies, as well as linewidths, amplitudes
and asymmetries). We present the first attempt to apply this method to
three instruments and three epochs. The derived sets of corrected mode
characteristics (frequencies, line widths, asymmetries and amplitudes)
are presented and compared.

---------------------------------------------------------
Title: Comparing the Internal Structure of the Sun During the Cycle
23 and Cycle 24 Minima
Authors: Basu, S.; Broomhall, A. -M.; Chaplin, W. J.; Elsworth, Y.;
Davies, G. R.; Schou, J.; Larson, T. P.
2013ASPC..478..161B Altcode:
The Birmingham Solar-Oscillations Network (BiSON) has been collecting
helioseismic data for the last three solar cycles. We use these data to
determine whether the internal properties of the Sun during the minimum
preceding cycle 24 was different compared to that preceding cycle 23.

---------------------------------------------------------
Title: The Torsional Oscillation and the Timing of the Solar Cycle:
Is it Maximum Yet?
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R.;
Larson, T. P.; Schou, J.; Thompson, M. J.
2013ASPC..478..303H Altcode:
After the late start to Cycle 24 there are some indications that
activity may have peaked as early as late 2011 and that the polar-field
reversal has already occurred in the North. We use helioseismic
measurements of the migrating zonal flow pattern known as the torsional
oscillation to estimate the length of the solar cycle, and find that
it has held steady at about 12.3 years since late 2009, which would
point to solar maximum in 2013 as expected.

---------------------------------------------------------
Title: Observing Open Clusters with a Sequence of Ages with Kepler
Authors: Guzik, Joyce A.; Bradley, Paul A.; Szabo, Robert; Molnar,
Laszlo; Pigulski, Andrzej; Zwintz, Konstanze; Ngeow, Chow-Choong;
Schou, Jesper; Handler, Gerald
2013arXiv1310.0772G Altcode:
We propose to observe with Kepler an age sequence of nearby uncrowded
open clusters. The current Kepler field contains very few (only 4)
somewhat distant and/or old clusters. Nearby open clusters are already
well characterized from ground- and many space-based observations. Our
proposal focuses on mid and upper main-sequence variables (main sequence
and pre-main sequence gamma Dor or delta Scuti stars, SPB/beta Cep
stars, Cepheids, or yellow supergiants), having periods of hours to
days accessible by longer cadence observations. Asteroseismology for
these objects to date is limited by the number of modes observable
from the ground, difficulty in obtaining spectroscopic or photometric
mode ID for the fainter targets that have been observed by Kepler,
uncertainties in interior and initial abundances, especially for stars
with abundance anomalies, uncertainties in distance/luminosity, and lack
of knowledge of prior evolution history. The additional constraints
of common age, distance and initial abundances in clusters will place
these variables in their evolutionary context, and help unlock some of
the science and reduce uncertainties for understanding star formation
and stellar evolution.

---------------------------------------------------------
Title: Global-Oscillation Eigenfunction Measurements of Solar
Meridional Flow
Authors: Woodard, M.; Schou, J.; Birch, A. C.; Larson, T. P.
2013SoPh..287..129W Altcode: 2012SoPh..tmp..179W
We describe and apply a new helioseismic method for measuring solar
subsurface axisymmetric meridional and zonal flow. The method is
based on a theoretical model of the response of global-oscillation
eigenfunctions to the flow velocity and uses cross spectra of the
time-varying coefficients in the spherical-harmonic expansion of
the photospheric Doppler-velocity field. Eigenfunction changes
modify the leakage matrix, which describes the sensitivity of the
spherical-harmonic coefficients to the global-oscillation modes. The
form of the leakage matrix in turn affects the theoretically expected
spherical-harmonic cross spectra. Estimates of internal meridional and
zonal flow were obtained by fitting the theoretical flow-dependent cross
spectra to spherical-harmonic cross spectra computed from approximately
500 days of full-disk Dopplergrams from the Helioseismic and Magnetic
Imager (HMI) on the SDO spacecraft. The zonal-flow measurements,
parameterized in the form of "a" coefficients, substantially agree
with measurements obtained from conventional global-mode-frequency
analysis. The meridional-flow estimates, in the form of depth-weighted
averages of the flow velocity, are similar to estimates obtained from
earlier analyses, for oscillation modes that penetrate the outermost
one-third of the convection zone. For more deeply penetrating modes,
the inferred flow velocity increases significantly with penetration
depth, indicating the need for either a modification of the simple
conveyor-belt picture of meridional flow or improvement in the
cross-spectral model.

---------------------------------------------------------
Title: Kepler White Paper: Asteroseismology of Solar-Like Oscillators
in a 2-Wheel Mission
Authors: Chaplin, W. J; Kjeldsen, H.; Christensen-Dalsgaard, J.;
Gilliland, R. L.; Kawaler, S. D.; Basu, S.; De Ridder, J.; Huber, D.;
Arentoft, T.; Schou, J.; Garcia, R. A.; Metcalfe, T. S.; Brogaard, K.;
Campante, T. L.; Elsworth, Y.; Miglio, A.; Appourchaux, T.; Bedding,
T. R.; Hekker, S.; Houdek, G.; Karoff, C.; Molenda-Zakowicz, J.;
Monteiro, M. J. P. F. G.; Silva Aguirre, V.; Stello, D.; Ball, W.;
Beck, P. G.; Birch, A. C.; Buzasi, D. L.; Casagrande, L.; Cellier,
T.; Corsaro, E.; Creevey, O. L.; Davies, G. R.; Deheuvels, S.; Dogan,
G.; Gizon, L.; Grundahl, F.; Guzik, J.; Handberg, R.; Jimenez, A.;
Kallinger, T.; Lund, M. N.; Lundkvist, M.; Mathis, S.; Mathur, S.;
Mazumdar, A.; Mosser, B.; Neiner, C.; Nielsen, M. B.; Palle, P. L.;
Pinsonneault, M. H.; Salabert, D.; Serenelli, A. M.; Shunker, H.;
White, T. R.
2013arXiv1309.0702C Altcode:
We comment on the potential for continuing asteroseismology of
solar-type and red-giant stars in a 2-wheel Kepler Mission. Our main
conclusion is that by targeting stars in the ecliptic it should be
possible to perform high-quality asteroseismology, as long as favorable
scenarios for 2-wheel pointing performance are met. Targeting the
ecliptic would potentially facilitate unique science that was not
possible in the nominal Mission, notably from the study of clusters
that are significantly brighter than those in the Kepler field. Our
conclusions are based on predictions of 2-wheel observations made by
a space photometry simulator, with information provided by the Kepler
Project used as input to describe the degraded pointing scenarios. We
find that elevated levels of frequency-dependent noise, consistent with
the above scenarios, would have a significant negative impact on our
ability to continue asteroseismic studies of solar-like oscillators in
the Kepler field. However, the situation may be much more optimistic
for observations in the ecliptic, provided that pointing resets of the
spacecraft during regular desaturations of the two functioning reaction
wheels are accurate at the &lt; 1 arcsec level. This would make it
possible to apply a post-hoc analysis that would recover most of the
lost photometric precision. Without this post-hoc correction---and the
accurate re-pointing it requires---the performance would probably be
as poor as in the Kepler-field case. Critical to our conclusions for
both fields is the assumed level of pointing noise (in the short-term
jitter and the longer-term drift). We suggest that further tests will
be needed to clarify our results once more detail and data on the
expected pointing performance becomes available, and we offer our
assistance in this work.

---------------------------------------------------------
Title: The Kepler-SEP Mission: Harvesting the South Ecliptic Pole
large-amplitude variables with Kepler
Authors: Szabó, R.; Molnár, L.; Kołaczkowski, Z.; Moskalik, P.;
Ivezić, Ž.; Udalski, A.; Szabados, L.; Kuehn, C.; Smolec, R.;
Pigulski, A.; Bedding, T.; Ngeow, C. C.; Guzik, J. A.; Ostrowski,
J.; De Cat, P.; Antoci, V.; Borkovits, T.; Soszyński, I.; Poleski,
R.; Kozłowski, Sz.; Pietrukowicz, P.; Skowron, J.; Szczygieł, D.;
Wyrzykowski, Ł.; Szymański, M.; Pietrzyński, G.; Ulaczyk, K.;
Plachy, E.; Schou, J.; Evans, N. R.; Kopaczki, G.
2013arXiv1309.0741S Altcode:
As a response to the white paper call, we propose to turn Kepler
to the South Ecliptic Pole (SEP) and observe thousands of large
amplitude variables for years with high cadence in the frame of
the Kepler-SEP Mission. The degraded pointing stability will still
allow observing these stars with reasonable (probably better
than mmag) accuracy. Long-term continuous monitoring already
proved to be extremely helpful to investigate several areas of
stellar astrophysics. Space-based missions opened a new window to
the dynamics of pulsation in several class of pulsating variable
stars and facilitated detailed studies of eclipsing binaries. The
main aim of this mission is to better understand the fascinating
dynamics behind various stellar pulsational phenomena (resonances,
mode coupling, chaos, mode selection) and interior physics (turbulent
convection, opacities). This will also improve the applicability of
these astrophysical tools for distance measurements, population and
stellar evolution studies. We investigated the pragmatic details of
such a mission and found a number of advantages: minimal reprogramming
of the flight software, a favorable field of view, access to both
galactic and LMC objects. However, the main advantage of the SEP
field comes from the large sample of well classified targets, mainly
through OGLE. Synergies and significant overlap (spatial, temporal
and in brightness) with both ground- (OGLE, LSST) and space-based
missions (GAIA, TESS) will greatly enhance the scientific value of
the Kepler-SEP mission. GAIA will allow full characterization of the
distance indicators. TESS will continuously monitor this field for at
least one year, and together with the proposed mission provide long time
series that cannot be obtained by other means. If Kepler-SEP program is
successful, there is a possibility to place one of the so-called LSST
"deep-drilling" fields in this region.

---------------------------------------------------------
Title: Accurate Characterization of High-degree Modes Using MDI
Observations
Authors: Korzennik, S. G.; Rabello-Soares, M. C.; Schou, J.; Larson,
T. P.
2013ApJ...772...87K Altcode:
We present the first accurate characterization of high-degree modes,
derived using the best Michelson Doppler Imager (MDI) full-disk
full-resolution data set available. A 90 day long time series of
full-disk 2 arcsec pixel-1 resolution Dopplergrams was
acquired in 2001, thanks to the high rate telemetry provided by the
Deep Space Network. These Dopplergrams were spatially decomposed using
our best estimate of the image scale and the known components of MDI's
image distortion. A multi-taper power spectrum estimator was used to
generate power spectra for all degrees and all azimuthal orders, up to
l = 1000. We used a large number of tapers to reduce the realization
noise, since at high degrees the individual modes blend into ridges and
thus there is no reason to preserve a high spectral resolution. These
power spectra were fitted for all degrees and all azimuthal orders,
between l = 100 and l = 1000, and for all the orders with substantial
amplitude. This fitting generated in excess of 5.2 × 106
individual estimates of ridge frequencies, line widths, amplitudes,
and asymmetries (singlets), corresponding to some 5700 multiplets
(l, n). Fitting at high degrees generates ridge characteristics,
characteristics that do not correspond to the underlying mode
characteristics. We used a sophisticated forward modeling to recover
the best possible estimate of the underlying mode characteristics (mode
frequencies, as well as line widths, amplitudes, and asymmetries). We
describe in detail this modeling and its validation. The modeling has
been extensively reviewed and refined, by including an iterative process
to improve its input parameters to better match the observations. Also,
the contribution of the leakage matrix on the accuracy of the
procedure has been carefully assessed. We present the derived set of
corrected mode characteristics, which includes not only frequencies,
but line widths, asymmetries, and amplitudes. We present and discuss
their uncertainties and the precision of the ridge-to-mode correction
schemes, through a detailed assessment of the sensitivity of the model
to its input set. The precision of the ridge-to-mode correction is
indicative of any possible residual systematic biases in the inferred
mode characteristics. In our conclusions, we address how to further
improve these estimates, and the implications for other data sets,
like GONG+ and HMI.

---------------------------------------------------------
Title: How much more can sunspots tell us about the solar dynamo?
Authors: Norton, Aimee A.; Jones, Eric H.; Liu, Y.; Hayashi, K.;
Hoeksema, J. T.; Schou, Jesper
2013IAUS..294...25N Altcode:
Sunspot observations inspired solar dynamo theory and continue
to do so. Simply counting them established the sunspot cycle
and its period. Latitudinal distributions introduced the tough
constraint that the source of sunspots moves equator-ward as the
cycle progresses. Observations of Hale's polarity law mandated
hemispheric asymmetry. How much more can sunspots tell us about
the solar dynamo? We draw attention to a few outstanding questions
raised by inherent sunspot properties. Namely, how to explain sunspot
rotation rates, the incoherence of follower spots, the longitudinal
spacing of sunspot groups, and brightness trends within a given sunspot
cycle. After reviewing the first several topics, we then present new
results on the brightness of sunspots in Cycle 24 as observed with
the Helioseismic Magnetic Imager (HMI). We compare these results
to the sunspot brightness observed in Cycle 23 with the Michelson
Doppler Imager (MDI). Next, we compare the minimum intensities of five
sunspots simultaneously observed by the Hinode Solar Optical Telescope
Spectropolarimeter (SOT-SP) and HMI to verify that the minimum
brightness of sunspot umbrae correlates well to the maximum field
strength. We then examine 90 and 52 sunspots in the north and south
hemisphere, respectively, from 2010 - 2012. Finally, we conclude that
the average maximum field strengths of umbra 40 Carrington Rotations
into Cycle 24 are 2690 Gauss, virtually indistinguishable from the
2660 Gauss value observed at a similar time in Cycle 23 with MDI.

---------------------------------------------------------
Title: Accurate characterization of high-degree modes using MDI data
Authors: Korzennik, S. G.; Rabello-Soares, M. C.; Schou, J.; Larson, T.
2013JPhCS.440a2016K Altcode: 2012arXiv1209.6414K
We present the first accurate characterization of high-degree modes
(i.e., l up to 1000), using the best MDI full-disk full-resolution
data set available (90-day long time series, acquired in
2001). The Dopplergrams were spatially decomposed using our best
estimate of the image scale and the known components of MDI's image
distortion. Multi-tapered power spectra were fitted for all degrees
and all azimuthal orders, between l = 100 and l = 1000, and for all
orders with substantial amplitude. Fitting at high degrees generates
ridge characteristics, characteristics that do not correspond to the
underlying mode characteristics. We used a sophisticated forward
modeling to recover the best possible estimate of the underlying
mode characteristics. We have derived a final set of corrected mode
characteristics (frequencies, line widths, asymmetries and amplitudes)
and their uncertainties.

---------------------------------------------------------
Title: The High-latitude Branch of the Solar Torsional Oscillation
in the Rising Phase of Cycle 24
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R.;
Larson, T. P.; Rempel, M.; Schou, J.; Thompson, M. J.
2013ApJ...767L..20H Altcode:
We use global heliseismic data from the Global Oscillation Network
Group, the Michelson Doppler Imager on board the Solar and Heliospheric
Observatory, and the Helioseismic and Magnetic Imager on board the Solar
Dynamics Observatory, to examine the behavior, during the rising phase
of Solar Cycle 24, of the migrating zonal flow pattern known as the
torsional oscillation. Although the high-latitude part of the pattern
appears to be absent in the new cycle when the flows are derived by
subtracting a mean across a full solar cycle, it can be seen if we
subtract the mean over a shorter period in the rising phase of each
cycle, and these two mean rotation profiles differ significantly
at high latitudes. This indicates that the underlying high-latitude
rotation has changed; we speculate that this is in response to weaker
polar fields, as suggested by a recent model.

---------------------------------------------------------
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: Using Distortion of Global Mode Eigenfunctions to Estimate
Large Scale Flows
Authors: Schou, J.; Woodard, M. F.; Baldner, C. S.; Larson, T. P.
2013enss.confE.103S Altcode:
The accurate measurement of large scale flows, such as differential
rotation and meridional flow, throughout the solar interior is important
for understanding the solar interior and the processes relevant for
the solar dynamo. Ordinarily normal modes would be expected to give
more accurate measurement of large scale flows than local helioseismic
techniques. Unfortunately, mode frequencies are not sensitive to the
meridional flow and so traditional methods do not work. Here we describe
our progress on using the distortion of the eigenfunctions to measure
flows. In particular on identifying the source of the large systematic
errors previously reported and determining the effect of light travel
time, center to limb phase variations and other physical effects.

---------------------------------------------------------
Title: Femtosecond-laser ablation dynamics of dielectrics: basics
and applications for thin films
Authors: Balling, P.; Schou, J.
2013RPPh...76c6502B Altcode:
Laser ablation of dielectrics by ultrashort laser pulses is
reviewed. The basic interaction between ultrashort light pulses and
the dielectric material is described, and different approaches to the
modeling of the femtosecond ablation dynamics are reviewed. Material
excitation by ultrashort laser pulses is induced by a combination
of strong-field excitation (multi-photon and tunnel excitation),
collisional excitation (potentially leading to an avalanche process),
and absorption in the plasma consisting of the electrons excited to
the conduction band. It is discussed how these excitation processes
can be described by various rate-equation models in combination with
different descriptions of the excited electrons. The optical properties
of the highly excited dielectric undergo a rapid change during the
laser pulse, which must be included in a detailed modeling of the
excitations. The material ejected from the dielectric following the
femtosecond-laser excitation can potentially be used for thin-film
deposition. The deposition rate is typically much smaller than that
for nanosecond lasers, but film production by femtosecond lasers
does possess several attractive features. First, the strong-field
excitation makes it possible to produce films of materials that are
transparent to the laser light. Second, the highly localized excitation
reduces the emission of larger material particulates. Third, lasers
with ultrashort pulses are shown to be particularly useful tools for
the production of nanocluster films. The important question of the
film stoichiometry relative to that of the target will be thoroughly
discussed in relation to the films reported in the literature.

---------------------------------------------------------
Title: Comparison Between Line-Of-Sight Observables And
Milne-Eddington Inversion Results From HMI: 24- And 12-hour Period
Oscillations
Authors: Couvidat, Sebastien; Liu, Yang; Scherrer, Philip H.; Schou,
Jesper; HMI Team
2013enss.confE..84C Altcode:
Oscillations with 12 and 24 hour periods are visible in sunspots
on line-of-sight (LOS) observables and, to a lesser extent, on
Milne-Eddington (ME) inversion results from the SDO/HMI instrument.Such
oscillations are artifacts produced by the LOS algorithm and ME
inversion procedure, and are not of solar origin. For instance, the LOS
algorithm depends on an Fe I line profile and on HMI filter transmission
profiles to convert the HMI intensities into Doppler velocities and LOS
magnetic-field strengths: one of the issues we encountered is that in
the presence of strong fields the Fe I profile used is inappropriate
and results in errors in the left and right circular polarization
velocity estimates. Here we present some properties of these 12-
and 24-hour period oscillations, and we discuss their origin and the
possibility of improving the LOS algorithm and ME inversion procedure
to reduce their amplitudes.

---------------------------------------------------------
Title: Progress in Near-Surface Flow Determinations: Minimizing
Systematics
Authors: Baldner, Charles S.; Basu, Sarbani; Bogart, Richard S.;
Schou, Jesper
2013enss.confE.116B Altcode:
One of the key achievements in helioseismology has been the measurement
of sub-surface dynamics. Measurements of both differential rotation
and meridional circulation exist from multiple instruments for more
than one and a half solar cycles. The precision of flow determinations
from various helioseismic techniques has been found to be greater than
their accuracy. Flows have been found to depend on the disk position
of the measurements, and some inconsistencies between measurements
from different instruments have also been found. In this work, we
report progress in understanding and correcting for these systematic
errors, and present the latest ring diagram determinations of shallow
subsurface flows with HMI/SDO data.

---------------------------------------------------------
Title: Three 2012 Transits of Venus: From Earth, Jupiter, and Saturn
Authors: Pasachoff, Jay M.; Schneider, G.; Babcock, B. A.; Lu, M.;
Edelman, E.; Reardon, K.; Widemann, T.; Tanga, P.; Dantowitz, R.;
Silverstone, M. D.; Ehrenreich, D.; Vidal-Madjar, A.; Nicholson,
P. D.; Willson, R. C.; Kopp, G. A.; Yurchyshyn, V. B.; Sterling,
A. C.; Scherrer, P. H.; Schou, J.; Golub, L.; McCauley, P.; Reeves, K.
2013AAS...22131506P Altcode:
We observed the 2012 June 6/5 transit seen from Earth (E/ToV),
simultaneously with Venus Express and several other spacecraft
not only to study the Cytherean atmosphere but also to provide an
exoplanet-transit analog. From Haleakala, the whole transit was visible
in coronal skies; among our instruments was one of the world-wide Venus
Twilight Experiment's nine coronagraphs. Venus's atmosphere became
visible before first contact. SacPeak/IBIS provided high-resolution
images at Hα/carbon-dioxide. Big Bear's NST also provided
high-resolution observations of the Cytherean atmosphere and black-drop
evolution. Our liaison with UH's Mees Solar Observatory scientists
provided magneto-optical imaging at calcium and potassium. Solar
Dynamics Observatory's AIA and HMI, and the Solar Optical Telescope
(SOT) and X-ray Telescope (XRT) on Hinode, and total-solar-irradiance
measurements with ACRIMSAT and SORCE/TIM, were used to observe the
event as an exoplanet-transit analog. On September 20, we imaged
Jupiter for 14 Hubble Space Telescope orbits, centered on a 10-hour
ToV visible from Jupiter (J/ToV), as an exoplanet-transit analog in
our own solar system, using Jupiter as an integrating sphere. Imaging
was good, although much work remains to determine if we can detect
the expected 0.01% solar irradiance decrease at Jupiter and the even
slighter differential effect between our violet and near-infrared
filters caused by Venus's atmosphere. We also give a first report on our
currently planned December 21 Cassini UVIS observations of a transit of
Venus from Saturn (S/ToV). Our E/ToV expedition was sponsored by the
Committee for Research and Exploration/National Geographic Society;
supplemented: NASA/AAS's Small Research Grant Program. We thank Rob
Ratkowski, Stan Truitt, Rob Lucas, Aram Friedman, and Eric Pilger
'82 at Haleakala, and Joseph Gangestad '06 at Big Bear for assistance,
and Lockheed Martin Solar and Astrophysics Lab and Hinode science and
operations teams for support for coordinated observations with NASA
satellites. Our J/ToV observations were based on observations made
with HST, operated by AURA, Inc., under NASA contract NAS 5-26555;
these observations are associated with program #13067.

---------------------------------------------------------
Title: Erratum: "On the Determination of Michelson
Doppler Imager High-degree Mode Frequencies" <A
href="/abs/2004ApJ...602..481K">(2004, ApJ, 602, 481)</A>
Authors: Korzennik, S. G.; Rabello-Soares, M. C.; Schou, J.
2012ApJ...760..156K Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Effects of Asymmetric Flows in Solar Convection on Oscillation
Modes
Authors: Baldner, Charles S.; Schou, Jesper
2012ApJ...760L...1B Altcode: 2012arXiv1210.1583B
Many helioseismic measurements suffer from substantial systematic
errors. A particularly frustrating one is that time-distance
measurements suffer from a large center to limb effect which looks
very similar to the finite light travel time, except that the magnitude
depends on the observable used and can have the opposite sign. This has
frustrated attempts to determine the deep meridional flow in the solar
convection zone, with Zhao et al. applying an ad hoc correction with
little physical basis to correct the data. In this Letter, we propose
that part of this effect can be explained by the highly asymmetrical
nature of the solar granulation which results in what appears to the
oscillation modes as a net radial flow, thereby imparting a phase shift
on the modes as a function of observing height and thus heliocentric
angle.

---------------------------------------------------------
Title: The 2012 Transit of Venus for Cytherean Atmospheric Studies
and as an Exoplanet Analog
Authors: Pasachoff, Jay M.; Schneider, G.; Babcock, B. A.; Lu, M.;
Reardon, K. P.; Widemann, T.; Tanga, P.; Dantowitz, R.; Willson,
R.; Kopp, G.; Yurchyshyn, V.; Sterling, A.; Scherrer, P.; Schou, J.;
Golub, L.; Reeves, K.
2012DPS....4450806P Altcode:
We worked to assemble as complete a dataset as possible for the
Cytherean atmosphere in collaboration with Venus Express in situ
and to provide an analog of spectral and total irradiance exoplanet
measurements. From Haleakala, the whole transit was visible in
coronal skies; our B images showed the evolution of the visibility
of Venus's atmosphere and of the black-drop effect, as part of the
Venus Twilight Experiment's 9 coronagraphs distributed worldwide
with BVRI. We imaged the Cytherean atmosphere over two minutes before
first contact, with subarcsecond resolution, with the coronagraph and
a separate refractor. The IBIS imaging spectrometer at Sacramento
Peak Observatory at H-alpha and carbon-dioxide also provided us
high-resolution imaging. The NST of Big Bear Solar Observatory
also provided high-resolution vacuum observations of the Cytherean
atmosphere and black drop evolution. Our liaison with UH's Mees Solar
Observatory scientists provided magneto-optical imaging at calcium
and potassium. Spaceborne observations included the Solar Dynamics
Observatory's AIA and HMI, and the Solar Optical Telescope (SOT)
and X-ray Telescope (XRT) on Hinode, and total-solar-irradiance
measurements with ACRIMSAT and SORCE/TIM, to characterize the
event as an exoplanet-transit analog. Our expedition was sponsored
by the Committee for Research and Exploration/National Geographic
Society. Some of the funds for the carbon-dioxide filter for IBIS were
provided by NASA through AAS's Small Research Grant Program. We thank
Rob Lucas, Aram Friedman, and Eric Pilger '82 for assistance with
Haleakala observing, Rob Ratkowski of Haleakala Amateur Astronomers
for assistance with equipment and with the site, Stan Truitt for the
loan of his Paramount ME, and Steve Bisque/Software Bisque for TheSky
X controller. We thank Joseph Gangestad '06 of Aerospace Corp., a
veteran of our 2004 expedition, for assistance at Big Bear. We thank
the Lockheed Martin Solar and Astrophysics Laboratory and Hinode
science and operations teams for planning and support.

---------------------------------------------------------
Title: A Determination of High-degree Mode Parameters Based on
MDI Observations
Authors: Korzennik, S. G.; Rabello-Soares, M. C.; Schou, J.; Larson,
T. P.
2012ASPC..462..189K Altcode:
We present the best to date determination of high-degree mode
parameters obtained from a ninety day long time series of full-disk
full-resolution Michelson Doppler Imager (MDI) Dopplergrams. These
Dopplergrams were decomposed using our best estimate of the image
scale and the known components of MDI's image distortion. The
spherical harmonic decomposition was carried out up to ℓ=1000,
and a high-order sine multi-taper power spectrum estimator was used
to generate power spectra. These power spectra were fitted for all
degrees and all azimuthal orders, for 100 ≤ ℓ ≤ 1000, and
for all radial orders with substantial amplitude, generating some
6 × 106 estimates of ridge frequencies, line-widths,
amplitudes and asymmetries. We used a sophisticated forward modeling
of the mode to ridge blending, to recover the best possible estimate
of the underlying mode characteristics.

---------------------------------------------------------
Title: Helioseismic Measurements of the Torsional Oscillation
Authors: Schou, J.; Howe, R.; Larson, T. P.
2012ASPC..462..352S Altcode:
The zonal flows known as the torsional oscillation have by now been
observed for more than 15 years using observations from the Michelson
Doppler Imager (MDI) and Global Oscillation Network Group (GONG). Even
with this limited set of data it is clear that there are significant
differences between the cycles, as also evidenced by the prolonged
recent solar minimum. Here we discuss some of these differences and
their significance. In particular we discuss the slower development of
the current solar cycle and the lack of a polar branch in the torsional
oscillation. We will also compare results from MDI and the Helioseismic
Magnetic Imager (HMI) and discuss the prospects for generating longer
consistent sets of observations.

---------------------------------------------------------
Title: Seismic Evidence for a Rapidly Rotating Core in a
Lower-giant-branch Star Observed with Kepler
Authors: Deheuvels, S.; García, R. A.; Chaplin, W. J.; Basu, S.;
Antia, H. M.; Appourchaux, T.; Benomar, O.; Davies, G. R.; Elsworth,
Y.; Gizon, L.; Goupil, M. J.; Reese, D. R.; Regulo, C.; Schou, J.;
Stahn, T.; Casagrande, L.; Christensen-Dalsgaard, J.; Fischer, D.;
Hekker, S.; Kjeldsen, H.; Mathur, S.; Mosser, B.; Pinsonneault, M.;
Valenti, J.; Christiansen, J. L.; Kinemuchi, K.; Mullally, F.
2012ApJ...756...19D Altcode: 2012arXiv1206.3312D
Rotation is expected to have an important influence on the structure
and the evolution of stars. However, the mechanisms of angular momentum
transport in stars remain theoretically uncertain and very complex to
take into account in stellar models. To achieve a better understanding
of these processes, we desperately need observational constraints on the
internal rotation of stars, which until very recently was restricted to
the Sun. In this paper, we report the detection of mixed modes—i.e.,
modes that behave both as g modes in the core and as p modes in
the envelope—in the spectrum of the early red giant KIC 7341231,
which was observed during one year with the Kepler spacecraft. By
performing an analysis of the oscillation spectrum of the star, we
show that its non-radial modes are clearly split by stellar rotation
and we are able to determine precisely the rotational splittings of
18 modes. We then find a stellar model that reproduces very well the
observed atmospheric and seismic properties of the star. We use this
model to perform inversions of the internal rotation profile of the
star, which enables us to show that the core of the star is rotating at
least five times faster than the envelope. This will shed new light on
the processes of transport of angular momentum in stars. In particular,
this result can be used to place constraints on the angular momentum
coupling between the core and the envelope of early red giants, which
could help us discriminate between the theories that have been proposed
over the last few decades.

---------------------------------------------------------
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: The Height of a White-light Flare and Its Hard X-Ray Sources
Authors: Martínez Oliveros, Juan-Carlos; Hudson, Hugh S.; Hurford,
Gordon J.; Krucker, Säm; Lin, R. P.; Lindsey, Charles; Couvidat,
Sebastien; Schou, Jesper; Thompson, W. T.
2012ApJ...753L..26M Altcode: 2012arXiv1206.0497M
We describe observations of a white-light (WL) flare
(SOL2011-02-24T07:35:00, M3.5) close to the limb of the Sun, from which
we obtain estimates of the heights of the optical continuum sources and
those of the associated hard X-ray (HXR) sources. For this purpose, we
use HXR images from the Reuven Ramaty High Energy Spectroscopic Imager
and optical images at 6173 Å from the Solar Dynamics Observatory. We
find that the centroids of the impulsive-phase emissions in WL and HXRs
(30-80 keV) match closely in central distance (angular displacement
from Sun center), within uncertainties of order 0farcs2. This directly
implies a common source height for these radiations, strengthening the
connection between visible flare continuum formation and the accelerated
electrons. We also estimate the absolute heights of these emissions
as vertical distances from Sun center. Such a direct estimation has
not been done previously, to our knowledge. Using a simultaneous 195
Å image from the Solar-Terrestrial RElations Observatory spacecraft
to identify the heliographic coordinates of the flare footpoints,
we determine mean heights above the photosphere (as normally defined;
τ = 1 at 5000 Å) of 305 ± 170 km and 195 ± 70 km, respectively, for
the centroids of the HXR and WL footpoint sources of the flare. These
heights are unexpectedly low in the atmosphere, and are consistent
with the expected locations of τ = 1 for the 6173 Å and the ~40 keV
photons observed, respectively.

---------------------------------------------------------
Title: Meridional Circulation From Normal Mode Analysis
Authors: Schou, Jesper; Woodard, M. F.; Larson, T. P.
2012AAS...22020505S Altcode:
We have recently been able to make significant progress in the
determining the solar meridional flow from the perturbations it causes
to the eigenfunctions of normal modes. In this poster we will describe
some of our recent progress and show that it will likely be possible to
determine the flow over a substantial part of the solar interior with
a precision sufficient to address important questions. However,
we also see what appears to be significant systematic errors and will
describe our efforts at understanding those.

---------------------------------------------------------
Title: Line-of-Sight Observables Algorithms for the Helioseismic
and Magnetic Imager (HMI) Instrument Tested with Interferometric
Bidimensional Spectrometer (IBIS) Observations
Authors: Couvidat, Sébastien; Rajaguru, S. P.; Wachter, Richard;
Sankarasubramanian, K.; Schou, Jesper; Scherrer, Philip H.
2012SoPh..278..217C Altcode:
The Helioseismic and Magnetic Imager (HMI) instrument onboard the
Solar Dynamics Observatory produces line-of-sight (LOS) observables
(Doppler velocity, magnetic-field strength, Fe I line width, line
depth, and continuum intensity) as well as vector magnetic-field maps
at the solar surface. The accuracy of LOS observables is dependent
on the algorithm used to translate a sequence of HMI filtergrams into
the corresponding observables. Using one hour of high-cadence imaging
spectropolarimetric observations of a sunspot in the Fe I line at 6173
Å through the Interferometric Bidimensional Spectrometer installed
at the Dunn Solar Telescope, and the Milne-Eddington inversion of the
corresponding Stokes vectors, we test the accuracy of the observables
algorithm currently implemented in the HMI data-analysis pipeline:
the MDI-like algorithm. In an attempt to improve the accuracy of
HMI observables, we also compare this algorithm to others that may be
implemented in the future: a least-squares fit with a Gaussian profile,
a least-squares fit with a Voigt profile, and the use of second Fourier
coefficients in the MDI-like algorithm.

---------------------------------------------------------
Title: Viewing Geometry, Line Height-of-Formation, and Helioseismic
Measurements
Authors: Baldner, Charles; Parchevsky, K.; Schou, J.; Larson, T.;
Couvidat, S.
2012AAS...22020506B Altcode:
Helioseismic mode parameters or travel times are commonly measured by
observing an absorption line in the solar atmosphere and determining
the line-of-sight velocity by means of the Doppler effect. Helioseismic
measurements are thus susceptible to a number of systematic errors
associated with the details of the line formation and wave propagation
in the atmosphere. Observing at different heights in the atmosphere
introduce errors through fairly simple geometric effects. In addition,
mode amplitudes and eigenfunction may change with height. More complex
potential sources of error include the fact that not all waves are
purely evanescent, the effects of convection, the acoustic source
depth and distribution, and non-adiabaticity. In this work, we report
progress in characterizing the effects on helioseismic measurements
by the differing height-of-formation across the disk. We consider the
effects of height-of-formation on global mode parameters, ring diagram
parameters, and time-distance travel time measurements.

---------------------------------------------------------
Title: Latest Results on the Torsional Oscillation and Solar Cycle 25
Authors: Hill, Frank; Howe, R.; Schou, J.; Thompson, M.; Larson, T.;
Komm, R.
2012AAS...22012302H Altcode:
The Torsional Oscillation in the Sun is a zonal (East-West) flow
that is slightly faster than the background differential rotation
profile. The location of this flow slowly migrates in latitude over
a period of several years. There are two branches of the flow: an
equatorward branch that underlies the active regions, and a poleward
branch. The timing of the equatorward migration is correlated with the
timing of the solar cycle such that the activity for a cycle appears
when the center of the flow reaches latitude 25 degrees. In addition,
the poleward branch appears about 12 years prior to the activity for
a cycle. Thus we should have observed the onset of Cycle 25 in 2008,
but did not. This poster will update the observations to 2012, and
present a new analysis that shows that the Cycle 25 flow appeared
in 2010, but was hidden by a change in the background differential
rotation profile. These results suggest that the next minimum will be
two years longer than average, and that Cycle 25 will begin in 2022.

---------------------------------------------------------
Title: Using HMI To Study Photospheric Footpoint Motions In X-class
Flares
Authors: Desai, Priyamvada; Bogart, R.; Couvidat, S.; Schou, J.
2012AAS...22020418D Altcode:
Recent investigations of flare-related changes in the photospheric absorption line (FeI 617.3 nm) profile of solar flares of varying
X-ray classes, observed using the Helioseismic and Magnetic
Imager (HMI) aboard the Solar Dynamic Observatory (SD0),
has indicated that a large percentage of them show distinct
continuum enhancement, along with a marked decrease in line-depth
(Desai,et al,submitted). Some of the X-class flares also showed a
reversal in the line profile, from absorption into emission during the
peak of the flare. We investigate the temporal and spatial variations
in the velocity of the photospheric foot point motions and track the
evolution of the flares of some of these X-class flares.

---------------------------------------------------------
Title: Direct Measurement Of The Height Of A White-light Flare
Authors: Hudson, Hugh S.; Martinez-Oliveros, J.; Krucker, S.; Hurford,
G.; Thompson, W.; Schou, J.; Couvidat, S.; Lindsey, C.
2012AAS...22020441H Altcode:
We have used RHESSI and HMI observations to observe hard X-ray and
white-light continuum sources of the limb flare SOL2011-02-24, and
find the source centroids to coincide within errors of about 0.2 arc s,
with the conclusion that the emissions form at the same height in the
atmosphere. This greatly strengthens the known association between
non-thermal electrons and white-light continuum formation. We also
use STEREO observations to find the heliographic coordinates of the
flare. This determines the projected height of the photosphere directly
below the flare emissions. With this information, the RHESSI metrology
determines the absolute height of the sources to be remarkably low
in the solar atmosphere: the two footpoints have comparable heights,
which we estimate at about 290 +- 138 km above the photosphere. This
location lies significantly below the visible-light limb height,
estimated at 500 km by Brown &amp; Christensen-Dalsgaard (1998), and
the height of optical depth unity to Thomson scattering, estimated
at a higher altitude. The results are not consistent with any current
models of these processes.

---------------------------------------------------------
Title: Two Years of Global Analysis with HMI
Authors: Larson, Timothy P.; Schou, J.
2012AAS...22020702L Altcode:
With HMI completing over two years of observations in May 2012,
we have the opportunity to see the beginning of any trends in the
global mode parameters and rotational inversions. In particular we
can investigate whether the one-year periodicity seen in the f-mode
frequencies measured by MDI is present in HMI. Furthermore, HMI gives
us an opportunity to examine the helioseismic signals for a long span
of time in observables contemporaneous with velocity, such as intensity
and line depth, which MDI was unable to provide. The high resolution
of the HMI instrument also gives us a chance to study in detail how
the apodization of the images affects the inferred mode parameters,
since our previous work has shown an as yet not understood systematic
error relating to the apodization.

---------------------------------------------------------
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: Wavelength Dependence of the Helioseismic and Magnetic Imager
(HMI) Instrument onboard the Solar Dynamics Observatory (SDO)
Authors: Couvidat, Sébastien; Schou, Jesper; Shine, Richard A.;
Bush, Rock I.; Miles, John W.; Scherrer, Philip H.; Rairden, Richard L.
2012SoPh..275..285C Altcode: 2011SoPh..tmp..150C; 2011SoPh..tmp..219C; 2011SoPh..tmp...33C;
2011SoPh..tmp..110C
The Helioseismic and Magnetic Imager (HMI) instrument will produce
Doppler-velocity and vector-magnetic-field maps of the solar surface,
whose accuracy is dependent on a thorough knowledge of the transmission
profiles of the components of the HMI optical-filter system. Here we
present a series of wavelength-dependence calibration tests, performed
on the instrument from 2005 onwards, to obtain these profiles. We
obtained the transmittances as a function of wavelength for the tunable
and non-tunable filter elements, as well as the variation of these
transmittances with temperature and the angle of incidence of rays of
light. We also established the presence of fringe patterns produced by
interferences inside the blocking filter and the front window, as well
as a change in transmitted intensity with the tuning position. This
thorough characterization of the HMI-filter system confirmed the very
high quality of the instrument, and showed that its properties are
well within the required specifications to produce superior data with
high spatial and temporal resolution.

---------------------------------------------------------
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: Polarization Calibration of the Helioseismic and Magnetic
Imager (HMI) onboard the Solar Dynamics Observatory (SDO)
Authors: Schou, J.; Borrero, J. M.; Norton, A. A.; Tomczyk, S.;
Elmore, D.; Card, G. L.
2012SoPh..275..327S Altcode:
As part of the overall ground-based calibration of the Helioseismic
and Magnetic Imager (HMI) instrument an extensive set of polarimetric
calibrations were performed. This paper describes the polarimetric
design of the instrument, the test setup, the polarimetric model,
the tests performed, and some results. It is demonstrated that HMI
achieves an accuracy of 1% or better on the crosstalks between Q,
U, and V and that our model can reproduce the intensities in our
calibration sequences to about 0.4%. The amount of depolarization
is negligible when the instrument is operated as intended which,
combined with the flexibility of the polarimeter design, means that
the polarimetric efficiency is excellent.

---------------------------------------------------------
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: HMI vector magnetic field products: the long-awaited release
has come! Now what?
Authors: Centeno, R.; Barnes, G.; Borrero, J.; Couvidat, S. P.;
Hayashi, K.; Hoeksema, J. T.; Leka, K. D.; Liu, Y.; Schou, J.; Schuck,
P. W.; Sun, X.; Tomczyk, S.
2011AGUFMSH31A1985C Altcode:
HMI vector magnetic field test products will be released, alongside
with the corresponding documentation, soon after the submission of this
abstract. These data represent a stage of the project at which the HMI
vector team has a large degree of confidence in the results. However,
longer-term research topics on how to improve certain aspects of the
data pipeline in general -and the spectral line inversion code in
particular- are being pursued as we get valuable input from the user
community. I will give a brief summary of the characteristics of the
released inversion data products and an update of where we stand now.

---------------------------------------------------------
Title: Sunspot Groups Simultaneously Observed with HMI and MDI
Authors: Norton, A. A.; Schou, J.; Liu, Y.; Hoeksema, J. T.
2011sdmi.confE..42N Altcode:
Simultaneous data from HMI and MDI is analyzed for active regions 11084
and 11087. We showcase the improved quality of HMI 45-second magnetogram
data over MDI magnetogram data due to higher spectral and spatial
sampling as well as better optical alignment and a magnetically more
sensitive spectral line. Specifically, HMI magnetogram data contains
less leakage of p-mode signal, umbrae do not show saturation at low
intensities, and HMI flux values are consistent with vector data. We
show comparisons of magnetic time series and power spectra observed
by HMI and MDI for sunspot, plage and quiet-Sun.

---------------------------------------------------------
Title: VFISV: Very Fast Inversion of the Stokes Vector for the
Helioseismic and Magnetic Imager
Authors: Borrero, J. M.; Tomczyk, S.; Kubo, M.; Socas-Navarro, H.;
Schou, J.; Couvidat, S.; Bogart, R.
2011SoPh..273..267B Altcode: 2009arXiv0901.2702B
In this paper we describe in detail the implementation and main
properties of a new inversion code for the polarized radiative transfer
equation (VFISV: Very Fast Inversion of the Stokes Vector). VFISV will
routinely analyze pipeline data from the Helioseismic and Magnetic
Imager (HMI) on-board of the Solar Dynamics Observatory (SDO). It
will provide full-disk maps (4096×4096 pixels) of the magnetic field
vector on the Solar Photosphere every ten minutes. For this reason
VFISV is optimized to achieve an inversion speed that will allow
it to invert sixteen million pixels every ten minutes with a modest
number (approx. 50) of CPUs. Here we focus on describing a number of
important details, simplifications and tweaks that have allowed us to
significantly speed up the inversion process. We also give details on
tests performed with data from the spectropolarimeter on-board of the
Hinode spacecraft.

---------------------------------------------------------
Title: Earth-Affecting Solar Causes Observatory (EASCO): a mission
at the Sun-Earth L5
Authors: Gopalswamy, Nat; Davila, Joseph M.; Auchère, Frédéric;
Schou, Jesper; Korendyke, Clarence M.; Shih, Albert; Johnston, Janet
C.; MacDowall, Robert J.; Maksimovic, Milan; Sittler, Edward; Szabo,
Adam; Wesenberg, Richard; Vennerstrom, Suzanne; Heber, Bernd
2011SPIE.8148E..0ZG Altcode: 2011SPIE.8148E..30G; 2011arXiv1109.2929G
Coronal mass ejections (CMEs) and corotating interaction regions
(CIRs) as well as their source regions are important because of
their space weather consequences. The current understanding of CMEs
primarily comes from the Solar and Heliospheric Observatory (SOHO)
and the Solar Terrestrial Relations Observatory (STEREO) missions,
but these missions lacked some key measurements: STEREO did not have a
magnetograph; SOHO did not have in-situ magnetometer. SOHO and other
imagers such as the Solar Mass Ejection Imager (SMEI) located on the
Sun-Earth line are also not well-suited to measure Earth-directed
CMEs. The Earth-Affecting Solar Causes Observatory (EASCO) is a
proposed mission to be located at the Sun-Earth L5 that overcomes
these deficiencies. The mission concept was recently studied at the
Mission Design Laboratory (MDL), NASA Goddard Space Flight Center,
to see how the mission can be implemented. The study found that the
scientific payload (seven remote-sensing and three in-situ instruments)
can be readily accommodated and can be launched using an intermediate
size vehicle; a hybrid propulsion system consisting of a Xenon ion
thruster and hydrazine has been found to be adequate to place the
payload at L5. Following a 2-year transfer time, a 4-year operation
is considered around the next solar maximum in 2025.

---------------------------------------------------------
Title: Photospheric signatures of Solar Flares
Authors: Desai, P.; Bogart, R.; Couvidat, S.; Schou, J.
2011sdmi.confE..32D Altcode:
White Light flares (WLF's) are enjoying a renewed interest in the
solar commmunity since the recent detections of white light continuum
emission of solar flares with TRACE (Hudson et.al, 2006; Fletcher
et.al, 2007) and HINODE(Wang, H., 2009). The Helioseismic and Magnetic
Imager (HMI) on the Solar Dynamic Observatory (SDO) produces a nearly
continuous stream of full disc images of the sun in a set of six
narrow wavelength bands around the FeI photospheric absorption line
at 6173 Angstrom with filtergrams made at a cadence of one every 1.85
seconds. Preliminary analysis shows that the photospheric signature of
Xray flares of sufficient intensity (Martinez Oliveros,J.C. et al, 2011)
can be readily detected in the HMI data. Motivated by this finding,
we have analyzed the HMI observables (in particular the Line Depth
and Continuum Intensity) and the corresponding line core intensity
during recent GOES M and X class flares (e.g. M6.6 on 2/13/2011,
X2.2 on 2/15/2011, X6.9 on 8/9/11).

---------------------------------------------------------
Title: Comparing Leakage Matrices
Authors: Larson, T. P.; Schou, J.; Korzennik, S. G.
2011sdmi.confE..44L Altcode:
The standard leakage matrix for global mode helioseismology is
calculated assuming a value of zero for the P-angle, B-angle, and CCD
offsets, and value of 1 AU for the observer distance. Since image center
is not constant we vary this parameter so see what effect is has on the
leaks and explore the possibility of using a leakage matrix averaged
over pixel offsets. Since the B-angle and observer distance vary in
a known way with time, we recompute the leakage matrix for realistic
values of these parameters and repeat the fits to find out how the mode
parameters are affected. Since previous studies have indicated certain
systematic errors are associated with the apodization, we also compute
leakage matrices for different apodizations, repeat the spherical
harmonic decomposition with those apodizations, and fit these to see
the effect on mode parameters. Lastly, we compare the leakage matrix
computed at Stanford with a completely independent calculation in order
to both verify our results and discover the source of any discrepancy.

---------------------------------------------------------
Title: Measuring Meridional Flow Using Global Modes
Authors: Schou, J.; Woodard, M. F.; Birch, A. C.; Larson, T. P.
2011sdmi.confE..71S Altcode:
In the past the meridional flow has been determined near the photosphere
by direct observations and below the solar surface using local
helioseismic methods. To first order normal mode frequencies are not
sensitive to the meridional flow, and so they are not useful for this
purpose. However, the eigenfunctions are sensitive to the meridional
flow to first order. Here we describe our progress on a project to
measure the eigenfunction perturbations and infer the meridional flow
with depth.

---------------------------------------------------------
Title: A determination of high degree mode parameters based on
MDI observations
Authors: Korzennik, S. G.; Rabello-Soares, M. C.; Schou, J.; Larson, T.
2011sdmi.confE..88K Altcode:
We present the best to date determination of high degree mode parameters
obtained from the longest full-disk high-resolution data set available
over the 13 years of MDI operations. A ninety day long time series
of full-disk two arc-second per pixel resolution dopplergrams were
acquired in 2001, thanks to the high rate telemetry provided by the
deep space network. These dopplergrams were decomposed using our best
estimate of the image scale and the known components of MDI's image
distortion. The spherical harmonics decomposition was carried out up
to l=1000, and a sine multi-taper power spectrum estimator was used
to generate power spectra for all degrees and all azimuthal orders up
to l=1000. We used a large number of tapers to reduce the realization
noise. Since at high degrees the individual modes blend into ridges,
there is no reason to preserve a high spectral resolution. These
power spectra were fitted for all degrees and all azimuthal orders,
between l=100 and l=1000, and for all orders with substantial amplitude,
generating in excess of 6 million individual estimate of frequencies,
line-widths amplitudes and asymmetries, corresponding to some 6,000
singlets. Fitting at high degrees generates characteristics of the
blended ridges, characteristics that do not correspond to the underlying
mode characteristics. We used a sophisticated forward modeling of the
mode to ridge blending to recover the best possible estimate of the
underlying mode characteristics for the mode frequency, as well as the
line-width, amplitude and asymmetry. We describe this modeling, as it
has been recently fine tuned, and the iterative process used to refine
its input parameters. Finally not only did we generate corrected mode
characteristics and their uncertainties, but we computed the sensitivity
of the model to its input set to best estimate the precision of the
ridge to mode correction itself. This was carried out to assess the
magnitude of any residual systematic errors in the final estimates of
the mode characteristics.

---------------------------------------------------------
Title: Temporal changes in the frequencies of the solar p-mode
oscillations during solar cycle 23
Authors: Rhodes, E. J.; Reiter, J.; Schou, J.; Larson, T.; Scherrer,
P.; Brooks, J.; McFaddin, P.; Miller, B.; Rodriguez, J.; Yoo, J.
2011IAUS..273..389R Altcode:
We present a study of the temporal changes in the sensitivities of the
frequencies of the solar p-mode oscillations to corresponding changes in
the levels of solar activity during Solar Cycle 23. From MDI and GONG++
full-disk Dopplergram three-day time series obtained between 1996 and
2008 we have computed a total of 221 sets of m-averaged power spectra
for spherical harmonic degrees ranging up to 1000. We have then fit
these 284 sets of m-averaged power spectra using our WMLTP fitting
code and both symmetric Lorentzian profiles for the peaks as well as
the asymmetric profile of Nigam and Kosovichev to obtain 568 tables
of p-mode parameters. We then inter-compared these 568 tables, and
we performed linear regression analyses of the differences in p-mode
frequencies, widths, amplitudes, and asymmetries as functions of the
differences in as many as ten different solar activity indices. From the
linear regression analyses that we performed on the frequency difference
data sets, we have discovered a new signature of the frequency shifts
of the p-modes. Specifically, we have discovered that the temporal
shifts of the solar oscillation frequencies are positively correlated
with the changes in solar activity below a limiting frequency. They
then become anti-correlated with the changes in activity for a range of
frequencies before once again becoming positively-correlated with the
activity changes at very high frequencies. We have also discovered
that the two frequencies where the sensitivities of the temporal
frequency shifts change sign also change in phase with the average
level of solar activity.

---------------------------------------------------------
Title: Transient Artifacts in SDO/HMI Flare Observations
Authors: Martinez Oliveros, Juan Carlos; Lindsey, C.; Hudson, H.;
Schou, J.; Couvidat, S.
2011SPD....42.2123M Altcode: 2011BAAS..43S.2123M
The Helioseismic and Magnetic Imager (HMI) on SDO provides a new
tool for the systematic observation of white-light flares, including
Doppler and magnetic information as well as continuum. In our initial
analysis of the highly impulsive gamma-ray flare SOL2010-06-12T00:57
(Martinez-Oliveros et al. 2011), we detected an apparently artifactual
blue shift in the two footpoint sources. We have now deployed the PASCAL
algorithm for the same flare as viewed in GONG++ data. This algorithm
makes it possible to obtain much better photometry (plus Doppler and
magnetic measurements) from the ground-based data. Using GONG++ we
have demonstrated the artifactual nature of the apparent blueshift,
finding instead weak redshifts at the foopoints. We discuss the flare
physics associated with these observations and describe the use of
PASCAL (with GONG++ or other ground-based data) as a complement to
the systematic SDO data.

---------------------------------------------------------
Title: A Comparison Of Solar High-degree p-mode Parameters From HMI
And MDI
Authors: Rabello-Soares, M. Cristina; Bogart, R.; Korzennik, S.;
Larson, T.; Reiter, J.; Rhodes, E.; Schou, J.
2011SPD....42.1606R Altcode: 2011BAAS..43S.1606R
Solar acoustic modes have been successfully used to make inferences
about the solar interior. The comparison of independent contemporaneous
data sets is important to test the reliability of our inferences. Here
we compare helioseismic data from the Michelson Doppler Imager (MDI)
on board SOHO with Helioseismic and Magnetic Imager (HMI) on board SDO
using spherical harmonic decomposition and ring-diagram analysis. We
will focus on the analysis of high-degree modes. They propagate through
the outer layers of the Sun giving valuable information about this
region. This interesting region is the seat of the near-surface shear
layer, where the excitation and damping mechanisms are believed to be
concentrated and where the effects of the equation of state are felt
most strongly.

---------------------------------------------------------
Title: The Height of White-light Flare Continuum Formation
Authors: Martinez Oliveros, J.; Hudson, Hugh; Krucker, S.; Schou,
J.; Couvidat, S.
2011SPD....42.2211M Altcode: 2011BAAS..43S.2211M
White-light continuum and hard X-ray emission in flares have strong
correlations in time and in horizontal position, but at present we do
not have a clear idea about their height structures. On 24 February
2011 a white-light flare (SOL2011-02-24T07:35) was observed on the
east limb, simultaneously by the Helioseismic Magnetic Imager (HMI)
on the Solar Dynamics Observatory (SDO), and by the Reuven Ramaty
High Energy Solar Spectroscopic Imager (RHESSI). This observation
gives us the opportunity to determine the heights of these emissions
directly, limited only by the limb references for the two spacecraft,
with almost no projection undertainty. HMI obtained clear images in
the pseudo-continuum around 6173A, and RHESSI obtained hard X-ray
images. For both data sets, the precision of centroid determination
is of order 0.1 arc s. We believe that the position of the white-light
limb, as a local reference, can also be understood at a corresponding
level of accuracy for the two data sets. We report the results of
this analysis and discuss our findings in terms of present models of
particle acceleration and energy transport in the impulsive phase.

---------------------------------------------------------
Title: Extending Global Helioseismic Measurements From MDI to HMI
Authors: Schou, Jesper; Larson, T. P.
2011SPD....42.1605S Altcode: 2011BAAS..43S.1605S
With the end of regular observations using MDI the question arises if
we can provide good continuity between MDI and HMI. In particular MDI
has shown intriguing differences in the torsional oscillation between
the two most recent 11-year solar cycles. In this poster we will discuss
some of the instrumental differences as well as examine the differences
in global mode parameters and inferred quantities between the two
during the period of overlap of MDI and HMI observations. This
work is supported by NASA contract NAS5-02139 to Stanford University.

---------------------------------------------------------
Title: Medium-l Global Helioseismology with MWO Dopplergrams
Authors: Larson, Timothy P.; Schou, J.; Rhodes, E. J.; Spinella, A.;
Irish, S.
2011SPD....42.1609L Altcode: 2011BAAS..43S.1609L
The 60-foot solar tower at Mount Wilson Observatory took high resolution
dopplergrams at a cadence of one minute between the summers of 1988 and
2006. Because this instrument overlaps with GONG and MDI, it provides
a unique opportunity to extend the inferences of those two projects
backwards in time to solar cycle 22. Furthermore, access to the MWO data
has been facilitated by its ingestion at the Joint Science Operations
Center (JSOC) at Stanford. For this initial study we choose for our
analysis a single summer in which MDI was also operating. By running
the MWO data through the same processing pipeline and comparing with
the results from MDI, we are able to determine how accurately the two
datasets can be combined. In future we will be able to use the MWO
data to compare the torsional oscillation during solar cycles 22 and 23.

---------------------------------------------------------
Title: Large-scale Zonal Flows During the Solar Minimum -- Where Is
Cycle 25?
Authors: Hill, Frank; Howe, R.; Komm, R.; Christensen-Dalsgaard, J.;
Larson, T. P.; Schou, J.; Thompson, M. J.
2011SPD....42.1610H Altcode: 2011BAAS..43S.1610H
The so-called torsional oscillation is a pattern of migrating zonal flow
bands that move from mid-latitudes towards the equator and poles as the
magnetic cycle progresses. Helioseismology allows us to probe these
flows below the solar surface. The prolonged solar minimum following
Cycle 23 was accompanied by a delay of 1.5 to 2 years in the migration
of bands of faster rotation towards the equator. During the rising phase
of Cycle 24, while the lower-level bands match those seen in the rising
phase of Cycle 23, the rotation rate at middle and higher latitudes
remains slower than it was at the corresponding phase in earlier cycles,
perhaps reflecting the weakness of the polar fields. In addition,
there is no evidence of the poleward flow associated with Cycle 25. We
will present the latest results based on nearly sixteen years of global
helioseismic observations from GONG and MDI, with recent results from
HMI, and discuss the implications for the development of Cycle 25.

---------------------------------------------------------
Title: HMI: First Results
Authors: Centeno, R.; Tomczyk, S.; Borrero, J. M.; Couvidat,
S. Hayashi, K.; Hoeksema, T.; Liu, Y.; Schou, J.
2011ASPC..437..147C Altcode: 2010arXiv1012.3796C
The Helioseismic and Magnetic Imager (HMI) has just started producing
data that will help determine what the sources and mechanisms of
variability in the Sun's interior are. The instrument measures the
Doppler shift and the polarization of the Fe I 6173 Å line, on the
entire solar disk at a relatively-high cadence, in order to study
the oscillations and the evolution of the full vector magnetic field
of the solar Photosphere. After the data are properly calibrated,
they are given to a Milne-Eddington inversion code (VFISV, Borrero et
al. 2010) whose purpose is to infer certain aspects of the physical
conditions in the Sun's Photosphere, such as the full 3-D topology
of the magnetic field and the line-of-sight velocity at the solar
surface. We will briefly describe the characteristics of the inversion
code, its advantages and limitations -both in the context of the model
atmosphere and the actual nature of the data-, and other aspects of its
performance on such a remarkable data load. Also, a cross-comparison
with near-simultaneous maps from the Spectro-Polarimeter (SP) onboard
Hinode will be made.

---------------------------------------------------------
Title: Imaging Spectroscopy of a White-Light Solar Flare
Authors: Martínez Oliveros, J. C.; Couvidat, S.; Schou, J.; Krucker,
S.; Lindsey, C.; Hudson, H. S.; Scherrer, P.
2011SoPh..269..269M Altcode: 2011SoPh..tmp....7M; 2010arXiv1012.0344M
We report observations of a white-light solar flare
(SOL2010-06-12T00:57, M2.0) observed by the Helioseismic Magnetic
Imager (HMI) on the Solar Dynamics Observatory (SDO) and the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The HMI data
give us the first space-based high-resolution imaging spectroscopy
of a white-light flare, including continuum, Doppler, and magnetic
signatures for the photospheric Fe I line at 6173.34 Å and its
neighboring continuum. In the impulsive phase of the flare, a bright
white-light kernel appears in each of the two magnetic footpoints. When
the flare occurred, the spectral coverage of the HMI filtergrams (six
equidistant samples spanning ±172 mÅ around nominal line center)
encompassed the line core and the blue continuum sufficiently far from
the core to eliminate significant Doppler crosstalk in the latter, which
is otherwise a possibility for the extreme conditions in a white-light
flare. RHESSI obtained complete hard X-ray and γ-ray spectra (this
was the first γ-ray flare of Cycle 24). The Fe I line appears to be
shifted to the blue during the flare but does not go into emission; the
contrast is nearly constant across the line profile. We did not detect
a seismic wave from this event. The HMI data suggest stepwise changes
of the line-of-sight magnetic field in the white-light footpoints.

---------------------------------------------------------
Title: First Global Rotation Inversions of HMI Data
Authors: Howe, R.; Larson, T. P.; Schou, J.; Hill, F.; Komm, R.;
Christensen-Dalsgaard, J.; Thompson, M. J.
2011JPhCS.271a2061H Altcode:
We present the first 2-dimensional global rotational inversions of
medium-degree p-mode data from the Helioseismic and Magnetic Imager,
and compare the results with inversions of Michelson Doppler Imager
data for the same time period. The inferred rotation profiles show
good agreement between the two instruments.

---------------------------------------------------------
Title: The torsional oscillation and the new solar cycle
Authors: Howe, R.; Hill, F.; Komm, R.; Christensen-Dalsgaard, J.;
Larson, T. P.; Schou, J.; Thompson, M. J.; Ulrich, R.
2011JPhCS.271a2074H Altcode:
We present updated observations of the pattern of migrating solar
zonal flows known as the torsional oscillation, covering 15 years of
helioseismic measurements with GONG and MDI and 30 years of surface
Doppler observations from Mount Wilson. We compare the behavior of the
flows during the extended solar minimum following Cycle 23 with that in
earlier minima. We demonstrate that the timing of the migration of the
zonal flow belts may be of some use in predicting the start of the new
cycle. We also note that the behavior of the high-latitude part of the
pattern currently differs from that seen early in the previous cycle,
with the high-latitude poleward-migrating branch still not established.

---------------------------------------------------------
Title: Rotation-rate variations at the tachocline: An update
Authors: Howe, R.; Komm, R.; Hill, F.; Christensen-Dalsgaard, J.;
Larson, T. P.; Schou, J.; Thompson, M. J.; Toomre, J.
2011JPhCS.271a2075H Altcode:
After 15 years of GONG and MDI observations of the solar interior
rotation, we revisit the issue of variations in the rotation rate near
the base of the convection zone. The 1.3-year period seen in the first
few years of the observations disappeared after 2000 and has still
not returned. On the other hand, the agreement between GONG and MDI
observations suggests that variations seen in this region have some
solar origin, whether a true rotation-rate change or possibly mere
stochastic variation; we present a numerical experiment supporting
this contention.

---------------------------------------------------------
Title: HMI global helioseismology data analysis pipeline
Authors: Larson, Tim; Schou, Jesper
2011JPhCS.271a2062L Altcode:
The HMI global helioseismology data analysis pipeline is based largely
on the MDI medium-l program. All of the modules that ran in the SOI
Science Support Center have been ported for use in the SDO Joint Science
Operations Center (JSOC) and given greater functionality. Many errors
and approximations which are present in the standard MDI pipeline
have been corrected and improvements have been added. Scripts have
been written to automate the submission of compute jobs to our local
cluster; it is now possible to go from dopplergrams to mode parameters
with the push of a button. JSOC dataseries have been created to
hold all intermediate data products, timeseries, window functions,
and mode parameters. Here we discuss the operation of the pipeline,
the structure of the data it generates, and access to the same.

---------------------------------------------------------
Title: Temporal changes in the frequencies and widths of the solar
p-mode oscillations
Authors: Rhodes, E. J., Jr.; Reiter, J.; Schou, J.; Larson, T.;
Scherrer, P.; Brooks, J.; McFaddin, P.; Miller, B.; Rodriguez, J.;
Yoo, J.
2011JPhCS.271a2029R Altcode:
We present a study of the temporal changes in the sensitivities of the
frequencies and widths of the solar p-mode oscillations to corresponding
changes in the levels of solar activity during Solar Cycle 23. From MDI
and GONG++ full-disk Dopplergram three-day time series obtained between
1996 and 2008 we have computed a total of 221 sets of m-averaged power
spectra for spherical harmonic degrees ranging up to 1000. We have then
fit these 221 sets of m-averaged power spectra using our WMLTP fitting
code and both symmetric Lorentzian profiles for the peaks as well as
the asymmetric profile of Nigam and Kosovichev to obtain 442 tables
of p-mode parameters. We then inter-compared these 442 tables which
comprise in excess of 5.3 million p-mode parameters, and we performed
linear regression analyses of the differences in p-mode frequencies
and widths as functions of the differences in as many as ten different
solar activity indices. From these linear regression analyses we have
discovered new signatures of the frequency shifts of the p-modes and
a similar, but slightly different, signature of the temporal shifts
in the widths of the oscillations.

---------------------------------------------------------
Title: Wavelike Properties of Supergranulation
Authors: Lee, S.; Beck, J. G.; Schou, J.; Stanford Solar Observatories
Group
2010AGUFMSH11A1606L Altcode:
Supergranulation has been a long studied feature of solar
convection. While the cellular pattern and horizontal outflows are
consistent with a convective phenomenon, the details of supergranulation
indicate that it can not be explained by a simple convective model. One
such aspect is its apparent super-rotation. Supergranulation appears to
rotate a few percent faster than the solar surface plasma. Some have
theorized that this super-rotation can be explained by a wave like
component in the supergranulation signal. Here, we present research
that cumulates into the discovery of two distinct components of the
supergranulation rotation rate using dopplergrams obtained from the
HMI instrument aboard the Solar Dynamics Observatory.

---------------------------------------------------------
Title: A Precise Asteroseismic Age and Radius for the Evolved Sun-like
Star KIC 11026764
Authors: Metcalfe, T. S.; Monteiro, M. J. P. F. G.; Thompson, M. J.;
Molenda-Żakowicz, J.; Appourchaux, T.; Chaplin, W. J.; Doǧan, G.;
Eggenberger, P.; Bedding, T. R.; Bruntt, H.; Creevey, O. L.; Quirion,
P. -O.; Stello, D.; Bonanno, A.; Silva Aguirre, V.; Basu, S.; Esch,
L.; Gai, N.; Di Mauro, M. P.; Kosovichev, A. G.; Kitiashvili, I. N.;
Suárez, J. C.; Moya, A.; Piau, L.; García, R. A.; Marques, J. P.;
Frasca, A.; Biazzo, K.; Sousa, S. G.; Dreizler, S.; Bazot, M.; Karoff,
C.; Frandsen, S.; Wilson, P. A.; Brown, T. M.; Christensen-Dalsgaard,
J.; Gilliland, R. L.; Kjeldsen, H.; Campante, T. L.; Fletcher, S. T.;
Handberg, R.; Régulo, C.; Salabert, D.; Schou, J.; Verner, G. A.;
Ballot, J.; Broomhall, A. -M.; Elsworth, Y.; Hekker, S.; Huber, D.;
Mathur, S.; New, R.; Roxburgh, I. W.; Sato, K. H.; White, T. R.;
Borucki, W. J.; Koch, D. G.; Jenkins, J. M.
2010ApJ...723.1583M Altcode: 2010arXiv1010.4329M
The primary science goal of the Kepler Mission is to provide
a census of exoplanets in the solar neighborhood, including the
identification and characterization of habitable Earth-like planets. The
asteroseismic capabilities of the mission are being used to determine
precise radii and ages for the target stars from their solar-like
oscillations. Chaplin et al. published observations of three bright
G-type stars, which were monitored during the first 33.5 days of science
operations. One of these stars, the subgiant KIC 11026764, exhibits a
characteristic pattern of oscillation frequencies suggesting that it
has evolved significantly. We have derived asteroseismic estimates of
the properties of KIC 11026764 from Kepler photometry combined with
ground-based spectroscopic data. We present the results of detailed
modeling for this star, employing a variety of independent codes and
analyses that attempt to match the asteroseismic and spectroscopic
constraints simultaneously. We determine both the radius and the age
of KIC 11026764 with a precision near 1%, and an accuracy near 2%
for the radius and 15% for the age. Continued observations of this
star promise to reveal additional oscillation frequencies that will
further improve the determination of its fundamental properties.

---------------------------------------------------------
Title: HMI Global Helioseismology Data Analysis Pipeline
Authors: Larson, Timothy P.; Schou, J.
2010AAS...21640205L Altcode: 2010BAAS...41..873L
The HMI global helioseismology data analysis pipeline is based largely
on the MDI medium-l program. All of the modules that ran in the SOI
Science Support Center have been ported for use in the SDO Joint Science
Operations Center (JSOC) and given greater functionality. Many errors
and approximations which are present in the standard MDI pipeline
have been corrected and improvements have been added. Scripts have
been written to automate the submission of compute jobs to our local
cluster; it is now possible to go from dopplergrams to mode parameters
with the push of a button. JSOC dataseries have been created to hold
all intermediate data products, timeseries, window functions, and mode
parameters. In this poster we discuss the operation of the pipeline,
the structure of the data it generates, and access to the same. This
work has been supported by NASA contract NAS5-02139.

---------------------------------------------------------
Title: First Data From the Helioseismic and Magnetic Imager
Authors: Schou, Jesper; HMI Team
2010AAS...21630801S Altcode:
With the successful launch of the Solar Dynamics Observatory on
February 11, 2010 and the HMI commissioning activities scheduled
to be completed, we expect to be producing science quality data by
the time of this meeting. In this talk we will start by describing
some of the results from the commissioning activities and show how
the measured performance compares to that expected. I will also
show a selection of science data including Dopplergrams, Line-of-Sight
magnetograms and vector magnetograms. This work was supported by NASA
through contract NAS5-02139 to Stanford University.

---------------------------------------------------------
Title: Improving Leakage Matrix Calculation
Authors: Larson, Timothy P.; Schou, J.
2010AAS...21640010L Altcode: 2010BAAS...41..856L
Previous work has revealed discrepancies between the analysis of MDI
full disk data and medium-l data from the same time period. Most
notably, the full disk analysis lacks some systematic errors
and generally results in better fits to the data. One of the
differences between the two analyses is the leakage matrix used in
the peakbagging. In this poster we discuss the effect of such things
as proper pixel integration, higher resolution spherical harmonics,
improved interpolation in the remapping, and various point spread
functions on the leakage matrix itself and the resulting mode
parameters. This work has been supported by NASA contract NAS5-02139.

---------------------------------------------------------
Title: The Asteroseismic Potential of Kepler: First Results for
Solar-Type Stars
Authors: Chaplin, W. J.; Appourchaux, T.; Elsworth, Y.; García,
R. A.; Houdek, G.; Karoff, C.; Metcalfe, T. S.; Molenda-Żakowicz,
J.; Monteiro, M. J. P. F. G.; Thompson, M. J.; Brown, T. M.;
Christensen-Dalsgaard, J.; Gilliland, R. L.; Kjeldsen, H.; Borucki,
W. J.; Koch, D.; Jenkins, J. M.; Ballot, J.; Basu, S.; Bazot, M.;
Bedding, T. R.; Benomar, O.; Bonanno, A.; Brandão, I. M.; Bruntt,
H.; Campante, T. L.; Creevey, O. L.; Di Mauro, M. P.; Doǧan,
G.; Dreizler, S.; Eggenberger, P.; Esch, L.; Fletcher, S. T.;
Frandsen, S.; Gai, N.; Gaulme, P.; Handberg, R.; Hekker, S.; Howe,
R.; Huber, D.; Korzennik, S. G.; Lebrun, J. C.; Leccia, S.; Martic,
M.; Mathur, S.; Mosser, B.; New, R.; Quirion, P. -O.; Régulo, C.;
Roxburgh, I. W.; Salabert, D.; Schou, J.; Sousa, S. G.; Stello, D.;
Verner, G. A.; Arentoft, T.; Barban, C.; Belkacem, K.; Benatti, S.;
Biazzo, K.; Boumier, P.; Bradley, P. A.; Broomhall, A. -M.; Buzasi,
D. L.; Claudi, R. U.; Cunha, M. S.; D'Antona, F.; Deheuvels, S.;
Derekas, A.; García Hernández, A.; Giampapa, M. S.; Goupil, M. J.;
Gruberbauer, M.; Guzik, J. A.; Hale, S. J.; Ireland, M. J.; Kiss,
L. L.; Kitiashvili, I. N.; Kolenberg, K.; Korhonen, H.; Kosovichev,
A. G.; Kupka, F.; Lebreton, Y.; Leroy, B.; Ludwig, H. -G.; Mathis, S.;
Michel, E.; Miglio, A.; Montalbán, J.; Moya, A.; Noels, A.; Noyes,
R. W.; Pallé, P. L.; Piau, L.; Preston, H. L.; Roca Cortés, T.;
Roth, M.; Sato, K. H.; Schmitt, J.; Serenelli, A. M.; Silva Aguirre,
V.; Stevens, I. R.; Suárez, J. C.; Suran, M. D.; Trampedach, R.;
Turck-Chièze, S.; Uytterhoeven, K.; Ventura, R.; Wilson, P. A.
2010ApJ...713L.169C Altcode: 2010arXiv1001.0506C
We present preliminary asteroseismic results from Kepler on three G-type
stars. The observations, made at one-minute cadence during the first
33.5 days of science operations, reveal high signal-to-noise solar-like
oscillation spectra in all three stars: about 20 modes of oscillation
may be clearly distinguished in each star. We discuss the appearance of
the oscillation spectra, use the frequencies and frequency separations
to provide first results on the radii, masses, and ages of the stars,
and comment in the light of these results on prospects for inference
on other solar-type stars that Kepler will observe.

---------------------------------------------------------
Title: Toward Eliminating Systematic Errors in Intermediate-Degree
p-Mode Measurements
Authors: Vorontsov, S. V.; Jefferies, S. M.; Giebink, C.; Schou, J.
2009ASPC..416..301V Altcode:
We report new measurements of p-mode frequency splittings from Michelson
Doppler Imager (MDI) in which systematic errors, previously seen,
appear to have been eliminated. We identify neglect of the effects
of mode coupling by differential rotation as a major source of the
systematic errors.

---------------------------------------------------------
Title: The internal dynamics and magnetism of the sun -- the
perspective from global helioseismology (Invited)
Authors: Howe, R.; Hill, F.; Komm, R.; Christensen-Dalsgaard, J.;
Schou, J.; Thompson, M. J.
2009AGUFMSH11B..04H Altcode:
Helioseismology allows us to probe the interior dynamics of the
Sun. Observations over the past three decades reveal the interior
rotation profile, with a near-surface shear layer, differential
rotation throughout the convection zone, a strong shear layer -- the
tachocline -- at the base of the convection zone, and approximately
uniform rotation in the radiative interior. Since the mid 1990's,
continuous observations from the Global Oscillations Network Group and
the Michelson Doppler Imager have allowed the study of subtle temporal
variations in the rotation within the convection zone. The so-called
"torsional oscillation" pattern of migrating zonal flows accompanying
the surface activity migration during the solar cycle has been shown to
penetrate deep within the convection zone. During the current extended
solar minimum, the flow bands can be seen to migrate more slowly towards
the equator than was seen in the previous minimum. There have also been
(still unconfirmed) findings of shorter-term variations in the rotation
rate close to the tachocline during the early years of the previous
solar cycle. This review will describe the important results and give
an update on the most recent observations of the interior dynamics as
we await the rise of solar cycle 24.

---------------------------------------------------------
Title: Variations in Global Mode Analysis
Authors: Larson, T.; Schou, J.
2009ASPC..416..311L Altcode:
As with any data analysis, the standard Michelson Doppler Imager (MDI)
medium-l analysis pipeline is based on approximations and somewhat
arbitrary choices in the processing. It is furthermore known that the
results of the standard analysis contain systematic errors, most notably
a bump in the normalized residuals of the a-coefficients around 3.4 mHz,
an annual variation in f-mode frequencies, and possibly a polar jet in
the rotation inversions. Our previous work has explored how these errors
are affected by making various corrections to the analysis. In this
article we extend our study to include a comparison of the results of
full disk data to those of the binned data we have previously used. We
go on to explore how several choices made in the analysis, such as
the amount of zero padding and the width of the fitting interval,
affect the mode parameters and the magnitude of the systematic errors.

---------------------------------------------------------
Title: The Torsional Oscillation and the Solar Minimum
Authors: Howe, R.; Hill, F.; Komm, R.; Christensen-Dalsgaard, J.;
Schou, J.; Thompson, M. J.
2009AGUFM.U34A..03H Altcode:
The so-called torsional oscillation is a pattern of zonal flow bands,
detected at the solar surface by direct Doppler measurements and within
the convection zone by helioseismic measurements such as those carried
out by the Global Oscillations Network Group and the Michelson Doppler
Imager, that migrates from mid-latitudes towards the equator and poles
with each solar cycle. In the current minimum the low-latitude branch
of the pattern can be seen to have taken at least a year longer to
migrate towards the equator than was the case in the previous minimum. A
flow configuration matching that of the previous minimum was reached
during 2008, and by early 2009 the fast-rotating belt associated with
the new cycle had reached the latitude at which the onset of activity
was seen in Cycle 23, but magnetic activity has remained low. We will
present the most recent results and consider the implications for the
new solar cycle.

---------------------------------------------------------
Title: The Torsional Oscillation and the Solar Cycle: Is it Minimum
Yet?
Authors: Howe, R.; Komm, R.; Hill, F.; Larson, T.; Schou, J.; Thompson,
M. J.; Ulrich, R. K.
2009ASPC..416..269H Altcode:
The torsional oscillation pattern of migrating zonal flows is related to
the solar activity cycle. In the approach to solar minimum, we compare
the current flow profile with that seen at the previous minimum, using
Global Oscillation Network Group (GONG) and Michelson Doppler Imager
(MDI) data as well as Mount Wilson Doppler observations that reach
further back in time. Will the flow pattern at the upcoming minimum
match that for the previous one?

---------------------------------------------------------
Title: Inferring Small-Scale Flatfields from Solar Rotation
Authors: Wachter, R.; Schou, J.
2009SoPh..258..331W Altcode:
We present a method to infer small-scale flatfields for imaging solar
instruments using only regular-observation intensity images with a
fixed field of view. The method is related to the flatfielding method
developed by Kuhn, Lin, and Loranz (Publ. Astron. Soc. Pac. 103,
1097 - 1108, 1991), but does not require image offsets. Instead, it
takes advantage of the fact that the solar image is changing in the CCD
reference frame due to solar rotation. We apply the method to data sets
of MDI filtergrams and compare the results to flat fields derived with
other methods. Finally, we discuss the planned implementation of this
method in the data processing for Helioseismic and Magnetic Imager on
the Solar Dynamics Observatory.

---------------------------------------------------------
Title: A Note on the Torsional Oscillation at Solar Minimum
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R.;
Schou, J.; Thompson, M. J.
2009ApJ...701L..87H Altcode: 2009arXiv0907.2965H
We examine the evolution of the zonal flow pattern in the upper solar
convection zone during the current extended solar minimum, and compare
it with that during the previous minimum. The results suggest that
a configuration matching that at the previous minimum was reached
during 2008, but that the flow band corresponding to the new cycle has
been moving more slowly toward the equator than was observed in the
previous cycle, resulting in a gradual increase in the apparent length
of the cycle during the 2007-2008 period. The current position of the
lower-latitude fast-rotating belt corresponds to that seen around the
onset of activity in the previous cycle.

---------------------------------------------------------
Title: Large Scale Flows From Eigenfunction Fitting
Authors: Schou, Jesper; Woodard, M. F.; Birch, A. C.
2009SPD....40.0705S Altcode:
It is well known that the eigenfunctions, as well as the
eigenfrequencies, of normal modes are perturbed by flows and other
asphericities. In the case of meridional flows, it is known that
while the perturbations to the frequencies are second order, the
eigenfunctions are perturbed at first order in the strength of the
meridional flow. This leads to the question of whether one might be
able to measure meridional flows using the observed eigenfunction
perturbations. Here we discuss how well we expect to be able to do
this and show some preliminary analysis of MDI data.

---------------------------------------------------------
Title: Exploring Data Discrepancies in Global Mode Analysis
Authors: Larson, Timothy P.; Schou, J.
2009SPD....40.0717L Altcode:
The MDI instrument produces both full resolution and binned
Dopplergrams. Although one might expect the analysis of these two
data products to give the same results in the medium-l range, in
fact significant differences are observed, depending on both spatial
resolution and image apodization. This can clearly be seen in raw
mode parameters such as amplitude as well as in inversions of mode
frequencies. Furthermore, the difference seems to vary from year to
year. In this poster we shall examine the first 13 dynamics runs taken
by MDI and try to account for these discrepancies.

---------------------------------------------------------
Title: The HMI Magnetic Field Pipeline
Authors: Hoeksema, Jon Todd; Liu, Y.; Schou, J.; Scherrer, P.; HMI
Science Team
2009SPD....40.1701H Altcode:
The Helioseismic and Magnetic Imager (HMI) will provide frequent
full-disk magnetic field data after launch of the Solar Dynamics
Observatory (SDO), currently scheduled for fall 2009. 16 megapixel
line-of-sight magnetograms (Blos) will be recorded every 45 seconds. A
full set of polarized filtergrams needed to determine the vector
magnetic field requires 90 seconds. Quick-look data will be available
within a few minutes of observation. Quick-look space weather and
browse products must have identified users, and the list currently
includes full disk magnetograms, feature identification and movies,
12-minute disambiguated vector fields in active region patches,
time evolution of AR indices, synoptic synchronic frames, potential
and MHD model results, and 1 AU predictions. A more complete set of
definitive science data products will be offered about a day later and
come in three types. "Pipeline” products, such as full disk vector
magnetograms, will be computed for all data on an appropriate cadence. A
larger menu of "On Demand” products, such as Non-Linear Force Free
Field snapshots of an evolving active region, will be produced whenever
a user wants them. Less commonly needed "On Request” products that
require significant project resources, such as a high resolution MHD
simulation of the global corona, will be created subject to availability
of resources. Further information can be found at the SDO Joint Science
Operations Center web page, jsoc.stanford.edu

---------------------------------------------------------
Title: Asteroseismology: The Next Frontier in Stellar Astrophysics
Authors: Giampapa, Mark S.; Aerts, Conny; Bedding, Tim; Bonanno,
Alfio; Brown, Timothy M.; Christensen-Dalsgaard, Jorgen; Dominik,
Martin; Ge, Jian; Gilliland, Ronald L.; Harvey, J. W.; Hill, Frank;
Kawaler, Steven D.; Kjeldsen, Hans; Kurtz, D. W.; Marcy, Geoffrey W.;
Matthews, Jaymie M.; Monteiro, Mario Joao P. F. G.; Schou, Jesper
2009astro2010S..91G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Improvements in global mode analysis
Authors: Larson, T. P.; Schou, J.
2008JPhCS.118a2083L Altcode:
As with any data analysis, the standard MDI medium-l analysis pipeline
is based on several approximations. Physical effects such as line
asymmetry, horizontal displacement at the solar surface, and distortion
of eigenfunctions have been ignored, as well as cubic distortion in the
optics and instrumental errors in the plate scale and orientation of
the CCD. Furthermore, we see several systematic errors in the results
of the analysis, most notably an annual variation in f-mode frequencies
and a bump in the normalized residuals of the a-coefficients around
3.4 mHz, which may relate to polar jets in the inversions. We have
reprocessed several years of data applying the above corrections, and
made improvements in the pipeline algorithm itself by recomputing the
locations of bad data points and using updated routines for detrending
and gapfilling. Along the way the pipeline has been almost entirely
automated. Here we discuss the resulting changes in mode parameters
and their effect on the magnitude of systematic errors.

---------------------------------------------------------
Title: High degree modes &amp; instrumental effects
Authors: Korzennik, S. G.; Rabello-Soares, M. C.; Schou, J.
2008JPhCS.118a2027K Altcode:
Full-disk observations taken with the Michelson Doppler Imager (MDI)
on board the Solar and Heliospheric Observatory (SOHO) spacecraft, or
the upgraded Global Oscillations Network Group (GONG) instruments, have
enough spatial resolution to resolve modes up to ι = 1000 if not ι =
1500. The inclusion of such high-degree modes (i.e., ι &lt;= 1000)
improves dramatically inferences near the surface. Unfortunately,
observational and instrumental effects cause the characterization
of high degree modes to be quite complicated. Indeed, the
characteristics of the solar acoustic spectrum are such that, for a
given order, mode lifetimes get shorter and spatial leaks get closer
in frequency as the degree of a mode increases. A direct consequence
of this property is that individual modes are resolved only at low
and intermediate degrees. At high degrees the individual modes blend
into ridges and the power distribution of the ridge defines the
ridge central frequency, masking the underlying mode frequency. An
accurate model of the amplitude of the peaks that contribute to the
ridge power distribution is needed to recover the underlying mode
frequency from fitting the ridge. We present a detailed discussion
of the modeling of the ridge power distribution, and the contribution
of the various observational and instrumental effects on the spatial
leakage, in the context of the MDI instrument. We have constructed a
physically motivated model (rather than an ad hoc correction scheme)
that results in a methodology that can produce unbiased estimates of
high-degree modes. This requires that the instrumental characteristics
are well understood, a task that has turned out to pose a major
challenge. We also present our latest results, where most of the
known instrumental and observational effects that affect specifically
high-degree modes were removed. These new results allow us to focus
our attention on changes with solar activity. Finally, we present
variations of mode frequencies resulting from solar activity over
most of solar cycle 23. We present the correlation of medium and high
degree modes with different solar indices. Our results confirm that
the frequency shift scaled by the relative mode inertia is a function
of frequency alone and follows a simple power law.

---------------------------------------------------------
Title: Analysis of MDI High-Degree Mode Frequencies and their
Rotational Splittings
Authors: Rabello-Soares, M. C.; Korzennik, S. G.; Schou, J.
2008SoPh..251..197R Altcode: 2008arXiv0808.2838R; 2008SoPh..tmp..133R
We present a detailed analysis of solar acoustic mode frequencies and
their rotational splittings for modes with degree up to 900. They were
obtained by applying spherical harmonic decomposition to full-disk
solar images observed by the Michelson Doppler Imager onboard the
Solar and Heliospheric Observatory spacecraft. Global helioseismology
analysis of high-degree modes is complicated by the fact that the
individual modes cannot be isolated, which has limited so far the use
of high-degree data for structure inversion of the near-surface layers
(r&gt;0.97R⊙). In this work, we took great care to recover
the actual mode characteristics using a physically motivated model
which included a complete leakage matrix. We included in our analysis
the following instrumental characteristics: the correct instantaneous
image scale, the radial and non-radial image distortions, the effective
position angle of the solar rotation axis, and a correction to the
Carrington elements. We also present variations of the mode frequencies
caused by the solar activity cycle. We have analyzed seven observational
periods from 1999 to 2005 and correlated their frequency shift with four
different solar indices. The frequency shift scaled by the relative mode
inertia is a function of frequency alone and follows a simple power law,
where the exponent obtained for the p modes is twice the value obtained
for the f modes. The different solar indices present the same result.

---------------------------------------------------------
Title: Analysis of MDI high-degree solar-p mode parameters
Authors: Rabello-Soares, M.; Korzennik, S.; Schou, J.
2008AGUSMSP21A..03R Altcode:
We present a detailed analysis of solar acoustic mode parameters for
modes with degree up to 900. They were obtained by applying spherical
harmonic decomposition to full-disk solar images observed by the
Michelson Doppler Imager (MDI) on board the Solar and Heliospheric
Observatory (SOHO) spacecraft. We have analyzed seven observational
periods (2-3 months long) from 1999 to 2005. A physically motivated
model including a complete leakage matrix was used to recover the
actual high-degree mode characteristics.

---------------------------------------------------------
Title: Helioseismic and Magnetic Imager Calibration and Expected
Performance
Authors: Schou, J.; Team, H.
2008AGUSMSP51B..14S Altcode:
The HMI instrument was delivered in November 2007 and has been
integrated onto the SDO spacecraft with launch planned for December
2008. During the integration and testing of the instrument an extensive
series of calibrations were performed. Here we describe some of the
calibrations performed and show selected results. We will also discuss
remaining efforts, such as on-orbit calibrations as well as describe the
expected performance of the instrument. The HMI project is supported
by the National Aeronautics and Space Administration under contract
NAS5-02139 to Stanford University.

---------------------------------------------------------
Title: Views of the Solar Torsional Oscillation
Authors: Howe, R.; Komm, R. W.; Hill, F.; Schou, J.; Thompson, M. J.
2008AGUSMSP41A..05H Altcode:
The pattern of zonal flows migrating towards the equator over the
solar cycle, known as the torsional oscillation, is well established
from both helioseismology and surface Doppler measurements. However,
the exact appearance of the pattern will vary depending on the form of
the overall rotation profile that has been subtracted, even when the
data cover a full eleven-year cycle. Here we compare the appearance
of the flow pattern when applying several different methods to the
MDI and GONG data for Solar Cycle 23.

---------------------------------------------------------
Title: Variations of the solar acoustic high-degree mode frequencies
over solar cycle 23
Authors: Rabello-Soares, M. C.; Korzennik, Sylvain G.; Schou, J.
2008AdSpR..41..861R Altcode: 2007arXiv0712.3608R
Using full-disk observations obtained with the Michelson Doppler Imager
(MDI) on board the Solar and Heliospheric Observatory (SOHO) spacecraft,
we present variations of the solar acoustic mode frequencies caused by
the solar activity cycle. High-degree (100 &lt; ℓ &lt; 900) solar
acoustic modes were analyzed using global helioseismology analysis
techniques over most of solar cycle 23. We followed the methodology
described in details in [Korzennik, S.G., Rabello-Soares, M.C., Schou,
J. On the determination of Michelson Doppler Imager high-degree mode
frequencies. ApJ 602, 481 515, 2004] to infer unbiased estimates
of high-degree mode parameters ([see also Rabello-Soares, M.C.,
Korzennik, S.G., Schou, J. High-degree mode frequencies: changes
with solar cycle. ESA SP-624, 2006]). We have removed most of the
known instrumental and observational effects that affect specifically
high-degree modes. We show that the high-degree changes are in good
agreement with the medium-degree results, except for years when the
instrument was highly defocused. We analyzed and discuss the effect
of defocusing on high-degree estimation. Our results for high-degree
modes confirm that the frequency shift scaled by the relative mode
inertia is a function of frequency and it is independent of degree.

---------------------------------------------------------
Title: Solar Cycle Changes Over 11 Years of Medium-Degree Helioseismic
Observations
Authors: Howe, Rachel; Komm, R. W.; Hill, F.; Christensen-Dalsgaard,
J.; Schou, J.; Thompson, M. J.
2007AAS...210.2218H Altcode: 2007BAAS...39..127H
The Global Oscillations Network Group (GONG) has now completed, and
the Michelson Doppler Imager (MDI) aboard SOHO will soon complete,
a full eleven years of continuous observations of the medium-degree
solar oscillations. This enables us to follow changes in the acoustic
mode parameters and interior dynamics over a full solar cycle. We
present results from observations of convection-zone dynamics, in
which the torsional oscillation pattern seen at the surface can be
followed throughout most of the bulk of the convection zone, and also
changes in the frequency, lifetime and amplitude of the modes which
can be shown to be closely related in space and time to the migrating
pattern of surface activity. This work utilizes data obtained by
the Global Oscillation Network Group (GONG) program, managed by the
National Solar Observatory, which is operated by AURA, Inc. under a
cooperative agreement with the National Science Foundation. SOHO is
a mission of international cooperation between ESA and NASA.

---------------------------------------------------------
Title: Helioseismic and Magnetic Imager Calibration Status
Authors: Schou, Jesper; HMI Calibration Team
2007AAS...210.2416S Altcode: 2007BAAS...39..131S
The HMI instrument is planned to be delivered shortly and launched
in August 2008. During the integration and testing of the instrument
we have performed an extensive series of calibrations. In this
poster we will start with a brief summary of the instrument status. We will then describe the calibrations performed so far including
some of the results. Finally we will describe what remains to be
done, including on-orbit calibrations, and how well we expect to be
able to calibrate the final data. The HMI project is supported
by the National Aeronautics and Space Administration under contract
NAS5-02139 to Stanford University.

---------------------------------------------------------
Title: Reduction of Systematic Errors in Global Mode Analysis
Authors: Larson, Timothy P.; Schou, J.
2007AAS...210.2202L Altcode: 2007BAAS...39..124L
In spite of the unprecedented success of the MDI Medium-l program, the
global mode analysis pipeline is known to contain errors. Physical
effects such as line assymmetry, horizontal displacement at the
solar surface, and distortion of eigenfunctions have been ignored, as
well as a wide array of instrumental effects. Additionally, certain
improvements in the pipeline algorithm itself are possible, most
notably in the gap-filling of time series. Perhaps unsurprisingly,
some features of the results seem to be the effect of systematic
errors. The most remarkable of these features are an annual variation
in f-mode frequency changes, a bump in the normalized residuals of the
a-coefficients around 3.4mHz, and polar jets in the inversions. In
this poster we discuss the application of a variety of corrections
to the analysis, the resulting changes in the mode parameters, and
the effect on the magnitude of systematic errors. We also describe
new software tools that been created to simplify and generalize the
pipeline, making it of greater utility to anyone interested in global
mode analysis. This work has been funded by NASA.

---------------------------------------------------------
Title: High-degree Mode Frequencies Using Global Helioseismology
Analysis Of MDI Observations And Their Variation With Solar Cycle.
Authors: Rabello-Soares, M. Cristina; Korzennik, S. G.; Schou, J.
2007AAS...210.2215R Altcode: 2007BAAS...39..126R
Using full-disk observations obtained with the Michelson Doppler
Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO)
spacecraft, we present solar acoustic mode frequencies and their
rotational splitting coefficients for modes with degree up to 900
determined using global helioseismology analysis. Most of the known
instrumental and observational effects that affect specifically the
high-degree modes have been removed. The structural and dynamical
properties of the near-surface layers of the Sun was analyzed through
the study of: (a) the difference between the observed and theoretical
frequencies; and (b) the solar rotation at different latitudes estimated
using a simple analytical method. We also present variations of
the mode frequencies resulting from solar activity over most of solar
cycle 23. We have studied the correlation of medium and high degree
modes with four different solar indices. Our results confirm that the
frequency shift scaled by the relative mode inertia is a function of
frequency alone and follows a simple power law.

---------------------------------------------------------
Title: Magnetic Field Vector Retrieval With the Helioseismic and
Magnetic Imager
Authors: Borrero, J. M.; Tomczyk, S.; Norton, A.; Darnell, T.; Schou,
J.; Scherrer, P.; Bush, R.; Liu, Y.
2007SoPh..240..177B Altcode: 2006astro.ph.11565B
We investigate the accuracy to which we can retrieve the solar
photospheric magnetic field vector using the Helioseismic and Magnetic
Imager (HMI) that will fly onboard of the Solar Dynamics Observatory by
inverting simulated HMI profiles. The simulated profiles realistically
take into account the effects of the photon noise, limited spectral
resolution, instrumental polarization modulation, solar p modes, and
temporal averaging. The accuracy of the determination of the magnetic
field vector is studied by considering the different operational modes
of the instrument.

---------------------------------------------------------
Title: Temporal variations in solar rotation at the bottom of the
convection zone: The current status
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R.;
Schou, J.; Thompson, M. J.; Toomre, J.
2007AdSpR..40..915H Altcode:
We present the most recent results on the short-period variations
in the solar rotation rate near the base of the convection zone. The
1.3-year period which was reported in the early years of solar cycle 23
appears not to persist after 2001, but there are hints of fluctuations
at a different period during the declining phase of the cycle.

---------------------------------------------------------
Title: Spectral Line Selection for HMI
Authors: Norton, A. A.; Pietarila Graham, J. D.; Ulrich, R. K.;
Schou, J.; Tomczyk, S.; Liu, Y.; Lites, B. W.; López Ariste, A.;
Bush, R. I.; Socas-Navarro, H.; Scherrer, P. H.
2006ASPC..358..193N Altcode:
We present information on two spectral lines, Fe I 6173 Å and Ni I 6768
Å, that were candidates for use in the Helioseismic and Magnetic Imager
(HMI) instrument. Both Fe I and Ni I profiles have clean continuum and
no blends that threaten performance. The higher Landé factor of Fe
I means its operational velocity range in regions of strong magnetic
field is smaller than for Ne I. Fe I performs better than Ni I for
vector magnetic field retrieval. Inversion results show that Fe I
consistently determines field strength and flux more accurately than
the Ni I line. Inversions show inclination and azimuthal errors are
recovered to ≈2° above 600 Mx/cm2 for Fe I and above
1000 Mx/cm2 for Ni I. The Fe I line was recommended, and
ultimately chosen, for use in HMI.

---------------------------------------------------------
Title: Magnetic Field Vector Retrieval with HMI
Authors: Borrero, J. M.; Tomczyk, S.; Norton, A. A.; Darnell, T.;
Schou, J.; Scherrer, P.; Bush, R. I.; Lui, Y.
2006ASPC..358..144B Altcode:
The Helioseismic and Magnetic Imager (HMI), on board the Solar Dynamics
Observatory (SDO), will begin data acquisition in 2008. It will provide
the first full-disk, high temporal cadence observations of the full
Stokes vector with a 0.5 arcsec pixel size. This will allow for a
continuous monitoring of the Solar magnetic-field vector. HMI data will
advance our understanding of the small- and large-scale magnetic field
evolution, its relation to the solar and global dynamic processes,
coronal field extrapolations, flux emergence, magnetic helicity, and
the nature of the polar magnetic fields. We summarize HMI's expected
operation modes, focusing on the polarization cross-talk induced
by the solar oscillations, and how this affects the magnetic-field
vector determination.

---------------------------------------------------------
Title: Spectral Line Selection for HMI: A Comparison of Fe I 6173
Å and Ni I 6768 Å
Authors: Norton, A. A.; Graham, J. Pietarila; Ulrich, R. K.; Schou,
J.; Tomczyk, S.; Liu, Y.; Lites, B. W.; Ariste, A. López; Bush,
R. I.; Socas-Navarro, H.; Scherrer, P. H.
2006SoPh..239...69N Altcode: 2006SoPh..tmp...88N; 2006astro.ph..8124N
We present a study of two spectral lines, Fe I 6173 Å and Ni I 6768 Å,
that were candidates to be used in the Helioseismic and Magnetic Imager
(HMI) for observing Doppler velocity and the vector magnetic field. The
line profiles were studied using the Mt. Wilson Observatory, the
Advanced Stokes Polarimeter and the Kitt Peak-McMath Pierce telescope
and one-meter Fourier transform spectrometer atlas. Both Fe I and Ni
I profiles have clean continua and no blends that threaten instrument
performance. The Fe I line is 2% deeper, 15% narrower, and has a 6%
smaller equivalent width than the Ni I line. The potential of each
spectral line to recover pre-assigned solar conditions is tested
using a least-squares minimization technique to fit Milne-Eddington
models to tens of thousands of line profiles that have been sampled
at five spectral positions across the line. Overall, the Fe I line
has a better performance than the Ni I line for vector-magnetic-field
retrieval. Specifically, the Fe I line is able to determine field
strength, longitudinal and transverse flux four times more accurately
than the Ni I line in active regions. Inclination and azimuthal angles
can be recovered to ≈2° above 600 Mx cm−2
for Fe I and above 1000 Mx cm−2 for Ni I. Therefore, the
Fe I line better determines the magnetic-field orientation in plage,
whereas both lines provide good orientation determination in penumbrae
and umbrae. We selected the Fe I spectral line for use in HMI due to
its better performance for magnetic diagnostics while not sacrificing
velocity information. The one exception to the better performance of the
Fe I line arises when high field strengths combine with high velocities
to move the spectral line beyond the effective sampling range. The
higher geff of Fe I means that its useful range of velocity
values in regions of strong magnetic field is smaller than Ni I.

---------------------------------------------------------
Title: High-degree mode frequencies: changes with solar cycle
Authors: Rabello-Soares, M. C.; Korzennik, S. G.; Schou, J.
2006ESASP.624E..71R Altcode: 2006soho...18E..71R
No abstract at ADS

---------------------------------------------------------
Title: Frequency, splitting, linewidth and amplitude estimates of
low-l p modes of alpha Cen A: analysis of WIRE photometry
Authors: Fletcher, S. T.; Chaplin, W. J.; Elsworth, Y.; Schou, J.;
Buzasi, D.
2006ESASP.624E..27F Altcode: 2006soho...18E..27F
No abstract at ADS

---------------------------------------------------------
Title: Solar Convection Zone Dynamics: How Sensitive Are Inversions
to Subtle Dynamo Features?
Authors: Howe, R.; Rempel, M.; Christensen-Dalsgaard, J.; Hill, F.;
Komm, R.; Larsen, R. M.; Schou, J.; Thompson, M. J.
2006ApJ...649.1155H Altcode:
The nearly 10 year span of medium-degree helioseismic data from the
Global Oscillation Network Group and the Michelson Doppler Imager has
allowed us to study the evolving flows in the solar convection zone
over most of solar cycle 23. Using two independent two-dimensional
rotation inversion techniques and extensive studies of the resolution
using artificial data from different assumed flow profiles, including
those generated from sample mean field dynamo models, we attempt to
assess the reality of certain features seen in the inferred rotation
profiles. Our results suggest that the findings from observations of
a substantial depth dependence of the phase of the zonal flow pattern
in the low latitudes, and the penetration of the flows deep into the
convection zone, are likely to be real rather than artifacts of the
inversion process.

---------------------------------------------------------
Title: Solar rotation and zonal flows from Mount Wilson 60 ft
tower data
Authors: Howe, R.; Bogart, R.; Christensen-Dalsgaard, J.; Rhodes,
E. J., Jr.; Rose, P.; Schou, J.
2006ESASP.624E..56H Altcode: 2006soho...18E..56H
No abstract at ADS

---------------------------------------------------------
Title: Line Shape Changes and Doppler Measurements in Solar Active
Regions. I. A Method for Correcting Dopplergrams from SOHO MDI
Authors: Wachter, R.; Schou, J.; Sankarasubramanian, K.
2006ApJ...648.1256W Altcode:
We present a method to correct MDI high-resolution Dopplergrams
in active regions for systematic observational errors due to the
spectral line shape changes. We use a measurement campaign with the
Advanced Stokes Polarimeter to investigate the influence of line shape
changes in active regions on MDI Dopplergram calibration. An estimate
of the width of the Ni I λ6768 absorption line used by MDI to measure
velocity provides a linear correction to the standard calibration. This
correction can be largely explained by the Zeeman broadening of the
absorption line.

---------------------------------------------------------
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: Frequency, splitting, linewidth and amplitude estimates
of low-ℓ p modes of α Cen A: analysis of Wide-Field Infrared
Explorer photometry
Authors: Fletcher, S. T.; Chaplin, W. J.; Elsworth, Y.; Schou, J.;
Buzasi, D.
2006MNRAS.371..935F Altcode: 2006astro.ph..7172F; 2006MNRAS.tmp..824F
We present results of fitting the 50-d time series of photometry of α
Cen A taken by the Wide-Field Infrared Explorer (WIRE) satellite in
1999. Both power spectrum and autocovariance function (ACF) fitting
techniques were used in an attempt to determine mode frequencies,
rotational splittings, lifetimes and amplitudes of low-l p modes. In
all, using both techniques, we managed to fit 18 modes (seven l = 0,
eight l = 1 and three l = 2) with frequencies determined to within
1-2 μHz. These estimates are shown to be 0.6 +/- 0.3 μHz lower, on
average, than the frequencies determined from two other more recent
studies, which used data gathered about 19 months after the WIRE
observations. This could be indicative of an activity cycle, although
due to the large uncertainty, more data would be needed to confirm
this. Over a range of 1700-2650 μHz, we were also able to use
the ACF fitting to determine an average lifetime of 3.9 +/- 1.4 d,
and an average rotational splitting of 0.54 +/- 0.22 μHz, which is
the first ever reliable estimate of this parameter. In contrast to
the ACF, the power spectrum fitting was shown to return significantly
biased results for these parameters.

---------------------------------------------------------
Title: The determination of global high-degree solar p-mode
parameters: challenges and new results
Authors: Rabello-Soares, M. C.; Korzennik, S. G.; Schou, J.
2006IAUJD..17E...7R Altcode:
We review the challenges of estimating unbiased mode parameters for
global high-degree solar acoustic modes (100 &lt; ℓ &lt; 1000), with
emphasis on the importance of knowing the instrumental characteristics
and how they affect mode parameters determination. We present new
estimates of the global high-degree mode parameters resulting from
incorporating our best knowledge of the MDI instrument on board SOHO
and we also present their dependence with the solar cycle.

---------------------------------------------------------
Title: Helioseismic and Magnetic Imager Calibration Status
Authors: Schou, Jesper; HMI Team
2006SPD....37.0606S Altcode: 2006BAAS...38..226S
The Helioseismic and Magnetic Imager (HMI) instrument, which is due to
be launched in August 2008 as part of the Solar Dynamics Observatory
spacecraft, is now nearly assembled and most of the ground based
calibrations have been performed.In this poster we will briefly
summarize the status of the instrument, describe the ground based
calibration efforts and outline the calibration efforts planned after
launch.Based on the calibration results we will also briefly discuss
the quality of the data expected from the instrument after launch.This
work is supported by the National Aeronautics and Space Administration
through contract NAS5-02139.

---------------------------------------------------------
Title: Changes Of The Solar Acoustic High-degree Mode Frequencies
Over The Solar Cycle
Authors: Korzennik, Sylvain G.; Rabello-Soares, M. C.; Schou, J.
2006SPD....37.0508K Altcode: 2006BAAS...38..224K
Using full-disk observations obtained with the Michelson Doppler
Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO)
spacecraft, we present frequency and frequency splitting variations
of high-degree (100 &lt; l &lt; 1000) solar acoustic modes over the
solar cycle using global helioseismology analysis techniques. Since we
analyzed high-degree modes, we focus on properties of the near-surface
solar region. We have corrected for most of the known instrumental
effects that affect the characterization of high-degree modes using
the methodology described in Korzennik et al. (2004) as to estimate
unbiased high-degree mode frequencies.

---------------------------------------------------------
Title: Large-Scale Zonal Flows Near the Solar Surface
Authors: Howe, R.; Komm, R.; Hill, F.; Ulrich, R.; Haber, D. A.;
Hindman, B. W.; Schou, J.; Thompson, M. J.
2006SoPh..235....1H Altcode:
Migrating bands of weak, zonal flow, associated with the activity
bands in the solar cycle, have been observed at the solar surface for
some time. More recently, these flows have been probed deep within the
convection zone using global helioseismology and examined in more detail
close to the surface with the techniques of local helioseismology. We
compare the near-surface results from global and local helioseismology
using data from the Michelson Doppler Imager and the Global Oscillation
Network Group with surface Doppler velocity measurements from the Mount
Wilson 150-foot tower and find that the results are in reasonable
agreement, with some explicable differences in detail. All of the
data sets show zones of faster rotation approaching the equator from
mid-latitudes during the solar cycle, with a variation at any given
location that can be approximately, but not completely, described by a
single sinusoid and an amplitude that does not drop off steeply below
the surface.

---------------------------------------------------------
Title: Variations of the solar acoustic high-degree mode parameters
over solar cycle 23
Authors: Rabello-Soares, M. C.; Korzennik, S. G.; Schou, J.
2006cosp...36.2668R Altcode: 2006cosp.meet.2668R
The structural and dynamical properties of the Sun as well as the
excitation and damping of the solar acoustic modes change with the solar
cycle The first two manifest themselves as changes in the acoustic mode
frequencies and frequency splittings While the last two are observed
as changes in the mode amplitudes and lifetimes Using full-disk
observations obtained with the Michelson Doppler Imager MDI on board
the Solar and Heliospheric Observatory SOHO spacecraft we present
the amplitude width frequency and frequency splitting variations of
high-degree 100 l 1000 solar acoustic modes over most of solar cycle
23 using global helioseismology analysis techniques By looking at
high-degree modes we focus on the properties of the near-surface solar
region Following the methodology described in detail in Korzennik
et al 2004 we have removed the known instrumental and observational
effects that affect specifically high-degree modes as well as possible
to infer unbiased estimates of high-degree mode parameters

---------------------------------------------------------
Title: Solar Convection-Zone Dynamics, 1995-2004
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R.;
Schou, J.; Thompson, M. J.
2005ApJ...634.1405H Altcode:
The nine-year span of medium-degree helioseismic data from the Global
Oscillation Network Group (GONG) and the Michelson Doppler Imager (MDI)
allows us to study the evolving zonal flows in the solar convection
zone over the rising phase, maximum, and early declining phase
of solar cycle 23. Using two independent two-dimensional rotation
inversion techniques, we investigate the depth profile of the flow
pattern known as the torsional oscillation. The observations suggest
that the flows penetrate deep within the convection zone-perhaps to
its base-even at low latitudes, and that the phase of the pattern is
approximately constant along lines of constant rotation rather than
lines of constant latitude.

---------------------------------------------------------
Title: How Sensitive are Rotation Inversions to Subtle Features of
the Dynamo?
Authors: Howe, R.; Rempel, M.; Christensen-Dalsgaard, J.; Schou, J.;
Thompson, M. J.; Komm, R.; Hill, F.
2005ASPC..346...99H Altcode:
Global rotation inversions can probe the pattern of zonal flows
well into the convection zone. In this paper, we test the ability
of the inversions to constrain the predictions of dynamo models. A
flux-transport dynamo model, including a mean-field theory of
differential rotation and allowing for feedback of the Lorentz force
on differential rotation and meridional flow, was used to produce a
22-year cycle of simulated rotation profiles. These were then subjected
to simulated inversions with realistic mode sets and errors, in order
to test how well the subtle subsurface features of the input profile
could be recovered. The preliminary results are quite encouraging.

---------------------------------------------------------
Title: Global, Local and Surface Measurements of Large-Scale Zonal
Flows Near the Solar Surface
Authors: Howe, R.; Komm, R. W.; Haber, D. A.; Hindman, B. W.; Ulrich,
R. K.; Schou, J.; Thompson, M. J.; Hill, F.
2005AGUSMSP32A..03H Altcode:
Migrating bands of weak zonal flow, associated with the activity
bands in the solar cycle, have been observed at the solar surface
for some time. More recently, these flows have been probed deep
within the convection zone using global helioseismology, and
examined in more detail close to the surface with the techniques of
local helioseismology. We compare the results from global and local
helioseismology using data from the Michelson Doppler Imager and the
GONG network and also Doppler measurements from Mount Wilson, and find
that the results are in reasonable agreement, with some explicable
differences in detail. This was work was supported by the National
Science Foundation and NASA.

---------------------------------------------------------
Title: Designing Data Services for the SDO AIA/HMI Joint Science
Operations Center
Authors: Larsen, R. M.; Bogart, R. S.; Scherrer, P. H.; Schou, J.;
Tian, K. Q.
2005AGUSMSH51B..04L Altcode:
The Joint Science Operations Center (JSOC) is designed to provide data
capture, archive, analysis, and distribution functions for both the
Helioseismic and Magnetic Imager and the Atmospheric Imaging Array
on the Solar Dynamics Observatory. These instruments are notable
for the large quantity of raw data they will generate, more than
doubling the total volume of all existing solar data in the first few
months alone. In addition, the JSOC will be required to handle a larger
component of higher-level data products than most previous missions. The
fundamental concept of the JSOC architecture is that the metadata and
bulk image data are stored separately, not being combined until the time
of use or export. This enables all metadata to be rapidly accessible
through a relational database. It permits flexible organization of the
data into virtual data sets or series, and the use of an abstracted
syntax, such as a URL, for data description and querying. Furthermore,
data organization details dictated by the requirements of efficient mass
storage can be hidden from the user. Multiple classes of access to the
processing and data are envisioned, ranging from production pipeline
modules generating standard mission products to workstation users
analyzing self-contained data products exported from the archive. The
JSOC design provides a rich set of primitives upon which VSO-like
services can be built, and should allow us to provide a unified view
of the data and data services to all classes of users.

---------------------------------------------------------
Title: New Steps Towards the Unbiased Characterization of High-Degree
Mode Frequencies
Authors: Rabello-Soares, M. C.; Korzennik, S. G.; Schou, J.
2005AGUSMSP11B..08R Altcode:
Bias in the characterization of high-degree mode frequency results
from the blending of individual modes into ridges as mode lifetimes get
shorter and spatial leaks get closer in frequency at high degrees. To
recover the actual underlying mode frequency from fitting the ridge,
an accurate model of the amplitude of the peaks that contribute to the
ridge power distribution is crucial. Such a model requires that the
instrumental characteristics be very well understood and very precisely
measured. We here present new results from our continuing effort to
estimate unbiased high-degree mode frequencies using full-disk data
from the Michelson Doppler Imager (MDI) on the Solar and Heliospheric
Observatory (SOHO). The methodology is based on the extensive analysis
presented in Korzennik et al (2004) - and will in turn be beneficial
to MDI, GONG and eventually HMI. The key improvement on our previous
work is the re-decomposition of the images onto spherical harmonic
component. This new spatial decomposition incorporates specific MDI
instrumental characteristics, like a more accurate plate scale, our
best model of the image distortion, the image orientation, etc... These
instrumental effects were introduced one at a time and their effect
carefully compared to the predictions of our model. As expected, by
including these corrections in the spatial decomposition, the residual
corrections that must be applied to the ridge frequency are reduced,
leading to a less biased frequency estimation.

---------------------------------------------------------
Title: Systematic errors in Dopplergrams of active regions
Authors: Wachter, R.; Schou, J.; Kosovichev, A. G.; Rajaguru, P.
2005AGUSMSP13A..07W Altcode:
Dopplergrams of magnetic regions are used in time-distance
helioseismology and for the investigation of oscillations in
sunspots. MDI Dopplergrams are produced onboard SOHO by measuring four
filtergrams which are sensitive to intensity fluctuations in the wings
of the Ni i absorption line at 6768 Å. The line of sight velocity
is inferred from a lookup table which is based on the shape of the
line in the quiet sun. In magnetic regions, however, the line shape
changes drastically and molecular absorption lines appear close to the
line's wavelength. This problem is equally relevant for the upcoming
Helioseismic and Magnetic Imager (HMI) which measures velocities based
on the same principles as MDI, using however a different absorption
line (Fe i at 6173 Å). Based on high spectral resolution images of
magnetic regions obtained by the ASP (Advanced Stokes Polarimeter)
instrument we show that the MDI Doppler velocities are systematically
underestimated in magnetic regions. We discuss possibilities to correct
the velocity measurements in magnetic regions using intensity and
magnetic field data.

---------------------------------------------------------
Title: The non-detection of oscillations in Procyon by MOST: Is it
really a surprise?
Authors: Bedding, T. R.; Kjeldsen, H.; Bouchy, F.; Bruntt, H.; Butler,
R. P.; Buzasi, D. L.; Christensen-Dalsgaard, J.; Frandsen, S.; Lebrun,
J. -C.; Martić, M.; Schou, J.
2005A&A...432L..43B Altcode: 2005astro.ph..1662B
We argue that the non-detection of oscillations in Procyon by the
MOST satellite reported by [CITE] is fully consistent with published
ground-based velocity observations of this star. We also examine the
claims that the MOST observations represent the best photometric
precision so far reported in the literature by about an order of
magnitude and are the most sensitive data set for asteroseismology
available for any star other than the Sun. These statements are
not correct, with the most notable exceptions being observations
of oscillations in α Cen A that are far superior. We further
disagree that the hump of excess power seen repeatedly from velocity
observations of Procyon can be explained as an artefact caused by
gaps in the data. The MOST observations failed to reveal oscillations
clearly because their noise level is too high, possibly from scattered
Earthlight in the instrument. We did find an excess of strong peaks in
the MOST amplitude spectrum that is inconsistent with a simple noise
source such as granulation, and may perhaps indicate oscillations at
roughly the expected level.

---------------------------------------------------------
Title: Helio- and Asteroseismic Analysis Methods
Authors: Schou, Jesper
2005HiA....13..415S Altcode:
Over the last few decades increasingly sophisticated analysis algorithms
have been developed in helioseismology. In this talk I will describe
some of these methods with an emphasis on those most relevant to the
study of oscillations in other stars and some of the relevant lessons
learned. I will then go on to discuss some of the properties of stellar
oscillations which distinguish them from solar oscillations and how
to address those in the analysis. Finally I will speculate on what we
may learn from the stellar oscillations.

---------------------------------------------------------
Title: How Sensitive are Rotation Inversions to Subtle Features of
the Dynamo?
Authors: Howe, R.; Rempel, M.; Christensen-Dalsgaard, J.; Hill, F.;
Komm, R. W.; Schou, J.; Thompson, M. J.
2004ESASP.559..468H Altcode: 2004soho...14..468H
No abstract at ADS

---------------------------------------------------------
Title: The Current Status of Analyzing High-Degree Modes
Authors: Reiter, J.; Rhodes, E. J., Jr.; Kosovichev, A. G.; Schou, J.
2004ESASP.559...61R Altcode: 2004soho...14...61R
No abstract at ADS

---------------------------------------------------------
Title: The Phase of the Torsional Oscillation Pattern
Authors: Howe, R.; Komm, R. W.; Hill, F.; Christensen-Dalsgaard, J.;
Schou, J.; Thompson, M. J.
2004ESASP.559..476H Altcode: 2004soho...14..476H
No abstract at ADS

---------------------------------------------------------
Title: Low Frequency Modes
Authors: Schou, J.
2004ESASP.559..134S Altcode: 2004soho...14..134S
No abstract at ADS

---------------------------------------------------------
Title: Convection-Zone Dynamics from GONG and MDI, 1995-2004
Authors: Howe, R.; Komm, R. W.; Hill, F.; Christensen-Dalsgaard, J.;
Haber, D. A.; Schou, J.; Thompson, M. J.
2004ESASP.559..472H Altcode: 2004soho...14..472H
No abstract at ADS

---------------------------------------------------------
Title: On the Determination of Michelson Doppler Imager High-Degree
Mode Frequencies
Authors: Korzennik, S. G.; Rabello-Soares, M. C.; Schou, J.
2004ApJ...602..481K Altcode: 2002astro.ph..7371K
The characteristics of the solar acoustic spectrum are such that mode
lifetimes get shorter and spatial leaks get closer in frequency as
the degree of a mode increases for a given order. A direct consequence
of this property is that individual p-modes are resolved only at low
and intermediate degrees and that at high degrees individual modes
blend into ridges. Once modes have blended into ridges, the power
distribution of the ridge defines the ridge central frequency, and it
will mask the true underlying mode frequency. An accurate model of the
amplitude of the peaks that contribute to the ridge power distribution
is needed to recover the underlying mode frequency from fitting the
ridge. We present the results of fitting high-degree power ridges
(up to l=900) computed from several 2-3 month long time series of
full-disk observations taken with the Michelson Doppler Imager (MDI) on
board the Solar and Heliospheric Observatory between 1996 and 1999. We
also present a detailed discussion of the modeling of the ridge power
distribution, and the contribution of the various observational
and instrumental effects on the spatial leakage, in the context
of the MDI instrument. We have constructed a physically motivated
model (rather than some ad hoc correction scheme) that we believe
results in a methodology that can produce an unbiased determination
of high-degree modes once the instrumental characteristics are well
understood. Finally, we present preliminary estimates of changes in
high-degree mode parameters with epoch and thus solar activity level and
discuss their significance. These estimates are preliminary because they
rely on a simple-if not simplistic-ridge-to-mode correction scheme to
account for errors in the plate scale used for the spherical harmonic
decomposition. Such a correction scheme produced residual systematics
that, as we show, are not always constant with time. These cannot be
properly corrected without reprocessing the data back to the level of
the spherical harmonic decomposition.

---------------------------------------------------------
Title: SOHO MDI CCD performance
Authors: Schou, J.
2004ESASP.538..411S Altcode: 2004sshp.conf..411S
The Michelson Doppler Imager (MDI) instrument on the Solar and
Heliospheric Observatory (SOHO) spacecraft is designed to study
solar oscillations. While MDI was not designed for high photometric
accuracy, I will describe how MDI data can be used to estimate the
photometric stability of a CCD camera in a space environment and show
that the system appears to be stable, at the relevant time-scales,
to a level well beyond that required for both planet finding and
asteroseismology. I will also present a simple minded analysis of
various types of radiation induced damage. Given the similarity of the
orbits (L1 for SOHO versus L2 for Eddington) some of these measurements
may be useful for the design of Eddington.

---------------------------------------------------------
Title: Wavelike Properties of Solar Supergranulation Detected in
Doppler Shift Data
Authors: Schou, J.
2003ApJ...596L.259S Altcode:
Recently, Gizon, Duvall, &amp; Schou suggested that supergranulation has
a wavelike component. In this Letter, I show that the same phenomenon
can be observed using surface Doppler shift data, thereby confirming
their observations, which were made using a helioseismic time-distance
technique. In addition to confirming those results, I show that the
wave motion is predominantly longitudinal (the fluid displacement is
in the direction of propagation), and I am able to extend the results
for the rotation and the meridional flow beyond +/-70° latitude. The
meridional flow results, which extend further than previous helioseismic
measurements, appear to show no sign of a second cell.

---------------------------------------------------------
Title: A Comparison of Solar p-Mode Parameters from MDI and GONG:
Mode Frequencies and Structure Inversions
Authors: Basu, S.; Christensen-Dalsgaard, J.; Howe, R.; Schou, J.;
Thompson, M. J.; Hill, F.; Komm, R.
2003ApJ...591..432B Altcode:
Helioseismic analysis of solar global oscillations allows investigation
of the internal structure of the Sun. One important test of the
reliability of the inferences from helioseismology is that the
results from independent sets of contemporaneous data are consistent
with one another. Here we compare mode frequencies from the Global
Oscillation Network Group and Michelson Doppler Imager on board SOHO
and resulting inversion results on the Sun's internal structure. The
average relative differences between the data sets are typically less
than 1×10-5, substantially smaller than the formal errors in
the differences; however, in some cases the frequency differences show
a systematic behavior that might nonetheless influence the inversion
results. We find that the differences in frequencies are not a result
of instrumental effects but are almost entirely related to the data
pipeline software. Inversion of the frequencies shows that their
differences do not result in any significant effects on the resulting
inferences on solar structure. We have also experimented with fitting
asymmetric profiles to the oscillation power spectra and find that,
compared with the symmetric fits, this causes no significant change
in the inversion results.

---------------------------------------------------------
Title: The LoHCo Project. 1 -- Comparison of Ring-Diagram Local
Helioseismology on GONG++, MDI and Mt. Wilson Data Sets
Authors: Bogart, R. S.; Schou, J.; Basu, S.; Bolding, J.; Hill, F.;
Howe, R.; Komm, R. W.; Leibacher, J. W.; Toner, C. G.; Corbard, T.;
Haber, D. A.; Hindman, B. W.; Toomre, J.; Rhodes, E. J.; Rose, P. J.;
LoHCo Project Team
2003SPD....34.0804B Altcode: 2003BAAS...35..822B
Full deployment of the GONG+ enhanced observing network in October
2001 and implementation of ring-diagram helioseismology in the
GONG++ analysis pipeline this year has enabled us to make a detailed
intercomparison of results obtained through multiple paths, from
observation through each of the analysis steps. Such comparisons
can provide a certain degree of validation of the implementations
of the analysis procedures, hints of systematic errors, and better
characterization of the observations, possibly leading to improved
calibrations. The Local Helioseismology Comparison (LoHCo) Project
has been established to provide standards for intercomparison of
results obtained with different local helioseismic analysis techniques
applied to the available observational data sources. We present here
a detailed comparison of ring-diagram determinations of localized
sub-surface flows and frequency shifts obtained from both MDI and
GONG in common observing intervals during Carrington Rotation 1988
(2002/3/30 -- 2002/4/26), using both the MDI and the GONG analysis
pipelines. We also present preliminary results of similar analyses of
data obtained by the Mt. Wilson MOF during the same times. This
work is partially supported by grants from NASA and NSF.

---------------------------------------------------------
Title: Time Variations of Meridional and Zonal Flows
Authors: Schou, J.
2003SPD....34.2603S Altcode: 2003BAAS...35Q.854S
The so-called torsional oscillation, which manifests itself as
bands of faster and slower rotation on top of the large-scale
differential rotation, is well known. Initially it was discovered
in surface Doppler shift observations, but it has since been seen
using a variety of helioseismic techniques and in the rotation rate
of the supergranulation. In this talk I will start by describing
some of these measurements and show how it has been possible to
detect the torsional oscillations to a significant depth. I will
also discuss some of the other variations which have been detected in
the solar rotation, in particular the 1.3 year oscillation near the
solar tachocline. Finally I will discuss recent measurements of the
meridional flow and its variations and what we may be able to learn in
the near future. This work was supported by NASA Grant NAG5-10483
to Stanford University.

---------------------------------------------------------
Title: Optimal Masks for Solar g-Mode Detection
Authors: Wachter, R.; Schou, J.; Kosovichev, A. G.; Scherrer, P. H.
2003ApJ...588.1199W Altcode:
The detection of gravity (g) modes of solar oscillations is important
for probing the physical conditions in the Sun's energy-generating
core. We have developed a new method of spatial masks optimized to
reveal solar g-modes of angular degree l=1-3 and applied it to Michelson
Doppler Imager data in the frequency range of 50-500 μHz. These
masks take into account the horizontal component of g-mode velocity
eigenfunctions and the variations in the level of noise across the
solar disk and adjust for the time-dependent mode projection properties
caused by the inclination of the Sun's axis of rotation. They allow us
to optimize the signal-to-noise ratio in the oscillation power spectra
for potential g-modes of various angular order and degree. The peaks
in the resulting spectra are analyzed in terms of their instrumental
origin, long-term stability, and correspondence to the theoretically
predicted g-mode spectrum. As a consequence of failing to detect
any g-mode candidates, new upper limits for the surface amplitude of
g-modes are obtained. The lowest upper limits in the range of 5-6 mm
s-1 are found for sectorial g-modes (l=m). These limits
are an order of magnitude higher than the theoretical prediction of
Kumar et al. in 1996.

---------------------------------------------------------
Title: Supergranular waves observed using MDI surface Doppler
shift data
Authors: Schou, Jesper
2003ESASP.517..381S Altcode: 2003soho...12..381S
Recently Gizon, Duvall and Schou (2002) suggested that supergranulation
has a wave-like component. Here I show that the phenomenon can also be
observed using surface Doppler shift data and extend their results. I
also show results for rotation and meridional flows beyond ±70°
latitude inferred using the supergranular waves and look for temporal
variations in the various properties.

---------------------------------------------------------
Title: On the characterization of high-degree modes: a lesson from MDI
Authors: Korzennik, Sylvain G.; Rabello-Soares, Cristina; Schou, Jesper
2003ESASP.517..145K Altcode: 2003soho...12..145K
High degree power ridges (up to l = 900) were computed and fitted for
several two to three-month-long time-series of full-disk observations
taken with the Michelson Doppler Imager (MDI) on-board the Solar and
Heliospheric Observatory between 1996 and 1999. A detailed discussion
of the modeling of the ridge power distribution, and the contribution
of the various observational and instrumental effects on the spatial
leakage, in the context of the MDI instrument, are presented. The result
of this work is a better understanding of the problems associated with
the characterization of high degree modes. We present the instrumental
and observational requirements needed to achieve a determination of high
degree mode frequencies whose residual systematic errors associated
with the accuracy of the ridge to mode correction scheme are smaller
than the uncertainty of the fitting itself.

---------------------------------------------------------
Title: High-degree p-modes and the sun's evolving surface
Authors: Rhodes, E. J., Jr.; Reiter, J.; Schou, J.
2003ESASP.517..173R Altcode: 2003soho...12..173R
Two of the most glaring problems in contemporary helioseismology are
the limited availability of high-degree p-mode frequencies for use in
inversions of solar internal structure and the lack of high-degree
frequency splitting coefficients for use in inversions of solar
internal dynamics. A third major problem is the lack of a consensus
regarding the mechanism underlying the temporal shifts of the p-mode
frequencies. The lack of high-degree frequencies and the lack of similar
high-degree frequency splittings have occurred because of the inherent
difficulties in measuring such frequencies and frequency splittings in
high-degree power spectra without the inclusion of numerous systematic
errors. We will first point out the importance of high-degree p-modes to
helioseismic inversions. Next, we will describe recent progress we have
made in estimating high-degree frequencies, including the use of both
asymmetric and symmetric profiles in our fits. We will then demonstrate
that the inclusion of corrections for the eigenfunction distortion
due to latitudinal differential rotation removes long-standing
discontinuities in the high-degree frequency splittings. We will go
on to describe our recent efforts of increasing the sensitivity of the
p-mode frequencies to changing levels of solar activity through the use
of observing runs which are as short as three days in duration. Finally,
we will also describe planned efforts at verifying recent discoveries in
solar internal dynamics through the reduction and analysis of full-disk
Dopplergrams obtained during Solar Cycle 22 prior to the beginning of
the GONG and MDI projects. Taken together, the recent improvements
in the estimation of high-degree frequencies, frequency splittings,
and the availability of useful data from Solar Cycle 22 indicate that
a renaissance in global helioseismology is now at hand.

---------------------------------------------------------
Title: Transient oscillations near the solar tachocline
Authors: Toomre, Juri; Christensen-Dalsgaard, Jorgen; Hill, Frank;
Howe, Rachel; Komm, Rudolf W.; Schou, Jesper; Thompson, Michael J.
2003ESASP.517..409T Altcode: 2003soho...12..409T
We report on further developments in the 1.3-yr quasi-periodic
oscillations reported by Howe et al. (2000). These are small (6 to 8
nHz peak-to-peak) oscillations in the inferred rotation rate near the
bottom of the convection zone and in the outer part of the radiative
interior. The oscillations are strongest and most coherent at about a
fractional radius of 0.72 in the equatorial region. Further monitoring
of the oscillations near the equator shows that they continued for a
period after the end of the data analyzed by Howe et al., but appear to
have now diminished in amplitude. This is reminiscent of the transient
behavior of similar (1.3 to 1.4 yr) periodicities in solar-wind and
geomagnetic datasets previously reported. We speculate that the near
tachocline oscillation is associated with the rising phase of the
solar cycle. We discuss tests performed to eliminate various possible
explanations of the oscillations due to systematic errors in the data
and in their analyses.

---------------------------------------------------------
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: Accurate measurements of SOI/MDI high-degree frequencies and
frequency splittings
Authors: Reiter, J.; Kosovichev, A. G.; Rhodes, E. J., Jr.; Schou, J.
2003ESASP.517..369R Altcode: 2003soho...12..369R
We present accurate measurements of high-degree p-mode frequencies
and frequency splittings obtained from the Full-Disk Program of
the Michelson Doppler Imager (MDI) experiment onboard the Solar
and Heliospheric Observatory (SOHO). The frequencies and frequency
splittings are computed from unaveraged zonal, tesseral, and sectoral
power spectra using a new fitting method of Reiter et al. (2002)
based upon a maximum-likelihood fitting approach. In this method,
both the spectral power distribution and contributions of the
various observational and instrumental effects to the spatial leakage
matrices are modelled accurately. We demonstrate that one of the most
long-standing problems in high-degree helioseismology, viz. the jumps
in the frequency splitting coefficients, can be solved by taking into
account the distortion of the leakage matrix by the solar differential
rotation. The results of inversion of the initial frequency set
determined using this new method in the range of angular degree l =
45-300 show a substantially better resolution of the subsurface layers
compared to the previous studies with l below 220.

---------------------------------------------------------
Title: Helio- and Asteroseismic Analysis Methods
Authors: Schou, Jesper
2003IAUJD..12E..34S Altcode:
Over the last few decades increasingly sophisticated analysis algorithms
have been developed in helioseismology. In this talk I will describe
some of these methods with an emphasis on those most relevant to the
study of oscillations in other stars and some of the relevant lessons
learned. I will then go on to discuss some of the properties of stellar
oscillations which distinguish them from solar oscillations and how
to address those in the analysis. Finally I will speculate on what we
may learn from the stellar oscillations.

---------------------------------------------------------
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: Helioseismic data analysis
Authors: Schou, Jesper
2003safd.book..247S Altcode:
The last decade has seen an impressive improvement in the quality and
quantity of helioseismic data. While much of the progress has come
from a new generation of instruments, such as GONG and MDI, data
analysis has also played a major role. In this review I will start
with a brief discussion of how the basic analysis of helioseismic
data is done. I will then discuss some of the data analysis problems,
their influence on our inferences about the Sun and speculate on what
improvements may be expected in the near future. Finally I will show
a selection of recent results.

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Title: Vector Magnetic Field Measurement Capabiliity of the
Helioseismic and Magnetic Imager on SDO
Authors: Bush, R.; Scherrer, P.; Schou, J.; Liu, Y.; Tomczyk, S.;
Graham, J.; Norton, A.
2002AGUFMSH52A0464B Altcode:
The Helioseismic and Magnetic Imager (HMI) instrument has been selected
as part of the payload complement of the Solar Dynamics Observatory
Spacecraft. In this poster we describe the observing technique for
measuring solar vector magnetic fields. The expected performance of
the HMI instrument will be discussed including results of modeling
the observing lines and instrument.

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Title: Solar Supergranulation Waves Detected in Surface Doppler Shift
Authors: Schou, J.
2002astro.ph..8387S Altcode:
Recently Gizon, Duvall and Schou (2002) suggested that supergranulation
has a wave-like component. In this paper I show that the same phenomenon
can be observed using surface Doppler shift data, thereby confirming
their observations. I am also able to measure the dispersion relation to
lower wavenumbers and to extend the results for rotation and meridional
flows beyond +/-70 degrees latitude.

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Title: A new method for measuring frequencies and splittings of
high-degree modes
Authors: Reiter, Johann; Rhodes, E. J., Jr.; Kosovichev, A. G.; Schou,
J.; Scherrer, P. H.
2002ESASP.508...91R Altcode: 2002soho...11...91R
A novel peak-bagging method is presented that operates by fitting a
theoretical profile (symmetric or asymmetric) to the separate peaks of
each multiplet within each unaveraged power spectrum. This new approach
allows a separate frequency, width, and amplitude to be obtained for
each m value at each value of l, n. Hence, the frequency splittings
due to solar rotation for each multiplet can be measured directly. We
present some of our initial results obtained with this new method
in the range 45 &lt;= l &lt;= 300, ν &lt;= 7 mHz when applied to
data from the Michelson Doppler Imager (MDI) onboard the Solar and
Heliospheric Observatory (SOHO). Also discussed are some instrumental
and methodological problems of high-degree mode measurements.

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Title: Effect of line asymmetry on determination of high-degree
mode frequencies
Authors: Reiter, Johann; Rhodes, E. J., Jr.; Kosovichev, A. G.; Schou,
J.; Scherrer, P. H.
2002ESASP.508...87R Altcode: 2002soho...11...87R
Accurate measurements of frequencies of high-degree p-modes are
important for diagnostics of the structure and dynamics of the
upper convective boundary layer, and understanding the nature of
the solar-cycle variations detected in low- and medium-degree mode
frequencies. Neglecting line asymmetry in the peak-bagging approach
may lead to systematic errors in the determination of the mode
characteristics and, hence, may affect the results of inversions. Here
we demonstrate how the p-mode frequencies are systematically changed in
the range of l &lt;= 1000, ν &lt;= 7mHz when line asymmetry is taken
into account in the fitting of the spectral power peaks. The results
reported are based upon spectra that were created from observations
obtained from the MDI Full-Disk Program during the 1996 SOHO/MDI
Dynamics Run.

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Title: Solar Cycle Variability of High-Frequency and High-Degree
p-Mode Oscillation Frequencies
Authors: Rhodes, E. J.; Reiter, J.; Schou, J.
2002AAS...200.0420R Altcode: 2002BAAS...34..949R
Most studies of the solar cycle dependence of the frequencies of the
p-mode oscillations have employed only the low- and intermediate-degree
modes having ν &lt;=4500μ Hz. Most of these past studies have also
employed lengthy observing runs covering two or more solar rotations. In
this poster we will present measurements of the frequencies of both
the high-ν and high-l oscillations which we have obtained during
widely-varying levels of solar activity over the past 13 years. In
contrast to the case of the lower-ν p-modes, the frequencies of the
higher-ν modes are anti-correlated with solar activity. Furthermore,
these frequencies can be estimated reliably from runs which are as
short as only a few days. Using such brief time series, we have been
able to obtain frequencies which show a higher sensitivity to changing
levels of activity than has been the case in the past. This higher
sensitivity gives us the expectation that such frequency shifts will
provide tighter constraints upon future estimates of their origin.

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Title: Solar g modes?
Authors: Schou, Jesper
2002ESASP.508...99S Altcode: 2002soho...11...99S
Due to the importance g modes for understanding the solar interior,
the search for them has been going on for decades, so far without any
definite detections. Mostly the search has been done at low degrees,
both for physical reasons and because these modes can be detected by the
very stable instruments which observe in integrated sunlight. However,
instead of searching for such g modes, I use MDI observations to follow
the f-mode and low order p-mode ridges towards lower degrees. It was
estimated (Schou, 1998) that at least half a solar cycle of data would
be required to detect these modes down to the first avoided crossing
with the g modes. The necessary observations are now available and I
will make such an attempt. I will also estimate mode parameters such
as frequencies, amplitudes and linewidths, the knowledge of which may
improve our inference regarding the deep solar interior and enable
better predictions for the amplitude of g modes.

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Title: Solar internal rotation as seen from SOHO MIDI data
Authors: Vorontsov, S. V.; Christensen-Dalsgaard, J.; Schou, J.;
Strakhov, V. N.; Thompson, M. J.
2002ESASP.508..111V Altcode: 2002soho...11..111V
We report results of inverting the rotational splittings of solar
p-mode frequencies measured from the MDI data, using an adaptive
regularization technique as described by Strakhov &amp; Vorontsov
(2001). We analyse the spatial and temporal structure of the variations
in the internal rotation, which constitute the 11-yr solar torsional
oscillations. We employ all the data collected over half a solar cycle
to analyse further the time-independent component of the differential
rotation, particularly focusing on the solar tachocline region. We
address systematic errors in the rotational splitting measurements,
as indicated by the inversion residuals, which appear to restrict the
accuracy of the current helioseismic measurements.

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Title: Solar cycle variability of high-frequency and high-degree
p-mode oscillation frequencies
Authors: Rhodes, E. J., Jr.; Reiter, J.; Schou, J.
2002ESASP.508...37R Altcode: 2002soho...11...37R
Most studies of the solar cycle dependence of the frequencies of the
p-mode oscillations have employed only the low- and intermediate-degree
modes having ν &lt;= 4500 μHz. Most of these past studies have
also employed lengthy observing runs covering two or more solar
rotations. In this paper we will present measurements of the frequencies
of both the high-ν and high-l oscillations which we have obtained
at widely-varying levels of solar activity over the past 13 years. In
contrast to the case of the lower-ν p-modes, the frequencies of the
higher-ν modes are anti-correlated with solar activity. Furthermore,
these frequencies can be estimated reliably from runs which are as
short as only a few days. Using such brief time series, we have been
able to obtain frequencies which show a higher sensitivity to changing
levels of activity than has been the case in the past. This higher
sensitivity gives us the expectation that such frequency shifts will
provide tighter constraints upon future estimates of their origin.

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Title: Optimal masks for g-mode detection in MIDI velocity data
Authors: Wachter, R.; Schou, J.; Kosovichev, A.; Scherrer, P. H.
2002ESASP.508..115W Altcode: 2002soho...11..115W
We are applying spatial masks to MDI velocity data that are optimized
for revealing g-modes in the frequency range 50 through 500 μHz. These
masks take into account the horizontal component of g-mode velocity
eigenfunctions as well as the time dependent mode projection properties
due to the changing solar B angle, and the varying noise level across
the solar disk. The solar noise, which is likely to be caused by
supergranulation in this frequency range is assumed to be uniformly
distributed over the solar surface, consisting of a dominant horizontal
component and a small radial component. The resulting time series are
examined for possible g-mode candidates and new upper limits for the
surface amplitude of g-modes are obtained.

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Title: Optimal masks for g-mode detection in MDI velocity data
Authors: Wachter, R.; Schou, J.; Kosovichev, A.; Scherrer, P. H.;
Phoebus Team
2002AAS...200.0412W Altcode: 2002BAAS...34..645W
We are applying spatial masks for MDI velocity data that are optimized
for revealing g-modes in the frequency range 50 through 500 μ
Hz. These masks take into account the horizontal component of g-mode
velocity eigenfunctions as well as the time dependent mode projection
properties due to the changing solar B angle, and the varying noise
level across the solar disk. The solar noise, which is likely to be
caused by supergranulation in this frequency range is assumed to be
uniformly distributed over the solar surface, consisting of a dominant
horizontal component and a smaller radial component. The optimal masks
are applied to the image and the resulting time series are examined
for possible g-mode candidates. Because no mode peak has been detected,
firm upper limits for the surface visibility of individual low degree
modes can be given.

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Title: Reduction of Systematic Errors in MDI Measurements
Authors: Schou, J.; Bogart, R. S.
2002AAS...200.0411S Altcode: 2002BAAS...34..645S
Several types of imperfections have been identified in the MDI
instrument and it has become clear that the resulting systematic
errors in the results limit our ability to make inferences about the
Sun. In this poster we will start by describing some of these problems
and how we have succeeded in quantifying them. We will then apply the
corresponding corrections to MDI data and show how various results are
improved. In particular we show the improvements to normal mode and
ring-diagram results and discuss how this will improve our inferences
about the Sun. The MDI project is supported by NASA grant NAG5-10483
to Stanford University.

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Title: Helioseismic Measurement of Solar Torsional Oscillations
Authors: Vorontsov, S. V.; Christensen-Dalsgaard, J.; Schou, J.;
Strakhov, V. N.; Thompson, M. J.
2002Sci...296..101V Altcode:
Bands of slower and faster rotation, the so-called torsional
oscillations, are observed at the Sun's surface to migrate in latitude
over the 11-year solar cycle. Here, we report on the temporal variations
of the Sun's internal rotation from solar p-mode frequencies obtained
over nearly 6 years by the Michelson Doppler Imager (MDI) instrument
on board the Solar and Heliospheric Observatory (SOHO) satellite. The
entire solar convective envelope appears to be involved in the torsional
oscillations, with phase propagating poleward and equatorward from
midlatitudes at all depths throughout the convective envelope.

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Title: A Comparison of Solar p-Mode Parameters from the Michelson
Doppler Imager and the Global Oscillation Network Group: Splitting
Coefficients and Rotation Inversions
Authors: Schou, J.; Howe, R.; Basu, S.; Christensen-Dalsgaard, J.;
Corbard, T.; Hill, F.; Komm, R.; Larsen, R. M.; Rabello-Soares, M. C.;
Thompson, M. J.
2002ApJ...567.1234S Altcode:
Using contemporaneous helioseismic data from the Global Oscillation
Network Group (GONG) and Michelson Doppler Imager (MDI) onboard SOHO,
we compare frequency-splitting data and resulting inversions about the
Sun's internal rotation. Helioseismology has been very successful in
making detailed and subtle inferences about the solar interior. But
there are some significant differences between inversion results
obtained from the MDI and GONG projects. It is important for making
robust inferences about the solar interior that these differences are
located and their causes eliminated. By applying the different analysis
pipelines developed by the projects not only to their own data but
also to the data from the other project, we conclude that the most
significant differences arise not from the observations themselves
but from the different frequency estimation analyses used by the
projects. We find that the GONG pipeline results in substantially fewer
fitted modes in certain regions. The most serious systematic differences
in the results, with regard to rotation, appear to be an anomaly in
the MDI odd-order splitting coefficients around a frequency of 3.5 mHz
and an underestimation of the low-degree rotational splittings in the
GONG algorithm.

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Title: Asteroseismology of the roAp Star α Cir Using the WIRE
Star Camera
Authors: Buzasi, D. L.; Kreidl, T. J.; Schou, J.; Preston, H. L.;
Laher, R.; Catanzarite, J.; Conrow, T.
2001AAS...19913311B Altcode: 2001BAAS...33.1501B
Using the camera on board the WIRE spacecraft, we observed α Cir
from 18 August through 29 September 2000. α Cir is the brightest
roAp star in the sky. Previous observers have detected a single
oscillation mode at 2442 μ Hz, split by the rotational frequency,
though Kurtz et al. (1994) reported the marginal detection of four
low-amplitude secondary frequencies. Our time series has considerably
improved signal-to-noise compared with past attempts. However, though
we do directly detect the rotation of α Cir, we see no sign of the
secondary frequencies reported earlier.

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Title: Solar-cycle variation of the sound-speed asphericity from
GONG and MDI data 1995-2000
Authors: Antia, H. M.; Basu, S.; Hill, F.; Howe, R.; Komm, R. W.;
Schou, J.
2001MNRAS.327.1029A Altcode: 2001astro.ph..9326A
We study the variation of the frequency splitting coefficients
describing the solar asphericity in both GONG and MDI data, and
use these data to investigate temporal sound-speed variations as a
function of both depth and latitude during the period 1995-2000 and a
little beyond. The temporal variations in even splitting coefficients
are found to be correlated to the corresponding component of magnetic
flux at the solar surface. We confirm that the sound-speed variations
associated with the surface magnetic field are superficial. Temporally
averaged results show a significant excess in sound speed around
r=0.92Rsolar and latitude of 60°.

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Title: Observed and Predicted Ratios of the Horizontal and Vertical
Components of the Solar p-Mode Velocity Eigenfunctions
Authors: Rhodes, Edward J., Jr.; Reiter, Johann; Schou, Jesper;
Kosovichev, Alexander G.; Scherrer, Philip H.
2001ApJ...561.1127R Altcode:
We present evidence that the observed ratios of the horizontal
and vertical components of the solar intermediate-degree p-mode
velocity eigenfunctions closely match theoretical predictions of
these ratios. This evidence comes from estimates of the observed
eigenfunction component ratios that were obtained from the fitting
of the p-mode oscillation peaks in low- and intermediate-degree
(l&lt;=200) m-averaged power spectra computed from two different
60.75 day time series of Global Oscillation Network Group (GONG)
project Dopplergrams obtained in late 1996 and early 1998. These fits
were carried out using a peak-fitting method in which we fitted each
observed p-mode multiplet with a model profile that included both the
target mode and its six nearest spatial sidelobes and which incorporated
the effects of the incomplete observational time series through the
convolution of the fitted profiles with the temporal window functions,
which were computed using the two actual GONG observing histories. The
fitted profile also included the effects of the spatial leakage of
the modes of differing degrees into the target spectrum through the
use of different sets of m-averaged spatial leakage matrices. In
order to study the sensitivity of the estimated component ratios to
the details of the computation of the m-averaged power spectra and of
the image-masking schemes employed by the GONG project, we generated
a total of 22 different sets of modal fits. We found that the best
agreement between the predicted and inferred ratios came from the use
of unweighted averaged power spectra that were computed using so-called
n-averaged frequency-splitting coefficients, which had been computed
by cross-correlating the 2l+1 zonal, tesseral, and sectoral power
spectra at each l over a wide range of frequencies. This comparison
yielded a total of 1906 pairs of predicted ct,theory and
fitted ct,fit eigenfunction component ratios. A linear
regression analysis of these pairs of ratios resulted in the following
regression equation: ct,fit=(0.0088+/-0.0013)+(0.9940+/-
0.0044)ct,theory. The resulting correlation coefficient
was 0.9817. This agreement between the predicted and inferred ratios
suggests that the predicted ratios should be used in the fitting of
high-degree power spectra where the ratios cannot be inferred because
of the blending together of individual modal peaks into broad ridges
of power.

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Title: Does the Sun Shrink with Increasing Magnetic Activity?
Authors: Dziembowski, W. A.; Goode, P. R.; Schou, J.
2001ApJ...553..897D Altcode: 2001astro.ph..1473D
We have analyzed the full set of Solar and Heliospheric Observatory
(SOHO) Michelson Doppler Imager (MDI) f- and p-mode oscillation
frequencies from 1996 to date in a search for evidence of solar radius
evolution during the rising phase of the current activity cycle. Just
as Antia et al. in 2000, we find that a significant fraction of the
f-mode frequency changes scale with frequency and that if these are
interpreted in terms of a radius change, it implies a shrinking Sun. Our
inferred rate of shrinkage is about 1.5 km yr-1, which is
somewhat smaller than found by Antia et al. We argue that this rate
does not refer to the surface but, rather, to a layer extending roughly
from 4 to 8 Mm beneath the visible surface. The rate of shrinking may
be accounted for by an increasing radial component of the rms random
magnetic field at a rate that depends on its radial distribution. If
it were uniform, the required field would be ~7 kG. However, if it
were inwardly increasing, then a 1 kG field at 8 Mm would suffice. To
assess contribution to the solar radius change arising above 4 Mm, we
analyzed the p-mode data. The evolution of the p-mode frequencies may be
explained by a magnetic field growing with activity. Our finding here
is very similar to that of Goldreich et al. (1991). If the change were
isotropic, then a 0.2 kG increase, from activity minimum to maximum,
is required at the photosphere, which would grow to about 1 kG at
1 Mm. If only the radial component of the field were to increase,
then the requirement for the photospheric field increase is reduced
to a modest 60-90 G. A relative decrease in temperature of the order
of 10-3 in the subphotospheric layers, or an equivalent
decrease in the turbulent energy, would have a similar effect to the
required inward growth of magnetic field change. The implications
of the near-surface magnetic field changes depend on the anisotropy
of the random magnetic field. If the field change is predominantly
radial, then we infer an additional shrinking at a rate between 1.1
and 1.3 km yr-1 at the photosphere. If, on the other hand,
the increase is isotropic, we find a competing expansion at a rate of
2.3 km yr-1. In any case, variations in the Sun's radius
in the activity cycle are at the level of 10-5 or less and,
hence, have a negligible contribution to the irradiance variations.

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Title: An Investigation of Systematic Errors in MDI Mode Parameters
Authors: Schou, J.; Rabello-Soares, M. C.
2001AGUSM..SP31A24S Altcode:
As described at this meeting by Howe et al., there are significant
differences between the p- and f-mode parameters fitted by the MDI
and GONG projects. Some of these problems seem to originate in the
MDI data and/or the analysis methods which has been used for most of
the MDI data analysis. Also subtle, but highly suspicious, features
are seen internally in the MDI data. In this poster we will examine
some of the possible explanations for these problems. In particular we
will examine the effects of the optical distortions in MDI and those
of the errors in some of the geometric parameters used for describing
the raw images. We will also examine some of the approximations made
in the analysis and see what effects they may have.

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Title: High Resolution Analysis of the Upper Convection Zone
Authors: Basu, S.; Bogart, R. S.; Schou, J.; Antia, H. M.
2001AGUSM..SP31A02B Altcode:
Plane-wave (ring-diagram) analysis of high-degree modes in data from the
SOI/MDI instrument on SOHO have enabled us us to determine the structure
and dynamics of about 0.1 solar radius below the photosphere, and study
how these quantities change with time. The bulk of the work done so far
has utilized full-disc data for which the analysis has the ability to
resolve structures with a scale of about 180 Mm. Additional data with
three times the spatial resolution over a small portion of the disc
are available for shorter durations at various times. Here we report
on details of various trends which not resolved by full-disc data.

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Title: Comparing Global Solar Rotation Results from MDI and GONG
Authors: Howe, R.; Komm, R. W.; Hill, F.; Christensen-Dalsgaard, J.;
Schou, J.; Thompson, M. J.; Corbard, T.
2001AGUSM..SP31A14H Altcode:
The GONG (Global Oscillations Network Group) project and the Solar
Oscillations Investigation (SOI) using the Michelson Doppler Imager
(MDI) instrument aboard the SOHO spacecraft have jointly accumulated
more than five years of data on medium-degree solar p-modes, including
nearly four years of contemporaneous observations. The inferences of
interior solar rotation from the two projects are broadly consistent
and show similar temporal variations, but there are also significant
systematic differences. We report here on the results of an ongoing
attempt to cross-compare the results and analysis techniques of the
two projects. Three 108-day periods, at low, medium and high solar
activity epochs, have been analysed, with both MDI and GONG analysis
being applied to each data set, and the results are compared.

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Title: Variations in Rotation Rate Within the Solar Convection Zone
From GONG and MDI 1995-2000
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R. W.;
Schou, J.; Thompson, M. J.; Toomre, J.
2001AGUSM..SP31A15H Altcode:
Helioseismic measurements with the Michelson Doppler Imager (MDI)
instrument aboard SOHO, and complementary measurements from the Global
Oscillation Network Group (GONG) project, are revealing changes deep
within the Sun as the solar cycle progresses. We will present the
latest results based on recent data from both experiments, including
flows in the upper part of the convection zone and variations in the
rotation rate near its base.

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Title: Challenges in High-Degree Helioseismology
Authors: Rhodes, E. J.; Reiter, J.; Schou, J.; Kosovichev, A. G.;
Scherrer, P. H.
2001AGUSM..SP21C06R Altcode:
Some of the most exciting results that the field of helioseismology
has provided in recent years have come from numerical inversions of
different properties of the solar p-mode oscillations. Such inversions
have been primarily of three types: 1) structural inversions which
have employed tables of the frequencies of various p-modes and
their associated uncertainties to infer different thermodynamic
properties of the solar interior as functions of radius and latitude,
2) rotational inversions which have employed tables of the frequency
splittings of the modes of different azimuthal order to measure the
internal angular velocity as functions of radius and latitude, and 3)
horizontal flow inversions which have employed sets of frequencies
of the rings that are observed in three-dimensional power spectra to
infer sub-photospheric horizontal flow vectors as functions of depth,
latitude and longitude. Unfortunately, the vast majority of such
inversions have only included frequencies or frequency splittings
of the low- and the intermediate-degree oscillations. Furthermore,
the horizontal flow inversions have been somewhat limited by the
difficulties in accurately fitting the rings of the higher-degree
power spectra. These limitations have prevented helioseismologists from
accurately inferring the sound speed, density, adiabatic gradient, and
helium abundance in the outermost three to four percent (by radius)
of the solar interior. In addition, the absence of high-l frequency
splittings from most past rotational inversions has limited the
accuracy with which we have been able to estimate the angular velocity
of the solar surface layers. These limitations have mainly come about
because for l&gt;= 200 the individual modal peaks blend together into
broad ridges of power. Fitting such ridges requires knowledge of the
amount of power which leaks into the sidelobes that are adjacent to
the true spectral peaks. Such leakage information requires detailed
knowledge of the spatial behavior of each different intrument, of
the ratio of horizontal and vertical components of the solar p-mode
eigenfunctions, and of the temporal window function of each dataset. In
this presentation we will demonstrate the high-l frequencies which we
have obtained from a new fitting technique which employs m-averaged
power spectra, temporal window functions, and spatial leakage matrices
to fit each mode or ridge with a total of seven peaks. We will also
demonstrate that we have obtained evidence from the fitting of GONG
power spectra that the true ratios of the eigenfunction components
match the theoretical predictions of these ratios. Finally, we will
also demonstrate that cross-correlations of the peaks and ridges in
the 2l+1 individual spectra at each l result in systematic jumps in the
frequency-splitting coefficients for l&gt;=200 due to the blending of
the peaks into ridges. We will point out that, unless some method can
be found which overcomes these detrimental effects of peak-blending,
we will not be able to provide measures of the latitudinal behavior
of the solar angular velocity close to the photosphere which will
be independent of the horizontal flow mesurements obtained with the
so-called “ring and trumpet” technique.

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Title: Supergranulation rotation
Authors: Schou, Jesper; Beck, John G.
2001ESASP.464..677S Altcode: 2001soho...10..677S
Simple convection models estimate the depth of supergranulation at
approximately 15,000 km which suggests that supergranules should rotate
at the rate of the plasma in the outer 2% of the Sun by radius. Previous
measurements (Snodgrass &amp; Ulrich, 1990; Beck &amp; Schou, 2000)
found that supergranules rotate significantly faster than this, with
a size-dependent rotation rate. We expand on previous work and show
that the torsional oscillation signal seen in the supergranules tracks
that obtained for normal modes. We also find that the amplitudes and
lifetimes of the supergranulation are size dependent.

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Title: Inversion for the solar internal rotation with an adaptive
regularization technique
Authors: Vorontsov, S. V.; Christensen-Dalsgaard, J.; Schou, J.;
Strakhov, V. N.; Thompson, M. J.
2001ESASP.464..559V Altcode: 2001soho...10..559V
We report the first preliminary results of applying the adaptive
regularization technique (Strakhov and Vorontsov 2000) to the inversion
for the solar internal rotation. The 360d SOHO MDI data set with 18-th
degree fit to the rotational splittings is used in the 1.5-D and 2-D
inversions, and we used the 72d data sets (Schou 1999) to trace the
possible variations with solar activity.

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Title: How correlated are f-mode frequencies with solar activity?
Authors: Antia, H. M.; Basu, Sarbani; Pintar, J.; Schou, Jesper
2001ESASP.464...27A Altcode: 2001soho...10...27A
Temporal variations of solar f-mode frequencies are studied using data
from the Global Oscillation Network Group (GONG) and the Michelson
Doppler Imager (MDI) covering the period from 1995 to 2000. The
frequencies show an increase with activity. There appears to be
one component in the time varying part of the frequencies which is
correlated with the solar activity indices. Superposed on this is an
oscillatory variation with a period of 1 year, whose origin is not
clear. The amplitudes of both the oscillatory and non-oscillatory
component increases with the degree (and hence frequency) of the mode.

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Title: Studying asphericity in the solar sound speed from MDI and
GONG data
Authors: Antia, H. M.; Basu, S.; Hill, F.; Howe, R.; Komm, R. W.;
Schou, J.
2001ESASP.464...45A Altcode: 2001soho...10...45A
We study the variation of the frequency splitting coefficients
describing the solar asphericity in both GONG and MDI data, and
use these data to investigate temporal sound-speed variations as a
function of both depth and latitude during the period 1995-2000. The
temporal variations in even splitting coefficients are found to
be correlated with the corresponding component of magnetic flux at
the solar surface. The sound-speed variations associated with the
surface magnetic field appear to be superficial. Temporally averaged
results show a significant excess in sound speed around r = 0.92
Rsolar and latitude of 60°.

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Title: Comparing mode frequencies from MDI and GONG
Authors: Howe, R.; Hill, F.; Basu, S.; Christensen-Dalsgaard, J.;
Komm, R. W.; Munk Larsen, R.; Roth, M.; Schou, J.; Thompson, M. J.;
Toomre, J.
2001ESASP.464..137H Altcode: 2001soho...10..137H
We present results of analyses of MDI and GONG time series covering
the same time intervals, and using both the MDI and GONG peakbagging
algorithms. We discuss some of the likely causes of differences between
the inferred frequencies and frequency splittings. In addition, we
consider the effect of these differences on the results of inversions
for the solar internal rotation and sound speed.

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Title: Solar cycle changes in convection zone dynamics from MDI and
GONG 1995 - 2000
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R. W.;
Munk Larsen, R.; Schou, J.; Thompson, M. J.; Toomre, J.
2001ESASP.464...19H Altcode: 2001soho...10...19H
The combined GONG and MDI medium-degree helioseismic data sets now cover
just over 5 years and allow us to probe the changing dynamics of the
convection zone in unprecedented detail. Here we present the latest
results from both projects, showing the evolution of the migrating
zonal flows close to the surface and also changes close to and below
the base of the convection zone.

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Title: Spatially-resolved Analysis of the the Upper Covnection Zone
Authors: Bogart, R. S.; Schou, J.; Basu, S.; Haber, D. A.; Hill, F.;
Antia, H. M.
2001IAUS..203..183B Altcode:
Plane-wave (ring-diagram) analysis of high-degree modes in data from the
SOI/MDI instrument on SOHO permits us to determine spatial and temporal
variations of the structure and dynamics of the upper convection zone,
to a depth of about 0.1 solar radius below the photosphere. The spatial
resolution achieved with full-disc data is at least 15 heliographic
degrees (180 Mm), and the temporal resolution is of order 1 day. Data
useful for such analysis cover at least two full Carrington rotations
in each year since 1996. Additional data with three times the spatial
resolution over a small portion of the disc are available for shorter
durations at various times. Analyses of the full-disc data from the
earlier years have already revealed systematic patterns in the global
meridional flow and flows associated with active regions during the
early phase of the solar cycle. Here we report on variations and trends
seen in the flows as the activity level of the cycle approaches maximum.

---------------------------------------------------------
Title: The determination of MDI high-degree mode frequencies
Authors: Rabello-Soares, M. C.; Korzennik, S. G.; Schou, J.
2001ESASP.464..129R Altcode: 2001soho...10..129R
As mode lifetimes get shorter and spatial leaks get closer in frequency,
individual p-modes can only be resolved up to some degree l (around
150). At higher degrees, individual modes blend into ridges and
the power distribution of the ridge masks the true underlying mode
frequency. To recover the underlying mode frequency from fitting the
ridge, an accurate model of the amplitude of the peaks that contribute
to the ridge power distribution is needed. Using full-disk data from the
Michelson Doppler Imager data on the Solar and Heliospheric Observatory,
we present and discuss the differences between the observations and
the spatial leakage calculation (including the horizontal component)
and estimate the horizontal-to-vertical displacement ratio for
medium-degree modes using sectoral modes for different observational
periods. We show how time variations in the instrument calibration
affect the spatial leakage and discuss their importance in the spatial
leakage calculation. By constructing a physically motivated model
(rather than some ad hoc correction scheme) can we hope to produce an
unbiased determination of the high-degree modes in the near future.

---------------------------------------------------------
Title: Observations of p-modes in α Cen
Authors: Schou, Jesper; Buzasi, Derek L.
2001ESASP.464..391S Altcode: 2001soho...10..391S
We present the results of an analysis of a 50 day time-series of
photometry of α Cen taken by the WIRE spacecraft. We see evidence of
p-modes similar to those observed in the Sun and present the results
obtained. We also discuss some of the implications of the measurements
and what can be observed in other stars.

---------------------------------------------------------
Title: Interior Solar-Cycle Changes Detected by Helioseismology
Authors: Howe, R.; Hill, F.; Komm, R. W.; Christensen-Dalsgaard, J.;
Munk Larsen, R.; Schou, J.; Thompson, M. J.; Toomre, J.
2001IAUS..203...40H Altcode:
Helioseismic measurements with the MDI instrument aboard SOHO,
and complementary measurements from the GONG network, are revealing
changes deep within the Sun as the solar cycle progresses. We will
present results based on recent data from both experiments, including
variations in the rotation rate deep inside the convection zone.

---------------------------------------------------------
Title: On the signature of the solar activity at the base of the
convective envelope
Authors: Monteiro, M. J. P. F. G.; Christensen-Dalsgaard, J.; Schou,
J.; Thompson, M. J.
2001ESASP.464..535M Altcode: 2001soho...10..535M
A we approach the solar maximum for the current cycle it is timely to
establish what helioseismology can tell us about the internal changes
the Sun undergoes as the cycle progresses. In this work we use SOI-MDI
data in order to identify changes occurring to the stratification of
the Sun at the base of the convective envelope. Both changes with
time (averaged over colatitude) and changes over time at different
colatitudes are addressed. These are discussed in terms of the changes
in the properties of the convection at the base of the envelope in
the region where the tachocline is present. Those variations could
be attributed to the interaction between convection, rotation and
magnetic fields, which must be intrinsically time-dependent phenomena,
possibly correlated with the observed changes occurring over the cycle
in the envelope up to the surface.

---------------------------------------------------------
Title: A New Class of Solar Oscillation Measurements
Authors: Schou, J.
2001IAUS..203...21S Altcode:
Over the last few years the quality and quantity of basic helioseismic
data have increased dramatically as instruments such as MDI on the
SOHO spacecraft and the GONG network have become operational. While the
data from these new instruments have led to a significant increase in
our ability to make inferences about the solar interior, it has become
apparent that the current analysis techniques are limiting our ability
to fully utilize these data. The high signal to noise ratio of these
data means that subtle details of the spectra, which were ignored in
earlier analysis, have to be properly modeled, both to extract all
the available information and to eliminate systematic errors. These
details include both solar effects and instrumental limitations and
artifacts. In this talk I will describe some recent results, some of
the problems involved in the analysis of normal modes and what progress
we may be able to make once we understand these problems better.

---------------------------------------------------------
Title: Observations of p-modes in α Cen
Authors: Schou, J.; Buzasi, D. L.
2000AAS...197.4604S Altcode: 2000BAAS...32.1477S
We present results of an analysis of time-series of photometry of α
Cen taken by the WIRE spacecraft in 1999 and 2000. We see evidence of
p-modes similar to those observed in the Sun and we present some of the
results obtained. We also discuss how the observed properties of the
modes relate to those expected and what can be observed in other stars.

---------------------------------------------------------
Title: Irradiation induced decomposition of the surface of icy
satellites and generation of oxidizing atmospheres
Authors: Baragiola, R. A.; Bahr, D. A.; Schou, J.; Shi, M.; Vidal,
R. A.
2000DPS....32.3408B Altcode: 2000BAAS...32.1056B
Past studies of sputtering of water ice, related to the question
of magnetospheric effects on icy satellites, have concluded that
desorption of intact water molecules dominates at low temperatures
(below ~ 100 K) and that emission of H2, and O2
become important at higher temperatures. The temperature dependence of
sputtering was predicted to change with the magnitude of the energy
deposition at the surface. However, our new results show that the
temperature dependence of the total sputtering yields for ions and
Lyman-alpha photons scales with energy deposition over five orders
of magnitude, against predictions. With new, absolute measurements of
the sputtered ejecta produced by 100 keV protons, we have discovered
that O and OH are primary species, even at low temperatures. This
surprising finding will require revision of models for sputter
generation of atmospheres. The presence of radicals imply strongly
oxidizing atmospheres, which in turn has implications on the state of
the surface of the satellites: formation of hydrogen peroxide from ice
and oxidation of non-ice components. Work supported by NASA-Office of
Space Research and Cassini mission, and by NSF-Astronomy Div.

---------------------------------------------------------
Title: Signatures of the Rise of Cycle 23
Authors: Dziembowski, W. A.; Goode, P. R.; Kosovichev, A. G.; Schou, J.
2000ApJ...537.1026D Altcode:
During the rise of Cycle 23, we have found a sizable, systematic
evolution of the Solar and Heliospheric Observatory/Michelson Doppler
Imager solar oscillation frequencies implying significant changes in the
spherically symmetric structure of the Sun's outer layers as well as in
its asphericity up to a P18 Legendre distortion. We conducted
a search for corresponding asymmetries in Ca II K data from Big Bear
Solar Observatory. We found tight temporal and angular correlations of
the respective asphericities up through P10. This result
emphasizes the role of the magnetic field in producing the frequency
changes. We carried out inversions of the frequency differences and
the splitting coefficients assuming that the source of the evolving
changes is a varying stochastic magnetic field. With respect to the
most recent activity minimum, we detected a significant perturbation in
the spherical part at a depth of 25-100 Mm, which may be interpreted
as being a result of a magnetic perturbation, &lt;B2&gt;,
of about (60KG)2 and/or a relative temperature perturbation
of about 1.2×10-4. Larger, although less statistically
significant, perturbations of the interior structure were found in
the aspherical distortion.

---------------------------------------------------------
Title: Studying Asphericity in the Solar Sound Speed from MDI and
GONG Data 1995-1999
Authors: Schou, J.; Antia, H. M.; Basu, S.; Howe, R.; Hill, F.; Komm,
R. W.
2000SPD....31.0111S Altcode: 2000BAAS...32..803S
We study the variation of the frequency splitting coefficients
describing the solar asphericity in both GONG and MDI data, and use
these data to investigate temporal sound-speed variations as a function
of both depth and latitude during the period 1995--99. We confirm that
the sound-speed variations associated with the surface magnetic field
are superficial.

---------------------------------------------------------
Title: Solar-Cycle Changes in Convection-Zone Dynamics from SOI and
GONG Data
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R. W.;
Larsen, R. M.; Schou, J.; Thompson, M. J.; Toomre, J.
2000SPD....31.0113H Altcode: 2000BAAS...32..803H
The combined GONG and MDI medium-degree helioseismic data sets now cover
more than 4.5 years and allow us to probe the changing dynamics of the
convection zone in unprecedented detail. Here we present the latest
results from both projects, showing the evolution of the migrating zonal
flows close to the surface and also changes close to and below the base
of the convection zone. This work utilizes data obtained by the Global
Oscillation Network Group (GONG) project, managed by the National Solar
Observatory, a Division of the National Optical Astronomy Observatories,
which is operated by AURA, Inc. under a cooperative agreement with the
National Science Foundation. SOHO is a joint project of ESA and NASA.

---------------------------------------------------------
Title: Seismology of the Sun and Other Stars
Authors: Schou, J.
2000SPD....31.0501S Altcode: 2000BAAS...32..836S
The last several years have seen impressive improvements in our
knowledge of the structure and dynamics of the solar interior. In
particular we have started seeing signs of temporal variations extending
to significant depths, in addition to the well known near surface
changes. Given these improvements it is natural to ask what we might
learn from future observations of global modes. Are there fundamental
questions we can expect to answer with the extended operations of
MDI and GONG? Are there other questions for which we will require
significantly different observations, such as observations of the
whole Sun or other ways of detecting the oscillations? In this talk I
will briefly review some of the recent observations and try to answer
some of these questions. Given that answering some of the fundamental
questions may not be feasible using solar oscillations, it is natural
to ask whether some of them might be answered by observing oscillations
in other stars. What might we learn from the asteroseismology missions
currently proposed or under development and what may be possible in
the more distant future using missions such as SISP, currently in
the Sun-Earth Connection Roadmap. The SOI--MDI project is supported
by NASA grant NAG5-3077 to Stanford University. SOHO is a mission of
international cooperation between ESA and NASA.

---------------------------------------------------------
Title: US Mission of Opportunity on MONS
Authors: Schou, J.; Scherrer, P. H.; Bogart, R. S.;
Christensen-Dalsgaard, J.; Kjeldsen, H.; Buzasi, D. L.
2000mons.proc..123S Altcode:
The Measuring Oscillations in Nearby Stars (MONS) Survey Telescope
(MONS-ST) is a Mission of Opportunity that will study the interior
of a large number of stars using stellar oscillations. MONS-ST will
provide additional hardware and/or extend the lifetime of the Danish
MONS micro satellite, which has been selected for flight in 2003. The
additional hardware provided by MONS-ST will enhance the MONS mission
by increasing the number of stars observable and extending the lifetime
of the mission. A Guest Investigator program, allowing a number of
scientists to propose targets and analyze data, will be part of the
mission.

---------------------------------------------------------
Title: Deeply Penetrating Banded Zonal Flows in the Solar Convection
Zone
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R. W.;
Larsen, R. M.; Schou, J.; Thompson, M. J.; Toomre, J.
2000ApJ...533L.163H Altcode: 2000astro.ph..3121H
Helioseismic observations have detected small temporal variations
of the rotation rate below the solar surface that correspond to the
so-called “torsional oscillations” known from Doppler measurements of
the surface. These appear as bands of slower- and faster-than-average
rotation moving equatorward. Here we establish, using complementary
helioseismic observations over 4 yr from the GONG network and from
the MDI instrument on board SOHO, that the banded flows are not
merely a near-surface phenomenon: rather, they extend downward at
least 60 Mm (some 8% of the total solar radius) and thus are evident
over a significant fraction of the nearly 200 Mm depth of the solar
convection zone.

---------------------------------------------------------
Title: Supergranulation rotation
Authors: Beck, John G.; Schou, Jesper
2000SoPh..193..333B Altcode:
Simple convection models estimate the depth of supergranulation at
approximately 7500 km which suggests that supergranules would rotate
at the rate of the plasma in the outer 1% of the solar radius. The
supergranulation rotation obtained from MDI dopplergrams shows that
supergranules rotate faster than the outer 5% of the convection
zone and show zonal flows matching results from inversions of f-mode
splittings. Additionally, the rotation rate depends on the size scale
of the features.

---------------------------------------------------------
Title: Does the tachocline show solar cycle related changes?
Authors: Basu, Sarbani; Schou, Jesper
2000SoPh..192..481B Altcode:
The tachocline at the base of the convection zone is generally believed
to be the seat of the solar dynamo. Here we investigate whether the
tachocline shows any detectable change using several 72 day time-series
of the Michelson Doppler Imager (MDI) Medium-l data. We do not find
any clear evidence of change with time.

---------------------------------------------------------
Title: Time Variability of Rotation in Solar Convection Zone From
soi-mdi
Authors: Toomre, J.; Christensen-Dalsgaard, J.; Howe, R.; Larsen,
R. M.; Schou, J.; Thompson, M. J.
2000SoPh..192..437T Altcode:
The variation of rotation in the convection zone over a period of two
years from mid-1996 is studied using inversions of SOI-MDI data. We
confirm the existence of near-surface banded zonal flows migrating
towards the equator from higher latitudes, and reveal that these banded
flows extend substantially beneath the surface, possibly to depths
as great as 70 Mm (10% of the solar radius). Our results also reveal
apparently significant temporal variations in the rotation rate at
high latitudes and in the vicinity of the tachocline over the period
of study.

---------------------------------------------------------
Title: Dynamic Variations at the Base of the Solar Convection Zone
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R. W.;
Larsen, R. M.; Schou, J.; Thompson, M. J.; Toomre, J.
2000Sci...287.2456H Altcode:
We have detected changes in the rotation of the sun near the base of
its convective envelope, including a prominent variation with a period
of 1.3 years at low latitudes. Such helioseismic probing of the deep
solar interior has been enabled by nearly continuous observation of
its oscillation modes with two complementary experiments. Inversion of
the global-mode frequency splittings reveals that the largest temporal
changes in the angular velocity Ω are of the order of 6 nanohertz
and occur above and below the tachocline that separates the sun's
differentially rotating convection zone (outer 30% by radius) from
the nearly uniformly rotating deeper radiative interior beneath. Such
changes are most pronounced near the equator and at high latitudes and
are a substantial fraction of the average 30-nanohertz difference in Ω
with radius across the tachocline at the equator. The results indicate
variations of rotation close to the presumed site of the solar dynamo,
which may generate the 22-year cycles of magnetic activity.

---------------------------------------------------------
Title: Helioseismic detection of temporal variations of solar rotation
rate near the base of the convection zone
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R. W.;
Larsen, R. M.; Schou, J.; Thompson, M. J.; Toomre, J.
1999AAS...19510702H Altcode: 1999BAAS...31R1530H
The differential rotation of the Sun and its ability to generate
large-scale magnetic fields through cyclic dynamo action appear to be
intimately linked. It is now commonly thought that the global dynamo
behavior responsible for the emergence of active regions is derived
from strong organized toroidal magnetic fields generated by rotational
shear in a thin region (the tachocline) at the base of the convection
zone. The magnetic field could well have a feedback effect on the fluid
motions in the region. We are thus motivated to use helioseismology
to look for changes in rotation profiles near the tachocline as the
Sun's magnetic cycle progresses. This approach has become possible
using frequency-splitting data for p- and f-mode oscillations derived
over four years (from May 1995 to Sept 1999) of full-disk Doppler
observations from the ground-based Global Oscillation Network Group
(GONG) project and from the Michelson Doppler Imager (MDI) experiment
aboard the SOHO spacecraft. Inversions using two different methods of
the splittings from these two independent data sets reveal systematic
variations of the rotation rate close to the base of the convection
zone, with different behavior at low and high latitudes. Notable
are variations of order 6 nHz in rotation rates near the equator,
to be compared with the radial angular velocity contrast across the
tachocline of about 30 nHz. These exhibit several nearly repetitive
changes with a period of about 1.2-1.4 years and appear to be real
changes in the deep convection zone and tachocline rotation rates that
need to be followed as the solar cycle progresses. The GONG project is
managed by the National Solar Observatory, a Division of the National
Optical Astronomy Observatories, which is operated by AURA, Inc. under
a cooperative agreement with the National Science Foundation. SOHO is
a joint project of ESA and NASA.

---------------------------------------------------------
Title: Helioseismic Constraints on the Structure of the Solar
Tachocline
Authors: Charbonneau, P.; Christensen-Dalsgaard, J.; Henning, R.;
Larsen, R. M.; Schou, J.; Thompson, M. J.; Tomczyk, S.
1999ApJ...527..445C Altcode:
This paper presents a series of helioseismic inversions aimed at
determining with the highest possible confidence and accuracy the
structure of the rotational shear layer (the tachocline) located
beneath the base of the solar convective envelope. We are particularly
interested in identifying features of the inversions that are robust
properties of the data, in the sense of not being overly influenced
by the choice of analysis methods. Toward this aim we carry out
two types of two-dimensional linear inversions, namely Regularized
Least-Squares (RLS) and Subtractive Optimally Localized Averages
(SOLA), the latter formulated in terms of either the rotation rate or
its radial gradient. We also perform nonlinear parametric least-squares
fits using a genetic algorithm-based forward modeling technique. The
sensitivity of each method is thoroughly tested on synthetic data. The
three methods are then used on the LOWL 2 yr frequency-splitting
data set. The tachocline is found to have an equatorial thickness
of w/Rsolar=0.039+/-0.013 and equatorial central radius
rc/Rsolar=0.693+/-0.002. All three techniques
also indicate that the tachocline is prolate, with a difference in
central radius Δrc/Rsolar~=0.024+/-0.004 between
latitude 60° and the equator. Assuming uncorrelated and normally
distributed errors, a strictly spherical tachocline can be rejected
at the 99% confidence level. No statistically significant variation
in tachocline thickness with latitude is found. Implications of these
results for hydrodynamical and magnetohydrodynamical models of the
solar tachocline are discussed.

---------------------------------------------------------
Title: Stellar and Planetary Explorer (SPEX)
Authors: Schou, J.; Bogart, R. S.; Bush, R. I.; Hoeksema, J. T.;
Scherrer, P. H.; Brown, T. M.; Buzasi, D. L.; Horner, S. D.; Korzennik,
S. G.
1999AAS...195.8808S Altcode: 1999BAAS...31.1506S
The Stellar and Planetary Explorer (SPEX) mission will search for
terrestrial inner planets around Sun-like (FGK, main sequence) stars
using photometric techniques as well as provide very long time series
for asteroseismology. If every Sun-like star had a planetary system
similar to ours, the proposed instrumentation would detect at least
100 terrestrial planets similar to the Earth or Venus and be able to
provide statistics on their diameters and orbital periods. SPEX will
accomplish this by continuously observing a large number of field stars
to detect planetary transits. The instrument consists of a fast Schmidt
camera with a mosaic of large CCD detectors. SPEX will observe a field
near the galactic plane from a geosynchronous orbit for a minimum of 3
years. The very long asteroseismic time series will allow inferences
on the interiors of more than 100 Sun-like stars with a variety of
masses and ages. This will allow us to substantially refine stellar
model calculations and in particular improve on the age estimates of
stars. This in turn is of considerable interest for the understanding
of the evolution of our galaxy and the universe as a whole. SPEX will
also provide new data for stellar activity and will be able to detect
the reflected light from large inner planets, such as those detected
using ground based Doppler velocity searches.

---------------------------------------------------------
Title: Migration of Zonal Flows Detected Using Michelson Doppler
Imager F-Mode Frequency Splittings
Authors: Schou, J.
1999ApJ...523L.181S Altcode:
The small-scale zonal flows in the outer part of the solar convection
zone were recently detected by Kosovichev &amp; Schou using f-mode
frequency splittings. Results from five 72 day time series (Schou
et al.) show a slight drift of the pattern toward the equator in a
manner similar to that seen in the so-called torsional oscillation
observed using surface Doppler shift measurements. Here I describe
results using 12 72 day time series of the Michelson Doppler Imager
medium-l data. These results show a clear drift of the pattern toward
the equator. Also, the near-surface rotation rate close to the solar
poles is observed to be slower than expected from an extrapolation
from lower latitudes and to be changing with time.

---------------------------------------------------------
Title: Rotation of the solar core from BiSON and LOWL frequency
observations
Authors: Chaplin, W. J.; Christensen-Dalsgaard, J.; Elsworth, Y.;
Howe, R.; Isaak, G. R.; Larsen, R. M.; New, R.; Schou, J.; Thompson,
M. J.; Tomczyk, S.
1999MNRAS.308..405C Altcode:
Determination of the rotation of the solar core requires very accurate
data on splittings for the low-degree modes which penetrate to the core,
as well as for modes of higher degree to suppress the contributions
from the rest of the Sun to the splittings of the low-degree modes. Here
we combine low-degree data based on 32 months of observations with the
BiSON network and data from the LOWL instrument. The data are analysed
with a technique that specifically aims at obtaining an inference of
rotation that is localized to the core. Our analysis provides what we
believe is the most stringent constraint to date on the rotation of
the deep solar interior.

---------------------------------------------------------
Title: Supergranules and Photospheric Motions Near the Solar Poles
Authors: Bogart, R. S.; Beck, J. G.; Bush, R. I.; Schou, J.
1999AAS...194.9303B Altcode: 1999BAAS...31Q.989B
Since the SOHO launch in 1995, there have been seven opportunities for
the Michelson Doppler Imager (MDI) to provide continuous Dopplergrams
of the solar poles over extended intervals of time. Because of the
nearly constant and uniform focus of the MDI images over small regions
it is possible to resolve and track supergranules across the pole at
these times. Seismic analysis of global modes from the MDI data has
previously hinted at the possibility of a polar vortex in the upper
convection zone. We report here on the first analysis of polar surface
motions inferred from MDI data covering a total of about 2000 hours
during five polar observing windows, including two full months around
the south polar apparition of 1998. Research supported by SOI-MDI NASA
grant NAG5-3077 at Stanford University.

---------------------------------------------------------
Title: Solar Internal Rotation as Measured by the SOHO SOI/MDI
Full-Disk Program
Authors: Rhodes, E. J., Jr.; Reiter, J.; Kosovichev, A. G.; Schou,
J.; Scherrer, P. H.
1999AAS...194.5602R Altcode: 1999BAAS...31..911R
We present estimates of the solar internal angular velocity obtained
from p-mode frequency splittings computed from observations of the
SOHO SOI/MDI Experiment's Full-Disk Program. Specifically, a time
series of full-disk Dopplergrams which was obtained during the 61-day
long 1996 Dynamics Run of the SOI/MDI Experiment were converted into
time series of spherical harmonic coeffificients for degrees ranging
up to 1000. These time series of spherical harmonic coefficients
were then converted into 1001 sets of zonal, sectoral, and tesseral
power spectra. Estimates of the rotationally-induced p-mode frequency
splittings for every degree between 1 and 1000 were then obtained from
these sets of power spectra through a cross-correlation of the 2l+1
spectra within each set. Because this cross-correlation was carried
out between the frequency limits of 1800 to 4800 mu Hz for each set
of spectra, the resulting splitting coefficients were effectively
averaged over the radial order, n, at each degree. Due to the blending
of individual p-mode spectral peaks for degrees above 200, the raw
frequency splitting coefficients for all degrees between 200 and 1000
had to be corrected before an inversion could be performed. We will
describe the method we adopted for correcting the raw splittings and
we will present both the raw and corrected splitting coefficients. We
will also present a two-dimensional inversion of the corrected
coefficients. Finally, we will also demonstrate how the inclusion of
the high-degree splittings allows us to obtain better estimates of the
solar internal angular velocity in the shallow sub-photospheric part
of the solar convection zone than have been possible in past studies
which included only the splittings of the low- and intermediate-degree
p-modes.

---------------------------------------------------------
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: Daily Variations and Average Structure of Solar Shear Flows
Deduced from Helioseismic Dense-Pack Samplings of Ring Diagrams
Authors: Haber, D. A.; Hindman, B. W.; Toomre, J.; Bogart, R. S.;
Schou, J.; Hill, F.
1999AAS...194.5611H Altcode: 1999BAAS...31..913H
We report on the daily variations and average behavior of large-scale
flows in the upper convection zone as determined by ring-diagram
helioseismic analysis applied to SOI-MDI full-disk velocity data from
the 1996 and 1998 Dynamics Campaigns. We have tracked many small regions
of 15 degrees diameter whose centers are spaced 7.5 degrees apart in
latitude and longitude, creating a mosaic of tiles that oversample
the spatial domain. The tiles cover the solar disk out to a distance
of 52.5 degrees from disk center. An individual dense-pack mosaic
is prepared by tracking each of 189 regions for 1664 minutes (27.7
hrs). Successive mosaics are prepared every 15 degrees in Carrington
longitude, roughly once every 1633 minutes. Such mosaics now cover
more than two full Carrington rotations in 1996 and one rotation in
1998. This is the best spatial and temporal coverage of any ring-diagram
study carried out to date. The longitudinally averaged meridional flow
varies with latitude but remains relatively constant with depth below
the upper shear layer at 2 Mm down to a depth of about 16 Mm. The
averaged zonal flow increases with depth within this same layer and
agrees well with the rotation rates found from global modes. However
with the high-degree wave field data from this analysis we are better
able to resolve that shear layer within the upper convection zone. We
see bands of faster averaged zonal flow near 30 degrees latitude both
in the northern and southern hemisphere that are present at all depths
studied. We also present movies of the daily variations in the flows
within this dense pack for given depths that show the evolution of
the complex velocity field. This research was supported by NASA grants
NAG5-3077 and NAG5-7996, and NSF grant AST-9417337.

---------------------------------------------------------
Title: Zonal Flows from the MDI Medium-l Data
Authors: Schou, J.
1999AAS...194.5612S Altcode: 1999BAAS...31..913S
The small scale zonal flows in the outer part of the solar convection
zone were recently detected by Kosovichev &amp; Schou (1997) using
f-mode frequency splittings. Results from five 72 day time-series
(Schou et al. 1998) showed a slight drift of the pattern towards the
equator in a manner similar to that seen in the so-called torsional
oscillation observed using surface Doppler shift measurements. Here
I describe results using several 72 day time-series of MDI Medium-l
data. These results show a clear drift of the pattern towards the
equator and that the near surface polar rotation rate appears to
change with time. This research is supported by the SOI-MDI NASA grant
NAG5-3077 at Stanford University.

---------------------------------------------------------
Title: A New Technique for Inversion of Helioseismic Data
Authors: Larsen, R. M.; Kosovichev, A. G.; Schou, J.
1999AAS...194.5604L Altcode: 1999BAAS...31..912L
Inversions of rotational frequency splittings derived from helioseismic
data obtained by the MDI instrument and the GONG network have given a
detailed picture of the differential rotation in the convection zone
(Schou et al. 1998). However, features associated with sharp gradients
of the rotation rate such as jets, near surface shear layers (torsional
oscillations) as well as the transition layer to the radiative interior
(the "tachocline") are usually not well resolved. This is due to the
smoothing applied by traditional inversion methods such as Regularized
Least Squares (RLS) and Optimally Localized Averages (OLA). In this work
we show how a generalized version of the method proposed by Thompson
(1990) can used be to study these features by inverting directly
for the radial or latitudinal derivative of the rotation rate. This
research is supported by the SOI-MDI NASA grant NAG5-3077 at Stanford
University. References: G.I. Marchuk, Methods of Numerical Mathematics,
New York, Springer-Verlag, 1975. Schou, J. et al., 1998, Astrophys J.,
505, 390. Thompson, M. J., 1990, Sol. Phys., 125, 1.

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Title: Time-Variability of the Inferred Rotation in the Upper
Convection Zone
Authors: Toomre, J.; Christensen-Dalsgaard, J.; Howe, R.; Larsen,
R. M.; Schou, J.; Thompson, M. J.
1999soho....9E..87T Altcode:
We present results of inverting for the rotation of the upper convection
zone, using frequency splittings derived from successive 72-day time
series of SOI-MDI observations. Schou (1999; also Schou et al. 1998)
has studied the evolution of the pattern of small-scale zonal flows
in the near-surface layers using f-mode splittings and has found that
this flow pattern migrates equatorward in a manner similar to that seen
in the so-called torsional oscillation observed in surface Doppler
measurements. In the present work we look at the time variability
of the rotation at greater depth, in the upper convection zone, by
inverting both f- and p-mode splittings. The evolution of the flow is
less regular than is seen at the surface, but we do observe apparently
significant variations in the inferred flow pattern, on latitudinal
scales similar to those seen at the surface. In particular, in the
subsurface shear layer we see intriguing variations, with the first
year's data showing apparent emergence of zonal flows of some 10-15
m/s amplitude at around 20 degrees latitude.

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Title: Supergranule Rotation: Too Fast For Convective Cells?
Authors: Beck, J. G.; Schou, J.
1999soho....9E..40B Altcode:
Supergranulation has been long suspected of being a convective
phenomenon. Conventional wisdom regarding convection suggests that
the depth of supergranulation is approximately one half of its width,
or 25,000 km. If supergranules are confined to the outer 25,000 km of
the convective zone, they should rotate at a rate consistent with that
range of depths. However, measurements suggest that supergranules rotate
faster than the plasma at any depth within the convection zone. We
will present supergranulation rotation measurements from improved data
and compare with the results of rotation inversions to determine the
anchor depth of supergranules and discuss the implications for models
of convection. This research is supported by NASA grant NAG5-3077 at
Stanford University.

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Title: Does the Tachocline Show Solar Cycle Related Changes?
Authors: Basu, Sarbani; Schou, Jesper
1999soho....9E..36B Altcode:
The tachocline at the base of the convection zone is generally believed
to be the seat of the solar dynamo. Here we investigate whether the
tachocline shows any detectable change using several 72 day time-series
of MDI Medium-l data.

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Title: Changes in High-Degree Oscillation Frequencies from 1996 to
1999 Determined from Ring-Diagram Analysis
Authors: Bogart, R. S.; Schou, J.; Haber, D. A.; Hindman, B. W.;
Toomre, J.; Hill, F.
1999soho....9E..45B Altcode:
Ring-diagram analysis has traditionally been used primarily as a
diagnostic for large-scale flows in the upper convection zone. It
also yields values for the unperturbed (rest) frequencies of the
local high-degree p-mode oscillations. These frequencies, positioned
predominantly in a regime where ridge-fitting of traditional
global modes is difficult, possess information about the average
near-surface temperature profile in the region being analyzed. As
the solar magnetic activity level increased from 1996 through 1999,
we might expect these frequencies to have changed correspondingly. We
present spatially and temporally averaged rest frequencies determined
from ring-diagram analysis of full-disk Doppler data for selected
intervals from each of the four annual SOI Dynamics campaigns covering
the rise in solar activity from 1996 through 1999. These analyses are
performed on a `dense-pack' mosaic of tracked tiles that oversample
the spatial domain with a resolution of 15 heliographic degrees (180
Mm). Tiles are individually tracked over time spans of 1664 minutes
(27.7 hr), so a given physical region on the Sun is sampled from 7 to
15 times depending on its latitude as it rotates across the visible
hemisphere. We discuss the frequency changes seen for comparable areas
on the disc over the years analyzed. This research is supported by
NASA grant NAG5-3077 at Stanford University.

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Title: Changes in the Near Surface Rotation Rate Observed by MDI
Authors: Schou, J.
1999soho....9E..80S Altcode:
Observations of solar f modes have shown that the small scale zonal
flows in the outer part of the solar convection zone vary with time in
a manner similar to that seen in surface Doppler shift measurements
of the so-called torsional oscillation. Using normal modes to study
these flows should allow their depth dependence to be determined which
could be an important clue for understanding their origin. Here I will
describe the results of using p and f modes to determine the depth and
time dependence of these flows. I will also examine the evolution of
the near polar rotation rate. This research is supported by NASA grant
NAG5-3077 at Stanford University.

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Title: Helioseismic Studies of the Solar Tachocline
Authors: Larsen, R. M.; Kosovichev, A. G.; Schou, J.
1999soho....9E..69L Altcode:
Accurate determinations of the depth and width of the solar tachocline
provide important observational constraints on theoretical models of
solar convection and the solar dynamo, which drives the Sun's magnetic
field. We present the tachocline parameters obtained from inversions
of frequency splittings from MDI, GONG and LOWL and compare this with
previously published results. Finally we invert frequency splittings
for individual GONG months and a number of 72 days sets from MDI, in
order to see if any variation can be seen in the tachocline parameters
with the solar cycle. Briefly, our method consists in inverting
the frequency splittings directly for the radial derivative of the
solar rotation rate using a full 2D SOLA technique. Subsequently the
radial derivative of a parametric model of the rotation profile in
the tachocline is convolved with the averaging kernels from the SOLA
inversion, and the parameters in the models are adjusted to obtain a
chi-squared fit of the model to the inversion result. This research
is supported by NASA grant NAG5-3077 at Stanford University.

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Title: Solar Shear Flows Deduced From Helioseismic Dense-Pack
Samplings of Ring Diagrams
Authors: Haber, D. A.; Hindman, B. W.; Toomre, J.; Bogart, R. S.;
Schou, J.; Hill, F.
1999soho....9E..62H Altcode:
Large-scale flows in the upper convection zone can be inferred
by ring-diagram helioseismic analysis, permitting the study of
both their daily variations and their longer temporal means. We use
selected full-disk SOI-MDI velocity data from the 1996, 1997, and 1998
Dynamics campaigns. We have tracked sets of regions (each 15 degrees
in diameter and spaced 7.5 degrees apart in latitude and longitude),
creating a `dense-pack' mosaic of such tiles that oversamples the
spatial domain. The tiles cover the solar disc to a distance of up to
52.5 degrees from center. A single dense-pack mosaic is prepared by
tracking each of 189 regions for 1664 minutes (27.7 hrs). Such mosaics
now cover more than two full Carrington rotations in 1996 and one-third
of a rotation each in both 1997 and 1998. This is the best spatial and
temporal coverage of any ring-diagram study carried out to date. We
are able to compare the mean flows determined over 9-day averages
for data from the different SOI-MDI Dynamics campaigns, as well as
examine the daily flow maps, allowing us to study possible changes
in the convection during the rising magnetic activity of the current
solar cycle. We also present movies of the daily variations in the
flows within this dense pack for given depths that show the evolution
of the complex velocity field. The longitudinally-averaged meridional
flow varies with latitude but remains relatively constant with depth
below the upper shear layer at 2 Mm down to a depth of about 16 Mm. The
averaged zonal flow increases with depth within this same layer and
agrees well with the rotation rates found from global modes. However,
with the high-degree wave-field data from this analysis we are better
able to resolve the shear layer within the upper convection zone. We
see bands of faster and slower average zonal flows in both hemispheres;
these are present at all depths studied.

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Title: Solar Cycle Onset Seen in SOHO Michelson Doppler Imager
Seismic Data
Authors: Dziembowski, W. A.; Goode, P. R.; di Mauro, M. P.; Kosovichev,
A. G.; Schou, J.
1998ApJ...509..456D Altcode:
We have analyzed time changes in centroid frequencies and multiplet
frequency splittings of solar oscillations determined with the Michelson
Doppler Imager instrument (MDI) on SOHO. The data were divided into
five consecutive 72 day sets covering the period from 1996 May 1
through 1997 April 25. We have detected a significant trend in the
a4 and a6 frequency splitting coefficients,
which reflects a decrease in the P4 distortion (described by
the fourth-degree Legendre polynomial of colatitude) and an increase
in the P6 distortion. The rise of the latter distortion
seems to coincide precisely with the rise in the number of new cycle
sunspots. Such sharp and detailed clues to activity onset are new and do
not exist in splitting data from the rising phase of the last cycle. The
relative differences among the solar radii inferred from the f-mode
frequencies from the five sets (at most 6 × 10-6 or 4 km)
are formally significant, reaching a minimum during the observed period.

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Title: Helioseismic Studies of Differential Rotation in the Solar
Envelope by the Solar Oscillations Investigation Using the Michelson
Doppler Imager
Authors: Schou, J.; Antia, H. M.; Basu, S.; Bogart, R. S.; Bush,
R. I.; Chitre, S. M.; Christensen-Dalsgaard, J.; Di Mauro, M. P.;
Dziembowski, W. A.; Eff-Darwich, A.; Gough, D. O.; Haber, D. A.;
Hoeksema, J. T.; Howe, R.; Korzennik, S. G.; Kosovichev, A. G.;
Larsen, R. M.; Pijpers, F. P.; Scherrer, P. H.; Sekii, T.; Tarbell,
T. D.; Title, A. M.; Thompson, M. J.; Toomre, J.
1998ApJ...505..390S Altcode:
The splitting of the frequencies of the global resonant acoustic modes
of the Sun by large-scale flows and rotation permits study of the
variation of angular velocity Ω with both radius and latitude within
the turbulent convection zone and the deeper radiative interior. The
nearly uninterrupted Doppler imaging observations, provided by the
Solar Oscillations Investigation (SOI) using the Michelson Doppler
Imager (MDI) on the Solar and Heliospheric Observatory (SOHO) spacecraft
positioned at the L1 Lagrangian point in continuous sunlight,
yield oscillation power spectra with very high signal-to-noise ratios
that allow frequency splittings to be determined with exceptional
accuracy. This paper reports on joint helioseismic analyses of
solar rotation in the convection zone and in the outer part of the
radiative core. Inversions have been obtained for a medium-l mode set
(involving modes of angular degree l extending to about 250) obtained
from the first 144 day interval of SOI-MDI observations in 1996. Drawing
inferences about the solar internal rotation from the splitting data
is a subtle process. By applying more than one inversion technique
to the data, we get some indication of what are the more robust
and less robust features of our inversion solutions. Here we have
used seven different inversion methods. To test the reliability and
sensitivity of these methods, we have performed a set of controlled
experiments utilizing artificial data. This gives us some confidence
in the inferences we can draw from the real solar data. The inversions
of SOI-MDI data have confirmed that the decrease of Ω with latitude
seen at the surface extends with little radial variation through much
of the convection zone, at the base of which is an adjustment layer,
called the tachocline, leading to nearly uniform rotation deeper
in the radiative interior. A prominent rotational shearing layer in
which Ω increases just below the surface is discernible at low to
mid latitudes. Using the new data, we have also been able to study the
solar rotation closer to the poles than has been achieved in previous
investigations. The data have revealed that the angular velocity
is distinctly lower at high latitudes than the values previously
extrapolated from measurements at lower latitudes based on surface
Doppler observations and helioseismology. Furthermore, we have found
some evidence near latitudes of 75° of a submerged polar jet which
is rotating more rapidly than its immediate surroundings. Superposed
on the relatively smooth latitudinal variation in Ω are alternating
zonal bands of slightly faster and slower rotation, each extending
some 10° to 15° in latitude. These relatively weak banded flows
have been followed by inversion to a depth of about 5% of the solar
radius and appear to coincide with the evolving pattern of “torsional
oscillations” reported from earlier surface Doppler studies.

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Title: Flows and Horizontal Displacements from Ring Diagrams
Authors: Schou, J.; Bogart, R. S.
1998ApJ...504L.131S Altcode:
A new algorithm for analyzing ring diagrams from helioseismic data
is presented. This method is applied to two months of Solar and
Heliospheric Observatory/Michelson Doppler Imager data from 1996
May 24 through July 24 to study flows and to measure the ratio of
the horizontal displacement to the vertical displacement for the
oscillations. We find that (1) the rotation rate agrees well with
that measured from global modes, extending the measurements closer to
the surface; (2) there is a 20 m s-1 poleward meridional
flow; (3) there are medium-scale velocity features persisting for
several days; (4) the radial gradient of the near surface rotation
rate decreases with latitude; and (5) meridional flow decreases with
depth. The measured horizontal-to-vertical displacement ratio is in
agreement with that expected from theoretical considerations.

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Title: Initial High-Degree p-Mode Frequencies and Rotational Frequency
Splittings from the SOHO SOI/MDI Experiment
Authors: Rhodes, E. J., Jr.; Reiter, J.; Kosovichev, A. G.; Schou,
J.; Scherrer, P. H.
1998AAS...192.1901R Altcode: 1998BAAS...30..845R
We present the first high-degree p-mode frequencies and
rotationally-induced frequency splittings obtained from the Full-Disk
Program of the SOHO Solar Oscillation Investigation/Michelson
Doppler Imager experiment. The frequencies and splittings which we
present here were computed from power spectra obtained during the
1996 SOI/MDI Dynamics Run. Specifically, a 60.75-day time series of
full-disk Dopplergrams was converted into sets of zonal, tesseral,
and sectoral power spectra covering the degree range of 0 through
1000. Estimates of the n-averaged frequency splittings were computed
for the frequency range of 1800 to 4800 microhertz at each degree and
these averaged splitting coefficients were then employed to compute an
average power spectrum for that degree. From these 1001 average power
spectra estimates were made of the frequencies, frequency uncertainties,
widths, peak power densities, and background power densities of a total
of 13664 separate peaks in the set of 1001 average power spectra. A
total of 2554 of these peaks were isolated enough in their respective
spectra to be fit as single p-modes. However, for the remaining 11110
peaks (mostly those above l =200), the individual p-mode peaks and
their spatial sidelobes were located so close together in frequency
that they appeared as ridges rather than as isolated modal peaks in the
average power spectra. For these p-mode ridges we obtained so-called
“ridge-fit” parameter estimates. Observed asymmetries in the p-mode
ridge shapes altered the fitted peak frequencies from their “true”
values and required that we correct the raw ridge-fit frequencies. Forty
sets of these power spectra were also processed to yield estimates of
the rotational splitting coefficients for individual p-mode ridges
for every 25th degree between l =25 and 1000. For l between 25
and 175 we will compare these Full-Disk program splittings with the
previously-published splittings from the 1996 SOI/MDI Medium-l Program
(Kosovichev et al., Solar Physics, 170, 43-61,1997).

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Title: The Rotation of the Solar Core Inferred by Genetic Forward
Modeling
Authors: Charbonneau, P.; Tomczyk, S.; Schou, J.; Thompson, M. J.
1998ApJ...496.1015C Altcode:
Genetic forward modeling is a genetic algorithm-based modeling technique
that can be used to perform helioseismic inversions of the Sun's
internal angular velocity profile. The method can easily accommodate
constraints such as positivity and monotonicity and readily lends itself
to the use of robust statistical goodness-of-fit estimators. After
briefly describing the technique, we ascertain its performance
by carrying out a series of inversions for artificial splitting
data generated from a set of synthetic internal rotation profiles
characterized by various small inward increases in angular velocity in
the deep solar core (r/R⊙ &lt;= 0.5). These experiments
indicate that the technique is accurate down to r/R⊙
~= 0.2, and retains useful sensitivity down to r/R⊙ ~=
0.1. We then use genetic forward modeling in conjunction with the
LOW degree L (LOWL) 2 year frequency-splitting data set to determine
the rotation rate in the deep solar core. We perform a large set
of one-dimensional and 1.5-dimensional inversions using regularized
least-squares minimization, conventional least-squares minimization
with a monotonicity constraint (∂Ω/∂r &lt;= 0), and inversions
using robust statistical estimators. These calculations indicate that
the solar core rotates very nearly rigidly down to r/R⊙ ~
0.1. More specifically, on spatial scales &gt;~0.04 R⊙ we
can rule out inward increases by more than 50% down to r/R⊙
= 0.2, and by more than a factor of 2 down to r/R⊙ =
0.1. Thorough testing of various possible sources of bias associated
with our technique indicates that these results are robust with respect
to intrinsic modeling assumptions. Consequences of our results for
models of the rotational evolution of the Sun and solar-type stars
are discussed.

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Title: Asymmetry in Velocity and Intensity Helioseismic Spectra:
A Solution to a Long-standing Puzzle
Authors: Nigam, R.; Kosovichev, A. G.; Scherrer, P. H.; Schou, J.
1998ApJ...495L.115N Altcode:
We give an explanation for the opposite sense of asymmetry of the
solar acoustic mode lines in velocity and intensity oscillation
power spectra, thereby solving the half-decade-old puzzle of Duvall
and coworkers. The solution came after comparing the velocity and
intensity oscillation data of medium angular degree l obtained from the
Michelson Doppler Imager instrument on board the Solar and Heliospheric
Observatory with the theoretical power spectra. We conclude that the
solar noise in the velocity and intensity spectra is made up of two
components: one is correlated to the source that is responsible for
driving the solar p-modes, and the other is an additive uncorrelated
background. The correlated component of the noise affects the line
profiles. The asymmetry of the intensity spectrum is reversed because
the correlated component is of a sufficiently large level, while the
asymmetry of the velocity spectrum remains unreversed because the
correlated component is smaller. This also explains the high-frequency
shift between velocity and intensity at and above the acoustic cutoff
frequency. A composite source consisting of a monopole term (mass term)
and a dipole term (force due to Reynolds stress) is found to explain
the observed spectra when it is located in the zone of superadiabatic
convection at a depth of 75+/-50 km below the photosphere.

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Title: Subphotospheric Convective Flows Determined by Ring-Diagram
Analyses of SOI-MDI Observations
Authors: Haber, D.; Hindman, B.; Toomre, J.; Bogart, R.; Schou, J.;
Hill, F.
1998ESASP.418..791H Altcode: 1998soho....6..791H
The variation of large-scale velocity flows with depth and location
on the sun places important constraints on theoretical models of the
solar convection zone and dynamo. High-degree oscillations can be
viewed as nearly plane waves that are advected and distorted by the
underlying flows. By conducting observations over limited regions
of the solar surface to obtain `ring diagram' power spectra, we can
deduce spatially-averaged horizontal flows with depth below that
region. Previous analyses of ring diagrams have already suggested
the presence of strong shearing flows below the surface. We have
now implemented a highly efficient technique for determining these
horizontal flows with depth and report here on a systematic analysis of
full-disk Doppler velocity data taken continuously with a one-minute
cadence during portions of the two-month dynamics observing program
with SOI-MDI in 1996. The square regions examined span about 15-circ,
and are studied for time intervals each of about 1536 mins (~25 hrs). A
lattice of such squares is considered: their centers are spaced 15-circ
apart in longitude and there are seven such regions across the solar
disk at +20-circ, 0-circ, -20-circ latitude. Another set of regions
is placed along the central meridian at 10-circ and 15-circ intervals
in latitude. Properties of the underlying large-scale subphotospheric
flows and their temporal variations so revealed are presented in detail.

---------------------------------------------------------
Title: Slow Poles and Shearing Flows from Heliospheric Observations
with MDI and GONG Spanning a Year
Authors: Schou, J.; Christensen-Dalsgaard, J.; Howe, R.; Larsen,
R. M.; Thompson, M. J.; Toomre, J.
1998ESASP.418..845S Altcode: 1998soho....6..845S
We invert one year of coeval high-resolution rotational splitting data
(up to degree l 250) from GONG and SOI-MDI. The first 4 months of
MDI data uncovered several new features in the rotation of the solar
convective envelope: surface and subsurface zonal bands corresponding
to the so-called torsional oscillations, superimposed on the overall
smooth latitudinal surface rotation; a drop in the near-polar surface
rotation rate below the rate extrapolated from lower latitudes; and an
indication of a prograde jet-like feature at high latitudes at a depth
of about 5 percent of the solar radius. Using the 1 year of data from
the MDI and GONG instruments, we test the robustness and stationarity of
these features. As an aid to testing the robustness of our inferences,
we use two independent inversion methods (2-D regularized least squares
and SOLA) and apply them to the splitting data obtained from both GONG
and MDI.

---------------------------------------------------------
Title: The Comparison of Simultaneous SOI/MDI and Mt. Wilson 60-foot
Tower Power Spectra and p-Mode Parameters
Authors: Rhodes, E. J., Jr.; Reiter, J.; Kosovichev, A. G.; Schou,
J.; Scherrer, P. H.; Rose, P. J.; Irish, S.; Jones, A. R.
1998ESASP.418..311R Altcode: 1998soho....6..311R
We present the results of the first detailed comparison between the
1996 SOHO SOI/MDI Dynamics Run program of full-disk Ni I Dopplergram
observations and a simultaneous time series of ground-based
observations obtained in the Na D lines at the 60-Foot Solar Tower
of the Mt. Wilson Observatory (MWO). Specifically, we will compare
sets of simultaneously-observed SOHO/MDI and MWO power spectra and
the high-degree p-mode frequencies, frequency splittings, widths, and
power densities which we obtained by fitting these two sets of power
spectra. Beginning on May 23, 1996, the SOI/MDI experiment began its
first high duty cycle run of 1024x1024 pixel images. this was the
1996 Dynamics Run. On all but three of the days of this 60.75-day
time series a second time series of simultaneous 1024x1024 pixel
full-disk Dopplergrams was obtained at MWO. From these simultaneous MWO
observations we have computed 601 sets of zonal, tesseral, and sectoral
power spectra which covered the degree range of 0 to 600. These sets
of power spectra were then analyzed in two different ways to yield
both frequencies and frequency splittings. First, estimates of the
frequency splittings were computed for the frequency range of 1800 to
4800 microhertz at each degree and these n-averaged splittings were
employed to compute an average power spectrum for that degree. Estimates
were then made of the frequencies, frequency uncertainties, widths, peak
power densities, and background power densities of the set of peaks in
these 601 average power spectra. As is described in our companion paper
on the MDI ridge-fit frequency measurements (Rhodes et al.,1998), we
also had to correct our raw MWO ridge-fit frequencies for the effects
the merger of individual p-mode peaks and sidelobes into ridges. We
could also directly compare the frequency dependence of the observed
power density in both the MDI and MWO power spectra. We have found
that there is a systematic difference such that the power density in
the chromospheric-level power spectra from MWO tends to be below that
of the photosheric-level MDI spectra at low frequencies and to rise
above the MDI power density as the frequency increases. All but the l
= 0 MWO power spectra were also processed to yield estimates of the
rotational splitting coefficients for individual p-mode ridges for
every degree between l = 4 and 600. We will compare these splittings
with both the previously-published splittings from the 1996 SOI/MDI
Medium-l Program (Kosovichev et al., 1997) and with the MDI high-degree
splittings presented in our companion MDI paper.

---------------------------------------------------------
Title: Comparison of SOHO-SOI/MDI and GONG Spectra
Authors: Komm, R. W.; Anderson, E.; Hill, F.; Howe, R.; Kosovichev,
A. G.; Scherrer, P. H.; Schou, J.; Fodor, I.; Stark, P.
1998ESASP.418..253K Altcode: 1998soho....6..253K
We compare solar p-mode parameters, such as central frequency, width,
and amplitude, derived from GONG and SOHO-SOI/MDI Medium-l Program
time series obtained during the same time period. With the excellent
data available now from GONG and SOHO-SOI/MDI, there exist data
sets long enough to make such a comparison useful. For this study,
we have chosen time series of three ell values (ell = 30, 65, and 100)
corresponding to GONG month 16 (Oct 28 -- Dec 2, 1996). For each time
series, we calculated multitaper power spectra using generalized
sine tapers to reduce the influence of the gap structure, which is
different for the two data sets. Then, we applied the GONG peakfitting
algorithm to the spectra to derive mode parameters and selected `good'
fits common to both MDI and GONG spectra, according to three selection
criteria. Preliminary results show that mode frequencies determined
from MDI spectra are essentially the same as the frequencies from
GONG spectra and that the difference is, in general, well within one
formal error bar. The background slope at frequencies above 5mHz is
different between MDI and GONG spectra depending on ell. At present,
we are analyzing 3-month time series of ell = 0 to ell = 150. We intend
to present the results of the on-going comparison.

---------------------------------------------------------
Title: Initial SOI/MDI High-Degree Frequencies and Frequency
Splittings
Authors: Rhodes, E. J., Jr.; Reiter, J.; Kosovichev, A. G.; Schou,
J.; Scherrer, P. H.
1998ESASP.418...73R Altcode: 1998soho....6...73R
We present the first high-degree p-mode frequencies and frequency
splittings obtained from the Full-Disk Program of the SOHO Solar
Oscillation Investigation/Michelson Doppler Imager experiment. The
frequencies and splittings which we present here were computed
from power spectra obtained during the 1996 SOI/MDI Dynamics
Run. Specifically, a time series of full-disk Dopplergrams, which
began on May 23, 1996, and which covered 87480 minutes (60.75 days),
was converted into sets of zonal, tesseral, and sectoral power spectra
covering the degree range of 0 through 1000. These sets of power spectra
were then analyzed in two different ways to yield both frequencies and
frequency splittings. First, estimates of the frequency splittings
were computed for the frequency range of 1800 to 4800 microhertz at
each degree and these n-averaged splittings were employed to compute an
average power spectrum for that degree. Estimates were then made of the
frequencies, frequency uncertainties, widths, peak power densities,
and background power densities of a total of 13664 separate peaks
in these 1001 average power spectra. A total of 2554 of these peaks
were isolated enough in their respective spectra to be fit as single
p-modes. However, for the remaining 11110 peaks (mostly those above l
= 200), the individual p-mode peaks and their spatial sidelobes were
located so close together in frequency that they appeared as ridges
rather than as isolated modal peaks in the average power spectra. For
these cases we were forced to employ a wider fitting range for our
frequency-estimation code and in so doing we obtained so-called
“ridge-fit” parameter estimates. Due to a degree-dependence in
the measured velocity power density, the observed p-mode ridges were
asymmetric in shape. These asymmetries in the p-mode ridge shapes,
which are in addition to the intinsic asymmetries caused by the
excitation mechanism of the p-modes themselves, alter the fitted peak
frequencies from their “true” values. For this reason, we had to
develop a mechanism which we then used to correct the raw ridge-fit
frequencies. Forty sets of these power spectra were also processed to
yield estimates of the rotational splitting coefficients for individual
p-mode ridges for every 25th degree between l = 25 and 1000. For l
between 25 and 175 we will compare these Full-Disk program splittings
with the previously-published splittings from the 1996 SOI/MDI Medium-l
Program (Kosovichev et al., Solar Physics, 170, 43-61,1997).

---------------------------------------------------------
Title: The Stellar and Planetary Explorer (SPEX) Mission
Authors: Schou, J.; Scherrer, P. H.; Brown, T. M.; Frandsen, S.;
Horner, S. D.; Korzennik, S. G.; Noyes, R. W.; Tarbell, T. D.; Title,
A. M.; Walker, A. B. C., II; Weiss, W. W.; Bogart, R. S.; Bush, R. I.;
Christensen-Dalsgaard, J.; Hoeksema, J. T.; Jones, A.; Kjeldsen, H.
1998ESASP.418..401S Altcode: 1998soho....6..401S
The Stellar and Planetary Explorer (SPEX) is a mission designed
to search for terrestrial sized planets around sun-like stars using
precise photometry. The planets will be detected by searching for the
decrease in brightness associated with transits of the planets in front
of their parent stars. One of the secondary scientific objective of
SPEX is to do asteroseismology on a number of sun-like stars. SPEX
is designed as a secondary payload on a commercial communications
satellite and will have a design life time of three years. We will
provide an overview of the SPEX scientific objectives and design,
with particular emphasis on the prospects for doing asteroseismology.

---------------------------------------------------------
Title: Solar Internal Rotation
Authors: Schou, J.; SOE Internal Rotation Team
1998IAUS..185..141S Altcode:
With the flood of high quality helioseismic data from the instruments
on the SOHO spacecraft (MDI/VIRGO/GOLF) and ground based instruments
(eg. GONG and LOWL) we have been able to get increasingly detailed
information on the rotation and other large scale flows in the solar
interior. In this talk I will discuss some of the highlights of what
we have learned so far and what we may expect to learn in the near
future. Among the recent advances have been tighter constraints on the
tachocline at the bottom of the convection zone, detection of details in
the surface rotation rate similar to the torsional oscillations found
in the surface Doppler shift and helioseismic evidence for meridional
flows. The MDI project is supported by NASA contract NAG5-3077 at
Stanford University.

---------------------------------------------------------
Title: Solar Asymmetries from SOHO/MDI Splitting Data
Authors: Goode, P. R.; Dziembowski, W. A.; DiMauro, M. P.; Kosovichev,
A. G.; Schou, J.
1998ESASP.418..887G Altcode: 1998soho....6..887G
No abstract at ADS

---------------------------------------------------------
Title: How Low Can We Get: The Quest for Ever Lower Frequencies
Authors: Schou, J.
1998ESASP.418..341S Altcode: 1998soho....6..341S
Almost since the dawn of helioseismology observers have tried to push
the low frequency limit of the observed modes down. Why is that? In
this poster I will ramble about the importance of these modes to our
inferences about the Sun. How much would it help to push the limit
down by another couple of hundred muHz? Do we have to see g-modes
in order to make a significant difference? Using nearly two years of
MDI data, devious analysis techniques and a bit of imagination I will
see how far down medium-degree modes can be observed. I will try to
determine what the noise sources are and what one might be able to do
to decrease their effect. What have we learned from the low frequency
modes we have seen so far? The f-mode observations have allowed us to
determine the acoustic radius of the Sun. Also low frequency modes have
made it possible to probe the so-called torsional oscillations. I will
present some recent results from these and other areas. Finally I will
speculate on the possibility of getting even lower in frequency. How
much does longer observations help? Would seeing the back side of the
Sun help? Are different analysis techniques needed? Are we ever going
to see g-modes?

---------------------------------------------------------
Title: Improved SOLA Inversions of MDI Data
Authors: Larsen, R. M.; Christensen-Dalsgaard, J.; Kosovichev, A. G.;
Schou, J.
1998ESASP.418..813L Altcode: 1998soho....6..813L
We present a new version of 2d-SOLA, where the target functions have
been modified to match the behavior of the mode kernels near the
rotation axis and to minimize near-surface contributions. Inversion
of artificial data show that these modifications significantly improve
the effective resolution near the pole, which allows us to assess the
reliability of the high-latitude features seen by other inversion
methods. Most importantly, our new inversions seem to confirm the
detection of a submerged polar jet previously seen in the 2d-RLS
inversions reported by Schou et al. 1998. A test of the robustness of
the improved method is carried out by inverting artificial data from
the MDI Hare and Hounds exercise. We analyze the averaging kernels and
error propagation of the method, and also describe the error-correlation
between different points in the solution, the latter being a potential
source of spurious features in the solutions as pointed out by Howe
and Thompson, 1996. So far, helioseismic datasets given in the form of
a-coefficients have been inverted under the assumption that the errors
in different a-coefficients are uncorrelated. The MDI peak-bagging
procedure, however, does produce estimates of the error-correlation
between a-coefficients within the same multiplet. Here we investigate
the effect of including this knowledge in the inversions.

---------------------------------------------------------
Title: Tests of Convective Frequency Effects with SOI/MDI High-Degree
Data
Authors: Rosenthal, C. S.; Christensen-Dalsgaard, J.; Kosovichev,
A. G.; Nordlund, A. A.; Reiter, J.; Rhodes, E. J., Jr.; Schou, J.;
Stein, R. F.; Trampedach, R.
1998ESASP.418..521R Altcode: 1998astro.ph..7066R; 1998soho....6..521R
Advances in hydrodynamical simulations have provided new insight into
the effects of convection on the frequencies of solar oscillations. As
more accurate observations become available, this may lead to an
improved understanding of the dynamics of convection and the interaction
between convection and pulsation (Rosenthal et al. 1999). Recent
high-resolution observations from the SOI/MDI instrument on the
SOHO spacecraft have provided the so-far most-detailed observations
of high-degree modes of solar oscillations, which are particularly
sensitive to the near-surface properties of the Sun. Here we present
preliminary results of a comparison between these observations and
frequencies computed for models based on realistic simulations of
near-surface convection. Such comparisons may be expected to help
in identifying the causes for the remaining differences between the
observed frequencies and those of solar models.

---------------------------------------------------------
Title: Spherical and aspherical structure of the sun: First year of
SOHO/MDI observations
Authors: Kosovichev, A. G.; Schou, J.; Scherrer, P. H.; Goode, P. H.;
Dziembowski, W. A.; Rhodes, E. J., Jr.; SOI Structure Inversion Team
1998IAUS..185..157K Altcode:
We report the results of one year of continuous observations of the
Sun's internal structure using data from the Medium-l Program of
the Michelson Doppler Imager (MDI) on board SOHO. The data provide
continuous coverage of p modes of angular degree l from 0 to 250,
and the f mode from l=100 to 250. The striking stability of solar
Dopplergrams measured by MDI, without an intervening atmosphere,
substantially decreases the noise in the solar oscillations power
spectrum compared with ground-based observations. This permits
detection of lower amplitude oscillations, extending the range and
precision of measured normal mode frequencies. We present new inversion
results for the radial and latitudinal seismic solar structures with
particular attention to zonal asphericity inferred with the high
angular resolution from the data. Using f-mode frequency splitting
we estimate the large-scale structure of the subsurface magnetic
fields. The variations of the solar structure observed during the
first year of MDI observations are also discussed.

---------------------------------------------------------
Title: Observations of Medium- and High-Degree Modes: Methods and
Sand-Traps
Authors: Schou, J.
1998ESASP.418...47S Altcode: 1998soho....6...47S
The spectrum of solar oscillations is often divided into low-, medium-
and high-degree (l) modes. Low-degree modes are those observable in
integrated sunlight, medium-degree modes are those for which the modes
with the same order (n) but adjacent l can be separated in a frequency
spectrum and high-degree modes are those for which the modes blend
together to form ridges. The medium-degree modes are those for which
most parameter estimates exist. Indeed, most of our knowledge of the
solar interior has been derived from observations of medium-degree
p- and f-mode frequencies and splittings. While much effort has been
spent on measuring the medium-degree mode parameters it is clear that
significant systematic errors remain and that further analysis is needed
to resolve those. I will discuss some of the methods used for observing
and analyzing medium-degree modes and describe some of the systematic
errors seen. It is likely that most of the errors come from the analysis
methods and the approximations they make. I will describe the general
idea behind some of the methods and try to identify some of the likely
causes for the systematic errors. For high-degree modes ridges are
formed and the instrumental problems become more important. Significant
progress has been made recently in the analysis of the high-degree
spectra. I will describe some of the problems encountered, summarize
the recent results and discuss what remains to be done.

---------------------------------------------------------
Title: Solar Asymmetries from SOHO/MDI Splitting Data
Authors: Dziembowski, W. A.; Goode, P. R.; Di Mauro, M. P.; Kosovichev,
A. G.; Schou, J.
1998ESASP.418..887D Altcode: 1998soho....6..887D
Systematic changes in p-mode frequencies through the solar cycle
have been discovered during the previous high activity phase. Most
significant changes were found in the even-a coefficients of
the fine structure in the oscillation spectra (Kuhn, 1988;
and Libbrecht and Woodard, 1990). We analyzed time changes in
frequencies determined with the SOHO/MDI instrument. The data were
divided into five 72-day sets covering (1) 5/1/96-7/11/96, (2)
7/12/96-9/21/96, (3) 9/22/96-12/2/96, (4) 12/3/96-2/12/97, and (5)
2/13/97-4/25/97. The splitting coefficients ak are defined by
nuvlosell,n,m-bar nuell,n = sum{k = 1}
ak {cal P}kell(m), where {cal P} are
are orthogonal polynomials (see Ritzwoller and Lavely 1991 and Schou,
et al. 1994). We analyzed behavior of the even order coefficients,
a2k, which arise from the respective, P2k
(cos θ), distortion of the Sun's structure. We found a significant
trend in behavior of the a4 and a6 coefficients,
which reflects a decrease of the P4 and an increase of the
P6 distortions. This trend is the same as seen in the BBSO
data (Libbrecht and Woodard, 1990) between 1986 and 1988 i.e. at the
onset of the previous activity phase. The trend in a2 is
not so apparent. The centroid frequencies, bar nuell,n, as
already reported by Kosovichev et al. (1998), exhibit small nonmonotonic
variations. The relative differences in solar radius inferred from the
f-mode frequencies in the five sets (at most 5 times 10-6)
are formally significant, but again there is no trend.

---------------------------------------------------------
Title: The SOI-MDI High-Latitude Jet: the Evidence For and Against
Authors: Howe, R.; Antia, H.; Basu, S.; Christensen-Dalsgaard, J.;
Korzennik, S. G.; Schou, J.; Thompson, M. J.
1998ESASP.418..803H Altcode: 1998soho....6..803H
The apparent detection of a prograde jet at latitude 75-circ and at a
radius of about 0.95Rodot in some inversions of rotation data
from SOI--MDI (Schou et al., 1998) has excited considerable interest,
but whether the jet really exists in the solar interior is certainly not
yet firmly established. The detection of the feature is sensitive both
to the inversion techniques used and to the methods of mode parameter
estimation used to generate the input data. In particular, the feature
is much more apparent in Regularized Least-Squares inversions than
in inversions using an Optimally Localized Average approach, and is
not detected at all in the present GONG data when analysed with the
GONG peakfinding algorithm, or indeed in SOI data when analysed with
the GONG algorithm. Therefore in this poster we examine critically
the current evidence for the source and existence of this jet in the
light of forward and inverse analyses.

---------------------------------------------------------
Title: Probing the Internal Structure of the Sun with the SOHO
Michelson Doppler Imager
Authors: Kosovichev, A. G.; Nigam, R.; Scherrer, P. H.; Schou, J.;
Reiter, J.; Rhodes, E. J., Jr.; Toutain, T.
1997AAS...191.7311K Altcode: 1997BAAS...29R1322K
The inference of the thermodynamic structure of the Sun from the
observed properties of the solar normal modes of oscillation is a
principal goal of helioseismology. We report the results of the first
year of continuous observations of the Sun's internal structure using
data from the Medium-l Program of the Michelson Doppler Imager (MDI)
on board ESA/NASA spacecraft SOHO. The data provide continuous coverage
of the acoustic (p) modes of angular degree l from 0 to 250, and the
fundamental (f) mode of the Sun from l=100 to 250. During two 2-month
intervals, the high-degree modes, up to l=1000, have been observed. The
great stability of solar Dopplergrams measured by MDI permits detection
of lower amplitude oscillations, extending the range and precision of
measured normal mode frequencies, and thus substantially increasing
the resolution and precision of helioseismic inversions. We present
new inversion results for the radial and latitudinal seismic solar
structures with particular attention to the transition region between
the radiative and convection zones and to the energy-generating core. We
discuss evidence for convective overshoot at the base of the convection
zone, and the significance of deviations in the core structure from
the standard evolutionary model. Comparing the f-mode frequencies
with the corresponding frequencies of the standard solar models, we
argue that the apparent photospheric solar radius (695.99 Mm) used to
calibrate the models should be reduced by approximately 0.3 Mm. The
discrepancy between the `seismic' and apparent photospheric radii is
not explained by the known systematic errors in the helioseismic and
photospheric measurements. If confirmed, this discrepancy represents
a new interesting challenge to theories of solar convection and solar
modeling. Using f-mode frequency splitting we estimate the large-scale
structure of the subsurface magnetic fields. The variations of the solar
oscillation frequencies during the first year of MDI observations are
also discussed.

---------------------------------------------------------
Title: Solar internal sound speed as inferred from combined BiSON
and LOWL oscillation frequencies
Authors: Basu, Sarbani; Christensen-Dalsgaard, J.; Chaplin, W. J.;
Elsworth, Y.; Isaak, G. R.; New, R.; Schou, J.; Thompson, M. J.;
Tomczyk, S.
1997MNRAS.292..243B Altcode: 1997astro.ph..2105B
Observations of the Sun with the LOWL instrument provide a homogeneous
set of solar p-mode frequencies from low to intermediate degree that
allow one to determine the structure of much of the solar interior
avoiding systematic errors that are introduced when different data sets
are combined, i.e., principally the effects of solar cycle changes on
the frequencies. Unfortunately, the LOWL data set contains very few of
the lowest-degree modes, which are essential for determining reliably
the structure of the solar core - in addition, these lowest-degree data
have fairly large associated uncertainties. However, observations made
by the Birmingham Solar-Oscillations Network (BiSON) in integrated
sunlight provide high-accuracy measurements of a large number of
low-degree modes. In this paper we demonstrate that the low-degree mode
set of the LOWL data can be successfully combined with the more accurate
BiSON data, provided the observations are contemporaneous for those
frequencies where the solar cycle induced effects are important. We
show that this leads to a factor of 2 decrease in the error on the
inferred sound speed in the solar core. We find that the solar sound
speed is higher than in solar models for r&lt;0.2Rsolar. The density
of the solar core is, however, lower than that in solar models.

---------------------------------------------------------
Title: Rotation and Zonal Flows in the Solar Envelope from the
SOHO/MDI Observations
Authors: Scherrer, P. H.; Schou, J.; Bogart, R. S.; Bush, R. I.;
Hoeksema, J. T.; Kosovichev, A. G.; Antia, H. M.; Chitre, S. M.;
Christensen-Dalsgaard, J.; Larsen, R. M.; Pijpers, F. P.; Eff-Darwich,
A.; Korzennik, S. G.; Gough, D. O.; Sekii, T.; Howe, R.; Tarbell,
T.; Title, A. M.; Thompson, M. J.; Toomre, J.
1997AAS...191.7310S Altcode: 1997BAAS...29.1322S
We report on the latest inferences concerning solar differential
rotation that have been drawn from the helioseismic data that are now
available from the Solar Oscillations Investigation (SOI) using the
Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory
(SOHO). That spacecraft is positioned in a halo orbit near the Sun-Earth
Lagrangian point L_1, in order to obtain continuous Doppler-imaged
observations of the sun with high spatial fidelity. Doppler velocity,
intensity and magnetic field images are recorded, based on modulations
of the 676.8 nm Ni I solar absorption line. The high spatial resolution
of MDI thereby permits the study of many millions of global resonant
modes of solar oscillation. Determination and subsequent inversion
of the frequencies of these modes, including the degeneracy-splitting
by the rotation of the sun, enables us to infer how the sun's angular
velocity varies throughout much of the interior. The current MDI data
are providing substantial refinements to the helioseismic deductions
that can be made about differential rotation both within the convection
zone and in its transition to the radiative interior. The shearing
layer evident in the angular velocity Omega just below the solar
surface is becoming better defined, as is the adjustment layer or
tachocline near the base of the convection zone. The MDI data are also
revealing a prominent decrease in Omega at high latitudes from the
rotation rate expressed by a simple three-term expansion in latitude
that was originally deduced from surface Doppler measurements. Further,
there are indications that a submerged polar vortex involving somewhat
faster Omega than its surroundings exists at about 75(deg) in latitudes.

---------------------------------------------------------
Title: Determination of the Sun's Seismic Radius from the SOHO
Michelson Doppler Imager
Authors: Schou, J.; Kosovichev, A. G.; Goode, P. R.; Dziembowski, W. A.
1997ApJ...489L.197S Altcode:
Dopplergrams from the Michelson Doppler Imager (MDI) instrument on board
the SOHO spacecraft have been used to accurately measure frequencies of
the Sun's fundamental (f) mode in the medium angular degree range, l =
88--250. The comparison of these frequencies with the corresponding
frequencies of the standard solar models suggests that the apparent
photospheric solar radius (695.99 Mm) used to calibrate the models
should be reduced by approximately 0.3 Mm. The precise value of the
seismologically determined solar radius depends on the description
of the subsurface layer of superadiabatic convection. The discrepancy
between the "seismic" and apparent photospheric radii is not explained
by the known systematic errors in the helioseismic and photospheric
measurements. If confirmed, this discrepancy represents an interesting
new challenge to theories of solar convection and solar modeling.

---------------------------------------------------------
Title: Measurements of Frequencies of Solar Oscillations from the
MDI Medium-l Program
Authors: Rhodes, E. J., Jr.; Kosovichev, A. G.; Schou, J.; Scherrer,
P. H.; Reiter, J.
1997SoPh..175..287R Altcode:
Inversions of solar internal structure employ both the frequencies
and the associated uncertainties of the solar oscillation modes
as input parameters. In this paper we investigate how systematic
errors in these input parameters may affect the resulting inferences
of the sun's internal structure. Such systematic errors are likely
to arise from inaccuracies in the theoretical models which are used
to represent the spectral lines in the observational power spectra,
from line blending, from asymmetries in the profiles of these lines,
and from other factors. In order to study such systematic effects we
have employed two different duration observing runs (one of 60 days
and the second of 144 days) obtained with the Medium-l Program of the
Michelson Doppler Imager experiment onboard the SOHO spacecraft. This
observing program provides continuous observations of solar oscillation
modes having angular degrees, l, ranging from 0 to ∼ 300. For this
study intermediate- and high-degree p-mode oscillations having degrees
less than 251 were employed.

---------------------------------------------------------
Title: Solar internal rotation from LOWL data. A 2D regularized
least-squares inversion using B-splines.
Authors: Corbard, T.; Berthomieu, G.; Morel, P.; Provost, J.; Schou,
J.; Tomczyk, S.
1997A&A...324..298C Altcode:
Observations of surface oscillations of the Sun can be analyzed to
probe the solar interior. We use data obtained by the LOWL instrument
(LOWL is an abbreviation for low degree with degree denoted by L)
installed on Mauna Loa, Hawaii, since 1994 to investigate solar
internal rotation. A 2 Dimensional Regularized Least-Squares (2D RLS)
inverse method based on an expansion of the solution on B-splines of
arbitrary order is presented and applied to a 2 year dataset. This
method insures the regularity of the solution in the center and
introduces surface constraints. The choice of trade-off parameters in
the regularization term is discussed using an L-curves analysis and
we discuss the influence of the choice of the order of derivatives in
the regularization terms for the description of the deep interior. We
study the latitudinal resolution of the inversion of a-coefficients
compared to that of the inversion of individual splittings built from
these coefficients. Compared to the previous inversion of the first
three months of LOWL data made by Tomczyk et al. (1995ApJ...448L..57T),
our solution is extended up to the surface by adding high degree modes
and constraining the rotation to fit the spectrographic observations
(Snodgrass, 1984SPh....94...13S). In the radiative zone we obtain
more rigid rotation and our solution is compatible with a rotation of
the solar core of the order or smaller than the surface rotation at
mid latitude.

---------------------------------------------------------
Title: Helioseismic measure of solar activity-meaning and
applications.
Authors: Dziembowski, W. A.; Philip, R. Goode; Schou, J.; Tomczyk,
Steve
1997A&A...323..231D Altcode:
We analyze the antisymmetric part of the fine structure in the LOWL
data, and find a remarkable agreement with the BBSO data taken during
the 1986 activity minimum. For both, the P_4_(cosθ) component of the
Sun's asphericity is dominant. We discuss the importance of measuring
this part of the fine structure as a global probe of the Sun's varying
magnetic activity. The asphericity affects oscillation frequencies in
a way that corrupts any inversion for the radial structure of the deep
solar interior. The results of inversion of the original and cleansed
data show that at the current minimal level of solar activity, the
effect is within the errors. However, this is not true in the case of
measurements taken in years of high activity. We mimic such measurements
by adding in appropriate frequency shifts evaluated from 1989 BBSO data.

---------------------------------------------------------
Title: Detection of Zonal Shear Flows beneath the Sun's Surface from
f-Mode Frequency Splitting
Authors: Kosovichev, A. G.; Schou, J.
1997ApJ...482L.207K Altcode:
We report on the first successful measure of the zonal variations of
the Sun's differential rotation (so-called torsional oscillations) by
helioseismology. Using new helioseismic data from the Michelson Doppler
Imager on board SOHO, we have detected zonal flow bands with velocity
variation of 5 m s-1 at a depth of 2-9 Mm beneath the
surface. The subsurface flow is inferred from rotational splitting of
frequencies of the fundamental mode of solar oscillations in the range
of angular degree l from 120 to 250, using a 144 day uninterrupted time
series of the Sun's Doppler velocities. The structure of the subsurface
shear flow resembles the pattern of the torsional oscillations observed
on the surface. Comparing with previous surface measurements, we found
evidence for migration of the flow bands towards the equator.

---------------------------------------------------------
Title: On the Importance of Various Sources of Systematic Errors in
Helioseismic Measurements
Authors: Schou, J.
1997SPD....28.0205S Altcode: 1997BAAS...29..893S
With the increasing precision of helioseismic measurements, the
importance of systematic errors are increasing too. In this poster
I will examine the importance of various possible systematic errors
in the measurements on which some of the recent discoveries in
helioseismology rely. Among the problems I will examine are the
effects of the distortion of the p-modes by differential rotation,
the horizontal displacement and the use of a-coefficients instead of
individual m splittings. All of these problems could potentially cause
the frequency splittings to be significantly biased. The research
presented here was supported by NASA contract NAG5-3077 to MDI at
Stanford University.

---------------------------------------------------------
Title: Results from the MDI Instrument
Authors: Schou, J.
1997AAS...190.0402S Altcode: 1997BAAS...29..782S
Using data from the MDI instrument on the SOHO satellite we have been
able to infer details in the structure and dynamics of the solar
interior which were not observed before. Here I will discuss some
of the highlights of what we have learned so far, such as details
of the near surface rotation rate and the structure and rotation
rate in the convection zone and below. I will also discuss some of
the results obtained about the near surface layers using local area
helioseismology. The MDI project is supported by NASA contract NAG5-3077
at Stanford University.

---------------------------------------------------------
Title: Analysis of Velocity and Intensity Helioseismic Spectra
from SOHO/MDI
Authors: Nigam, R.; Kosovichev, A. G.; Scherrer, P. H.; Schou, J.
1997SPD....28.0904N Altcode: 1997BAAS...29..913N
We give an explanation for the cause of the asymmetry of spectral lines
of solar oscillation power spectrum. We also explain the cause of the
opposite sense of asymmetry in velocity and intensity oscillation power
spectra, thereby resolving a half-decade old puzzle. The motivation for
the investigation came after comparing the velocity and intensity data
obtained from the Michelson Doppler Imager (MDI) instrument on board the
Solar and Heliospheric Observatory (SOHO). The analysis is based on a
theoretical model of wave excitation with viscous damping in conjunction
with a spherically symmetric solar model. Neglecting asymmetry can
lead to systematic errors in the eigenfrequency measurements, which
in turn leads to errors in inversion. This research was supported by
NASA grant NAG5-3077 at Stanford University.

---------------------------------------------------------
Title: Solar Internal Rotation From SOHO and Ground Based
Helioseismology Experiments
Authors: Schou, J.
1997SPD....28.0903S Altcode: 1997BAAS...29..913S
With the quality of the data from the helioseismology instruments on
SOHO and from ground based instruments we have begun to see intriguing
details in the solar rotation rate that could not be detected before. In
this talk I will briefly discuss some of the highlights of what we
have learned so far, such as details of the near surface rotation rate,
the properties of the tachocline at the bottom of the convection zone
and the rotation of the solar core. The MDI project is supported by
NASA contract NAG5-3077 at Stanford University.

---------------------------------------------------------
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: Persistence of Large-Scale Flows Beneath Quiet Sun: Local-Area
Analysis Using MDI Doppler Data
Authors: Haber, D.; Toomre, J.; Bogart, R.; Schou, J.; Gonzalez, I.;
Hill, F.
1997SPD....28.0201H Altcode: 1997BAAS...29..893H
Knowing the large-scale flows that occur in the upper convection zone
is critical to our understanding of the processes that govern the
solar cycle. Here we apply solar oscillation ring-diagram analysis to
several small tracked regions on the sun, approximately 15(deg) on a
side, as they rotate across the solar disk, in order to determine the
persistence and depth variation of the large-scale flows beneath these
regions. We use the Doppler velocity images from the Michelson Doppler
Imager (MDI) instrument aboard the Solar Heliospheric Observatory
(SOHO) satellite using quiet-sun data taken during the MDI Dynamics
campaign of 1996. Three regions at different latitudes were extracted
from full-disk Doppler images of 1024 x 1024 pixels (pixel size ~
2” square) with a one-minute temporal cadence. Eight sequential
1536-minute time intervals were tracked, remapped onto great-circle
grids, spatially and temporally filtered, and apodized in space and
time. They were then Fourier transformed in two spatial dimensions
and time. The resulting power spectra show characteristic rings at
each frequency slice. Shifts in the center positions of the rings are
caused by underlying flow fields and can be inverted to map these flows
with depth. We use several techniques to fit these shifts in order to
assess the stability of the results.

---------------------------------------------------------
Title: The Effects of Systematic Errors in the Estimation of p-Mode
Frequencies on the Inversion of Solar Internal Structure
Authors: Rhodes, E. J., Jr.; Appourchaux, T.; Bachmann, K.; Kosovichev,
A. G.; Scherrer, P. H.; Schou, J.; Reiter, J.
1997SPD....28.0901R Altcode: 1997BAAS...29..913R
The frequencies and associated uncertainties of the low-, intermdeiate-,
and high-degree p-mode oscillations are the input quantities for the
inversion programs which infer the thermodynamic structure of the solar
interior. In this review we will attempt to demonstrate the different
possible systematic errors that are currently present in our estimation
of both the modal frequencies and their uncertainties. We will also
demonstrate the effects of some of these errors upon the inferred
radial profile of the solar internal sound speed. Among the different
possible systematic errors which we will discuss are the effects of:
1)the asymmetric shapes of the peaks in observational power spectra,
2)the realization noise which is present in the case of the low-degree
modes, 3)the different frequency estimation methods used on different
types of power spectra (i.e., on either tesseral power spectra or
on m-averaged power spectra), 4) the differences in the frequencies
which are estimated from velocity- and intensity-based power spectra,
5) the blending of individual p-modes into so-called "ridges" of
observed power at both high degrees and high frequencies, and 6) the
spatial and temporal aliasing which occurs at both high degrees and
at high frequencies. We will demonstrate these different errors using
results obtained with the VIRGO and MDI experiments onboard the SOHO
spacecraft. We will also compare some of these space-based results
with the results of similar estimates obtained from co-temporaneous
ground-based observations, such as from the Mt. Wilson 60-Foot Solar
Tower. We will include the results from different structural inversions
carried out with different sets of input frequencies and uncertainties
in order to demonstrate the effects of these different systematic
errors upon the inverted internal sound speed profile.

---------------------------------------------------------
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: Frequencies of solar oscillations and the seismic structure
of the Sun from SOHO/MDI.
Authors: Rhodes, E. J., Jr.; Kosovichev, A. G.; Scherrer, P. H.;
Schou, J.; Reiter, J.
1997AGAb...13..163R 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: Solar structure as revealed by 1 year LOWL data
Authors: Basu, Sarbani; Christensen-Dalsgaard, J.; Schou, J.; Thompson,
M. J.; Tomczyk, S.
1996BASI...24..147B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Results from the LOWL instrument
Authors: Schou, J.; Tomczyk, S.; Thompson, M. J.
1996BASI...24..375S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Low-degree frequency splitting measurements and the rotation
rate of the solar core
Authors: Tomczyk, S.; Schou, J.; Thompson, M. J.
1996BASI...24..245T Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Measurement of the Rotation Rate in the Deep Solar Interior
Authors: Tomczyk, S.; Schou, J.; Thompson, M. J.
1996AAS...188.6903T Altcode: 1996BAAS...28..935T
We present a measurement of the rotation rate in the interior of the
Sun based on two years of observations with the LOWL instrument. LOWL
allows the observation of solar oscillations with degrees from 0
to 100, thus providing a homogeneous low- and intermediate-degree
dataset. Significantly, it is able to make spatially resolved
observations of low degree modes, thereby making it possible to separate
the different modes within a given multiplet. This reduces the potential
for systematic errors compared to observations using integrated
sunlight. We have used observations of the frequency splittings
of modes with degrees from 1 to 100 to infer the rotation rate in
the solar interior with some radial resolution and without excessive
errors. Over most of the interior we have also been able to estimate
the latitudinal variation of the rotation rate. We confirm earlier
findings that near the base of the convection zone the solar rotation
profile undergoes a transition from surface-like differential rotation
to a rotation rate that is independent of latitude. Additionally,
we find that below the base of the convection zone our measurement is
consistent with rigid body rotation at a rate somewhat lower than the
surface equatorial rate. This measurement provides strong constraints
on the theories of angular momentum transport in solar-type stars.

---------------------------------------------------------
Title: Differential Rotation and Dynamics of the Solar Interior
Authors: Thompson, M. J.; Toomre, J.; Anderson, E. R.; Antia, H. M.;
Berthomieu, G.; Burtonclay, D.; Chitre, S. M.; Christensen-Dalsgaard,
J.; Corbard, T.; De Rosa, M.; Genovese, C. R.; Gough, D. O.; Haber,
D. A.; Harvey, J. W.; Hill, F.; Howe, R.; Korzennik, S. G.; Kosovichev,
A. G.; Leibacher, J. W.; Pijpers, F. P.; Provost, J.; Rhodes, E. J.,
Jr.; Schou, J.; Sekii, T.; Stark, P. B.; Wilson, P. R.
1996Sci...272.1300T Altcode:
Splitting of the sun's global oscillation frequencies by large-scale
flows can be used to investigate how rotation varies with radius
and latitude within the solar interior. The nearly uninterrupted
observations by the Global Oscillation Network Group (GONG) yield
oscillation power spectra with high duty cycles and high signal-to-noise
ratios. Frequency splittings derived from GONG observations confirm
that the variation of rotation rate with latitude seen at the surface
carries through much of the convection zone, at the base of which is
an adjustment layer leading to latitudinally independent rotation at
greater depths. A distinctive shear layer just below the surface is
discernible at low to mid-latitudes.

---------------------------------------------------------
Title: The Shape of Solar Oscillation Eigenfunctions
Authors: Schou, J.
1996AAS...188.6906S Altcode: 1996BAAS...28..936S
Helioseismic images of the Sun have generally been processed
assuming that their observed surface behavior is given by perfect
spherical harmonics, although it is well known that this is only
an approximation. Among the neglected effects are the distortion of
the modes by differential rotation and the neglect of the transverse
component of the surface velocity in the reduction. In this poster I
will discuss these approximations and their possible effects on the
mode power spectra and the deduced frequencies. While some of the
effects are rather small others are easily visible in the observed
power spectra. I will show results of analyzing various datasets to
look for these effects. While it appears that some of the results are
as theoretically expected others are more difficult to understand. It
is the hope that we will be able to obtain more reliable and precise
mode parameters by taking these effects into account in the analysis of
data from projects such as SOI/MDI and GONG. This research is supported
by the SOI-MDI NASA contract NAG5-3077 at Stanford University.

---------------------------------------------------------
Title: GONG Data: Implications for the Sun's Interior and Near
Surface Magnetic Field
Authors: Goode, P. R.; Dziembowski, W. A.; Rhodes, E. J., Jr.; Tomczyk,
S.; Schou, J.; GONG Magnetic Effects Team
1996AAS...188.5307G Altcode: 1996BAAS...28..904G
The solar oscillation spectrum and the fine structure in it from the
first complete month of GONG data have been used to place a limit
on the Sun's internal magnetic field. The limit is consistent with
the magnetic pressure being no more than 1/1000 of the gas pressure
between the Sun's deep interior and its surface. This conclusion is
consistent with earlier results. The GONG data are from a time near
magnetic activity minimum. The effect of the near surface magnetic
field on the fine structure in the oscillation spectrum reflects
a perturbation of quadrupole toroidal symmetry. This geometry also
dominated at the last activity minimum. The meaning of this result is
discussed. The near surface magnetic perturbation is not spherically
symmetric. This corrupts the results of inversions designed to probe
the Sun's deep interior. The solution to this problem is presented.

---------------------------------------------------------
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: The Sun's Hydrostatic Structure from LOWL Data
Authors: Basu, Sarbani; Christensen-Dalsgaard, J.; Schou, J.; Thompson,
M. J.; Tomczyk, S.
1996ApJ...460.1064B Altcode:
Recent observations with the LOWL (Low-Degree [l] Oscillations
Experiment) instrument have for the first time provided a uniform set of
frequencies that allow detailed inversion for the structure of much of
the Sun's deep interior. We present the results of inverting the LOWL
data and compare them with the corresponding results obtained using
inhomogeneous data sets from more than one instrument. Furthermore,
improvements in the description of the required physics motivates the
calculation of new solar models. Thus, we present results of inversions
of the LOWL data against several reference models using up-to-date
physics. In models including the gravitational settling of helium, the
sound speed and density agree with the Sun to within substantially
better than 1%. We test various modifications to the physics of
the models in order to see if the remaining small (but significant)
discrepancy between the Sun and the models can be removed. We find
that none of the modifications tested can adequately account for the
remaining discrepancy, though a small increase in helium diffusion
in the core gives a modest improvement over the standard diffusion
model. Finally, we find that the seismic data support theoretical
calculations that indicate that settling of heavier elements has
reduced the present surface value of Z by about 8% relative to its
mean interior value.

---------------------------------------------------------
Title: Inversion for the Velocity Field in the Solar Interior
Authors: Schou, Jesper
1996imie.conf...48S Altcode:
No abstract at ADS

---------------------------------------------------------
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: Inversion for the Velocity Field in the Solar Interior
Authors: Schou, J.
1996LNES...63...48S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Solar Oscillations Investigation - Michelson Doppler Imager
Authors: Scherrer, P. H.; Bogart, R. S.; Bush, R. I.; Hoeksema, J. T.;
Kosovichev, A. G.; Schou, J.; Rosenberg, W.; Springer, L.; Tarbell,
T. D.; Title, A.; Wolfson, C. J.; Zayer, I.; MDI Engineering Team
1995SoPh..162..129S Altcode:
The Solar Oscillations Investigation (SOI) uses the Michelson Doppler
Imager (MDI) instrument to probe the interior of the Sun by measuring
the photospheric manifestations of solar oscillations. Characteristics
of the modes reveal the static and dynamic properties of the
convection zone and core. Knowledge of these properties will improve
our understanding of the solar cycle and of stellar evolution. Other
photospheric observations will contribute to our knowledge of the
solar magnetic field and surface motions. The investigation consists
of coordinated efforts by several teams pursuing specific scientific
objectives.

---------------------------------------------------------
Title: Results From the LOWL Instrument
Authors: Schou, J.; Tomczyk, S.; Thompson, M. J.
1995AAS...18710101S Altcode: 1995BAAS...27.1426S
In this poster we will present various results from one year of
observations with the LOWL instrument. The LOWL instrument is designed
to observe oscillations with degrees l from 0 to about 100 providing a
homogeneous dataset for inversions. Given the spatial resolution of the
instrument it is possible to separate different azimuthal orders m even
for the lowest degree modes, hopefully giving more accurate splittings
than those determined using integrated sunlight for a similar time
period. We will show the results of an inversion for the solar rotation
rate between 0.1R_sun and 0.95R_sun. Given the long duration and the
large number of terms used to describe the m-dependence of the mode
frequencies we have been able to obtain very good resolution in both
the radial direction and in latitude while keeping the statistical
errors low, thereby obtaining more accurate results than previously
possible. We will also present results of analyzing different parts of
the time-series and compare the temporal variations in mode frequencies
and splittings with variations in the solar activity.

---------------------------------------------------------
Title: Measurement of the Rotation Rate in the Deep Solar Interior
Authors: Tomczyk, S.; Schou, J.; Thompson, M. J.
1995ApJ...448L..57T Altcode:
We present a measurement of solar internal rotation based
on observations obtained over 3 months in early 1994 with a new
instrument called LOWL. This instrument allows for the simultaneous
observation of low- and intermediate-degree solar oscillations with
spatial resolution. We have measured the frequency splitting of 673
multiplets with degrees ranging from 1 to 80 and inverted these to
derive an estimate of the solar internal rotation profile between 0.2
and 0.85 R⊙. The accuracy of this measurement ranges from ~1% in the
outer regions to ~5% at 0.2 R⊙ and thus places better constraints than
hitherto on the rotation in the deep solar interior. We confirm earlier
findings that near the base of the convection zone the solar rotation
profile undergoes a transition from surface-like differential rotation
to a rotation rate that is independent of latitude. In addition, we
find that from the base of the convection zone down to 0.2 R⊙ our
measurement is consistent with rigid body rotation at a rate somewhat
lower than the surface equatorial rate. The accuracy of our measurement
in the deep solar interior provides a strong constraint to theories
of solar and stellar angular momentum transport.

---------------------------------------------------------
Title: The Sensitivity of Various Mode Sets for Probing Differential
Rotation Shear Zones
Authors: Christensen-Dalsgaard, J.; Schou, J.; Thompson, M. J.;
Toomre, J.
1995ESASP.376b..41C Altcode: 1995soho....2...41C; 1995help.confP..41C
The potential of forthcoming datasets from GONG and SOI for resolving
long-lived azimuthal jets and shearing flows is investigated. The
authors construct various artificial datasets, containing noise
resembling that for a one-year observing run. These are then inverted,
using a 2-D regularized least squares (RLS) inversion. In particular,
the authors investigate the ability of the RLS method to form
well-localized averages of the rotation rate, as measured by the
averaging kernels, using an extensive mode set as well as subsets
thereof. The authors show that it is possible to keep the noise in
the solution down to a few nanohertz in much of the solar interior,
while obtaining very reasonable resolution, for a GONG-like dataset.

---------------------------------------------------------
Title: Solar Structure Inversion with LOWL Data
Authors: Basu, S.; Christensen-Dalsgaard, J.; Schou, J.; Thompson,
M. J.; Tomczyk, S.
1995ESASP.376b..25B Altcode: 1995help.confP..25B; 1995soho....2...25B
Presents inversion results for the radial hydrostatic structure of
the Sun, using six months of oscillation data obtained with the LOWL
instrument. Using modes with degrees from 0 to 90 and frequencies
between 1.5 and 3.5 mHz, the authors have inferred the variation with
depth of the sound speed, density ρ and u = p/ρ (p being pressure)
from r = 0.05 Rsun to 0.85 Rsun. They find that
in this region the sound speed in the Sun is within 0.15% of that of a
model constructed using the MHD equation of state and OPAL opacities
and incorporating helium diffusion. The density difference between
Sun and model is less than 0.8%. Given the small error bars on the
inversion results these small differences are significant, however. The
solar sound speed appears to be higher than in the model for r &lt;
0.2 Rsun . The authors speculate that this might indicate
interesting physics in the inner core.

---------------------------------------------------------
Title: Constraining Solar Core Rotation with Genetic Forward Modelling
Authors: Tomczyk, S.; Charbonneau, P.; Schou, J.; Thompson, M. J.
1995ESASP.376b.271T Altcode: 1995soho....2..271T; 1995help.confP.271T
No abstract at ADS

---------------------------------------------------------
Title: Results from the LOWL Instrument
Authors: Schou, J.; Tomczyk, S.; Thompson, M. J.
1995SPD....26..402S Altcode: 1995BAAS...27..954S
No abstract at ADS

---------------------------------------------------------
Title: Michelson Doppler Imager (MDI) Performance Characteristics
Authors: Zayer, I.; Morrison, M.; Pope, T.; Rosenberg, W.; Tarbell,
T.; Title, A.; Wolfson, J.; Bogart, R. S.; Hoeksema, J. T.; Milford,
P.; Scherrer, P. H.; Schou, J.
1995ASPC...76..456Z Altcode: 1995gong.conf..456Z
No abstract at ADS

---------------------------------------------------------
Title: Optimally Localized Kernels for 2D Helioseismic Inversion
Authors: Christensen-Dalsgaard, J.; Larsen, R. M.; Schou, J.; Thompson,
M. J.
1995ASPC...76...70C Altcode: 1995gong.conf...70C
No abstract at ADS

---------------------------------------------------------
Title: Sputtering of water ice surfaces and the production of extended
neutral atmospheres
Authors: Shi, M.; Baragiola, R. A.; Grosjean, D. E.; Johnson, R. E.;
Jurac, S.; Schou, J.
1995JGR...10026387S Altcode:
Plasma and UV photon bombardment of an icy object in the outer
solar system can lead to ejection of atoms and molecules from the
surface which can, in turn, produce an extended neutral atmosphere. We
present new laboratory studies of the sputtering of water ice by keV
ions (H+ through Ne+) made using a sensitive
microbalance technique that allows measurements at very low ion
fluences. These results for the sputtering yield of ice by keV
O+ ions, the dominant sputtering agents in the Saturnian
magnetosphere, are much larger than those used previously to model the
neutral cloud associated with the icy satellites. The data presented
are used to recalculate previously published sputtering rates for
the icy satellites of Jupiter and Saturn, and for the E-ring grains
at Saturn. The new results can account, in part, for the discrepancy
between the predicted and observed OH cloud near Tethys in Saturn's
inner magnetosphere. We compare the yields induced by the incident
ions to the recently measured UV photosputtering yield, and discuss
possible synergism between UV photon and plasma ion induced erosion.

---------------------------------------------------------
Title: Some Aspects of Helioseismic Time-Series Analysis
Authors: Schou, J.; Christensen-Dalsgaard, J.; Thompson, M. J.
1995ASPC...76..528S Altcode: 1995gong.conf..528S
No abstract at ADS

---------------------------------------------------------
Title: Status of the Solar Oscillations Investigation - Michelson
Doppler Imager
Authors: Scherrer, P. H.; Bogart, R. S.; Bush, R. I.; Hoeksema,
J. T.; Milford, P.; Schou, J.; Pope, T.; Rosenberg, W.; Springer,
L.; Tarbell, T.; Title, A.; Wolfson, J.; Zayer, I.
1995ASPC...76..402S Altcode: 1995gong.conf..402S
No abstract at ADS

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Title: Preliminary Results from Observations with the LOWL Instrument
Authors: Schou, J.; Tomczyk, S.
1995ASPC...76..448S Altcode: 1995gong.conf..448S
No abstract at ADS

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Title: Hunting for Azimuthal Jets and Shearing Flows in the Solar
Convection Zone
Authors: Christensen-Dalsgaard, J.; Schou, J.; Thompson, M. J.;
Toomre, J.
1995ASPC...76..212C Altcode: 1995gong.conf..212C
No abstract at ADS

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Title: A Measurement of the Rotation Rate in the Deep Solar Interior
Authors: Schou, J.; Tomczyk, S.; Thompson, M. J.
1994AAS...185.4401S Altcode: 1994BAAS...26Q1377S
Measurements of the rotation rate in the deep solar interior using
helioseismology have given inconsistent results, presumably due to
problems with the algorithms used for the analysis of spatially
unresolved observations of the oscillations. Here we present a
measurement of the rotation rate in the interior of the Sun based
on observations with a new instrument called LOWL. Unlike earlier
instruments this instrument allows the observation of oscillations
with degrees from 0 to ~ 80. In particular it is able to make spatially
resolved observations of low degree modes, thereby making it possible to
spatially separate the different modes within a given multiplet. This
reduces the potential for systematic errors compared to observations
using integrated sunlight. We have used observations of the frequency
splittings of modes with degrees from 1 to 80 to infer the rotation rate
in the solar interior down to ~ 0.2 R_⊙ with some radial resolution
and without excessive errors. Over part of the range we have also been
able to estimate the latitudinal variation of the rotation rate. This
measurement provides strong constraints on the theories of solar and
stellar angular momentum transport.

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Title: Generation of artificial helioseismic time-series.
Authors: Schou, J.; Brown, T. M.
1994A&AS..107..541S Altcode:
We present an outline of an algorithm to generate artificial
helioseismic time-series, taking into account as much as possible of
the knowledge we have on solar oscillations. The hope is that it will
be possible to find the causes of some of the systematic errors in
analysis algorithms by testing them with such artificial time-series.

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Title: On the Rotation Rate in the Solar Convection Zone
Authors: Schou, Jesper; Brown, Timothy M.
1994ApJ...434..378S Altcode:
Recently Gough et al. (1993) have argued that the rotation rate in parts
of the solar convection zone may be constant on cylinders as predicted
by models of the convection zone, contrary to the inferences generally
made from helioseismology. Here we consider models similar to those
suggested by Gough et al. and show that they are either inconsistent
with observations made by Fourier Tachometer or require unphysical
rotation rates in other parts of the Sun. These observations use a more
detailed model of the effects of the solar rotation on the observed
frequencies than that used in reducing previous observations. We
also show the results of an inversion of the Fourier Tachometer
observations and compare it with an inversion of data similar to that
used previously. The result of this inversion generally confirms the
conclusions from previous inversions.

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Title: On Comparing Helioseismic Two-dimensional Inversion Methods
Authors: Schou, J.; Christensen-Dalsgaard, J.; Thompson, M. J.
1994ApJ...433..389S Altcode:
We consider inversion techniques for investigating the structure and
dynamics of the solar interior as functions of radius and latitude. In
particular, we look at the problem of inferring the radial and
latitudinal dependence of the Sun's internal rotation, using a fully
two-dimensional least-squares inversion algorithm. Concepts such as
averaging kernels, measures of resolution, and trade-off curves, which
have previously been used in the one-dimensional case, are generalized
to facilitate a comparison of two-dimensional methods. We investigate
the weighting given to different modes and discuss the implications of
this for observational strategies. As an illustration we use a mode
set whose properties are similar to those expected for data from the
GONG network.

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Title: Generation of artificial helioseismic time-series
Authors: Schou, J.; Brown, T. M.
1993STIN...9415968S Altcode:
We present an outline of an algorithm to generate artificial
helioseismic time-series, taking into account as much as possible of
the knowledge we have on solar oscillations. The hope is that it will
be possible to find the causes of some of the systematic errors in
analysis algorithms by testing them with such artificial time-series.

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Title: Observations of Intermediate Degree Solar Oscillations:
1989 April--June
Authors: Bachmann, Kurt T.; Schou, Jesper; Brown, Timothy M.
1993ApJ...412..870B Altcode:
Frequencies, splittings, and line widths from 85 d of full disk
Doppler observations of solar p-modes taken between April 4 and June
30, 1989 are presented. Comparison of the present mode parameters
with published Big Bear Solar Observatory (BBSO) results yields good
agreement in general and is thus a confirmation of their work using an
independent instrument and set of analysis routines. Average differences
in p-mode frequencies measured by the two experiments in spring-summer
1989 are explained as a result of differences in the exact periods of
data collection during a time of rapidly changing solar activity. It
is shown that the present a(1) splitting coefficients for p-modes
with nu/L less than 45 micro-Hz suffer from a significant systematic
error. Evidence is presented to the effect that a detector distortion
or alignment problem, not a problem with the power spectra analysis,
is the most likely explanation of this a(1) anomaly.

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Title: Requirements for the Observation of Low-Degree Solar
Oscillations
Authors: Veitzer, S. A.; Tomczyk, S.; Schou, J.
1993ASPC...42..465V Altcode: 1993gong.conf..465V
No abstract at ADS

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Title: Observations of Intermediate Degree Solar Oscillations -
1989APR-JUN
Authors: Bachmann, K. T.; Schou, J.; Brown, T. M.
1993ASPC...42..197B Altcode: 1993gong.conf..197B
No abstract at ADS

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Title: Two-dimensional helioseismic inversions
Authors: Schou, J.; Christensen-Dalsgaard, J.; Thompson, M. J.
1993ASPC...40...72S Altcode: 1993IAUCo.137...72S; 1993ist..proc...72S
No abstract at ADS

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Title: On the analysis of helioseismic data
Authors: Schou, Jesper
1993PhDT.......436S Altcode:
No abstract at ADS

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Title: Preliminary results from observations with the Fourier
tachometer.
Authors: Schou, J.; Brown, T. M.; Bachman, K. T.
1993ASPC...40...90S Altcode: 1993IAUCo.137...90S; 1993ist..proc...90S
The authors present preliminary results from an analysis of two ≍3
month observation runs with the Fourier Tachometer (which was operated
by HAO and NSO) from 1987 and 1989. The analysis was done with two
different methods in order to test for systematic errors. It is shown
that the mode frequencies change in a manner similar to that reported
by Libbrecht and Woodard. The authors also present results for the
frequency splittings caused by the solar rotation.

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Title: Two-Dimensional Helioseismic Inversions
Authors: Schou, J.; Christensen-Dalsgaard, J.; Thompson, M. J.
1993ASPC...42..205S Altcode: 1993gong.conf..205S
No abstract at ADS

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Title: The Resolving Power of Current Helioseismic Inversions for
the Sun's Internal Rotation
Authors: Schou, J.; Christensen-Dalsgaard, J.; Thompson, M. J.
1992ApJ...385L..59S Altcode:
An inversion of the currently available rotational splitting data
which is representative of other recent inversions for the sun's
internal rotation is analyzed. The so-called polar rate is found to be
an extrapolation from lower latitudes. The basic conclusion that the
average radial gradient of the rotation rate in the solar convection
zone is small is corroborated.

---------------------------------------------------------
Title: On the Analysis of Helioseismic Data
Authors: Schou, J.
1992PhDT.......380S Altcode:
No abstract at ADS

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Title: On the Analysis of Helioseismic Time-Series
Authors: Schou, J.; Brown, T. M.
1991BAAS...23.1390S Altcode:
No abstract at ADS

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Title: On the 2-Dimensional Rotational Inversion Problem
Authors: Schou, Jesper
1991LNP...388...93S Altcode: 1991ctsm.conf...93S
Inspired by the GONG hound and hare exercise, where 2-dimensional
inversions are being performed, I have started writing a set of programs
to do both the forward and inverse problem, in both the 1-dimensional
case (with possible expansion in latitude and the full 2-dimensional
case. Some preliminary results of running these programs are presented
here, together with an outline of the whole package.

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Title: An Inversion for the Rotation Rate in the Solar Interior
Authors: Schou, Jesper
1991LNP...388...81S Altcode: 1991ctsm.conf...81S
The results of an inversion of a set of preliminary rotational
splittings from 1986 and 1988 from Big Bear Solar Observatory are
presented. The splittings and the results of the inversions for the
1986 case are essentially identical to those in Christensen-Dalsgaard
and Schou (1986, and there appear to be no significant differences
between the two years. The results from both years seem to confirm
the earlier result that the rotation rate is surface-like and constant
on radii in the part of the convection zone that is resolved by this
mode set and independent of both radius and latitude in the radiative
interior. Since the observations used here only span degrees from 5 -
60 it has not been possible to perform an inversion for the rotation
rate inside approximately 0.4R and in the outer part of the convection
zone, and it is therefore not possible to see whether there is a
bump in the rotation rate near the surface, as indicated by Hill et
al. (1988). There are, however, indications that the rotation rate is
not constant throughout the convection zone.

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Title: A comparison of methods for inverting helioseismic data
Authors: Christensen-Dalsgaard, J.; Schou, J.; Thompson, M. J.
1990MNRAS.242..353C Altcode:
Consideration is given to the inversion techniques used to study the
properties of the solar interior with observed frequencies of solar
oscillation. Linear inversion schemes for helioseismic problems are
compared in terms of resolutions and error properties. Measures of the
error amplification resulting from the inversion and the resolution
achieved are defined. Although the study includes only inversions for
a spherically symmetric angular velocity distribution, it is suggested
that the methods may be used to study kernels for other properties of
the solar interior.

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Title: Differential rotation in the solar interior.
Authors: Christensen-Dalsgaard, J.; Schou, J.
1988ESASP.286..149C Altcode: 1988ssls.rept..149C
The authors present an estimate of the angular velocity in the
outer half of the Sun, as a function of depth and latitude. This
was obtained by applying inversion by means of optimally localized
averages to rotational splitting coefficients observed by Kent
Libbrecht at BBSO. The results seem to indicate that the surface
differential rotation persists through the convection zone; beneath
the convection zone there appears to be a transition, which occurs
within our resolution width of about 0.1 R, to solid-body rotation. The
authors were unable to perform the inversion in the core, due to the
lack of adequate data for modes of low degree.