Author name code: sekii
ADS astronomy entries on 2022-09-14
author:"Sekii, Takashi"
------------------------------------------------------------------------
Title: Bayesian Rotation Inversion of KIC 11145123
Authors: Hatta, Yoshiki; Sekii, Takashi; Benomar, Othman; Takata, Masao
Bibcode: 2022ApJ...927...40H
Altcode: 2021arXiv211212341H
A scheme of Bayesian rotation inversion, which allows us to compute
the probability of a model of a stellar rotational profile, is
developed. The validation of the scheme with simple rotational profiles
and the corresponding sets of artificially generated rotational shifts
has been successfully carried out, and we can correctly distinguish
the (right) rotational model, prepared beforehand for generating
the artificial rotational shifts, from the other (wrong) rotational
model. The Bayesian scheme is applied to a γ Dor-δ Sct-type hybrid
star, KIC 11145123, leading to a result that the convective core
of the star might be rotating much faster (~10 times faster) than
the other regions of the star. The result is consistent with that
previously suggested by Hatta et al. based on a three-zone modeling,
further strengthening their argument from a Bayesian point of view.
Title: Nonstandard Modeling of a Possible Blue Straggler Star,
KIC 11145123
Authors: Hatta, Yoshiki; Sekii, Takashi; Takata, Masao; Benomar, Othman
Bibcode: 2021ApJ...923..244H
Altcode: 2021arXiv211006926H
Nonstandard modeling of KIC 11145123, a possible blue straggler star,
has been asteroseismically carried out based on a scheme to compute
stellar models with the chemical compositions in their envelopes
arbitrarily modified, mimicking the effects of some interactions with
other stars through which blue straggler stars are thought to be
born. We have constructed a nonstandard model of the star with the
following parameters: M = 1.36 M ⊙, Y init
= 0.26, Z init = 0.002, and f ovs = 0.027,
where f ovs is the extent of overshooting described as an
exponentially decaying diffusive process. The modification is down
to the depth of r/R ~ 0.6 and the extent ΔX, which is a difference
in surface hydrogen abundance between the envelope-modified and
unmodified models, is 0.06. The residuals between the model and the
observed frequencies are comparable with those for the previous model
computed assuming standard single-star evolution, suggesting that it
is possible that the star was born with a relatively ordinary initial
helium abundance of ~0.26 compared with that of the previous models
(~0.30-0.40), then experienced some modification of the chemical
compositions and gained helium in the envelope. Detailed analyses
of the nonstandard model have implied that the elemental diffusion
in the deep radiative region of the star might be much weaker than
that assumed in current stellar evolutionary calculations; we need
some extra mechanisms inside the star, rendering the star a much more
intriguing target to be further investigated.
Title: A journey of exploration to the polar regions of a star:
probing the solar poles and the heliosphere from high helio-latitude
Authors: Harra, Louise; Andretta, Vincenzo; Appourchaux, Thierry;
Baudin, Frédéric; Bellot-Rubio, Luis; Birch, Aaron C.; Boumier,
Patrick; Cameron, Robert H.; Carlsson, Matts; Corbard, Thierry;
Davies, Jackie; Fazakerley, Andrew; Fineschi, Silvano; Finsterle,
Wolfgang; Gizon, Laurent; Harrison, Richard; Hassler, Donald M.;
Leibacher, John; Liewer, Paulett; Macdonald, Malcolm; Maksimovic,
Milan; Murphy, Neil; Naletto, Giampiero; Nigro, Giuseppina; Owen,
Christopher; Martínez-Pillet, Valentín; Rochus, Pierre; Romoli,
Marco; Sekii, Takashi; Spadaro, Daniele; Veronig, Astrid; Schmutz, W.
Bibcode: 2021ExA...tmp...93H
Altcode: 2021arXiv210410876H
A mission to view the solar poles from high helio-latitudes (above 60°)
will build on the experience of Solar Orbiter as well as a long heritage
of successful solar missions and instrumentation (e.g. SOHO Domingo et
al. (Solar Phys. 162(1-2), 1-37 1995), STEREO Howard et al. (Space
Sci. Rev. 136(1-4), 67-115 2008), Hinode Kosugi et al. (Solar
Phys. 243(1), 3-17 2007), Pesnell et al. Solar Phys. 275(1-2),
3-15 2012), but will focus for the first time on the solar poles,
enabling scientific investigations that cannot be done by any other
mission. One of the major mysteries of the Sun is the solar cycle. The
activity cycle of the Sun drives the structure and behaviour of the
heliosphere and of course, the driver of space weather. In addition,
solar activity and variability provides fluctuating input into the
Earth climate models, and these same physical processes are applicable
to stellar systems hosting exoplanets. One of the main obstructions
to understanding the solar cycle, and hence all solar activity,
is our current lack of understanding of the polar regions. In this
White Paper, submitted to the European Space Agency in response to the
Voyage 2050 call, we describe a mission concept that aims to address
this fundamental issue. In parallel, we recognise that viewing the Sun
from above the polar regions enables further scientific advantages,
beyond those related to the solar cycle, such as unique and powerful
studies of coronal mass ejection processes, from a global perspective,
and studies of coronal structure and activity in polar regions. Not
only will these provide important scientific advances for fundamental
stellar physics research, they will feed into our understanding of
impacts on the Earth and other planets' space environment.
Title: A Journey of Exploration to the Polar Regions of a Star:
Probing the Solar Poles and the Heliosphere from High Helio-Latitude
Authors: Finsterle, W.; Harra, L.; Andretta, V.; Appourchaux, T.;
Baudin, F.; Bellot Rubio, L.; Birch, A.; Boumier, P.; Cameron, R. H.;
Carlsson, M.; Corbard, T.; Davies, J. A.; Fazakerley, A. N.; Fineschi,
S.; Gizon, L. C.; Harrison, R. A.; Hassler, D.; Leibacher, J. W.;
Liewer, P. C.; Macdonald, M.; Maksimovic, M.; Murphy, N.; Naletto, G.;
Nigro, G.; Owen, C. J.; Martinez-Pillet, V.; Rochus, P. L.; Romoli,
M.; Sekii, T.; Spadaro, D.; Veronig, A.
Bibcode: 2020AGUFMSH0110005F
Altcode:
A mission to view the solar poles from high helio-latitudes (above
60°) will build on the experience of Solar Orbiter as well as a long
heritage of successful solar missions and instrumentation (e.g. SOHO,
STEREO, Hinode, SDO), but will focus for the first time on the solar
poles, enabling scientific investigations that cannot be done by
any other mission. One of the major mysteries of the Sun is the solar
cycle. The activity cycle of the Sun drives the structure and behaviour
of the heliosphere and is, of course, the driver of space weather. In
addition, solar activity and variability provides fluctuating input
into the Earth climate models, and these same physical processes
are applicable to stellar systems hosting exoplanets. One of the
main obstructions to understanding the solar cycle, and hence all
solar activity, is our current lack of understanding of the polar
regions. We describe a mission concept that aims to address this
fundamental issue. In parallel, we recognise that viewing the Sun
from above the polar regions enables further scientific advantages,
beyond those related to the solar cycle, such as unique and powerful
studies of coronal mass ejection processes, from a global perspective,
and studies of coronal structure and activity in polar regions. Not
only will these provide important scientific advances for fundamental
stellar physics research, they will feed into our understanding of
impacts on the Earth and other planets' space environment.
Title: Asteroseismic Study of KIC 11145123: Its Structure and Rotation
Authors: Hatta, Yoshiki; Sekii, Takashi; Takata, Masao; Kurtz,
Donald W.
Bibcode: 2020ASSP...57..243H
Altcode:
KIC 11145123 is one of the Kepler targets that has been actively
studied asteroseismically. Its well-resolved frequency splittings
for p, g, and mixed modes have enabled us to infer the 1-dimensional
surface-to-core rotation of the star (Kurtz et al., Mon Not R Astron
Soc 444:102, 2014) and to measure the asphericity of the star sensed
by the modes (Gizon et al., Sci Adv 2:e1601777, 2016). These have been
the first such attempts for main-sequence stars other than the Sun. In
this paper, we report further detailed asteroseismic analyses of KIC
11145123, focusing on inferring 2-dimensional rotation profile and on
fine-tuning the equilibrium model of the star. Our main results are:
(1) the convective core might be rotating six times faster than
the other parts of the star, and (2) adopting diffusion "weaker"
than the usual settings used in the standard 1-d stellar evolution
calculations leads to a better agreement with the observed g-mode
period spacing pattern. These two results suggest that the rotational
velocity shear might be causing some extra mixing around the boundary
between convective core and radiative region above, and the extra
mixing weakens, in effect, the diffusion process there. This resembles
a mechanism thought to be at work around the solar tachocline.
Title: The Two-dimensional Internal Rotation of KIC 11145123
Authors: Hatta, Yoshiki; Sekii, Takashi; Takata, Masao; Kurtz,
Donald W.
Bibcode: 2019ApJ...871..135H
Altcode: 2021arXiv211106853H
The 2D internal rotation of KIC 11145123 has been inferred via
asteroseismology. Based on the optimally localized averaging method and
a simple three-zone modeling of the internal rotation, we have found
evidence for a contrast between the internal rotation of the radiative
region and that of the convective core; the radiative region rotates
almost uniformly throughout the region, but the convective core may be
rotating about 6 times faster than the radiative region above. We have
also found marginally significant evidence of latitudinal differential
rotation in the outer envelope. These newly indicated features of the
internal rotation of the star can help us further constrain the theory
of angular momentum transport inside stars, as well as understand the
complex physical properties of the star, which was once thought to
be a main-sequence A-type star but recently has been proposed to be
a blue straggler, based on spectroscopy.
Title: Asteroseismic detection of latitudinal differential rotation
in 13 Sun-like stars
Authors: Benomar, O.; Bazot, M.; Nielsen, M. B.; Gizon, L.; Sekii,
T.; Takata, M.; Hotta, H.; Hanasoge, S.; Sreenivasan, K. R.;
Christensen-Dalsgaard, J.
Bibcode: 2018Sci...361.1231B
Altcode: 2018arXiv180907938B
The differentially rotating outer layers of stars are thought to
play a role in driving their magnetic activity, but the underlying
mechanisms that generate and sustain differential rotation are
poorly understood. We report the measurement using asteroseismology
of latitudinal differential rotation in the convection zones of 40
Sun-like stars. For the most significant detections, the stars’
equators rotate approximately twice as fast as their midlatitudes. The
latitudinal shear inferred from asteroseismology is much larger than
predictions from numerical simulations.
Title: Can high angular degree non-radial pulsations be observed in
roAp stars?
Authors: Mathys, Gautier; Shibahashi, Hiromoto; Quintero Noda, Carlos;
Sekii, Takashi
Bibcode: 2018phos.confE..43M
Altcode:
In the presence of a magnetic field, stellar spectral lines may
appear systematically broader in one circular polarisation than in
the opposite one. This rotational crossover effect, which is observed
in some Ap stars, results from a correlation between the rotational
Doppler shift and the different Zeeman shifts of the circularly
polarised components. Crossover of non-rotational origin has been
detected in a number of roAp stars as well as in some noAp stars. The
most plausible interpretation is that it is induced by the pulsational
velocity gradients across the photospheric layer. Pulsational crossover
is expected to be detectable even in the case of high angular degree
pulsation modes, contrary to luminosity variations. Thus, it may open
a new window into unexplored physics in roAp stars.
Title: Spectroscopic and asteroseismic analysis of the remarkable
main-sequence A star KIC 11145123
Authors: Takada-Hidai, Masahide; Kurtz, Donald W.; Shibahashi,
Hiromoto; Murphy, Simon J.; Takata, Masao; Saio, Hideyuki; Sekii,
Takashi
Bibcode: 2017MNRAS.470.4908T
Altcode: 2017arXiv170604314T
A spectroscopic analysis was carried out to clarify the properties of
KIC 11145123 - the first main-sequence star with a directly measured
core-to-surface rotation profile - based on spectra observed with
the High Dispersion Spectrograph (HDS) of the Subaru telescope. The
atmospheric parameters (Teff = 7600 K, log g = 4.2, ξ =
3.1 km s-1 and [Fe/H] = -0.71 dex), the radial and rotation
velocities, and elemental abundances were obtained by analysing line
strengths and fitting line profiles, which were calculated with a 1D
LTE model atmosphere. The main properties of KIC 11145123 are: (1) a low
[Fe/H] = -0.71 ± 0.11 dex and a high radial velocity of -135.4 ± 0.2
km s-1. These are remarkable among late-A stars. Our best
asteroseismic models with this low [Fe/H] have slightly high helium
abundance and low masses of 1.4 M⊙. All of these results
strongly suggest that KIC 11145123 is a Population II blue straggler;
(2) the projected rotation velocity confirms the asteroseismically
predicted slow rotation of the star; (3) comparisons of abundance
patterns between KIC 11145123 and Am, Ap, and blue stragglers show that
KIC 11145123 is neither an Am star nor an Ap star, but has abundances
consistent with a blue straggler. We conclude that the remarkably long
100-d rotation period of this star is a consequence of it being a blue
straggler, but both pathways for the formation of blue stragglers -
merger and mass loss in a binary system - pose difficulties for our
understanding of the exceedingly slow rotation. In particular, we show
that there is no evidence of any secondary companion star, and we put
stringent limits on the possible mass of any such purported companion
through the phase modulation technique.
Title: The double-ridge structure of the high-frequency time-distance
crosscorrelation function in local helioseismology
Authors: Kambara, Nagaaki; Sekii, Takashi
Bibcode: 2017SPD....4810902K
Altcode:
We model helioseismic high-frequency cross-correlation function and
carry out comparison with observational data.We also discuss the
source depth of the acoustic waves, one of the model parameters.It
has been reported that when time-distance analysis is applied to the
high-frequency acoustic waves, with frequencies above the critical
cutoff frequency, time-distance cross-correlation function exhibits
double-ridge structure. It has been pointed out, however, that in such
analyses subcritical components (frequency < 5.3 MHz) may not be
completely filtered out, and a hypothesis is that the double ridges
are generated as artificial interference patterns of the subcritical
waves and the supercritical waves. We test this hypothesis using
SDO/HMI data.The data are put through a frequency filter before the
cross-correlation function is computed. We vary the width and central
frequency of the filter and examine when double ridges appear. When both
the supercritical and the subcritical components are present in the
filtered power spectrum, double ridges appear. When there is only one
of the components, however, double ridges do not appear, confirming
that interference between the two components is necessary for the
double ridges.Next, we construct a simple model of cross-correlation
function by ray-tracing the waves generated at a certain depth. The
model reproduces the double-ridge structure well, indeed by interference
between the supercritical part and the subcritical part, each of which
by itself exhibits only a single ridge. We find that the successful
reproduction of the observations depends sharply on the source depth
of the acoustic wave, one of the input parameters to the model.This
indicates a possibility that we can measure the source depth of the
acoustic waves precisely, using the double ridges.
Title: SOLARIS: Solar Sail Investigation of the Sun
Authors: Appourchaux, Thierry; Auchère, Frédéric; Antonucci, Ester;
Gizon, Laurent; MacDonald, Malcolm; Hara, Hirohisa; Sekii, Takashi;
Moses, Daniel; Vourlidas, Angelos
Bibcode: 2017arXiv170708193A
Altcode:
In this paper, we detail the scientific objectives and outline
a strawman payload of the SOLAR sail Investigation of the Sun
(SOLARIS). The science objectives are to study the 3D structure
of the solar magnetic and velocity field, the variation of total
solar irradiance with latitude, and the structure of the corona. We
show how we can meet these science objective using solar-sail
technologies currently under development. We provide a tentative
mission profile considering several trade-off approaches. We also
provide a tentative mass budget breakdown and a perspective for a
programmatic implementation.
Title: Helioseismology and Dynamics of the Solar Interior
Authors: Thompson, M. J.; Brun, A. S.; Culhane, J. L.; Gizon, L.;
Roth, M.; Sekii, T.
Bibcode: 2017hdsi.book.....T
Altcode:
No abstract at ADS
Title: Future Mission Concepts for Helioseismology
Authors: Sekii, Takashi; Appourchaux, Thierry; Fleck, Bernhard;
Turck-Chièze, Sylvaine
Bibcode: 2017hdsi.book..291S
Altcode:
No abstract at ADS
Title: Preface: Helioseismology and Dynamics of the Solar Interior
Authors: Gizon, Laurent; Thompson, Michael J.; Brun, A. Sacha; Culhane,
J. Len; Roth, Markus; Sekii, Takashi
Bibcode: 2017hdsi.book....1G
Altcode:
No abstract at ADS
Title: Shape of a slowly rotating star measured by asteroseismology
Authors: Gizon, L.; Sekii, T.; Takata, M.; Kurtz, D. W.; Shibahashi,
H.; Bazot, M.; Benomar, O.; Birch, A. C.; Sreenivasan, K. R.
Bibcode: 2016SciA....2E1777G
Altcode: 2016arXiv161106435G
Stars are not perfectly spherically symmetric. They are deformed by
rotation and magnetic fields. Until now, the study of stellar shapes
has only been possible with optical interferometry for a few of the
fastest-rotating nearby stars. We report an asteroseismic measurement,
with much better precision than interferometry, of the asphericity of
an A-type star with a rotation period of 100 days. Using the fact that
different modes of oscillation probe different stellar latitudes, we
infer a tiny but significant flattening of the star's shape of $\Delta
R/R = (1.8 \pm 0.6) \times 10^{-6}$. For a stellar radius $R$ that is
$2.24$ times the solar radius, the difference in radius between the
equator and the poles is $\Delta R = 3 \pm 1$ km. Because the observed
$\Delta R/R$ is only one-third of the expected rotational oblateness,
we conjecture the presence of a weak magnetic field on a star that
does not have an extended convective envelope. This calls to question
the origin of the magnetic field.
Title: Statistics of the two-point cross-covariance function of
solar oscillations
Authors: Nagashima, Kaori; Sekii, Takashi; Gizon, Laurent; Birch,
Aaron C.
Bibcode: 2016A&A...593A..41N
Altcode: 2016arXiv160606497N
Context. The cross-covariance of solar oscillations observed at
pairs of points on the solar surface is a fundamental ingredient in
time-distance helioseismology. Wave travel times are extracted from
the cross-covariance function and are used to infer the physical
conditions in the solar interior.
Aims: Understanding the
statistics of the two-point cross-covariance function is a necessary
step towards optimizing the measurement of travel times.
Methods:
By modeling stochastic solar oscillations, we evaluate the variance
of the cross-covariance function as function of time-lag and distance
between the two points.
Results: We show that the variance of
the cross-covariance is independent of both time-lag and distance in
the far field, that is, when they are large compared to the coherence
scales of the solar oscillations.
Conclusions: The constant
noise level for the cross-covariance means that the signal-to-noise
ratio for the cross-covariance is proportional to the amplitude of
the expectation value of the cross-covariance. This observation is
important for planning data analysis efforts.
Title: Future Mission Concepts for Helioseismology
Authors: Sekii, Takashi; Appourchaux, Thierry; Fleck, Bernhard;
Turck-Chièze, Sylvaine
Bibcode: 2015SSRv..196..285S
Altcode: 2015SSRv..tmp...15S
Future space-mission concepts currently discussed in the helioseismology
community are reviewed. One popular idea is to observe the Sun from high
latitudes, to explore the polar regions as well as to probe the deep
interior using stereoscopic techniques, by combining observations from
high latitudes with observations from within the ecliptic plane. Another
idea is to stay within the ecliptic plane but still aim for stereoscopic
helioseismology for deep layers. A new instrument and a novel mission
concept for studying the solar core regions are also discussed.
Title: Asteroseismic measurement of surface-to-core rotation in a
main-sequence star
Authors: Kurtz, Donald W.; Saio, Hideyuki; Takata, Masao; Shibahashi,
Hiromoto; Murphy, Simon J.; Sekii, Takashi
Bibcode: 2015EPJWC.10101007K
Altcode:
We have discovered rotationally split core g-mode triplets and surface
p-mode triplets and quintuplets in a terminal age main-sequence A
star, KIC 11145123, that shows both δ Sct p-mode pulsations and γ
Dor g-mode pulsations. This gives the first robust determination of
the rotation of the deep core and surface of a main-sequence star,
essentially model-independently. We find its rotation to be nearly
uniform with a period near 100 d, but we show with high confidence that
the surface rotates slightly faster than the core. A strong angular
momentum transfer mechanism must be operating to produce the nearly
rigid rotation, and a mechanism other than viscosity must be operating
to produce a more rapidly rotating surface than core. Our asteroseismic
result, along with previous asteroseismic constraints on internal
rotation in some B stars, and measurements of internal rotation in
some subgiant, giant and white dwarf stars, has made angular momentum
transport in stars throughout their lifetimes an observational science.
Title: Recent discoveries of structures and physical processes in
local helioseismology
Authors: Sekii, T.; Shibahashi, H.
Bibcode: 2015exse.book..180S
Altcode:
No abstract at ADS
Title: Asteroseismic measurement of slow, nearly uniform
surface-to-core rotation in the main-sequence F star KIC 9244992
Authors: Saio, Hideyuki; Kurtz, Donald W.; Takata, Masao; Shibahashi,
Hiromoto; Murphy, Simon J.; Sekii, Takashi; Bedding, Timothy R.
Bibcode: 2015MNRAS.447.3264S
Altcode: 2014arXiv1412.5362S
We have found a rotationally split series of core g-mode triplets
and surface p-mode multiplets in a main-sequence F star, KIC
9244992. Comparison with models shows that the star has a mass of about
1.45 M⊙, and is at an advanced stage of main-sequence
evolution in which the central hydrogen abundance mass fraction is
reduced to about 0.1. This is the second case, following KIC 11145123,
of an asteroseismic determination of the rotation of the deep core
and surface of an A-F main-sequence star. We have found, essentially
model independently, that the rotation near the surface, obtained
from p-mode splittings, is 66 d, slightly slower than the rotation
of 64 d in the core, measured by g-mode splittings. KIC 9244992 is
similar to KIC 11145123 in that both are near the end of main-sequence
stage with very slow and nearly uniform rotation. This indicates the
angular momentum transport in the interior of an A-F star during the
main-sequence stage is much stronger than that expected from standard
theoretical formulations.
Title: Solar rotation inferred from radial velocities of the
Sun-as-a-star during the 2012 May 21 eclipse*
Authors: Takeda, Yoichi; Ohshima, Osamu; Kambe, Eiji; Toda, Hiroyuki;
Koyano, Hisashi; Sato, Bun'ei; Nakamura, Yasuhisa; Narita, Norio;
Sekii, Takashi
Bibcode: 2015PASJ...67...10T
Altcode: 2014arXiv1411.1481T; 2015PASJ..tmp..147T
With the aim of examining how much information of solar rotation can be
obtained purely spectroscopically by observing the Sun-as-a-star during
the 2012 May 21 eclipse at Okayama Astrophysical Observatory, we studied
the variation of radial velocities (Vr), which were derived
by using the iodine-cell technique based on a set of 184 high-dispersion
spectra consecutively obtained over a time span of ∼ 4 hr. The
resulting Vr(t) was confirmed to show the characteristic
variation (Rossiter-McLaughlin effect) caused by time-varying visibility
of the solar disk. By comparing the observed Vr(t) curve
with the theoretical ones [which were simulated with the latitude (ψ)
dependent solar rotation law ωsidereal(ψ) = A + B sin
2ψ (degree d-1)] we found that the relation B
≃ -5.5A + 77 gives the best fit, though separate determinations of A
and B were not possible. Since this relationship is consistent with the
real values known for the Sun (A ≃ 14.5, B ≃ -2.8), we may state
that our analysis yielded satisfactory results. This consequence may
provide the prospect of getting useful information on stellar rotation
of eclipsing binaries from radial-velocity studies during eclipse,
if many spectra of sufficiently high time-resolution are available.
Title: Asteroseismic measurement of surface-to-core rotation in a
main-sequence A star, KIC 11145123
Authors: Kurtz, Donald W.; Saio, Hideyuki; Takata, Masao; Shibahashi,
Hiromoto; Murphy, Simon J.; Sekii, Takashi
Bibcode: 2014MNRAS.444..102K
Altcode: 2014arXiv1405.0155K
We have discovered rotationally split core g-mode triplets and surface
p-mode triplets and quintuplets in a terminal age main-sequence A star,
KIC 11145123, that shows both δ Sct p-mode pulsations and γ Dor g-mode
pulsations. This gives the first robust determination of the rotation
of the deep core and surface of a main-sequence star, essentially model
independently. We find its rotation to be nearly uniform with a period
near 100 d, but we show with high confidence that the surface rotates
slightly faster than the core. A strong angular momentum transfer
mechanism must be operating to produce the nearly rigid rotation,
and a mechanism other than viscosity must be operating to produce a
more rapidly rotating surface than core. Our asteroseismic result,
along with previous asteroseismic constraints on internal rotation in
some B stars, and measurements of internal rotation in some subgiant,
giant and white dwarf stars, has made angular momentum transport in
stars throughout their lifetimes an observational science.
Title: Avoided Crossing and Synchronization
Authors: Sekii, T.; Shibahashi, H.
Bibcode: 2013ASPC..479..573S
Altcode:
We examine avoided crossing of stellar pulsations in the nonlinear
regime, where synchronization may occur, based on a simple model of
weakly coupled van der Pol oscillators with close frequencies. For this
simple case, avoided crossing is unaffected in the sense that there is
a frequency difference between the symmetric and antisymmetric modes,
but as a result of synchronization, unlike the linear oscillations case,
the system can vibrate in only one of the modes.
Title: A Statistical Study of Coronal Active Events in the North
Polar Region
Authors: Sako, Nobuharu; Shimojo, Masumi; Watanabe, Tetsuya; Sekii,
Takashi
Bibcode: 2013ApJ...775...22S
Altcode:
In order to study the relationship between characteristics of polar
coronal active events and the magnetic environment in which such events
take place, we analyze 526 X-ray jets and 1256 transient brightenings
in the polar regions and in regions around the equatorial limbs. We
calculate the occurrence rates of these polar coronal active events
as a function of distance from the boundary of coronal holes, and
find that most events in the polar quiet regions occur adjacent to
and equatorward of the coronal hole boundaries, while events in the
polar coronal holes occur uniformly within them. Based primarily on
the background intensity, we define three categories of regions that
produce activity: polar coronal holes, coronal hole boundary regions,
and polar quiet regions. We then investigate the properties of the
events produced in these regions. We find no significant differences
in their characteristics, for example, length and lifetime, but there
are differences in the occurrence rates. The mean occurrence rate of
X-ray jets around the boundaries of coronal holes is higher than that
in the polar quiet regions, equatorial quiet regions, and polar coronal
holes. Furthermore, the mean occurrence rate of transient brightenings
is also higher in these regions. We make comparison with the occurrence
rates of emerging and canceling magnetic fields in the photosphere
reported in previous studies, and find that they do not agree with
the occurrence rates of transient brightenings found in this study.
Title: Probing the Shallow Convection Zone: Rising Motion of
Subsurface Magnetic Fields in the Solar Active Region
Authors: Toriumi, Shin; Ilonidis, Stathis; Sekii, Takashi; Yokoyama,
Takaaki
Bibcode: 2013ApJ...770L..11T
Altcode: 2013arXiv1305.3023T
In this Letter, we present a seismological detection of a rising motion
of magnetic flux in the shallow convection zone of the Sun, and show
estimates of the emerging speed and its decelerating nature. In order to
evaluate the speed of subsurface flux that creates an active region, we
apply six Fourier filters to the Doppler data of NOAA AR 10488, observed
with the Solar and Heliospheric Observatory/Michelson Doppler Imager,
to detect the reduction of acoustic power at six different depths from
-15 to -2 Mm. All the filtered acoustic powers show reductions, up to 2
hr before the magnetic flux first appears at the visible surface. The
start times of these reductions show a rising trend with a gradual
deceleration. The obtained velocity is first several km s-1
in a depth range of 15-10 Mm, then ~1.5 km s-1 at 10-5 Mm,
and finally ~0.5 km s-1 at 5-2 Mm. If we assume that the
power reduction is actually caused by the magnetic field, the velocity
of the order of 1 km s-1 is well in accordance with previous
observations and numerical studies. Moreover, the gradual deceleration
strongly supports the theoretical model that the emerging flux slows
down in the uppermost convection zone before it expands into the
atmosphere to build an active region.
Title: Helioseismic Detection of the Pre-emerging Magnetic Flux in
the Shallow Convection Zone
Authors: Toriumi, S.; Ilonidis, S.; Sekii, T.; Yokoyama, T.
Bibcode: 2013enss.confE..25T
Altcode:
We detect the rising magnetic flux in the shallower convection zone
of the Sun by observing acoustic power reduction, and evaluate its
rising speed. Here we aim to reveal the rising speed of the magnetic
flux in the shallow convection zone, before the active region are
created. We apply six different Fourier filters to the Doppler data of
NOAA AR 10488 taken by SOHO/MDI, to detect the reduction of acoustic
power at six different depths from -15 to -2 Mm. The filtered powers
show reductions before the start of flux appearance at the visible
surface. The start times of these reductions show a rising trend,
first at several km/s in a depth range of 15-10 Mm, then ∼1.5 km/s
at 10-5 Mm, and finally at ∼0.5 km/s at 5-2 Mm. If we assume that
the power reduction is actually caused by the rising magnetic flux,
the rising rate of the order of 1 km/s is well in accordance with
previous observations and numerical simulations. Moreover, the gradual
deceleration supports our simulations and theoretical model that the
rising flux slows down in the uppermost convection zone, just before
its further emergence into the solar atmosphere.
Title: Local-Helioseismology Study of Supergranulation in the
Polar Region
Authors: Nagashima, K.; Zhao, J.; Kosovichev, A. G.; Sekii, T.
Bibcode: 2012ASPC..454...19N
Altcode:
Hinode/SOT data have been used to study supergranulation in the polar
region. Although foreshortening generally makes it difficult to observe
the polar region in detail, to partially overcome the difficulty we
use the high-resolution Hinode/SOT observations of the polar regions
during the period of the highest inclination of the solar axis to the
ecliptic. By time-distance helioseismology we have found 'alignment'
of the supergranular cells peculiar in the polar region. This
might be an indication of the giant-cell structure in the polar region.
Title: The 3rd Hinode Science Meeting
Authors: Sekii, T.; Watanabe, T.; Sakurai, T.
Bibcode: 2012ASPC..454.....S
Altcode:
No abstract at ADS
Title: Helioseismic Investigation of Sub-Photospheric Properties of
a Coronal Hole
Authors: Zharkov, S.; Harra, L. K.; Sekii, T.
Bibcode: 2012ASPC..454...27Z
Altcode:
We present initial results of our investigation into sub-photospheric
properties of an equatorial coronal hole obtained via helioseismic
analysis using Hinode and MDI observational data. As at photospheric
level coronal holes are characterised by open magnetic field we look
for seismic signatures of such fields and compare those to the ones
observed in plages.
Title: Effects of Spectral Line Formation Height in Time-Distance
Helioseismology
Authors: Nagashima, K.; Parchevsky, K. V.; Zhao, J.; Duvall, T. L.,
Jr.; Kosovichev, A. G.; Sekii, T.
Bibcode: 2012ASPC..456...57N
Altcode:
To understand the effect of the formation-height difference in
time-distance helioseismology analyses, we consider the wave behavior
above the surface. We show that by using the numerically-simulated
wavefields at two different heights this difference may cause
travel-time shifts due to the non-stationary character of waves excited
by near-surface acoustic sources. This needs to be taken into account
in multi-wavelength helioseismology and measurements close to the
solar limb.
Title: Links between photospheric and chromospheric oscillations
Authors: Kosovichev, A. G.; Kitiashvili, I. N.; Mitra-Kraev, U.;
Sekii, T.
Bibcode: 2012decs.confE..97K
Altcode:
Oscillations excited by turbulent convection play important in the
dynamics and energetics of the solar atmosphere. Oscillations below the
acoustic cut-off frequency form photospheric resonant modes trapped
in the interior but also penetrating into the chromosphere. Above
the frequency cut-off, the oscillations represent traveling waves
in the chromosphere that form pseudo-modes due to interference with
waves coming from the interior. The physics of the chromospheric
oscillations, their coupling to the photospheric oscillations, and
their role in the chromospheric dynamics and energetics are not fully
understood. The observed oscillation properties strongly depend on
the excitation mechanism, interaction with turbulence and radiation,
and local structure and dynamics of the chromosphere. Significant
advances can be made through multi-wavelength observations of
atmospheric oscillations and realistic numerical radiative hydrodynamics
simulations. Using Hinode/SOT data we investigate the basic properties
of solar oscillations observed at two levels in the solar atmosphere,
in the G-band (formed in the photosphere) and in the CaII H line
(chromospheric emission). We analyzed the data by calculating the
individual power spectra as well as the cross-spectral properties,
i.e., coherence and phase shift. The observational properties are
compared with theoretical models and numerical simulations. The
results reveal significant frequency shifts between the CaII H and
G-band spectra, in particular above the acoustic cutoff frequency for
pseudo-modes. The cross-spectrum phase shows peaks associated with
the acoustic oscillation (p-mode) lines, and begins to increase with
frequency around the acoustic cut-off. However, we find no phase shift
for the (surface gravity wave) f-mode. The observed properties for
the p-modes are qualitatively reproduced in a model that includes a
correlated background due to radiative effects. Our results show that
multi-wavelength observations of solar oscillations, in combination
with radiative hydrodynamics modeling, help to understand the coupling
between photospheric and chromospheric oscillations.
Title: Measuring Acoustic Travel Times in Higher-Latitude Regions
of the Sun using Hinode and SDO Data
Authors: Nagashima, K.; Duvall, T.; Zhao, J.; Kosovichev, A. G.;
Parchevsky, K.; Sekii, T.
Bibcode: 2011AGUFMSH51B2016N
Altcode:
The interior structure and dynamics of the Sun can be probed by
measuring and inverting travel times of acoustic waves, widely
known as time-distance helioseismology. Recent high-resolution
observations of solar oscillations with Hinode/SOT and SDO/HMI
provide us with an opportunity to investigate the flow dynamics in
higher-latitude regions of the Sun. Of particular interest is the
meridional circulation flow, which is crucial for understanding the
solar dynamo mechanism and predicting the solar activity cycles. We
investigate systematic uncertainties of the travel times due to
the center-to-limb variations, which may significantly affect the
helioseismic inferences of the meridional flows. We present the results
of analysis of the cross-correlations in the Doppler velocity, line
core and intensity observations, and the corresponding travel-time fits
for various positions on the solar disk. We discuss the origin of the
center-to-limb variations, including the foreshortening effect, the
difference in the line formation height, and other effects. For better
understanding of the relative role of these effects we use 3D numerical
simulations of solar oscillations in a realistic model of the Sun.
Title: Global Helioseismology
Authors: Sekii, Takashi
Bibcode: 2011sdmi.confE..12S
Altcode:
Global helioseismology has been extremely successful in probing the
solar interior. Unlike local helioseismology, which was more recently
developed, global helioseismology relies on precise determination of
eigenfrequencies and their inversions, which renders the global approach
suited for measuring long-term averages of symmetric structures in the
sun. Strengths and limitations of global helioseismology are discussed,
as well as its main results and their impacts on our understanding of
the sun.
Title: Multi-wavelength time-distance helioseismology analyses
Authors: Nagashima, Kaori; Zhao, Junwei; Duvall, Thomas, Jr.;
Kosovichev, Alexander G.; Parchevsky, Konstantin; Sekii, Takashi
Bibcode: 2011sdmi.confE..37N
Altcode:
Travel times of the acoustic waves in the Sun tell us the structure
and the dynamics of the Sun. This information have been used to probe
the solar interior. If we exploit multi-layer observation datasets,
however, it will provide us with means to study the wave propagation
between the layers as well (Nagashima et al. 2009). In this study, using
multi-wavelength datasets obtained by Hinode/SOT, SDO/HMI, and SDO/AIA
we calculate the cross-correlation function of the wavefield and carry
out time-distance helioseismology analyses. Our preliminary results show
that when we cross-correlate the wavefields of two different layers
the cross-correlation functions between these layers are different
from the cross-correlation functions of both single layers, and this
provides us with an insight of wave propagation properties. We also
use numerical simulations of solar oscillations to help interpret our
observational results.
Title: The SOLAR-C mission: current status
Authors: Shimizu, Toshifumi; Tsuneta, Saku; Hara, Hirohisa; Ichimoto,
Kiyoshi; Kusano, Kanya; Sakao, Taro; Sekii, Takashi; Suematsu,
Yoshinori; Watanabe, Tetsuya
Bibcode: 2011SPIE.8148E..0BS
Altcode: 2011SPIE.8148E..10S
Two mission concepts (plan A: out-of-ecliptic mission and plan B:
high resolution spectroscopic mission) have been studied for the next
Japanese-led solar mission Solar-C, which will follow the scientific
success of the Hinode mission. The both mission concepts are concluded
as equally important and attractive for the promotion of space solar
physics. In the meantime we also had to make efforts for prioritizing
the two options, in order to proceed to next stage of requesting the
launch of Solar-C mission at the earliest opportunity. This paper
briefly describes the two mission concepts and the current status
on our efforts for prioritizing the two options. More details are
also described for the plan B option as the first-priority Solar-C
mission. The latest report from the Solar-C mission concept studies
was documented as "Interim Report on the Solar-C Mission Concept."
Title: Detection of Supergranulation Alignment in Polar Regions of
the Sun by Helioseismology
Authors: Nagashima, Kaori; Zhao, Junwei; Kosovichev, Alexander G.;
Sekii, Takashi
Bibcode: 2011ApJ...726L..17N
Altcode: 2010arXiv1011.1025N
We report on a new phenomenon of "alignment" of supergranulation cells
in the polar regions of the Sun. Recent high-resolution data sets
obtained by the Solar Optical Telescope on board the Hinode satellite
enabled us to investigate supergranular structures in high-latitude
regions of the Sun. We have carried out a local helioseismology
time-distance analysis of the data and detected acoustic travel-time
variations due to the supergranular flows. The supergranulation cells
in both the north and south polar regions show systematic alignment
patterns in the north-south direction. The south-pole data sets obtained
in a month-long Hinode campaign indicate that the supergranulation
alignment property may be quite common in the polar regions. We also
discuss the latitudinal dependence of the supergranulation cell sizes;
the data show that the east-west cell size decreases toward higher
latitudes.
Title: Helioseismology Study of Subsurface Dynamics in the Polar
Regions of the Sun
Authors: Nagashima, K.; Zhao, J.; Kosovichev, A. G.; Sekii, T.
Bibcode: 2010AGUFM.S32A..03N
Altcode:
We report on our time-distance helioseismology study of the
subsurface dynamics in the polar regions of the Sun. It is generally
difficult to observe the polar regions in details because of severe
foreshortening. The high-resolution data obtained by Solar Optical
Telescope (SOT) onboard the Hinode satellite, however, enabled
us to investigate dynamics in the regions with up to 80 degrees
in latitude. The measurements are obtained by calculating the
cross-covariance function of the random solar oscillations observed
as fluctuations of intensity of the Ca II H line, and by fitting a
Gabor-wavelet function for estimating the phase and group travel times
of solar acoustic waves. We obtain the maps of subsurface velocity field
by inverting the differences of the phase travel times, calculated for
the cross-correlations with positive and negative lag times, using a
ray-path approximation. Among the subsurface dynamical processes in the
polar region, we focus on supergranulation in this study. Supergranules
are thought to be one of convective cells in the convective envelope
of the Sun. Typical temporal and spatial scales of supergranular cells
are 1 day and 30 Mm, respectively. The supergranulation is considered
to play important roles in the magnetic flux transport and formation
of the magnetic network. However, we still do not have sufficient
knowledge of their origin and properties. In our study, we have observed
a curious alignment of the supergranular cells in the polar regions
approximately in the North-South direction. The alignment was seen
in both northern and southern polar regions. We discuss properties
and temporal evolution of the supergranular structures in the region,
as well as the possibilities of measuring the differential rotation
and meridional flows in the polar regions, which are critical for the
solar dynamo theories.
Title: Supergranulation in the Polar Regions Observed by Hinode/SOT
Authors: Nagashima, Kaori; Zhao, J.; Kosovichev, A.; Sekii, T.
Bibcode: 2010AAS...21640002N
Altcode: 2010BAAS...41..855N
We report on our investigation of the supergranular structure in the
polar regions of the Sun by local helioseismology. Supergranules
represent large-scale convective cells: the horizontal spatial
scale is about 30 Mm and the lifetime is about 1 day. They play
important role in the magnetic flux transport and formation of the
magnetic network. Recent helioseismological studies have found
that the depth of the cells is much smaller than the horizontal
scale, and that the supergranulation pattern exhibits a wave-like
behavior. However, we still do not have sufficient knowledge of
the origin and properties of the supergranulation. In this work, we
have carried out a new time-distance helioseismology analysis using
high-resolution datasets of the polar regions of the Sun obtained
by the Solar Optical Telescope (SOT) onboard the Hinode satellite
during the periods of the high inclination of the solar axis to the
ecliptic. Because of the foreshortening such measurements are not
currently possible with any other helioseismology instrument. We have
measured the travel-time shifts of acoustic waves traveling to various
depth below the photosphere, and obtain the subphotospheric horizontal
flow maps by inversion. We have detected the supergranular cells in
the polar regions and studied their properties. For comparison we did a
similar analysis for low-latitude regions at the East limb of the Sun,
and at the disk center. Comparing with the cells in the lower-latitude
regions, we have observed a curious alignment of the cells in the polar
regions, approximately in the North-South direction. This `alignment'
has been found in both the North and the South polar regions. In
the presentation, we discuss the alignment phenomenon as well as the
physical properties of the supergranular cells in the polar regions.
Title: The quest for the solar g modes
Authors: Appourchaux, T.; Belkacem, K.; Broomhall, A. -M.; Chaplin,
W. J.; Gough, D. O.; Houdek, G.; Provost, J.; Baudin, F.; Boumier,
P.; Elsworth, Y.; García, R. A.; Andersen, B. N.; Finsterle, W.;
Fröhlich, C.; Gabriel, A.; Grec, G.; Jiménez, A.; Kosovichev, A.;
Sekii, T.; Toutain, T.; Turck-Chièze, S.
Bibcode: 2010A&ARv..18..197A
Altcode: 2010A&ARv.tmp....1A; 2009arXiv0910.0848A
Solar gravity modes (or g modes)—oscillations of the solar interior
on which buoyancy acts as the restoring force—have the potential
to provide unprecedented inference on the structure and dynamics of
the solar core, inference that is not possible with the well-observed
acoustic modes (or p modes). The relative high amplitude of the g-mode
eigenfunctions in the core and the evanesence of the modes in the
convection zone make the modes particularly sensitive to the physical
and dynamical conditions in the core. Owing to the existence of the
convection zone, the g modes have very low amplitudes at photospheric
levels, which makes the modes extremely hard to detect. In this article,
we review the current state of play regarding attempts to detect g
modes. We review the theory of g modes, including theoretical estimation
of the g-mode frequencies, amplitudes and damping rates. Then we go
on to discuss the techniques that have been used to try to detect g
modes. We review results in the literature, and finish by looking to
the future, and the potential advances that can be made—from both
data and data-analysis perspectives—to give unambiguous detections
of individual g modes. The review ends by concluding that, at the time
of writing, there is indeed a consensus amongst the authors that there
is currently no undisputed detection of solar g modes.
Title: High-Resolution Helioseismic Imaging of Subsurface Structures
and Flows of a Solar Active Region Observed by Hinode
Authors: Zhao, Junwei; Kosovichev, Alexander G.; Sekii, Takashi
Bibcode: 2010ApJ...708..304Z
Altcode: 2009arXiv0911.1161Z
We analyze a solar active region observed by the Hinode Ca II H line
using the time-distance helioseismology technique, and infer wave-speed
perturbation structures and flow fields beneath the active region
with a high spatial resolution. The general subsurface wave-speed
structure is similar to the previous results obtained from Solar and
Heliospheric Observatory/Michelson Doppler Imager observations. The
general subsurface flow structure is also similar, and the downward
flows beneath the sunspot and the mass circulations around the sunspot
are clearly resolved. Below the sunspot, some organized divergent flow
cells are observed, and these structures may indicate the existence of
mesoscale convective motions. Near the light bridge inside the sunspot,
hotter plasma is found beneath, and flows divergent from this area
are observed. The Hinode data also allow us to investigate potential
uncertainties caused by the use of phase-speed filter for short travel
distances. Comparing the measurements with and without the phase-speed
filtering, we find out that inside the sunspot, mean acoustic travel
times are in basic agreement, but the values are underestimated by a
factor of 20%-40% inside the sunspot umbra for measurements with the
filtering. The initial acoustic tomography results from Hinode show
a great potential of using high-resolution observations for probing
the internal structure and dynamics of sunspots.
Title: Recent Progress and Future Directions for Helioseismology
Authors: Kosovichev, A.; Zhao, J.; Sekii, T.; Nagashima, K.;
Mitra-Kraev, U.
Bibcode: 2009ASPC..415..399K
Altcode:
Hinode/SOT observations provide unique data for high-resolution
helioseismology. These data have allowed us for the first
time to resolve the subsurface convective boundary layer, obtain
high-resolution images of structures and mass flows beneath a sunspot,
detect flare-generated MHD waves in the sunspot umbra, carry out
multi-wavelength studies of solar oscillations, and obtain unique
helioseismic data for probing the subsurface dynamics in near-polar
regions. Future directions in helioseismology will be focused on
understanding the mechanism of solar dynamo, diagnostics of emerging
magnetic flux, formation and evolution of sunspot regions and their
flaring activity. Of particular interest are investigations of solar
convection dynamics, differential rotation and meridional flows
in the near polar regions. These tasks require development of new
helioseismology methods for probing conditions in strong magnetic
field regions and improving temporal and spatial resolutions. The new
developments in helioseismology will be supported by realistic MHD
simulations and based on massive data analysis from Hinode and Solar
Dynamics Observatory.
Title: Travel-Time Analyses of an Emerging-Flux Region
Authors: Nagashima, K.; Sekii, T.; Kosovichev, A. G.; Zhao, J.;
Tarbell, T. D.
Bibcode: 2009ASPC..415..417N
Altcode:
Travel-time analyses of a newly-formed plage region are presented. The
dataset has been obtained from the 12-hr Hinode observation of an
emerging-flux region (to be NOAA AR 10975) close to the disc center
on 23 November 2007. The SOT provides data in Ca II H line and in
Fe I 557.6nm line; we use both chromospheric intensity oscillation
data and photospheric Dopplergrams for travel-time measurement by
a cross-correlation method. In the plage region, we have detected
a travel-time anomaly in the chromospheric data, but not in the
photospheric data. This can be interpreted as a signature of downflows
in the chromosphere. This result illustrates how time-distance
techniques can be used to study chromospheric flows.
Title: Hinode/SOT Helioseismic Observations
Authors: Sekii, T.
Bibcode: 2009ASPC..415..405S
Altcode:
Helioseismic observations by the Hinode Solar Optical Telescope
(SOT) are reviewed. The benefit of the SOT high-resolution
capability for local helioseismology has already been demonstrated
in mapping subsurface flows and {revealing the fine structure of
umbral flashes}. The phase relations between the photospheric and
the chromospheric oscillations in the sub-critical frequency range
are also discussed, as are possible future directions of the SOT
helioseismology programme.
Title: New Results of High-Resolution Helioseismology from Hinode
Authors: Kosovichev, A.; Zhao, J.; Sekii, T.; Nagashima, K.;
Mitra-Kraev, U.
Bibcode: 2009ASPC..416...41K
Altcode:
The Solar Optical Telescope of Hinode provides unique multi-wavelength
high-resolution data for local helioseismic diagnostics of the
sub-surface structure and dynamics of the Sun. The helioseismology data
from Hinode have allowed us for the first time to observe oscillations
of very high angular degree and high frequencies, and substantially
improve the spatial resolution of time-distance helioseismology in
near-surface layers of the Sun, compared to the previous SOHO/MDI
data. The Hinode data have also provided important insight on the
nature of sunspot oscillations, and the correlated component of
stochastic excitation. Initial attempts have been made to investigate
the dynamics of the polar regions, critical for dynamo modeling, but
previously unaccessible for helioseismology. In addition, a new type
of flare-excited MHD oscillations was detected from Hinode observations
of the solar flare of December 13, 2006.
Title: Subsurface Structures and Flow Fields of an Active Region
Observed by Hinode
Authors: Zhao, J.; Kosovichev, A. G.; Sekii, T.
Bibcode: 2009ASPC..415..411Z
Altcode:
We analyze a solar active region observed by Hinode Ca II H line using
the time-distance helioseismology technique, and derive the subsurface
structure and flow fields of this active region. The basic subsurface
wave speed structure is essentially the same as the previous results
obtained from MDI observations. The subsurface flow structure is
also similar to the previous results, but the downward flows are more
resolved. Additionally, vertical mass circulations outside the sunspot
are more clearly detected, although no mass conservation constraints
are imposed in the inversion procedure. Near a light bridge area,
hotter plasma is found beneath, and flows divergent from this area
are observed. Inside the sunspot umbra and penumbra, some organized
divergent flow cells of an intermediate size between granulation and
supergranulation are also observed. These initial results demonstrate
the potential and importance of high-resolution helioseismology of
sunspots.
Title: Subsurface Flows in Solar Active Regions and Polar Areas from
Hinode Observations
Authors: Zhao, Junwei; Kosovichev, A. G.; Sekii, T.
Bibcode: 2009SPD....40.0706Z
Altcode:
High-resolution observations of Ca II H line intensity made by Hinode
have provided us an opportunity to study subsurface flow fields of
solar active regions with an unprecedented high spatial resolution,
and a possibility to study subsurface meridional flows in the solar
polar areas. For a large active region, we have found a strong
downdraft below the sunspot, a mass circulation around the spot, and
an evidence of magnetoconvection beneath the strong field area. For
the polar regions, we have been able to derive a poleward meridional
flow of an order of 5 m/s for the first time using the time-distance
helioseismology technique.
Title: Helioseismic Signature of Chromospheric Downflows in Acoustic
Travel-Time Measurements From Hinode
Authors: Nagashima, Kaori; Sekii, Takashi; Kosovichev, Alexander G.;
Zhao, Junwei; Tarbell, Theodore D.
Bibcode: 2009ApJ...694L.115N
Altcode: 2009arXiv0903.1323N
We report on a signature of chromospheric downflows in two emerging
flux regions detected by time-distance helioseismology analysis. We
use both chromospheric intensity oscillation data in the Ca II H line
and photospheric Dopplergrams in the Fe I 557.6 nm line obtained
by Hinode/SOT for our analyses. By cross-correlating the Ca II
oscillation signals, we have detected a travel-time anomaly in the
plage regions; outward travel times are shorter than inward travel
times by 0.5-1 minute. However, such an anomaly is absent in the Fe
I data. These results can be interpreted as evidence of downflows in
the lower chromosphere. The downflow speed is estimated to be below
10 km s-1. This result demonstrates a new possibility of
studying chromospheric flows by time-distance analysis.
Title: Solar Dynamo and Magnetic Self-Organization
Authors: Kosovichev, A. G.; Arlt, R.; Bonanno, A.; Brandenburg,
A.; Brun, A. S.; Busse, F.; Dikpati, M.; Hill, F.; Gilman, P. A.;
Nordlund, A.; Ruediger, G.; Stein, R. F.; Sekii, T.; Stenflo, J. O.;
Ulrich, R. K.; Zhao, J.
Bibcode: 2009astro2010S.160K
Altcode:
No abstract at ADS
Title: High-Resolution Helioseismology from Hinode
Authors: Kosovichev, A. G.; Zhao, J.; Sekii, T.; Nagashima, K.;
Mitra-Kraev, U.
Bibcode: 2008AGUFMSH41B1627K
Altcode:
The Solar Optical Telescope (SOT) on the Hinode space mission provides
unique multi-wavelength high-resolution data for local helioseismic
diagnostics of the sub-photospheric structure and dynamics of the
Sun. The helioseismology data from Hinode have allowed us for the
first time to observe oscillations of very high angular degree and
high frequencies. The Hinde data provide a potential for substantial
improvement of the spatial resolution of time-distance helioseismology
in near-surface layers of the Sun, compared to the previous SOHO/MDI
data. The Hinode data have also provided important insight in the nature
of sunspot oscillations. Simultaneous observations of solar oscillations
in two different spectral interval have allowed us to investigate the
mode physics and the correlated component of stochastic excitation. In
addition, a new type of flare-excited MHD oscillations was detected
from Hinode observations of the solar flare of December 13, 2006.
Title: Helioseismic Measurement of Subsurface Flows at Solar High
Latitude
Authors: Zhao, J.; Kosovichev, A. G.; Sekii, T.
Bibcode: 2008AGUFMSH44A..03Z
Altcode:
The solar polar magnetic field is of great interest as it is where
solar magnetic field reversal starts. Surface and subsurface plasma
flows are very important in understanding the field reversal because
magnetic field flux is transported to solar high latitude from lower
latitude according to flux transport theory. Local helioseismology
has been able to derive subsurface flow fields, rotation rates,
and meridional flows up to 30 Mm in depth. The results obtained by
time-distance helioseismology during Solar Cycle 23 from SOHO/MDI have
also revealed significant changes of the speed and the longitudinal
structure of the flows. We used these measurements to compare with the
magnetic flux transport determined from the magnetic field synoptic
data. Furthermore, by use of MDI dynamic campaign observations and
a recent high resolution observation of solar South Pole by Hinode,
we explore the possibility to detect subsurface flow fields in solar
high latitude.
Title: AsteroFLAG — from the Sun to the stars
Authors: Chaplin, W. J.; Appourchaux, T.; Arentoft, T.; Ballot, J.;
Baudin, F.; Bazot, M.; Bedding, T. R.; Christensen-Dalsgaard, J.;
Creevey, O. L.; Duez, V.; Elsworth, Y.; Fletcher, S. T.; García,
R. A.; Gough, D. O.; Jiménez, A.; Jiménez-Reyes, S. J.; Houdek, G.;
Kjeldsen, H.; Lazrek, M.; Leibacher, J. W.; Monteiro, M. J. P. F. G.;
Neiner, C.; New, R.; Régulo, C.; Salabert, D.; Samadi, R.; Sekii,
T.; Sousa, S. G.; Toutain, T.; Turck-Chièze, S.
Bibcode: 2008JPhCS.118a2048C
Altcode:
We stand on the threshold of a critical expansion of asteroseismology
of Sun-like stars, the study of stellar interiors by observation
and analysis of their global acoustic modes of oscillation. The
Sun-like oscillations give a very rich spectrum allowing the internal
structure and dynamics to be probed down into the stellar cores to
very high precision. Asteroseismic observations of many stars will
allow multiple-point tests of crucial aspects of stellar evolution
and dynamo theory. The aims of the asteroFLAG collaboration are to
help the community to refine existing, and to develop new, methods
for analysis of the asteroseismic data on the Sun-like oscillators.
Title: Loop Morphology and Flows and their Relation to the Magnetic
Field
Authors: Teriaca, L.; Wiegelmann, T.; Lagg, A.; Solanki, S. K.; Curdt,
W.; Sekii, T.
Bibcode: 2008ASPC..397..196T
Altcode:
In November 2006 we obtained several rasters of a large sunspot
and its trailing region using the SUMER spectrometer on SOHO. The
observations consist of spectroheliograms in the continuum around
142 nm and in several spectral lines formed between 80000 K and
0.6 MK, covering the temperature range from the chromosphere to
the lower corona. The observed profiles provide LOS velocity and
Doppler width maps. TRACE images in the EUV passbands and in the 160
nm continuum provide a clear picture of the coronal loops and the
chromosphere near their footpoints. The same target was also observed
by all the instruments aboard Hinode and, in particular, by the SOT
spectro-polarimeter measuring the photospheric magnetic vector. We
combined SOT and MDI data (covering a larger FOV) to infer the coronal
magnetic field of the active region by a nonlinear force-free field
extrapolation. The observed radiance and velocity patterns at the
various heights/temperatures throughout the solar atmosphere are
compared with the field topology.
Title: AsteroFLAG: First results from hare-and-hounds Exercise #1
Authors: Chaplin, W. J.; Appourchaux, T.; Arentoft, T.; Ballot, J.;
Christensen-Dalsgaard, J.; Creevey, O. L.; Elsworth, Y.; Fletcher,
S. T.; García, R. A.; Houdek, G.; Jiménez-Reyes, S. J.; Kjeldsen,
H.; New, R.; Régulo, C.; Salabert, D.; Sekii, T.; Sousa, S. G.;
Toutain, T.; rest of asteroFLAG Group
Bibcode: 2008AN....329..549C
Altcode: 2008arXiv0803.4143C
We report on initial results from the first phase of Exercise #1 of the
asteroFLAG hare and hounds. The asteroFLAG group is helping to prepare
for the asteroseismology component of NASA's Kepler mission, and the
first phase of Exercise #1 is concerned with testing extraction of
estimates of the large and small frequency spacings of the low-degree p
modes from Kepler-like artificial data. These seismic frequency spacings
will provide key input for complementing the exoplanet search data.
Title: Initial Helioseismology Results from Hinode
Authors: Kosovichev, A.; Zhao, J.; Sekii, T.; Nagashima, K.
Bibcode: 2008AGUSMSP21A..01K
Altcode:
Solar Optical Telescope of Hinode provides unique multi-wavelength
high-resolution data for local helioseismic diagnostics of the
sub-surface structure and dynamics of the Sun. The helioseismology data
from Hinode have allowed us for the first time to observe oscillations
of very high angular degree and high frequencies, and substantially
improve the spatial resolution of time-distance helioseismology in
near-surface layers of the Sun, compared to the previous SOHO/MDI
data. The Hinode data have also provided important insight in the
nature of sunspot oscillations. Initial attempts have been made to
investigate the dynamics of the polar regions, previously unaccessible
for helioseismology. We present the first results and discuss the
potential and perspective of the Hinode helioseismology program.
Title: First Detection of MHD Oscillations Excited by Solar Flare
in Sunspot Umbra
Authors: Kosovichev, A.; Sekii, T.
Bibcode: 2008AGUSMSP21B..07K
Altcode:
We report on detection of a new type of flare-excited oscillations
from Hinode observations of the solar flare of December 13, 2006. The
oscillations observed in Ca II H images appeared in the sunspot
umbra immediately after the impulsive phase of the flare. They had
the amplitude 2-4 times larger than the pre-flare oscillations in
the umbra. Also, their frequency seemed to be higher. There is an
evidence that during the first 30-40 min the oscillations represent
waves traveling through the umbra in the direction away from the flare
ribbon with a speed of 50-100 km/s. Then, the oscillation become more
irregular with some occasional wave packets. The lifetime of these
oscillations is probable about 8 hours. The estimated speed indicates
that the waves are of an MHD type, and if their speed is of the order
of magnitude of the Alfven speed then they should propagate rather low
in the sunspot chromosphere. Sunspot oscillations have been studied
intensively for many years but the Hinode observations are the first
that show enhanced oscillations in the umbra, associated with a solar
flare. Further investigations of these oscillations are of great
interest for understanding the processes in solar flares and sunspots.
Title: Properties of high-degree oscillation modes of the Sun observed
with Hinode/SOT
Authors: Mitra-Kraev, U.; Kosovichev, A. G.; Sekii, T.
Bibcode: 2008A&A...481L...1M
Altcode: 2007arXiv0711.2210M
Aims:With the Solar Optical Telescope on Hinode, we investigate
the basic properties of high-degree solar oscillations observed at
two levels in the solar atmosphere, in the G-band (formed in the
photosphere) and in the Ca II H line (chromospheric emission).
Methods: We analyzed the data by calculating the individual power
spectra as well as the cross-spectral properties, i.e., coherence and
phase shift. The observational properties are compared with a simple
theoretical model, which includes the effects of correlated noise.
Results: The results reveal significant frequency shifts between the
Ca II H and G-band spectra, in particular above the acoustic cut-off
frequency for pseudo-modes. The cross-spectrum phase shows peaks
associated with the acoustic oscillation (p-mode) lines, and begins to
increase with frequency around the acoustic cut-off. However, we find
no phase shift for the (surface gravity wave) f-mode. The observed
properties for the p-modes are qualitatively reproduced in a simple
model with a correlated background if the correlated noise level in
the Ca II H data is higher than in the G-band data. These results
suggest that multi-wavelength observations of solar oscillations,
in combination with the traditional intensity-velocity observations,
may help to determine the level of the correlated background noise
and to determine the type of wave excitation sources on the Sun.
Title: Development of Iodine Cells for Subaru HDS and Okayama
HIDES. III. An Improvement on the Radial-Velocity Measurement
Technique
Authors: Kambe, Eiji; Ando, Hiroyasu; Sato, Bun'ei; Izumiura, Hideyuki;
Sekii, Takashi; Paulson, Daine B.; Yanagisawa, Kenshi; Masuda, Seiji;
Shibahashi, Hiromoto; Hatzes, Artie P.; Martic, Milena; Lebrun,
Jean-Claude; Mkrtichian, David E.; Kiss, Laszlo L.; Bruntt, Hans;
O'Toole, Simon J.; Bedding, Timothy R.
Bibcode: 2008PASJ...60...45K
Altcode:
An improvement of the radial-velocity measurement accuracy is crucial
for the detection of tiny stellar oscillations and exoplanets. Through
the analysis of week-long extensive observations of solar-type stars
(Procyon in 2000, 2002, and 2006/2007 and τ Cet in 2002 and 2006/2007),
we have carefully examined, revised, and finely tuned the widely used
multiple Gaussian IP fitting method for the spectrograph, HIDES. By
determining a necessary and sufficient number of free parameters in the
model as well as introducing an iterative process in the radial-velocity
analysis, we can reach a precision of below 3ms-1, which
is much smaller than the precision of 6ms-1 officially
announced so far for HIDES. We also make our technique refined for the
2002 McDonald Procyon data. Even with our revised method, slow radial
velocity variations with an amplitude of about 10ms-1
are left in the Procyon data. We emphasize that it is neither due
to particular observing instruments nor radial-velocity analysis,
and thus could be due to stellar origin. The analysis presented
here makes the foundations of our next scientific analysis of the
radial-velocity variations of Procyon, which will be presented in our
forthcoming papers.
Title: Hinode SOT observations of plume upflows and cascading
downflows in quiescent solar prominences
Authors: Berger, T.; Shine, R.; Slater, G.; Tarbell, T.; Title, A.;
Lites, B.; Tsuneta, S.; Okamoto, T. J.; Ichimoto, K.; Katsukawa, Y.;
Sekii, T.; Suematsu, Y.; Shimizu, T.
Bibcode: 2007AGUFMSH53A1065B
Altcode:
We present several Hinode SOT filtergram movies of quiescent solar
prominences that show newly discovered "plume-like" upflows and
cascading "waterfall-like" downflows that persist for the entire
multi-hour duration of the observations. The flow speeds are on the
order of 10 km/sec with typical widths of 400-700 km. Preliminary
calculations show that if the upflows are buoyancy driven, the
associated thermal perturbation is on the order of 10,000 K, sufficient
to explain the dark appearance of the upflows in the interference
filter passbands. In addition we observe rotational vortices and
body oscillations within the prominences. These new observations
challenge current magnetostatic models of solar prominences by showing
that prominence plasmas are in constant motion, often in directions
perpendicular to the magnetic field lines proposed by the models. TRACE,
Hinode/EIS, and Hinode/XRT observations are used to investigate the
differential topology of the flows across temperature regimes.
Title: Initial Observations of Sunspot Oscillations Excited by
Solar Flare
Authors: Kosovichev, A. G.; Sekii, T.
Bibcode: 2007ApJ...670L.147K
Altcode: 2007arXiv0710.1808K
Observations of a large solar flare on 2006 December 13 using Solar
Optical Telescope (SOT) on board the Hinode spacecraft revealed
high-frequency oscillations excited by the flare in the sunspot
chromosphere. These oscillations are observed in the region of strong
magnetic field of the sunspot umbra and may provide a new diagnostic
tool for probing the structure of sunspots and understanding physical
processes in solar flares.
Title: Initial Helioseismic Observations by Hinode/SOT
Authors: Sekii, Takashi; Kosovichev, Alexander G.; Zhao, Junwei;
Tsuneta, Saku; Shibahashi, Hiromoto; Berger, Thomas E.; Ichimoto,
Kiyoshi; Katsukawa, Yukio; Lites, Bruce; Nagata, Shin'ichi; Shimizu,
Toshifumi; Shine, Richard A.; Suematsu, Yoshinori; Tarbell, Theodore
D.; Title, Alan M.
Bibcode: 2007PASJ...59S.637S
Altcode: 2007arXiv0709.1806S
Results from initial helioseismic observations by the Solar Optical
Telescope on-board Hinode are reported. It has been demonstrated
that intensity oscillation data from the Broadband Filter Imager
can be used for various helioseismic analyses. The k - ω power
spectra, as well as the corresponding time-distance cross-correlation
function, which promise high-resolution time-distance analysis below
the 6-Mm travelling distance, were obtained for G-band and CaII-H
data. Subsurface supergranular patterns were observed from our first
time-distance analysis. The results show that the solar oscillation
spectrum is extended to much higher frequencies and wavenumbers, and
the time-distance diagram is extended to much shorter travel distances
and times than were observed before, thus revealing great potential
for high-resolution helioseismic observations from Hinode.
Title: Observations of Sunspot Oscillations in G Band and CaII H
Line with Solar Optical Telescope on Hinode
Authors: Nagashima, Kaori; Sekii, Takashi; Kosovichev, Alexander G.;
Shibahashi, Hiromoto; Tsuneta, Saku; Ichimoto, Kiyoshi; Katsukawa,
Yukio; Lites, Bruce; Nagata, Shin'ichi; Shimizu, Toshifumi; Shine,
Richard A.; Suematsu, Yoshinori; Tarbell, Theodore D.; Title, Alan M.
Bibcode: 2007PASJ...59S.631N
Altcode: 2007arXiv0709.0569N
Exploiting high-resolution observations made by the Solar Optical
Telescope on board Hinode, we investigate the spatial distribution
of the power spectral density of the oscillatory signal in and around
the active region NOAA 10935. The G-band data show that in the umbra
the oscillatory power is suppressed in all frequency ranges. On
the other hand, in CaII H intensity maps oscillations in the umbra,
so-called umbral flashes, are clearly seen with the power peaking around
5.5mHz. The CaII H power distribution shows the enhanced elements with
the spatial scale of the umbral flashes over most of the umbra, but
there is a region with suppressed power at the center of the umbra. The
origin and property of this node-like feature remain unexplained.
Title: Calibration of SOT Dopplergrams
Authors: Katsukawa, Y.; Ichimoto, K.; Sekii, T.; Suematsu, Y.; Tsuneta,
S.; Shine, R. A.; Tarbell, T. D.
Bibcode: 2007ASPC..369...43K
Altcode:
Narrow-band Filter Imager on SOT provides Dopplergrams (DGs) which
are images of Doppler (line- of-sight) velocities. Observations with
DGs are critically important in studies of photospheric dynamics and
helioseismology. The primary photospheric line used for DGs is Fe I 5576
Å which is a line insensitive to Zeeman effect. We made a calibration
function for the 5576 Å DGs to get actual Doppler velocities from
velocity indexes using an atlas spectrum and simulated transmission
profiles for the tunable filter (TF) on SOT. Using data sets taken
in the natural sun-light test, we quantitatively evaluated accuracy
of the DGs by comparing the rotational speed of the Sun measured
with DGs with the expected one. There was a little systematic error
in the velocity obtained by SOT, but the error was less than 20 %
of the predicted velocities.
Title: Distribution of MDI p-mode power correlation coefficients
Authors: Y Chang, H.; Sekii, T.
Bibcode: 2006ESASP.624E.132Y
Altcode: 2006soho...18E.132Y
No abstract at ADS
Title: The internal structure of the Sun inferred from g modes and
low-frequency p modes
Authors: Elsworth, Y. P.; Baudin, F.; Chaplin, W; Andersen, B;
Appourchaux, T.; Boumier, P.; Broomhall, A. -M.; Corbard, T.;
Finsterle, W.; Fröhlich, C.; Gabriel, A.; García, R. A.; Gough,
D. O.; Grec, G.; Jiménez, A.; Kosovichev, A.; Provost, J.; Sekii,
T.; Toutain, T.; Turck-Chièze, S.
Bibcode: 2006ESASP.624E..22E
Altcode: 2006soho...18E..22E
The Phoebus group is an international collaboration of
helioseismologists, its aim being to detect low-frequency solar g
modes. Here, we report on recent work, including the development and
application of new techniques based on the detection of coincidences
in contemporaneous datasets and the asymptotic properties of the g-mode
frequencies. The length of the time series available to the community is
now more than ten years, and this has reduced significantly the upper
detection limits on the g-mode amplitudes. Furthermore, low-degree p
modes can now be detected clearly at frequencies below 1000 μHz.
Title: Rotation inversions of artificial solarFLAG Sun-as-a-star data
Authors: Chaplin, W. J.; Sekii, T.; Appourchaux, T.; Baudin, F.;
Boumier, P.; Corbard, T.; Elsworth, Y.; Fletcher, S. T.; Garcia,
R. A.; Jiménez-Reyes, S. J.; Lazrek, M.; New, R.; Salabert, D.;
Toutain, T.; Wachter, R.
Bibcode: 2006ESASP.624E..82C
Altcode: 2006soho...18E..82C
No abstract at ADS
Title: Recent Progresses on g-Mode Search
Authors: Appourchaux, T.; Andersen, B.; Baudin, F.; Boumier, P.;
Broomhall, A. -M.; Chaplin, W.; Corbard, T.; Elsworth, Y.; Finsterle,
W.; Fröhlich, C.; Gabriel, A.; Garcia, R.; Gough, D. O.; Grec, G.;
Jiménez, A.; Kosovichev, A.; Provost, J.; Sekii, T.; Toutain, T.;
Turck-Chièze, S.
Bibcode: 2006ESASP.617E...2A
Altcode: 2006soho...17E...2A
No abstract at ADS
Title: Seismology of Solar Internal Rotation
Authors: Sekii, Takashi
Bibcode: 2005HiA....13..424S
Altcode:
Helioseismologists have been highly successful in revealing how
the solar interior differentially rotates through observations of
rotationally split eigen p modes of the sun. What we currently know
about the internal rotation of the sun including how it varies over
the solar cycle is reviewed. Seismic inference of internal rotation
based on low-degree modes is also discussed.
Title: Annual Report of the National Astronomical Observatory of
Japan, Volume 7 Fiscal 2004
Authors: Tanikawa, Kiyotaka; Imanishi, Masatoshi; Ueda, Akitoshi; Oe,
Masafumi; Sekii, Takashi; Sôma, Mitsuru; Miyoshi, Makoto; Yamashita,
Yoshiko
Bibcode: 2005ARAOJ...7.....T
Altcode:
No abstract at ADS
Title: Annual Report of the National Astronomical Observatory of
Japan, Volume 6 Fiscal 2003
Authors: Tanikawa, Kiyotaka; Imanishi, Masatoshi; Ueda, Akitoshi; Oe,
Masashi; Sekii, Takashi; Sôma, Mitsuru; Miyoshi, Makoto; Yamashita,
Yoshiko
Bibcode: 2005naoj.book.....T
Altcode:
No abstract at ADS
Title: On the detectability of a rotation-rate gradient in the
solar core
Authors: Chaplin, W. J.; Sekii, T.; Elsworth, Y.; Gough, D. O.
Bibcode: 2004MNRAS.355..535C
Altcode: 2004MNRAS.tmp..451C
We consider the problem of uncovering a possible gradient of rotation
in the solar core using seismic frequency splittings of low-degree
p-modes. The data are presumed to come from full-disc observations of
the Sun. In particular, we formulate an approach for determining the
diagnostic potential of a low-l splitting set to uncover, by inversion,
a difference in rotation between two target radii: one deep in the core,
the other in the mid-regions of the radiative zone. Our formulation
assumes the underlying rotation rate to be flat in the outer part
of the zone, but to vary linearly in the core (with the breakpoint
in behaviour located between the two target radii). Artificial data
are used to test the formulation, and to investigate the relative
importance at low l of deeply penetrating, high-order modes and
relatively shallow, low-order modes. Our results suggest that in
order to detect a significant difference between the rotation at r=
0.1R and 0.35R- with the input splitting data coming from a 10-yr
set of high-quality observations - that difference would need to be
a significant fraction of the rotation rate itself.
Title: SOT Local-Helioseismology Programme
Authors: Sekii, T.
Bibcode: 2004ASPC..325...87S
Altcode:
Local helioseismology is proving to be a powerful tool in revealing
small-scale flow and inhomogeneity in subsurface layers. The Solar
Optical Telescope (SOT) onboard SolarB is capable of producing
high-resolution Doppler-grams that can be used for this new method
of helioseismology. It is reviewed and discussed what can be learned
from SOT local helioseismology, as well as what should be done for a
successful programme.
Title: The Solar-B Mission and the Forefront of Solar Physics
Authors: Sakurai, T.; Sekii, T.
Bibcode: 2004ASPC..325.....S
Altcode:
No abstract at ADS
Title: An Alternative Interpretation of the Double-Ridge Structure
in the - Frequency Time-Distance Autocorrelation Function?
Authors: Sekii, T.; Shibahashi, H.; Jefferies, S. M.
Bibcode: 2004ESASP.559..619S
Altcode: 2004soho...14..619S
No abstract at ADS
Title: Helioseismic measurement of solar rotation
Authors: Sekii, Takashi
Bibcode: 2004ceme.symp...25S
Altcode:
Helioseismology has been successful in investigating the otherwise
invisible solar interior, and one of its greatest success is in
revealing how the interior of the sun rotates. It is briefly reviewed
how this is done, what we have learnt, and what they imply.
Title: Annual Report of the National Astronomical Observatory of
Japan, Volume 5 Fiscal 2002
Authors: Tanikawa, Kiyotaka; Imanishi, Masatoshi; Miyoshi, Makoto;
Muramatsu, Toshiya; Sekii, Takashi; Sôma, Mitsuru; Ueda, Akitoshi;
Yamashita, Yoshiko; Yasuda, Naoki
Bibcode: 2004naoj.book.....T
Altcode:
No abstract at ADS
Title: Autocorrelation analysis of MDI high-frequency data
Authors: Sekii, T.; Shibahashi, H.; Kosovichev, A. G.
Bibcode: 2003ESASP.517..385S
Altcode: 2003soho...12..385S
We have analyzed MDI data using time-distance autocorrelation function,
in a high-frequency range above the acoustic cut-off frequency of the
solar atmosphere. The MDI velocity, intensity and line-depth signals
were looked at. The wave reflection rate at the photosphere has been
found to be around 10 per cent for l = 125 and ν = 6.75mHz. The result
is compared with a previous measurement.
Title: On averaged time-distance autocorrelation diagrams
Authors: Sekii, T.; Shibahashi, H.
Bibcode: 2003ESASP.517..389S
Altcode: 2003soho...12..389S
To calculate time-distance autocorrelation for local-helioseismic
analyses, the correlation function is often put in a form which permits
application of the convolution theorem so that CPU-intensive direct
integration is alleviated. It is normally justified by a statistical
argument with a certain assumption on properties of power distribution
of the wavefield; the results are then interpreted as ensemble
averages of the autocorrelation. However, the same results can also
be obtained by taking spatial averages without any assumption on the
power distribution, thereby providing a different interpretation to the
results obtained through convolution theorem. It is straightforward
to demonstrate the above in the case of wavefield in infinite
two-dimensional space. Here we present a direct demonstration of the
same equivalence of the averages in the case of spherical geometry,
which has turned out to be less straightforward.
Title: Annual Report of the National Astronomical Observatory of
Japan, Volume 4 Fiscal 2001
Authors: Tanikawa, Kiyotaka; Honma, Mareki; Imanishi, Masatoshi; Kuno,
Nario; Muramatsu, Toshiya; Sekii, Takashi; Sôma, Mitsuru; Yamashita,
Yoshiko; Yasuda, Naoki
Bibcode: 2003naoj.book.....T
Altcode:
No abstract at ADS
Title: Seismology of solar rotation
Authors: Sekii, Takashi
Bibcode: 2003safd.book..263S
Altcode:
Helioseismology provides us with means to investigate the otherwise
invisible solar interior. The seismic approach is indispensable for the
study of internal structure and evolution of the sun. It is even more
so, however, for the study of dynamical aspects of the sun, because
of the lack of other reliable means. The current status of seismology
of solar rotation is reviewed and outstanding problems are discussed.
Title: Seismology of Solar Internal Rotation
Authors: Sekii, Takashi
Bibcode: 2003IAUJD..12E..36S
Altcode:
Helioseismologists have been highly successful in revealing how
the solar interior differentially rotates through observations of
rotationally split eigen p modes of the sun. What we currently know
about the internal rotation of the sun including how it varies over
the solar cycle is reviewed. Seismic inference of internal rotation
based on low-degree modes is also discussed.
Title: Acoustic Tomography of the Sun's Interior with SDO:
Possibilities and Limitations
Authors: Kosovichev, A. G.; Duvall, T. L.; Birch, A. C.; Gizon, L.;
Zhao, J.; Sekii, T.; Shibahashi, H.
Bibcode: 2002AGUFMSH21C..06K
Altcode:
Helioseismic and Magnetic Imager on board SDO will significantly expand
the possibilities for imaging plasma flows and structures in the solar
interior. It will provide for the first time high-resolution data for
continuous monitoring of emerging flux and developing active regions in
the upper convection zone. It will also allow us to look for localized
structures and flows in the tachocline region and deeper interior,
and also investigate the near-polar regions. The expected results may
give important clues of how the solar dynamo works and active regions
develop. HMI will observe the entire spectrum of the solar acoustic
and surface gravity waves, and provide the most comprehensive data
for global and local helioseismology. The high-frequency part of the
oscillation spectrum will be used for studying seismic properties of
the solar atmosphere in the quiet Sun and active regions. The method
of acoustic tomography or time-distance helioseismology is one of the
primary tools of the HMI investigation. It is based on measurements and
inversions of travel-time delays of solar waves, caused by plasma flows
and variations of temperature and magnetic field. The data analysis
requires deep understanding of the physics of wave propagation in the
Sun and substantial computer resources. One of the important goals
is to provide the flow and sound-speed maps of the upper convection
zone in near-real time for space weather applications. We present the
current status of the field, and discuss plans and challenges for the
HMI data analyses and interpretation.
Title: On the effect of error correlation on linear inversions
Authors: Gough, D. O.; Sekii, T.
Bibcode: 2002MNRAS.335..170G
Altcode:
We have examined the effect on linear helioseismic inversions of
correlations in data errors, taking an example from one-dimensional
rotational splitting inversion. Artificial data with correlated errors
were generated and then inverted with or without using the proper
covariance matrix. The effects of using incorrect covariance matrices,
on solutions as well as on trade-offs, are discussed. It is found
that improper account of the correlations can be deleterious to the
faithfulness of the inversions, and yields incorrect error estimates,
which under some circumstances can lead to misleading inferences.
Title: What have we learnt with the Luminosity Oscillations Imager
over the past 6 years?
Authors: Appourchaux, T.; Andersen, B.; Sekii, T.
Bibcode: 2002ESASP.508...47A
Altcode: 2002soho...11...47A
We summarize what we achieved with 6 years of LOI data. We present
old as well as new results regarding the p-mode parameters dependence
upon solar activity. We have also derived the dependence of the solar
background noise upon solar activity and solar disk position. Inversions
done using LOI frequencies and higher-degree mode frequencies from
GONG confirm previous velocity inversions.
Title: Local-area helioseismology by SOT on-board Solar-B
Authors: Sekii, T.; Shibahashi, H.; Kosovichev, A. G.; Duvall, T. L.,
Jr.; Berger, T. E.; Bush, R.; Scherrer, P. H.
Bibcode: 2001ESASP.464..327S
Altcode: 2001soho...10..327S
Solar-B satellite, a successor to Yohkoh, will be launched
in 2005. Placed in a sun-synchronous orbit, it will carry out
multi-wavelength observation in optical, EUV and X-ray ranges. One of
the instruments on Solar-B, Solar Optical Telescope (SOT), a Japan/US
collaboration, aims at measuring the magnetic field and the Doppler
velocity field in the solar photosphere. Although it is not specifically
designed for helioseismic observations, the high-resolution Dopplergram
produced by SOT is potentially a very powerful tool for detailed
seismic investigation of subsurface magnetic and thermal structures
and associated mass flows. If successful, these measurements will be
an important contribution to the main goal of the Solar-B project:
understanding the origin and dynamics of the basic magnetic structures
and their effects on the solar corona. We discuss the prospect and
challenges of local-area helioseismology by SOT.
Title: Comparison of phase inversion and time-distance analysis of
one-dimensional artificial seismic data
Authors: Gough, D. O.; Sekii, T.; Toomre, J.
Bibcode: 2001ESASP.464..207G
Altcode: 2001soho...10..207G
We have studied the application of a phase inversion technique (Gough,
Merryfield and Toomre 1991, 1993, 1998; Gough, Sekii and Toomre 1998,
2000) to stochastically excited damped oscillations in an inhomogeneous
one-dimensional loop, using artificial seismic data (Gough, Sekii and
Toomre 2000). It has been found that within a parameter range that might
be relevant to the solar case, strong damping can significantly degrade
inversions for the wave propagation speed. Here we analyse the same
artificial data by a time-distance technique, to compare how the two
techniques differ in their response to the presence of strong damping.
Title: g-mode detection: Where do we stand?
Authors: Appourchaux, T.; Andersen, B.; Berthomieu, G.; Chaplin, W.;
Elsworth, Y.; Finsterle, W.; Frölich, C.; Gough, D. O.; Hoeksema,
T.; Isaak, G.; Kosovichev, A.; Provost, J.; Scherrer, P.; Sekii, T.;
Toutain, T.
Bibcode: 2001ESASP.464..467A
Altcode: 2001soho...10..467A
We review the recent developments in determining the upper limits to
g-mode amplitudes obtained by SOHO instruments, GONG and BiSON. We
address how this limit can be improved by way of new helioseismic
instruments and/or new collaborations, hopefully providing in the not
too distant future unambiguous g-mode detection.
Title: On measuring low-degree p-mode frequency splitting with
full-disc integrated data
Authors: Appourchaux, T.; Chang, H. -Y.; Gough, D. O.; Sekii, T.
Bibcode: 2000MNRAS.319..365A
Altcode:
The standard method of measuring rotational splitting from solar
full-disc oscillation data, based on maximum-likelihood fitting of
multi-Lorentzian profiles to oscillation power spectra, systematically
overestimates the splitting. One of the reasons is that the maximum
likelihood estimators (MLE) become unbiased only asymptotically as
the number of data tends to infinity; for a finite data set they
are often biased, inducing a systematic error. In this paper we
assess by Monte Carlo simulations the amount of systematic error
in the splitting measurement, using artificially generated power
spectra. The simulations are carried out for multiplets of degree
S(t) = ΣkSk(t), l=1, 2 and 3 with various
signal-to-noise ratios, linewidths and observing times. We address
the possible use of non-MLE estimators that could provide a smaller
or negligible systematic error. The implication for asteroseismology
is also discussed.
Title: Solar Interior: Inversions of Helioseismic Data
Authors: Sekii, T.
Bibcode: 2000eaa..bookE2248S
Altcode:
Once the eigenfrequencies of the solar oscillations are measured (see
HELIOSEISMIC OBSERVATIONS), they are analysed further to investigate
the solar interior. For example, we can use these frequencies to reveal
how the adiabatic soundspeed varies with depth, or how the interior
of the Sun rotates differentially, both in depth and in latitude. This
procedure is called inversion, because the proced...
Title: Phase inversion of one-dimensional artificial seismic data
Authors: Gough, D. O.; Sekii, T.; Toomre, J.
Bibcode: 2000SoPh..195....1G
Altcode:
Oscillations of an inhomogeneous one-dimensional loop have been
simulated for the purpose of examining the effect of excitation and
damping on the sound-speed inversion based on phase analysis. It has
been demonstrated that the procedure is robust against the realization
noise arising from frequent, stochastic excitation of weakly damped
waves, but that strong damping can spoil the inversion.
Title: Observational Upper Limits to Low-Degree Solar g-Modes
Authors: Appourchaux, T.; Fröhlich, C.; Andersen, B.; Berthomieu, G.;
Chaplin, W. J.; Elsworth, Y.; Finsterle, W.; Gough, D. O.; Hoeksema,
J. T.; Isaak, G. R.; Kosovichev, A. G.; Provost, J.; Scherrer, P. H.;
Sekii, T.; Toutain, T.
Bibcode: 2000ApJ...538..401A
Altcode:
Observations made by the Michelson Doppler Imager (MDI) and Variability
of solar IRradiance and Gravity Oscillations (VIRGO) on the Solar and
Heliospheric Observatory (SOHO) and by the ground-based Birmingham
Solar Oscillations Network (BiSON) and Global Oscillations Network
Group (GONG) have been used in a concerted effort to search for solar
gravity oscillations. All spectra are dominated by solar noise in the
frequency region from 100 to 1000 μHz, where g-modes are expected to be
found. Several methods have been used in an effort to extract any g-mode
signal present. These include (1) the correlation of data-both full-disk
and imaged (with different spatial-mask properties)-collected over
different time intervals from the same instrument, (2) the correlation
of near-contemporaneous data from different instruments, and (3) the
extraction-through the application of complex filtering techniques-of
the coherent part of data collected at different heights in the solar
atmosphere. The detection limit is set by the loss of coherence
caused by the temporal evolution and the motion (e.g., rotation)
of superficial structures. Although we cannot identify any g-mode
signature, we have nevertheless set a firm upper limit to the amplitudes
of the modes: at 200 μHz, they are below 10 mm s-1 in
velocity, and below 0.5 parts per million in intensity. The velocity
limit corresponds very approximately to a peak-to-peak vertical
displacement of δR/Rsolar=2.3×10-8 at the
solar surface. These levels which are much lower than prior claims,
are consistent with theoretical predictions.
Title: Prospects for Measuring Differential Rotation in White Dwarfs
through Asteroseismology
Authors: Kawaler, Steven D.; Sekii, Takashi; Gough, Douglas
Bibcode: 1999ApJ...516..349K
Altcode: 1998astro.ph.11286K
We examine the potential of asteroseismology for exploring the internal
rotation of white dwarf stars. Data from global observing campaigns
have revealed a wealth of frequencies, some of which show the signature
of rotational splitting. Tools developed for helioseismology to use
many solar p-mode frequencies for inversion of the rotation rate with
depth are adapted to the case of more limited numbers of modes of
low degree. We find that the small number of available modes in white
dwarfs, coupled with the similarity between the rotational-splitting
kernels of the modes, renders direct inversion unstable. Accordingly,
we adopt what we consider to be plausible functional forms for the
differential rotation profile; this is sufficiently restrictive to
enable us to carry out a useful calibration. We show examples of this
technique for PG 1159 stars and pulsating DB white dwarfs. Published
frequency splittings for white dwarfs are currently not accurate
enough for meaningful inversions; reanalysis of existing data can
provide splittings of sufficient accuracy when the frequencies of
individual peaks are extracted via least-squares fitting or multipeak
decompositions. We find that, when mode trapping is evident in the
period spacing of g modes, the measured splittings can constrain dΩ/dr.
Title: Phase Inversion of One-Dimensional Artificial Seismic Data
Authors: Gough, D. O.; Sekii, T.; Toomre, J.
Bibcode: 1999soho....9E..30G
Altcode:
It has been pointed out (Sekii 1997; Gough, Sekii & Toomre 1998,
1999) that the application of the phase inversion technique (Gough,
Merryfield & Toomre 1991, 1993, 1998) to the solar high-degree
sectoral data, for detecting the background inhomogeneity in the
solar equatorial region, may be facing difficulty arising from the
presence of excitation and damping. We have studied the implication of
the effects of these, using a simple model of stochastically excited
damped oscillations of a one-dimensional loop. The results of phase
inversions for various cases will be presented and various strategies
to overcome the difficulty will be discussed.
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.
Bibcode: 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.
Title: The Solar Internal Rotation from GOLF Splittings
Authors: Corbard, T.; Di Mauro, M. P.; Sekii, T.; GOLF Team
Bibcode: 1998ESASP.418..741C
Altcode: 1998soho....6..741C; 1998astro.ph..6254C
The low degree splittings obtained from one year of GOLF data analysis
are combined with the MDI medium-l 144-day splittings in order to
infer the solar internal rotation as a function of the radius down to
0.2R_sun. Several inverse methods are applied to the same data and the
uncertainties on the solution as well as the resolution reachable are
discussed. The results are compared with the one obtained from the
low degree splittings estimated from GONG network.
Title: Helioseismic Determination of the Solar Tachocline Thickness
Authors: Elliott, J. R.; Gough, D. O.; Sekii, T.
Bibcode: 1998ESASP.418..763E
Altcode: 1998soho....6..763E
Inversions of rotational splitting data reveal the existence of a
shear layer at the base of the convection zone, which is known as the
tachocline. The angular velocity in the convection zone varies with
latitude, whereas the angular velocity in the outer layers of the
radiative envelope is more nearly uniform. The differential rotation
of the convection zone is no doubt driven by anisotropic Reynolds
stresses associated with the turbulent convection. It is likely
that the radiative envelope is held rigid by a large-scale magnetic
field. The transition between the two is too sharp to be resolved
by the seismic splitting data. However, it must necessarily support
an Ekman circulation (Spiegel & Zahn, 1992), which advects heavy
elements that have settled under gravity back into the convection zone,
causing an abrupt discontinuity in chemical composition at the base of
the tachocline, thereby modifying the hydrostatic stratification. We
have calibrated theoretical solar models incorporating the tachocline
mixing against the latest seismic data from SOI/MDI, from which we have
obtained a value for the tachocline thickness which is substantially
more precise than the formal resolving power of the seismic data.
Title: Some Comments on Phase Inversions
Authors: Gough, D. O.; Sekii, T.; Toomre, J.
Bibcode: 1998ESASP.418..789G
Altcode: 1998soho....6..789G
The method of phase inversion have been proposed and tested for simple
cases by Gough, Merryfield and Toomre(1991,1993,1998) for detection of
inhomogeneity in media by observing wave propagation. We discuss some of
the difficulties that are encountered with the procedure in practice,
and what might be done to overcome them in transferring the technique
to the solar case, such as in the study of horizontal inhomogeneity in
the solar cavity along the equator using the MDI sectoral-mode data. A
complication seems to arise, aside from observational problems, from
the fact that the waves are not only scattered by inhomogeneity, but
are also excited and damped, as is observed in the broadening of the
ridges in the k-ω diagram.
Title: Observational Upper Limits for Low-Degree Solar g-modes
Authors: Fröhlich, C.; Finsterle, W.; Andersen, B.; Appourchaux, T.;
Chaplin, W. J.; Elsworth, Y.; D. O. Gough; Hoeksema, J. T.; Isaak,
G. R.; Kosovichev, A. G.; Provost, J.; Scherrer, P. H.; Sekii, T.;
Toutain, T.
Bibcode: 1998ESASP.418...67F
Altcode: 1998soho....6...67F
No abstract at ADS
Title: Recent topics in helioseismology.
Authors: Sekii, T.
Bibcode: 1998AstHe..91...92S
Altcode:
Selected topics from the recent development in helioseismology are
reviewed. The emphasis is on those findings from the SOHO satellite.
Title: On the Influence of Unaccounted Error Correlations of Linear
Inversions
Authors: Gough, D. O.; Sekii, T.
Bibcode: 1998ESASP.418..787G
Altcode: 1998soho....6..787G
Continuing our previous study (Gough 1996, also see Kosovichev et
al. 1995) on the effects of correlated errors on linear inversions,
we investigate how the optimally localized averaging method and the
regularized least-square fitting method, both of which are `optimal'
in different ways, respond differently to the use of wrong covariance
matrices in one-dimensional rotation inversions. Our results demonstrate
how important it is to exercise care in estimating error correlations
when trying to extract mode frequencies from helioseismic data.
Title: Comparative Studies of Low-Order and Low-Degree Solar p Modes
Authors: Appourchaux, T.; Andersen, B.; Chaplin, W.; Elsworth, Y.;
Finsterle, W.; Frohlich, C.; Gough, D.; Hoeksema, J. T.; Isaak, G.;
Kosovichev, A.; Provost, J.; Scherrer, P.; Sekii, T.; Toutain, T.
Bibcode: 1998ESASP.418...95A
Altcode: 1998soho....6...95A
The amplitudes of solar p-modes decrease steeply with decreasing
radial order below about 17. The background solar signal (solar noise)
in general increases steadily with decreasing frequency. For the
irradiance and radiance measurements with VIRGO or SOI/MDI on SOHO this
combination makes it difficult to detect low degree modes below about
1.8 mHz. The solar noise as observed in velocity with SOI/MDI or the
ground based BISON network is significantly lower in this region than
in intensity measurements. This allows low degree modes to be observed
close to 1 mHz. We present results of detection and charaterization
of the lowest order observable p-modes both in velocity and intensity
measurements. Where applicable the properties of the modes observed
with the two methods are compared.
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.
Bibcode: 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: Structure and Rotation of the Solar Interior: Initial Results
from the MDI Medium-L Program
Authors: Kosovichev, A. G.; Schou, J.; Scherrer, P. H.; Bogart, R. S.;
Bush, R. I.; Hoeksema, J. T.; Aloise, J.; Bacon, L.; Burnette, A.; de
Forest, C.; Giles, P. M.; Leibrand, K.; Nigam, R.; Rubin, M.; Scott,
K.; Williams, S. D.; Basu, Sarbani; Christensen-Dalsgaard, J.; Dappen,
W.; Rhodes, E. J., Jr.; Duvall, T. L., Jr.; Howe, R.; Thompson, M. J.;
Gough, D. O.; Sekii, T.; Toomre, J.; Tarbell, T. D.; Title, A. M.;
Mathur, D.; Morrison, M.; Saba, J. L. R.; Wolfson, C. J.; Zayer, I.;
Milford, P. N.
Bibcode: 1997SoPh..170...43K
Altcode:
The medium-l program of the Michelson Doppler Imager instrument on board
SOHO provides continuous observations of oscillation modes of angular
degree, l, from 0 to ∽ 300. The data for the program are partly
processed on board because only about 3% of MDI observations can be
transmitted continuously to the ground. The on-board data processing,
the main component of which is Gaussian-weighted binning, has been
optimized to reduce the negative influence of spatial aliasing of the
high-degree oscillation modes. The data processing is completed in a
data analysis pipeline at the SOI Stanford Support Center to determine
the mean multiplet frequencies and splitting coefficients. The initial
results show that the noise in the medium-l oscillation power spectrum
is substantially lower than in ground-based measurements. This enables
us to detect lower amplitude modes and, thus, to extend the range of
measured mode frequencies. This is important for inferring the Sun's
internal structure and rotation. The MDI observations also reveal the
asymmetry of oscillation spectral lines. The line asymmetries agree
with the theory of mode excitation by acoustic sources localized in the
upper convective boundary layer. The sound-speed profile inferred from
the mean frequencies gives evidence for a sharp variation at the edge
of the energy-generating core. The results also confirm the previous
finding by the GONG (Gough et al., 1996) that, in a thin layer just
beneath the convection zone, helium appears to be less abundant than
predicted by theory. Inverting the multiplet frequency splittings from
MDI, we detect significant rotational shear in this thin layer. This
layer is likely to be the place where the solar dynamo operates. In
order to understand how the Sun works, it is extremely important to
observe the evolution of this transition layer throughout the 11-year
activity cycle.
Title: Structure inversions with the VIRGO data
Authors: Appourchaux, Th.; Sekii, Takashi; Gough, Douglas; Lee, Umin;
Wehrli, Christoph; Virgo Team
Bibcode: 1997IAUS..181..159A
Altcode:
No abstract at ADS
Title: Internal structure and rotation of the Sun: First results
from MDI data
Authors: Kosovichev, A. G.; Schou, J.; Scherrer, P. H.; Bogart, R. S.;
Bush, R. I.; Hoeksema, J. T.; Aloise, J.; Bacon, L.; Burnette, A.;
De Forest, C.; Giles, P. M.; Leibrand, K.; Nigam, R.; Rubin, M.;
Scott, K.; Williams, S. D.; Basu, Sarbani; Christensen-Dalsgaard,
J.; Däppen, W.; Rhodes, E. J., Jr.; Duvall, T. L., Jr.; Howe, R.;
Thompson, M. J.; Gough, D. O.; Sekii, T.; Toomre, J.; Tarbell, T. D.;
Title, A. M.; Mathur, D.; Morrison, M.; Saba, J. L. R.; Wolfson,
C. J.; Zayer, I.; Milford, P. N.
Bibcode: 1997IAUS..181..203K
Altcode:
No abstract at ADS
Title: Internal Solar rotation
Authors: Sekii, T.
Bibcode: 1997IAUS..181..189S
Altcode:
No abstract at ADS
Title: First results from VIRGO on SoHO
Authors: Frohlich, C.; Andersen, B. N.; Appourchaux, T.; Berthomieu,
G.; Crommelynck, D. A.; Domingo, V.; Fichot, A.; Finsterle, W.;
Gómez, M. F.; Gough, D.; Jiménez, A.; Leifsen, T.; Lombaerts, M.;
Pap, J. M.; Provost, J.; Roca Cortés, T.; Romero, J.; Roth, H. -J.;
Sekii, T.; Telljohann, U.; Toutain, T.; Wehrli, C.
Bibcode: 1997IAUS..181...67F
Altcode:
No abstract at ADS
Title: First Results from VIRGO, the Experiment for Helioseismology
and Solar Irradiance Monitoring on SOHO
Authors: Fröhlich, Claus; Andersen, Bo N.; Appourchaux, Thierry;
Berthomieu, Gabrielle; Crommelynck, Dominique A.; Domingo, Vicente;
Fichot, Alain; Finsterle, Wolfgang; Gómez, Maria F.; Gough, Douglas;
Jiménez, Antonio; Leifsen, Torben; Lombaerts, Marc; Pap, Judit M.;
Provost, Janine; Roca Cortés, Teodoro; Romero, José; Roth, Hansjörg;
Sekii, Takashi; Telljohann, Udo; Toutain, Thierry; Wehrli, Christoph
Bibcode: 1997SoPh..170....1F
Altcode:
First results from the VIRGO experiment (Variability of solar IRradiance
and Gravity Oscillations) on the ESA/NASA Mission SOHO (Solar and
Heliospheric Observatory) are reported. The observations started
mid-January 1996 for the radiometers and sunphotometers and near the
end of March for the luminosity oscillation imager. The performance of
all the instruments is very good, and the time series of the first 4-6
months are evaluated in terms of solar irradiance variability, solar
background noise characteristics and p-mode oscillations. The solar
irradiance is modulated by the passage of active regions across the
disk, but not all of the modulation is straightforwardly explained in
terms of sunspot flux blocking and facular enhancement. Helioseismic
inversions of the observed p-mode frequencies are more-or-less in
agreement with the latest standard solar models. The comparison of
VIRGO results with earlier ones shows evidence that magnetic activity
plays a significant role in the dynamics of the oscillations beyond
its modulation of the resonant frequencies. Moreover, by comparing
the amplitudes of different components ofp -mode multiplets, each of
which are influenced differently by spatial inhomogeneity, we have
found that activity enhances excitation.
Title: The seismic structure of the Sun from GONG
Authors: Anderson, E.; Antia, H. M.; Basu, S.; Chaboyer, B.; Chitre,
S. M.; Christensen-Dalsgaard, J.; Eff-Darwich, A.; Elliott, J. R.;
Giles, P. M.; Gough, D. O.; Guzik, J. A.; Harvey, J. W.; Hill,
F.; Leibacher, J. W.; Kosovichev, A. G.; Monteiro, M. J. P. F. G.;
Richard, O.; Sekii, T.; Shibahashi, H.; Takata, M.; Thompson, M. J.;
Toomre, J.; Vauclair, S.; Vorontsov, S. V.
Bibcode: 1997IAUS..181..151A
Altcode:
No abstract at ADS
Title: The Seismic Structure of the Sun
Authors: Gough, D. O.; Kosovichev, A. G.; Toomre, J.; Anderson,
E.; Antia, H. M.; Basu, S.; Chaboyer, B.; Chitre, S. M.;
Christensen-Dalsgaard, J.; Dziembowski, W. A.; Eff-Darwich, A.;
Elliott, J. R.; Giles, P. M.; Goode, P. R.; Guzik, J. A.; Harvey,
J. W.; Hill, F.; Leibacher, J. W.; Monteiro, M. J. P. F. G.; Richard,
O.; Sekii, T.; Shibahashi, H.; Takata, M.; Thompson, M. J.; Vauclair,
S.; Vorontsov, S. V.
Bibcode: 1996Sci...272.1296G
Altcode:
Global Oscillation Network Group data reveal that the internal
structure of the sun can be well represented by a calibrated standard
model. However, immediately beneath the convection zone and at the
edge of the energy-generating core, the sound-speed variation is
somewhat smoother in the sun than it is in the model. This could be a
consequence of chemical inhomogeneity that is too severe in the model,
perhaps owing to inaccurate modeling of gravitational settling or to
neglected macroscopic motion that may be present in the sun. Accurate
knowledge of the sun's structure enables inferences to be made about
the physics that controls the sun; for example, through the opacity,
the equation of state, or wave motion. Those inferences can then be
used elsewhere in astrophysics.
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.
Bibcode: 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: Inferring Spatial Variation of Solar Properties from
Helioseismic Data
Authors: Gough, D. O.; Sekii, T.; Stark, P. B.
Bibcode: 1996ApJ...459..779G
Altcode:
A common method to infer that solar properties vary with position is
to compare linear estimates of averages of those properties centered
at different locations. If some of the confidence intervals for the
averages do not overlap, one concludes that the property varies. In
order for this conclusion to be statistically valid, the lengths of
the intervals must be adjusted to obtain the correct "simultaneous
coverage probability." We illustrate the notion of simultaneous coverage
probability using coin tossing as an example. We present four methods
for adjusting the lengths of confidence intervals for linear estimates,
and a complementary approach to infer changes based on constructing
a linear estimator that is directly sensitive to changes. The first
method for constructing simultaneous confidence intervals is based
on Bonferroni's inequality, and applies generally to confidence
intervals for any set of parameters, from dependent or independent
observations. The second method is based on a 2 measure of fit to the
data, which allows one to compute s1multaneous confidence intervals for
any number of linear functionals of the model. The third method uses a
^{2 }distribution in the space of estimates, which yields "Scheffe'"
confidence intervals for the functionals. The fourth method, which
produces the shortest confidence intervals, uses the infinity-norm
in the space of estimates to construct "maximum-modulus" confidence
intervals. We apply the four methods to search for radial changes in
averages of solar angular velocity, using data from Big Bear Solar
Observatory (BBSO) averaged for the 4 yr 1986, 1988-1990. Finally,
we apply the new differencing estimator to the BBSO data, finding
strong evidence that the average solar angular velocity is lower near
the poles than near the equator over a range of depths, as is observed
at the surface as well.
Title: On Frequency Determination and Temporal Data Sampling
Authors: Chang, H. -Y.; Gough, D.; Sekii, T.
Bibcode: 1995ESASP.376b.175C
Altcode: 1995soho....2..175C; 1995help.confP.175C
The authors have further developed the procedure described by Chang
and Gough (1995) in GONG94 to analyse low-degree helioseismic time
series. An idealized model was used to construct artifical data
representing a superposition of undamped oscillations in a cavity which
is varying slowly with time in an unknown manner. The procedure involves
matching a model of the data to the actual data by minimizing a measure
of misfit. This requires a multidimensional parameter search which,
if a generic procedure such as the simplex algorithm used for GONG94
is employed, is computationally expensive. The authors have developed
an efficient scheme tailored explicitly to the problem in hand, which
uses a combination of an approximate factorization of the parameter
space with an alternation of a jumping algorithm between local minima
and Newton-Ralphson iteration.
Title: Observational Constraints on the Internal Solar Angular
Velocity
Authors: Sekii, T.; Genovese, C. R.; Gough, D. O.; Stark, P. B.
Bibcode: 1995ESASP.376b.279S
Altcode: 1995help.confP.279S; 1995soho....2..279S
No abstract at ADS
Title: The 1 ⊗ 1 Rotation Inversion with Expansion Coefficients
Authors: Sekii, T.
Bibcode: 1995ESASP.376b.285S
Altcode: 1995help.confP.285S; 1995soho....2..285S
No abstract at ADS
Title: Linear Inversions in Helioseismology: Testing New
Regularization Techniques for Solving Large Systems of Linear
Algebraic Equations
Authors: Strakhov, V. N.; Vorontsov, S. V.; Sekii, T.
Bibcode: 1995ASPC...76..492S
Altcode: 1995gong.conf..492S
No abstract at ADS
Title: Angular Inversions of Rotational Splitting Coefficients
Authors: Sekii, T.
Bibcode: 1995ASPC...76...74S
Altcode: 1995gong.conf...74S
No abstract at ADS
Title: Asymptotic Signatures of Jovian Discontinuities
Authors: Gough, D. O.; Sekii, T.
Bibcode: 1995ASPC...76..374G
Altcode: 1995gong.conf..374G
No abstract at ADS
Title: Working Group 9 - Interior Structure and Inversions
Authors: Kosovichev, A. G.; Basu, S.; Christensen-Dalsgaard, J.;
Eff-Darwich, A.; Gough, D. O.; Iglesias, C. A.; Pérez-Hernández,
F.; Rogers, F.; Sekii, T.; Shibahashi, H.
Bibcode: 1995ESASP.376a.211K
Altcode: 1995heli.conf..211K
No abstract at ADS
Title: Inversions of BBSO Rotational Splitting Data
Authors: Sekii, T.; Gough, D. O.; Kosovichev, A. G.
Bibcode: 1995ASPC...76...59S
Altcode: 1995gong.conf...59S
No abstract at ADS
Title: How Regularization Influences Rotation Inversion
Authors: Sekii, T.
Bibcode: 1995ASPC...76...67S
Altcode: 1995gong.conf...67S
No abstract at ADS
Title: Invitation to helioseismology II.
Authors: Shibahashi, H.; Sekii, T.
Bibcode: 1993AstHe..86..461S
Altcode:
No abstract at ADS
Title: A new strategy for 2D inversion for solar rotation
Authors: Sekii, Takashi
Bibcode: 1993MNRAS.264.1018S
Altcode:
In the linearized integral formula for rotational splitting of solar
p-mode frequencies, the dominant term in the kernel is a product of
two terms, one depending on the radial coordinate alone, and the other
depending solely on the colatitude. By retaining only this term in the
equation, two-dimensional inverse techniques for the solar internal
rotation can be formulated in such a way that the angular and radial
inversions can be carried out separately. Thus the 2D inversion is
reduced to two successive 1D inversions. This strategy leads to a
dramatic reduction in computational burden when one is inverting
huge data sets. It is also possible to improve the quality of the
inversions significantly by a modification to the radial inversion
procedure. In this paper, regularized least-squares data fitting and
a modified optimally localized averaging are applied to the angular
inversion and the radial inversion, respectively. The quality of the
inversions is illustrated by means of the resulting averaging kernels.
Title: Invitation to helioseismology I.
Authors: Shibahashi, H.; Sekii, T.
Bibcode: 1993AstHe..86..416S
Altcode:
No abstract at ADS
Title: Seismic evidence of modulation of the structure and angular
velocity of the Sun associated with the solar cycle
Authors: Gough, D. O.; Kosovichev, A. G.; Sekii, T.; Libbrecht, K. G.;
Woodard, M. F.
Bibcode: 1993ASPC...40...93G
Altcode: 1993IAUCo.137...93G; 1993ist..proc...93G
No abstract at ADS
Title: The Form of the Angular Velocity in the Solar Convection Zone
Authors: Gough, D. O.; Kosovichev, A. G.; Sekii, T.; Libbrecht, K. G.;
Woodard, M. F.
Bibcode: 1993ASPC...42..213G
Altcode: 1993gong.conf..213G
No abstract at ADS
Title: on the Detection of Convective Overshoot
Authors: Gough, D. O.; Sekii, T.
Bibcode: 1993ASPC...42..177G
Altcode: 1993gong.conf..177G
No abstract at ADS
Title: A procedure for two-dimensional asymptotic rotational-splitting
inversion
Authors: Sekii, T.; Gough, D. O.
Bibcode: 1993ASPC...40..569S
Altcode: 1993IAUCo.137..569S; 1993ist..proc..569S
No abstract at ADS
Title: on an R/1XR/1 Inversion Technique for Solar Rotation
Authors: Sekii, T.
Bibcode: 1993ASPC...42..237S
Altcode: 1993gong.conf..237S
No abstract at ADS
Title: Two-Dimensional Inversion for Solar Internal Rotation
Authors: Sekii, Takashi
Bibcode: 1991PASJ...43..381S
Altcode:
The internal rotation of the sun is inferred by solving a fully
two-dimensional inverse problem of the rotational splitting data of
Libbrecht (1989, AAA 080.006). We solve a set of integral equations
which relate the rotational splittings of the eigenfrequencies of
the sun to the internal rotation rate of the sun, the latter being
considered as a function of both the distance from the center and the
latitude. For this purpose we suggest an inverse method and apply
it to the present problem. The integral equations are descretized
and reduced to a set of linear equations. First, we impose an
error-weighted least-squares condition with some boundary constraints
at the surface. Second, we impose a flatness condition: the average of
the first derivatives of the rotation rate is required to be as small as
possible under the first constraint. Inversions were carried out while
fully utilizing singular-value decomposition. In order to stabilize
the solution to observational and numeric errors, we discarded small
singular values. The results of numerical experiments show that the
best resolution is obtained within a depth range of 0.6 <~ r/R <~
0.9 in the low-latitude region. By inverting the observed splittings,
we found that the solar internal rotation in the low-latitude region
is slightly faster than the surface rotation rate in the outer ten
percent of the radius, and decreases to about five percent less than
the surface rotation rate within the lower levels of the convection
zone. In the region 0.7 <~ r/R <~ 0.8, the rotation rate within
the low-latitude zone is as fast as the surface rate, but is slower
than the surface rate at r/R <~ 0.7. The rotation rate within the
mid-latitude region decreases with depth in the outer twenty percent
of the radius, and gradually increases inward to be faster than the
surface rate.
Title: Two-Dimensional Inversion of Rotational Splitting Data
Authors: Sekii, Takashi
Bibcode: 1990LNP...367..337S
Altcode: 1990psss.conf..337S
We solve a two-dimensional inverse problem of the rotational frequency
splitting to infer the rotation rate in the sun as a function of both
the radius and the latitude. We use Libbrecht's (1989) observational
data of the solar p-mode frequency splitting. We discretize a set
of linear integral equations for rotational splittings and reduce
them a set of linear algebraic equations. We solve the resultant
algebraic equations by imposing an error-weighted least squares
condition cooperated with boundary constraint at the surface. In order
to stabilize the solution to observational and numerical errors, we
discard small singular values of the coefficient matrix, and this keeps
some parameters undetermined. To determine these parameters we impose
a flatness condition. The inverted results show the solar internal
rotation becomes slower at low latitudes and faster at high latitudes
with increasing depth. The most significant deviation from this trend
is that rotation is slow at low latitudes in the convection zone.
Title: An asymptotic inversion method of inferring the sound velocity
distribution in the sun from the spectrum of p-mode oscillations
Authors: Sekii, Takashi; Shibahashi, Hiromoto
Bibcode: 1989PASJ...41..311S
Altcode:
An inversion method for inferring the sound velocity distribution in
the sun from its p-mode oscillation data is presented. The method is
applied to a set of theoretically calculated eigenfrequencies of a
solar model. It is shown that the method improves the accuracy and
effectiveness of the asymptotic inversion method of inferring the
internal structure of the sun, in particular, the deep interior of
the sun.
Title: An inversion method based on the Moore-Penrose generalized
inverse matrix.
Authors: Sekii, T.; Shibahashi, H.
Bibcode: 1988ESASP.286..521S
Altcode: 1988ssls.rept..521S
The authors present an inversion method based on solving the algebraic
equations by means of the Moore-Penrose generalized inverse matrix of
inferring the solar internal rotation, and evaluate its validity.
Title: Sound velocity distribution in the Sun inferred from the
asymptotic inversion of p-mode spectra.
Authors: Shibahashi, H.; Sekii, T.
Bibcode: 1988ESASP.286..471S
Altcode: 1988ssls.rept..471S
The authors solve the inverse problem of the solar p-mode spectrum
based on a recently developed asymptotic inversion method, and infer
the sound velocity distribution with the accuracy better than a few
percent. In this method, the equation governing high order p-mode
oscillations has been reduced to a form of the Schrödinger equation
in quantum mechanics. By using the quantization rule based on the
WKBJ method, the authors formulated an integral equation to infer
the form of the "acoustic potential". The acoustic potential itself
consists of two parts. One of them is dependent on the degree l and
the sound velocity and is the dominant term in the deep interior while
the other is independent of l and dominates in the outer envelope. By
examining the l-dependence of the acoustic potential thus inferred,
the authors separate these two terms and then they infer the sound
velocity distribution in the Sun. They apply this method to the real
observational data of Duvall et al. and Libbrecht and Kaufman and obtain
the sound velocity distribution in the Sun. They compare the result
with their model and find that there is a small discrepancy between
the model and the inferred result in the range of 0.3 < r/R < 0.4.
Title: White Dwarf Seismology - Inverse Problem of G-Mode Oscillations
Authors: Shibahashi, H.; Sekii, T.; Kawaler, S.
Bibcode: 1988LNP...305...86S
Altcode: 1988IAUCo.108...86S; 1988adse.conf...86S
No abstract at ADS
Title: Inverse Problem of Solar Oscillations
Authors: Sekii, Takashi; Shibahashi, Hiromoto
Bibcode: 1987LNP...274..322S
Altcode: 1987stpu.conf..322S
We present some preliminary results of numerical simulation to
infer the sound velocity distribution in the solar interior from the
oscillation data of the Sun as the inverse problem. We analyze the
acoustic potential itself by taking account of some factors other than
the sound velocity, and we can infer fairly well the sound velocity
distribution in the deep interior of the Sun.