Author name code: gizon
ADS astronomy entries on 2022-09-14
author:"Gizon, Laurent"
------------------------------------------------------------------------
Title: Evolution of dipolar mixed-mode coupling factor in red giant
stars: impact of buoyancy spike
Authors: Jiang, C.; Cunha, M.; Christensen-Dalsgaard, J.; Zhang,
Q. S.; Gizon, L.
Bibcode: 2022MNRAS.515.3853J
Altcode: 2022arXiv220709878J; 2022MNRAS.tmp.1954J
Mixed modes observed in red giants allow for investigation of the
stellar interior structures. One important feature in these structures
is the buoyancy spike caused by the discontinuity of the chemical
gradient left behind during the first dredge-up. The buoyancy spike
emerges at the base of the convective zone in low-luminosity red
giants and later becomes a glitch when the g-mode cavity expands
to encompass the spike. Here, we study the impact of the buoyancy
spike on the dipolar mixed modes using stellar models with different
properties. We find that the applicability of the asymptotic formalisms
for the coupling factor, q, varies depending on the location of the
evanescent zone, relative to the position of the spike. Significant
deviations between the value of q inferred from fitting the oscillation
frequencies and either of the formalisms proposed in the literature are
found in models with a large frequency separation in the interval 5-15
μHz, with evanescent zones located in a transition region that may be
thin or thick. However, it is still possible to reconcile q with the
predictions from the asymptotic formalisms, by choosing which formalism
to use according to the value of q. For stars approaching the luminosity
bump, the buoyancy spike becomes a glitch and strongly affects the mode
frequencies. Fitting the frequencies without accounting for the glitch
leads to unphysical variations in the inferred q, but we show that this
is corrected when properly accounting for the glitch in the fitting.
Title: Erratum: "Faculae Cancel out on the Surfaces of Active Suns"
(2022, ApJL, 934, L23)
Authors: Nèmec, N. -E.; Shapiro, A. I.; Işık, E.; Sowmya, K.;
Solanki, S. K.; Krivova, N. A.; Cameron, R. H.; Gizon, L.
Bibcode: 2022ApJ...936L..17N
Altcode:
No abstract at ADS
Title: KIC 6951642: confirmed Kepler $\gamma$ Doradus-$\delta$ Scuti
star with intermediate to fast rotation in a possible single-lined
binary system
Authors: Samadi-Ghadim, A.; Lampens, P.; Gizon, L.
Bibcode: 2022arXiv220904651S
Altcode:
KIC 6951642 has been reported as a candidate hybrid pulsator of
type-$\gamma$ Doradus-$\delta$ Scuti from observations of the first
quarters of the Kepler mission. We aim to investigate the pulsating
nature of KIC 6951642 and to search for the signature of rotation and/or
activity in the light curves. We performed an iterative frequency
search of both Fourier spectra, and searched for regular patterns
in them. We applied spectrum synthesis to determine the atmospheric
stellar parameters. Since KIC 6951642 was reported to belong to a
spectroscopic binary system, we fitted the time delays derived from
the light curves with the radial velocities obtained from published
as well as new spectra in an attempt to improve the quality of the
first orbit. Follow-up spectroscopy showed that KIC 6951642 is a
fast-rotating F0-type star in a possible single-lined binary with a
period of $\sim$4.8 yr. In the low-frequency regime, we identified
the frequencies of 0.721 d$^{-1}$ as well as of 0.0087 d$^{-1}$. We
attribute the first frequency to stellar rotation and the second one
to stellar activity with a cycle. We also detected $g$ modes, with the
strongest mode located at 2.238 d$^{-1}$, as well as three asymmetric
multiplets (with a mean spacing of 0.675$\pm$0.044 d$^{-1}$). In
the high-frequency regime, we detected frequencies of type-$\delta$
Scuti, with the strongest mode located at 13.96 d$^{-1}$, as well as
seven asymmetric multiplets (with a mean spacing of 0.665$\pm$0.084
d$^{-1}$). We subsequently identified a few more frequencies that
appear to be combinations of a $g$ or $p$ mode and one of the higher
cited frequencies not due to pulsations. We propose that KIC 6951642
accommodates for a fast-rotating $\gamma$ Dor-$\delta$ Sct hybrid star
with various rotationally split multiplets of $g$ and $p$ modes and that
it also displays a cycle lasting years of (possible) stellar activity.
Title: Faculae Cancel out on the Surfaces of Active Suns
Authors: Nèmec, N. -E.; Shapiro, A. I.; Işık, E.; Sowmya, K.;
Solanki, S. K.; Krivova, N. A.; Cameron, R. H.; Gizon, L.
Bibcode: 2022ApJ...934L..23N
Altcode: 2022arXiv220706816N
Surfaces of the Sun and other cool stars are filled with magnetic
fields, which are either seen as dark compact spots or more
diffuse bright structures like faculae. Both hamper detection and
characterization of exoplanets, affecting stellar brightness and
spectra, as well as transmission spectra. However, the expected facular
and spot signals in stellar data are quite different, for instance,
they have distinct temporal and spectral profiles. Consequently,
corrections of stellar data for magnetic activity can greatly benefit
from the insight on whether the stellar signal is dominated by spots or
faculae. Here, we utilize a surface flux transport model to show that
more effective cancellation of diffuse magnetic flux associated with
faculae leads to spot area coverages increasing faster with stellar
magnetic activity than that by faculae. Our calculations explain the
observed dependence between solar spot and facular area coverages and
allow its extension to stars that are more active than the Sun. This
extension enables anticipating the properties of stellar signal and its
more reliable mitigation, leading to a more accurate characterization
of exoplanets and their atmospheres.
Title: Viscous inertial modes on a differentially rotating sphere:
Comparison with solar observations
Authors: Fournier, Damien; Gizon, Laurent; Hyest, Laura
Bibcode: 2022A&A...664A...6F
Altcode: 2022arXiv220413412F
Context. In a previous paper, we studied the effect of latitudinal
rotation on solar equatorial Rossby modes in the β-plane
approximation. Since then, a rich spectrum of inertial modes has been
observed on the Sun, which is not limited to the equatorial Rossby
modes and includes high-latitude modes.
Aims: Here we extend
the computation of toroidal modes in 2D to spherical geometry using
realistic solar differential rotation and including viscous damping. The
aim is to compare the computed mode spectra with the observations and to
study mode stability.
Methods: At a fixed radius, we solved the
eigenvalue problem numerically using a spherical harmonics decomposition
of the velocity stream function.
Results: Due to the presence
of viscous critical layers, the spectrum consists of four different
families: Rossby modes, high-latitude modes, critical-latitude modes,
and strongly damped modes. For each longitudinal wavenumber m ≤ 3,
up to three Rossby-like modes are present on the sphere, in contrast
to the equatorial β plane where only the equatorial Rossby mode is
present. The least damped modes in the model have eigenfrequencies
and eigenfunctions that resemble the observed modes; the comparison
improves when the radius is taken in the lower half of the convection
zone. For radii above 0.75 R⊙ and Ekman numbers E <
10−4, at least one mode is unstable. For either m = 1 or
m = 2, up to two Rossby modes (one symmetric and one antisymmetric)
are unstable when the radial dependence of the Ekman number follows a
quenched diffusivity model (E ≈ 2 × 10−5 at the base
of the convection zone). For m = 3, up to two Rossby modes can be
unstable, including the equatorial Rossby mode.
Conclusions:
Although the 2D model discussed here is highly simplified, the
spectrum of toroidal modes appears to include many of the observed
solar inertial modes. The self-excited modes in the model have
frequencies close to those of the observed modes with the largest
amplitudes. Movies associated to Fig. 2 are available at https://www.aanda.org
Title: Impact of spatially correlated fluctuations in sunspots on
metrics related to magnetic twist
Authors: Baumgartner, C.; Birch, A. C.; Schunker, H.; Cameron, R. H.;
Gizon, L.
Bibcode: 2022A&A...664A.183B
Altcode: 2022arXiv220702135B
Context. The twist of the magnetic field above a sunspot is an
important quantity in solar physics. For example, magnetic twist
plays a role in the initiation of flares and coronal mass ejections
(CMEs). Various proxies for the twist above the photosphere have been
found using models of uniformly twisted flux tubes, and are routinely
computed from single photospheric vector magnetograms. One class of
proxies is based on αz, the ratio of the vertical current
to the vertical magnetic field. Another class of proxies is based on the
so-called twist density, q, which depends on the ratio of the azimuthal
field to the vertical field. However, the sensitivity of these proxies
to temporal fluctuations of the magnetic field has not yet been well
characterized.
Aims: We aim to determine the sensitivity of twist
proxies to temporal fluctuations in the magnetic field as estimated
from time-series of SDO/HMI vector magnetic field maps.
Methods:
To this end, we introduce a model of a sunspot with a peak vertical
field of 2370 Gauss at the photosphere and a uniform twist density
q = −0.024 Mm−1. We add realizations of the temporal
fluctuations of the magnetic field that are consistent with SDO/HMI
observations, including the spatial correlations. Using a Monte-Carlo
approach, we determine the robustness of the different proxies to the
temporal fluctuations.
Results: The temporal fluctuations of
the three components of the magnetic field are correlated for spatial
separations up to 1.4 Mm (more than expected from the point spread
function alone). The Monte-Carlo approach enables us to demonstrate that
several proxies for the twist of the magnetic field are not biased in
each of the individual magnetograms. The associated random errors on
the proxies have standard deviations in the range between 0.002 and
0.006 Mm−1, which is smaller by approximately one order
of magnitude than the mean value of q.
Title: Contribution of flows around active regions to the north-south
helioseismic travel-time measurements
Authors: Poulier, P. -L.; Liang, Z. -C.; Fournier, D.; Gizon, L.
Bibcode: 2022A&A...664A.189P
Altcode: 2022arXiv220610751P
Context. In local helioseismology, the travel times of acoustic waves
propagating in opposite directions along the same meridian inform us
about horizontal flows in the north-south direction. The longitudinal
averages of the north-south helioseismic travel-time shifts vary with
the sunspot cycle.
Aims: We aim to study the contribution of
inflows into solar active regions to this solar-cycle variation.
Methods: To do so, we identified the local flows around active regions
in the horizontal flow maps obtained from correlation tracking of
granulation in continuum images of the Helioseismic and Magnetic Imager
onboard the Solar Dynamics Observatory. We computed the forward-modeled
travel-time perturbations caused by these inflows using 3D sensitivity
kernels. In order to compare with the observations, we averaged these
forward-modeled travel-time perturbations over longitude and time
in the same way as the measured travel times.
Results: The
forward-modeling approach shows that the inflows associated with active
regions may account for only a fraction of the solar-cycle variations
in the north-south travel-time measurements.
Conclusions: The
travel-time perturbations caused by the large-scale inflows surrounding
the active regions do not explain in full the solar-cycle variations
seen in the helioseismic measurements of the meridional circulation.
Title: Theory of solar oscillations in the inertial frequency range:
Amplitudes of equatorial modes from a nonlinear rotating convection
simulation
Authors: Bekki, Yuto; Cameron, Robert H.; Gizon, Laurent
Bibcode: 2022arXiv220811081B
Altcode:
Several types of inertial modes have been detected on the
Sun. Properties of these inertial modes have been studied in the
linear regime but have not been studied in nonlinear simulations
of solar rotating convection. Comparing the nonlinear simulations,
the linear theory, and the solar observations is important to better
understand the differences between the models and the real Sun. We
wish to detect and characterize the modes present in a nonlinear
numerical simulation of solar convection, in particular to understand
the amplitudes and lifetimes of the modes. We developed a code with
a Yin-Yang grid to carry out fully-nonlinear numerical simulations
of rotating convection in a spherical shell. The stratification is
solar-like up to 0.96R. The simulations cover a duration of about
15 solar years. Various large-scale modes at low frequencies are
extracted from the simulation. Their characteristics are compared to
those from the linear model and to the observations. Among other modes,
both the equatorial Rossby modes and the columnar convective modes
are seen in the simulation. The columnar convective modes contain
most of the large-scale velocity power outside the tangential
cylinder and substantially contribute to the heat and angular
momentum transport. Equatorial Rossby modes with no radial node (n=0)
are also found: They have the same spatial structures as the linear
eigenfunctions. They are stochastically excited by convection and have
the amplitudes of a few m/s and mode linewidths of about 20-30 nHz,
which are comparable to those observed on the Sun. We also confirm the
existence of the mixed modes between the equatorial Rossby modes and
the columnar convective modes in our nonlinear simulation, as predicted
by the linear eigenmode analysis. We also see the high-latitude mode
with m=1 in our nonlinear simulation but its amplitude is much weaker
than that observed on the Sun.
Title: The on-ground data reduction and calibration pipeline for
SO/PHI-HRT
Authors: Sinjan, J.; Calchetti, D.; Hirzberger, J.; Orozco Suárez,
D.; Albert, K.; Albelo Jorge, N.; Appourchaux, T.; Alvarez-Herrero,
A.; Blanco Rodríguez, J.; Gandorfer, A.; Germerott, D.; Guerrero,
L.; Gutierrez Marquez, P.; Kahil, F.; Kolleck, M.; Solanki, S. K.; del
Toro Iniesta, J. C.; Volkmer, R.; Woch, J.; Fiethe, B.; Gómez Cama,
J. M.; Pérez-Grande, I.; Sanchis Kilders, E.; Balaguer Jiménez,
M.; Bellot Rubio, L. R.; Carmona, M.; Deutsch, W.; Fernandez-Rico,
G.; Fernández-Medina, A.; García Parejo, P.; Gasent Blesa, J. L.;
Gizon, L.; Grauf, B.; Heerlein, K.; Korpi-Lagg, A.; Lange, T.; López
Jiménez, A.; Maue, T.; Meller, R.; Michalik, H.; Moreno Vacas, A.;
Müller, R.; Nakai, E.; Schmidt, W.; Schou, J.; Schühle, U.; Staub,
J.; Strecker, H.; Torralbo, I.; Valori, G.
Bibcode: 2022arXiv220814904S
Altcode:
The ESA/NASA Solar Orbiter space mission has been successfully launched
in February 2020. Onboard is the Polarimetric and Helioseismic Imager
(SO/PHI), which has two telescopes, a High Resolution Telescope
(HRT) and the Full Disc Telescope (FDT). The instrument is designed
to infer the photospheric magnetic field and line-of-sight velocity
through differential imaging of the polarised light emitted by the
Sun. It calculates the full Stokes vector at 6 wavelength positions
at the Fe I 617.3 nm absorption line. Due to telemetry constraints,
the instrument nominally processes these Stokes profiles onboard,
however when telemetry is available, the raw images are downlinked and
reduced on ground. Here the architecture of the on-ground pipeline
for HRT is presented, which also offers additional corrections not
currently available on board the instrument. The pipeline can reduce
raw images to the full Stokes vector with a polarimetric sensitivity
of $10^{-3}\cdot I_{c}$ or better.
Title: Stellar limb darkening. A new MPS-ATLAS library for Kepler,
TESS, CHEOPS, and PLATO passbands
Authors: Kostogryz, N. M.; Witzke, V.; Shapiro, A. I.; Solanki, S. K.;
Maxted, P. F. L.; Kurucz, R. L.; Gizon, L.
Bibcode: 2022arXiv220606641K
Altcode:
The detection of the first exoplanet paved the way into the era of
transit photometry space missions with a revolutionary photometric
precision that aim at discovering new exoplanetary systems around
different types of stars. With this high precision, it is possible
to derive very accurately the radii of exoplanets which is crucial
for constraining their type and composition. However, it requires an
accurate description of host stars, especially their center-to-limb
variation of intensities (so called limb darkening) as it affects the
planet-to-star radius ratio determination. We aim at improving the
accuracy of limb darkening calculations for stars with a wide range
of fundamental parameters. We used the recently developed 1D MPS-ATLAS
code to compute model atmosphere structures and to synthesize stellar
limb darkening on a very fine grid of stellar parameters. For the
computations we utilized the most accurate information on chemical
element abundances and mixing length parameters including convective
overshoot. The stellar limb darkening was fitted using the two most
accurate limb darkening laws: the power-2 and 4-parameters non-linear
laws. We present a new extensive library of stellar model atmospheric
structures, the synthesized stellar limb darkening curves, and the
coefficients of parameterized limb-darkening laws on a very fine grid of
stellar parameters in the Kepler, TESS, CHEOPS, and PLATO passbands. The
fine grid allows overcoming the sizable errors introduced by the need
to interpolate. Our computations of solar limb darkening are in a
good agreement with available solar measurements at different view
angles and wavelengths. Our computations of stellar limb darkening
agree well with available measurements of Kepler stars. A new grid of
stellar model structures, limb darkening and their fitted coefficients
in different broad filters is provided in CDS.
Title: Theory of solar oscillations in the inertial frequency range:
Linear modes of the convection zone
Authors: Bekki, Yuto; Cameron, Robert H.; Gizon, Laurent
Bibcode: 2022A&A...662A..16B
Altcode: 2022arXiv220304442B
Context. Several types of global-scale inertial modes of oscillation
have been observed on the Sun. These include the equatorial Rossby
modes, critical-latitude modes, and high-latitude modes. However,
the columnar convective modes (predicted by simulations and also
known as banana cells or thermal Rossby waves) remain elusive.
Aims: We aim to investigate the influence of turbulent diffusivities,
non-adiabatic stratification, differential rotation, and a latitudinal
entropy gradient on the linear global modes of the rotating solar
convection zone.
Methods: We numerically solved for the
eigenmodes of a rotating compressible fluid inside a spherical
shell. The model takes into account the solar stratification, turbulent
diffusivities, differential rotation (determined by helioseismology),
and the latitudinal entropy gradient. As a starting point, we restricted
ourselves to a superadiabaticity and turbulent diffusivities that
are uniform in space. We identified modes in the inertial frequency
range, including the columnar convective modes as well as modes of
a mixed character. The corresponding mode dispersion relations and
eigenfunctions are computed for azimuthal orders of m ≤ 16.
Results: The three main results are as follows. Firstly, we find
that, for m ≳ 5, the radial dependence of the equatorial Rossby
modes with no radial node (n = 0) is radically changed from the
traditional expectation (rm) for turbulent diffusivities
≳1012 cm2 s−1. Secondly,
we find mixed modes, namely, modes that share properties of the
equatorial Rossby modes with one radial node (n = 1) and the columnar
convective modes, which are not substantially affected by turbulent
diffusion. Thirdly, we show that the m = 1 high-latitude mode in the
model is consistent with the solar observations when the latitudinal
entropy gradient corresponding to a thermal wind balance is included
(baroclinically unstable mode).
Conclusions: To our knowledge,
this work is the first realistic eigenvalue calculation of the global
modes of the rotating solar convection zone. This calculation reveals
a rich spectrum of modes in the inertial frequency range, which can
be directly compared to the observations. In turn, the observed modes
can inform us about the solar convection zone.
Title: ESA's PLATO mission: Development status and upcoming milestones
Authors: Heras, Ana María; Rauer, Heike; Aerts, Conny; Deleuil,
Magali; Gizon, Laurent; Goupil, Marie-Jo; Mas-Hesse, Miguel; Pagano,
Isabella; Piotto, Giampaolo; Pollacco, Don; Ragazzoni, Roberto;
Ramsay, Gavin; Udry, Stephane
Bibcode: 2022BAAS...54e5401H
Altcode:
PLATO is the third medium class mission in ESA's Cosmic Vision
programme, with a planned launch date by end 2026. Its main objective
is the detection and bulk characterisation of exoplanets down to
Earth size, with emphasis on planets orbiting up to the habitable
zone of bright solar-like stars. PLATO will study host stars using
asteroseismology, allowing us to determine the stellar properties
with high accuracy and substantially enhance our knowledge of stellar
structure and evolution. Following the successful Critical Milestone
Review, ESA has given green light to continue the implementation of
the spacecraft and the payload, which includes the serial production
of its 26 cameras. We will present the status of the satellite and
the ground segment development, and report on the progress made on
the PLATO Input Catalogue, the sky field selection, the ground-based
follow-up observations programme, and on other activities associated
with the scientific preparation. We will also give an overview of
the main upcoming milestones in the mission development and in the
observing strategy definition.
Title: A New Method for Calculating Solar Irradiance at Mars
Authors: De Oliveira, I.; Shapiro, A. I.; Sowmya, K.; Medvedev, A.;
Nèmec, N. -E.; Gizon, L.
Bibcode: 2022mamo.conf.1535D
Altcode:
No abstract at ADS
Title: ET White Paper: To Find the First Earth 2.0
Authors: Ge, Jian; Zhang, Hui; Zang, Weicheng; Deng, Hongping; Mao,
Shude; Xie, Ji-Wei; Liu, Hui-Gen; Zhou, Ji-Lin; Willis, Kevin; Huang,
Chelsea; Howell, Steve B.; Feng, Fabo; Zhu, Jiapeng; Yao, Xinyu; Liu,
Beibei; Aizawa, Masataka; Zhu, Wei; Li, Ya-Ping; Ma, Bo; Ye, Quanzhi;
Yu, Jie; Xiang, Maosheng; Yu, Cong; Liu, Shangfei; Yang, Ming; Wang,
Mu-Tian; Shi, Xian; Fang, Tong; Zong, Weikai; Liu, Jinzhong; Zhang, Yu;
Zhang, Liyun; El-Badry, Kareem; Shen, Rongfeng; Tam, Pak-Hin Thomas;
Hu, Zhecheng; Yang, Yanlv; Zou, Yuan-Chuan; Wu, Jia-Li; Lei, Wei-Hua;
Wei, Jun-Jie; Wu, Xue-Feng; Sun, Tian-Rui; Wang, Fa-Yin; Zhang,
Bin-Bin; Xu, Dong; Yang, Yuan-Pei; Li, Wen-Xiong; Xiang, Dan-Feng;
Wang, Xiaofeng; Wang, Tinggui; Zhang, Bing; Jia, Peng; Yuan, Haibo;
Zhang, Jinghua; Xuesong Wang, Sharon; Gan, Tianjun; Wang, Wei; Zhao,
Yinan; Liu, Yujuan; Wei, Chuanxin; Kang, Yanwu; Yang, Baoyu; Qi, Chao;
Liu, Xiaohua; Zhang, Quan; Zhu, Yuji; Zhou, Dan; Zhang, Congcong;
Yu, Yong; Zhang, Yongshuai; Li, Yan; Tang, Zhenghong; Wang, Chaoyan;
Wang, Fengtao; Li, Wei; Cheng, Pengfei; Shen, Chao; Li, Baopeng; Pan,
Yue; Yang, Sen; Gao, Wei; Song, Zongxi; Wang, Jian; Zhang, Hongfei;
Chen, Cheng; Wang, Hui; Zhang, Jun; Wang, Zhiyue; Zeng, Feng; Zheng,
Zhenhao; Zhu, Jie; Guo, Yingfan; Zhang, Yihao; Li, Yudong; Wen, Lin;
Feng, Jie; Chen, Wen; Chen, Kun; Han, Xingbo; Yang, Yingquan; Wang,
Haoyu; Duan, Xuliang; Huang, Jiangjiang; Liang, Hong; Bi, Shaolan; Gai,
Ning; Ge, Zhishuai; Guo, Zhao; Huang, Yang; Li, Gang; Li, Haining;
Li, Tanda; Yuxi; Lu; Rix, Hans-Walter; Shi, Jianrong; Song, Fen;
Tang, Yanke; Ting, Yuan-Sen; Wu, Tao; Wu, Yaqian; Yang, Taozhi; Yin,
Qing-Zhu; Gould, Andrew; Lee, Chung-Uk; Dong, Subo; Yee, Jennifer C.;
Shvartzvald, Yossi; Yang, Hongjing; Kuang, Renkun; Zhang, Jiyuan;
Liao, Shilong; Qi, Zhaoxiang; Yang, Jun; Zhang, Ruisheng; Jiang, Chen;
Ou, Jian-Wen; Li, Yaguang; Beck, Paul; Bedding, Timothy R.; Campante,
Tiago L.; Chaplin, William J.; Christensen-Dalsgaard, Jørgen; García,
Rafael A.; Gaulme, Patrick; Gizon, Laurent; Hekker, Saskia; Huber,
Daniel; Khanna, Shourya; Li, Yan; Mathur, Savita; Miglio, Andrea;
Mosser, Benoît; Ong, J. M. Joel; Santos, Ângela R. G.; Stello,
Dennis; Bowman, Dominic M.; Lares-Martiz, Mariel; Murphy, Simon; Niu,
Jia-Shu; Ma, Xiao-Yu; Molnár, László; Fu, Jian-Ning; De Cat, Peter;
Su, Jie; consortium, the ET
Bibcode: 2022arXiv220606693G
Altcode:
We propose to develop a wide-field and ultra-high-precision photometric
survey mission, temporarily named "Earth 2.0 (ET)". This mission is
designed to measure, for the first time, the occurrence rate and the
orbital distributions of Earth-sized planets. ET consists of seven
30cm telescopes, to be launched to the Earth-Sun's L2 point. Six
of these are transit telescopes with a field of view of 500 square
degrees. Staring in the direction that encompasses the original Kepler
field for four continuous years, this monitoring will return tens of
thousands of transiting planets, including the elusive Earth twins
orbiting solar-type stars. The seventh telescope is a 30cm microlensing
telescope that will monitor an area of 4 square degrees toward the
galactic bulge. This, combined with simultaneous ground-based KMTNet
observations, will measure masses for hundreds of long-period and
free-floating planets. Together, the transit and the microlensing
telescopes will revolutionize our understandings of terrestrial
planets across a large swath of orbital distances and free space. In
addition, the survey data will also facilitate studies in the fields
of asteroseismology, Galactic archeology, time-domain sciences, and
black holes in binaries.
Title: Revised extinctions and radii for 1.5 million stars observed
by APOGEE, GALAH, and RAVE
Authors: Yu, Jie; Khanna, Shourya; Themessl, Nathalie; Hekker, Saskia;
Dréau, Guillaume; Gizon, Laurent; Bi, Shaolan
Bibcode: 2022arXiv220600046Y
Altcode:
Asteroseismology has become widely accepted as a benchmark for
accurate and precise fundamental stellar properties. It can therefore
be used to validate and calibrate stellar parameters derived from
other approaches. Meanwhile, one can leverage archival and ongoing
large-volume surveys in photometry, spectroscopy, and astrometry to
infer stellar parameters over a wide range of evolutionary stages,
independently of asteroseismology. Our pipeline, $\texttt{SEDEX}$,
compares the spectral energy distribution predicted by the MARCS
and BOSZ model spectra with 32 photometric bandpasses, combining
data from 9 major, large-volume photometric surveys. We restrict
the analysis to targets with available spectroscopy to lift the
temperature-extinction degeneracy. Validation of our method and
results with CHARA interferometry, HST CALSPEC spectrophotometry,
and asteroseismology, shows that we achieve high precision and
accuracy. We present improved interstellar extinction ($\sigma_{A_V}
\simeq$ 0.08 mag) and stellar radii ($\sigma_R/R \simeq$ 3.6%) for
$\sim$1.5 million stars in the low- to high-extinction ($A_V \lesssim
6 $ mag) fields observed by the APOGEE, GALAH, and RAVE spectroscopic
surveys. We derive global extinctions for 191 Gaia DR2 open clusters. We
confirm the differential extinction in NGC 6791 ($A_V=0.2$ to $0.6$
mag) and NGC 6819 ($A_V=0.4$ to $0.6$ mag) that have been subject to
extensive asteroseismic analysis.
Title: The magnetic drivers of campfires seen by the Polarimetric
and Helioseismic Imager (PHI) on Solar Orbiter
Authors: Kahil, F.; Hirzberger, J.; Solanki, S. K.; Chitta, L. P.;
Peter, H.; Auchère, F.; Sinjan, J.; Orozco Suárez, D.; Albert,
K.; Albelo Jorge, N.; Appourchaux, T.; Alvarez-Herrero, A.; Blanco
Rodríguez, J.; Gandorfer, A.; Germerott, D.; Guerrero, L.; Gutiérrez
Márquez, P.; Kolleck, M.; del Toro Iniesta, J. C.; Volkmer, R.;
Woch, J.; Fiethe, B.; Gómez Cama, J. M.; Pérez-Grande, I.; Sanchis
Kilders, E.; Balaguer Jiménez, M.; Bellot Rubio, L. R.; Calchetti,
D.; Carmona, M.; Deutsch, W.; Fernández-Rico, G.; Fernández-Medina,
A.; García Parejo, P.; Gasent-Blesa, J. L.; Gizon, L.; Grauf, B.;
Heerlein, K.; Lagg, A.; Lange, T.; López Jiménez, A.; Maue, T.;
Meller, R.; Michalik, H.; Moreno Vacas, A.; Müller, R.; Nakai,
E.; Schmidt, W.; Schou, J.; Schühle, U.; Staub, J.; Strecker, H.;
Torralbo, I.; Valori, G.; Aznar Cuadrado, R.; Teriaca, L.; Berghmans,
D.; Verbeeck, C.; Kraaikamp, E.; Gissot, S.
Bibcode: 2022A&A...660A.143K
Altcode: 2022arXiv220213859K
Context. The Extreme Ultraviolet Imager (EUI) on board the Solar Orbiter
(SO) spacecraft observed small extreme ultraviolet (EUV) bursts,
termed campfires, that have been proposed to be brightenings near the
apexes of low-lying loops in the quiet-Sun atmosphere. The underlying
magnetic processes driving these campfires are not understood.
Aims: During the cruise phase of SO and at a distance of 0.523
AU from the Sun, the Polarimetric and Helioseismic Imager on Solar
Orbiter (SO/PHI) observed a quiet-Sun region jointly with SO/EUI,
offering the possibility to investigate the surface magnetic field
dynamics underlying campfires at a spatial resolution of about 380
km.
Methods: We used co-spatial and co-temporal data of the
quiet-Sun network at disc centre acquired with the High Resolution
Imager of SO/EUI at 17.4 nm (HRIEUV, cadence 2 s) and the
High Resolution Telescope of SO/PHI at 617.3 nm (HRT, cadence 2.5
min). Campfires that are within the SO/PHI−SO/EUI common field
of view were isolated and categorised according to the underlying
magnetic activity.
Results: In 71% of the 38 isolated events,
campfires are confined between bipolar magnetic features, which seem to
exhibit signatures of magnetic flux cancellation. The flux cancellation
occurs either between the two main footpoints, or between one of the
footpoints of the loop housing the campfire and a nearby opposite
polarity patch. In one particularly clear-cut case, we detected the
emergence of a small-scale magnetic loop in the internetwork followed
soon afterwards by a campfire brightening adjacent to the location
of the linear polarisation signal in the photosphere, that is to
say near where the apex of the emerging loop lays. The rest of the
events were observed over small scattered magnetic features, which
could not be identified as magnetic footpoints of the campfire hosting
loops.
Conclusions: The majority of campfires could be driven
by magnetic reconnection triggered at the footpoints, similar to the
physical processes occurring in the burst-like EUV events discussed
in the literature. About a quarter of all analysed campfires, however,
are not associated to such magnetic activity in the photosphere, which
implies that other heating mechanisms are energising these small-scale
EUV brightenings.
Title: Testing solar surface flux transport models in the first days
after active region emergence
Authors: Gottschling, N.; Schunker, H.; Birch, A. C.; Cameron, R.;
Gizon, L.
Bibcode: 2022A&A...660A...6G
Altcode: 2021arXiv211101896G
Context. Active regions (ARs) play an important role in the magnetic
dynamics of the Sun. Solar surface flux transport models (SFTMs) are
used to describe the evolution of the radial magnetic field at the solar
surface. The models are kinematic in the sense that the radial component
of the magnetic field behaves as passively advected corks. There is,
however, uncertainty about using these models in the early stage of
AR evolution, where dynamic effects might be important.
Aims:
We aim to test the applicability of SFTMs in the first days after the
emergence of ARs by comparing them with observations. The models we
employ range from passive evolution to models where the inflows around
ARs are included.
Methods: We simulated the evolution of the
surface magnetic field of 17 emerging ARs using a local surface flux
transport simulation. The regions were selected such that they did not
form fully fledged sunspots that exhibit moat flows. The simulation
included diffusion and advection by a velocity field, for which we
tested different models. For the flow fields, we used observed flows
from local correlation tracking of solar granulation, as well as
parametrizations of the inflows around ARs based on the gradient of
the magnetic field. To evaluate our simulations, we measured the cross
correlation between the observed and the simulated magnetic field, as
well as the total unsigned flux of the ARs, over time. We also tested
the validity of our simulations by varying the starting time relative
to the emergence of flux.
Results: We find that the simulations
using observed surface flows can reproduce the evolution of the observed
magnetic flux. The effect of buffeting the field by supergranulation can
be described as a diffusion process. The SFTM is applicable after 90%
of the peak total unsigned flux of the AR has emerged. Diffusivities
in the range between D = 250-720 km2 s−1 are
consistent with the evolution of the AR flux in the first five days
after this time. We find that the converging flows around emerging
ARs are not important for the evolution of the total flux of the AR
in these first five days; their effect of increasing flux cancellation
is balanced by the decrease in flux transport away from the AR.
Title: HiRISE - High-Resolution Imaging and Spectroscopy Explorer
- Ultrahigh resolution, interferometric and external occulting
coronagraphic science
Authors: Erdélyi, Robertus; Damé, Luc; Fludra, Andrzej; Mathioudakis,
Mihalis; Amari, T.; Belucz, B.; Berrilli, F.; Bogachev, S.; Bolsée,
D.; Bothmer, V.; Brun, S.; Dewitte, S.; de Wit, T. Dudok; Faurobert,
M.; Gizon, L.; Gyenge, N.; Korsós, M. B.; Labrosse, N.; Matthews,
S.; Meftah, M.; Morgan, H.; Pallé, P.; Rochus, P.; Rozanov, E.;
Schmieder, B.; Tsinganos, K.; Verwichte, E.; Zharkov, S.; Zuccarello,
F.; Wimmer-Schweingruber, R.
Bibcode: 2022ExA...tmp...21E
Altcode:
Recent solar physics missions have shown the definite role of waves and
magnetic fields deep in the inner corona, at the chromosphere-corona
interface, where dramatic and physically dominant changes occur. HiRISE
(High Resolution Imaging and Spectroscopy Explorer), the ambitious new
generation ultra-high resolution, interferometric, and coronagraphic,
solar physics mission, proposed in response to the ESA Voyage 2050
Call, would address these issues and provide the best-ever and most
complete solar observatory, capable of ultra-high spatial, spectral,
and temporal resolution observations of the solar atmosphere, from the
photosphere to the corona, and of new insights of the solar interior
from the core to the photosphere. HiRISE, at the L1 Lagrangian
point, would provide meter class FUV imaging and spectro-imaging,
EUV and XUV imaging and spectroscopy, magnetic fields measurements,
and ambitious and comprehensive coronagraphy by a remote external
occulter (two satellites formation flying 375 m apart, with a
coronagraph on a chaser satellite). This major and state-of-the-art
payload would allow us to characterize temperatures, densities, and
velocities in the solar upper chromosphere, transition zone, and inner
corona with, in particular, 2D very high resolution multi-spectral
imaging-spectroscopy, and, direct coronal magnetic field measurement,
thus providing a unique set of tools to understand the structure and
onset of coronal heating. HiRISE's objectives are natural complements
to the Parker Solar Probe and Solar Orbiter-type missions. We present
the science case for HiRISE which will address: i) the fine structure
of the chromosphere-corona interface by 2D spectroscopy in FUV at
very high resolution; ii) coronal heating roots in the inner corona by
ambitious externally-occulted coronagraphy; iii) resolved and global
helioseismology thanks to continuity and stability of observing at the
L1 Lagrange point; and iv) solar variability and space climate with,
in addition, a global comprehensive view of UV variability.
Title: The PLATO field selection process. I. Identification and
content of the long-pointing fields
Authors: Nascimbeni, V.; Piotto, G.; Börner, A.; Montalto, M.;
Marrese, P. M.; Cabrera, J.; Marinoni, S.; Aerts, C.; Altavilla,
G.; Benatti, S.; Claudi, R.; Deleuil, M.; Desidera, S.; Fabrizio,
M.; Gizon, L.; Goupil, M. J.; Granata, V.; Heras, A. M.; Magrin, D.;
Malavolta, L.; Mas-Hesse, J. M.; Ortolani, S.; Pagano, I.; Pollacco,
D.; Prisinzano, L.; Ragazzoni, R.; Ramsay, G.; Rauer, H.; Udry, S.
Bibcode: 2022A&A...658A..31N
Altcode: 2021arXiv211013924N
PLAnetary Transits and Oscillations of stars is an ESA M-class
satellite planned for launch by the end of 2026 and dedicated to the
wide-field search of transiting planets around bright and nearby
stars, with a strong focus on discovering habitable rocky planets
hosted by solar-like stars. The choice of the fields to be pointed
at is a crucial task since it has a direct impact on the scientific
return of the mission. In this paper, we describe and discuss the formal
requirements and the key scientific prioritization criteria that have to
be taken into account in the Long-duration Observation Phase (LOP) field
selection, and apply a quantitative metric to guide us in this complex
optimization process. We identify two provisional LOP fields, one for
each hemisphere (LOPS1 and LOPN1), and we discuss their properties and
stellar content. While additional fine-tuning shall be applied to LOP
selection before the definitive choice, which is set to be made two
years before launch, we expect that their position will not move by
more than a few degrees with respect to what is proposed in this paper.
Title: Efficient computation of modal Green's kernels with application
to helioseismology
Authors: Faucher, Florian; Barucq, Helene; Fournier, Damien; Gizon,
Laurent; Pham, Ha
Bibcode: 2021AGUFM.S13C..01F
Altcode:
The computation of Greens kernels occupies an important place in
terrestrial and extraterrestrial seismology which aim to reconstruct
the interior from data on the surface. Currently, time-distance
helioseismology ([3]) relies on single-source computations which
use Greens kernel G(r, s) with source s at fixed height. However,
observed solar oscillations should be considered as an average over
all depths weighted by transparency. This requires information of the
full Greens kernel, i.e., for both r and s in the entire discretized
region, [1]. Our algorithm solves for two regular solutions and
the full kernel is assembled from these two simulations. It is thus
independent of source position, in contrast to classical approaches
which require different discretization as the source height varies,
[1]. Additionally, the behavior of the kernel at the source position
is described analytically and circumvents dealing with a singular
source problem. We provide numerical illustration of our method for
solar background given by model S in the interior and an isothermal
atmospheric model AtmoI. We first ignore gravity and work with a
scalar wave equation, for which Greens kernels and power spectra
are computed and serve to investigate p-modes. Here, the outgoing
behaviour of the Greens kernel is characterized by radiation boundary
condition based on the exact Dirichlet-to-Neumann map. Secondly,
to include gravity, we solve the simplified Galbrun's wave equation,
[2], and compute corresponding power spectra which exhibit the solar
g and f -modes. References [1] H. Barucq, F. Faucher, D. Fournier,
L. Gizon and H. Pham, Efficient and Accurate Algorithm for the Full
Modal Greens Kernel of the Scalar Wave Equation in Helioseismology,
SIAM Journal on Applied Mathematics, 80 (6), 2020. [2] H. Barucq,
F. Faucher, D. Fournier, L. Gizon and H. Pham, Outgoing modal solutions
for Galbruns equation in helioseismology, Journal of Differential
Equations, 286, 2021. [3] J. Christensen-Dalsgaard, Lecture notes on
stellar oscillations, 2014.
Title: Solar inertial modes: observations and modeling
Authors: Gizon, Laurent
Bibcode: 2021AGUFMSH34D..05G
Altcode:
The oscillations of a slowly rotating star have long been classified
into spheroidal and toroidal modes. The spheroidal modes include
the well-known 5-min acoustic modes used in helioseismology. Here
we report observations of the Suns toroidal modes, for which the
restoring force is the Coriolis force and whose periods are of order
of the solar rotation period. By comparison with the normal modes of
a differentially rotating spherical shell, we find an identification
for many of the observed modes. These are the high-lattitude inertial
modes, the critical-latitude inertial modes, and the equatorial Rossby
modes. Some of these modes are self-excited in the model. We show that
the inertial mode eigenfunctions are sensitive to the superadiabaticity
and the turbulent viscosity in the deep convection zone -- quantities
that are poorly constrained by p-mode helioseismology. Publication:
Astronomy and Astrophysics, Forthcoming Article (accepted 1 July 2021),
https://doi.org/10.1051/0004-6361/202141462 Authors: Laurent Gizon,
Robert H. Cameron, Yuto Bekki, Aaron C. Birch, Richard S. Bogart,
Allan Sacha Brun, Cilia Damiani, Damien Fournier, Laura Hyest, Kiran
Jain, B. Lekshmi, Zhi-Chao Liang, and Bastian Proxauf
Title: BESTP - An automated Bayesian modeling tool for
asteroseismology
Authors: Jiang, Chen; Gizon, Laurent
Bibcode: 2021RAA....21..226J
Altcode: 2021arXiv210503728J
Asteroseismic observations are crucial to constrain stellar models
with precision. Bayesian Estimation of STellar Parameters (BESTP) is a
tool that utilizes Bayesian statistics and nested sampling Monte Carlo
algorithm to search for the stellar models that best match a given
set of classical and asteroseismic constraints from observations. The
computation and evaluation of models are efficiently performed in
an automated and multi-threaded way. To illustrate the capabilities
of BESTP, we estimate fundamental stellar properties for the Sun and
the red-giant star HD 222076. In both cases, we find models that are
consistent with observations. We also evaluate the improvement in the
precision of stellar parameters when the oscillation frequencies of
individual modes are included as constraints, compared to the case when
only the large frequency separation is included. For the solar case,
the uncertainties of estimated masses, radii and ages are reduced by
0.7%, 0.3% and 8% respectively. For HD 222076, they are reduced even
more noticeably by 2%, 0.5% and 4.7% respectively. We also note an
improvement of 10% for the age of HD 222076 when the Gaia parallax
is included as a constraint compared to the case when only the large
separation is included as a constraint.
Title: Habitability of the early Earth: Liquid water under a faint
young Sun facilitated by strong tidal heating due to a closer Moon
Authors: Heller, René; Duda, Jan-Peter; Winkler, Max; Reitner,
Joachim; Gizon, Laurent
Bibcode: 2021PalZ...95..563H
Altcode: 2020arXiv200703423H
Geological evidence suggests liquid water near the Earth's surface as
early as 4.4 billion years ago when the faint young Sun only radiated
about 70% of its modern power output. At this point, the Earth should
have been a global snowball if it possessed atmospheric properties
similar to those of the modern Earth. An extreme atmospheric greenhouse
effect, an initially more massive Sun, release of heat acquired during
the accretion process of protoplanetary material, and radioactivity of
the early Earth material have been proposed as reservoirs or traps for
heat. For now, the faint-young-sun paradox persists as an important
problem in our understanding of the origin of life on Earth. Here we
use the constant-phase-lag tidal theory to explore the possibility
that the new-born Moon, which formed about 69 million years after the
ignition of the Sun, generated extreme tidal friction - and therefore
heat - in the Hadean and possibly the Archean Earth. We show that
the Earth-Moon system has lost about $3~ \times ~10^{31}$ J (99% of
its initial mechanical energy budget) as tidal heat. Tidal heating
of roughly 10 W/m$^{-2}$ through the surface on a time scale of 100
million years could have accounted for a temperature increase of up
to 5 degrees Celsius on the early Earth. This heating effect alone
does not solve the faint-young-sun paradox but it could have played
a key role in combination with other effects. Future studies of the
interplay of tidal heating, the evolution of the solar power output,
and the atmospheric (greenhouse) effects on the early Earth could help
in solving the faint-young-sun paradox.
Title: PLATO Ground Segment: A high level description
Authors: O'Rourke, Laurence; Gizon, Laurent
Bibcode: 2021plat.confE.102O
Altcode:
We describe the different elements of the PLATO Ground Segment,
depending on both ESA and the PLATO Mission Consortium.
Title: The Complicated Case of δ Scu3 Pulsa3ons and Rota3on in KIC
6951642; a long-orbit Single-lined Spectroscopic Binary Star
Authors: Samadi-Ghadim, Anya; Lampens, Patricia; Gizon, Laurent
Bibcode: 2021plat.confE..63S
Altcode:
Abstract: More than four years of HERMES observations have
confirmed KIC 6951642 is a very long orbit (≍1770 d) single-lined
spectroscopic binary (F0-type) with a fast-rotating companion (vsin
i = 123±3 Km/s). The Fourier spectrum of its four-year photometric
observa3ons includes plenty of significant frequencies (594) in low-
and high-frequency regions. The high-frequency modes appear with various
time-delay patterns. We detected several rota3onally split 𝛿 Scuti
pulsa3ons cantered at 13.96 per day (and average frequency spacing of
Δ𝑓= 0.723±0.006 per day) for KIC 6951642. The detailed study of
all significant low frequencies, extended from 0.72 to 3.60 per day,
revealed that the two most dominant frequencies (with the same amplitude
and larger than of p-modes) are a combina3on the lowest-frequency modes
(𝑓3 = 𝑓< 0.17 per day), i.e. 𝑓orhrm + 𝑚𝑓orhrm( 𝑚 =
12,14. We suggest the lowest-frequency modes are very large harmonics
(orders of 10) of orbital frequency (≍0.0006 𝑑!"). We verified
the other most dominant low-frequencies as harmonics of rota3on
frequency 0.721 per day and its combinations. Finally, we reject the
probability of hybrid pulsations in the fast-rotating companion of
KIC 6951642. We introduce it as a 𝛿 Scuti pulsator with a candidate
rotation frequency of 0.721 per day .
Title: The Complicated Case of δ Scuti Pulsations and Rotation in
KIC 6951642; a long-orbit Single-lined Spectroscopic Binary Star
Authors: Samadi-Ghadim, Anya; Lampens, Patricia; Gizon, Laurent
Bibcode: 2021plat.confE..67S
Altcode:
Abstract: More than four years of HERMES observations have
confirmed KIC 6951642 is a very long orbit (≍1770 d) single-lined
spectroscopic binary (F0-type) with a fast-rotating companion (vsin
i = 123±3 Km/s). The Fourier spectrum of its four-year photometric
observations includes plenty of significant frequencies (594) in low-
and high-frequency regions. The high-frequency modes appear with
various time-delay patterns. We detected several rotationally split
𝛿 Scuti pulsations centered at 13.96 per day (and average frequency
spacing of Δ𝑓= 0.723±0.006 per day) for KIC 6951642. The detailed
study of all significant low frequencies, extended from 0.72 to 3.60
per day, revealed that the two most dominant frequencies (with the
same amplitude and larger than of p-modes) are a combination of the
lowest-frequency modes (𝑓3 = 𝑓< 0.17 per day),
i.e. 𝑓orhrm + 𝑚𝑓orhrm (𝑚 = 12,14). We
suggest the lowest-frequency modes are very large harmonics (orders
of 10) of orbital frequency (≍0.0006 per day). We verified the other
most dominant low-frequencies as harmonics of rota3on frequency 0.721
per day and its combinations. Finally, we reject the probability of
hybrid pulsations in the fast-rotating companion of KIC 6951642. We
introduce it as a 𝛿 Scuti pulsator with a candidate rotation
frequency of 0.721 per day.
Title: Modelling continuum intensity perturbations caused by solar
acoustic oscillations
Authors: Kostogryz, N. M.; Fournier, D.; Gizon, L.
Bibcode: 2021A&A...654A...1K
Altcode: 2021arXiv210707220K
Context. Helioseismology is the study of the Sun's interior
using observations of oscillations at the surface. It suffers
from systematic errors, for instance a center-to-limb error in
travel-time measurements. Understanding these errors requires an
adequate understanding of the nontrivial relationship between wave
displacement and helioseismic observables (intensity or velocity).
Aims: The wave displacement causes perturbations in the atmospheric
thermodynamical quantities which, in turn, perturb the opacity, the
optical depth, the source function, and the local ray geometry, thus
affecting the emergent intensity. We aim to establish the most complete
relationship achieved to date between the wave displacement and the
emergent intensity perturbation by solving the radiative transfer
problem in the perturbed atmosphere.
Methods: We derived an
expression for the emergent intensity perturbation caused by acoustic
oscillations at any point on the solar disk by applying a first-order
perturbation theory. As input perturbations, we considerd adiabatic
modes of oscillation of different degrees in a spherically-symmetric
solar model. The background and the perturbed intensities are computed
by solving the radiative transfer equation considering the main sources
of opacity in the continuum (absorption and scattering).
Results:
We find that for all modes, the perturbations to the thermodynamical
quantities are not sufficient to model the intensity perturbations:
the geometrical effects due to the wave displacement must always
be taken into account as they lead to a difference in amplitude and
a phase shift between temperature perturbations at the surface and
emergent intensity perturbations. The closer to the limb, the greater
the differences. For modes with eigenfrequencies around 3 mHz, we found
that the radial and horizontal components of the wave displacement are
important, in particular, for high-degree modes.
Conclusions:
This work presents improvements for the computation of the intensity
perturbations, in particular, for high-degree modes. Here, we explain
the differences in intensity computations seen in earlier works. The
phase shifts and amplitude differences between the temperature and
intensity perturbations increase toward the limb. This should prove
helpful when interpreting some of the systematic centre-to-limb
effects observed in local helioseismology. The computations are fast
(3 s for 2000 positions and one frequency for one core) and can be
parallelised. This work can be extended to models of the line-of-sight
velocity observable.
Title: How to Estimate the Far-Side Open Flux Using STEREO Coronal
Holes
Authors: Heinemann, Stephan G.; Temmer, Manuela; Hofmeister, Stefan
J.; Stojakovic, Aleksandar; Gizon, Laurent; Yang, Dan
Bibcode: 2021SoPh..296..141H
Altcode: 2021arXiv210902375H
Global magnetic field models use as input synoptic data, which usually
show "aging effects" as the longitudinal 360∘ information
is not obtained simultaneously. Especially during times of increased
solar activity, the evolution of the magnetic field may yield large
uncertainties. A significant source of uncertainty is the Sun's magnetic
field on the side of the Sun invisible to the observer. Various methods
have been used to complete the picture: synoptic charts, flux-transport
models, and far side helioseismology. In this study, we present a new
method to estimate the far-side open flux within coronal holes using
STEREO EUV observations. First, we correlate the structure of the
photospheric magnetic field as observed with the Helioseismic and
Magnetic Imager on board the Solar Dynamics Observatory (HMI/SDO)
with features in the transition region. From the 304 Å intensity
distribution, which we found to be specific to coronal holes, we derive
an empirical estimate for the open flux. Then we use a large sample of
313 SDO coronal hole observations to verify this relation. Finally,
we perform a cross-instrument calibration from SDO to STEREO data
to enable the estimation of the open flux at solar longitudes not
visible from Earth. We find that the properties of strong unipolar
magnetic elements in the photosphere, which determine the coronal
hole's open flux, can be approximated by open fields in the transition
region. We find that structures below a threshold of 78 % (STEREO)
or 94 % (SDO) of the solar disk median intensity as seen in 304 Å
filtergrams are reasonably well correlated with the mean magnetic
flux density of coronal holes (cc=sp 0.59 ). Using the area covered
by these structures (AOF) and the area of the coronal hole
(ACH), we model the open magnetic flux of a coronal hole
as |ΦCH|=0.25 ACHexp (0.032 AOF)
with an estimated uncertainty of 40 to 60 %.
Title: The all-sky PLATO input catalogue
Authors: Montalto, M.; Piotto, G.; Marrese, P. M.; Nascimbeni, V.;
Prisinzano, L.; Granata, V.; Marinoni, S.; Desidera, S.; Ortolani, S.;
Aerts, C.; Alei, E.; Altavilla, G.; Benatti, S.; Börner, A.; Cabrera,
J.; Claudi, R.; Deleuil, M.; Fabrizio, M.; Gizon, L.; Goupil, M. J.;
Heras, A. M.; Magrin, D.; Malavolta, L.; Mas-Hesse, J. M.; Pagano,
I.; Paproth, C.; Pertenais, M.; Pollacco, D.; Ragazzoni, R.; Ramsay,
G.; Rauer, H.; Udry, S.
Bibcode: 2021A&A...653A..98M
Altcode: 2021arXiv210813712M
Context. The ESA PLAnetary Transits and Oscillations of stars (PLATO)
mission will search for terrestrial planets in the habitable zone of
solar-type stars. Because of telemetry limitations, PLATO targets
need to be pre-selected.
Aims: In this paper, we present
an all sky catalogue that will be fundamental to selecting the best
PLATO fields and the most promising target stars, deriving their basic
parameters, analysing the instrumental performances, and then planing
and optimising follow-up observations. This catalogue also represents
a valuable resource for the general definition of stellar samples
optimised for the search of transiting planets.
Methods:
We used Gaia Data Release 2 astrometry and photometry and 3D maps
of the local interstellar medium to isolate FGK (V ≤ 13) and M
(V ≤ 16) dwarfs and subgiant stars.
Results: We present the
first public release of the all-sky PLATO input catalogue (asPIC1.1)
containing a total of 2 675 539 stars including 2 378 177 FGK dwarfs
and subgiants and 297 362 M dwarfs. The median distance in our sample
is 428 pc for FGK stars and 146 pc for M dwarfs, respectively. We
derived the reddening of our targets and developed an algorithm to
estimate stellar fundamental parameters (Teff, radius, mass)
from astrometric and photometric measurements.
Conclusions:
We show that the overall (internal+external) uncertainties on the
stellar parameter determined in the present study are ∼230 K (4%)
for the effective temperatures, ∼0.1 R⊙ (9%) for
the stellar radii, and ∼0.1 M⊙ (11%) for the stellar
mass. We release a special target list containing all known planet hosts
cross-matched with our catalogue. The catalogue described in this
article is only available at MAST as a High Level Science Product via https://dx.doi.org/10.17909/t9-8msm-xh08
and https://archive.stsci.edu/hlsp/aspic,
in the SSDC tools page (https://tools.ssdc.asi.it/asPICtool/)
and at the CDS via anonymous ftp to cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/cgi-bin/viz-bin/cat/J/A+A/653/A98
Title: The PLATO mission: Overview and status
Authors: Rauer, Heike; Pagano, Isabella; Mas-Hesse, Miguel; Aerts,
Conny; Deleuil, Magali; Gizon, Laurent; Goupil, Marie-Jo; María
Heras, Ana; Piotto, Giampaolo; Pollacco, Don; Ragazzoni, Roberto;
Ramsay, Gavin; Udry, Stéphane
Bibcode: 2021EPSC...15...90R
Altcode:
PLATO is an ESA mission dedicated to the study of exoplanets and stars,
with a planned launch date in 2026. By performing photometric monitoring
of about 250 000 bright stars (mV < 13), PLATO will be able to
discover and characterise hundreds of exoplanets, including small
planets orbiting up to the habitable zone of solar-like stars. PLATO's
precision will also allow for a precise characterisation of the host
stars through asteroseismology. These objectives require both a wide
field of view and high sensitivity, which are achieved with a payload
comprising 24 cameras with partially overlapping fields of view. They
are complemented by 2 more cameras optimised for brighter stars that
will also be used as fine guidance sensor. The PLATO development
phase started after the mission adoption in July 2017. The Mission
Preliminary Design Review (PDR) was declared successful in October
2020. The implementation and delivery to ESA of the flight model CCDs
for all cameras (4 CCDs per camera) has been completed. Currently
the Structural Thermal Model (STM) of the payload optical bench is
being manufactured, while the STM of a single camera has already
been successfully tested. In parallel, a first engineering model of
a complete, fully functional camera is being integrated, to verify
its performance under operational conditions, and the qualification
models of the different payload units are being built.We will present
the status of the PLATO payload implementation in the context of the
satellite development. In particular, we will describe the payload
manufacturing, integration, and tests that will be reviewed at the
Critical Milestone in the second half of 2021. We will also summarise
the progress made in the science preparation activities, as well as
on the ground segment.
Title: Detection of Rossby modes with even azimuthal orders using
helioseismic normal-mode coupling
Authors: Mandal, K.; Hanasoge, S. M.; Gizon, L.
Bibcode: 2021A&A...652A..96M
Altcode: 2021arXiv210603971M
Context. Retrograde Rossby waves, measured to have significant
amplitudes in the Sun, likely have notable implications for various
solar phenomena.
Aims: Rossby waves create small-amplitude,
very-low frequency motions, on the order of the rotation rate and lower,
which in turn shift the resonant frequencies and eigenfunctions of the
acoustic modes of the Sun. The detection of even azimuthal orders Rossby
modes using mode coupling presents additional challenges and prior work
therefore only focused on odd orders. Here, we successfully extend the
methodology to measure even azimuthal orders as well.
Methods:
We analyze 4 and 8 years of Helioseismic and Magnetic Imager (HMI)
data and consider coupling between different-degree acoustic modes
(of separations 1 and 3 in the harmonic degree). The technique uses
couplings between different frequency bins to capture the temporal
variability of the Rossby modes.
Results: We observe significant
power close to the theoretical dispersion relation for sectoral Rossby
modes, where the azimuthal order is the same as the harmonic degree,
s = |t|. Our results are consistent with prior measurements of Rossby
modes with azimuthal orders over the range t = 4 to 16 with maximum
power occurring at mode t = 8. The amplitudes of these modes vary
from 1 to 2 m s−1. We place an upper bound of 0.2 m
s−1 on the sectoral t = 2 mode, which we do not detect
in our measurements.
Conclusions: This effort adds credence to
the mode-coupling methodology in helioseismology.
Title: Evolution of solar surface inflows around emerging active
regions
Authors: Gottschling, N.; Schunker, H.; Birch, A. C.; Löptien, B.;
Gizon, L.
Bibcode: 2021A&A...652A.148G
Altcode: 2021arXiv210510501G
Context. Solar active regions are associated with Evershed
outflows in sunspot penumbrae, moat outflows surrounding sunspots,
and extended inflows surrounding active regions. Extended inflows
have been identified around established active regions with various
methods. The evolution of these inflows and their dependence on active
region properties as well as their effect on the global magnetic
field are not yet understood.
Aims: We aim to understand
the evolution of the average inflows around emerging active regions
and to derive an empirical model for these inflows. We expect that
this can be used to better understand how the inflows act on the
diffusion of the magnetic field in active regions.
Methods:
We analyzed horizontal flows at the surface of the Sun using local
correlation tracking of solar granules observed in continuum images
of the Helioseismic and Magnetic Imager on board the Solar Dynamics
Observatory. We measured average flows of a sample of 182 isolated
active regions up to seven days before and after their emergence
onto the solar surface with a cadence of 12 h. About half of the
active regions in the sample developed sunspots with moat flows in
addition to the surrounding inflows. We investigated the average
inflow properties with respect to active region characteristics of
total flux and latitude. We fit a model to these observed inflows for
a quantitative analysis.
Results: We find that converging flows
of about 20-30 m s−1 are first visible one day prior to
emergence, in agreement with recent results. These converging flows
are present regardless of the active region properties of latitude
or flux. We confirm a recently found prograde flow of about 40 m
s−1 at the leading polarity during emergence. We find
that the time after emergence when the latitudinal inflows increase in
amplitude depends on the flux of the active region, ranging from one
to four days after emergence and increasing with flux. The largest
extent of the inflows is up to about 7 ± 1° away from the center
of the active region within the first six days after emergence. The
inflow velocities have amplitudes of about 50 m s−1. Supplementary material associated to Appendix D is available at https://dx.doi.org/10.17617/3.6h
Title: Solar inertial modes: Observations, identification, and
diagnostic promise
Authors: Gizon, Laurent; Cameron, Robert H.; Bekki, Yuto; Birch,
Aaron C.; Bogart, Richard S.; Brun, Allan Sacha; Damiani, Cilia;
Fournier, Damien; Hyest, Laura; Jain, Kiran; Lekshmi, B.; Liang,
Zhi-Chao; Proxauf, Bastian
Bibcode: 2021A&A...652L...6G
Altcode: 2021arXiv210709499G
The oscillations of a slowly rotating star have long been classified
into spheroidal and toroidal modes. The spheroidal modes include
the well-known 5-min acoustic modes used in helioseismology. Here
we report observations of the Sun's toroidal modes, for which the
restoring force is the Coriolis force and whose periods are on the
order of the solar rotation period. By comparing the observations
with the normal modes of a differentially rotating spherical shell,
we are able to identify many of the observed modes. These are the
high-latitude inertial modes, the critical-latitude inertial modes,
and the equatorial Rossby modes. In the model, the high-latitude
and critical-latitude modes have maximum kinetic energy density at
the base of the convection zone, and the high-latitude modes are
baroclinically unstable due to the latitudinal entropy gradient. As
a first application of inertial-mode helioseismology, we constrain
the superadiabaticity and the turbulent viscosity in the deep
convection zone. Movie associated to Fig. 2 is available at https://www.aanda.org
Title: Radiative Transfer with Opacity Distribution Functions:
Application to Narrowband Filters
Authors: Anusha, L. S.; Shapiro, A. I.; Witzke, V.; Cernetic, M.;
Solanki, S. K.; Gizon, L.
Bibcode: 2021ApJS..255....3A
Altcode: 2021arXiv210413661A
Modeling of stellar radiative intensities in various spectral passbands
plays an important role in stellar physics. At the same time, direct
calculation of the high-resolution spectrum and then integration of it
over the given spectral passband is computationally demanding due to
the vast number of atomic and molecular lines. This is particularly so
when employing three-dimensional (3D) models of stellar atmospheres. To
accelerate the calculations, one can employ approximate methods, e.g.,
the use of opacity distribution functions (ODFs). Generally, ODFs
provide a good approximation of traditional spectral synthesis, i.e.,
computation of intensities through filters with strictly rectangular
transmission functions. However, their performance strongly deteriorates
when the filter transmission noticeably changes within its passband,
which is the case for almost all filters routinely used in stellar
physics. In this context, the aims of this paper are (a) to generalize
the ODFs method for calculating intensities through filters with
arbitrary transmission functions, and (b) to study the performance of
the standard and generalized ODFs methods for calculating intensities
emergent from 3D models of stellar atmospheres. For this purpose we
use the newly developed MPS-ATLAS radiative transfer code to compute
intensities emergent from 3D cubes simulated with the radiative
magnetohydrodynamics code MURaM. The calculations are performed
in the 1.5D regime, i.e., along many parallel rays passing through
the simulated cube. We demonstrate that the generalized ODFs method
allows accurate and fast syntheses of spectral intensities and their
center-to-limb variations.
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: VizieR Online Data Catalog: asPIC1.1 catalogue (Montalto+,
2021)
Authors: Montalto, M.; Piotto, G.; Marrese, P. M.; Nascimbeni, V.;
Prisinzano, L.; Granata, V.; Marinoni, S.; Desidera, S.; Ortolani, S.;
Aerts., C.; Alei, E.; Altavilla, G.; Benatti, S.; Borner, A.; Cabrera,
J.; Claudi, R.; Deleuil, M.; Frabrizio, M.; Gizon, L.; Goupil, M. J.;
Heras, A. M.; Magrin, D.; Malavolta, L.; Mas-Hesse, J. M.; Pagano,
I.; Paproth, C.; Pertenais, M.; Pollacco, D.; Ragazzoni, G.; Ramsay,
R.; Rauer, H.; Udry, S.
Bibcode: 2021yCat..36530098M
Altcode:
The all sky PLATO Input Catalog (asPIC1.1) contains 2 675 539 entries
and 70 columns. The aspic1_1.dat size is ~1.9GB. (1 data file).
Title: Helioseismological determination of the subsurface spatial
spectrum of solar convection: Demonstration using numerical
simulations
Authors: Böning, Vincent G. A.; Birch, Aaron C.; Gizon, Laurent;
Duvall, Thomas L.
Bibcode: 2021A&A...649A..59B
Altcode: 2021arXiv210208603B
Context. Understanding convection is important in stellar physics, for
example, when it is an input in stellar evolution models. Helioseismic
estimates of convective flow amplitudes in deeper regions of the
solar interior disagree by orders of magnitude among themselves and
with simulations.
Aims: We aim to assess the validity of an
existing upper limit of solar convective flow amplitudes at a depth
of 0.96 solar radii obtained using time-distance helioseismology and
several simplifying assumptions.
Methods: We generated synthetic
observations for convective flow fields from a magnetohydrodynamic
simulation (MURaM) using travel-time sensitivity functions and a noise
model. We compared the estimates of the flow amplitude with the actual
value of the flow.
Results: For the scales of interest (ℓ
< 100), we find that the current procedure for obtaining an upper
limit gives the correct order of magnitude of the flow for the given
flow fields. We also show that this estimate is not an upper limit
in a strict sense because it underestimates the flow amplitude at the
largest scales by a factor of about two because the scale dependence
of the signal-to-noise ratio has to be taken into account. After
correcting for this and after taking the dependence of the measurements
on direction in Fourier space into account, we show that the obtained
estimate is indeed an upper limit.
Conclusions: We conclude that
time-distance helioseismology is able to correctly estimate the order of
magnitude (or an upper limit) of solar convective flows in the deeper
interior when the vertical correlation function of the different flow
components is known and the scale dependence of the signal-to-noise
ratio is taken into account. We suggest that future work should include
information from different target depths to better separate the effect
of near-surface flows from those at greater depths. In addition, the
measurements are sensitive to all three flow directions, which should
be taken into account.
Title: Autocorrelation of the Ground Vibrations Recorded by the SEIS
InSight Seismometer on Mars
Authors: Compaire, N.; Margerin, L.; Garcia, R. F.; Pinot, B.; Calvet,
M.; Orhand-Mainsant, G.; Kim, D.; Lekic, V.; Tauzin, B.; Schimmel,
M.; Stutzmann, E.; Knapmeyer-Endrun, B.; Lognonné, P.; Pike, W. T.;
Schmerr, N.; Gizon, L.; Banerdt, W. B.
Bibcode: 2021JGRE..12606498C
Altcode:
Since early February 2019, the SEIS (Seismic Experiment for Interior
Structure) seismometer deployed at the surface of Mars in the framework
of the InSight mission has been continuously recording the ground
motion at Elysium Planitia. In this study, we take advantage of this
exceptional data set to put constraints on the crustal properties
of Mars using seismic interferometry (SI). To carry out this task,
we first examine the continuous records from the very broadband
seismometer. Several deterministic sources of environmental noise
are identified and specific preprocessing strategies are presented to
mitigate their influence. Applying the principles of SI to the single
station configuration of InSight, we compute, for each Sol and each
hour of the martian day, the diagonal elements of the time domain
correlation tensor of random ambient vibrations recorded by SEIS. A
similar computation is performed on the diffuse waveforms generated
by more than a hundred Marsquakes. A careful signal to noise ratio
analysis and an inter comparison between the two datasets suggest
that the results from SI are most reliable in a narrow frequency band
around 2.4 Hz, where an amplification of both ambient vibrations and
seismic events is observed. The average autocorrelation functions (ACFs)
contain well identifiable seismic arrivals, that are very consistent
between the two datasets. Interpreting the vertical and horizontal ACFs
as, respectively, the P and S seismic reflectivity below InSight, we
propose a simple stratified velocity model of the crust, which is mostly
compatible with previous results from receiver function analysis. Our
results are discussed and compared to recent works from the literature.
Title: Autocorrelation of the ground vibration recorded by the
SEIS-InSight seismometer on Mars for imaging and monitoring
applications
Authors: Compaire, Nicolas; Margerin, Ludovic; Garcia, Raphaël F.;
Calvet, Marie; Pinot, Baptiste; Orhand-Mainsant, Guénolé; Kim,
Doyeon; Lekic, Vedran; Tauzin, Benoit; Schimmel, Martin; Stutzmann,
Eléonore; Knapmeyer-Endrun, Brigitte; Lognonné, Philippe; Pike,
William T.; Schmerr, Nicholas; Gizon, Laurent; Banerdt, Bruce
Bibcode: 2021EGUGA..2312292C
Altcode:
Since early February 2019, the SEIS seismometer deployed at the
surface of Mars in the framework of the NASA-InSight mission has been
continuously recording the ground motion at Elysium Planitia. In this
work, we take advantage of this exceptional dataset to put constraints
on the crustal properties of Mars using seismic interferometry
(SI). This method use the seismic waves, either from background
vibrations of the planet or from quakes, that are scattered in
the medium in order to recover the ground response between two
seismic sensors. Applying the principles of SI to the single-station
configuration of SEIS, we compute, for each Sol (martian day) and each
local hour, all the components of the time-domain autocorrelation
tensor of random ambient vibrations in various frequency bands. A
similar computation is performed on the diffuse waveforms generated
by more than a hundred Marsquakes. For imaging application a careful
signal-to-noise ratio analysis and an inter-comparison between the two
datasets are applied. These analyses suggest that the reconstructed
ground responses are most reliable in a relatively narrow frequency
band around 2.4Hz, where an amplification of both ambient vibrations
and seismic events is observed. The average Auto-Correlation Functions
(ACFs) from both ambient vibrations and seismic events contain well
identifiable seismic arrivals, that are very consistent between the two
datasets. We interpret the vertical and horizontal ACFs as the ground
reflection response below InSight for the compressional waves and the
shear waves respectively. We propose a simple stratified velocity model
of the crust, which is most compatible with the arrival times of the
detected phases, as well as with previous seismological studies of
the SEIS record. The hourly computation of the ACFs over one martian
year also allows us to study the diurnal and seasonal variations of
the reconstructed ground response with a technique call Passive Image
Interferometry (PII). In this study we present measurements of the
relative stretching coefficient between consecutive ACF waveforms and
discuss the potential origins of the observed temporal variations.
Title: Asteroseismology of luminous red giants with Kepler -
II. Dependence of mass-loss on pulsations and radiation
Authors: Yu, Jie; Hekker, Saskia; Bedding, Timothy R.; Stello, Dennis;
Huber, Daniel; Gizon, Laurent; Khanna, Shourya; Bi, Shaolan
Bibcode: 2021MNRAS.501.5135Y
Altcode: 2020MNRAS.tmp.3723Y; 2020arXiv201212414Y
Mass-loss by red giants is an important process to understand the
final stages of stellar evolution and the chemical enrichment of the
interstellar medium. Mass-loss rates are thought to be controlled by
pulsation-enhanced dust-driven outflows. Here, we investigate the
relationships between mass-loss, pulsations, and radiation, using
3213 luminous Kepler red giants and 13 5000 ASAS-SN semiregulars and
Miras. Mass-loss rates are traced by infrared colours using 2MASS and
Wide-field Infrared Survey Explorer(WISE) and by observed-to-model
WISE fluxes, and are also estimated using dust mass-loss rates from
literature assuming a typical gas-to-dust mass ratio of 400. To specify
the pulsations, we extract the period and height of the highest peak in
the power spectrum of oscillation. Absolute magnitudes are obtained from
the 2MASS Ks band and the Gaia DR2 parallaxes. Our results
follow. (i) Substantial mass-loss sets in at pulsation periods above
∼60 and ∼100 d, corresponding to Asymptotic-Giant-Branch stars at
the base of the period-luminosity sequences C' and C. (ii)
The mass-loss rate starts to rapidly increase in semiregulars for which
the luminosity is just above the red-giant-branch tip and gradually
plateaus to a level similar to that of Miras. (iii) The mass-loss rates
in Miras do not depend on luminosity, consistent with pulsation-enhanced
dust-driven winds. (iv) The accumulated mass-loss on the red giant
branch consistent with asteroseismic predictions reduces the masses of
red-clump stars by 6.3 per cent, less than the typical uncertainty on
their asteroseismic masses. Thus mass-loss is currently not a limitation
of stellar age estimates for galactic archaeology studies.
Title: Accurate Short-Characteristics Radiative Transfer in A
Numerical Tool for Astrophysical RESearch (ANTARES)
Authors: Kostogryz, Nadiia M.; Kupka, Friedrich; Piskunov, Nikolai;
Fabbian, Damian; Krüger, Daniel; Gizon, Laurent
Bibcode: 2021SoPh..296...46K
Altcode:
We aim to improve the accuracy of radiative energy transport in
three-dimensional radiation hydrodynamical simulations in ANTARES
(A Numerical Tool for Astrophysical RESearch). We implement in the
ANTARES short-characteristics numerical schemes a modification of
the Bézier interpolant solver. This method yields a smoother surface
structure in simulations of solar convection and reduces the artifacts
appearing due to the limited number of rays along which the integration
is done. Reducing such artifacts leads to increased stability of the
code. We show that our new implementation achieves a better agreement
of the temperature structure and its gradient with a semi-empirical
model derived from observations, as well as of synthetic spectral-line
profiles with the observed solar spectrum.
Title: Rossby Waves in Astrophysics
Authors: Zaqarashvili, T. V.; Albekioni, M.; Ballester, J. L.;
Bekki, Y.; Biancofiore, L.; Birch, A. C.; Dikpati, M.; Gizon, L.;
Gurgenashvili, E.; Heifetz, E.; Lanza, A. F.; McIntosh, S. W.; Ofman,
L.; Oliver, R.; Proxauf, B.; Umurhan, O. M.; Yellin-Bergovoy, R.
Bibcode: 2021SSRv..217...15Z
Altcode:
Rossby waves are a pervasive feature of the large-scale motions of the
Earth's atmosphere and oceans. These waves (also known as planetary
waves and r-modes) also play an important role in the large-scale
dynamics of different astrophysical objects such as the solar
atmosphere and interior, astrophysical discs, rapidly rotating stars,
planetary and exoplanetary atmospheres. This paper provides a review
of theoretical and observational aspects of Rossby waves on different
spatial and temporal scales in various astrophysical settings. The
physical role played by Rossby-type waves and associated instabilities
is discussed in the context of solar and stellar magnetic activity,
angular momentum transport in astrophysical discs, planet formation,
and other astrophysical processes. Possible directions of future
research in theoretical and observational aspects of astrophysical
Rossby waves are outlined.
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: Power spectrum of turbulent convection in the solar photosphere
Authors: Yelles Chaouche, L.; Cameron, R. H.; Solanki, S. K.;
Riethmüller, T. L.; Anusha, L. S.; Witzke, V.; Shapiro, A. I.;
Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; van Noort,
M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez,
D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
Bibcode: 2020A&A...644A..44Y
Altcode: 2020arXiv201009037Y
The solar photosphere provides us with a laboratory for understanding
turbulence in a layer where the fundamental processes of transport
vary rapidly and a strongly superadiabatic region lies very closely
to a subadiabatic layer. Our tools for probing the turbulence are
high-resolution spectropolarimetric observations such as have recently
been obtained with the two balloon-borne SUNRISE missions, and numerical
simulations. Our aim is to study photospheric turbulence with the
help of Fourier power spectra that we compute from observations
and simulations. We also attempt to explain some properties of the
photospheric overshooting flow with the help of its governing equations
and simulations. We find that quiet-Sun observations and smeared
simulations are consistent with each other and exhibit a power-law
behavior in the subgranular range of their Doppler velocity power
spectra with a power-law index of ≈ - 2. The unsmeared simulations
exhibit a power law that extends over the full range between the
integral and Taylor scales with a power-law index of ≈ - 2.25. The
smearing, reminiscent of observational conditions, considerably reduces
the extent of the power-law-like portion of the power spectra. This
suggests that the limited spatial resolution in some observations
might eventually result in larger uncertainties in the estimation of
the power-law indices. The simulated vertical velocity power spectra
as a function of height show a rapid change in the power-law index
(at the subgranular range) from roughly the optical depth unity layer,
that is, the solar surface, to 300 km above it. We propose that the
cause of the steepening of the power-law index is the transition from
a super- to a subadiabatic region, in which the dominant source of
motions is overshooting convection. A scale-dependent transport of
the vertical momentum occurs. At smaller scales, the vertical momentum
is more efficiently transported sideways than at larger scales. This
results in less vertical velocity power transported upward at small
scales than at larger scales and produces a progressively steeper
vertical velocity power law below 180 km. Above this height, the
gravity work progressively gains importance at all relevant scales,
making the atmosphere progressively more hydrostatic and resulting
in a gradually less steep power law. Radiative heating and cooling of
the plasma is shown to play a dominant role in the plasma energetics
in this region, which is important in terms of nonadiabatic damping
of the convective motions.
Title: Solar east-west flow correlations that persist for months at
low latitudes are dominated by active region inflows
Authors: Hanson, Chris S.; Duvall, Thomas L.; Birch, Aaron C.; Gizon,
Laurent; Sreenivasan, Katepalli R.
Bibcode: 2020A&A...644A.103H
Altcode: 2020arXiv201013052H
Context. Giant-cell convection is believed to be an important
component of solar dynamics. For example, it is expected to play a
crucial role in maintaining the Sun's differential rotation.
Aims: We reexamine early reports of giant convective cells detected
using a correlation analysis of Dopplergrams. We extend this analysis
using 19 years of space- and ground-based observations of near-surface
horizontal flows.
Methods: Flow maps are derived through the
local correlation tracking of granules and helioseismic ring-diagram
analysis. We compute temporal auto-correlation functions of the
east-west flows at fixed latitude.
Results: Correlations in the
east-west velocity can be clearly seen up to five rotation periods. The
signal consists of features with longitudinal wavenumbers up to m = 9
at low latitudes. Comparison with magnetic images indicates that these
flow features are associated with magnetic activity. The signal is not
seen above the noise level during solar minimum.
Conclusions:
Our results show that the long-term correlations in east-west flows
at low latitudes are predominantly due to inflows into active regions
and not to giant convective cells.
Title: Acoustic wave propagation through solar granulation: Validity
of effective-medium theories, coda waves
Authors: Poulier, P. -L.; Fournier, D.; Gizon, L.; Duvall, T. L.
Bibcode: 2020A&A...643A.168P
Altcode: 2020arXiv201001174P
Context. The frequencies, lifetimes, and eigenfunctions of solar
acoustic waves are affected by turbulent convection, which is random
in space and in time. Since the correlation time of solar granulation
and the periods of acoustic waves (∼5 min) are similar, the medium
in which the waves propagate cannot a priori be assumed to be time
independent.
Aims: We compare various effective-medium solutions
with numerical solutions in order to identify the approximations
that can be used in helioseismology. For the sake of simplicity, the
medium is one dimensional.
Methods: We consider the Keller
approximation, the second-order Born approximation, and spatial
homogenization to obtain theoretical values for the effective
wave speed and attenuation (averaged over the realizations of the
medium). Numerically, we computed the first and second statistical
moments of the wave field over many thousands of realizations of
the medium (finite-amplitude sound-speed perturbations are limited
to a 30 Mm band and have a zero mean).
Results: The effective
wave speed is reduced for both the theories and the simulations. The
attenuation of the coherent wave field and the wave speed are best
described by the Keller theory. The numerical simulations reveal the
presence of coda waves, trailing the ballistic wave packet. These
late arrival waves are due to multiple scattering and are easily
seen in the second moment of the wave field.
Conclusions: We
find that the effective wave speed can be calculated, numerically and
theoretically, using a single snapshot of the random medium (frozen
medium); however, the attenuation is underestimated in the frozen
medium compared to the time-dependent medium. Multiple scattering
cannot be ignored when modeling acoustic wave propagation through solar
granulation. Movies associated to Figs. 3 and 9 are available at https://www.aanda.org
Title: Effect of latitudinal differential rotation on solar Rossby
waves: Critical layers, eigenfunctions, and momentum fluxes in the
equatorial β plane
Authors: Gizon, L.; Fournier, D.; Albekioni, M.
Bibcode: 2020A&A...642A.178G
Altcode: 2020arXiv200802185G
Context. Retrograde-propagating waves of vertical vorticity with
longitudinal wavenumbers between 3 and 15 have been observed on the Sun
with a dispersion relation close to that of classical sectoral Rossby
waves. The observed vorticity eigenfunctions are symmetric in latitude,
peak at the equator, switch sign near 20°-30°, and decrease at higher
latitudes.
Aims: We search for an explanation that takes solar
latitudinal differential rotation into account.
Methods: In
the equatorial β plane, we studied the propagation of linear Rossby
waves (phase speed c < 0) in a parabolic zonal shear flow, U = - U̅
ξ2 < 0, where U̅ = 244 m s-1, and ξ is the
sine of latitude.
Results: In the inviscid case, the eigenvalue
spectrum is real and continuous, and the velocity stream functions are
singular at the critical latitudes where U = c. We add eddy viscosity
to the problem to account for wave attenuation. In the viscous case,
the stream functions solve a fourth-order modified Orr-Sommerfeld
equation. Eigenvalues are complex and discrete. For reasonable values
of the eddy viscosity corresponding to supergranular scales and above
(Reynolds number 100 ≤ Re ≤ 700), all modes are stable. At fixed
longitudinal wavenumber, the least damped mode is a symmetric mode
whose real frequency is close to that of the classical Rossby mode,
which we call the R mode. For Re ≈ 300, the attenuation and the
real part of the eigenfunction is in qualitative agreement with the
observations (unlike the imaginary part of the eigenfunction, which has
a larger amplitude in the model).
Conclusions: Each longitudinal
wavenumber is associated with a latitudinally symmetric R mode trapped
at low latitudes by solar differential rotation. In the viscous model,
R modes transport significant angular momentum from the dissipation
layers toward the equator.
Title: The Solar Orbiter Science Activity Plan. Translating solar
and heliospheric physics questions into action
Authors: Zouganelis, I.; De Groof, A.; Walsh, A. P.; Williams, D. R.;
Müller, D.; St Cyr, O. C.; Auchère, F.; Berghmans, D.; Fludra,
A.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.;
Owen, C. J.; Rodríguez-Pacheco, J.; Romoli, M.; Solanki, S. K.;
Watson, C.; Sanchez, L.; Lefort, J.; Osuna, P.; Gilbert, H. R.;
Nieves-Chinchilla, T.; Abbo, L.; Alexandrova, O.; Anastasiadis, A.;
Andretta, V.; Antonucci, E.; Appourchaux, T.; Aran, A.; Arge, C. N.;
Aulanier, G.; Baker, D.; Bale, S. D.; Battaglia, M.; Bellot Rubio,
L.; Bemporad, A.; Berthomier, M.; Bocchialini, K.; Bonnin, X.; Brun,
A. S.; Bruno, R.; Buchlin, E.; Büchner, J.; Bucik, R.; Carcaboso,
F.; Carr, R.; Carrasco-Blázquez, I.; Cecconi, B.; Cernuda Cangas, I.;
Chen, C. H. K.; Chitta, L. P.; Chust, T.; Dalmasse, K.; D'Amicis, R.;
Da Deppo, V.; De Marco, R.; Dolei, S.; Dolla, L.; Dudok de Wit, T.;
van Driel-Gesztelyi, L.; Eastwood, J. P.; Espinosa Lara, F.; Etesi,
L.; Fedorov, A.; Félix-Redondo, F.; Fineschi, S.; Fleck, B.; Fontaine,
D.; Fox, N. J.; Gandorfer, A.; Génot, V.; Georgoulis, M. K.; Gissot,
S.; Giunta, A.; Gizon, L.; Gómez-Herrero, R.; Gontikakis, C.; Graham,
G.; Green, L.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler,
D. M.; Hirzberger, J.; Ho, G. C.; Hurford, G.; Innes, D.; Issautier,
K.; James, A. W.; Janitzek, N.; Janvier, M.; Jeffrey, N.; Jenkins,
J.; Khotyaintsev, Y.; Klein, K. -L.; Kontar, E. P.; Kontogiannis,
I.; Krafft, C.; Krasnoselskikh, V.; Kretzschmar, M.; Labrosse, N.;
Lagg, A.; Landini, F.; Lavraud, B.; Leon, I.; Lepri, S. T.; Lewis,
G. R.; Liewer, P.; Linker, J.; Livi, S.; Long, D. M.; Louarn, P.;
Malandraki, O.; Maloney, S.; Martinez-Pillet, V.; Martinovic, M.;
Masson, A.; Matthews, S.; Matteini, L.; Meyer-Vernet, N.; Moraitis,
K.; Morton, R. J.; Musset, S.; Nicolaou, G.; Nindos, A.; O'Brien,
H.; Orozco Suarez, D.; Owens, M.; Pancrazzi, M.; Papaioannou, A.;
Parenti, S.; Pariat, E.; Patsourakos, S.; Perrone, D.; Peter, H.;
Pinto, R. F.; Plainaki, C.; Plettemeier, D.; Plunkett, S. P.; Raines,
J. M.; Raouafi, N.; Reid, H.; Retino, A.; Rezeau, L.; Rochus, P.;
Rodriguez, L.; Rodriguez-Garcia, L.; Roth, M.; Rouillard, A. P.;
Sahraoui, F.; Sasso, C.; Schou, J.; Schühle, U.; Sorriso-Valvo, L.;
Soucek, J.; Spadaro, D.; Stangalini, M.; Stansby, D.; Steller, M.;
Strugarek, A.; Štverák, Š.; Susino, R.; Telloni, D.; Terasa, C.;
Teriaca, L.; Toledo-Redondo, S.; del Toro Iniesta, J. C.; Tsiropoula,
G.; Tsounis, A.; Tziotziou, K.; Valentini, F.; Vaivads, A.; Vecchio,
A.; Velli, M.; Verbeeck, C.; Verdini, A.; Verscharen, D.; Vilmer, N.;
Vourlidas, A.; Wicks, R.; Wimmer-Schweingruber, R. F.; Wiegelmann,
T.; Young, P. R.; Zhukov, A. N.
Bibcode: 2020A&A...642A...3Z
Altcode: 2020arXiv200910772Z
Solar Orbiter is the first space mission observing the solar plasma
both in situ and remotely, from a close distance, in and out of the
ecliptic. The ultimate goal is to understand how the Sun produces
and controls the heliosphere, filling the Solar System and driving
the planetary environments. With six remote-sensing and four in-situ
instrument suites, the coordination and planning of the operations are
essential to address the following four top-level science questions:
(1) What drives the solar wind and where does the coronal magnetic field
originate?; (2) How do solar transients drive heliospheric variability?;
(3) How do solar eruptions produce energetic particle radiation that
fills the heliosphere?; (4) How does the solar dynamo work and drive
connections between the Sun and the heliosphere? Maximising the
mission's science return requires considering the characteristics
of each orbit, including the relative position of the spacecraft
to Earth (affecting downlink rates), trajectory events (such
as gravitational assist manoeuvres), and the phase of the solar
activity cycle. Furthermore, since each orbit's science telemetry
will be downloaded over the course of the following orbit, science
operations must be planned at mission level, rather than at the level
of individual orbits. It is important to explore the way in which those
science questions are translated into an actual plan of observations
that fits into the mission, thus ensuring that no opportunities are
missed. First, the overarching goals are broken down into specific,
answerable questions along with the required observations and the
so-called Science Activity Plan (SAP) is developed to achieve this. The
SAP groups objectives that require similar observations into Solar
Orbiter Observing Plans, resulting in a strategic, top-level view of
the optimal opportunities for science observations during the mission
lifetime. This allows for all four mission goals to be addressed. In
this paper, we introduce Solar Orbiter's SAP through a series of
examples and the strategy being followed.
Title: The Polarimetric and Helioseismic Imager on Solar Orbiter
Authors: Solanki, S. K.; del Toro Iniesta, J. C.; Woch, J.; Gandorfer,
A.; Hirzberger, J.; Alvarez-Herrero, A.; Appourchaux, T.; Martínez
Pillet, V.; Pérez-Grande, I.; Sanchis Kilders, E.; Schmidt, W.;
Gómez Cama, J. M.; Michalik, H.; Deutsch, W.; Fernandez-Rico, G.;
Grauf, B.; Gizon, L.; Heerlein, K.; Kolleck, M.; Lagg, A.; Meller, R.;
Müller, R.; Schühle, U.; Staub, J.; Albert, K.; Alvarez Copano, M.;
Beckmann, U.; Bischoff, J.; Busse, D.; Enge, R.; Frahm, S.; Germerott,
D.; Guerrero, L.; Löptien, B.; Meierdierks, T.; Oberdorfer, D.;
Papagiannaki, I.; Ramanath, S.; Schou, J.; Werner, S.; Yang, D.;
Zerr, A.; Bergmann, M.; Bochmann, J.; Heinrichs, J.; Meyer, S.;
Monecke, M.; Müller, M. -F.; Sperling, M.; Álvarez García, D.;
Aparicio, B.; Balaguer Jiménez, M.; Bellot Rubio, L. R.; Cobos
Carracosa, J. P.; Girela, F.; Hernández Expósito, D.; Herranz, M.;
Labrousse, P.; López Jiménez, A.; Orozco Suárez, D.; Ramos, J. L.;
Barandiarán, J.; Bastide, L.; Campuzano, C.; Cebollero, M.; Dávila,
B.; Fernández-Medina, A.; García Parejo, P.; Garranzo-García, D.;
Laguna, H.; Martín, J. A.; Navarro, R.; Núñez Peral, A.; Royo, M.;
Sánchez, A.; Silva-López, M.; Vera, I.; Villanueva, J.; Fourmond,
J. -J.; de Galarreta, C. Ruiz; Bouzit, M.; Hervier, V.; Le Clec'h,
J. C.; Szwec, N.; Chaigneau, M.; Buttice, V.; Dominguez-Tagle, C.;
Philippon, A.; Boumier, P.; Le Cocguen, R.; Baranjuk, G.; Bell,
A.; Berkefeld, Th.; Baumgartner, J.; Heidecke, F.; Maue, T.; Nakai,
E.; Scheiffelen, T.; Sigwarth, M.; Soltau, D.; Volkmer, R.; Blanco
Rodríguez, J.; Domingo, V.; Ferreres Sabater, A.; Gasent Blesa,
J. L.; Rodríguez Martínez, P.; Osorno Caudel, D.; Bosch, J.; Casas,
A.; Carmona, M.; Herms, A.; Roma, D.; Alonso, G.; Gómez-Sanjuan, A.;
Piqueras, J.; Torralbo, I.; Fiethe, B.; Guan, Y.; Lange, T.; Michel,
H.; Bonet, J. A.; Fahmy, S.; Müller, D.; Zouganelis, I.
Bibcode: 2020A&A...642A..11S
Altcode: 2019arXiv190311061S
Aims: This paper describes the Polarimetric and Helioseismic
Imager on the Solar Orbiter mission (SO/PHI), the first magnetograph and
helioseismology instrument to observe the Sun from outside the Sun-Earth
line. It is the key instrument meant to address the top-level science
question: How does the solar dynamo work and drive connections between
the Sun and the heliosphere? SO/PHI will also play an important role
in answering the other top-level science questions of Solar Orbiter,
while hosting the potential of a rich return in further science.
Methods: SO/PHI measures the Zeeman effect and the Doppler shift
in the Fe I 617.3 nm spectral line. To this end, the instrument
carries out narrow-band imaging spectro-polarimetry using a tunable
LiNbO3 Fabry-Perot etalon, while the polarisation modulation
is done with liquid crystal variable retarders. The line and the nearby
continuum are sampled at six wavelength points and the data are recorded
by a 2k × 2k CMOS detector. To save valuable telemetry, the raw data
are reduced on board, including being inverted under the assumption of
a Milne-Eddington atmosphere, although simpler reduction methods are
also available on board. SO/PHI is composed of two telescopes; one,
the Full Disc Telescope, covers the full solar disc at all phases of
the orbit, while the other, the High Resolution Telescope, can resolve
structures as small as 200 km on the Sun at closest perihelion. The high
heat load generated through proximity to the Sun is greatly reduced by
the multilayer-coated entrance windows to the two telescopes that allow
less than 4% of the total sunlight to enter the instrument, most of
it in a narrow wavelength band around the chosen spectral line.
Results: SO/PHI was designed and built by a consortium having partners
in Germany, Spain, and France. The flight model was delivered to
Airbus Defence and Space, Stevenage, and successfully integrated into
the Solar Orbiter spacecraft. A number of innovations were introduced
compared with earlier space-based spectropolarimeters, thus allowing
SO/PHI to fit into the tight mass, volume, power and telemetry budgets
provided by the Solar Orbiter spacecraft and to meet the (e.g. thermal)
challenges posed by the mission's highly elliptical orbit.
Title: PMI: The Photospheric Magnetic Field Imager
Authors: Staub, Jan; Fernandez-Rico, German; Gandorfer, Achim; Gizon,
Laurent; Hirzberger, Johann; Kraft, Stefan; Lagg, Andreas; Schou,
Jesper; Solanki, Sami K.; del Toro Iniesta, Jose Carlos; Wiegelmann,
Thomas; Woch, Joachim
Bibcode: 2020JSWSC..10...54S
Altcode:
We describe the design and the capabilities of the Photospheric Magnetic
field Imager (PMI), a compact and lightweight vector magnetograph,
which is being developed for ESA's Lagrange mission to the Lagrange
L5 point. After listing the design requirements and give a scientific
justification for them, we describe the technical implementation and
the design solution capable of fulfilling these requirements. This is
followed by a description of the hardware architecture as well as the
operations principle. An outlook on the expected performance concludes
the paper.
Title: The PLATO mission: Studying the diversity of exoplanets
orbiting up to the habitable zone of Sun-like stars
Authors: Heras, Ana Maria; Rauer, Heike; Aerts, Conny; Deleuil, Magali;
Gizon, Laurent; Goupil, Marie-Jo; Mas-Hesse, Miguel; Piotto, Giampaolo;
Pollacco, Don; Ragazzoni, Roberto; Ramsay, Gavin; Udry, Stéphane
Bibcode: 2020EPSC...14..396H
Altcode:
The ESA PLATO mission will provide unprecedented data to study the
diversity of planets orbiting up to the habitable zone of bright
Sun-like stars. PLATO will detect and characterise exoplanets using the
transit method combined with ground-based radial velocity measurements,
and study the host stars with asteroseismology. PLATO"s core observing
sample consists of Sun-like stars of V < 11. For statistical studies,
PLATO will also monitor a large sample of Sun-like stars with V <
13 and cool late-type dwarfs with V < 16. To benefit from PLATO"s
advanced photometric capabilities, the general community will be invited
to submit proposals on complementary science topics in the framework of
a guest observer"s programme. The PLATO payload consists of four groups
of six cameras each that overlap covering a total field of about 2150
deg2 with four different sensitivities. Two additional cameras will
observe the brightest stars (V < 8.5) in two-colours, and will be
used as fine guidance sensor. PLATO is the third medium-class mission in
ESA"s Cosmic Vision programme, with a planned launch date in 2026. The
satellite will operate in an orbit around the second Lagrange point,
L2. We will present the status of the mission science definition and
performance, and of the satellite and ground-segment developments.
Title: Detection of exomoons in simulated light curves with a
regularized convolutional neural network
Authors: Alshehhi, Rasha; Rodenbeck, Kai; Gizon, Laurent; Sreenivasan,
Katepalli R.
Bibcode: 2020A&A...640A..41A
Altcode: 2020arXiv200513035A
Context. Many moons have been detected around planets in our Solar
System, but none has been detected unambiguously around any of the
confirmed extrasolar planets.
Aims: We test the feasibility of a
supervised convolutional neural network to classify photometric transit
light curves of planet-host stars and identify exomoon transits, while
avoiding false positives caused by stellar variability or instrumental
noise.
Methods: Convolutional neural networks are known to have
contributed to improving the accuracy of classification tasks. The
network optimization is typically performed without studying the
effect of noise on the training process. Here we design and optimize
a 1D convolutional neural network to classify photometric transit
light curves. We regularize the network by the total variation loss
in order to remove unwanted variations in the data features.
Results: Using numerical experiments, we demonstrate the benefits
of our network, which produces results comparable to or better than
the standard network solutions. Most importantly, our network clearly
outperforms a classical method used in exoplanet science to identify
moon-like signals. Thus the proposed network is a promising approach
for analyzing real transit light curves in the future.
Title: Average motion of emerging solar active region
polarities. II. Joy's law
Authors: Schunker, H.; Baumgartner, C.; Birch, A. C.; Cameron, R. H.;
Braun, D. C.; Gizon, L.
Bibcode: 2020A&A...640A.116S
Altcode: 2020arXiv200605565S
Context. The tilt of solar active regions described by Joy's law
is essential for converting a toroidal field to a poloidal field in
Babcock-Leighton dynamo models. In thin flux tube models the Coriolis
force causes what we observe as Joy's law, acting on east-west flows
as they rise towards the surface.
Aims: Our goal is to measure
the evolution of the average tilt angle of hundreds of active regions
as they emerge, so that we can constrain the origins of Joy's law.
Methods: We measured the tilt angle of the primary bipoles in 153
emerging active regions (EARs) in the Solar Dynamics Observatory
Helioseismic Emerging Active Region survey. We used line-of-sight
magnetic field measurements averaged over 6 h to define the polarities
and measure the tilt angle up to four days after emergence.
Results: We find that at the time of emergence the polarities are on
average aligned east-west, and that neither the separation nor the
tilt depends on latitude. We do find, however, that EARs at higher
latitudes have a faster north-south separation speed than those closer
to the equator at the emergence time. After emergence, the tilt
angle increases and Joy's law is evident about two days later. The
scatter in the tilt angle is independent of flux until about one day
after emergence, when we find that higher-flux regions have a smaller
scatter in tilt angle than lower-flux regions.
Conclusions:
Our finding that active regions emerge with an east-west alignment
is consistent with earlier observations, but is still surprising
since thin flux tube models predict that tilt angles of rising flux
tubes are generated below the surface. Previously reported tilt angle
relaxation of deeply anchored flux tubes can be largely explained
by the change in east-west separation. We conclude that Joy's law is
caused by an inherent north-south separation speed present when the
flux first reaches the surface, and that the scatter in the tilt angle
is consistent with buffeting of the polarities by supergranulation.
Title: Predicting frequency changes of global-scale solar Rossby
modes due to solar cycle changes in internal rotation
Authors: Goddard, C. R.; Birch, A. C.; Fournier, D.; Gizon, L.
Bibcode: 2020A&A...640L..10G
Altcode: 2020arXiv200714387G
Context. Large-scale equatorial Rossby modes have been observed on the
Sun over the last two solar cycles.
Aims: We investigate the
impact of the time-varying zonal flows on the frequencies of Rossby
modes.
Methods: A first-order perturbation theory approach
is used to obtain an expression for the expected shift in the mode
frequencies due to perturbations in the internal rotation rate.
Results: Using the time-varying rotation from helioseismic inversions
we predict the changes in Rossby mode frequencies with azimuthal
orders from m = 1 to m = 15 over the last two solar cycles. The
peak-to-peak frequency change is less than 1 nHz for the m = 1 mode,
grows with m, and reaches 25 nHz for m = 15.
Conclusions: Given
the observational uncertainties on mode frequencies due to the finite
mode lifetimes, we find that the predicted frequency shifts are near
the limit of detectability.
Title: Exomoon indicators in high-precision transit light curves
Authors: Rodenbeck, Kai; Heller, René; Gizon, Laurent
Bibcode: 2020A&A...638A..43R
Altcode: 2020arXiv200402259R
Context. While the Solar System contains about 20 times more
moons than planets, no moon has been confirmed around any of the
thousands of extrasolar planets discovered so far. Considering the
large computational load required for the statistical vetting of
exomoon candidates in a star-planet-moon framework, tools for an
uncomplicated identification of the most promising exomoon candidates
could be beneficial to streamline follow-up studies.
Aims:
Here we study three exomoon indicators that emerge if well-established
planet-only models are fitted to a planet-moon transit light curve:
transit timing variations (TTVs), transit duration variations (TDVs),
and apparent planetary transit radius variations (TRVs). We re-evaluate
under realistic conditions the previously proposed exomoon signatures
in the TTV and TDV series.
Methods: We simulated light curves of
a transiting exoplanet with a single moon, taking into account stellar
limb darkening, orbital inclinations, planet-moon occultations, and
noise from both stellar granulation and instrumental effects. These
model light curves were then fitted with a planet-only transit model
whilst pretending there were no moon, and we explored the resulting
TTV, TDV, and TRV series for evidence of the moon.
Results:
The previously described ellipse in the TTV-TDV diagram of an exoplanet
with a moon emerges only for high-density moons. However, low-density
moons distort the sinusoidal shapes of the TTV and the TDV series
due to their photometric contribution to the combined planet-moon
transit. Sufficiently large moons can nevertheless produce periodic
apparent TRVs of their host planets that could be observable. We
find that Kepler and PLATO have similar performances in detecting the
exomoon-induced TRV effect around simulated bright (mV = 8)
stars. Although these stars are rare in the Kepler sample, they will be
abundant in the PLATO sample. Moreover, PLATO's higher cadence yields
a stronger TTV signal. We detect substantial TRVs of the Saturn-sized
planet Kepler-856 b although an exomoon could only ensure Hill stability
in a very narrow orbital range.
Conclusions: The periodogram of
the sequence of transit radius measurements can indicate the presence
of a moon. The TTV and TDV series of exoplanets with moons could be
more complex than previously assumed. We propose that TRVs could be
a more promising means to identify exomoons in large exoplanet surveys.
Title: Meridional flow in the Sun’s convection zone is a single
cell in each hemisphere
Authors: Gizon, Laurent; Cameron, Robert H.; Pourabdian, Majid; Liang,
Zhi-Chao; Fournier, Damien; Birch, Aaron C.; Hanson, Chris S.
Bibcode: 2020Sci...368.1469G
Altcode:
The Sun’s magnetic field is generated by subsurface motions of the
convecting plasma. The latitude at which the magnetic field emerges
through the solar surface (as sunspots) drifts toward the equator
over the course of the 11-year solar cycle. We use helioseismology to
infer the meridional flow (in the latitudinal and radial directions)
over two solar cycles covering 1996-2019. Two data sources are used,
which agree during their overlap period of 2001-2011. The time-averaged
meridional flow is shown to be a single cell in each hemisphere,
carrying plasma toward the equator at the base of the convection zone
with a speed of ~4 meters per second at 45° latitude. Our results
support the flux-transport dynamo model, which explains the drift of
sunspot-emergence latitudes through the meridional flow.
Title: Rossby modes in slowly rotating stars: depth dependence in
distorted polytropes with uniform rotation
Authors: Damiani, C.; Cameron, R. H.; Birch, A. C.; Gizon, L.
Bibcode: 2020A&A...637A..65D
Altcode: 2020arXiv200305276D
Context. Large-scale Rossby waves have recently been discovered based on
measurements of horizontal surface and near-surface solar flows.
Aims: We are interested in understanding why it is only equatorial modes
that are observed and in modelling the radial structure of the observed
modes. To this aim, we have characterised the radial eigenfunctions
of r modes for slowly rotating polytropes in uniform rotation.
Methods: We followed Provost et al. (1981, A&A, 94, 126) and
considered a linear perturbation theory to describe quasi-toroidal
stellar adiabatic oscillations in the inviscid case. We used
perturbation theory to write the solutions to the fourth order in the
rotational frequency of the star. We numerically solved the eigenvalue
problem, concentrating on the type of behaviour exhibited where the
stratification is nearly adiabatic.
Results: We find that for
free-surface boundary conditions on a spheroid of non-vanishing surface
density, r modes can only exist for ℓ = m spherical harmonics in the
inviscid case and we compute their depth dependence and frequencies to
leading order. For quasi-adiabatic stratification, the sectoral modes
with no radial nodes are the only modes which are almost toroidal and
the depth dependence of the corresponding horizontal motion scales as
rm. For all r modes, except the zero radial order sectoral
ones, non-adiabatic stratification plays a crucial role in the radial
force balance.
Conclusions: The lack of quasi-toroidal solutions
when stratification is close to neutral, except for the sectoral modes
without nodes in radius, follows from the need for both horizontal
and radial force balance. In the absence of super- or sub-adiabatic
stratification and viscosity, both the horizontal and radial parts of
the force balance independently determine the pressure perturbation. The
only quasi-toroidal cases in which these constraints on the pressure
perturbation are consistent are the special cases where ℓ = m and
the horizontal displacement scales with rm.
Title: The Solaris Solar Polar Mission
Authors: Hassler, Donald M.; Newmark, Jeff; Gibson, Sarah; Harra,
Louise; Appourchaux, Thierry; Auchere, Frederic; Berghmans, David;
Colaninno, Robin; Fineschi, Silvano; Gizon, Laurent; Gosain, Sanjay;
Hoeksema, Todd; Kintziger, Christian; Linker, John; Rochus, Pierre;
Schou, Jesper; Viall, Nicholeen; West, Matt; Woods, Tom; Wuelser,
Jean-Pierre
Bibcode: 2020EGUGA..2217703H
Altcode:
The solar poles are one of the last unexplored regions of the solar
system. Although Ulysses flew over the poles in the 1990s, it did
not have remote sensing instruments onboard to probe the Sun's polar
magnetic field or surface/sub-surface flows.We will discuss Solaris,
a proposed Solar Polar MIDEX mission to revolutionize our understanding
of the Sun by addressing fundamental questions that can only be answered
from a polar vantage point. Solaris uses a Jupiter gravity assist to
escape the ecliptic plane and fly over both poles of the Sun to >75
deg. inclination, obtaining the first high-latitude, multi-month-long,
continuous remote-sensing solar observations. Solaris will address key
outstanding, breakthrough problems in solar physics and fill holes in
our scientific understanding that will not be addressed by current
missions.With focused science and a simple, elegant mission design,
Solaris will also provide enabling observations for space weather
research (e.g. polar view of CMEs), and stimulate future research
through new unanticipated discoveries.
Title: Solar-cycle irradiance variations over the last four billion
years
Authors: Shapiro, Anna V.; Shapiro, Alexander I.; Gizon, Laurent;
Krivova, Natalie A.; Solanki, Sami K.
Bibcode: 2020A&A...636A..83S
Altcode: 2020arXiv200208806S
Context. The variability of the spectral solar irradiance (SSI) over the
course of the 11-year solar cycle is one of the manifestations of solar
magnetic activity. There is strong evidence that the SSI variability
has an effect on the Earth's atmosphere. The faster rotation of the
Sun in the past lead to a more vigorous action of solar dynamo and
thus potentially to larger amplitude of the SSI variability on the
timescale of the solar activity cycle. This could lead to a stronger
response of the Earth's atmosphere as well as other solar system
planets' atmospheres to the solar activity cycle.
Aims: We
calculate the amplitude of the SSI and total solar irradiance (TSI)
variability over the course of the solar activity cycle as a function
of solar age.
Methods: We employed the relationship between the
stellar magnetic activity and the age based on observations of solar
twins. Using this relation, we reconstructed solar magnetic activity
and the corresponding solar disk area coverages by magnetic features
(i.e., spots and faculae) over the last four billion years. These disk
coverages were then used to calculate the amplitude of the solar-cycle
SSI variability as a function of wavelength and solar age.
Results: Our calculations show that the young Sun was significantly
more variable than the present Sun. The amplitude of the solar-cycle TSI
variability of the 600 Myr old Sun was about ten times larger than that
of the present Sun. Furthermore, the variability of the young Sun was
spot-dominated (the Sun being brighter at the activity minimum than
in the maximum), that is, the Sun was overall brighter at activity
minima than at maxima. The amplitude of the TSI variability decreased
with solar age until it reached a minimum value at 2.8 Gyr. After this
point, the TSI variability is faculae-dominated (the Sun is brighter
at the activity maximum) and its amplitude increases with age.
Title: Solar Rossby waves observed in GONG++ ring-diagram flow maps
Authors: Hanson, Chris S.; Gizon, Laurent; Liang, Zhi-Chao
Bibcode: 2020A&A...635A.109H
Altcode: 2020arXiv200201194H
Context. Solar Rossby waves have only recently been unambiguously
identified in Helioseimsic and Magnetic Imager (HMI) and Michelson
Doppler Imager maps of flows near the solar surface. So far this
has not been done with the Global Oscillation Network Group (GONG)
ground-based observations, which have different noise properties.
Aims: We use 17 years of GONG++ data to identify and characterize solar
Rossby waves using ring-diagram helioseismology. We compare directly
with HMI ring-diagram analysis.
Methods: Maps of the radial
vorticity were obtained for flows within the top 2 Mm of the surface
for 17 years of GONG++ data. The data were corrected for systematic
effects including the annual periodicity related to the B0
angle. We then computed the Fourier components of the radial vorticity
of the flows in the co-rotating frame. We performed the same analysis
on the HMI data that overlap in time.
Results: We find that the
solar Rossby waves have measurable amplitudes in the GONG++ sectoral
power spectra for azimuthal orders between m = 3 and m = 15. The
measured mode characteristics (frequencies, lifetimes, and amplitudes)
from GONG++ are consistent with the HMI measurements in the overlap
period from 2010 to 2018 for m ≤ 9. For higher-m modes the amplitudes
and frequencies agree within two sigmas. The signal-to-noise ratio
of modes in GONG++ power spectra is comparable to those of HMI for 8
≤ m ≤ 11, but is lower by a factor of two for other modes.
Conclusions: The GONG++ data provide a long and uniform data set that
can be used to study solar global-scale Rossby waves from 2001.
Title: Characterizing the spatial pattern of solar supergranulation
using the bispectrum
Authors: Böning, Vincent G. A.; Birch, Aaron C.; Gizon, Laurent;
Duvall, Thomas L.; Schou, Jesper
Bibcode: 2020A&A...635A.181B
Altcode: 2020arXiv200208262B
Context. The spatial power spectrum of supergranulation does not
fully characterize the underlying physics of turbulent convection. For
example, it does not describe the non-Gaussianity in the horizontal flow
divergence.
Aims: Our aim is to statistically characterize the
spatial pattern of solar supergranulation beyond the power spectrum. The
next-order statistic is the bispectrum. It measures correlations
of three Fourier components and is related to the nonlinearities in
the underlying physics. It also characterizes how a skewness in the
dataset is generated by the coupling of three Fourier components.
Methods: We estimated the bispectrum of supergranular horizontal surface
divergence maps that were obtained using local correlation tracking
(LCT) and time-distance helioseismology (TD) from one year of data
from the helioseismic and magnetic imager on-board the solar dynamics
observatory starting in May 2010.
Results: We find significantly
nonzero and consistent estimates for the bispectrum using LCT and
TD. The strongest nonlinearity is present when the three coupling
wave vectors are at the supergranular scale. These are the same wave
vectors that are present in regular hexagons, which have been used in
analytical studies of solar convection. At these Fourier components,
the bispectrum is positive, consistent with the positive skewness in
the data and consistent with supergranules preferentially consisting
of outflows surrounded by a network of inflows. We use the bispectral
estimates to generate synthetic divergence maps that are very similar to
the data. This is done by a model that consists of a Gaussian term and
a weaker quadratic nonlinear component. Using this method, we estimate
the fraction of the variance in the divergence maps from the nonlinear
component to be of the order of 4-6%.
Conclusions: We propose
that bispectral analysis is useful for understanding the dynamics of
solar turbulent convection, for example for comparing observations
and numerical models of supergranular flows. This analysis may also
be useful to generate synthetic flow fields.
Title: On long-duration 3D simulations of stellar convection using
ANTARES
Authors: Kupka, F.; Fabbian, D.; Krüger, D.; Kostogryz, N.; Gizon, L.
Bibcode: 2020IAUGA..30..373K
Altcode:
We present initial results from three-dimensional (3-D) radiation
hydrodynamical simulations for the Sun and targeted Sun-like stars. We
plan to extend these simulations up to several stellar days to study
p-mode excitation and damping processes. The level of variation of
irradiance on the time scales spanned by our 3-D simulations will
be studied too. Here we show results from a first analysis of the
computational data we produced so far.
Title: Asteroseismology of luminous red giants with Kepler I:
long-period variables with radial and non-radial modes
Authors: Yu, Jie; Bedding, Timothy R.; Stello, Dennis; Huber, Daniel;
Compton, Douglas L.; Gizon, Laurent; Hekker, Saskia
Bibcode: 2020MNRAS.493.1388Y
Altcode: 2020arXiv200110878Y; 2020MNRAS.tmp..257Y
While long-period variables (LPVs) have been extensively investigated,
especially with MACHO and OGLE data for the Magellanic Clouds, there
still exist open questions in their pulsations regarding the excitation
mechanisms, radial order, and angular degree assignment. Here, we
perform asteroseismic analyses on LPVs observed by the 4-year Kepler
mission. Using a cross-correlation method, we detect unambiguous
pulsation ridges associated with radial fundamental modes (n = 1) and
overtones (n ≥ 2), where the radial order assignment is made using
theoretical frequencies and observed frequencies. Our results confirm
that the amplitude variability seen in semiregulars is consistent with
oscillations being solar-like. We identify that the dipole modes,
l = 1, are dominant in the radial orders of 3 ≤ n ≤ 6, and that
quadrupole modes, l = 2, are dominant in the first overtone n = 2. A
test of seismic scaling relations using Gaia DR2 parallaxes reveals
the possibility that the relations break down when νmax
≲ 3 $\mu {\rm Hz}$ (R ≳ 40 R⊙, or log $\rm L/L_{\odot
}$ ≳ 2.6). Our homogeneous measurements of pulsation amplitude and
period for 3213 LPVs will be valuable for probing effects of pulsation
on mass-loss, in particular in those stars with periods around 60 d,
which has been argued as a threshold of substantial pulsation-triggered
mass-loss.
Title: Exploring the latitude and depth dependence of solar Rossby
waves using ring-diagram analysis
Authors: Proxauf, B.; Gizon, L.; Löptien, B.; Schou, J.; Birch,
A. C.; Bogart, R. S.
Bibcode: 2020A&A...634A..44P
Altcode: 2019arXiv191202056P
Context. Global-scale equatorial Rossby waves have recently been
unambiguously identified on the Sun. Like solar acoustic modes, Rossby
waves are probes of the solar interior.
Aims: We study the
latitude and depth dependence of the Rossby wave eigenfunctions.
Methods: By applying helioseismic ring-diagram analysis and granulation
tracking to observations by HMI aboard SDO, we computed maps of
the radial vorticity of flows in the upper solar convection zone
(down to depths of more than 16 Mm). The horizontal sampling of the
ring-diagram maps is approximately 90 Mm (∼7.5°) and the temporal
sampling is roughly 27 hr. We used a Fourier transform in longitude
to separate the different azimuthal orders m in the range 3 ≤ m ≤
15. At each m we obtained the phase and amplitude of the Rossby waves
as functions of depth using the helioseismic data. At each m we also
measured the latitude dependence of the eigenfunctions by calculating
the covariance between the equator and other latitudes.
Results:
We conducted a study of the horizontal and radial dependences of the
radial vorticity eigenfunctions. The horizontal eigenfunctions are
complex. As observed previously, the real part peaks at the equator
and switches sign near ±30°, thus the eigenfunctions show significant
non-sectoral contributions. The imaginary part is smaller than the real
part. The phase of the radial eigenfunctions varies by only ±5° over
the top 15 Mm. The amplitude of the radial eigenfunctions decreases
by about 10% from the surface down to 8 Mm (the region in which
ring-diagram analysis is most reliable, as seen by comparing with the
rotation rate measured by global-mode seismology).
Conclusions:
The radial dependence of the radial vorticity eigenfunctions deduced
from ring-diagram analysis is consistent with a power law down to 8 Mm
and is unreliable at larger depths. However, the observations provide
only weak constraints on the power-law exponents. For the real part,
the latitude dependence of the eigenfunctions is consistent with
previous work (using granulation tracking). The imaginary part is
smaller than the real part but significantly nonzero.
Title: Efficient and Accurate Algorithm for the Full Modal Green's
Kernel of the Scalar Wave Equation in Helioseismology
Authors: Barucq, Hélène; Faucher, Florian; Fournier, Damien; Gizon,
Laurent; Pham, Ha
Bibcode: 2020SJAM...80.2657B
Altcode: 2021SJAM...80.2657B
In this work, we provide an algorithm to compute efficiently and
accurately the full outgoing modal Green's kernel for the scalar wave
equation in local helioseismology under spherical symmetry. Due to the
high computational cost of a full Green's function, current helioseismic
studies only use its values at a single depth. However, a more realistic
modelisation of the helioseismic products (cross-covariance and power
spectrum) requires the full Green's kernel. In the classical approach,
the Dirac source is discretized and one simulation gives the Green's
function on a line. Here, we propose a two-step algorithm which, with
two simulations, provides the full kernel on the domain. Moreover,
our method is more accurate as the singularity of the solution due
to the Dirac source is described exactly. In addition, it is coupled
with the exact Dirichlet-to-Neumann boundary condition, providing
optimal accuracy in approximating the outgoing Green's kernel,
which we demonstrate in our experiments. In addition, we show that
high-frequency approximations of the nonlocal radiation boundary
conditions can represent accurately the helioseismic products.
Title: An improved multi-ridge fitting method for ring-diagram
helioseismic analysis
Authors: Nagashima, Kaori; Birch, Aaron C.; Schou, Jesper; Hindman,
Bradley W.; Gizon, Laurent
Bibcode: 2020A&A...633A.109N
Altcode: 2019arXiv191107772N
Context. There is a wide discrepancy in current estimates of
the strength of convection flows in the solar interior obtained
using different helioseismic methods applied to observations from
the Helioseismic and Magnetic Imager onboard the Solar Dynamics
Observatory. The cause for these disparities is not known.
Aims: As one step in the effort to resolve this discrepancy, we aim to
characterize the multi-ridge fitting code for ring-diagram helioseismic
analysis that is used to obtain flow estimates from local power spectra
of solar oscillations.
Methods: We updated the multi-ridge
fitting code developed by Greer et al. (2014, Sol. Phys., 289, 2823)
to solve several problems we identified through our inspection of the
code. In particular, we changed the (1) merit function to account for
the smoothing of the power spectra, (2) model for the power spectrum,
and (3) noise estimates. We used Monte Carlo simulations to generate
synthetic data and to characterize the noise and bias of the updated
code by fitting these synthetic data.
Results: The bias in
the output fit parameters, apart from the parameter describing the
amplitude of the p-mode resonances in the power spectrum, is below
what can be measured from the Monte-Carlo simulations. The amplitude
parameters are underestimated; this is a consequence of choosing to
fit the logarithm of the averaged power. We defer fixing this problem
as it is well understood and not significant for measuring flows in the
solar interior. The scatter in the fit parameters from the Monte-Carlo
simulations is well-modeled by the formal error estimates from the
code.
Conclusions: We document and demonstrate a reliable
multi-ridge fitting method for ring-diagram analysis. The differences
between the updated fitting results and the original results are less
than one order of magnitude and therefore we suspect that the changes
will not eliminate the aforementioned orders-of-magnitude discrepancy
in the amplitude of convective flows in the solar interior.
Title: Asteroseismic Signature of a Large Active Region
Authors: Papini, Emanuele; Gizon, Laurent
Bibcode: 2019FrASS...6...72P
Altcode: 2019arXiv191111812P
Axisymmetric magnetic activity on the Sun and Sun-like stars increases
the frequencies of the modes of acoustic oscillation. However, it is
unclear how a corotating patch of activity affects the oscillations,
since such a perturbation is unsteady in the frame of the observer. In
this paper we qualitatively describe the asteroseismic signature of
a large active region in the power spectrum of the dipole (l = 1) and
quadrupole (l = 2) p modes. First we calculate the frequencies and the
relative amplitudes of the azimuthal modes of oscillation in a frame
that corotates with the active region, using first-order perturbation
theory. For the sake of simplicity, the influence of the active region
is approximated by a near-surface increase in sound speed. In the
corotating frame the perturbations due to (differential) rotation and
the active region completely lift the (2l + 1)-fold azimuthal degeneracy
of the frequency spectrum of modes with harmonic degree l. Then we
transform to an inertial frame to obtain the observed power spectrum. In
the frame of the observer, the unsteady nature of the perturbation
leads to the appearance of (2l + 1)^2 peaks in the power spectrum of a
multiplet. These peaks blend into each other to form asymmetric line
profiles. In the limit of a small active region (angular diameter
less than 30°), we approximate the power spectrum of a multiplet
in terms of 2 × (2l + 1) peaks, whose amplitudes and frequencies
depend on the latitude of the active region and the inclination angle
of the star's rotation axis. In order to check the results and to
explore the nonlinear regime, we perform numerical simulations using
the 3D time-domain pseudo-spectral linear pulsation code GLASS. For
small sound-speed perturbations, we find a good agreement between
the simulations and linear theory. Larger perturbation amplitudes
will induce mode mixing and lead to additional complex changes in the
predicted power spectrum. However linear perturbation theory provides
useful guidance to search for the observational signature of large
individual active regions in stellar oscillation power spectra.
Title: Solar irradiance variability over last four billion years
Authors: Shapiro, Anna V.; Shapiro, Alexander I.; Gizon, Laurent;
Krivova, Natalie A.; Solanki, Sami K.
Bibcode: 2019EPSC...13.2071S
Altcode:
The action of dynamo generates magnetic field in the solar
interior. This field then travels through the convective zone and
emerges on the solar surface, leading to a various manifestations
of solar magnetic activity. One of the most appealing among them
is the variations of Spectral Solar Irradiance (SSI). There is an
evidence that these variations have substantial effect on the Earth's
climate system. The faster rotation of the Sun in the past led to
a more vigorous dynamo and consequently larger amplitude of solar
spectral irradiance variability. This could led to a stronger effect
of the SSI variability on the Earth. The main goal of our study is to
calculate the amplitude of the SSI variability over the course of the
solar activity cycle (which presently lasts 11 years but could have
different duration in the past) as a function of solar age. We utilise
recently published relation between the stellar chromospheric activity
and stellar age to reconstruct solar chromospheric activity back in
time. It is used to calculate solar disk coverages by magnetic features,
i.e. solar spots and faculae. Corresponding brightness variations are
then computed using the SATIRE (which stands for Spectral and Total
Irradiance Reconstruction) approach. Our study shows that the facular
component of the irradiance variability over the solar activity cycle
decreases slower with the solar age than the spot component. This
makes the dependence of the amplitude of the solar variability on the
age non-monotonic. The am- plitude decreases for the young Sun till
it reaches minimum value and then gradually increases again. The
variability of the Total Solar Irradiance (TSI, i.e. irradiance
integrated over the entire spectral domain) changes from being spot- to
facular-dominated at the solar age of about 2.8 Gyr. Our calculations
show that the amplitude of the TSI variability of 600-Myr Sun was one
order of magnitude larger than the present-day value. We have found that
the age of the transition between spot- and facular-dominated regimes
of the variability depends on the wavelength. For example, it is about
1.3 Gyr for the 210-400 nm spectral domain and becomes approximately
3.7 Gyr for the 400-700 nm spectral range. Our calculations of the past
solar irradiance variability on the activity cycle timescale might be
of interest for paleoclimate researchers as well as for modelling of
atmospheres of exoplanets.
Title: Signature of solar g modes in first-order p-mode frequency
shifts
Authors: Böning, Vincent G. A.; Hu, Huanchen; Gizon, Laurent
Bibcode: 2019A&A...629A..26B
Altcode: 2019arXiv190702379B
Context. Solar gravity modes (g modes) are buoyancy waves that are
trapped in the solar radiative zone and have been very difficult to
detect at the surface. Solar g modes would complement solar pressure
modes (p modes) in probing the central regions of the Sun, for example
the rotation rate of the core.
Aims: A detection of g modes using
changes in the large frequency separation of p modes has recently been
reported. However, it is unclear how p and g modes interact. The aim
of this study is to evaluate to what extent g modes can perturb the
frequencies of p modes.
Methods: We computed the first-order
perturbation to global p-mode frequencies due to a flow field and
perturbations to solar structure (e.g. density and sound speed) caused
by a g mode. We focused on long-period g modes and assumed that the
g-mode perturbations are constant in time. The surface amplitude of
g modes is assumed to be 1 mm s-1, which is close to the
observational limit set by Doppler observations.
Results:
Gravity modes do perturb p-mode frequencies to first order if the
harmonic degree of the g mode is even and if its azimuthal order is
zero. The effect is extremely small. For dipole and quadrupole p modes,
all frequency shifts are smaller than 0.1 nHz, or 2 × 10-8
in relative numbers. This is because the relative perturbation to
solar structure quantities caused by a g mode of realistic amplitude
is of the order of 10-6-10-5. Additionally, we
find that structural changes dominate over advection. Surprisingly,
the interaction of g and p modes takes place to a large part near the
surface, where p modes spend most of their propagation times and g
modes generate the largest relative changes to solar structure. This
is due to the steep density stratification, which compensates the
evanescent behaviour of g modes in the convection zone.
Conclusions: It appears to be impossible to detect g modes solely
through their signature in p-mode frequency shifts. Whether g modes
leave a detectable signature in p-mode travel times under a given
observational setup remains an open question.
Title: Average surface flows before the formation of solar active
regions and their relationship to the supergranulation pattern
Authors: Birch, A. C.; Schunker, H.; Braun, D. C.; Gizon, L.
Bibcode: 2019A&A...628A..37B
Altcode:
Context. The emergence of solar active regions is an important but
poorly understood aspect of the solar dynamo.
Aims: Knowledge
of the flows associated with the rise of active-region-forming
magnetic concentrations through the near-surface layers will help
determine the mechanisms of active region formation.
Methods:
We used helioseismic holography and granulation tracking to measure
the horizontal flows at the surface that precede the emergence of
active regions. We then averaged these flows over about sixty emerging
active regions to reduce the noise, selecting active regions that
emerge into relatively quiet Sun. To help interpret the results,
we constructed a simple model flow field by generating synthetic
"emergence locations" that are probabilistically related to the
locations of supergranulation-scale convergence regions in the quiet
Sun.
Results: The flow maps obtained from helioseismology and
granulation tracking are very similar (correlation coefficients for
single maps around 0.96). We find that active region emergence is,
on average, preceded by converging horizontal flows of amplitude
about 40 m s-1. The convergence region extends over about
40 Mm in the east-west direction and about 20 Mm in the north-south
direction and is centered in the retrograde direction relative to the
emergence location. This flow pattern is largely reproduced by a model
in which active region emergence occurs preferentially in the prograde
direction relative to supergranulation inflows.
Conclusions:
Averaging over many active regions reveals a statistically significant
pattern of near-surface flows prior to emergence. The qualitative
success of our simple model suggests that rising flux concentrations
and supergranule-scale flows interact during the emergence process.
Title: Evolution of Flows around Emerging Active Regions
Authors: Gottschling, Nils; Schunker, Hannah; Birch, Aaron C.;
Gizon, Laurent
Bibcode: 2019AAS...23440201G
Altcode:
Inflows associated with established active regions have been measured
with velocities of about 30 m/s and extending up to 10° from the
active regions, but have so far been included in surface flux transport
models only in a simple form. How these flows develop as active regions
emerge has not yet been studied. We measure the flows surrounding 182
emerging active regions observed by the SDO/HMI instrument using local
correlation tracking of the granulation as they evolve from seven
days before to seven days after emergence. We find flows converging
towards the trailing polarity at the time when magnetic flux first
emerges. Three days later, extended inflows form towards the center of
the active region predominantly in the north-south direction, together
with outflows at the leading polarity, due to moat flows around
sunspots. The flows have velocities of 20 to 30 m/s and increase in
extent from about 2° to about 7°. At later times, the flows resemble
those from previous studies of established active regions. These results
will help constrain models of the surface evolution of magnetic fields.
Title: On the latitude dependence of Rossby waves in the Sun
Authors: Proxauf, Bastian Severin Niklas; Gizon, Laurent; Löptien,
Björn; Schou, Jesper; Birch, Aaron C.; Bogart, Richard S.
Bibcode: 2019AAS...23431801P
Altcode:
We study the latitude and depth dependence of solar Rossby waves. We
use horizontal flows from local helioseismology (ring-diagram analysis)
at different depths in the solar interior. From these we compute maps
of the radial vorticity. We confirm the existence of solar Rossby waves
in the sectoral (m = l) power spectra at all depths down to 17 Mm below
the surface. The depth dependence of the eigenfunctions is consistent
with rm, although this is a weak constraint due to the noise
level. The latitudinal eigenfunctions are observed to be more narrow
than |sin(θ)|m, likely indicating that the modes sense the
latitudinal differential rotation. Furthermore, we detect a non-zero
imaginary component of the latitudinal eigenfunctions, possibly related
to viscous dissipation. These new observations provide additional
constraints on the physics of large-scale Rossby waves in the Sun.
Title: Time-distance helioseismology of solar Rossby waves
Authors: Liang, Zhi-Chao; Gizon, Laurent; Birch, Aaron C.; Duvall,
Thomas L.
Bibcode: 2019A&A...626A...3L
Altcode: 2018arXiv181207413L
Context. Solar Rossby waves (r modes) have recently been discovered
in the near-surface horizontal flow field using the techniques of
granulation-tracking and ring-diagram analysis applied to six years of
SDO/HMI data.
Aims: Here we apply time-distance helioseismology
to the combined SOHO/MDI and SDO/HMI data sets, which cover 21 years
of observations from May 1996 to April 2017. The goal of this study
is to provide an independent confirmation over two solar cycles and in
deeper layers of the Sun.
Methods: We have measured south-north
helioseismic travel times along the equator, which are sensitive to
subsurface north-south flows. To reduce noise, the travel times were
averaged over travel distances from 6° to 30°; the mean distance
corresponds to a p-mode lower turning point of 0.91 R⊙. The
21-year time series of travel-time measurements was split into
three seven-year subsets and transformed to obtain power spectra in a
corotating frame.
Results: The power spectra all show peaks near
the frequencies of the classical sectoral Rossby waves for azimuthal
wavenumbers in the range 3 ≤ m ≤ 15. The mode frequencies and
linewidths of the modes with m ≤ 9 are consistent with a previous
study whereas modes with m ≥ 10 are shifted toward less negative
frequencies by 10-20 nHz. While most of these modes have e-folding
lifetimes on the order of a few months, the longest lived mode, m = 3,
has an e-folding lifetime of more than one year. For each mode, the rms
velocity at the equator is in the range of 1-3 m s-1, with
the largest values for m ∼ 10. No evidence for the m = 2 sectoral mode
is found in the power spectrum, implying that the rms velocity of this
mode is below ∼0.5 m s-1.
Conclusions: This work
confirms the existence of equatorial global Rossby waves in the solar
interior over the past two solar cycles and shows that time-distance
helioseismology is a promising technique to study them deep in the
convection zone. The movie associated to Fig. 1 is available at https://www.aanda.org
Title: Modeling observables for local helioseismology.
Authors: Kostogryz, Nadiia; Fournier, Damien; Gizon, Laurent
Bibcode: 2019AAS...23430705K
Altcode:
Local helioseismology provides different techniques to study flows
in the solar interior. However, all of them suffer from systematic
errors, which occur because of the nontrivial relationship between
wave displacement and helioseismic observables, such as intensity
and Doppler velocity. In this study, we solve the radiative transfer
equation in a perturbed solar atmosphere including flows caused by
acoustic oscillations. The adiabatic oscillations for normal modes
of low and high degree are computed using the ADIPLS code that
solves an eigenvalue problem in a standard solar model assuming
spherically symmetric background quantities. The wave displacement
causes perturbations in atmospheric thermodynamical quantities that,
in turn, perturb opacity and emergent intensity. These perturbations
depend on the center to the limb distance. In addition, the oscillations
modify the shape of the solar surface and thus the direction of the
normal to the surface. For low-degree modes this geometrical effect
is negligible, however, this effect matters for high-degree modes with
a large horizontal component of wave displacement. We investigate the
contribution of such perturbations on emergent intensity and velocity
and estimate their impact on helioseismic observables.
Title: Average motion of emerging solar active region
polarities. I. Two phases of emergence
Authors: Schunker, H.; Birch, A. C.; Cameron, R. H.; Braun, D. C.;
Gizon, L.; Burston, R. B.
Bibcode: 2019A&A...625A..53S
Altcode: 2019arXiv190311839S
Aims: Our goal is to constrain models of active region
formation by tracking the average motion of active region polarity
pairs as they emerge onto the surface.
Methods: We measured
the motion of the two main opposite polarities in 153 emerging active
regions using line-of-sight magnetic field observations from the Solar
Dynamics Observatory Helioseismic Emerging Active Region (SDO/HEAR)
survey. We first measured the position of each of the polarities
eight hours after emergence, when they could be clearly identified,
using a feature recognition method. We then tracked their location
forwards and backwards in time.
Results: We find that, on
average, the polarities emerge with an east-west orientation and the
separation speed between the polarities increases. At about 0.1 days
after emergence, the average separation speed reaches a peak value
of 229 ± 11 ms-1, and then starts to decrease. About
2.5 days after emergence the polarities stop separating. We also
find that the separation and the separation speed in the east-west
direction are systematically larger for active regions that have
higher flux. The scatter in the location of the polarities increases
from about 5 Mm at the time of emergence to about 15 Mm at two days
after emergence.
Conclusions: Our results reveal two phases of
the emergence process defined by the rate of change of the separation
speed as the polarities move apart. Phase 1 begins when the opposite
polarity pairs first appear at the surface, with an east-west alignment
and an increasing separation speed. We define Phase 2 to begin when
the separation speed starts to decrease, and ends when the polarities
have stopped separating. This is consistent with a previous study: the
peak of a flux tube breaks through the surface during Phase 1. During
Phase 2 the magnetic field lines are straightened by magnetic tension,
so that the polarities continue to move apart, until they eventually
lie directly above their anchored subsurface footpoints. The scatter
in the location of the polarities is consistent with the length and
timescales of supergranulation, supporting the idea that convection
buffets the polarities as they separate.
Title: Sectoral r modes and periodic radial velocity variations of
Sun-like stars
Authors: Lanza, A. F.; Gizon, L.; Zaqarashvili, T. V.; Liang, Z. -C.;
Rodenbeck, K.
Bibcode: 2019A&A...623A..50L
Altcode: 2019arXiv190108777L
Context. Radial velocity (RV) measurements are used to search for
planets orbiting late-type main-sequence stars and to confirm the
transiting planets.
Aims: The most advanced spectrometers are now
approaching a precision of 10 cm s-1, which implies the need
to identify and correct for all possible sources of RV oscillations
intrinsic to the star down to this level and possibly beyond. The
recent discovery of global-scale equatorial Rossby waves in the
Sun, also called r modes, prompted us to investigate their possible
signature in stellar RV measurements. These r modes are toroidal
modes of oscillation whose restoring force is the Coriolis force;
they propagate in the retrograde direction in a frame that co-rotates
with the star. The solar r modes with azimuthal orders 3 ≤ m ≲ 15
were identified unambiguously because of their dispersion relation and
their long e-folding lifetimes of hundreds of days.
Methods:
In this paper, we simulate the RV oscillations produced by sectoral
r modes with 2 ≤ m ≤ 5 by assuming a stellar rotation period of
25.54 days and a maximum amplitude of the surface velocity of each
mode of 2 m s-1. This amplitude is representative of the
solar measurements except for the m = 2 mode, which has not yet been
observed on the Sun.
Results: Sectoral r modes with azimuthal
orders m = 2 and 3 would produce RV oscillations with amplitudes of
76.4 and 19.6 cm s-1 and periods of 19.16 and 10.22 days,
respectively, for a star with an inclination of the rotation axis to the
line of sight i = 60°. Therefore, they may produce rather sharp peaks
in the Fourier spectrum of the radial velocity time series that could
lead to spurious planetary detections.
Conclusions: Sectoral
r modes may represent a source of confusion in the case of slowly
rotating inactive stars that are preferential targets for RV planet
search. The main limitation of the present investigation is the lack of
observational constraints on the amplitude of the m = 2 mode on the Sun.
Title: VizieR Online Data Catalog: Asymmetry of oscillations in 43
Kepler stars (Benomar+, 2018)
Authors: Benomar, O.; Goupil, M.; Belkacem, K.; Appourchaux, T.;
Nielsen, M. B.; Bazot, M.; Gizon, L.; Hanasoge, S.; Sreenivasan,
K. R.; Marchand, B.
Bibcode: 2019yCat..18570119B
Altcode:
In this work, the considered ensemble of stars is a subset of the Kepler
LEGACY sample (Lund+ 2017, J/ApJ/835/172). We selected 43 stars for
analysis out of 66 of the LEGACY sample. The current analysis uses the
unweighted power spectra provided by the Kepler Asteroseismic Science
Operations Center (KASOC) pipeline (http://kasoc.phys.au.dk/). (2
data files).
Title: Latitudinal differential rotation in the solar analogues 16
Cygni A and B
Authors: Bazot, M.; Benomar, O.; Christensen-Dalsgaard, J.; Gizon,
L.; Hanasoge, S.; Nielsen, M.; Petit, P.; Sreenivasan, K. R.
Bibcode: 2019A&A...623A.125B
Altcode: 2019arXiv190201676B
Context. Asteroseismology has undergone a profound transformation
as a scientific field following the CoRoT and Kepler space
missions. The latter is now yielding the first measurements of
latitudinal differential rotation obtained directly from oscillation
frequencies. Differential rotation is a fundamental mechanism of the
stellar dynamo effect.
Aims: Our goal is to measure the amount
of differential rotation in the solar analogues 16 Cyg A and B, which
are the components of a binary system. These stars are the brightest
observed by Kepler and have therefore been extensively observed, with
exquisite precision on their oscillation frequencies.
Methods:
We modelled the acoustic power spectrum of 16 Cyg A and B using a model
that takes into account the contribution of differential rotation to
the rotational frequency splitting. The estimation was carried out in a
Bayesian setting. We then inverted these results to obtain the rotation
profile of both stars under the assumption of a solar-like functional
form.
Results: We observe that the magnitude of latitudinal
differential rotation has a strong chance of being solar-like for
both stars, their rotation rates being higher at the equator than at
the pole. The measured latitudinal differential rotation, defined as
the difference of rotation rate between the equator and the pole, is
320 ± 269 nHz and 440-383+363 nHz for 16 Cyg
A and B, respectively, confirming that the rotation rates of these
stars are almost solar-like. Their equatorial rotation rates are 535
± 75 nHz and 565-129+150 nHz. Our results are
in good agreement with measurements obtained from spectropolarimetry,
spectroscopy, and photometry.
Conclusions: We present the first
conclusive measurement of latitudinal differential rotation for solar
analogues. Their rotational profiles are very close to those of the
Sun. These results depend weakly on the uncertainties of the stellar
parameters.
Title: Starspot rotation rates versus activity cycle phase: Butterfly
diagrams of Kepler stars are unlike that of the Sun
Authors: Nielsen, M. B.; Gizon, L.; Cameron, R. H.; Miesch, M.
Bibcode: 2019A&A...622A..85N
Altcode: 2018arXiv181206414N
Context. During the solar magnetic activity cycle the emergence
latitudes of sunspots change, leading to the well-known butterfly
diagram. This phenomenon is poorly understood for other stars since
starspot latitudes are generally unknown. The related changes in
starspot rotation rates caused by latitudinal differential rotation can,
however, be measured.
Aims: Using the set of 3093 Kepler stars
with measured activity cycles, we aim to study the temporal change in
starspot rotation rates over magnetic activity cycles, and how this
relates to the activity level, the mean rotation rate of the star, and
its effective temperature.
Methods: We measured the photometric
variability as a proxy for the magnetic activity and the spot rotation
rate in each quarter over the duration of the Kepler mission. We
phase-folded these measurements with the cycle period. To reduce random
errors, we performed averages over stars with comparable mean rotation
rates and effective temperature at fixed activity-cycle phases.
Results: We detect a clear correlation between the variation of activity
level and the variation of the starspot rotation rate. The sign and
amplitude of this correlation depends on the mean stellar rotation and -
to a lesser extent - on the effective temperature. For slowly rotating
stars (rotation periods between 15 - 28 days), the starspot rotation
rates are clearly anti-correlated with the level of activity during
the activity cycles. A transition is observed around rotation periods
of 10 - 15 days, where stars with an effective temperature above 4200 K
instead show positive correlation.
Conclusions: Our measurements
can be interpreted in terms of a stellar "butterfly diagram",
but these appear different from that of the Sun since the starspot
rotation rates are either in phase or anti-phase with the activity
level. Alternatively, the activity cycle periods observed by Kepler are
short (around 2.5 years) and may therefore be secondary cycles, perhaps
analogous to the solar quasi-biennial oscillations. Rotation and
activity tables are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/622/A85
Title: Supervised neural networks for helioseismic ring-diagram
inversions
Authors: Alshehhi, Rasha; Hanson, Chris S.; Gizon, Laurent; Hanasoge,
Shravan
Bibcode: 2019A&A...622A.124A
Altcode: 2019arXiv190101505A
Context. The inversion of ring fit parameters to obtain subsurface
flow maps in ring-diagram analysis for eight years of SDO observations
is computationally expensive, requiring ∼3200 CPU hours.
Aims: In this paper we apply machine-learning techniques to the
inversion step of the ring diagram pipeline in order to speed up the
calculations. Specifically, we train a predictor for subsurface flows
using the mode fit parameters and the previous inversion results to
replace future inversion requirements.
Methods: We utilize
artificial neural networks (ANNs) as a supervised learning method
for predicting the flows in 15° ring tiles. We discuss each step
of the proposed method to determine the optimal approach. In order
to demonstrate that the machine-learning results still contain the
subtle signatures key to local helioseismic studies, we use the
machine-learning results to study the recently discovered solar
equatorial Rossby waves.
Results: The ANN is computationally
efficient, able to make future flow predictions of an entire Carrington
rotation in a matter of seconds, which is much faster than the current
∼31 CPU hours. Initial training of the networks requires ∼3 CPU
hours. The trained ANN can achieve a rms error equal to approximately
half that reported for the velocity inversions, demonstrating the
accuracy of the machine learning (and perhaps the overestimation of the
original errors from the ring-diagram pipeline). We find the signature
of equatorial Rossby waves in the machine-learning flows covering
six years of data, demonstrating that small-amplitude signals are
maintained. The recovery of Rossby waves in the machine-learning flow
maps can be achieved with only one Carrington rotation (27.275 days) of
training data.
Conclusions: We show that machine learning can be
applied to and perform more efficiently than the current ring-diagram
inversion. The computation burden of the machine learning includes 3
CPU hours for initial training, then around 10-4 CPU hours
for future predictions.
Title: Signal and noise in helioseismic holography
Authors: Gizon, Laurent; Fournier, Damien; Yang, Dan; Birch, Aaron C.;
Barucq, Hélène
Bibcode: 2018A&A...620A.136G
Altcode: 2018arXiv181000402G
Context. Helioseismic holography is an imaging technique used to study
heterogeneities and flows in the solar interior from observations of
solar oscillations at the surface. Holographic images contain noise
due to the stochastic nature of solar oscillations.
Aims:
We aim to provide a theoretical framework for modeling signal and
noise in Porter-Bojarski helioseismic holography.
Methods:
The wave equation may be recast into a Helmholtz-like equation, so
as to connect with the acoustics literature and define the holography
Green's function in a meaningful way. Sources of wave excitation are
assumed to be stationary, horizontally homogeneous, and spatially
uncorrelated. Using the first Born approximation we calculated
holographic images in the presence of perturbations in sound-speed,
density, flows, and source covariance, as well as the noise level as a
function of position. This work is a direct extension of the methods
used in time-distance helioseismology to model signal and noise.
Results: To illustrate the theory, we compute the holographic
image intensity numerically for a buried sound-speed perturbation
at different depths in the solar interior. The reference Green's
function is obtained for a spherically-symmetric solar model using a
finite-element solver in the frequency domain. Below the pupil area on
the surface, we find that the spatial resolution of the holographic
image intensity is very close to half the local wavelength. For a
sound-speed perturbation of size comparable to the local spatial
resolution, the signal-to-noise ratio is approximately constant with
depth. Averaging the image intensity over a number N of frequencies
above 3 mHz increases the signal-to-noise ratio by a factor nearly
equal to the square root of N. This may not be the case at lower
frequencies, where large variations in the holographic signal are due
to the contributions from the long-lived modes of oscillation.
Title: Twenty-one-year helioseismic measurement of solar meridional
circulation from SOHO/MDI and SDO/HMI: Anomalous northern hemisphere
during cycle 24
Authors: Liang, Zhi-Chao; Gizon, Laurent; Birch, Aaron C.; Duvall,
Thomas L., Jr.; Rajaguru, S. P.
Bibcode: 2018csc..confE..59L
Altcode:
We apply time-distance helioseismology to MDI and HMI medium-degree
Dopplergrams covering May 1996-April 2017, i.e., 12-yr of cycle 23
and 9-yr of cycle 24. Our data analysis takes several systematic
effects into account, including the P-angle error, surface magnetic
field effects, and the center-to-limb variations. For comparison,
forward-modeled travel-time differences are computed in the ray
approximation for representative meridional flow models. The measured
travel-time differences are similar in the southern hemisphere for
cycles 23 and 24. However, they differ in the northern hemisphere
between cycles 23 and 24. Except for cycle 24's northern hemisphere,
the measurements favor a single-cell meridional circulation model where
the poleward flows persist down to about 0.8 solar radii, accompanied
by local inflows toward the activity belts in the near-surface
layers. Cycle 24's northern hemisphere is found to be anomalous:
travel-time differences are significantly smaller when travel distances
are greater than 20 deg. This asymmetry between northern and southern
hemispheres during cycle 24 was not present in previous measurements
(e.g., Rajaguru & Antia 2015), which assumed a different P-angle
error correction where south-north travel-time differences are shifted
to zero at the equator for all travel distances. In our measurements,
the travel-time differences at the equator are zero for travel distances
less than about 30 deg, but they do not vanish for larger travel
distances. Rather than a P-angle error, this equatorial offset for
large travel distances might be caused by the asymmetrical near-surface
flows around the end points of the acoustic ray paths.
Title: Towards improved multi-ridge fitting method for ring-diagram
analysis
Authors: Nagashima, Kaori; Birch, Aaron C.; Schou, Jesper; Hindman,
Bradley; Gizon, Laurent
Bibcode: 2018csc..confE..50N
Altcode:
Ring-diagram analysis is one of the important methods of local
helioseismology for probing subsurface flows. In ring-diagram analysis
the Doppler shifts of oscillation mode frequencies due to flows
are measured by fitting a model function to the local oscillation
power spectra. Here we propose alteration of the multi-ridge fitting
method developed by Greer et al. (2014). It is well known that the
solar oscillation power is chi-square distributed (with two degrees
of freedom), and the fitting in the existing multi-ridge fitting is
done with the maximum likelihood method based on this probability
distribution function. However, the power is in practice remapped
from Cartesian to polar coordinates and/or smoothed in azimuth
of the wavevector. The smoothed power is approximately normally
distributed. We demonstrate that the probability distribution function
of the logarithm of the normally-distributed power is approximated
by a normal distribution with a variance that is independent of the
expectation value of the power. Therefore, we alter the fitting method
using the logarithm of the power with a least-square method. In this
presentation we report the bias and noise levels in the updated fitting
results as well as the crosstalk between the parameters using a Monte
Carlo simulation of the power spectra.
Title: Statistical constraints on active region emergence from the
surface motion of the polarities
Authors: Schunker, Hannah; Birch, Aaron; Cameron, Robert; Braun,
Doug; Gizon, Laurent
Bibcode: 2018csc..confE..45S
Altcode:
We measured the motion of the two main opposite polarities in
154 emerging active regions using line-of-sight magnetograms from
SDO/HMI. Our results reveal two phases of the emergence process defined
by the rate of change of the separation speed as the polarities move
apart. Phase one begins when the opposite polarity pairs first appear at
the surface, with an east-west alignment and an increasing separation
speed of 1.6 +/- 0.4 km/s. Phase two begins when the separation speed
starts to decrease, about 0.1 days after emergence, and ends about 2.5
days after emergence when the polarities have stopped separating. This
is consistent with the picture of Chen, Rempel, & Fan (2017):
during phase one, the peak of a flux tube breaks through the surface
and then, during phase two, the magnetic field lines are straightened
by magnetic tension to eventually lie directly above their subsurface
footpoints. The scatter in the location of the polarities is consistent
with the length and time scales of supergranulation, supporting the idea
that convection buffets the polarities as they separate. On average,
the polarities emerge with an east-west orientation with the tilt angle
developing over time independent of flux, in contrast to predictions
from thin flux tube theory.
Title: Rossby waves in the solar convection zone measured by
deep-focus time-distance helioseismology
Authors: Duvall, T. L., Jr.; Birch, A. C.; Liang, Z. -C.; Gizon, L.
Bibcode: 2018csc..confE..57D
Altcode:
Recent work by Loeptien et al. has shown spectral signatures of
equatorial Rossby waves in the solar photosphere (via correlation
tracking of granulation) and in the outer 20 Mm of the convection zone
(via helioseismic ring diagrams). This result is potentially extremely
important for understanding convection zone dynamics and as such should
be studied by all available techniques. To this end we have searched
for these Rossby waves using deep-focus time-distance helioseismology
in 8 years of HMI medium resolution (medium l) Dopplergrams. We also
see the signatures of equatorial Rossby waves for focus depths of 0 Mm
(photosphere) down to 70 Mm below the surface. At 105 Mm (mid convection
zone) and 210 Mm (bottom of convection zone) no such signatures are
seen, although whether this is a s/n issue is not determined. We will
hopefully be able to determine the radial eigenfunctions of the Rossby
waves from this type of measurement.
Title: Solar meridional circulation from twenty-one years of SOHO/MDI
and SDO/HMI observations. Helioseismic travel times and forward
modeling in the ray approximation
Authors: Liang, Zhi-Chao; Gizon, Laurent; Birch, Aaron C.; Duvall,
Thomas L.; Rajaguru, S. P.
Bibcode: 2018A&A...619A..99L
Altcode: 2018arXiv180808874L
Context. The solar meridional flow is an essential ingredient in
flux-transport dynamo models. However, no consensus on its subsurface
structure has been reached.
Aims: We merge the data sets from
SOHO/MDI and SDO/HMI with the aim of achieving a greater precision
on helioseismic measurements of the subsurface meridional flow.
Methods: The south-north travel-time differences are measured by
applying time-distance helioseismology to the MDI and HMI medium-degree
Dopplergrams covering May 1996-April 2017. Our data analysis corrects
for several sources of systematic effects: P-angle error, surface
magnetic field effects, and center-to-limb variations. For HMI data,
we used the P-angle correction provided by the HMI team based on
the Venus and Mercury transits. For MDI data, we used a P-angle
correction estimated from the correlation of MDI and HMI data during
the period of overlap. The center-to-limb effect is estimated from
the east-west travel-time differences and is different for MDI and
HMI observations. An interpretation of the travel-time measurements is
obtained using a forward-modeling approach in the ray approximation.
Results: In the latitude range 20°-35°, the travel-time
differences are similar in the southern hemisphere for cycles 23 and
24. However, they differ in the northern hemisphere between cycles 23
and 24. Except for cycle 24's northern hemisphere, the measurements
favor a single-cell meridional circulation model where the poleward
flows persist down to ∼0.8 R⊙, accompanied by local
inflows toward the activity belts in the near-surface layers. Cycle
24's northern hemisphere is anomalous: travel-time differences are
significantly smaller when travel distances are greater than 20°. This
asymmetry between northern and southern hemispheres during cycle 24
was not present in previous measurements, which assumed a different
P-angle error correction where south-north travel-time differences
are shifted to zero at the equator for all travel distances. In our
measurements, the travel-time differences at the equator are zero for
travel distances less than ∼30°, but they do not vanish for larger
travel distances. This equatorial offset for large travel distances
need not be interpreted as a deep cross-equator flow; it could be
due to the presence of asymmetrical local flows at the surface near
the end points of the acoustic ray paths.
Conclusions: The
combined MDI and HMI helioseismic measurements presented here contain a
wealth of information about the subsurface structure and the temporal
evolution of the meridional circulation over 21 years. To infer the
deep meridional flow, it will be necessary to model the contribution
from the complex time-varying flows in the near-surface layers. The data are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/619/A99
Title: Revisiting helioseismic constraints on subsurface convection
Authors: Birch, Aaron; Duvall, Tom; Gizon, Laurent; Hanasoge, Shravan;
Hindman, Bradley; Nagashima, Kaori; Sreenivasan, Katepalli
Bibcode: 2018csc..confE..42B
Altcode:
There is disagreement by orders of magnitude between different
helioseismic measurements of the the amplitude of subsurface
convective flows. In addition, there are enormous differences between
some measurements and simulations of subsurface convection. Further
observational and theoretical work on the topic of solar subsurface
convection is crucial. Motivated by the need to establish a
clear baseline for future work, we present a uniform view of the
existing results by expressing upper limits and flow estimates as
root-mean-square velocity per multiplet for all cases. The disagreements
between the upper limit of Hanasoge, Duvall, and Sreenivasan (2012), the
ASH simulations of Miesch et al. (2008), and the helioseismic analysis
of Greer et al. (2015) remain, but are reduced in amplitude. Reconciling
the helioseismic masurements may involve reconsidering the assumptions
about the vertical correlations of the flow field and the methods for
separating signal and noise.
Title: VizieR Online Data Catalog: Starspot rotation rates
vs. activity cycle phase (Nielsen+, 2019)
Authors: Nielsen, M. B.; Gizon, L.; Cameron, R. H.; Miesch, M.
Bibcode: 2018yCat..36220085N
Altcode:
Activity cycle parameters for 3093 stars observed by Kepler, with
measured cycle periods from Reinhold et al. (2017A&A...603A..52R,
Cat. J/A+A/603/A52). The integral, A, of the power density
spectrum around the mean rotation rate (nurot, from McQuillan et
al. (2014ApJS..211...24M, Cat. J/ApJS/211/24)) is used as proxy for
magnetic activity. This and the rotation rate, nu, are traced from
quarters Q1 to Q17 of Kepler observations. (1 data file).
Title: On the depth dependence of solar equatorial Rossby waves
Authors: Proxauf, Bastian; Gizon, Laurent; Löptien, Björn; Birch,
Aaron C.; Schou, Jesper; Bogart, Richard S.
Bibcode: 2018csc..confE..43P
Altcode:
Here we use local helioseismology and local correlation tracking of
granulation to infer horizontal flows on the solar surface and in the
interior. From these flows, we compute maps of the radial vorticity
at different depths in order to study Rossby waves. We show that
the frequencies of these waves agree well with a simple theoretical
dispersion relation. Also, we show that Rossby waves have significant
amplitudes in the first 20 Mm below the surface and investigate
the dependence of the Rossby waves on depth. We find an unexpected,
presumably spurious dip in the wave power and a depth-independent
phase and we conclude that further studies are needed.
Title: Supervised Neural Networks for Helioseismic Ring-diagram
Inversions
Authors: Alshehhi, Rasha; Hanson, Chris S.; Gizon, Laurent
Bibcode: 2018csc..confE..83A
Altcode:
The inversion of ring fit parameters to obtain subsurface flow maps
in ring-diagram analysis for SDO observations is computationally
expensive. We apply machine learning techniques to the inversion
step of the pipeline, to replace future inversion requirements. We
utilize Artificial Neural Networks as a supervised learning method
for predicting the flows in 15° ring tiles. To demonstrate that the
machine learning results still contain the subtle signatures key to
local helioseismic studies, we use the machine learning results to
re-detect equatorial Rossby waves. We find the Artificial Neural Network
is computationally efficient, can achieve a root mean-square error
of half that reported for the observations, and reduce computational
burden by two orders of magnitude. We find that the signatures of the
Rossby waves are still in the machine learning results, showing that
important helioseismic signatures are maintained.
Title: Butterfly diagram of a Sun-like star observed using
asteroseismology
Authors: Bazot, M.; Nielsen, M. B.; Mary, D.; Christensen-Dalsgaard,
J.; Benomar, O.; Petit, P.; Gizon, L.; Sreenivasan, K. R.; White, T. R.
Bibcode: 2018A&A...619L...9B
Altcode: 2018arXiv181008630B
Stellar magnetic fields are poorly understood, but are known to be
important for stellar evolution and exoplanet habitability. They
drive stellar activity, which is the main observational
constraint on theoretical models for magnetic field generation and
evolution. Starspots are the main manifestation of the magnetic fields
at the stellar surface. In this study we measured the variation in
their latitude with time, called a butterfly diagram in the solar case,
for the solar analogue HD 173701 (KIC 8006161). To this end, we used
Kepler data to combine starspot rotation rates at different epochs and
the asteroseismically determined latitudinal variation in the stellar
rotation rates. We observe a clear variation in the latitude of the
starspots. It is the first time such a diagram has been constructed
using asteroseismic data.
Title: Sensitivity kernels for time-distance
helioseismology. Efficient computation for spherically symmetric
solar models
Authors: Fournier, Damien; Hanson, Chris S.; Gizon, Laurent; Barucq,
Hélène
Bibcode: 2018A&A...616A.156F
Altcode: 2018arXiv180506141F
Context. The interpretation of helioseismic measurements, such
as wave travel-time, is based on the computation of kernels that
give the sensitivity of the measurements to localized changes in
the solar interior. These kernels are computed using the ray or
the Born approximation. The Born approximation is preferable as it
takes finite-wavelength effects into account, although it can be
computationally expensive.
Aims: We propose a fast algorithm
to compute travel-time sensitivity kernels under the assumption that
the background solar medium is spherically symmetric.
Methods:
Kernels are typically expressed as products of Green's functions
that depend upon depth, latitude, and longitude. Here, we compute
the spherical harmonic decomposition of the kernels and show that the
integrals in latitude and longitude can be performed analytically. In
particular, the integrals of the product of three associated Legendre
polynomials can be computed.
Results: The computations are fast
and accurate and only require the knowledge of the Green's function
where the source is at the pole. The computation time is reduced
by two orders of magnitude compared to other recent computational
frameworks.
Conclusions: This new method allows flexible and
computationally efficient calculations of a large number of kernels,
required in addressing key helioseismic problems. For example, the
computation of all the kernels required for meridional flow inversion
takes less than two hours on 100 cores.
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: Evolution and wave-like properties of the average solar
supergranule
Authors: Langfellner, J.; Birch, A. C.; Gizon, L.
Bibcode: 2018A&A...617A..97L
Altcode: 2018arXiv180512522L
Context. Solar supergranulation presents us with many mysteries. For
example, previous studies in spectral space have found that
supergranulation has wave-like properties.
Aims: Here we study,
in real space, the wave-like evolution of the average supergranule
over a range of spatial scales (from 10 to 80 Mm). We complement this
by characterizing the evolution of the associated network magnetic
field.
Methods: We used one year of data from the Helioseismic
and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory to
measure horizontal near-surface flows near the solar equator by applying
time-distance helioseismology (TD) on Dopplergrams and granulation
tracking (LCT) on intensity images. The average supergranule outflow
(or inflow) was constructed by averaging over 10 000 individual outflows
(or inflows). The contemporaneous evolution of the magnetic field was
studied with HMI line-of-sight observations.
Results: We confirm
and extend previous measurements of the supergranular wave dispersion
relation to angular wavenumbers in the range 50 < kR⊙
< 270. We find a plateau for kR⊙ > 120. In real
space, larger supergranules undergo oscillations with longer periods
and lifetimes than smaller cells. We find excellent agreement between TD
and LCT and obtain wave properties that are independent of the tracking
rate. The observed network magnetic field follows the oscillations
of the supergranular flows with a six-hour time lag. This behavior
can be explained by computing the motions of corks carried by the
supergranular flows.
Conclusions: Signatures of supergranular
waves in surface horizontal flows near the solar equator can be
observed in real space. These oscillatory flows control the evolution
of the network magnetic field, in particular they explain the recently
discovered east-west anisotropy of the magnetic field around the average
supergranule. Background flow measurements that we obtain from Doppler
frequency shifts do not favor shallow models of supergranulation. The movies associated to Figs. B.1 and B.2 are available at https://www.aanda.org/
Title: Revisiting the exomoon candidate signal around Kepler-1625 b
Authors: Rodenbeck, Kai; Heller, René; Hippke, Michael; Gizon, Laurent
Bibcode: 2018A&A...617A..49R
Altcode: 2018arXiv180604672R
Context. Transit photometry of the Jupiter-sized exoplanet candidate
Kepler-1625 b has recently been interpreted as showing hints of
a moon. This exomoon, the first of its kind, would be as large as
Neptune and unlike any moon we know from the solar system.
Aims: We aim to clarify whether the exomoon-like signal is indeed
caused by a large object in orbit around Kepler-1625 b, or whether it
is caused by stellar or instrumental noise or by the data detrending
procedure.
Methods: To prepare the transit data for model
fitting, we explore several detrending procedures using second-,
third-, and fourth-order polynomials and an implementation of the
Cosine Filtering with Autocorrelation Minimization (CoFiAM). We then
supply a light curve simulator with the co-planar orbital dynamics of
the system and fit the resulting planet-moon transit light curves to
the Kepler data. We employ the Bayesian information criterion (BIC)
to assess whether a single planet or a planet-moon system is a more
likely interpretation of the light curve variations. We carry out
a blind hare-and-hounds exercise using many noise realizations by
injecting simulated transits into different out-of-transit parts of
the original Kepler-1625 light curve: (1) 100 sequences with three
synthetic transits of a Kepler-1625 b-like Jupiter-size planet and (2)
100 sequences with three synthetic transits of a Kepler-1625 b-like
planet with a Neptune-sized moon.
Results: The statistical
significance and characteristics of the exomoon-like signal strongly
depend on the detrending method (polynomials versus cosines), the
data chosen for detrending, and the treatment of gaps in the light
curve. Our injection-retrieval experiment shows evidence of moons
in about 10% of those light curves that do not contain an injected
moon. Strikingly, many of these false-positive moons resemble the
exomoon candidate, that is, a Neptune-sized moon at about 20 Jupiter
radii from the planet. We recover between about one third and one
half of the injected moons, depending on the detrending method, with
radii and orbital distances broadly corresponding to the injected
values.
Conclusions: A ΔBIC of - 4.9 for the CoFiAM-based
detrending is indicative of an exomoon in the three transits of
Kepler-1625 b. This solution, however, is only one out of many and
we find very different solutions depending on the details of the
detrending method. We find it concerning that the detrending is so
clearly key to the exomoon interpretation of the available data of
Kepler-1625 b. Further high-accuracy transit observations may overcome
the effects of red noise but the required amount of additional data
might be large. A movie associated to Fig. 4 is available at https://www.aanda.org.
Title: VizieR Online Data Catalog: Helioseismic measurements of
solar meridional flow (Liang+, 2018)
Authors: Liang, Z. -C.; Gizon, L.; Birch, A. C.; Duvall, T. L. Jr;
Rajaguru, S. P.
Bibcode: 2018yCat..36190099L
Altcode:
The measured travel-time shifts are averaged over three periods,
cycle 23 (May 1996 to April 2008; 3051 days used), cycle 24 (May 2008
to April 2017; 2833 days used), and both the cycles 23 and 24 (May
1996 to April 2017; 5884 days used), as a function of latitude and
travel distance. Also provided are the standard errors of the temporal
means over the three periods. They are all in units of seconds. The
coordinates of these maps can be obtained from the WCS keywords in
the headers; that is, latitude = (i - CRPIX1)*CDELT1 + CRVAL1 [deg],
and distance = (j - CRPIX2)*CDELT2 + CRVAL2 [deg], where i=1..200 and
j=1..61. (7 data files).
Title: Fragile Detection of Solar g -Modes by Fossat et al.
Authors: Schunker, Hannah; Schou, Jesper; Gaulme, Patrick; Gizon,
Laurent
Bibcode: 2018SoPh..293...95S
Altcode: 2018arXiv180404407S
The internal gravity modes of the Sun are notoriously difficult to
detect, and the claimed detection of gravity modes presented by Fossat
et al. (Astron. Astrophys.604, A40, 2017) is thus very exciting. Given
the importance of these modes for understanding solar structure and
dynamics, the results must be robust. While Fossat et al. described
their method and parameter choices in detail, the sensitivity of their
results to several parameters was not presented. Therefore, we test the
sensitivity of the results to a selection of the parameters. The most
concerning result is that the detection vanishes when we adjust the
start time of the 16.5-year velocity time-series by a few hours. We
conclude that this reported detection of gravity modes is extremely
fragile and should be treated with utmost caution.
Title: Probing sunspots with two-skip time-distance helioseismology
Authors: Duvall, Thomas L., Jr.; Cally, Paul S.; Przybylski, Damien;
Nagashima, Kaori; Gizon, Laurent
Bibcode: 2018A&A...613A..73D
Altcode: 2018arXiv180601032D
Context. Previous helioseismology of sunspots has been sensitive to
both the structural and magnetic aspects of sunspot structure.
Aims: We aim to develop a technique that is insensitive to the magnetic
component so the two aspects can be more readily separated.
Methods: We study waves reflected almost vertically from the underside
of a sunspot. Time-distance helioseismology was used to measure travel
times for the waves. Ray theory and a detailed sunspot model were used
to calculate travel times for comparison.
Results: It is shown
that these large distance waves are insensitive to the magnetic field
in the sunspot. The largest travel time differences for any solar
phenomena are observed.
Conclusions: With sufficient modeling
effort, these should lead to better understanding of sunspot structure.
Title: Global-scale equatorial Rossby waves as an essential component
of solar internal dynamics
Authors: Löptien, Björn; Gizon, Laurent; Birch, Aaron C.; Schou,
Jesper; Proxauf, Bastian; Duvall, Thomas L.; Bogart, Richard S.;
Christensen, Ulrich R.
Bibcode: 2018NatAs...2..568L
Altcode: 2018NatAs.tmp...54L; 2018arXiv180507244L
The Sun's complex dynamics is controlled by buoyancy and rotation
in the convection zone. Large-scale flows are dominated by vortical
motions1 and appear to be weaker than expected in the solar
interior2. One possibility is that waves of vorticity
due to the Coriolis force, known as Rossby waves3 or
r modes4, remove energy from convection at the largest
scales5. However, the presence of these waves in the Sun
is still debated. Here, we unambiguously discover and characterize
retrograde-propagating vorticity waves in the shallow subsurface layers
of the Sun at azimuthal wavenumbers below 15, with the dispersion
relation of textbook sectoral Rossby waves. The waves have lifetimes
of several months, well-defined mode frequencies below twice the solar
rotational frequency, and eigenfunctions of vorticity that peak at the
equator. Rossby waves have nearly as much vorticity as the convection
at the same scales, thus they are an essential component of solar
dynamics. We observe a transition from turbulence-like to wave-like
dynamics around the Rhines scale6 of angular wavenumber
of approximately 20. This transition might provide an explanation for
the puzzling deficit of kinetic energy at the largest spatial scales.
Title: Comparison of Travel-Time and Amplitude Measurements for
Deep-Focusing Time-Distance Helioseismology
Authors: Pourabdian, Majid; Fournier, Damien; Gizon, Laurent
Bibcode: 2018SoPh..293...66P
Altcode: 2018arXiv180402311P
The purpose of deep-focusing time-distance helioseismology is
to construct seismic measurements that have a high sensitivity
to the physical conditions at a desired target point in the solar
interior. With this technique, pairs of points on the solar surface are
chosen such that acoustic ray paths intersect at this target (focus)
point. Considering acoustic waves in a homogeneous medium, we compare
travel-time and amplitude measurements extracted from the deep-focusing
cross-covariance functions. Using a single-scattering approximation,
we find that the spatial sensitivity of deep-focusing travel times to
sound-speed perturbations is zero at the target location and maximum
in a surrounding shell. This is unlike the deep-focusing amplitude
measurements, which have maximum sensitivity at the target point. We
compare the signal-to-noise ratio for travel-time and amplitude
measurements for different types of sound-speed perturbations, under
the assumption that noise is solely due to the random excitation of the
waves. We find that, for highly localized perturbations in sound speed,
the signal-to-noise ratio is higher for amplitude measurements than
for travel-time measurements. We conclude that amplitude measurements
are a useful complement to travel-time measurements in time-distance
helioseismology.
Title: Asymmetry of Line Profiles of Stellar Oscillations Measured
by Kepler for Ensembles of Solar-like Oscillators: Impact on Mode
Frequencies and Dependence on Effective Temperature
Authors: Benomar, O.; Goupil, Mjo.; Belkacem, K.; Appourchaux, T.;
Nielsen, M. B.; Bazot, M.; Gizon, L.; Hanasoge, S.; Sreenivasan,
K. R.; Marchand, B.
Bibcode: 2018ApJ...857..119B
Altcode: 2018arXiv180406117B
Oscillation properties are usually measured by fitting symmetric
Lorentzian profiles to the power spectra of Sun-like stars. However,
the line profiles of solar oscillations have been observed to be
asymmetrical for the Sun. The physical origin of this line asymmetry is
not fully understood; though, it should depend on the depth dependence
of the source of wave excitation (convective turbulence) and details of
the observable (velocity or intensity). For oscillations of the Sun,
it has been shown that neglecting the asymmetry leads to systematic
errors in the frequency determination. This could subsequently
affect the results of seismic inferences of the solar internal
structure. Using light curves from the Kepler spacecraft, we have
measured mode asymmetries in 43 stars. We confirm that neglecting the
asymmetry leads to systematic errors that can exceed the 1σ confidence
intervals for seismic observations longer than one year. Therefore,
the application of an asymmetric Lorentzian profile should be favored
to improve the accuracy of the internal stellar structure and stellar
fundamental parameters. We also show that the asymmetry changes sign
between cool Sun-like stars and hotter stars. This provides the best
constraints to date on the location of the excitation sources across
the Hertzsprung-Russel diagram.
Title: Atmospheric-radiation boundary conditions for high-frequency
waves in time-distance helioseismology
Authors: Fournier, D.; Leguèbe, M.; Hanson, C. S.; Gizon, L.; Barucq,
H.; Chabassier, J.; Duruflé, M.
Bibcode: 2017A&A...608A.109F
Altcode: 2017arXiv170902156F
The temporal covariance between seismic waves measured at two
locations on the solar surface is the fundamental observable in
time-distance helioseismology. Above the acoustic cut-off frequency (
5.3 mHz), waves are not trapped in the solar interior and the covariance
function can be used to probe the upper atmosphere. We wish to implement
appropriate radiative boundary conditions for computing the propagation
of high-frequency waves in the solar atmosphere. We consider recently
developed and published radiative boundary conditions for atmospheres
in which sound-speed is constant and density decreases exponentially
with radius. We compute the cross-covariance function using a finite
element method in spherical geometry and in the frequency domain. The
ratio between first- and second-skip amplitudes in the time-distance
diagram is used as a diagnostic to compare boundary conditions and to
compare with observations. We find that a boundary condition applied 500
km above the photosphere and derived under the approximation of small
angles of incidence accurately reproduces the "infinite atmosphere"
solution for high-frequency waves. When the radiative boundary condition
is applied 2 Mm above the photosphere, we find that the choice of
atmospheric model affects the time-distance diagram. In particular,
the time-distance diagram exhibits double-ridge structure when using
a Vernazza Avrett Loeser atmospheric model.
Title: Recovery of subsurface profiles of supergranular flows via
iterative inversion of synthetic travel times
Authors: Bhattacharya, Jishnu; Hanasoge, Shravan M.; Birch, Aaron C.;
Gizon, Laurent
Bibcode: 2017A&A...607A.129B
Altcode:
Aims: We develop a helioseismic inversion algorithm that can
be used to recover subsurface vertical profiles of two-dimensional
supergranular flows from surface measurements of synthetic wave
travel times.
Methods: We carried out seismic wave-propagation
simulations with a two-dimensional section of a flow profile that
resembles an average supergranule and a starting model that only
has flows at the surface. We assumed that the wave measurements are
entirely without realization noise for the purpose of our test. We
expanded the vertical profile of the supergranule stream function on a
basis of B-splines. We iteratively updated the B-spline coefficients
of the supergranule model to reduce the travel-time differences
observed between the two simulations. We performed the exercise for
four different vertical profiles peaking at different depths below the
solar surface.
Results: We are able to accurately recover depth
profiles of four supergranule models at depths up to 8-10 Mm below the
solar surface using f-p4 modes under the assumption that
there is no realization noise. We are able to obtain the peak depth
and the depth of the return flow for each model.
Conclusions:
A basis-resolved inversion performs significantly better than an
inversion in which the flow field is inverted at each point in the
radial grid. This is an encouraging result and might act as a guide
in developing more realistic inversion strategies that can be applied
to supergranular flows in the Sun.
Title: The Maximum Entropy Limit of Small-scale Magnetic Field
Fluctuations in the Quiet Sun
Authors: Gorobets, A. Y.; Berdyugina, S. V.; Riethmüller, T. L.;
Blanco Rodríguez, J.; Solanki, S. K.; Barthol, P.; Gandorfer, A.;
Gizon, L.; Hirzberger, J.; van Noort, M.; Del Toro Iniesta, J. C.;
Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
Bibcode: 2017ApJS..233....5G
Altcode: 2017arXiv171008361G
The observed magnetic field on the solar surface is characterized by a
very complex spatial and temporal behavior. Although feature-tracking
algorithms have allowed us to deepen our understanding of this behavior,
subjectivity plays an important role in the identification and tracking
of such features. In this paper, we continue studies of the temporal
stochasticity of the magnetic field on the solar surface without relying
either on the concept of magnetic features or on subjective assumptions
about their identification and interaction. We propose a data analysis
method to quantify fluctuations of the line-of-sight magnetic field by
means of reducing the temporal field’s evolution to the regular Markov
process. We build a representative model of fluctuations converging to
the unique stationary (equilibrium) distribution in the long time limit
with maximum entropy. We obtained different rates of convergence to the
equilibrium at fixed noise cutoff for two sets of data. This indicates
a strong influence of the data spatial resolution and mixing-polarity
fluctuations on the relaxation process. The analysis is applied to
observations of magnetic fields of the relatively quiet areas around an
active region carried out during the second flight of the Sunrise/IMaX
and quiet Sun areas at the disk center from the Helioseismic and
Magnetic Imager on board the Solar Dynamics Observatory satellite.
Title: Promoting access to and use of seismic data in a large
scientific community. SpaceInn data handling and archiving
Authors: Michel, Eric; Belkacem, Kevin; Samadi, Reza; Assis Peralta,
Raphael de; Renié, Christian; Abed, Mahfoudh; Lin, Guangyuan;
Christensen-Dalsgaard, Jørgen; Houdek, Günter; Handberg, Rasmus;
Gizon, Laurent; Burston, Raymond; Nagashima, Kaori; Pallé, Pere;
Poretti, Ennio; Rainer, Monica; Mistò, Angelo; Panzera, Maria Rosa;
Roth, Markus
Bibcode: 2017EPJWC.16001011M
Altcode:
The growing amount of seismic data available from space missions
(SOHO, CoRoT, Kepler, SDO,…) but also from ground-based facilities
(GONG, BiSON, ground-based large programmes…), stellar modelling
and numerical simulations, creates new scientific perspectives such as
characterizing stellar populations in our Galaxy or planetary systems
by providing model-independent global properties of stars such as mass,
radius, and surface gravity within several percent accuracy, as well as
constraints on the age. These applications address a broad scientific
community beyond the solar and stellar one and require combining
indices elaborated with data from different databases (e.g. seismic
archives and ground-based spectroscopic surveys). It is thus a basic
requirement to develop a simple and effcient access to these various
data resources and dedicated tools. In the framework of the European
project SpaceInn (FP7), several data sources have been developed or
upgraded. The Seismic Plus Portal has been developed, where synthetic
descriptions of the most relevant existing data sources can be found,
as well as tools allowing to localize existing data for given objects
or period and helping the data query. This project has been developed
within the Virtual Observatory (VO) framework. In this paper, we
give a review of the various facilities and tools developed within
this programme. The SpaceInn project (Exploitation of Space Data for
Innovative Helio- and Asteroseismology) has been initiated by the
European Helio- and Asteroseismology Network (HELAS).
Title: A view into the core of α Cen A
Authors: Bazot, Michaël; Christensen-Dalsgaard, Jørgen; Benomar,
Othman; Gizon, Laurent
Bibcode: 2017EPJWC.16003006B
Altcode:
We present results of modelling of α Cen A. In order to estimate
the physical parameters of this star, we modelled spectroscopic,
interferometric, astrometric and asteroseismic data. To that effect we
chose to use a Bayesian approach to parameter estimation, which allowed
us, in particular, to define our prior knowledge on the parameters. An
important question we wanted to address was to assess whether or not
α Cen A has a convective core. We found that the data we used give
indecisive results on this issue. If the star has a convective core,
and provided that overshooting is taken into account, there is a
possibility for the star to be in the peculiar state in which the ppII
chain is the main driver of nuclear energy generation. We also found
a non-negligible probability for α Cen A to be a very early subgiant.
Title: Measuring solar active region inflows with local correlation
tracking of granulation
Authors: Löptien, B.; Birch, A. C.; Duvall, T. L.; Gizon, L.; Proxauf,
B.; Schou, J.
Bibcode: 2017A&A...606A..28L
Altcode: 2017arXiv170508833L
Context. Sixteen years ago local helioseismology detected spatially
extended converging surface flows into solar active regions. These
flows play an important role in flux-transport models of the solar
dynamo.
Aims: We aim to validate the existence of the inflows by
deriving horizontal flow velocities around active regions with local
correlation tracking of granulation.
Methods: We generate a
six-year time series of full-disk maps of the horizontal velocity at
the solar surface by tracking granules in continuum intensity images
provided by the Helioseismic and Magnetic Imager (HMI) on board
the Solar Dynamics Observatory (SDO).
Results: On average,
active regions are surrounded by inflows extending up to 10° from
the center of the active region of magnitudes of 20-30 m/s, reaching
locally up to 40 m/s, which is in agreement with results from local
helioseismology. By computing an ensemble average consisting of 243
individual active regions, we show that the inflows are not azimuthally
symmetric, but converge predominantly towards the trailing polarity
of the active region with respect to the longitudinally and temporally
averaged flow field.
Title: Sensitivity of helioseismic measurements of normal-mode
coupling to flows and sound-speed perturbations
Authors: Hanasoge, Shravan M.; Woodard, Martin; Antia, H. M.; Gizon,
Laurent; Sreenivasan, Katepalli R.
Bibcode: 2017MNRAS.470.1404H
Altcode: 2017arXiv170508204H
In this article, we derive and compute the sensitivity of measurements
of coupling between normal modes of oscillation in the Sun to underlying
flows. The theory is based on first-born perturbation theory, and the
analysis is carried out using the formalism described by Lavely &
Ritzwoller (1992). Albeit tedious, we detail the derivation and compute
the sensitivity of specific pairs of coupled normal modes to anomalies
in the interior. Indeed, these kernels are critical for the accurate
inference of convective flow amplitudes and large-scale circulations in
the solar interior. We resolve some inconsistencies in the derivation
of Lavely & Ritzwoller (1992) and reformulate the fluid-continuity
condition. We also derive and compute sound-speed kernels, paving the
way for inverting for thermal anomalies alongside flows.
Title: Supergranular waves revisited
Authors: Langfellner, Jan; Birch, Aaron; Gizon, Laurent
Bibcode: 2017SPD....4840102L
Altcode:
Solar supergranules remain a mysterious phenomenon, half a century after
their discovery. One particularly interesting aspect of supergranulation
is its wave-like nature detected in Fourier space. Using SDO/HMI local
helioseismology and granulation tracking, we provide new evidence for
supergranular waves. We also discuss their influence on the evolution
of the network magnetic field using cork simulations.
Title: Comparison of acoustic travel-time measurement of solar
meridional circulation from SDO/HMI and SOHO/MDI
Authors: Duvall, Thomas L.; Liang, Zhi-Chao; Birch, Aaron; Gizon,
Laurent; Schou, Jesper
Bibcode: 2017SPD....4840103D
Altcode:
Time-distance helioseismology is one of the primary tools for studying
the solar meridional circulation. However, travel-time measurements
of the subsurface meridional flow suffer from a variety of systematic
errors, such as a center-to-limb variation and an offset due to the
P-angle uncertainty of solar images. Here we apply the time-distance
technique to contemporaneous medium-degree Dopplergrams produced by
SOHO/MDI and SDO/HMI to obtain the travel-time difference caused by
meridional circulation throughout the solar convection zone. The P-angle
offset in MDI images is measured by cross-correlating MDI and HMI
images. The travel-time measurements in the south-north and east-west
directions are averaged over the same observation period for the two
data sets and then compared to examine the consistency of MDI and HMI
travel times after correcting the systematic errors.The offsets in the
south-north travel-time difference from MDI data induced by the P-angle
error gradually diminish with increasing travel distance. However,
these offsets become noisy for travel distances corresponding to
waves that reach the base of the convection zone. This suggests that
a careful treatment of the P-angle problem is required when studying a
deep meridional flow. After correcting the P-angle and the removal of
the center-to-limb effect, the travel-time measurements from MDI and
HMI are consistent within the error bars for meridional circulation
covering the entire convection zone. The fluctuations observed in both
data sets are highly correlated and thus indicate their solar origin
rather than an instrumental origin. Although our results demonstrate
that the ad hoc correction is capable of reducing the wide discrepancy
in the travel-time measurements from MDI and HMI, we cannot exclude
the possibility that there exist other systematic effects acting on
the two data sets in the same way.
Title: Iterative inversion of synthetic travel times successful at
recovering sub-surface profiles of supergranular flows
Authors: Bhattacharya, Jishnu; Hanasoge, Shravan M.; Birch, Aaron C.;
Gizon, Laurent
Bibcode: 2017arXiv170803464B
Altcode:
We develop a helioseismic inversion algorithm that can be used to
recover sub-surface vertical profiles of 2-dimensional supergranular
flows from surface measurements of synthetic wave travel times. We
carry out seismic wave-propagation simulations through a 2-dimensional
section of a flow profile that resembles an averaged supergranule, and
a starting model that has flows only at the surface. We assume that the
wave measurements are entirely without realization noise for the purpose
of our test. We expand the vertical profile of the supergranule stream
function on a basis of B-splines. We iteratively update the B-spline
coefficients of the supergranule model to reduce the travel-times
differences observed between the two simulations. We carry out the
exercise for four different vertical profiles peaking at different
depths below the solar surface. We are able to accurately recover depth
profiles of four supergranule models at depths up to $8-10\,\text{Mm}$
below the solar surface using $f-p_4$ modes, under the assumption that
there is no realization noise. We are able to obtain the peak depth and
the depth of the return flow for each model. A basis-resolved inversion
performs significantly better than one where the flow field is inverted
for at each point in the radial grid. This is an encouraging result and
might act as a guide in developing more realistic inversion strategies
that can be applied to supergranular flows in the Sun.
Title: Evidence for photometric activity cycles in 3203 Kepler stars
Authors: Reinhold, Timo; Cameron, Robert H.; Gizon, Laurent
Bibcode: 2017A&A...603A..52R
Altcode: 2017arXiv170503312R
Context. In recent years it has been claimed that the length of stellar
activity cycles is determined by the stellar rotation rate. It has been
observed that the cycle period increases with rotation period along
two distinct sequences, known as the active and inactive sequences. In
this picture the Sun occupies a solitary position between the two
sequences. Whether the Sun might undergo a transitional evolutionary
stage is currently under debate.
Aims: Our goal is to measure
cyclic variations of the stellar light curve amplitude and the rotation
period using four years of Kepler data. Periodic changes in the light
curve amplitude or the stellar rotation period are associated with
an underlying activity cycle.
Methods: Using a recent sample
of active stars we compute the rotation period and the variability
amplitude for each individual Kepler quarter and search for periodic
variations of both time series. To test for periodicity in each
stellar time series we consider Lomb-Scargle periodograms and use a
selection based on a false alarm probability (FAP).
Results:
We detect amplitude periodicities in 3203 stars between 0.5 <
Pcyc < 6 yr covering rotation periods between 1 <
Prot < 40 days. Given our sample size of 23 601 stars
and our selection criteria that the FAP is less than 5%, this number
is almost three times higher than that expected from pure noise. We do
not detect periodicities in the rotation period beyond those expected
from noise. Our measurements reveal that the cycle period shows a weak
dependence on rotation rate, slightly increasing for longer rotation
periods. We further show that the shape of the variability deviates from
a pure sine curve, consistent with observations of the solar cycle. The
cycle shape does not show a statistically significant dependence on
effective temperature.
Conclusions: We detect activity cycles
in more than 13% of our final sample with a FAP of 5% (calculated by
randomly shuffling the measured 90-day variability measurements for
each star). Our measurements do not support the existence of distinct
sequences in the Prot-Pcyc plane, although there
is some evidence for the inactive sequence for rotation periods between
5-25 days. Unfortunately, the total observing time is too short to draw
sound conclusions on activity cycles with similar lengths to that of the
solar cycle. A table containing all cycle periods and time series
is only available in electronic form at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr
(130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/603/A52
Title: PLATO as it is : A legacy mission for Galactic archaeology
Authors: Miglio, A.; Chiappini, C.; Mosser, B.; Davies, G. R.;
Freeman, K.; Girardi, L.; Jofré, P.; Kawata, D.; Rendle, B. M.;
Valentini, M.; Casagrande, L.; Chaplin, W. J.; Gilmore, G.; Hawkins,
K.; Holl, B.; Appourchaux, T.; Belkacem, K.; Bossini, D.; Brogaard,
K.; Goupil, M. -J.; Montalbán, J.; Noels, A.; Anders, F.; Rodrigues,
T.; Piotto, G.; Pollacco, D.; Rauer, H.; Prieto, C. Allende; Avelino,
P. P.; Babusiaux, C.; Barban, C.; Barbuy, B.; Basu, S.; Baudin, F.;
Benomar, O.; Bienaymé, O.; Binney, J.; Bland-Hawthorn, J.; Bressan,
A.; Cacciari, C.; Campante, T. L.; Cassisi, S.; Christensen-Dalsgaard,
J.; Combes, F.; Creevey, O.; Cunha, M. S.; Jong, R. S.; Laverny, P.;
Degl'Innocenti, S.; Deheuvels, S.; Depagne, É.; Ridder, J.; Matteo,
P. Di; Mauro, M. P. Di; Dupret, M. -A.; Eggenberger, P.; Elsworth,
Y.; Famaey, B.; Feltzing, S.; García, R. A.; Gerhard, O.; Gibson,
B. K.; Gizon, L.; Haywood, M.; Handberg, R.; Heiter, U.; Hekker,
S.; Huber, D.; Ibata, R.; Katz, D.; Kawaler, S. D.; Kjeldsen, H.;
Kurtz, D. W.; Lagarde, N.; Lebreton, Y.; Lund, M. N.; Majewski, S. R.;
Marigo, P.; Martig, M.; Mathur, S.; Minchev, I.; Morel, T.; Ortolani,
S.; Pinsonneault, M. H.; Plez, B.; Moroni, P. G. Prada; Pricopi, D.;
Recio-Blanco, A.; Reylé, C.; Robin, A.; Roxburgh, I. W.; Salaris,
M.; Santiago, B. X.; Schiavon, R.; Serenelli, A.; Sharma, S.; Aguirre,
V. Silva; Soubiran, C.; Steinmetz, M.; Stello, D.; Strassmeier, K. G.;
Ventura, P.; Ventura, R.; Walton, N. A.; Worley, C. C.
Bibcode: 2017AN....338..644M
Altcode: 2017arXiv170603778M
Deciphering the assembly history of the Milky Way is a formidable
task, which becomes possible only if one can produce high-resolution
chrono-chemo-kinematical maps of the Galaxy. Data from large-scale
astrometric and spectroscopic surveys will soon provide us with a
well-defined view of the current chemo-kinematical structure of the
Milky Way, but will only enable a blurred view on the temporal sequence
that led to the present-day Galaxy. As demonstrated by the (ongoing)
exploitation of data from the pioneering photometric missions CoRoT,
Kepler, and K2, asteroseismology provides the way forward: solar-like
oscillating giants are excellent evolutionary clocks thanks to the
availability of seismic constraints on their mass and to the tight
age-initial-mass relation they adhere to. In this paper we identify
five key outstanding questions relating to the formation and evolution
of the Milky Way that will need precise and accurate ages for large
samples of stars to be addressed, and we identify the requirements
in terms of number of targets and the precision on the stellar
properties that are needed to tackle such questions. By quantifying
the asteroseismic yields expected from PLATO for red-giant stars, we
demonstrate that these requirements are within the capabilities of the
current instrument design, provided that observations are sufficiently
long to identify the evolutionary state and allow robust and precise
determination of acoustic-mode frequencies. This will allow us to
harvest data of sufficient quality to reach a 10% precision in age. This
is a fundamental pre-requisite to then reach the more ambitious goal
of a similar level of accuracy, which will only be possible if we
have to hand a careful appraisal of systematic uncertainties on age
deriving from our limited understanding of stellar physics, a goal
which conveniently falls within the main aims of PLATO's core science.
Title: Problems in computational helioseismology
Authors: Gizon, Laurent; Fournier, Damien; Hohage, Thorsten
Bibcode: 2017arXiv170708566G
Altcode:
We discuss current advances in forward and inverse modeling for local
helioseismology. We report theoretical uniqueness results, in particular
the Novikov-Agaltsov reconstruction algorithm, which is relevant to
solving the non-linear inverse problem of time-distance helioseismology
(finite amplitude pertubations to the medium). Numerical experiments
were conducted to determine the number of frequencies required to
reconstruct density and sound speed in the solar interior.
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: Limits on radial differential rotation in Sun-like stars from
parametric fits to oscillation power spectra
Authors: Nielsen, M. B.; Schunker, H.; Gizon, L.; Schou, J.; Ball,
W. H.
Bibcode: 2017A&A...603A...6N
Altcode: 2017arXiv170510517N
Context. Rotational shear in Sun-like stars is thought to be
an important ingredient in models of stellar dynamos. Thanks to
helioseismology, rotation in the Sun is characterized well, but the
interior rotation profiles of other Sun-like stars are not so well
constrained. Until recently, measurements of rotation in Sun-like stars
have focused on the mean rotation, but little progress has been made on
measuring or even placing limits on differential rotation.
Aims:
Using asteroseismic measurements of rotation we aim to constrain the
radial shear in five Sun-like stars observed by the NASA Kepler mission:
KIC 004914923, KIC 005184732,
KIC 006116048, KIC 006933899,
and KIC 010963065.
Methods: We used stellar
structure models for these five stars from previous works. These models
provide the mass density, mode eigenfunctions, and the convection
zone depth, which we used to compute the sensitivity kernels for the
rotational frequency splitting of the modes. We used these kernels as
weights in a parametric model of the stellar rotation profile of each
star, where we allowed different rotation rates for the radiative
interior and the convective envelope. This parametric model was
incorporated into a fit to the oscillation power spectrum of each
of the five Kepler stars. This fit included a prior on the rotation
of the envelope, estimated from the rotation of surface magnetic
activity measured from the photometric variability.
Results:
The asteroseismic measurements without the application of priors are
unable to place meaningful limits on the radial shear. Using a prior
on the envelope rotation enables us to constrain the interior rotation
rate and thus the radial shear. In the five cases that we studied,
the interior rotation rate does not differ from the envelope by more
than approximately ± 30%. Uncertainties in the rotational splittings
are too large to unambiguously determine the sign of the radial shear.
Title: VizieR Online Data Catalog: Activity cycles in 3203 Kepler
stars (Reinhold+, 2017)
Authors: Reinhold, T.; Cameron, R. H.; Gizon, L.
Bibcode: 2017yCat..36030052R
Altcode:
Rvar time series, sine fit parameters, mean rotation periods, and
false alarm probabilities of all 3203 Kepler stars are presented. For
simplicity, the KIC number and the fit parameters of a certain star
are repeated in each line. The fit function to the Rvar(t) time
series equals y_fit=Acyc*sin(2*pi/(Pcyc*365)*(t-t0))+Offset. (2
data files).
Title: Comparison of acoustic travel-time measurements of solar
meridional circulation from SDO/HMI and SOHO/MDI
Authors: Liang, Zhi-Chao; Birch, Aaron C.; Duvall, Thomas L., Jr.;
Gizon, Laurent; Schou, Jesper
Bibcode: 2017A&A...601A..46L
Altcode: 2017arXiv170400475L
Context. Time-distance helioseismology is one of the primary tools
for studying the solar meridional circulation, especially in the lower
convection zone. However, travel-time measurements of the subsurface
meridional flow suffer from a variety of systematic errors, such as
a center-to-limb variation and an offset due to the position angle
(P-angle) uncertainty of solar images. It has been suggested that the
center-to-limb variation can be removed by subtracting east-west from
south-north travel-time measurements. This ad hoc method for the removal
of the center-to-limb effect has been adopted widely but not tested
for travel distances corresponding to the lower convection zone.
Aims: We explore the effects of two major sources of the systematic
errors, the P-angle error arising from the instrumental misalignment and
the center-to-limb variation, on the acoustic travel-time measurements
in the south-north direction.
Methods: We apply the time-distance
technique to contemporaneous medium-degree Dopplergrams produced by
SOHO/MDI and SDO/HMI to obtain the travel-time difference caused by
meridional circulation throughout the solar convection zone. The
P-angle offset in MDI images is measured by cross-correlating MDI
and HMI images. The travel-time measurements in the south-north and
east-west directions are averaged over the same observation period
(May 2010 to Apr. 2011) for the two data sets and then compared to
examine the consistency of MDI and HMI travel times after applying
the above-mentioned corrections.
Results: The offsets in the
south-north travel-time difference from MDI data induced by the P-angle
error gradually diminish with increasing travel distance. However,
these offsets become noisy for travel distances corresponding to
waves that reach the base of the convection zone. This suggests that
a careful treatment of the P-angle problem is required when studying a
deep meridional flow. After correcting the P-angle and the removal of
the center-to-limb effect, the travel-time measurements from MDI and
HMI are consistent within the error bars for meridional circulation
covering the entire convection zone. The fluctuations observed in both
data sets are highly correlated and thus indicate their solar origin
rather than an instrumental origin. Although our results demonstrate
that the ad hoc correction is capable of reducing the wide discrepancy
in the travel-time measurements from MDI and HMI, we cannot exclude
the possibility that there exist other systematic effects acting on
the two data sets in the same way.
Title: Erratum: Morphological Properties of
Slender CaII H Fibrils Observed by sunrise II (ApJS 229, 1, 6)
Authors: Gafeira, R.; Lagg, A.; Solanki, S. K.; Jafarzadeh, S.;
van Noort, M.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta,
J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco
Suárez, D.; Riethmüller, T. L.; Schmidt, W.
Bibcode: 2017ApJS..230...11G
Altcode:
No abstract at ADS
Title: Kepler observations of the asteroseismic binary HD 176465
Authors: White, T. R.; Benomar, O.; Silva Aguirre, V.; Ball, W. H.;
Bedding, T. R.; Chaplin, W. J.; Christensen-Dalsgaard, J.; Garcia,
R. A.; Gizon, L.; Stello, D.; Aigrain, S.; Antia, H. M.; Appourchaux,
T.; Bazot, M.; Campante, T. L.; Creevey, O. L.; Davies, G. R.;
Elsworth, Y. P.; Gaulme, P.; Handberg, R.; Hekker, S.; Houdek,
G.; Howe, R.; Huber, D.; Karoff, C.; Marques, J. P.; Mathur, S.;
McQuillan, A.; Metcalfe, T. S.; Mosser, B.; Nielsen, M. B.; Régulo,
C.; Salabert, D.; Stahn, T.
Bibcode: 2017A&A...601A..82W
Altcode: 2016arXiv160909581W; 2016A&A...601A..82W
Binary star systems are important for understanding stellar structure
and evolution, and are especially useful when oscillations can be
detected and analysed with asteroseismology. However, only four
systems are known in which solar-like oscillations are detected in
both components. Here, we analyse the fifth such system, HD 176465,
which was observed by Kepler. We carefully analysed the system's
power spectrum to measure individual mode frequencies, adapting our
methods where necessary to accommodate the fact that both stars
oscillate in a similar frequency range. We also modelled the two
stars independently by fitting stellar models to the frequencies and
complementaryparameters. We are able to cleanly separate the oscillation
modes in both systems. The stellar models produce compatible ages and
initial compositions for the stars, as is expected from their common
and contemporaneous origin. Combining the individual ages, the system
is about 3.0 ± 0.5 Gyr old. The two components of HD 176465 are young
physically-similar oscillating solar analogues, the first such system
to be found, and provide important constraints for stellar evolution
and asteroseismology.
Title: Computational helioseismology in the frequency domain:
acoustic waves in axisymmetric solar models with flows
Authors: Gizon, Laurent; Barucq, Hélène; Duruflé, Marc; Hanson,
Chris S.; Leguèbe, Michael; Birch, Aaron C.; Chabassier, Juliette;
Fournier, Damien; Hohage, Thorsten; Papini, Emanuele
Bibcode: 2017A&A...600A..35G
Altcode: 2016arXiv161101666G
Context. Local helioseismology has so far relied on semi-analytical
methods to compute the spatial sensitivity of wave travel times to
perturbations in the solar interior. These methods are cumbersome
and lack flexibility.
Aims: Here we propose a convenient
framework for numerically solving the forward problem of time-distance
helioseismology in the frequency domain. The fundamental quantity to
be computed is the cross-covariance of the seismic wavefield.
Methods: We choose sources of wave excitation that enable us to relate
the cross-covariance of the oscillations to the Green's function in a
straightforward manner. We illustrate the method by considering the
3D acoustic wave equation in an axisymmetric reference solar model,
ignoring the effects of gravity on the waves. The symmetry of the
background model around the rotation axis implies that the Green's
function can be written as a sum of longitudinal Fourier modes, leading
to a set of independent 2D problems. We use a high-order finite-element
method to solve the 2D wave equation in frequency space. The computation
is embarrassingly parallel, with each frequency and each azimuthal
order solved independently on a computer cluster.
Results:
We compute travel-time sensitivity kernels in spherical geometry for
flows, sound speed, and density perturbations under the first Born
approximation. Convergence tests show that travel times can be computed
with a numerical precision better than one millisecond, as required
by the most precise travel-time measurements.
Conclusions:
The method presented here is computationally efficient and will be
used to interpret travel-time measurements in order to infer, e.g.,
the large-scale meridional flow in the solar convection zone. It
allows the implementation of (full-waveform) iterative inversions,
whereby the axisymmetric background model is updated at each iteration.
Title: Surface-effect corrections for oscillation frequencies of
evolved stars
Authors: Ball, W. H.; Gizon, L.
Bibcode: 2017A&A...600A.128B
Altcode: 2017arXiv170202570B
Context. Accurate modelling of solar-like oscillators requires that
modelled mode frequencies are corrected for the systematic shift caused
by improper modelling of the near-surface layers, known as the surface
effect. Several parametrizations of the surface effect are now available
but they have not yet been systematically compared with observations
of stars showing modes with mixed g- and p-mode character.
Aims: We investigate how much additional uncertainty is introduced
to stellar model parameters by our uncertainty about the functional
form of the surface effect. At the same time, we test whether any of
the parametrizations is significantly better or worse at modelling
observed subgiants and low-luminosity red giants.
Methods: We
model six stars observed by Kepler that show clear mixed modes. We fix
the input physics of the stellar models and vary the choice of surface
correction between five parametrizations.
Results: Models using a
solar-calibrated power law correction consistently fit the observations
more poorly than the other four corrections. Models with the remaining
four corrections generally fit the observations about equally well,
with the combined surface correction by Ball & Gizon perhaps being
marginally superior. The fits broadly agree on the model parameters
within about the 2σ uncertainties, with discrepancies between the
modified Lorentzian and free power law corrections occasionally
exceeding the 3σ level. Relative to the best-fitting values, the
total uncertainties on the masses, radii and ages of the stars are
all less than 2, 1 and 6 per cent, respectively.
Conclusions:
A solar-calibrated power law, as formulated by Kjeldsen et al., appears
unsuitable for use with more evolved solar-like oscillators. Among
the remaining surface corrections, the uncertainty in the model
parameters introduced by the surface effects is about twice as large
as the uncertainty in the individual fits for these six stars. Though
the fits are thus somewhat less certain because of our uncertainty of
how to manage the surface effect, these results also demonstrate that
it is feasible to model the individual mode frequencies of subgiants
and low-luminosity red giants, and hence also use these individual
stars to help to constrain stellar models.
Title: Slender Ca II H Fibrils Mapping Magnetic Fields in the Low
Solar Chromosphere
Authors: Jafarzadeh, S.; Rutten, R. J.; Solanki, S. K.; Wiegelmann, T.;
Riethmüller, T. L.; van Noort, M.; Szydlarski, M.; Blanco Rodríguez,
J.; Barthol, P.; del Toro Iniesta, J. C.; Gandorfer, A.; Gizon, L.;
Hirzberger, J.; Knölker, M.; Martínez Pillet, V.; Orozco Suárez,
D.; Schmidt, W.
Bibcode: 2017ApJS..229...11J
Altcode: 2016arXiv161003104J
A dense forest of slender bright fibrils near a small solar active
region is seen in high-quality narrowband Ca II H images from the SuFI
instrument onboard the Sunrise balloon-borne solar observatory. The
orientation of these slender Ca II H fibrils (SCF) overlaps with the
magnetic field configuration in the low solar chromosphere derived
by magnetostatic extrapolation of the photospheric field observed
with Sunrise/IMaX and SDO/HMI. In addition, many observed SCFs are
qualitatively aligned with small-scale loops computed from a novel
inversion approach based on best-fit numerical MHD simulation. Such
loops are organized in canopy-like arches over quiet areas that differ
in height depending on the field strength near their roots.
Title: Magneto-static Modeling from Sunrise/IMaX: Application to an
Active Region Observed with Sunrise II
Authors: Wiegelmann, T.; Neukirch, T.; Nickeler, D. H.; Solanki, S. K.;
Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller,
T. L.; van Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.;
Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
Bibcode: 2017ApJS..229...18W
Altcode: 2017arXiv170101458N; 2017arXiv170101458W
Magneto-static models may overcome some of the issues facing force-free
magnetic field extrapolations. So far they have seen limited use
and have faced problems when applied to quiet-Sun data. Here we
present a first application to an active region. We use solar vector
magnetic field measurements gathered by the IMaX polarimeter during
the flight of the Sunrise balloon-borne solar observatory in 2013
June as boundary conditions for a magneto-static model of the higher
solar atmosphere above an active region. The IMaX data are embedded
in active region vector magnetograms observed with SDO/HMI. This work
continues our magneto-static extrapolation approach, which was applied
earlier to a quiet-Sun region observed with Sunrise I. In an active
region the signal-to-noise-ratio in the measured Stokes parameters
is considerably higher than in the quiet-Sun and consequently the
IMaX measurements of the horizontal photospheric magnetic field allow
us to specify the free parameters of the model in a special class of
linear magneto-static equilibria. The high spatial resolution of IMaX
(110-130 km, pixel size 40 km) enables us to model the non-force-free
layer between the photosphere and the mid-chromosphere vertically
by about 50 grid points. In our approach we can incorporate some
aspects of the mixed beta layer of photosphere and chromosphere, e.g.,
taking a finite Lorentz force into account, which was not possible with
lower-resolution photospheric measurements in the past. The linear model
does not, however, permit us to model intrinsic nonlinear structures
like strongly localized electric currents.
Title: The Second Flight of the Sunrise Balloon-borne Solar
Observatory: Overview of Instrument Updates, the Flight, the Data,
and First Results
Authors: Solanki, S. K.; Riethmüller, T. L.; Barthol, P.; Danilovic,
S.; Deutsch, W.; Doerr, H. -P.; Feller, A.; Gandorfer, A.; Germerott,
D.; Gizon, L.; Grauf, B.; Heerlein, K.; Hirzberger, J.; Kolleck, M.;
Lagg, A.; Meller, R.; Tomasch, G.; van Noort, M.; Blanco Rodríguez,
J.; Gasent Blesa, J. L.; Balaguer Jiménez, M.; Del Toro Iniesta,
J. C.; López Jiménez, A. C.; Orozco Suarez, D.; Berkefeld, T.;
Halbgewachs, C.; Schmidt, W.; Álvarez-Herrero, A.; Sabau-Graziati,
L.; Pérez Grande, I.; Martínez Pillet, V.; Card, G.; Centeno, R.;
Knölker, M.; Lecinski, A.
Bibcode: 2017ApJS..229....2S
Altcode: 2017arXiv170101555S
The Sunrise balloon-borne solar observatory, consisting of a 1 m
aperture telescope that provides a stabilized image to a UV filter
imager and an imaging vector polarimeter, carried out its second science
flight in 2013 June. It provided observations of parts of active regions
at high spatial resolution, including the first high-resolution images
in the Mg II k line. The obtained data are of very high quality, with
the best UV images reaching the diffraction limit of the telescope
at 3000 Å after Multi-Frame Blind Deconvolution reconstruction
accounting for phase-diversity information. Here a brief update is
given of the instruments and the data reduction techniques, which
includes an inversion of the polarimetric data. Mainly those aspects
that evolved compared with the first flight are described. A tabular
overview of the observations is given. In addition, an example time
series of a part of the emerging active region NOAA AR 11768 observed
relatively close to disk center is described and discussed in some
detail. The observations cover the pores in the trailing polarity of
the active region, as well as the polarity inversion line where flux
emergence was ongoing and a small flare-like brightening occurred in
the course of the time series. The pores are found to contain magnetic
field strengths ranging up to 2500 G, and while large pores are clearly
darker and cooler than the quiet Sun in all layers of the photosphere,
the temperature and brightness of small pores approach or even exceed
those of the quiet Sun in the upper photosphere.
Title: A Tale of Two Emergences: Sunrise II Observations of Emergence
Sites in a Solar Active Region
Authors: Centeno, R.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.;
Solanki, S. K.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger,
J.; Riethmüller, T. L.; van Noort, M.; Orozco Suárez, D.; Berkefeld,
T.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
Bibcode: 2017ApJS..229....3C
Altcode: 2016arXiv161003531C
In 2013 June, the two scientific instruments on board the second Sunrise
mission witnessed, in detail, a small-scale magnetic flux emergence
event as part of the birth of an active region. The Imaging Magnetograph
Experiment (IMaX) recorded two small (∼ 5\prime\prime )
emerging flux patches in the polarized filtergrams of a photospheric Fe
I spectral line. Meanwhile, the Sunrise Filter Imager (SuFI) captured
the highly dynamic chromospheric response to the magnetic fields pushing
their way through the lower solar atmosphere. The serendipitous capture
of this event offers a closer look at the inner workings of active
region emergence sites. In particular, it reveals in meticulous detail
how the rising magnetic fields interact with the granulation as they
push through the Sun’s surface, dragging photospheric plasma in
their upward travel. The plasma that is burdening the rising field
slides along the field lines, creating fast downflowing channels at
the footpoints. The weight of this material anchors this field to the
surface at semi-regular spatial intervals, shaping it in an undulatory
fashion. Finally, magnetic reconnection enables the field to release
itself from its photospheric anchors, allowing it to continue its
voyage up to higher layers. This process releases energy that lights
up the arch-filament systems and heats the surrounding chromosphere.
Title: Photospheric Response to an Ellerman Bomb-like Event—An
Analogy of Sunrise/IMaX Observations and MHD Simulations
Authors: Danilovic, S.; Solanki, S. K.; Barthol, P.; Gandorfer,
A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van Noort, M.;
Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez, D.;
Schmidt, W.; Martínez Pillet, V.; Knölker, M.
Bibcode: 2017ApJS..229....5D
Altcode: 2016arXiv160903817D
Ellerman Bombs are signatures of magnetic reconnection, which is an
important physical process in the solar atmosphere. How and where they
occur is a subject of debate. In this paper, we analyze Sunrise/IMaX
data, along with 3D MHD simulations that aim to reproduce the exact
scenario proposed for the formation of these features. Although
the observed event seems to be more dynamic and violent than the
simulated one, simulations clearly confirm the basic scenario for the
production of EBs. The simulations also reveal the full complexity of
the underlying process. The simulated observations show that the Fe I
525.02 nm line gives no information on the height where reconnection
takes place. It can only give clues about the heating in the aftermath
of the reconnection. However, the information on the magnetic field
vector and velocity at this spatial resolution is extremely valuable
because it shows what numerical models miss and how they can be
improved.
Title: Transverse Oscillations in Slender Ca II H Fibrils Observed
with Sunrise/SuFI
Authors: Jafarzadeh, S.; Solanki, S. K.; Gafeira, R.; van Noort, M.;
Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta, J. C.; Gandorfer,
A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco Suárez, D.;
Riethmüller, T. L.; Schmidt, W.
Bibcode: 2017ApJS..229....9J
Altcode: 2016arXiv161007449J
We present observations of transverse oscillations in slender Ca II
H fibrils (SCFs) in the lower solar chromosphere. We use a 1 hr long
time series of high- (spatial and temporal-) resolution seeing-free
observations in a 1.1 Å wide passband covering the line core of Ca
II H 3969 Å from the second flight of the Sunrise balloon-borne solar
observatory. The entire field of view, spanning the polarity inversion
line of an active region close to the solar disk center, is covered with
bright, thin, and very dynamic fine structures. Our analysis reveals
the prevalence of transverse waves in SCFs with median amplitudes and
periods on the order of 2.4 ± 0.8 km s-1 and 83 ± 29 s,
respectively (with standard deviations given as uncertainties). We
find that the transverse waves often propagate along (parts of) the
SCFs with median phase speeds of 9 ± 14 km s-1. While the
propagation is only in one direction along the axis in some of the
SCFs, propagating waves in both directions, as well as standing waves
are also observed. The transverse oscillations are likely Alfvénic
and are thought to be representative of magnetohydrodynamic kink
waves. The wave propagation suggests that the rapid high-frequency
transverse waves, often produced in the lower photosphere, can
penetrate into the chromosphere with an estimated energy flux of ≈15
kW m-2. Characteristics of these waves differ from those
reported for other fibrillar structures, which, however, were observed
mainly in the upper solar chromosphere.
Title: The amplitude of the cross-covariance function of solar
oscillations as a diagnostic tool for wave attenuation and geometrical
spreading
Authors: Nagashima, Kaori; Fournier, Damien; Birch, Aaron C.; Gizon,
Laurent
Bibcode: 2017A&A...599A.111N
Altcode: 2016arXiv161208991N
Context. In time-distance helioseismology, wave travel times are
measured from the two-point cross-covariance function of solar
oscillations and are used to image the solar convection zone in three
dimensions. There is, however, also information in the amplitude
of the cross-covariance function, for example, about seismic wave
attenuation.
Aims: We develop a convenient procedure to
measure the amplitude of the cross-covariance function of solar
oscillations.
Methods: In this procedure, the amplitude of the
cross-covariance function is linearly related to the cross-covariance
function and can be measured even for high levels of noise.
Results: As an example application, we measure the amplitude
perturbations of the seismic waves that propagate through the sunspot
in active region NOAA 9787. We can recover the amplitude variations
due to the scattering and attenuation of the waves by the sunspot
and associated finite-wavelength effects.
Conclusions: The
proposed definition of cross-covariance amplitude is robust to noise,
can be used to relate measured amplitudes to 3D perturbations in the
solar interior under the Born approximation, and provides independent
information from the travel times.
Title: Kinematics of Magnetic Bright Features in the Solar Photosphere
Authors: Jafarzadeh, S.; Solanki, S. K.; Cameron, R. H.; Barthol, P.;
Blanco Rodríguez, J.; del Toro Iniesta, J. C.; Gandorfer, A.; Gizon,
L.; Hirzberger, J.; Knölker, M.; Martínez Pillet, V.; Orozco Suárez,
D.; Riethmüller, T. L.; Schmidt, W.; van Noort, M.
Bibcode: 2017ApJS..229....8J
Altcode: 2016arXiv161007634J
Convective flows are known as the prime means of transporting magnetic
fields on the solar surface. Thus, small magnetic structures are good
tracers of turbulent flows. We study the migration and dispersal
of magnetic bright features (MBFs) in intergranular areas observed
at high spatial resolution with Sunrise/IMaX. We describe the flux
dispersal of individual MBFs as a diffusion process whose parameters are
computed for various areas in the quiet-Sun and the vicinity of active
regions from seeing-free data. We find that magnetic concentrations
are best described as random walkers close to network areas (diffusion
index, γ =1.0), travelers with constant speeds over a supergranule
(γ =1.9{--}2.0), and decelerating movers in the vicinity of flux
emergence and/or within active regions (γ =1.4{--}1.5). The three
types of regions host MBFs with mean diffusion coefficients of 130
km2 s-1, 80-90 km2 s-1,
and 25-70 km2 s-1, respectively. The MBFs in
these three types of regions are found to display a distinct kinematic
behavior at a confidence level in excess of 95%.
Title: Spectropolarimetric Evidence for a Siphon Flow along an
Emerging Magnetic Flux Tube
Authors: Requerey, Iker S.; Ruiz Cobo, B.; Del Toro Iniesta, J. C.;
Orozco Suárez, D.; Blanco Rodríguez, J.; Solanki, S. K.; Barthol,
P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.;
van Noort, M.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
Bibcode: 2017ApJS..229...15R
Altcode: 2016arXiv161106732R
We study the dynamics and topology of an emerging magnetic flux
concentration using high spatial resolution spectropolarimetric data
acquired with the Imaging Magnetograph eXperiment on board the sunrise
balloon-borne solar observatory. We obtain the full vector magnetic
field and the line of sight (LOS) velocity through inversions of
the Fe I line at 525.02 nm with the SPINOR code. The derived vector
magnetic field is used to trace magnetic field lines. Two magnetic flux
concentrations with different polarities and LOS velocities are found
to be connected by a group of arch-shaped magnetic field lines. The
positive polarity footpoint is weaker (1100 G) and displays an upflow,
while the negative polarity footpoint is stronger (2200 G) and shows
a downflow. This configuration is naturally interpreted as a siphon
flow along an arched magnetic flux tube.
Title: Morphological Properties of Slender Ca II H Fibrils Observed
by SUNRISE II
Authors: Gafeira, R.; Lagg, A.; Solanki, S. K.; Jafarzadeh, S.;
van Noort, M.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta,
J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco
Suárez, D.; Riethmüller, T. L.; Schmidt, W.
Bibcode: 2017ApJS..229....6G
Altcode: 2016arXiv161200319G
We use seeing-free high spatial resolution Ca II H data obtained by
the SUNRISE observatory to determine properties of slender fibrils
in the lower solar chromosphere. In this work we use intensity images
taken with the SuFI instrument in the Ca II H line during the second
scientific flight of the SUNRISE observatory to identify and track
elongated bright structures. After identification, we analyze theses
structures to extract their morphological properties. We identify
598 slender Ca II H fibrils (SCFs) with an average width of around
180 km, length between 500 and 4000 km, average lifetime of ≈400
s, and average curvature of 0.002 arcsec-1. The maximum
lifetime of the SCFs within our time series of 57 minutes is ≈2000
s. We discuss similarities and differences of the SCFs with other
small-scale, chromospheric structures such as spicules of type I and
II, or Ca II K fibrils.
Title: A New MHD-assisted Stokes Inversion Technique
Authors: Riethmüller, T. L.; Solanki, S. K.; Barthol, P.; Gandorfer,
A.; Gizon, L.; Hirzberger, J.; van Noort, M.; Blanco Rodríguez, J.;
Del Toro Iniesta, J. C.; Orozco Suárez, D.; Schmidt, W.; Martínez
Pillet, V.; Knölker, M.
Bibcode: 2017ApJS..229...16R
Altcode: 2016arXiv161105175R
We present a new method of Stokes inversion of spectropolarimetric
data and evaluate it by taking the example of a Sunrise/IMaX
observation. An archive of synthetic Stokes profiles is obtained
by the spectral synthesis of state-of-the-art magnetohydrodynamics
(MHD) simulations and a realistic degradation to the level of the
observed data. The definition of a merit function allows the archive
to be searched for the synthetic Stokes profiles that best match the
observed profiles. In contrast to traditional Stokes inversion codes,
which solve the Unno-Rachkovsky equations for the polarized radiative
transfer numerically and fit the Stokes profiles iteratively, the new
technique provides the full set of atmospheric parameters. This gives
us the ability to start an MHD simulation that takes the inversion
result as an initial condition. After a relaxation process of half an
hour solar time we obtain physically consistent MHD data sets with
a target similar to the observation. The new MHD simulation is used
to repeat the method in a second iteration, which further improves
the match between observation and simulation, resulting in a factor
of 2.2 lower mean {χ }2 value. One advantage of the new
technique is that it provides the physical parameters on a geometrical
height scale. It constitutes a first step toward inversions that give
results consistent with the MHD equations.
Title: Oscillations on Width and Intensity of Slender Ca II H Fibrils
from Sunrise/SuFI
Authors: Gafeira, R.; Jafarzadeh, S.; Solanki, S. K.; Lagg, A.;
van Noort, M.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta,
J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco
Suárez, D.; Riethmüller, T. L.; Schmidt, W.
Bibcode: 2017ApJS..229....7G
Altcode: 2017arXiv170102801G
We report the detection of oscillations in slender Ca II H fibrils
(SCFs) from high-resolution observations acquired with the Sunrise
balloon-borne solar observatory. The SCFs show obvious oscillations in
their intensity, but also their width. The oscillatory behaviors are
investigated at several positions along the axes of the SCFs. A large
majority of fibrils show signs of oscillations in intensity. Their
periods and phase speeds are analyzed using a wavelet analysis. The
width and intensity perturbations have overlapping distributions
of the wave period. The obtained distributions have median values
of the period of 32 ± 17 s and 36 ± 25 s, respectively. We
find that the fluctuations of both parameters propagate in
the SCFs with speeds of {11}-11+49 km
s-1 and {15}-15+34 km s-1,
respectively. Furthermore, the width and intensity oscillations have a
strong tendency to be either in anti-phase or, to a smaller extent, in
phase. This suggests that the oscillations of both parameters are caused
by the same wave mode and that the waves are likely propagating. Taking
all the evidence together, the most likely wave mode to explain all
measurements and criteria is the fast sausage mode.
Title: Solar Coronal Loops Associated with Small-scale Mixed Polarity
Surface Magnetic Fields
Authors: Chitta, L. P.; Peter, H.; Solanki, S. K.; Barthol, P.;
Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van
Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco
Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
Bibcode: 2017ApJS..229....4C
Altcode: 2016arXiv161007484C
How and where are coronal loops rooted in the solar lower
atmosphere? The details of the magnetic environment and its evolution
at the footpoints of coronal loops are crucial to understanding the
processes of mass and energy supply to the solar corona. To address
the above question, we use high-resolution line-of-sight magnetic
field data from the Imaging Magnetograph eXperiment instrument on the
Sunrise balloon-borne observatory and coronal observations from the
Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory
of an emerging active region. We find that the coronal loops are
often rooted at the locations with minor small-scale but persistent
opposite-polarity magnetic elements very close to the larger dominant
polarity. These opposite-polarity small-scale elements continually
interact with the dominant polarity underlying the coronal loop through
flux cancellation. At these locations we detect small inverse Y-shaped
jets in chromospheric Ca II H images obtained from the Sunrise Filter
Imager during the flux cancellation. Our results indicate that magnetic
flux cancellation and reconnection at the base of coronal loops due
to mixed polarity fields might be a crucial feature for the supply of
mass and energy into the corona.
Title: Moving Magnetic Features around a Pore
Authors: Kaithakkal, A. J.; Riethmüller, T. L.; Solanki, S. K.; Lagg,
A.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; vanNoort,
M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez,
D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
Bibcode: 2017ApJS..229...13K
Altcode: 2016arXiv160905664K
Spectropolarimetric observations from Sunrise/IMaX, obtained in 2013
June, are used for a statistical analysis to determine the physical
properties of moving magnetic features (MMFs) observed near a pore. MMFs
of the same and opposite polarity, with respect to the pore, are found
to stream from its border at an average speed of 1.3 km s-1
and 1.2 km s-1, respectively, with mainly same-polarity MMFs
found further away from the pore. MMFs of both polarities are found to
harbor rather weak, inclined magnetic fields. Opposite-polarity MMFs
are blueshifted, whereas same-polarity MMFs do not show any preference
for up- or downflows. Most of the MMFs are found to be of sub-arcsecond
size and carry a mean flux of ∼1.2 × 1017 Mx.
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: 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: Interpretation of Helioseismic Travel Times
Authors: Burston, Raymond; Gizon, Laurent; Birch, Aaron C.
Bibcode: 2017hdsi.book..207B
Altcode:
No abstract at ADS
Title: Helioseismology with Solar Orbiter
Authors: Löptien, Björn; Birch, Aaron C.; Gizon, Laurent; Schou,
Jesper; Appourchaux, Thierry; Blanco Rodríguez, Julián; Cally,
Paul S.; Dominguez-Tagle, Carlos; Gandorfer, Achim; Hill, Frank;
Hirzberger, Johann; Scherrer, Philip H.; Solanki, Sami K.
Bibcode: 2017hdsi.book..257L
Altcode:
No abstract at ADS
Title: Intensity contrast of the average supergranule
Authors: Langfellner, J.; Birch, A. C.; Gizon, L.
Bibcode: 2016A&A...596A..66L
Altcode: 2016arXiv160909308L
While the velocity fluctuations of supergranulation dominate the
spectrum of solar convection at the solar surface, very little is known
about the fluctuations in other physical quantities like temperature
or density at supergranulation scale. Using observations from the
Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO), we characterize the intensity contrast of solar
supergranulation at the solar surface. We identify the positions of
104 outflow and inflow regions at supergranulation scales,
from which we construct average flow maps and co-aligned intensity
and magnetic field maps. In the average outflow center, the maximum
intensity contrast is (7.8 ± 0.6) × 10-4 (there is no
corresponding feature in the line-of-sight magnetic field). This
corresponds to a temperature perturbation of about 1.1 ± 0.1 K, in
agreement with previous studies. We discover an east-west anisotropy,
with a slightly deeper intensity minimum to the east of the outflow
center. The evolution is asymmetric in time: the intensity excess is
larger eight hours before the reference time (the time of maximum
outflow), while it has almost disappeared eight hours after the
reference time. In the average inflow region, the intensity contrast
mostly follows the magnetic field distribution, except for an east-west
anisotropic component that dominates eight hours before the reference
time. We suggest that the east-west anisotropy in the intensity is
related to the wave-like properties of supergranulation.
Title: An all-sky catalogue of solar-type dwarfs for exoplanetary
transit surveys
Authors: Nascimbeni, V.; Piotto, G.; Ortolani, S.; Giuffrida, G.;
Marrese, P. M.; Magrin, D.; Ragazzoni, R.; Pagano, I.; Rauer, H.;
Cabrera, J.; Pollacco, D.; Heras, A. M.; Deleuil, M.; Gizon, L.;
Granata, V.
Bibcode: 2016MNRAS.463.4210N
Altcode: 2016arXiv160903037N; 2016MNRAS.tmp.1422N
Most future surveys designed to discover transiting exoplanets,
including TESS and PLATO, will target bright (V ≲ 13) and nearby
solar-type stars having a spectral type later than F5. In order to
enhance the probability of identifying transits, these surveys must
cover a very large area on the sky, because of the intrinsically low
areal density of bright targets. Unfortunately, no existing catalogue
of stellar parameters is both deep and wide enough to provide a
homogeneous input list. As the first Gaia data release exploitable
for this purpose is expected to be released not earlier than late
2017, we have devised an improved reduced-proper-motion (RPM) method
to discriminate late field dwarfs and giants by combining the fourth
U.S. Naval Observatory CCD Astrograph Catalog (UCAC4) proper motions
with AAVSO Photometric All-Sky Survey DR6 photometry, and relying on
Radial Velocity Experiment DR4 as an external calibrator. The output,
named UCAC4-RPM, is a publicly available, complete all-sky catalogue of
solar-type dwarfs down to V ≃ 13.5, plus an extension to log g >
3.0 subgiants. The relatively low amount of contamination (defined as
the fraction of false positives; <30 per cent) also makes UCAC4-RPM
a useful tool for the past and ongoing ground-based transit surveys,
which need to discard candidate signals originating from early-type
or giant stars. As an application, we show how UCAC4-RPM may support
the preparation of the TESS (that will map almost the entire sky)
input catalogue and the input catalogue of PLATO, planned to survey
more than half of the whole sky with exquisite photometric precision.
Title: Far side Helioseismology with Solar Orbiter
Authors: Appourchaux, T.; Birch, A.; Gizon, L. C.; Löptien, B.;
Schou, J.; Solanki, S. K.; del Toro Iniesta, J. C.; Gandorfer, A.;
Hirzberger, J.; Alvarez-Herrero, A.; Woch, J. G.; Schmidt, W.
Bibcode: 2016AGUFMSH43A2554A
Altcode:
The Solar Orbiter mission, to be launched in October 2018, will
carry a suite of remote sensing and in-situ instruments, including
the Polarimetric and Helioseismic Imager (PHI). PHI will deliver
high-cadence images of the Sun in intensity and Doppler velocity
suitable for carrying out novel helioseismic studies. The orbit
of the Solar Orbiter spacecraft will reach a solar latitude up to
34 degrees by the end of the extended mission and thus will enable
the first local helioseismology studies of the polar regions. The
full range of Earth-Sun-spacecraft angles provided by the orbit will
enable helioseismology from two vantage points by combining PHI with
another instrument: stereoscopic helioseismology will allow the study
of the deep solar interior and a better understanding of the physics
of solar oscillations in both quiet Sun and sunspots. In this paper
we will review the helioseismic objectives achievable with PHI, and
will also give a short status report of the development of the Flight
Model of PHI.
Title: 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: SDO/HMI survey of emerging active regions for helioseismology
Authors: Schunker, H.; Braun, D. C.; Birch, A. C.; Burston, R. B.;
Gizon, L.
Bibcode: 2016A&A...595A.107S
Altcode: 2016arXiv160808005S
Context. Observations from the Solar Dynamics Observatory (SDO) have
the potential for allowing the helioseismic study of the formation of
hundreds of active regions, which would enable us to perform statistical
analyses.
Aims: Our goal is to collate a uniform data set of
emerging active regions observed by the SDO/HMI instrument suitable for
helioseismic analysis, where each active region is centred on a 60° ×
60° area and can be observed up to seven days before emergence.
Methods: We restricted the sample to active regions that were visible in
the continuum and emerged into quiet Sun largely avoiding pre-existing
magnetic regions. As a reference data set we paired a control region
(CR), with the same latitude and distance from central meridian, with
each emerging active region (EAR). The control regions do not have
any strong emerging flux within 10° of the centre of the map. Each
region was tracked at the Carrington rotation rate as it crossed
the solar disk, within approximately 65° from the central meridian
and up to seven days before, and seven days after, emergence. The
mapped and tracked data, consisting of line-of-sight velocity,
line-of-sight magnetic field, and intensity as observed by SDO/HMI,
are stored in datacubes that are 410 min in duration and spaced 320
min apart. We call this data set, which is currently comprised of 105
emerging active regions observed between May 2010 and November 2012,
the SDO Helioseismic Emerging Active Region (SDO/HEAR) survey.
Results: To demonstrate the utility of a data set of a large number of
emerging active regions, we measure the relative east-west velocity of
the leading and trailing polarities from the line-of-sight magnetogram
maps during the first day after emergence. The latitudinally averaged
line-of-sight magnetic field of all the EARs shows that, on average,
the leading (trailing) polarity moves in a prograde (retrograde)
direction with a speed of 121 ± 22 m s-1 (-70 ± 13 m
s-1) relative to the Carrington rotation rate in the first
day. However, relative to the differential rotation of the surface
plasma, the east-west velocity is symmetric, with a mean of 95 ± 13
m s-1.
Conclusions: The SDO/HEAR data set will not
only be useful for helioseismic studies, but will also be useful to
study other features such as the surface magnetic field evolution of
a large sample of EARs. We intend to extend this survey forwards in
time to include more EARs observed by SDO/HMI.
Title: Solar-cycle variation of the rotational shear near the
solar surface
Authors: Barekat, A.; Schou, J.; Gizon, L.
Bibcode: 2016A&A...595A...8B
Altcode: 2016arXiv160807101B
Context. Helioseismology has revealed that the angular velocity of the
Sun increases with depth in the outermost 35 Mm of the Sun. Recently,
we have shown that the logarithmic radial gradient (dlnΩ/dlnr) in the
upper 10 Mm is close to -1 from the equator to 60° latitude.
Aims: We aim to measure the temporal variation of the rotational shear
over solar cycle 23 and the rising phase of cycle 24 (1996-2015).
Methods: We used f mode frequency splitting data spanning 1996 to
2011 from the Michelson Doppler Imager (MDI) and 2010 to 2015 from
the Helioseismic Magnetic Imager (HMI). In a first for such studies,
the f mode frequency splitting data were obtained from 360-day time
series. We used the same method as in our previous work for measuring
dlnΩ/dlnr from the equator to 80° latitude in the outer 13 Mm of
the Sun. Then, we calculated the variation of the gradient at annual
cadence relative to the average over 1996 to 2015.
Results:
We found the rotational shear at low latitudes (0° to 30°) to vary
in-phase with the solar activity, varying by ~± 10% over the period
1996 to 2015. At high latitudes (60° to 80°), we found rotational
shear to vary in anti-phase with the solar activity. By comparing
the radial gradient obtained from the splittings of the 360-day and
the corresponding 72-day time series of HMI and MDI data, we suggest
that the splittings obtained from the 72-day HMI time series suffer
from systematic errors.
Conclusions: We provide a quantitative
measurement of the temporal variation of the outer part of the near
surface shear layer which may provide useful constraints on dynamo
models and differential rotation theory.
Title: VizieR Online Data Catalog: All-sky catalog of solar-type
dwarfs (Nascimbeni+, 2016)
Authors: Nascimbeni, V.; Piotto, G.; Ortolani, S.; Giuffrida, G.;
Marrese, P. M.; Magrin, D.; Ragazzoni, R.; Pagano, I.; Rauer, H.;
Cabrera, J.; Pollacco, D.; Heras, A. M.; Deleuil, M.; Gizon, L.;
Granata, V.
Bibcode: 2016yCat..74634210N
Altcode:
We devised a new RPM-based algorithm to assign a luminosity class
to field stars by knowing only their proper motions and two optical
magnitudes. By applying this optimal algorithm on a new stellar
catalogue compiled by matching UCAC4, APASS DR6 and Tycho-2, we ended
up with UCAC4-RPM - an all-sky sample of solar-type dwarf stars complete
down to at least V~=13. (2 data files).
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: MESA meets MURaM. Surface effects in main-sequence solar-like
oscillators computed using three-dimensional radiation hydrodynamics
simulations
Authors: Ball, W. H.; Beeck, B.; Cameron, R. H.; Gizon, L.
Bibcode: 2016A&A...592A.159B
Altcode: 2016arXiv160602713B
Context. Space-based observations of solar-like oscillators have
identified large numbers of stars in which many individual mode
frequencies can be precisely measured. However, current stellar models
predict oscillation frequencies that are systematically affected by
simplified modelling of the near-surface layers.
Aims: We use
three-dimensional radiation hydrodynamics simulations to better model
the near-surface equilibrium structure of dwarfs with spectral types F3,
G2, K0 and K5, and examine the differences between oscillation mode
frequencies computed in stellar models with and without the improved
near-surface equilibrium structure.
Methods: We precisely match
stellar models to the simulations' gravities and effective temperatures
at the surface, and to the temporally- and horizontally-averaged
densities and pressures at their deepest points. We then replace
the near-surface structure with that of the averaged simulation and
compute the change in the oscillation mode frequencies. We also fit
the differences using several parametric models currently available
in the literature.
Results: The surface effect in the stars of
solar-type and later is qualitatively similar and changes steadily
with decreasing effective temperature. In particular, the point of
greatest frequency difference decreases slightly as a fraction of
the acoustic cut-off frequency and the overall scale of the surface
effect decreases. The surface effect in the hot, F3-type star follows
the same trend in scale (I.e. it is larger in magnitude) but shows
a different overall variation with mode frequency. We find that a
two-term fit using the cube and inverse of the frequency divided by
the mode inertia is best able to reproduce the surface terms across
all four spectral types, although the scaled solar term and a modified
Lorentzian function also match the three cooler simulations reasonably
well.
Conclusions: Three-dimensional radiation hydrodynamics
simulations of near-surface convection can be averaged and combined with
stellar structure models to better predict oscillation mode frequencies
in solar-like oscillators. Our simplified results suggest that the
surface effect is generally larger in hotter stars (and correspondingly
smaller in cooler stars) and of similar shape in stars of solar type
and cooler. However, we cannot presently predict whether this will
remain so when other components of the surface effect are included.
Title: On the uncertain nature of the core of α Cen A
Authors: Bazot, M.; Christensen-Dalsgaard, J.; Gizon, L.; Benomar, O.
Bibcode: 2016MNRAS.460.1254B
Altcode: 2016MNRAS.tmp..725B; 2016arXiv160307583B
High-quality astrometric, spectroscopic, interferometric and,
importantly, asteroseismic observations are available for α Cen A,
which is the closest binary star system to earth. Taking all these
constraints into account, we study the internal structure of the star
by means of theoretical modelling. Using the Aarhus STellar Evolution
Code (ASTEC) and the tools of Computational Bayesian Statistics, in
particular a Markov chain Monte Carlo algorithm, we perform statistical
inferences for the physical characteristics of the star. We find that
α Cen A has a probability of approximately 40 per cent of having
a convective core. This probability drops to few per cent if one
considers reduced rates for the 14N(p,γ)15O
reaction. These convective cores have fractional radii less than 8 per
cent when overshoot is neglected. Including overshooting also leads
to the possibility of a convective core mostly sustained by the ppII
chain energy output. We finally show that roughly 30 per cent of the
stellar models describing α Cen A are in the subgiant regime.
Title: A low upper limit on the subsurface rise speed of solar
active regions
Authors: Birch, A. C.; Schunker, H.; Braun, D. C.; Cameron, R.; Gizon,
L.; Lo ptien, B.; Rempel, M.
Bibcode: 2016SciA....2E0557B
Altcode: 2016arXiv160705250B
Magnetic field emerges at the surface of the Sun as sunspots and active
regions. This process generates a poloidal magnetic field from a rising
toroidal flux tube, it is a crucial but poorly understood aspect of
the solar dynamo. The emergence of magnetic field is also important
because it is a key driver of solar activity. We show that measurements
of horizontal flows at the solar surface around emerging active regions,
in combination with numerical simulations of solar magnetoconvection,
can constrain the subsurface rise speed of emerging magnetic flux. The
observed flows imply that the rise speed of the magnetic field is
no larger than 150 m/s at a depth of 20 Mm, that is, well below the
prediction of the (standard) thin flux tube model but in the range
expected for convective velocities at this depth. We conclude that
convective flows control the dynamics of rising flux tubes in the upper
layers of the Sun and cannot be neglected in models of flux emergence.
Title: Sensitivity Kernels for Flows in Time-Distance Helioseismology:
Extension to Spherical Geometry
Authors: Böning, Vincent G. A.; Roth, Markus; Zima, Wolfgang; Birch,
Aaron C.; Gizon, Laurent
Bibcode: 2016ApJ...824...49B
Altcode: 2016arXiv160403803B
We extend an existing Born approximation method for calculating the
linear sensitivity of helioseismic travel times to flows from Cartesian
to spherical geometry. This development is necessary for using the
Born approximation for inferring large-scale flows in the deep solar
interior. As first sanity check, we compare two f-mode kernels from our
spherical method and from an existing Cartesian method. The horizontal
and total integrals agree to within 0.3%. As a second consistency test,
we consider a uniformly rotating Sun and a travel distance of 42°. The
analytical travel-time difference agrees with the forward-modeled
travel-time difference to within 2%. In addition, we evaluate the impact
of different choices of filter functions on the kernels for a meridional
travel distance of 42°. For all filters, the sensitivity is found to
be distributed over a large fraction of the convection zone. We show
that the kernels depend on the filter function employed in the data
analysis process. If modes of higher harmonic degree (90 ≲ l ≲ 170)
are permitted, a noisy pattern of a spatial scale corresponding to l
≈ 260 appears near the surface. When mainly low-degree modes are used
(l ≲ 70), the sensitivity is concentrated in the deepest regions and
it visually resembles a ray-path-like structure. Among the different
low-degree filters used, we find the kernel for phase-speed-filtered
measurements to be best localized in depth.
Title: The shrinking Sun: A systematic error in local correlation
tracking of solar granulation
Authors: Löptien, B.; Birch, A. C.; Duvall, T. L.; Gizon, L.;
Schou, J.
Bibcode: 2016A&A...590A.130L
Altcode: 2016arXiv160404469L
Context. Local correlation tracking of granulation (LCT) is an important
method for measuring horizontal flows in the photosphere. This method
exhibits a systematic error that looks like a flow converging toward
disk center, which is also known as the shrinking-Sun effect.
Aims: We aim to study the nature of the shrinking-Sun effect for
continuum intensity data and to derive a simple model that can explain
its origin.
Methods: We derived LCT flow maps by running the
LCT code Fourier Local Correlation Tracking (FLCT) on tracked and
remapped continuum intensity maps provided by the Helioseismic and
Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). We
also computed flow maps from synthetic continuum images generated from
STAGGER code simulations of solar surface convection. We investigated
the origin of the shrinking-Sun effect by generating an average
granule from synthetic data from the simulations.
Results: The
LCT flow maps derived from the HMI data and the simulations exhibit
a shrinking-Sun effect of comparable magnitude. The origin of this
effect is related to the apparent asymmetry of granulation originating
from radiative transfer effects when observing with a viewing angle
inclined from vertical. This causes, in combination with the expansion
of the granules, an apparent motion toward disk center.
Title: Data compression for local correlation tracking of solar
granulation
Authors: Löptien, B.; Birch, A. C.; Duvall, T. L.; Gizon, L.;
Schou, J.
Bibcode: 2016A&A...587A...9L
Altcode: 2015arXiv151203243L
Context. Several upcoming and proposed space missions, such as
Solar Orbiter, will be limited in telemetry and thus require data
compression.
Aims: We test the impact of data compression on
local correlation tracking (LCT) of time series of continuum intensity
images. We evaluate the effect of several lossy compression methods
(quantization, JPEG compression, and a reduced number of continuum
images) on measurements of solar differential rotation with LCT.
Methods: We applied the different compression methods to tracked
and remapped continuum intensity maps obtained by the Helioseismic
and Magnetic Imager (HMI) on board the Solar Dynamics Observatory. We
derived 2D vector velocities using the local correlation tracking code
Fourier Local Correlation Tracking (FLCT) and determined the additional
bias and noise introduced by compression to differential rotation.
Results: We find that probing differential rotation with LCT is
very robust to lossy data compression when using quantization. Our
results are severely affected by systematic errors of the LCT method
and the HMI instrument. The sensitivity of LCT to systematic errors
is a concern for Solar Orbiter.
Title: Asteroseismic inversions for radial differential rotation of
Sun-like stars: ensemble fits
Authors: Schunker, H.; Schou, J.; Ball, W. H.; Nielsen, M. B.;
Gizon, L.
Bibcode: 2016A&A...586A..79S
Altcode: 2015arXiv151207169S
Context. Radial differential rotation is an important parameter
for stellar dynamo theory and for understanding angular momentum
transport.
Aims: We investigate the potential of using a large
number of similar stars simultaneously to constrain their average radial
differential rotation gradient: we call this "ensemble fitting".
Methods: We use a range of stellar models along the main sequence,
each with a synthetic rotation profile. The rotation profiles are step
functions with a step of ΔΩ = -0.35 μHz, which is located at the base
of the convection zone. These models are used to compute the rotational
splittings of the p modes and to model their uncertainties. We then
fit an ensemble of stars to infer the average ΔΩ.
Results: All
the uncertainties on the inferred ΔΩ for individual stars are of the
order 1 μHz. Using 15 stellar models in an ensemble fit, we show that
the uncertainty on the average ΔΩ is reduced to less than the input
ΔΩ, which allows us to constrain the sign of the radial differential
rotation. We show that a solar-like ΔΩ ≈ 30 nHz can be constrained
by an ensemble fit of thousands of main-sequence stars. Observing the
number of stars required to successfully exploit the ensemble fitting
method will be possible with future asteroseismology missions, such
as PLATO. We demonstrate the potential of ensemble fitting by showing
that any systematic differences in the average ΔΩ between F, G,
and K-type stars larger than 100 nHz could be detected.
Title: Seismic Sounding of Convection in the Sun
Authors: Hanasoge, Shravan; Gizon, Laurent; Sreenivasan, Katepalli R.
Bibcode: 2016AnRFM..48..191H
Altcode: 2015arXiv150307961H
Thermal convection is the dominant mechanism of energy transport in the
outer envelope of the Sun (one-third by radius). It drives global fluid
circulations and magnetic fields observed on the solar surface. Vigorous
surface convection excites a broadband spectrum of acoustic waves
that propagate within the interior and set up modal resonances. These
acoustic waves, also called seismic waves in this context, are observed
at the surface of the Sun by space- and ground-based telescopes. Seismic
sounding, the study of these seismic waves to infer the internal
properties of the Sun, constitutes helioseismology. Here we review
our knowledge of solar convection, especially that obtained through
seismic inference. Several characteristics of solar convection,
such as differential rotation, anisotropic Reynolds stresses,
the influence of rotation on convection, and supergranulation, are
considered. On larger scales, several inferences suggest that convective
velocities are substantially smaller than those predicted by theory
and simulations. This discrepancy challenges the models of internal
differential rotation that rely on convective stresses as a driving
mechanism and provide an important benchmark for numerical simulations.
Title: Interpretation of Helioseismic Travel Times. Sensitivity to
Sound Speed, Pressure, Density, and Flows
Authors: Burston, Raymond; Gizon, Laurent; Birch, Aaron C.
Bibcode: 2015SSRv..196..201B
Altcode: 2015arXiv150309005B; 2015SSRv..tmp...18B
Time-distance helioseismology uses cross-covariances of wave
motions on the solar surface to determine the travel times of wave
packets moving from one surface location to another. We review the
methodology to interpret travel-time measurements in terms of small,
localised perturbations to a horizontally homogeneous reference solar
model. Using the first Born approximation, we derive and compute
3D travel-time sensitivity (Fréchet) kernels for perturbations in
sound-speed, density, pressure, and vector flows. While kernels for
sound speed and flows had been computed previously, here we extend
the calculation to kernels for density and pressure, hence providing a
complete description of the effects of solar dynamics and structure on
travel times. We treat three thermodynamic quantities as independent
and do not assume hydrostatic equilibrium. We present a convenient
approach to computing damped Green's functions using a normal-mode
summation. The Green's function must be computed on a wavenumber grid
that has sufficient resolution to resolve the longest lived modes. The
typical kernel calculations used in this paper are computer intensive
and require on the order of 600 CPU hours per kernel. Kernels are
validated by computing the travel-time perturbation that results
from horizontally-invariant perturbations using two independent
approaches. At fixed sound-speed, the density and pressure kernels
are approximately related through a negative multiplicative factor,
therefore implying that perturbations in density and pressure are
difficult to disentangle. Mean travel-times are not only sensitive to
sound-speed, density and pressure perturbations, but also to flows,
especially vertical flows. Accurate sensitivity kernels are needed to
interpret complex flow patterns such as convection.
Title: The EChO science case
Authors: Tinetti, Giovanna; Drossart, Pierre; Eccleston, Paul; Hartogh,
Paul; Isaak, Kate; Linder, Martin; Lovis, Christophe; Micela, Giusi;
Ollivier, Marc; Puig, Ludovic; Ribas, Ignasi; Snellen, Ignas; Swinyard,
Bruce; Allard, France; Barstow, Joanna; Cho, James; Coustenis, Athena;
Cockell, Charles; Correia, Alexandre; Decin, Leen; de Kok, Remco;
Deroo, Pieter; Encrenaz, Therese; Forget, Francois; Glasse, Alistair;
Griffith, Caitlin; Guillot, Tristan; Koskinen, Tommi; Lammer, Helmut;
Leconte, Jeremy; Maxted, Pierre; Mueller-Wodarg, Ingo; Nelson, Richard;
North, Chris; Pallé, Enric; Pagano, Isabella; Piccioni, Guseppe;
Pinfield, David; Selsis, Franck; Sozzetti, Alessandro; Stixrude,
Lars; Tennyson, Jonathan; Turrini, Diego; Zapatero-Osorio, Mariarosa;
Beaulieu, Jean-Philippe; Grodent, Denis; Guedel, Manuel; Luz, David;
Nørgaard-Nielsen, Hans Ulrik; Ray, Tom; Rickman, Hans; Selig,
Avri; Swain, Mark; Banaszkiewicz, Marek; Barlow, Mike; Bowles, Neil;
Branduardi-Raymont, Graziella; du Foresto, Vincent Coudé; Gerard,
Jean-Claude; Gizon, Laurent; Hornstrup, Allan; Jarchow, Christopher;
Kerschbaum, Franz; Kovacs, Géza; Lagage, Pierre-Olivier; Lim, Tanya;
Lopez-Morales, Mercedes; Malaguti, Giuseppe; Pace, Emanuele; Pascale,
Enzo; Vandenbussche, Bart; Wright, Gillian; Ramos Zapata, Gonzalo;
Adriani, Alberto; Azzollini, Ruymán; Balado, Ana; Bryson, Ian;
Burston, Raymond; Colomé, Josep; Crook, Martin; Di Giorgio, Anna;
Griffin, Matt; Hoogeveen, Ruud; Ottensamer, Roland; Irshad, Ranah;
Middleton, Kevin; Morgante, Gianluca; Pinsard, Frederic; Rataj, Mirek;
Reess, Jean-Michel; Savini, Giorgio; Schrader, Jan-Rutger; Stamper,
Richard; Winter, Berend; Abe, L.; Abreu, M.; Achilleos, N.; Ade, P.;
Adybekian, V.; Affer, L.; Agnor, C.; Agundez, M.; Alard, C.; Alcala,
J.; Allende Prieto, C.; Alonso Floriano, F. J.; Altieri, F.; Alvarez
Iglesias, C. A.; Amado, P.; Andersen, A.; Aylward, A.; Baffa, C.;
Bakos, G.; Ballerini, P.; Banaszkiewicz, M.; Barber, R. J.; Barrado,
D.; Barton, E. J.; Batista, V.; Bellucci, G.; Belmonte Avilés,
J. A.; Berry, D.; Bézard, B.; Biondi, D.; Błęcka, M.; Boisse, I.;
Bonfond, B.; Bordé, P.; Börner, P.; Bouy, H.; Brown, L.; Buchhave,
L.; Budaj, J.; Bulgarelli, A.; Burleigh, M.; Cabral, A.; Capria,
M. T.; Cassan, A.; Cavarroc, C.; Cecchi-Pestellini, C.; Cerulli,
R.; Chadney, J.; Chamberlain, S.; Charnoz, S.; Christian Jessen,
N.; Ciaravella, A.; Claret, A.; Claudi, R.; Coates, A.; Cole, R.;
Collura, A.; Cordier, D.; Covino, E.; Danielski, C.; Damasso, M.;
Deeg, H. J.; Delgado-Mena, E.; Del Vecchio, C.; Demangeon, O.; De Sio,
A.; De Wit, J.; Dobrijévic, M.; Doel, P.; Dominic, C.; Dorfi, E.;
Eales, S.; Eiroa, C.; Espinoza Contreras, M.; Esposito, M.; Eymet,
V.; Fabrizio, N.; Fernández, M.; Femenía Castella, B.; Figueira,
P.; Filacchione, G.; Fletcher, L.; Focardi, M.; Fossey, S.; Fouqué,
P.; Frith, J.; Galand, M.; Gambicorti, L.; Gaulme, P.; García López,
R. J.; Garcia-Piquer, A.; Gear, W.; Gerard, J. -C.; Gesa, L.; Giani,
E.; Gianotti, F.; Gillon, M.; Giro, E.; Giuranna, M.; Gomez, H.;
Gomez-Leal, I.; Gonzalez Hernandez, J.; González Merino, B.; Graczyk,
R.; Grassi, D.; Guardia, J.; Guio, P.; Gustin, J.; Hargrave, P.; Haigh,
J.; Hébrard, E.; Heiter, U.; Heredero, R. L.; Herrero, E.; Hersant,
F.; Heyrovsky, D.; Hollis, M.; Hubert, B.; Hueso, R.; Israelian, G.;
Iro, N.; Irwin, P.; Jacquemoud, S.; Jones, G.; Jones, H.; Justtanont,
K.; Kehoe, T.; Kerschbaum, F.; Kerins, E.; Kervella, P.; Kipping, D.;
Koskinen, T.; Krupp, N.; Lahav, O.; Laken, B.; Lanza, N.; Lellouch,
E.; Leto, G.; Licandro Goldaracena, J.; Lithgow-Bertelloni, C.; Liu,
S. J.; Lo Cicero, U.; Lodieu, N.; Lognonné, P.; Lopez-Puertas,
M.; Lopez-Valverde, M. A.; Lundgaard Rasmussen, I.; Luntzer, A.;
Machado, P.; MacTavish, C.; Maggio, A.; Maillard, J. -P.; Magnes, W.;
Maldonado, J.; Mall, U.; Marquette, J. -B.; Mauskopf, P.; Massi, F.;
Maurin, A. -S.; Medvedev, A.; Michaut, C.; Miles-Paez, P.; Montalto,
M.; Montañés Rodríguez, P.; Monteiro, M.; Montes, D.; Morais, H.;
Morales, J. C.; Morales-Calderón, M.; Morello, G.; Moro Martín,
A.; Moses, J.; Moya Bedon, A.; Murgas Alcaino, F.; Oliva, E.; Orton,
G.; Palla, F.; Pancrazzi, M.; Pantin, E.; Parmentier, V.; Parviainen,
H.; Peña Ramírez, K. Y.; Peralta, J.; Perez-Hoyos, S.; Petrov, R.;
Pezzuto, S.; Pietrzak, R.; Pilat-Lohinger, E.; Piskunov, N.; Prinja,
R.; Prisinzano, L.; Polichtchouk, I.; Poretti, E.; Radioti, A.; Ramos,
A. A.; Rank-Lüftinger, T.; Read, P.; Readorn, K.; Rebolo López,
R.; Rebordão, J.; Rengel, M.; Rezac, L.; Rocchetto, M.; Rodler, F.;
Sánchez Béjar, V. J.; Sanchez Lavega, A.; Sanromá, E.; Santos,
N.; Sanz Forcada, J.; Scandariato, G.; Schmider, F. -X.; Scholz,
A.; Scuderi, S.; Sethenadh, J.; Shore, S.; Showman, A.; Sicardy, B.;
Sitek, P.; Smith, A.; Soret, L.; Sousa, S.; Stiepen, A.; Stolarski,
M.; Strazzulla, G.; Tabernero, H. M.; Tanga, P.; Tecsa, M.; Temple,
J.; Terenzi, L.; Tessenyi, M.; Testi, L.; Thompson, S.; Thrastarson,
H.; Tingley, B. W.; Trifoglio, M.; Martín Torres, J.; Tozzi, A.;
Turrini, D.; Varley, R.; Vakili, F.; de Val-Borro, M.; Valdivieso,
M. L.; Venot, O.; Villaver, E.; Vinatier, S.; Viti, S.; Waldmann,
I.; Waltham, D.; Ward-Thompson, D.; Waters, R.; Watkins, C.; Watson,
D.; Wawer, P.; Wawrzaszk, A.; White, G.; Widemann, T.; Winek, W.;
Wiśniowski, T.; Yelle, R.; Yung, Y.; Yurchenko, S. N.
Bibcode: 2015ExA....40..329T
Altcode: 2015ExA...tmp...67T; 2015arXiv150205747T
The discovery of almost two thousand exoplanets has revealed an
unexpectedly diverse planet population. We see gas giants in few-day
orbits, whole multi-planet systems within the orbit of Mercury,
and new populations of planets with masses between that of the Earth
and Neptune—all unknown in the Solar System. Observations to date
have shown that our Solar System is certainly not representative of
the general population of planets in our Milky Way. The key science
questions that urgently need addressing are therefore: What are
exoplanets made of? Why are planets as they are? How do planetary
systems work and what causes the exceptional diversity observed as
compared to the Solar System? The EChO (Exoplanet Characterisation
Observatory) space mission was conceived to take up the challenge to
explain this diversity in terms of formation, evolution, internal
structure and planet and atmospheric composition. This requires
in-depth spectroscopic knowledge of the atmospheres of a large and
well-defined planet sample for which precise physical, chemical and
dynamical information can be obtained. In order to fulfil this ambitious
scientific program, EChO was designed as a dedicated survey mission
for transit and eclipse spectroscopy capable of observing a large,
diverse and well-defined planet sample within its 4-year mission
lifetime. The transit and eclipse spectroscopy method, whereby the
signal from the star and planet are differentiated using knowledge of
the planetary ephemerides, allows us to measure atmospheric signals
from the planet at levels of at least 10-4 relative to
the star. This can only be achieved in conjunction with a carefully
designed stable payload and satellite platform. It is also necessary
to provide broad instantaneous wavelength coverage to detect as many
molecular species as possible, to probe the thermal structure of the
planetary atmospheres and to correct for the contaminating effects of
the stellar photosphere. This requires wavelength coverage of at least
0.55 to 11 μm with a goal of covering from 0.4 to 16 μm. Only modest
spectral resolving power is needed, with R ~ 300 for wavelengths less
than 5 μm and R ~ 30 for wavelengths greater than this. The transit
spectroscopy technique means that no spatial resolution is required. A
telescope collecting area of about 1 m2 is sufficiently
large to achieve the necessary spectro-photometric precision: for the
Phase A study a 1.13 m2 telescope, diffraction limited at 3
μm has been adopted. Placing the satellite at L2 provides a cold and
stable thermal environment as well as a large field of regard to allow
efficient time-critical observation of targets randomly distributed over
the sky. EChO has been conceived to achieve a single goal: exoplanet
spectroscopy. The spectral coverage and signal-to-noise to be achieved
by EChO, thanks to its high stability and dedicated design, would be
a game changer by allowing atmospheric composition to be measured
with unparalleled exactness: at least a factor 10 more precise and
a factor 10 to 1000 more accurate than current observations. This
would enable the detection of molecular abundances three orders of
magnitude lower than currently possible and a fourfold increase from
the handful of molecules detected to date. Combining these data with
estimates of planetary bulk compositions from accurate measurements
of their radii and masses would allow degeneracies associated with
planetary interior modelling to be broken, giving unique insight
into the interior structure and elemental abundances of these alien
worlds. EChO would allow scientists to study exoplanets both as a
population and as individuals. The mission can target super-Earths,
Neptune-like, and Jupiter-like planets, in the very hot to temperate
zones (planet temperatures of 300-3000 K) of F to M-type host stars. The
EChO core science would be delivered by a three-tier survey. The EChO
Chemical Census: This is a broad survey of a few-hundred exoplanets,
which allows us to explore the spectroscopic and chemical diversity of
the exoplanet population as a whole. The EChO Origin: This is a deep
survey of a subsample of tens of exoplanets for which significantly
higher signal to noise and spectral resolution spectra can be obtained
to explain the origin of the exoplanet diversity (such as formation
mechanisms, chemical processes, atmospheric escape). The EChO Rosetta
Stones: This is an ultra-high accuracy survey targeting a subsample
of select exoplanets. These will be the bright "benchmark" cases
for which a large number of measurements would be taken to explore
temporal variations, and to obtain two and three dimensional spatial
information on the atmospheric conditions through eclipse-mapping
techniques. If EChO were launched today, the exoplanets currently
observed are sufficient to provide a large and diverse sample. The
Chemical Census survey would consist of > 160 exoplanets with a range
of planetary sizes, temperatures, orbital parameters and stellar host
properties. Additionally, over the next 10 years, several new ground-
and space-based transit photometric surveys and missions will come
on-line (e.g. NGTS, CHEOPS, TESS, PLATO), which will specifically focus
on finding bright, nearby systems. The current rapid rate of discovery
would allow the target list to be further optimised in the years prior
to EChO's launch and enable the atmospheric characterisation of hundreds
of planets.
Title: Helioseismology with Solar Orbiter
Authors: Löptien, Björn; Birch, Aaron C.; Gizon, Laurent; Schou,
Jesper; Appourchaux, Thierry; Blanco Rodríguez, Julián; Cally,
Paul S.; Dominguez-Tagle, Carlos; Gandorfer, Achim; Hill, Frank;
Hirzberger, Johann; Scherrer, Philip H.; Solanki, Sami K.
Bibcode: 2015SSRv..196..251L
Altcode: 2014arXiv1406.5435L; 2014SSRv..tmp...31L
The Solar Orbiter mission, to be launched in July 2017, will
carry a suite of remote sensing and in-situ instruments, including
the Polarimetric and Helioseismic Imager (PHI). PHI will deliver
high-cadence images of the Sun in intensity and Doppler velocity
suitable for carrying out novel helioseismic studies. The orbit of
the Solar Orbiter spacecraft will reach a solar latitude of up to
21∘ (up to 34∘ by the end of the extended
mission) and thus will enable the first local helioseismology studies of
the polar regions. Here we consider an array of science objectives to be
addressed by helioseismology within the baseline telemetry allocation
(51 Gbit per orbit, current baseline) and within the science observing
windows (baseline 3×10 days per orbit). A particularly important
objective is the measurement of large-scale flows at high latitudes
(rotation and meridional flow), which are largely unknown but play an
important role in flux transport dynamos. For both helioseismology
and feature tracking methods convection is a source of noise in
the measurement of longitudinally averaged large-scale flows, which
decreases as T -1/2 where T is the total duration of the
observations. Therefore, the detection of small amplitude signals (e.g.,
meridional circulation, flows in the deep solar interior) requires long
observation times. As an example, one hundred days of observations at
lower spatial resolution would provide a noise level of about three m/s
on the meridional flow at 80∘ latitude. Longer time-series
are also needed to study temporal variations with the solar cycle. The
full range of Earth-Sun-spacecraft angles provided by the orbit will
enable helioseismology from two vantage points by combining PHI with
another instrument: stereoscopic helioseismology will allow the study
of the deep solar interior and a better understanding of the physics
of solar oscillations in both quiet Sun and sunspots. We have used a
model of the PHI instrument to study its performance for helioseismology
applications. As input we used a 6 hr time-series of realistic solar
magneto-convection simulation (Stagger code) and the SPINOR radiative
transfer code to synthesize the observables. The simulated power
spectra of solar oscillations show that the instrument is suitable for
helioseismology. In particular, the specified point spread function,
image jitter, and photon noise are no obstacle to a successful mission.
Title: Rotation, differential rotation, and gyrochronology of active
Kepler stars
Authors: Reinhold, Timo; Gizon, Laurent
Bibcode: 2015A&A...583A..65R
Altcode: 2015arXiv150707757R
Context. In addition to the discovery of hundreds of exoplanets, the
high-precision photometry from the CoRoT and Kepler satellites has
led to measurements of surface rotation periods for tens of thousands
of stars, which can potentially be used to infer stellar ages via
gyrochronology.
Aims: Our main goal is to derive ages of
thousands of field stars using consistent rotation period measurements
derived by different methods. Multiple rotation periods are interpreted
as surface differential rotation (DR). We study the dependence of DR
with rotation period and effective temperature.
Methods: We
reanalyze a previously studied sample of 24 124 Kepler stars using
different approaches based on the Lomb-Scargle periodogram. Each
quarter (Q1-Q14) is treated individually using a prewhitening
approach. Additionally, the full time series and their different
segments are analyzed.
Results: For more than 18 500 stars
our results are consistent with the rotation periods from McQuillan
et al. (2014, ApJS, 211, 24). Of these, more than 12 300 stars show
multiple significant peaks, which we interpret as DR. Dependencies
of the DR with rotation period and effective temperature could
be confirmed, e.g., the relative DR increases with rotation
period. Gyrochronology ages between 100 Myr and 10 Gyr were derived
for more than 17 000 stars using different gyrochronology relations,
most of them with uncertainties dominated by period variations. We
find a bimodal age distribution for Teff between 3200-4700
K. The derived ages reveal an empirical activity-age relation using
photometric variability as stellar activity proxy. Additionally, we
found 1079 stars with extremely stable (mostly short) periods. Half
of these periods may be associated with rotation stabilized by
non-eclipsing companions, the other half might be due to pulsations.
Conclusions: The derived gyrochronology ages are well constrained
since more than ~93.0% of the stars seem to be younger than the Sun
where calibration is most reliable. Explaining the bimodality in
the age distribution is challenging, and limits accurate stellar age
predictions. The relation between activity and age is interesting, and
requires further investigation. The existence of cool stars with almost
constant rotation period over more than three years of observation might
be explained by synchronization with stellar companions, or a dynamo
mechanism keeping the spot configurations extremely stable. Full
Tables 2 and 4 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/583/A65
Title: A seismic and gravitationally bound double star observed by
Kepler. Implication for the presence of a convective core
Authors: Appourchaux, T.; Antia, H. M.; Ball, W.; Creevey, O.;
Lebreton, Y.; Verma, K.; Vorontsov, S.; Campante, T. L.; Davies,
G. R.; Gaulme, P.; Régulo, C.; Horch, E.; Howell, S.; Everett, M.;
Ciardi, D.; Fossati, L.; Miglio, A.; Montalbán, J.; Chaplin, W. J.;
García, R. A.; Gizon, L.
Bibcode: 2015A&A...582A..25A
Altcode:
Context. Solar-like oscillations have been observed by Kepler and CoRoT
in many solar-type stars, thereby providing a way to probe stars using
asteroseismology.
Aims: The derivation of stellar parameters
has usually been done with single stars. The aim of the paper is to
derive the stellar parameters of a double-star system (HIP 93511),
for which an interferometric orbit has been observed along with
asteroseismic measurements.
Methods: We used a time series
of nearly two years of data for the double star to detect the two
oscillation-mode envelopes that appear in the power spectrum. Using
a new scaling relation based on luminosity, we derived the radius and
mass of each star. We derived the age of each star using two proxies:
one based upon the large frequency separation and a new one based
upon the small frequency separation. Using stellar modelling, the mode
frequencies allowed us to derive the radius, the mass, and the age of
each component. In addition, speckle interferometry performed since 2006
has enabled us to recover the orbit of the system and the total mass
of the system.
Results: From the determination of the orbit,
the total mass of the system is 2.34-0.33+0.45
M⊙. The total seismic mass using scaling relations
is 2.47 ± 0.07 M⊙. The seismic age derived using the
new proxy based upon the small frequency separation is 3.5 ± 0.3
Gyr. Based on stellar modelling, the mean common age of the system is
2.7-3.9 Gyr. The mean total seismic mass of the system is 2.34-2.53
M⊙ consistent with what we determined independently
with the orbit. The stellar models provide the mean radius, mass,
and age of the stars as RA = 1.82-1.87R⊙,
MA = 1.25-1.39 M⊙, AgeA = 2.6-3.5
Gyr; RB = 1.22-1.25 R⊙, MB =
1.08-1.14 M⊙, AgeB = 3.35-4.21 Gyr. The
models provide two sets of values for Star A: [1.25-1.27]
M⊙ and [1.34-1.39] M⊙. We detect a
convective core in Star A, while Star B does not have any. For the
metallicity of the binary system of Z ≈ 0.02, we set the limit
between stars having a convective core in the range [1.14-1.25]
M⊙. Appendices are available in electronic form at http://www.aanda.org
Title: Constraining differential rotation of Sun-like stars from
asteroseismic and starspot rotation periods
Authors: Nielsen, M. B.; Schunker, H.; Gizon, L.; Ball, W. H.
Bibcode: 2015A&A...582A..10N
Altcode: 2015arXiv150802164N
In previous work, we identified six Sun-like stars observed by Kepler
with exceptionally clear asteroseismic signatures of rotation. Here,
we show that five of these stars exhibit surface variability suitable
for measuring rotation. We compare the rotation periods obtained from
light-curve variability with those from asteroseismology in order to
further constrain differential rotation. The two rotation measurement
methods are found to agree within uncertainties, suggesting that radial
differential rotation is weak, as is the case for the Sun. Furthermore,
we find significant discrepancies between ages from asteroseismology
and from three different gyrochronology relations, implying that
stellar age estimation is problematic even for Sun-like stars.
Title: Asteroseismology of Solar-Type Stars with K2: Detection of
Oscillations in C1 Data
Authors: Chaplin, W. J.; Lund, M. N.; Handberg, R.; Basu, S.;
Buchhave, L. A.; Campante, T. L.; Davies, G. R.; Huber, D.; Latham,
D. W.; Latham, C. A.; Serenelli, A.; Antia, H. M.; Appourchaux, T.;
Ball, W. H.; Benomar, O.; Casagrande, L.; Christensen-Dalsgaard, J.;
Coelho, H. R.; Creevey, O. L.; Elsworth, Y.; García, R. A.; Gaulme,
P.; Hekker, S.; Kallinger, T.; Karoff, C.; Kawaler, S. D.; Kjeldsen,
H.; Lundkvist, M. S.; Marcadon, F.; Mathur, S.; Miglio, A.; Mosser,
B.; Régulo, C.; Roxburgh, I. W.; Silva Aguirre, V.; Stello, D.;
Verma, K.; White, T. R.; Bedding, T. R.; Barclay, T.; Buzasi, D. L.;
Dehuevels, S.; Gizon, L.; Houdek, G.; Howell, S. B.; Salabert, D.;
Soderblom, D. R.
Bibcode: 2015PASP..127.1038C
Altcode: 2015arXiv150701827C
We present the first detections by the NASA K2 Mission of oscillations
in solar-type stars, using short-cadence data collected during
K2 Campaign\,1 (C1). We understand the asteroseismic detection
thresholds for C1-like levels of photometric performance, and we
can detect oscillations in subgiants having dominant oscillation
frequencies around $1000\,\rm \mu Hz$. Changes to the operation of the
fine-guidance sensors are expected to give significant improvements
in the high-frequency performance from C3 onwards. A reduction in the
excess high-frequency noise by a factor of two-and-a-half in amplitude
would bring main-sequence stars with dominant oscillation frequencies as
high as ${\simeq 2500}\,\rm \mu Hz$ into play as potential asteroseismic
targets for K2.
Title: Spatially resolved vertical vorticity in solar supergranulation
using helioseismology and local correlation tracking
Authors: Langfellner, J.; Gizon, L.; Birch, A. C.
Bibcode: 2015A&A...581A..67L
Altcode: 2015arXiv150400223L
Flow vorticity is a fundamental property of turbulent convection in
rotating systems. Solar supergranules exhibit a preferred sense of
rotation, which depends on the hemisphere. This is due to the Coriolis
force acting on the diverging horizontal flows. We aim to spatially
resolve the vertical flow vorticity of the average supergranule at
different latitudes, both for outflow and inflow regions. To measure the
vertical vorticity, we use two independent techniques: time-distance
helioseismology (TD) and local correlation tracking of granules in
intensity images (LCT) using data from the Helioseismic and Magnetic
Imager (HMI) on board the Solar Dynamics Observatory (SDO). Both maps
are corrected for center-to-limb systematic errors. We find that 8 h
TD and LCT maps of vertical vorticity are highly correlated at large
spatial scales. Associated with the average supergranule outflow, we
find tangential (vortical) flows that reach about 10 m s-1
in the clockwise direction at 40° latitude. In average inflow
regions, the tangential flow reaches the same magnitude, but in the
anticlockwise direction. These tangential velocities are much smaller
than the radial (diverging) flow component (300 m s-1 for the
average outflow and 200 m s-1 for the average inflow). The
results for TD and LCT as measured from HMI are in excellent agreement
for latitudes between -60° and 60°. From HMI LCT, we measure the
vorticity peak of the average supergranule to have a full width at
half maximum of about 13 Mm for outflows and 8 Mm for inflows. This
is larger than the spatial resolution of the LCT measurements
(about 3 Mm). On the other hand, the vorticity peak in outflows is
about half the value measured at inflows (e.g., 4 × 10-6
s-1 clockwise compared to 8 × 10-6 s-1
anticlockwise at 40° latitude). Results from the Michelson Doppler
Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO)
obtained in 2010 are biased compared to the HMI/SDO results for
the same period. Appendices are available in electronic form at http://www.aanda.orgThe
azimuthally averaged velocity components vr and
vt for supergranular outflows and inflows at various
latitudes are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/581/A67
Title: VizieR Online Data Catalog: Solar supergranular velocity
profiles (Langfellner+, 2015)
Authors: Langfellner, J.; Gizon, L.; Birch, A. C.
Bibcode: 2015yCat..35810067L
Altcode:
These tables give the azimuthally averaged horizontal velocity
components vr and vt for the average supergranular outflows and inflows
at various latitudes, as displayed for 40° latitude in Fig. 13 in
the paper. The velocities have been computed by the method of local
correlation tracking using intensity images from the Helioseismic and
Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO)
spacecraft. The structures of all data files are identical. The error
estimates were computed by splitting the whole observation period
(112 days) in eight parts. (14 data files).
Title: Anisotropy of the solar network magnetic field around the
average supergranule
Authors: Langfellner, J.; Gizon, L.; Birch, A. C.
Bibcode: 2015A&A...579L...7L
Altcode: 2015arXiv150501427L
Supergranules in the quiet Sun are outlined by a web-like structure of
enhanced magnetic field strength, the so-called magnetic network. We
aim to map the magnetic network field around the average supergranule
near disk center. We use observations of the line-of-sight component
of the magnetic field from the Helioseismic and Magnetic Imager (HMI)
onboard the Solar Dynamics Observatory (SDO). The average supergranule
is constructed by coaligning and averaging over 3000 individual
supergranules. We determine the positions of the supergranules with an
image segmentation algorithm that we apply to maps of the horizontal
flow divergence measured using time-distance helioseismology. In the
center of the average supergranule, the magnetic (intranetwork) field
is weaker by about 2.2 Gauss than the background value (3.5 Gauss),
whereas it is enhanced in the surrounding ring of horizontal inflows
(by about 0.6 Gauss on average). We find that this network field is
significantly stronger west (prograde) of the average supergranule than
in the east (by about 0.3 Gauss). With time-distance helioseismology, we
find a similar anisotropy. The observed anisotropy of the magnetic field
adds to the mysterious dynamical properties of solar supergranulation.
Title: Simulating acoustic waves in spotted stars
Authors: Papini, Emanuele; Birch, Aaron C.; Gizon, Laurent; Hanasoge,
Shravan M.
Bibcode: 2015A&A...577A.145P
Altcode: 2015arXiv150306032P
Acoustic modes of oscillation are affected by stellar activity,
however it is unclear how starspots contribute to these changes. Here
we investigate the nonmagnetic effects of starspots on global modes
with angular degree ℓ ≤ 2 in highly active stars, and characterize
the spot seismic signature on synthetic light curves. We perform
3D time-domain simulations of linear acoustic waves to study their
interaction with a model starspot. We model the spot as a 3D change
in the sound speed stratification with respect to a convectively
stable stellar background, built from solar Model S. We perform a
parametric study by considering different depths and perturbation
amplitudes. Exact numerical simulations allow the investigation of the
wavefield-spot interaction beyond first order perturbation theory. The
interaction of the axisymmetric modes with the starspot is strongly
nonlinear. As mode frequency increases, the frequency shifts for radial
modes exceed the value predicted by linear theory, while the shifts for
the ℓ = 2,m = 0 modes are smaller than predicted by linear theory,
with avoided-crossing-like patterns forming between the m = 0 and m =
1 mode frequencies. The nonlinear behavior increases with increasing
spot amplitude and/or decreasing depth. Linear theory still reproduces
the correct shifts for nonaxisymmetric modes. In the nonlinear regime
the mode eigenfunctions are not pure spherical harmonics, but rather
a mixture of different spherical harmonics. This mode mixing, together
with the frequency changes, may lead to misidentification of the modes
in the observed acoustic power spectra.
Title: Magnetic Flux Transport at the Solar Surface
Authors: Jiang, J.; Hathaway, D. H.; Cameron, R. H.; Solanki, S. K.;
Gizon, L.; Upton, L.
Bibcode: 2015sac..book..491J
Altcode:
No abstract at ADS
Title: Magnetic Flux Transport at the Solar Surface
Authors: Jiang, J.; Hathaway, D. H.; Cameron, R. H.; Solanki, S. K.;
Gizon, L.; Upton, L.
Bibcode: 2014SSRv..186..491J
Altcode: 2014SSRv..tmp...43J; 2014arXiv1408.3186J
After emerging to the solar surface, the Sun's magnetic field displays a
complex and intricate evolution. The evolution of the surface field is
important for several reasons. One is that the surface field, and its
dynamics, sets the boundary condition for the coronal and heliospheric
magnetic fields. Another is that the surface evolution gives us insight
into the dynamo process. In particular, it plays an essential role
in the Babcock-Leighton model of the solar dynamo. Describing this
evolution is the aim of the surface flux transport model. The model
starts from the emergence of magnetic bipoles. Thereafter, the model is
based on the induction equation and the fact that after emergence the
magnetic field is observed to evolve as if it were purely radial. The
induction equation then describes how the surface flows—differential
rotation, meridional circulation, granular, supergranular flows,
and active region inflows—determine the evolution of the field (now
taken to be purely radial). In this paper, we review the modeling of
the various processes that determine the evolution of the surface
field. We restrict our attention to their role in the surface flux
transport model. We also discuss the success of the model and some of
the results that have been obtained using this model.
Title: Image compression in local helioseismology
Authors: Löptien, B.; Birch, A. C.; Gizon, L.; Schou, J.
Bibcode: 2014A&A...571A..42L
Altcode: 2014arXiv1409.4176L
Context. Several upcoming helioseismology space missions are
very limited in telemetry and will have to perform extensive data
compression. This requires the development of new methods of data
compression.
Aims: We give an overview of the influence of lossy
data compression on local helioseismology. We investigate the effects
of several lossy compression methods (quantization, JPEG compression,
and smoothing and subsampling) on power spectra and time-distance
measurements of supergranulation flows at disk center.
Methods:
We applied different compression methods to tracked and remapped
Dopplergrams obtained by the Helioseismic and Magnetic Imager onboard
the Solar Dynamics Observatory. We determined the signal-to-noise
ratio of the travel times computed from the compressed data as a
function of the compression efficiency.
Results: The basic
helioseismic measurements that we consider are very robust to lossy
data compression. Even if only the sign of the velocity is used,
time-distance helioseismology is still possible. We achieve the
best results by applying JPEG compression on spatially subsampled
data. However, our conclusions are only valid for supergranulation
flows at disk center and may not be valid for all helioseismology
applications.
Title: The PLATO 2.0 mission
Authors: Rauer, H.; Catala, C.; Aerts, C.; Appourchaux, T.; Benz,
W.; Brandeker, A.; Christensen-Dalsgaard, J.; Deleuil, M.; Gizon,
L.; Goupil, M. -J.; Güdel, M.; Janot-Pacheco, E.; Mas-Hesse,
M.; Pagano, I.; Piotto, G.; Pollacco, D.; Santos, Ċ.; Smith, A.;
Suárez, J. -C.; Szabó, R.; Udry, S.; Adibekyan, V.; Alibert, Y.;
Almenara, J. -M.; Amaro-Seoane, P.; Eiff, M. Ammler-von; Asplund, M.;
Antonello, E.; Barnes, S.; Baudin, F.; Belkacem, K.; Bergemann, M.;
Bihain, G.; Birch, A. C.; Bonfils, X.; Boisse, I.; Bonomo, A. S.;
Borsa, F.; Brandão, I. M.; Brocato, E.; Brun, S.; Burleigh, M.;
Burston, R.; Cabrera, J.; Cassisi, S.; Chaplin, W.; Charpinet, S.;
Chiappini, C.; Church, R. P.; Csizmadia, Sz.; Cunha, M.; Damasso, M.;
Davies, M. B.; Deeg, H. J.; Díaz, R. F.; Dreizler, S.; Dreyer, C.;
Eggenberger, P.; Ehrenreich, D.; Eigmüller, P.; Erikson, A.; Farmer,
R.; Feltzing, S.; de Oliveira Fialho, F.; Figueira, P.; Forveille,
T.; Fridlund, M.; García, R. A.; Giommi, P.; Giuffrida, G.; Godolt,
M.; Gomes da Silva, J.; Granzer, T.; Grenfell, J. L.; Grotsch-Noels,
A.; Günther, E.; Haswell, C. A.; Hatzes, A. P.; Hébrard, G.; Hekker,
S.; Helled, R.; Heng, K.; Jenkins, J. M.; Johansen, A.; Khodachenko,
M. L.; Kislyakova, K. G.; Kley, W.; Kolb, U.; Krivova, N.; Kupka, F.;
Lammer, H.; Lanza, A. F.; Lebreton, Y.; Magrin, D.; Marcos-Arenal,
P.; Marrese, P. M.; Marques, J. P.; Martins, J.; Mathis, S.; Mathur,
S.; Messina, S.; Miglio, A.; Montalban, J.; Montalto, M.; Monteiro,
M. J. P. F. G.; Moradi, H.; Moravveji, E.; Mordasini, C.; Morel, T.;
Mortier, A.; Nascimbeni, V.; Nelson, R. P.; Nielsen, M. B.; Noack,
L.; Norton, A. J.; Ofir, A.; Oshagh, M.; Ouazzani, R. -M.; Pápics,
P.; Parro, V. C.; Petit, P.; Plez, B.; Poretti, E.; Quirrenbach, A.;
Ragazzoni, R.; Raimondo, G.; Rainer, M.; Reese, D. R.; Redmer, R.;
Reffert, S.; Rojas-Ayala, B.; Roxburgh, I. W.; Salmon, S.; Santerne,
A.; Schneider, J.; Schou, J.; Schuh, S.; Schunker, H.; Silva-Valio,
A.; Silvotti, R.; Skillen, I.; Snellen, I.; Sohl, F.; Sousa, S. G.;
Sozzetti, A.; Stello, D.; Strassmeier, K. G.; Švanda, M.; Szabó,
Gy. M.; Tkachenko, A.; Valencia, D.; Van Grootel, V.; Vauclair,
S. D.; Ventura, P.; Wagner, F. W.; Walton, N. A.; Weingrill, J.;
Werner, S. C.; Wheatley, P. J.; Zwintz, K.
Bibcode: 2014ExA....38..249R
Altcode: 2014ExA...tmp...41R; 2013arXiv1310.0696R
PLATO 2.0 has recently been selected for ESA's M3 launch opportunity
(2022/24). Providing accurate key planet parameters (radius, mass,
density and age) in statistical numbers, it addresses fundamental
questions such as: How do planetary systems form and evolve? Are there
other systems with planets like ours, including potentially habitable
planets? The PLATO 2.0 instrument consists of 34 small aperture
telescopes (32 with 25 s readout cadence and 2 with 2.5 s candence)
providing a wide field-of-view (2232 deg 2) and a large
photometric magnitude range (4-16 mag). It focusses on bright (4-11
mag) stars in wide fields to detect and characterize planets down to
Earth-size by photometric transits, whose masses can then be determined
by ground-based radial-velocity follow-up measurements. Asteroseismology
will be performed for these bright stars to obtain highly accurate
stellar parameters, including masses and ages. The combination of
bright targets and asteroseismology results in high accuracy for
the bulk planet parameters: 2 %, 4-10 % and 10 % for planet radii,
masses and ages, respectively. The planned baseline observing strategy
includes two long pointings (2-3 years) to detect and bulk characterize
planets reaching into the habitable zone (HZ) of solar-like stars
and an additional step-and-stare phase to cover in total about 50 %
of the sky. PLATO 2.0 will observe up to 1,000,000 stars and detect
and characterize hundreds of small planets, and thousands of planets
in the Neptune to gas giant regime out to the HZ. It will therefore
provide the first large-scale catalogue of bulk characterized planets
with accurate radii, masses, mean densities and ages. This catalogue
will include terrestrial planets at intermediate orbital distances,
where surface temperatures are moderate. Coverage of this parameter
range with statistical numbers of bulk characterized planets is unique
to PLATO 2.0. The PLATO 2.0 catalogue allows us to e.g.: - complete
our knowledge of planet diversity for low-mass objects, - correlate the
planet mean density-orbital distance distribution with predictions from
planet formation theories,- constrain the influence of planet migration
and scattering on the architecture of multiple systems, and - specify
how planet and system parameters change with host star characteristics,
such as type, metallicity and age. The catalogue will allow us to study
planets and planetary systems at different evolutionary phases. It
will further provide a census for small, low-mass planets. This will
serve to identify objects which retained their primordial hydrogen
atmosphere and in general the typical characteristics of planets
in such low-mass, low-density range. Planets detected by PLATO 2.0
will orbit bright stars and many of them will be targets for future
atmosphere spectroscopy exploring their atmosphere. Furthermore,
the mission has the potential to detect exomoons, planetary rings,
binary and Trojan planets. The planetary science possible with PLATO
2.0 is complemented by its impact on stellar and galactic science via
asteroseismology as well as light curves of all kinds of variable stars,
together with observations of stellar clusters of different ages. This
will allow us to improve stellar models and study stellar activity. A
large number of well-known ages from red giant stars will probe the
structure and evolution of our Galaxy. Asteroseismic ages of bright
stars for different phases of stellar evolution allow calibrating
stellar age-rotation relationships. Together with the results of ESA's
Gaia mission, the results of PLATO 2.0 will provide a huge legacy to
planetary, stellar and galactic science.
Title: Time-distance helioseismology: A new averaging scheme for
measuring flow vorticity
Authors: Langfellner, J.; Gizon, L.; Birch, A. C.
Bibcode: 2014A&A...570A..90L
Altcode: 2014arXiv1408.4669L
Context. Time-distance helioseismology provides information about
vector flows in the near-surface layers of the Sun by measuring wave
travel times between points on the solar surface. Specific spatial
averages of travel times have been proposed for distinguishing between
flows in the east-west and north-south directions and measuring the
horizontal divergence of the flows. No specific measurement technique
has, however, been developed to measure flow vorticity.
Aims:
Here we propose a new measurement technique tailored to measuring
the vertical component of vorticity. Fluid vorticity is a fundamental
property of solar convection zone dynamics and of rotating turbulent
convection in particular.
Methods: The method consists of
measuring the travel time of waves along a closed contour on the
solar surface in order to approximate the circulation of the flow
along this contour. Vertical vorticity is related to the difference
between clockwise and anti-clockwise travel times.
Results:
We applied the method to characterize the vortical motions of solar
convection using helioseismic data from the Helioseismic and Magnetic
Imager onboard the Solar Dynamics Observatory (SDO/HMI) and from the
Michelson Doppler Imager onboard the Solar and Heliospheric Observatory
(SOHO/MDI). Away from the equator, a clear correlation between vertical
vorticity and horizontal divergence is detected. Horizontal outflows
are associated with negative vorticity in the northern hemisphere
and positive vorticity in the southern hemisphere. The signal is much
stronger for HMI than for MDI observations. We characterize the spatial
power spectrum of the signal by comparison with a noise model. Vertical
vorticity at horizontal wavenumbers below 250 /R⊙ can be
probed with this helioseismic technique.
Title: The radial gradient of the near-surface shear layer of the Sun
Authors: Barekat, A.; Schou, J.; Gizon, L.
Bibcode: 2014A&A...570L..12B
Altcode: 2014arXiv1410.3162B
Context. Helioseismology has provided unprecedented information about
the internal rotation of the Sun. One of the important achievements was
the discovery of two radial shear layers: one near the bottom of the
convection zone (the tachocline) and one near the surface. These shear
layers may be important ingredients for explaining the magnetic cycle of
the Sun.
Aims: We measure the logarithmic radial gradient of the
rotation rate (dlnΩ/dlnr) near the surface of the Sun using 15 years of
f mode rotational frequency splittings from the Michelson Doppler Imager
(MDI) and four years of data from the Helioseismic and Magnetic Imager
(HMI).
Methods: We model the angular velocity of the Sun in the
upper ~10 Mm as changing linearly with depth and use a multiplicative
optimally localized averaging inversion to infer the gradient of the
rotation rate as a function of latitude.
Results: Both the MDI
and HMI data show that dlnΩ/dlnr is close to -1 from the equator
to 60° latitude and stays negative up to 75° latitude. However,
the value of the gradient is different for MDI and HMI for latitudes
above 60°. Additionally, there is a significant difference between
the value of dlnΩ/dlnr using an older and recently reprocessed MDI
data for latitudes above 30°.
Conclusions: We could reliably
infer the value of dlnΩ/dlnr up to 60°, but not above this latitude,
which will hopefully constrain theories of the near-surface shear
layer and dynamo. Furthermore, the recently reprocessed MDI splitting
data are more reliable than the older versions which contained clear
systematic errors in the high degree f modes.
Title: Interpreting the Helioseismic and Magnetic Imager (HMI)
Multi-Height Velocity Measurements
Authors: Nagashima, Kaori; Löptien, Björn; Gizon, Laurent; Birch,
Aaron C.; Cameron, Robert; Couvidat, Sebastien; Danilovic, Sanja;
Fleck, Bernhard; Stein, Robert
Bibcode: 2014SoPh..289.3457N
Altcode: 2014arXiv1404.3569N; 2014SoPh..tmp...84N
The Solar Dynamics Observatory/Helioseismic and Magnetic Imager
(SDO/HMI) filtergrams, taken at six wavelengths around the Fe I 6173.3
Å line, contain information about the line-of-sight velocity over
a range of heights in the solar atmosphere. Multi-height velocity
inferences from these observations can be exploited to study wave
motions and energy transport in the atmosphere. Using realistic
convection-simulation datasets provided by the STAGGER and MURaM
codes, we generate synthetic filtergrams and explore several methods
for estimating Dopplergrams. We investigate at which height each
synthetic Dopplergram correlates most strongly with the vertical
velocity in the model atmospheres. On the basis of the investigation,
we propose two Dopplergrams other than the standard HMI-algorithm
Dopplergram produced from HMI filtergrams: a line-center Dopplergram
and an average-wing Dopplergram. These two Dopplergrams correlate most
strongly with vertical velocities at the heights of 30 - 40 km above
(line center) and 30 - 40 km below (average wing) the effective height
of the HMI-algorithm Dopplergram. Therefore, we can obtain velocity
information from two layers separated by about a half of a scale height
in the atmosphere, at best. The phase shifts between these multi-height
Dopplergrams from observational data as well as those from the simulated
data are also consistent with the height-difference estimates in the
frequency range above the photospheric acoustic-cutoff frequency.
Title: A new correction of stellar oscillation frequencies for
near-surface effects (Corrigendum)
Authors: Ball, Warrick H.; Gizon, L.
Bibcode: 2014A&A...569C...2B
Altcode:
No abstract at ADS
Title: Rotational splitting as a function of mode frequency for six
Sun-like stars
Authors: Nielsen, M. B.; Gizon, L.; Schunker, H.; Schou, J.
Bibcode: 2014A&A...568L..12N
Altcode: 2014arXiv1408.4307N
Asteroseismology offers the prospect of constraining differential
rotation in Sun-like stars. Here we have identified six high
signal-to-noise main-sequence Sun-like stars in the Kepler field,
which all have visible signs of rotational splitting of their p-mode
frequencies. For each star, we extract the rotational frequency
splitting and inclination angle from separate mode sets (adjacent modes
with l = 2, 0, and 1) spanning the p-mode envelope. We use a Markov
chain Monte Carlo method to obtain the best fit and errors associated
with each parameter. We are able to make independent measurements of
rotational splittings of ~8 radial orders for each star. For all six
stars, the measured splittings are consistent with uniform rotation,
allowing us to exclude large radial differential rotation. This work
opens the possibility of constraining internal rotation of Sun-like
stars. Table 2 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/568/L12
Title: A new correction of stellar oscillation frequencies for
near-surface effects
Authors: Ball, W. H.; Gizon, L.
Bibcode: 2014A&A...568A.123B
Altcode: 2014arXiv1408.0986B
Context. Space-based observations of solar-like oscillations present
an opportunity to constrain stellar models using individual mode
frequencies. However, current stellar models are inaccurate near
the surface, which introduces a systematic difference that must be
corrected.
Aims: We introduce and evaluate two parametrizations
of the surface corrections based on formulae given by Gough (1990,
LNP, 367, 283). The first we call a cubic term proportional to
ν3/ ℐ and the second has an additional inverse term
proportional to ν-1/ ℐ, where ν and ℐ are the frequency
and inertia of an oscillation mode.
Methods: We first show that
these formulae accurately correct model frequencies of two different
solar models (Model S and a calibrated MESA model) when compared to
observed BiSON frequencies. In particular, even the cubic form alone
fits significantly better than a power law. We then incorporate the
parametrizations into a modelling pipeline that simultaneously fits the
surface effects and the underlying stellar model parameters. We apply
this pipeline to synthetic observations of a Sun-like stellar model,
solar observations degraded to typical asteroseismic uncertainties, and
observations of the well-studied CoRoT target HD 52265. For comparison,
we also run the pipeline with the scaled power-law correction proposed
by Kjeldsen et al. (2008, ApJ, 683, L175).
Results: The fits
to synthetic and degraded solar data show that the method is unbiased
and produces best-fit parameters that are consistent with the input
models and known parameters of the Sun. Our results for HD 52265
are consistent with previous modelling efforts and the magnitude of
the surface correction is similar to that of the Sun. The fit using a
scaled power-law correction is significantly worse but yields consistent
parameters, suggesting that HD 52265 is sufficiently Sun-like for the
same power-law to be applicable.
Conclusions: We find that the
cubic term alone is suitable for asteroseismic applications and it is
easy to implement in an existing pipeline. It reproduces the frequency
dependence of the surface correction better than a power-law fit,
both when comparing calibrated solar models to BiSON observations and
when fitting stellar models using the individual frequencies. This
parametrization is thus a useful new way to correct model frequencies
so that observations of individual mode frequencies can be exploited.
Title: VizieR Online Data Catalog: Rotational frequency splitting
in Sun-like stars (Nielsen+, 2014)
Authors: Nielsen, M. B.; Gizon, L.; Schunker, H.; Schou, J.
Bibcode: 2014yCat..35689012N
Altcode:
We used short-cadence (~58s) white light observations from the NASA
Kepler mission from March 2009 until the end of the mission in early
2013. The data were obtained from the Mikulski Archive for Space
Telescopes. We fit the power spectrum with a model consisting
of a constant noise level, two frequency-dependent Harvey-like noise
terms (see Eq. (1) in Aigrain et al. 2004A&A...414.1139A), in
addition to the individual oscillation modes. We model these as a
sum of Lorentzian profiles as per Eq. (10) in Handberg & Campante
(2011A&A...527A..56H), each consisting of mode power, frequency,
and linewidth. (2 data files).
Title: Generalization of the noise model for time-distance
helioseismology
Authors: Fournier, D.; Gizon, L.; Hohage, T.; Birch, A. C.
Bibcode: 2014A&A...567A.137F
Altcode: 2014arXiv1406.5335F
Context. In time-distance helioseismology, information about the
solar interior is encoded in measurements of travel times between
pairs of points on the solar surface. Travel times are deduced from
the cross-covariance of the random wave field. Here, we consider
travel times and also products of travel times as observables. They
contain information about the statistical properties of convection
in the Sun.
Aims: We derive analytic formulae for the noise
covariance matrix of travel times and products of travel times.
Methods: The basic assumption of the model is that noise is the result
of the stochastic excitation of solar waves, a random process that
is stationary and Gaussian. We generalize the existing noise model
by dropping the assumption of horizontal spatial homogeneity. Using
a recurrence relation, we calculate the noise covariance matrices
for the moments of order 4, 6, and 8 of the observed wave field,
for the moments of order 2, 3 and 4 of the cross-covariance, and for
the moments of order 2, 3 and 4 of the travel times.
Results:
All noise covariance matrices depend only on the expectation value
of the cross-covariance of the observed wave field. For products of
travel times, the noise covariance matrix consists of three terms
proportional to 1 /T, 1 /T2, and 1 /T3, where T
is the duration of the observations. For typical observation times of
a few hours, the term proportional to 1 /T2 dominates and
Cov [ τ1τ2,τ3τ4
] ≈ Cov [ τ1,τ3 ] Cov [
τ2,τ4 ] + Cov [ τ1,τ4
] Cov [ τ2,τ3 ], where the τi are
arbitrary travel times. This result is confirmed for p1
travel times by Monte Carlo simulations and comparisons with
SDO/HMI observations.
Conclusions: General and accurate
formulae have been derived to model the noise covariance matrix
of helioseismic travel times and products of travel times. These
results could easily be generalized to other methods of local
helioseismology, such as helioseismic holography and ring diagram
analysis. Appendices are available in electronic form at http://www.aanda.org
Title: Structure and evolution of solar supergranulation using
SDO/HMI data
Authors: Roudier, Th.; Švanda, M.; Rieutord, , M.; Malherbe, J. M.;
Burston, R.; Gizon, L.
Bibcode: 2014A&A...567A.138R
Altcode: 2014arXiv1407.0196R
Context. Studying the motions on the solar surface is fundamental
for understanding how turbulent convection transports energy and how
magnetic fields are distributed across the solar surface.
Aims:
From horizontal velocity measurements all over the visible disc of the
Sun and using data from the Solar Dynamics Observatory/Helioseismic and
Magnetic Imager (SDO/HMI), we investigate the structure and evolution
of solar supergranulation.
Methods: Horizontal velocity fields
were measured by following the proper motions of solar granules
using a newly developed version of the coherent structure tracking
code. With this tool, maps of horizontal divergence were computed. We
then segmented and identified supergranular cells and followed their
histories by using spatio-temporal labelling. With this data set we
derived the fundamental properties of supergranulation, including their
motion.
Results: We find values of the fundamental parameters
of supergranulation similar to previous studies: a mean lifetime of
1.5 days and a mean diameter of 25 Mm. The tracking of individual
supergranular cells reveals the solar differential rotation and a
poleward circulation trend of the meridional flow. The shape of the
derived differential rotation and meridional flow does not depend on
the cell size. If there is a background magnetic field, the diverging
flows in supergranules are weaker.
Conclusions: This study
confirms that supergranules are suitable tracers that may be used to
investigate the large-scale flows of the solar convection as long as
they are detectable enough on the surface.
Title: Propagating Linear Waves in Convectively Unstable Stellar
Models: A Perturbative Approach
Authors: Papini, E.; Gizon, L.; Birch, A. C.
Bibcode: 2014SoPh..289.1919P
Altcode: 2013arXiv1312.4183P
Linear time-domain simulations of acoustic oscillations are unstable
in the stellar convection zone. To overcome this problem it is
customary to compute the oscillations of a stabilized background
stellar model. The stabilization affects the result, however. Here we
propose to use a perturbative approach (running the simulation twice)
to approximately recover the acoustic wave field while preserving
seismic reciprocity. To test the method we considered a 1D standard
solar model. We found that the mode frequencies of the (unstable)
standard solar model are well approximated by the perturbative approach
within 1 μHz for low-degree modes with frequencies near 3 mHz. We
also show that the perturbative approach is appropriate for correcting
rotational-frequency kernels. Finally, we comment that the method can
be generalized to wave propagation in 3D magnetized stellar interiors
because the magnetic fields have stabilizing effects on convection.
Title: Seismic analysis of HD 43587Aa, a solar-like oscillator in
a multiple system
Authors: Boumier, P.; Benomar, O.; Baudin, F.; Verner, G.; Appourchaux,
T.; Lebreton, Y.; Gaulme, P.; Chaplin, W.; García, R. A.; Hekker,
S.; Regulo, C.; Salabert, D.; Stahn, T.; Elsworth, Y.; Gizon, L.;
Hall, M.; Mathur, S.; Michel, E.; Morel, T.; Mosser, B.; Poretti,
E.; Rainer, M.; Roxburgh, I.; do Nascimento, J. -D., Jr.; Samadi,
R.; Auvergne, M.; Chaintreuil, S.; Baglin, A.; Catala, C.
Bibcode: 2014A&A...564A..34B
Altcode: 2014arXiv1402.5053B
Context. The object HD 43587Aa is a G0V star observed during the
145-day LRa03 run of the COnvection, ROtation and planetary Transits
space mission (CoRoT), for which complementary High Accuracy Radial
velocity Planet Searcher (HARPS) spectra with S/N > 300 were also
obtained. Its visual magnitude is 5.71, and its effective temperature is
close to 5950 K. It has a known companion in a highly eccentric orbit
and is also coupled with two more distant companions.
Aims:
We undertake a preliminary investigation of the internal structure
of HD 43587Aa.
Methods: We carried out a seismic analysis of
the star, using maximum likelihood estimators and Markov chain Monte
Carlo methods.
Results: We established the first table of the
eigenmode frequencies, widths, and heights for HD 43587Aa. The star
appears to have a mass and a radius slightly larger than the Sun, and is
slightly older (5.6 Gyr). Two scenarios are suggested for the geometry
of the star: either its inclination angle is very low, or the rotation
velocity of the star is very low.
Conclusions: A more detailed
study of the rotation and of the magnetic and chromospheric activity for
this star is needed, and will be the subject of a further study. New
high resolution spectrometric observations should be performed for at
least several months in duration.
Title: Seismic constraints on the radial dependence of the internal
rotation profiles of six Kepler subgiants and young red giants
Authors: Deheuvels, S.; Doğan, G.; Goupil, M. J.; Appourchaux, T.;
Benomar, O.; Bruntt, H.; Campante, T. L.; Casagrande, L.; Ceillier,
T.; Davies, G. R.; De Cat, P.; Fu, J. N.; García, R. A.; Lobel,
A.; Mosser, B.; Reese, D. R.; Regulo, C.; Schou, J.; Stahn, T.;
Thygesen, A. O.; Yang, X. H.; Chaplin, W. J.; Christensen-Dalsgaard,
J.; Eggenberger, P.; Gizon, L.; Mathis, S.; Molenda-Żakowicz, J.;
Pinsonneault, M.
Bibcode: 2014A&A...564A..27D
Altcode: 2014arXiv1401.3096D
Context. We still do not understand which physical mechanisms are
responsible for the transport of angular momentum inside stars. The
recent detection of mixed modes that contain the clear signature of
rotation in the spectra of Kepler subgiants and red giants gives us
the opportunity to make progress on this question.
Aims: Our
aim is to probe the radial dependence of the rotation profiles for a
sample of Kepler targets. For this purpose, subgiants and early red
giants are particularly interesting targets because their rotational
splittings are more sensitive to the rotation outside the deeper core
than is the case for their more evolved counterparts.
Methods:
We first extracted the rotational splittings and frequencies of the
modes for six young Kepler red giants. We then performed a seismic
modeling of these stars using the evolutionary codes Cesam2k and
astec. By using the observed splittings and the rotational kernels
of the optimal models, we inverted the internal rotation profiles
of the six stars.
Results: We obtain estimates of the core
rotation rates for these stars, and upper limits to the rotation in
their convective envelope. We show that the rotation contrast between
the core and the envelope increases during the subgiant branch. Our
results also suggest that the core of subgiants spins up with time,
while their envelope spins down. For two of the stars, we show that a
discontinuous rotation profile with a deep discontinuity reproduces
the observed splittings significantly better than a smooth rotation
profile. Interestingly, the depths that are found to be most probable
for the discontinuities roughly coincide with the location of the
H-burning shell, which separates the layers that contract from those
that expand.
Conclusions: We characterized the differential
rotation pattern of six young giants with a range of metallicities, and
with both radiative and convective cores on the main sequence. This will
bring observational constraints to the scenarios of angular momentum
transport in stars. Moreover, if the existence of sharp gradients in
the rotation profiles of young red giants is confirmed, it is expected
to help in distinguishing between the physical processes that could
transport angular momentum in the subgiant and red giant branches. Appendices and Tables 3-9 are available in electronic form at http://www.aanda.org
Title: The PLATO 2.0 Mission
Authors: Pagano, I.; Rauer, H.; Aerts, C.; Appourchaux, T.; Benz,
W.; Brandeker, A.; Christensen-Dalsgaard, J.; Deleuil, M.; Gizon, L.;
Goupil, M. -J.; Guedel, M.; Heras, A.; Janot-Pacheco, E.; Mas-Hesse,
M.; Piotto, G.; Pollaco, D.; Ragazzoni, R.; Santos, N. C.; Smith,
A.; Suarez, J. C.; Szabo, R.; Udry, S.
Bibcode: 2014EPSC....9..789P
Altcode:
PLATO 2.0 is the M class mission selected by ESA for its M3 launch
slot in the framework of the Cosmic Vision 2015-2025 program. The main
goals of PLATO 2.0 are the detection of terrestrial exoplanets in the
habitable zone of solar-type stars and the characterization of their
bulk properties needed to determine their habitability. Moreover,
PLATO 2.0 will be key in understanding the formation, architecture,
and evolution of planetary systems thanks to a thorough inventory
of the physical properties of thousands of rocky, icy, and gaseous
giant planets. We will illustrate the PLATO 2.0 science goals, how
the instrument is conceived to fulfil the science requirements, and
how the project is organized to implement the instrument, plan the
observations, and exploit the resulting data.
Title: Comparison between Mg II k and Ca II H Images Recorded by
SUNRISE/SuFI
Authors: Danilovic, S.; Hirzberger, J.; Riethmüller, T. L.; Solanki,
S. K.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Gizon, L.; Knölker,
M.; Schmidt, W.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.
Bibcode: 2014ApJ...784...20D
Altcode:
We present a comparison of high-resolution images of the solar surface
taken in the Mg II k and Ca II H channels of the Filter Imager on the
balloon-borne solar observatory SUNRISE. The Mg and Ca lines are sampled
with 0.48 nm and 0.11 nm wide filters, respectively. The two channels
show remarkable qualitative and quantitative similarities in the quiet
Sun, in an active region plage and during a small flare. However, the Mg
filtergrams display 1.4-1.7 times higher intensity contrast and appear
more smeared and smoothed in the quiet Sun. In addition, the fibrils
in a plage are wider. Although the exposure time is 100 times longer
for Mg images, the evidence suggests that these differences cannot be
explained only with instrumental effects or the evolution of the solar
scene. The differences at least partially arise because of different
line-formation heights, the stronger response of Mg k emission peaks
to the higher temperatures, and the larger height range sampled by
the broad Mg filter used here. This is evidently manifested during
the flare when a surge in Mg evolves differently than in Ca.
Title: Physical causes of solar cycle amplitude variability
Authors: Cameron, R. H.; Jiang, J.; Schüssler, M.; Gizon, L.
Bibcode: 2014JGRA..119..680C
Altcode:
The level of solar activity varies from cycle to cycle. This
variability is probably caused by a combination of nonlinear and
random effects. Based on surface flux transport simulations, we
show that the observed inflows into active regions and toward the
activity belts provide an important nonlinearity in the framework of
Babcock-Leighton model for the solar dynamo. Inclusion of these inflows
also leads to a reproduction of the observed proportionality between
the open heliospheric flux during activity minima and the maximum
sunspot number of the following cycle. A substantial component of
the random variability of the cycle strength is associated with the
cross-equatorial flux plumes that occur when large, highly tilted
sunspot groups emerge close to the equator. We show that the flux
transported by these events is important for the amplitude of the polar
fields and open flux during activity minima. The combined action of
inflows and cross-equatorial flux plumes provides an explanation for
the weakness of the polar fields at the end of solar cycle 23 (and
hence for the relative weakness of solar cycle 24).
Title: Precise and accurate interpolated stellar oscillation
frequencies on the main sequence
Authors: Ball, Warrick H.; Schou, Jesper; Gizon, Laurent; Marques,
João P. C.
Bibcode: 2014IAUS..301..379B
Altcode: 2013arXiv1309.3044B
High-quality data from space-based observatories present an opportunity
to fit stellar models to observations of individually-identified
oscillation frequencies, not just the large and small frequency
separations. But such fits require the evaluation of a large number of
accurate stellar models, which remains expensive. Here, we show that
global-mode oscillation frequencies interpolated in a grid of stellar
models are precise and accurate, at least in the neighbourhood of a
solar model.
Title: Helioseismic and Magnetic Imager Multi-height Dopplergrams
Authors: Nagashima, Kaori; Gizon, Laurent; Birch, Aaron; Löptien,
Björn; Couvidat, Sebastien; Fleck, Bernhard
Bibcode: 2014arXiv1401.3182N
Altcode:
We study Doppler velocity measurements at multiple heights in the solar
atmosphere using a set of six filtergrams obtained by the Helioseismic
magnetic Imager on board the Solar Dynamics Observatory. There are clear
and significant phase differences between core and wing Dopplergrams in
the frequency range above the photospheric acoustic cutoff frequency,
which indicates that these are really "multi-height" datasets.
Title: VizieR Online Data Catalog: Asteroseismic study of solar-type
stars (Chaplin+, 2014)
Authors: Chaplin, W. J.; Basu, S.; Huber, D.; Serenelli, A.;
Casagrande, L.; Silva Aguirre, V.; Ball, W. H.; Creevey, O. L.;
Gizon, L.; Handberg, R.; Karoff, C.; Lutz, R.; Marques, J. P.;
Miglio, A.; Stello, D.; Suran, M. D.; Pricopi, D.; Metcalfe, T. S.;
Monteiro, M. J. P. F. G.; Molenda-Zakowicz, J.; Appourchaux, T.;
Christensen-Dalsgaard, J.; Elsworth, Y.; Garcia, R. A.; Houdek, G.;
Kjeldsen, H.; Bonanno, A.; Campante, T. L.; Corsaro, E.; Gaulme, P.;
Hekker, S.; Mathur, S.; Mosser, B.; Regulo, C.; Salabert, D.
Bibcode: 2014yCat..22100001C
Altcode:
During the first 10 months of science operations more than 2000
solar-type stars were selected by the Kepler Asteroseismic Science
Consortium (KASC) to be observed as part of an asteroseismic survey
of the Sun-like population in the Kepler field of view. Solar-like
oscillations were detected by Kepler in more than 500 stars (Chaplin
et al. 2011Sci...332..213C), and from these data robust global or
average asteroseismic parameters were determined for all targets in the
sample. These asteroseismic parameters allow us to estimate fundamental
properties of the stars. In this paper we present stellar properties
- namely masses, radii, surface gravities, mean densities and ages -
of this asteroseismic sample of main-sequence and subgiant stars. (5 data files).
Title: Asteroseismic Fundamental Properties of Solar-type Stars
Observed by the NASA Kepler Mission
Authors: Chaplin, W. J.; Basu, S.; Huber, D.; Serenelli, A.;
Casagrande, L.; Silva Aguirre, V.; Ball, W. H.; Creevey, O. L.;
Gizon, L.; Handberg, R.; Karoff, C.; Lutz, R.; Marques, J. P.;
Miglio, A.; Stello, D.; Suran, M. D.; Pricopi, D.; Metcalfe, T. S.;
Monteiro, M. J. P. F. G.; Molenda-Żakowicz, J.; Appourchaux, T.;
Christensen-Dalsgaard, J.; Elsworth, Y.; García, R. A.; Houdek, G.;
Kjeldsen, H.; Bonanno, A.; Campante, T. L.; Corsaro, E.; Gaulme, P.;
Hekker, S.; Mathur, S.; Mosser, B.; Régulo, C.; Salabert, D.
Bibcode: 2014ApJS..210....1C
Altcode: 2013arXiv1310.4001C
We use asteroseismic data obtained by the NASA Kepler mission to
estimate the fundamental properties of more than 500 main-sequence and
sub-giant stars. Data obtained during the first 10 months of Kepler
science operations were used for this work, when these solar-type
targets were observed for one month each in survey mode. Stellar
properties have been estimated using two global asteroseismic parameters
and complementary photometric and spectroscopic data. Homogeneous sets
of effective temperatures, T eff, were available for the
entire ensemble from complementary photometry; spectroscopic estimates
of T eff and [Fe/H] were available from a homogeneous
analysis of ground-based data on a subset of 87 stars. We adopt
a grid-based analysis, coupling six pipeline codes to 11 stellar
evolutionary grids. Through use of these different grid-pipeline
combinations we allow implicitly for the impact on the results of
stellar model dependencies from commonly used grids, and differences
in adopted pipeline methodologies. By using just two global parameters
as the seismic inputs we are able to perform a homogenous analysis
of all solar-type stars in the asteroseismic cohort, including many
targets for which it would not be possible to provide robust estimates
of individual oscillation frequencies (due to a combination of low
signal-to-noise ratio and short dataset lengths). The median final
quoted uncertainties from consolidation of the grid-based analyses
are for the full ensemble (spectroscopic subset) approximately 10.8%
(5.4%) in mass, 4.4% (2.2%) in radius, 0.017 dex (0.010 dex) in log g,
and 4.3% (2.8%) in mean density. Around 36% (57%) of the stars have
final age uncertainties smaller than 1 Gyr. These ages will be useful
for ensemble studies, but should be treated carefully on a star-by-star
basis. Future analyses using individual oscillation frequencies will
offer significant improvements on up to 150 stars, in particular for
estimates of the ages, where having the individual frequency data is
most important.
Title: Measuring Stellar Rotation Periods with Kepler
Authors: Nielsen, M. B.; Gizon, L.; Schunker, H.; Karoff, C.
Bibcode: 2013ASPC..479..137N
Altcode: 2015arXiv150309042N
We measure rotation periods for 12151 stars in the Kepler field,
based on photometric variability caused by stellar activity. Our
analysis returns stable rotation periods over at least six out of
eight quarters of Kepler data. This large sample of stars enables us
to study rotation periods as a function of spectral type. We find good
agreement with previous studies and v sin i measurements for F, G, and
K stars. Combining rotation periods, (B-V) color, and gyrochronology
relations, we find that cool stars in our sample are predominantly
younger than ∼ 1 Gyr.
Title: Helioseismic and Magnetic Imager Multi-height Dopplergrams
Authors: Nagashima, K.; Gizon, L.; Birch, A.; Löptien, B.; Couvidat,
S.; Fleck, B.
Bibcode: 2013ASPC..479..429N
Altcode:
We study Doppler velocity measurements at multiple heights in the solar
atmosphere using a set of six filtergrams, obtained by the Helioseismic
Magnetic Imager on board the Solar Dynamics Observatory. There are clear
and significant phase differences between core and wing Dopplergrams in
the frequency range above the photospheric acoustic cutoff frequency,
which indicates that these are really “multi-height” datasets.
Title: Helioseismology of sunspots: defocusing, folding, and healing
of wavefronts
Authors: Liang, Z. -C.; Gizon, L.; Schunker, H.; Philippe, T.
Bibcode: 2013A&A...558A.129L
Altcode:
We observe and characterize the scattering of acoustic wave packets by
a sunspot in a regime where the wavelength is comparable to the size
of the sunspot. Spatial maps of wave travel times and amplitudes are
measured from the cross-covariance function of the random wave field
observed by SOHO/MDI around the sunspot in active region NOAO 9787. We
consider separately incoming plane wave packets consisting of f modes
and p modes with radial orders up to four. Observations show that the
travel-time perturbations diminish with distance far away from the
sunspot - a finite-wavelength phenomenon known as wavefront healing in
scattering theory. Observations also show a reduction of the amplitude
of the waves after their passage through the sunspot. We suggest
that a significant fraction of this amplitude reduction is due to the
defocusing of wave energy by the fast wave-speed perturbation introduced
by the sunspot. This "geometrical attenuation" will contribute to the
wave amplitude reduction in addition to the physical absorption of
waves by sunspots. We also observe an enhancement of wave amplitude
away from the central path: diffracted rays intersect with unperturbed
rays (caustics) and wavefronts fold and triplicate. Wave amplitude
measurements in time-distance helioseismology provide independent
information that can be used in concert with travel-time measurements.
Title: Helioseismology of sunspots: how sensitive are travel times
to the Wilson depression and to the subsurface magnetic field?
Authors: Schunker, H.; Gizon, L.; Cameron, R. H.; Birch, A. C.
Bibcode: 2013A&A...558A.130S
Altcode: 2013arXiv1303.6307S
To assess the ability of helioseismology to probe the subsurface
structure and magnetic field of sunspots, we need to determine
how helioseismic travel times depend on perturbations to
sunspot models. Here we numerically simulate the propagation of f,
p1, and p2 wave packets through magnetic sunspot
models. Among the models we considered, a ±50 km change in the height
of the Wilson depression and a change in the subsurface magnetic
field geometry can both be detected above the observational noise
level. We also find that the travel-time shifts due to changes in a
sunspot model must be modeled by computing the effects of changing the
reference sunspot model, and not by computing the effects of changing
the subsurface structure in the quiet-Sun model. For p1
modes, the latter is wrong by a factor of four. In conclusion, numerical
modeling of MHD wave propagation is an essential tool for interpreting
the effects of sunspots on seismic waveforms.
Title: First High-resolution Images of the Sun in the 2796 Å Mg II
k Line
Authors: Riethmüller, T. L.; Solanki, S. K.; Hirzberger, J.;
Danilovic, S.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Gizon, L.;
Knölker, M.; Schmidt, W.; Del Toro Iniesta, J. C.
Bibcode: 2013ApJ...776L..13R
Altcode: 2013arXiv1309.5213R
We present the first high-resolution solar images in the Mg II k 2796
Å line. The images, taken through a 4.8 Å broad interference filter,
were obtained during the second science flight of Sunrise in 2013 June
by the Sunrise Filter Imager (SuFI) instrument. The Mg II k images
display structures that look qualitatively very similar to images taken
in the core of Ca II H. The Mg II images exhibit reversed granulation
(or shock waves) in the internetwork regions of the quiet Sun, at
intensity contrasts that are similar to those found in Ca II H. Very
prominent in Mg II are bright points, both in the quiet Sun and in plage
regions, particularly near the disk center. These are much brighter than
at other wavelengths sampled at similar resolution. Furthermore, Mg II k
images also show fibril structures associated with plage regions. Again,
the fibrils are similar to those seen in Ca II H images, but tend to
be more pronounced, particularly in weak plage.
Title: Kepler White Paper: Asteroseismology of Solar-Like Oscillators
in a 2-Wheel Mission
Authors: Chaplin, W. J; Kjeldsen, H.; Christensen-Dalsgaard, J.;
Gilliland, R. L.; Kawaler, S. D.; Basu, S.; De Ridder, J.; Huber, D.;
Arentoft, T.; Schou, J.; Garcia, R. A.; Metcalfe, T. S.; Brogaard, K.;
Campante, T. L.; Elsworth, Y.; Miglio, A.; Appourchaux, T.; Bedding,
T. R.; Hekker, S.; Houdek, G.; Karoff, C.; Molenda-Zakowicz, J.;
Monteiro, M. J. P. F. G.; Silva Aguirre, V.; Stello, D.; Ball, W.;
Beck, P. G.; Birch, A. C.; Buzasi, D. L.; Casagrande, L.; Cellier,
T.; Corsaro, E.; Creevey, O. L.; Davies, G. R.; Deheuvels, S.; Dogan,
G.; Gizon, L.; Grundahl, F.; Guzik, J.; Handberg, R.; Jimenez, A.;
Kallinger, T.; Lund, M. N.; Lundkvist, M.; Mathis, S.; Mathur, S.;
Mazumdar, A.; Mosser, B.; Neiner, C.; Nielsen, M. B.; Palle, P. L.;
Pinsonneault, M. H.; Salabert, D.; Serenelli, A. M.; Shunker, H.;
White, T. R.
Bibcode: 2013arXiv1309.0702C
Altcode:
We comment on the potential for continuing asteroseismology of
solar-type and red-giant stars in a 2-wheel Kepler Mission. Our main
conclusion is that by targeting stars in the ecliptic it should be
possible to perform high-quality asteroseismology, as long as favorable
scenarios for 2-wheel pointing performance are met. Targeting the
ecliptic would potentially facilitate unique science that was not
possible in the nominal Mission, notably from the study of clusters
that are significantly brighter than those in the Kepler field. Our
conclusions are based on predictions of 2-wheel observations made by
a space photometry simulator, with information provided by the Kepler
Project used as input to describe the degraded pointing scenarios. We
find that elevated levels of frequency-dependent noise, consistent with
the above scenarios, would have a significant negative impact on our
ability to continue asteroseismic studies of solar-like oscillators in
the Kepler field. However, the situation may be much more optimistic
for observations in the ecliptic, provided that pointing resets of the
spacecraft during regular desaturations of the two functioning reaction
wheels are accurate at the < 1 arcsec level. This would make it
possible to apply a post-hoc analysis that would recover most of the
lost photometric precision. Without this post-hoc correction---and the
accurate re-pointing it requires---the performance would probably be
as poor as in the Kepler-field case. Critical to our conclusions for
both fields is the assumed level of pointing noise (in the short-term
jitter and the longer-term drift). We suggest that further tests will
be needed to clarify our results once more detail and data on the
expected pointing performance becomes available, and we offer our
assistance in this work.
Title: Rotation periods of 12 000 main-sequence Kepler stars:
Dependence on stellar spectral type and comparison with v sin
i observations
Authors: Nielsen, M. B.; Gizon, L.; Schunker, H.; Karoff, C.
Bibcode: 2013A&A...557L..10N
Altcode: 2013arXiv1305.5721N
Aims: We aim to measure the starspot rotation periods of
active stars in the Kepler field as a function of spectral type and
to extend reliable rotation measurements from F-, G-, and K-type to
M-type stars.
Methods: Using the Lomb-Scargle periodogram
we searched more than 150 000 stellar light curves for periodic
brightness variations. We analyzed periods between 1 and 30 days
in eight consecutive Kepler quarters, where 30 days is an estimated
maximum for the validity of the PDC_MAP data correction pipeline. We
selected stable rotation periods, i.e., periods that do not vary
from the median by more than one day in at least six of the eight
quarters. We averaged the periods for each stellar spectral class
according to B - V color and compared the results to archival vsini
data, using stellar radii estimates from the Kepler Input Catalog.
Results: We report on the stable starspot rotation periods of 12
151 Kepler stars. We find good agreement between starspot velocities
and vsini data for all F-, G- and early K-type stars. The 795 M-type
stars in our sample have a median rotation period of 15.4 days. We
find an excess of M-type stars with periods less than 7.5 days
that are potentially fast-rotating and fully convective. Measuring
photometric variability in multiple Kepler quarters appears to be a
straightforward and reliable way to determine the rotation periods
of a large sample of active stars, including late-type stars. Table 1 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/557/L10
Title: Seismic constraints on rotation of Sun-like star and mass
of exoplanet
Authors: Gizon, L.; Ballot, J.; Michel, E.; Stahn, T.; Vauclair, G.;
Bruntt, H.; Quirion, P. -O.; Benomar, O.; Vauclair, S.; Appourchaux,
T.; Auvergne, M.; Baglin, A.; Barban, C.; Baudin, F.; Bazot, M.;
Campante, T.; Catala, C.; Chaplin, W.; Creevey, O.; Deheuvels, S.;
Dolez, N.; Elsworth, Y.; Garcia, R.; Gaulme, P.; Mathis, S.; Mathur,
S.; Mosser, B.; Regulo, C.; Roxburgh, I.; Salabert, D.; Samadi, R.;
Sato, K.; Verner, G.; Hanasoge, S.; Sreenivasan, K. R.
Bibcode: 2013PNAS..11013267G
Altcode: 2013arXiv1308.4352G
Rotation is thought to drive cyclic magnetic activity in the Sun
and Sun-like stars. Stellar dynamos, however, are poorly understood
owing to the scarcity of observations of rotation and magnetic fields
in stars. Here, inferences are drawn on the internal rotation of a
distant Sun-like star by studying its global modes of oscillation. We
report asteroseismic constraints imposed on the rotation rate and the
inclination of the spin axis of the Sun-like star HD 52265, a principal
target observed by the CoRoT satellite that is known to host a planetary
companion. These seismic inferences are remarkably consistent with an
independent spectroscopic observation (rotational line broadening)
and with the observed rotation period of star spots. Furthermore,
asteroseismology constrains the mass of exoplanet HD 52265b. Under
the standard assumption that the stellar spin axis and the axis of the
planetary orbit coincide, the minimum spectroscopic mass of the planet
can be converted into a true mass of 1.85 (+0.52,-0.42) M_Jupiter,
which implies that it is a planet, not a brown dwarf.
Title: VizieR Online Data Catalog: Rotation periods of 12000 Kepler
stars (Nielsen+, 2013)
Authors: Nielsen, M. B.; Gizon, L.; Schunker, H.; Karoff, C.
Bibcode: 2013yCat..35579010N
Altcode:
Rotation periods of 12253 stars in the Kepler field. The periods are
determined by the brightness variations, from star spots or active
regions, in the light curves of the white light photometry obtained by
the Kepler spacecraft. The median absolute deviation from the median
(MAD) of the periods shows the scatter of periods for each star, over
6 or more (out of 8 analyzed) Kepler quarters. The g-r color index,
E(B-V), radius, surface gravity, and effective temperature are from
the Kepler Input Catalog (KIC). Column 9 (TF) indicates whether or
not the msMAP data for a given star satisfies the selection criteria
described in section 2. Of these, there are 86 stars with periods
from the msMAP data that differ from the period derived from the
PDCMAP data by more than one frequency resolution element
(1/90d-1). For these stars the msMAP periods are therefore
given in column 10 as a none-zero value. (1 data file).
Title: Propagation of Seismic Waves Through a Spatio-temporally
Fluctuating Medium: Homogenization
Authors: Hanasoge, Shravan M.; Gizon, Laurent; Bal, Guillaume
Bibcode: 2013ApJ...773..101H
Altcode: 2013arXiv1306.3620H
Measurements of seismic wave travel times at the photosphere of the
Sun have enabled inferences of its interior structure and dynamics. In
interpreting these measurements, the simplifying assumption that waves
propagate through a temporally stationary medium is almost universally
invoked. However, the Sun is in a constant state of evolution, on
a broad range of spatio-temporal scales. At the zero-wavelength
limit, i.e., when the wavelength is much shorter than the scale
over which the medium varies, the WKBJ (ray) approximation may be
applied. Here, we address the other asymptotic end of the spectrum, the
infinite-wavelength limit, using the technique of homogenization. We
apply homogenization to scenarios where waves are propagating through
rapidly varying media (spatially and temporally), and derive effective
models for the media. One consequence is that a scalar sound speed
becomes a tensorial wave speed in the effective model and anisotropies
can be induced depending on the nature of the perturbation. The second
term in this asymptotic two-scale expansion, the so-called corrector,
contains contributions due to higher-order scattering, leading to the
decoherence of the wave field. This decoherence may be causally linked
to the observed wave attenuation in the Sun. Although the examples
we consider here consist of periodic arrays of perturbations to the
background, homogenization may be extended to ergodic and stationary
random media. This method may have broad implications for the manner
in which we interpret seismic measurements in the Sun and for modeling
the effects of granulation on the scattering of waves and distortion
of normal-mode eigenfunctions.
Title: Comparison of Solar Surface Flows Inferred from Time-Distance
Helioseismology and Coherent Structure Tracking Using HMI/SDO
Observations
Authors: Švanda, Michal; Roudier, Thierry; Rieutord, Michel; Burston,
Raymond; Gizon, Laurent
Bibcode: 2013ApJ...771...32S
Altcode: 2013arXiv1305.0875S
We compare measurements of horizontal flows on the surface of the
Sun using helioseismic time-distance inversions and coherent structure
tracking of solar granules. Tracking provides two-dimensional horizontal
flows on the solar surface, whereas the time-distance inversions
estimate the full three-dimensional velocity flows in the shallow
near-surface layers. Both techniques use Helioseismic and Magnetic
Imager observations as input. We find good correlations between the
various measurements resulting from the two techniques. Further, we
find a good agreement between these measurements and the time-averaged
Doppler line-of-sight velocity, and also perform sanity checks on the
vertical flow that resulted from the three-dimensional time-distance
inversion.
Title: Comparison of solar horizontal velocity fields from SDO/HMI
and Hinode data
Authors: Roudier, Th.; Rieutord, M.; Prat, V.; Malherbe, J. M.; Renon,
N.; Frank, Z.; Švanda, M.; Berger, T.; Burston, R.; Gizon, L.
Bibcode: 2013A&A...552A.113R
Altcode: 2013arXiv1303.4271R
Context. The measurement of the Sun's surface motions with a high
spatial and temporal resolution is still a challenge.
Aims:
We wish to validate horizontal velocity measurements all over the
visible disk of the Sun from Solar Dynamics Observatory/ Helioseismic
and Magnetic Imager (SDO/HMI) data.
Methods: Horizontal velocity
fields are measured by following the proper motions of solar granules
using a newly developed version of the coherent structure tracking
(CST) code. The comparison of the surface flows measured at high
spatial resolution (Hinode, 0.1 arcsec) and low resolution (SDO/HMI,
0.5 arcsec) allows us to determine corrections to be applied to
the horizontal velocity measured from HMI white light data.
Results: We derive horizontal velocity maps with spatial and temporal
resolutions of respectively 2.5 Mm and 30 min. From the two components
of the horizontal velocity vx and vy measured
in the sky plane and the simultaneous line of sight component from
SDO/HMI dopplergrams vD, we derive the spherical velocity
components (vr, vθ, vϕ). The
azimuthal component vϕ gives the solar differential rotation
with a high precision (± 0.037 km s-1) from a temporal
sequence of only three hours.
Conclusions: By following the
proper motions of the solar granules, we can revisit the dynamics of
the solar surface at high spatial and temporal resolutions from hours
to months and years with the SDO data.
Title: Extracting multi-height velocity information from SDO/HMI
Dopplergrams
Authors: Nagashima, Kaori; Gizon, Laurent; Birch, Aaron; Loeptien,
Bjoern; Couvidat, Sebastien; Fleck, Bernhard; Stein, Robert
Bibcode: 2013enss.confE..76N
Altcode:
Multi-height velocity information in the solar atmosphere is useful for
many studies of the structure and dynamics of the solar atmosphere. We
discuss the possibility of measuring the vertical velocity at multiple
layers in the solar atmosphere using the six filtergrams of the Fe
I 6173A absorption line obtained by SDO/HMI. In the standard HMI
pipeline processing, these filtergrams are combined to estimate a
single Doppler velocity. Here we construct three Dopplergrams by
computing pair-wise differences between intensities in the blue
and red wings of the line. We use realistic numerical simulations
of convection to evaluate the range of heights that contribute to
each of our multi-height velocity estimates. The cross-spectra of
the Dopplergrams contain interesting information about vertical wave
propagation in the solar atmosphere.
Title: Probing vortical motions in the Sun with time-distance
helioseismology
Authors: Langfellner, Jan; Gizon, Laurent; Birch, Aaron; Schunker,
Hannah
Bibcode: 2013enss.confE..69L
Altcode:
Fluid vorticity describes fundamental aspects of the dynamics of the
solar convection zone. For example, vortical flows play an important
role in dynamo models and inform us about the dynamical effects of
rotation on convection and active region flows. However, no specific
helioseismology technique has so far been developed to infer vorticity
in the solar interior. Here we propose to use a new geometrical scheme
for time-distance helioseismology, which is particularly sensitive to
vertical vorticity. It consists of measuring the difference in wave
travel time (clockwise minus counterclockwise) around a circular array
of points. We will demonstrate the capabilities of the technique using
SDO/HMI observations and discuss future applications.
Title: Pathway to the Square Kilometre Array - The German White
Paper -
Authors: Aharonian, F.; Arshakian, T. G.; Allen, B.; Banerjee, R.;
Beck, R.; Becker, W.; Bomans, D. J.; Breitschwerdt, D.; Brüggen,
M.; Brunthaler, A.; Catinella, B.; Champion, D.; Ciardi, B.; Crocker,
R.; de Avillez, M. A.; Dettmar, R. J.; Engels, D.; Enßlin, T.; Enke,
H.; Fieseler, T.; Gizon, L.; Hackmann, E.; Hartmann, B.; Henkel, C.;
Hoeft, M.; Iapichino, L.; Innes, D.; James, C.; Jasche, J.; Jones,
D.; Kagramanova, V.; Kauffmann, G.; Keane, E.; Kerp, J.; Klöckner,
H. -R.; Kokkotas, K.; Kramer, M.; Krause, M.; Krause, M.; Krupp, N.;
Kunz, J.; Lämmerzahl, C.; Lee, K. J.; List, M.; Liu, K.; Lobanov,
A.; Mann, G.; Merloni, A.; Middelberg, E.; Niemeyer, J.; Noutsos,
A.; Perlick, V.; Reich, W.; Richter, P.; Roy, A.; Saintonge, A.;
Schäfer, G.; Schaffner-Bielich, J.; Schinnerer, E.; Schleicher, D.;
Schneider, P.; Schwarz, D. J.; Sedrakian, A.; Sesana, A.; Smolčić,
V.; Solanki, S.; Tuffs, R.; Vetter, M.; Weber, E.; Weller, J.; Wex,
N.; Wucknitz, O.; Zwaan, M.
Bibcode: 2013arXiv1301.4124A
Altcode:
The Square Kilometre Array (SKA) is the most ambitious radio telescope
ever planned. With a collecting area of about a square kilometre,
the SKA will be far superior in sensitivity and observing speed to
all current radio facilities. The scientific capability promised by
the SKA and its technological challenges provide an ideal base for
interdisciplinary research, technology transfer, and collaboration
between universities, research centres and industry. The SKA in
the radio regime and the European Extreme Large Telescope (E-ELT)
in the optical band are on the roadmap of the European Strategy Forum
for Research Infrastructures (ESFRI) and have been recognised as the
essential facilities for European research in astronomy. This "White
Paper" outlines the German science and R&D interests in the SKA
project and will provide the basis for future funding applications to
secure German involvement in the Square Kilometre Array.
Title: Seismic Probes of Solar Interior Magnetic Structure
Authors: Hanasoge, Shravan; Birch, Aaron; Gizon, Laurent; Tromp, Jeroen
Bibcode: 2012PhRvL.109j1101H
Altcode: 2012arXiv1207.4352H
Sun spots are prominent manifestations of solar magnetoconvection,
and imaging their subsurface structure is an outstanding problem
of wide physical importance. Travel times of seismic waves that
propagate through these structures are typically used as inputs to
inversions. Despite the presence of strongly anisotropic magnetic
waveguides, these measurements have always been interpreted in terms
of changes to isotropic wave speeds and flow-advection-related Doppler
shifts. Here, we employ partial-differential-equation-constrained
optimization to determine the appropriate parametrization of the
structural properties of the magnetic interior. Seven different
wave speeds fully characterize helioseismic wave propagation: the
isotropic sound speed, a Doppler-shifting flow-advection velocity,
and an anisotropic magnetic velocity. The structure of magnetic media
is sensed by magnetoacoustic slow and fast modes and Alfvén waves,
each of which propagates at a different wave speed. We show that even
in the case of weak magnetic fields, significant errors may be incurred
if these anisotropies are not accounted for in inversions. Translation
invariance is demonstrably lost. These developments render plausible
the accurate seismic imaging of magnetoconvection in the Sun.
Title: Seismic Evidence for a Rapidly Rotating Core in a
Lower-giant-branch Star Observed with Kepler
Authors: Deheuvels, S.; García, R. A.; Chaplin, W. J.; Basu, S.;
Antia, H. M.; Appourchaux, T.; Benomar, O.; Davies, G. R.; Elsworth,
Y.; Gizon, L.; Goupil, M. J.; Reese, D. R.; Regulo, C.; Schou, J.;
Stahn, T.; Casagrande, L.; Christensen-Dalsgaard, J.; Fischer, D.;
Hekker, S.; Kjeldsen, H.; Mathur, S.; Mosser, B.; Pinsonneault, M.;
Valenti, J.; Christiansen, J. L.; Kinemuchi, K.; Mullally, F.
Bibcode: 2012ApJ...756...19D
Altcode: 2012arXiv1206.3312D
Rotation is expected to have an important influence on the structure
and the evolution of stars. However, the mechanisms of angular momentum
transport in stars remain theoretically uncertain and very complex to
take into account in stellar models. To achieve a better understanding
of these processes, we desperately need observational constraints on the
internal rotation of stars, which until very recently was restricted to
the Sun. In this paper, we report the detection of mixed modes—i.e.,
modes that behave both as g modes in the core and as p modes in
the envelope—in the spectrum of the early red giant KIC 7341231,
which was observed during one year with the Kepler spacecraft. By
performing an analysis of the oscillation spectrum of the star, we
show that its non-radial modes are clearly split by stellar rotation
and we are able to determine precisely the rotational splittings of
18 modes. We then find a stellar model that reproduces very well the
observed atmospheric and seismic properties of the star. We use this
model to perform inversions of the internal rotation profile of the
star, which enables us to show that the core of the star is rotating at
least five times faster than the envelope. This will shed new light on
the processes of transport of angular momentum in stars. In particular,
this result can be used to place constraints on the angular momentum
coupling between the core and the envelope of early red giants, which
could help us discriminate between the theories that have been proposed
over the last few decades.
Title: Helioseismology challenges models of solar convection
Authors: Gizon, Laurent; Birch, Aaron C.
Bibcode: 2012PNAS..10911896G
Altcode: 2012arXiv1208.6154G
Convection is the mechanism by which energy is transported through the
outermost 30% of the Sun. Solar turbulent convection is notoriously
difficult to model across the entire convection zone where the density
spans many orders of magnitude. In this issue of PNAS, Hanasoge et
al. (2012) employ recent helioseismic observations to derive stringent
empirical constraints on the amplitude of large-scale convective
velocities in the solar interior. They report an upper limit that is far
smaller than predicted by a popular hydrodynamic numerical simulation.
Title: Solar results in the stellar context
Authors: Gizon, Laurent; Marques, Joao
Bibcode: 2012cosp...39..625G
Altcode: 2012cosp.meet..625G
No abstract at ADS
Title: Precise modeling of the exoplanet host star and CoRoT main
target HD 52265
Authors: Escobar, M. E.; Théado, S.; Vauclair, S.; Ballot, J.;
Charpinet, S.; Dolez, N.; Hui-Bon-Hoa, A.; Vauclair, G.; Gizon, L.;
Mathur, S.; Quirion, P. O.; Stahn, T.
Bibcode: 2012A&A...543A..96E
Altcode: 2012arXiv1205.6424E
Aims: This paper presents a detailed and precise study of the
characteristics of the exoplanet host star and CoRoT main target HD
52265, derived from asteroseismic studies. We compare our results
with previous estimates, and provide a comprehensive summary and
discussion.
Methods: Our basic method is similar to that
previously used by the Toulouse group for solar-type stars. Models are
computed with various initial chemical compositions and the computed
p-mode frequencies are compared with the observed ones. All models
include atomic diffusion and we discuss the importance of radiative
accelerations. Several tests are used, including the usual frequency
combinations and the fits to the échelle diagrams. Possible surface
effects are introduced and discussed. Automatic codes are also used
to identify the best-fit model for this star (SEEK and AMP) and their
results are compared with those obtained with the detailed method.
Results: We find precise results for the mass, radius, and age of
this star, as well as its effective temperature and luminosity. We
also estimate the initial helium abundance. These results are important
for the characterization of the star-planet system.
Title: Some Dynamic Analysis of the Photosphere from Hinode/SOT and
SDO/HMI Observations
Authors: Roudier, T.; Malherbe, J.; Rieutord, M.; Berger, T.; Frank,
Z.; Prat, V.; Renon, N.; Gizon, L.; Svanda, M.
Bibcode: 2012ASPC..456...65R
Altcode:
We first present the important role played by the families of granule
(or Tree of Fragmenting granules) in the formation of the photospheric
network. Then, we describe the occurence and characteristics of
acoustic events (AE), defined as spatially concentrated energy flux,
in the quiet Sun. Finally, we present how horizontal velocities obtained
from SDO/HMI data are calibrated by using Hinode/SOT observations.
Title: Quasi full-disk maps of solar horizontal velocities using
SDO/HMI data
Authors: Roudier, Th.; Rieutord, M.; Malherbe, J. M.; Renon, N.;
Berger, T.; Frank, Z.; Prat, V.; Gizon, L.; Švanda, M.
Bibcode: 2012A&A...540A..88R
Altcode: 2012arXiv1203.0514R
Aims: For the first time, the motion of granules (solar plasma
on the surface on scales larger than 2.5 Mm) has been followed over
the entire visible surface of the Sun, using SDO/HMI white-light
data.
Methods: Horizontal velocity fields are derived from image
correlation tracking using a new version of the coherent structure
tracking algorithm. The spatial and temporal resolutions of the
horizontal velocity map are 2.5 Mm and 30 min, respectively.
Results: From this reconstruction, using the multi-resolution
analysis, one can obtain to the velocity field at different scales
with its derivatives such as the horizontal divergence or the vertical
component of the vorticity. The intrinsic error on the velocity is
~0.25 km s-1 for a time sequence of 30 min and a mesh size
of 2.5 Mm. This is acceptable compared to the granule velocities, which
range between 0.3 km s-1 and 1.8 km s-1. A high
correlation between velocities computed from Hinode and SDO/HMI has
been found (85%). From the data we derive the power spectrum of the
supergranulation horizontal velocity field, the solar differential
rotation, and the meridional velocity.
Title: Asteroseismology of the Solar Analogs 16 Cyg A and B from
Kepler Observations
Authors: Metcalfe, T. S.; Chaplin, W. J.; Appourchaux, T.; García,
R. A.; Basu, S.; Brandão, I.; Creevey, O. L.; Deheuvels, S.; Doǧan,
G.; Eggenberger, P.; Karoff, C.; Miglio, A.; Stello, D.; Yıldız,
M.; Çelik, Z.; Antia, H. M.; Benomar, O.; Howe, R.; Régulo, C.;
Salabert, D.; Stahn, T.; Bedding, T. R.; Davies, G. R.; Elsworth, Y.;
Gizon, L.; Hekker, S.; Mathur, S.; Mosser, B.; Bryson, S. T.; Still,
M. D.; Christensen-Dalsgaard, J.; Gilliland, R. L.; Kawaler, S. D.;
Kjeldsen, H.; Ibrahim, K. A.; Klaus, T. C.; Li, J.
Bibcode: 2012ApJ...748L..10M
Altcode: 2012arXiv1201.5966M
The evolved solar-type stars 16 Cyg A and B have long been studied
as solar analogs, yielding a glimpse into the future of our own
Sun. The orbital period of the binary system is too long to provide
meaningful dynamical constraints on the stellar properties, but
asteroseismology can help because the stars are among the brightest
in the Kepler field. We present an analysis of three months of nearly
uninterrupted photometry of 16 Cyg A and B from the Kepler space
telescope. We extract a total of 46 and 41 oscillation frequencies
for the two components, respectively, including a clear detection
of octupole (l = 3) modes in both stars. We derive the properties
of each star independently using the Asteroseismic Modeling Portal,
fitting the individual oscillation frequencies and other observational
constraints simultaneously. We evaluate the systematic uncertainties
from an ensemble of results generated by a variety of stellar evolution
codes and fitting methods. The optimal models derived by fitting each
component individually yield a common age (t = 6.8 ± 0.4 Gyr) and
initial composition (Z i = 0.024 ± 0.002, Y i =
0.25 ± 0.01) within the uncertainties, as expected for the components
of a binary system, bolstering our confidence in the reliability of
asteroseismic techniques. The longer data sets that will ultimately
become available will allow future studies of differential rotation,
convection zone depths, and long-term changes due to stellar activity
cycles.
Title: Multichannel Three-Dimensional SOLA Inversion for Local
Helioseismology
Authors: Jackiewicz, J.; Birch, A. C.; Gizon, L.; Hanasoge, S. M.;
Hohage, T.; Ruffio, J. -B.; Švanda, M.
Bibcode: 2012SoPh..276...19J
Altcode: 2011arXiv1109.2712J
Inversions for local helioseismology are an important and necessary step
for obtaining three-dimensional maps of various physical quantities
in the solar interior. Frequently, the full inverse problems that
one would like to solve prove intractable because of computational
constraints. Due to the enormous seismic data sets that already
exist and those forthcoming, this is a problem that needs to be
addressed. To this end, we present a very efficient linear inversion
algorithm for local helioseismology. It is based on a subtractive
optimally localized averaging (SOLA) scheme in the Fourier domain,
utilizing the horizontal-translation invariance of the sensitivity
kernels. In Fourier space the problem decouples into many small
problems, one for each horizontal wave vector. This multichannel
SOLA method is demonstrated for an example problem in time-distance
helioseismology that is small enough to be solved both in real and
Fourier space. We find that both approaches are successful in solving
the inverse problem. However, the multichannel SOLA algorithm is much
faster and can easily be parallelized.
Title: The adjoint method applied to time-distance helioseismology
Authors: Hanasoge, S.; Birch, A. C.; Gizon, L. C.; Tromp, J.
Bibcode: 2011AGUFMSH51B2002H
Altcode:
For a given misfit function, a specified optimality measure of a model,
its gradient describes the manner in which one may alter properties of
the system to march towards a stationary point. The adjoint method,
arising from partial-differential-equation-constrained optimization,
describes a means of extracting derivatives of a misfit function with
respect to model parameters through finite computation. It relies on
the accurate calculation of wavefields that are driven by two types
of sources, namely the average wave-excitation spectrum, resulting
in the forward wavefield, and differences between predictions and
observations, resulting in an adjoint wavefield. All sensitivity
kernels relevant to a given measurement emerge directly from the
evaluation of an interaction integral involving these wavefields. The
technique facilitates computation of sensitivity kernels relative to
three-dimensional heterogeneous background models with magnetic fields,
thereby paving the way for non-linear iterative inversions. We present
flow, sound-speed and magnetic-field kernels.
Title: The Adjoint Method Applied to Time-distance Helioseismology
Authors: Hanasoge, Shravan M.; Birch, Aaron; Gizon, Laurent; Tromp,
Jeroen
Bibcode: 2011ApJ...738..100H
Altcode: 2011arXiv1105.4263H
For a given misfit function, a specified optimality measure of a model,
its gradient describes the manner in which one may alter properties
of the system to march toward a stationary point. The adjoint method,
arising from partial-differential-equation-constrained optimization,
describes a means of extracting derivatives of a misfit function with
respect to model parameters through finite computation. It relies on
the accurate calculation of wavefields that are driven by two types
of sources, namely, the average wave-excitation spectrum, resulting
in the forward wavefield, and differences between predictions and
observations, resulting in an adjoint wavefield. All sensitivity
kernels relevant to a given measurement emerge directly from the
evaluation of an interaction integral involving these wavefields. The
technique facilitates computation of sensitivity kernels (Fréchet
derivatives) relative to three-dimensional heterogeneous background
models, thereby paving the way for nonlinear iterative inversions. An
algorithm to perform such inversions using as many observations as
desired is discussed.
Title: Erratum: ``Linear sensitivity of helioseismic travel times
to local flows''
Authors: Birch, A. C.; Gizon, L.; Burston, R.
Bibcode: 2011AN....332..658B
Altcode:
We fix three typographical errors in ``Linear Sensitivity of
Helioseismic Travel Times to Local Flows'' (Astronomische Nachrichten,
Vol. 328, Issue 3, p. 228).
Title: Constructing and Characterising Solar Structure Models for
Computational Helioseismology
Authors: Schunker, H.; Cameron, R. H.; Gizon, L.; Moradi, H.
Bibcode: 2011SoPh..271....1S
Altcode: 2011SoPh..tmp..124S; 2011arXiv1105.0219S; 2011SoPh..tmp..179S;
2011SoPh..tmp..248S
In local helioseismology, numerical simulations of wave propagation
are useful to model the interaction of solar waves with perturbations
to a background solar model. However, the solution to the linearised
equations of motion include convective modes that can swamp the
helioseismic waves that we are interested in. In this article,
we construct background solar models that are stable against
convection, by modifying the vertical pressure gradient of Model S
(Christensen-Dalsgaard et al., 1996, Science272, 1286) relinquishing
hydrostatic equilibrium. However, the stabilisation affects the
eigenmodes that we wish to remain as close to Model S as possible. In
a bid to recover the Model S eigenmodes, we choose to make additional
corrections to the sound speed of Model S before stabilisation. No
stabilised model can be perfectly solar-like, so we present three
stabilised models with slightly different eigenmodes. The models are
appropriate to study the f and p1 to p4 modes with
spherical harmonic degrees in the range from 400 to 900. Background
model CSM has a modified pressure gradient for stabilisation and has
eigenfrequencies within 2% of Model S. Model CSM_A has an additional 10%
increase in sound speed in the top 1 Mm resulting in eigenfrequencies
within 2% of Model S and eigenfunctions that are, in comparison with
CSM, closest to those of Model S. Model CSM_B has a 3% decrease in
sound speed in the top 5 Mm resulting in eigenfrequencies within 1%
of Model S and eigenfunctions that are only marginally adversely
affected. These models are useful to study the interaction of
solar waves with embedded three-dimensional heterogeneities, such
as convective flows and model sunspots. We have also calculated the
response of the stabilised models to excitation by random near-surface
sources, using simulations of the propagation of linear waves. We find
that the simulated power spectra of wave motion are in good agreement
with an observed SOHO/MDI power spectrum. Overall, our convectively
stabilised background models provide a good basis for quantitative
numerical local helioseismology. The models are available for download
from http://www.mps.mpg.de/projects/seismo/NA4/.
Title: Validated helioseismic inversions for 3D vector flows
Authors: Švanda, M.; Gizon, L.; Hanasoge, S. M.; Ustyugov, S. D.
Bibcode: 2011A&A...530A.148S
Altcode: 2011arXiv1104.4083S
Context. According to time-distance helioseismology, information
about internal fluid motions is encoded in the travel times of solar
waves. The inverse problem consists of inferring three-dimensional
vector flows from a set of travel-time measurements. While only few
tests of the inversions have been done, it is known that the retrieval
of the small-amplitude vertical flow velocities is problematic. A
thorough study of biases and noise has not been carried out in
realistic conditions.
Aims: Here we investigate the potential
of time-distance helioseismology to infer three-dimensional convective
velocities in the near-surface layers of the Sun. We developed a new
subtractive optimally localised averaging (SOLA) code suitable for
pipeline pseudo-automatic processing. Compared to its predecessor,
the code was improved by accounting for additional constraints in
order to get the right answer within a given noise level. The main
aim of this study is to validate results obtained by our inversion
code.
Methods: We simulate travel-time maps using a snapshot
from a numerical simulation of solar convective flows, realistic Born
travel-time sensitivity kernels, and a realistic model of travel-time
noise. These synthetic travel times are inverted for flows and the
results compared with the known input flow field. Additional constraints
are implemented in the inversion: cross-talk minimization between flow
components and spatial localization of inversion coefficients.
Results: Using modes f, p1 through p4, we show
that horizontal convective flow velocities can be inferred without
bias, at a signal-to-noise ratio greater than one in the top 3.5 Mm,
provided that observations span at least four days. The vertical
component of velocity (vz), if it were to be weak, is
more difficult to infer and is seriously affected by cross-talk from
horizontal velocity components. We emphasise that this cross-talk
must be explicitly minimised in order to retrieve vz
in the top 1 Mm. We also show that statistical averaging over many
different areas of the Sun allows for reliably measuring of average
properties of all three flow components in the top 5.5 Mm of the
convection zone. Figures 16-28 are available in electronic form
at http://www.aanda.org
Title: Accurate p-mode measurements of the G0V metal-rich CoRoT
target HD 52265
Authors: Ballot, J.; Gizon, L.; Samadi, R.; Vauclair, G.; Benomar, O.;
Bruntt, H.; Mosser, B.; Stahn, T.; Verner, G. A.; Campante, T. L.;
García, R. A.; Mathur, S.; Salabert, D.; Gaulme, P.; Régulo, C.;
Roxburgh, I. W.; Appourchaux, T.; Baudin, F.; Catala, C.; Chaplin,
W. J.; Deheuvels, S.; Michel, E.; Bazot, M.; Creevey, O.; Dolez, N.;
Elsworth, Y.; Sato, K. H.; Vauclair, S.; Auvergne, M.; Baglin, A.
Bibcode: 2011A&A...530A..97B
Altcode: 2011arXiv1105.3551B
Context. The star HD 52265 is a G0V metal-rich
exoplanet-host star observed in the seismology field of the CoRoT
space telescope from November 2008 to March 2009. The satellite
collected 117 days of high-precision photometric data on this star,
showing that it presents solar-like oscillations. HD 52265 was also
observed in spectroscopy with the Narval spectrograph at the same
epoch.
Aims: We characterise HD 52265 using both spectroscopic
and seismic data.
Methods: The fundamental stellar parameters
of HD 52265 were derived with the semi-automatic software VWA, and
the projected rotational velocity was estimated by fitting synthetic
profiles to isolated lines in the observed spectrum. The parameters
of the observed p modes were determined with a maximum-likelihood
estimation. We performed a global fit of the oscillation spectrum,
over about ten radial orders, for degrees l = 0 to 2. We also derived
the properties of the granulation, and analysed a signature of the
rotation induced by the photospheric magnetic activity.
Results:
Precise determinations of fundamental parameters have been obtained:
Teff = 6100 ± 60 K, log g = 4.35 ± 0.09, [M/H] = 0.19 ±
0.05, as well as vsini=3.6+0.3-1.0kms. We have
measured a mean rotation period Prot = 12.3 ± 0.15 days,
and find a signature of differential rotation. The frequencies of 31
modes are reported in the range 1500-2550 μHz. The large separation
exhibits a clear modulation around the mean value Dnu=98.3 ± 0.1
μHz. Mode widths vary with frequency along an S-shape with a clear
local maximum around 1800 μHz. We deduce lifetimes ranging between
0.5 and 3 days for these modes. Finally, we find a maximal bolometric
amplitude of about 3.96 ± 0.24 ppm for radial modes. The CoRoT
space mission, launched on December 27th 2006, has been developed
and is operated by CNES, with the contribution of Austria, Belgium,
Brazil, ESA (RSSD and Science Programme), Germany and Spain.
Title: SLiM: A Code for the Simulation of Wave Propagation through
an Inhomogeneous, Magnetised Solar Atmosphere
Authors: Cameron, R.; Gizon, L.; Daiffallah, K.
Bibcode: 2011ascl.soft05004C
Altcode:
The semi-spectral linear MHD (SLiM) code follows the interaction
of linear waves through an inhomogeneous three-dimensional solar
atmosphere. The background model allows almost arbitrary perturbations
of density, temperature, sound speed as well as magnetic and velocity
fields. The code is useful in understanding the helioseismic signatures
of various solar features, including sunspots.
Title: 3D Numerical Simulations of f-Mode Propagation Through Magnetic
Flux Tubes
Authors: Daiffallah, K.; Abdelatif, T.; Bendib, A.; Cameron, R.;
Gizon, L.
Bibcode: 2011SoPh..268..309D
Altcode: 2010SoPh..tmp..204D; 2010SoPh..tmp..228D; 2010arXiv1008.2531D
Three-dimensional numerical simulations have been used to study the
scattering of a surface-gravity wave packet by vertical magnetic-flux
tubes, with radii from 200 km to 3 Mm, embedded in stratified polytropic
atmosphere. The scattered wave has been found to consist primarily of
m=0 (axisymmetric) and m=1 modes. The ratio of the amplitude of these
two modes was found to be strongly dependent on the radius of the flux
tube. The kink mode is the dominant mode excited in tubes with a small
radius, while the sausage mode is dominant for large tubes. Simulations
of this type provide a simple, efficient, and robust way to start to
understand the seismic signature of flux tubes, which have recently
begun to be observed.
Title: Constructing Semi-Empirical Sunspot Models for Helioseismology
Authors: Cameron, R. H.; Gizon, L.; Schunker, H.; Pietarila, A.
Bibcode: 2011SoPh..268..293C
Altcode: 2010arXiv1003.0528C; 2010SoPh..tmp..167C
One goal of helioseismology is to determine the subsurface structure
of sunspots. In order to do so, it is important to understand
first the near-surface effects of sunspots on solar waves, which are
dominant. Here we construct simplified, cylindrically-symmetric sunspot
models that are designed to capture the magnetic and thermodynamics
effects coming from about 500 km below the quiet-Sun τ5000=1
level to the lower chromosphere. We use a combination of existing
semi-empirical models of sunspot thermodynamic structure (density,
temperature, pressure): the umbral model of Maltby et al. (1986,
Astrophys. J. 306, 284) and the penumbral model of Ding and Fang (1989,
Astron. Astrophys. 225, 204). The OPAL equation-of-state tables are used
to derive the sound-speed profile. We smoothly merge the near-surface
properties to the quiet-Sun values about 1 Mm below the surface. The
umbral and penumbral radii are free parameters. The magnetic field is
added to the thermodynamic structure, without requiring magnetostatic
equilibrium. The vertical component of the magnetic field is assumed
to have a Gaussian horizontal profile, with a maximum surface field
strength fixed by surface observations. The full magnetic-field vector
is solenoidal and determined by the on-axis vertical field, which,
at the surface, is chosen such that the field inclination is 45° at
the umbral - penumbral boundary. We construct a particular sunspot
model based on SOHO/MDI observations of the sunspot in active region
NOAA 9787. The helioseismic signature of the model sunspot is studied
using numerical simulations of the propagation of f, p1,
and p2 wave packets. These simulations are compared
against cross-covariances of the observed wave field. We find that
the sunspot model gives a helioseismic signature that is similar to
the observations.
Title: Four years of HELAS
Authors: Roth, M.; Lühe, O. v. d.; Aerts, C.; Christensen-Dalsgaard,
J.; Corbard, T.; Daszyńska-Daszkiewicz, J.; Di Mauro, M. P.; Gizon,
L.; Jiménez-Reyes, S.; Monteiro, M. J. P. F. G.; Pallé, P. L.;
Thompson, M. J.
Bibcode: 2010AN....331.1084R
Altcode:
The European Coordination Action on HELio- and ASteroseismology
(HELAS) has completed its fourth and final year of initial funding by
the European Commission. Set up as a network which combines solar and
stellar physics communities in the important and vigorously evolving
field of seismology, HELAS has been able to coordinate the efforts of
European astronomers with remarkable success. Four large international
conferences including the HELAS-IV conference on Lanzarote as well as
many workshops were organized with a substantial contribution from
HELAS. About a dozen workshops, addressing specialized questions in
global and local helioseismology and asteroseismology were entirely
organized by HELAS. Data analysis tools to prepare the European
communities for the upcoming influx of data from new missions have
been prepared, tested and demonstrated. Lecture notes and outreach
material have been assembled and prepared for general access. As a
result, HELAS has an important impact on the scientific output of the
astrophysics seismology communities and significantly increased the
visibility of European research in this field. This paper summarizes
the activities and accomplishments of HELAS.
Title: Modeling the Subsurface Structure of Sunspots
Authors: Moradi, H.; Baldner, C.; Birch, A. C.; Braun, D. C.; Cameron,
R. H.; Duvall, T. L.; Gizon, L.; Haber, D.; Hanasoge, S. M.; Hindman,
B. W.; Jackiewicz, J.; Khomenko, E.; Komm, R.; Rajaguru, P.; Rempel,
M.; Roth, M.; Schlichenmaier, R.; Schunker, H.; Spruit, H. C.;
Strassmeier, K. G.; Thompson, M. J.; Zharkov, S.
Bibcode: 2010SoPh..267....1M
Altcode: 2009arXiv0912.4982M; 2010SoPh..tmp..171M
While sunspots are easily observed at the solar surface, determining
their subsurface structure is not trivial. There are two main
hypotheses for the subsurface structure of sunspots: the monolithic
model and the cluster model. Local helioseismology is the only means
by which we can investigate subphotospheric structure. However, as
current linear inversion techniques do not yet allow helioseismology to
probe the internal structure with sufficient confidence to distinguish
between the monolith and cluster models, the development of physically
realistic sunspot models are a priority for helioseismologists. This
is because they are not only important indicators of the variety of
physical effects that may influence helioseismic inferences in active
regions, but they also enable detailed assessments of the validity of
helioseismic interpretations through numerical forward modeling. In
this article, we provide a critical review of the existing sunspot
models and an overview of numerical methods employed to model wave
propagation through model sunspots. We then carry out a helioseismic
analysis of the sunspot in Active Region 9787 and address the serious
inconsistencies uncovered by Gizon et al. (2009a, 2009b). We find that
this sunspot is most probably associated with a shallow, positive
wave-speed perturbation (unlike the traditional two-layer model)
and that travel-time measurements are consistent with a horizontal
outflow in the surrounding moat.
Title: An absorbing boundary formulation for the stratified,
linearized, ideal MHD equations based on an unsplit, convolutional
perfectly matched layer
Authors: Hanasoge, S. M.; Komatitsch, D.; Gizon, L.
Bibcode: 2010A&A...522A..87H
Altcode: 2010arXiv1003.0725H
Perfectly matched layers are a very efficient way to absorb waves on
the outer edges of media. We present a stable convolutional unsplit
perfectly matched formulation designed for the linearized stratified
Euler equations. The technique as applied to the Magneto-hydrodynamic
(MHD) equations requires the use of a sponge, which, despite placing
the perfectly matched status in question, is still highly efficient at
absorbing outgoing waves. We study solutions of the equations in the
backdrop of models of linearized wave propagation in the Sun. We test
the numerical stability of the schemes by integrating the equations
over a large number of wave periods.
Title: Erratum: Erratum to: Helioseismology of Sunspots: A Case
Study of NOAA Region 9787
Authors: Gizon, L.; Schunker, H.; Baldner, C. S.; Basu, S.; Birch,
A. C.; Bogart, R. S.; Braun, D. C.; Cameron, R.; Duvall, T. L.;
Hanasoge, S. M.; Jackiewicz, J.; Roth, M.; Stahn, T.; Thompson, M. J.;
Zharkov, S.
Bibcode: 2010SSRv..156..257G
Altcode: 2010SSRv..tmp...99G
No abstract at ADS
Title: Scattering of helioseismic waves by a sunspot: wavefront
healing and folding
Authors: Liang, Z. -C.; Gizon, L.; Schunker, H.
Bibcode: 2010arXiv1010.0216L
Altcode:
We observe and characterize the scattering of acoustic wave packets
by a sunspot, in a regime where the wavelength is comparable to the
size of the sunspot. Spatial maps of wave traveltimes and amplitudes
are measured from the cross-covariance function of the random wave
field. The averaging procedure is such that incoming wave packets
are plane wave packets. Observations show that the magnitude of the
traveltime perturbation caused by the sunspot diminishes as waves
propagate away from the sunspot -- a finite-wavelength phenomenon
known as wavefront healing. Observations also show a reduction of the
amplitude of the waves after their passage through the sunspot. A
significant fraction of this amplitude reduction is due to the
defocusing of wave energy by the fast wave-speed perturbation introduced
by the sunspot. This ``geometrical attenuation'' will contribute to
the wave amplitude reduction in addition to the physical absorption of
waves. In addition, we observe an enhancement of wave amplitude away
from the central path: diffracted rays intersect with unperturbed rays
(caustics) and wavefronts fold and triplicate. Thus we find that ray
tracing is useful to interpret these phenomena, although it cannot
explain wavefront healing.
Title: Local Helioseismology: Three-Dimensional Imaging of the
Solar Interior
Authors: Gizon, Laurent; Birch, Aaron C.; Spruit, Henk C.
Bibcode: 2010ARA&A..48..289G
Altcode: 2010arXiv1001.0930G
The Sun supports a rich spectrum of internal waves that are continuously
excited by turbulent convection. The Global Oscillation Network
Group (GONG) network and the SOHO/MDI (Solar and Heliospheric
Observatory/Michelson Doppler Imager) space instrument provide
an exceptional database of spatially resolved observations of
solar oscillations, covering more than an entire sunspot cycle (11
years). Local helioseismology is a set of tools for probing the solar
interior in three dimensions using measurements of wave travel times
and local mode frequencies. Local helioseismology has discovered
(a) near-surface vector flows associated with convection, (b) 250 m
s-1 subsurface horizontal outflows around sunspots, (c)
∼50 m s-1 extended horizontal flows around active regions
(converging near the surface and diverging below), (d) the effect of
the Coriolis force on convective flows and active region flows, (e)
the subsurface signature of the 15 m s-1 poleward meridional
flow, (f) a ±5 m s-1 time-varying depth-dependent component
of the meridional circulation around the mean latitude of activity,
and (g) magnetic activity on the farside of the Sun.
Title: Developing Physics-Based Procedures for Local Helioseismic
Probing of Sunspots and Magnetic Regions
Authors: Birch, Aaron; Braun, D. C.; Crouch, A.; Rempel, M.; Fan,
Y.; Centeno, R.; Toomre, J.; Haber, D.; Hindman, B.; Featherstone,
N.; Duvall, T., Jr.; Jackiewicz, J.; Thompson, M.; Stein, R.; Gizon,
L.; Cameron, R.; Saidi, Y.; Hanasoge, S.; Burston, R.; Schunker, H.;
Moradi, H.
Bibcode: 2010AAS...21630805B
Altcode:
We have initiated a project to test and improve the local helioseismic
techniques of time-distance and ring-diagram analysis. Our goals are
to develop and implement physics-based methods that will (1) enable the
reliable determinations of subsurface flow, magnetic field, and thermal
structure in regions of strong magnetic fields and (2) be quantitatively
tested with realistic solar magnetoconvection simulations in the
presence of sunspot-like magnetic fields. We are proceeding through a
combination of improvements in local helioseismic measurements, forward
modeling of the helioseismic wavefield, kernel computations, inversions,
and validation through numerical simulations. As improvements over
existing techniques are made they will be applied to the SDO/HMI
observations. This work is funded through the the NASA Heliophysics
Science Division through the Solar Dynamics Observatory (SDO) Science
Center program.
Title: German Science Center for the Solar Dynamics Observatory
Authors: Saidi, Y.; Burston, R.; Moradi, H.; Gizon, L.
Bibcode: 2010arXiv1004.4312S
Altcode:
A data and computation center for helioseismology has been set up
at the Max Planck Institute for Solar System Research in Germany to
prepare for the SDO mission. Here we present the system infrastructure
and the scientific aims of this project, which is funded through grants
from the German Aerospace Center and the European Research Council.
Title: Convectively stabilised background solar models for local
helioseismology
Authors: Schunker, H.; Cameron, R.; Gizon, L.
Bibcode: 2010arXiv1002.1969S
Altcode:
In local helioseismology numerical simulations of wave propagation
are useful to model the interaction of solar waves with perturbations
to a background solar model. However, the solution to the equations
of motions include convective modes that can swamp the waves we are
interested in. For this reason, we choose to first stabilise the
background solar model against convection by altering the vertical
pressure gradient. Here we compare the eigenmodes of our convectively
stabilised model with a standard solar model (Model S) and find a
good agreement.
Title: Numerical Simulations of Quiet Sun Oscillations
Authors: Schunker, H.; Cameron, R.; Gizon, L.
Bibcode: 2009ASPC..416...49S
Altcode:
We develop a quiet Sun background model to be used for the numerical
simulation of solar oscillations and explore the properties of this
model using the three-dimensional Semi-spectral Linear MHD (SLiM)
code. We first suggest criteria for defining a convectively stable,
but solar-like, background model. A first step in the development
of such a solar-like model is presented and we demonstrate that it
meets the first of the criteria by comparing the power spectrum of
the simulation with SoHO/MDI observations.
Title: Multi-channel OLA Inversion for Local Helioseismology
Authors: Jackiewicz, Jason; Birch, A.; Gizon, L.
Bibcode: 2009SPD....40.0714J
Altcode:
We present preliminary results of a three-dimensional inversion
for local helioseismology, in particular utilizing the time-distance
technique. This inversion is unique in that in combines a Fourier-space
multi-channel deconvolution with the optimally localized averages (OLA)
method. The result is a very computationally efficient procedure that
is fully parallelizable and suited for large-scale inversions needed
for future studies. A simple example of the inversion is shown using
point-to-point Born approximation kernels for sound-speed perturbations
and the travel-time noise covariance. A step-by-step comparison is shown
with a recently developed real-space OLA inversion to demonstrate the
benefits of a Fourier-space formulation.
Title: Probing the Solar Interior in Three Dimensions
Authors: Gizon, Laurent
Bibcode: 2009SPD....40.0201G
Altcode:
The ability to probe the solar interior with sound waves has opened
a vast domain of applications in solar, stellar, and fundamental
physics. Helioseismology has enabled us to test and refine the
standard solar model, and also to explore phenomena that we do not
understand at all. In this talk I will discuss two topics in local
helioseismology: solar near-surface convection and sunspots. First,
I will present recent inversions of helioseismic travel times (MDI/SOHO
observations), which describe the plasma flow in the top 4 Mm of the
convection zone. For daily averages, the flows are mostly horizontal
and dominated by supergranulation. The inferred vertical flows are not
unreasonable: upward flows are typically found in regions of horizontal
outflows, with amplitudes that are consistent with mass conservation. A
statistical study reveals other subtle effects, such as the influence
of the Coriolis force on the flows. Second, I will comment on
sunspot seismology, which offers unique prospects for studying the
subsurface structure, formation, and evolution of sunspots. Solar waves
are known to interact strongly with sunspots, as seen for example in the
correlations of the random wave field at the solar surface. Because the
effects of the magnetic field on the waves cannot be assumed to be weak,
it is however a challenge to interpret the observations. I will argue
that three-dimensional numerical simulations of wave propagation through
sunspot models are an essential tool to help solve this problem. I
will show an example of a simple sunspot model for which the numerical
simulations mostly agree with the seismic observations. With the
continued operations of SOHO and dedicated ground-based facilities,
the upcoming launch of the Solar Dynamics Observatory of NASA, and the
preparations for the Solar Orbiter mission of ESA, helioseismology is,
today, a very active branch of solar physics.
Title: Helioseismology of Sunspots: A Case Study of NOAA Region 9787
Authors: Gizon, L.; Schunker, H.; Baldner, C. S.; Basu, S.; Birch,
A. C.; Bogart, R. S.; Braun, D. C.; Cameron, R.; Duvall, T. L.;
Hanasoge, S. M.; Jackiewicz, J.; Roth, M.; Stahn, T.; Thompson, M. J.;
Zharkov, S.
Bibcode: 2009SSRv..144..249G
Altcode: 2008SSRv..tmp..188G; 2010arXiv1002.2369G
Various methods of helioseismology are used to study the subsurface
properties of the sunspot in NOAA Active Region 9787. This sunspot
was chosen because it is axisymmetric, shows little evolution during
20-28 January 2002, and was observed continuously by the MDI/SOHO
instrument. AR 9787 is visible on helioseismic maps of the farside
of the Sun from 15 January, i.e. days before it crossed the East
limb. Oscillations have reduced amplitudes in the sunspot at all
frequencies, whereas a region of enhanced acoustic power above 5.5 mHz
(above the quiet-Sun acoustic cutoff) is seen outside the sunspot and
the plage region. This enhanced acoustic power has been suggested to
be caused by the conversion of acoustic waves into magneto-acoustic
waves that are refracted back into the interior and re-emerge as
acoustic waves in the quiet Sun. Observations show that the sunspot
absorbs a significant fraction of the incoming p and f modes around 3
mHz. A numerical simulation of MHD wave propagation through a simple
model of AR 9787 confirmed that wave absorption is likely to be due
to the partial conversion of incoming waves into magneto-acoustic
waves that propagate down the sunspot. Wave travel times and mode
frequencies are affected by the sunspot. In most cases, wave packets
that propagate through the sunspot have reduced travel times. At
short travel distances, however, the sign of the travel-time shifts
appears to depend sensitively on how the data are processed and,
in particular, on filtering in frequency-wavenumber space. We carry
out two linear inversions for wave speed: one using travel-times
and phase-speed filters and the other one using mode frequencies
from ring analysis. These two inversions give subsurface wave-speed
profiles with opposite signs and different amplitudes. The travel-time
measurements also imply different subsurface flow patterns in the
surface layer depending on the filtering procedure that is used. Current
sensitivity kernels are unable to reconcile these measurements, perhaps
because they rely on imperfect models of the power spectrum of solar
oscillations. We present a linear inversion for flows of ridge-filtered
travel times. This inversion shows a horizontal outflow in the upper
4 Mm that is consistent with the moat flow deduced from the surface
motion of moving magnetic features. From this study of AR 9787, we
conclude that we are currently unable to provide a unified description
of the subsurface structure and dynamics of the sunspot.
Title: POLAR investigation of the Sun—POLARIS
Authors: Appourchaux, T.; Liewer, P.; Watt, M.; Alexander, D.;
Andretta, V.; Auchère, F.; D'Arrigo, P.; Ayon, J.; Corbard, T.;
Fineschi, S.; Finsterle, W.; Floyd, L.; Garbe, G.; Gizon, L.; Hassler,
D.; Harra, L.; Kosovichev, A.; Leibacher, J.; Leipold, M.; Murphy,
N.; Maksimovic, M.; Martinez-Pillet, V.; Matthews, B. S. A.; Mewaldt,
R.; Moses, D.; Newmark, J.; Régnier, S.; Schmutz, W.; Socker, D.;
Spadaro, D.; Stuttard, M.; Trosseille, C.; Ulrich, R.; Velli, M.;
Vourlidas, A.; Wimmer-Schweingruber, C. R.; Zurbuchen, T.
Bibcode: 2009ExA....23.1079A
Altcode: 2008ExA...tmp...40A; 2008arXiv0805.4389A
The POLAR Investigation of the Sun (POLARIS) mission uses a combination
of a gravity assist and solar sail propulsion to place a spacecraft
in a 0.48 AU circular orbit around the Sun with an inclination of 75°
with respect to solar equator. This challenging orbit is made possible
by the challenging development of solar sail propulsion. This first
extended view of the high-latitude regions of the Sun will enable
crucial observations not possible from the ecliptic viewpoint or from
Solar Orbiter. While Solar Orbiter would give the first glimpse of
the high latitude magnetic field and flows to probe the solar dynamo,
it does not have sufficient viewing of the polar regions to achieve
POLARIS’s primary objective: determining the relation between the
magnetism and dynamics of the Sun’s polar regions and the solar cycle.
Title: Astrodynamical Space Test of Relativity Using Optical Devices
I (ASTROD I)—A class-M fundamental physics mission proposal for
Cosmic Vision 2015-2025
Authors: Appourchaux, Thierry; Burston, Raymond; Chen, Yanbei; Cruise,
Michael; Dittus, Hansjörg; Foulon, Bernard; Gill, Patrick; Gizon,
Laurent; Klein, Hugh; Klioner, Sergei; Kopeikin, Sergei; Krüger, Hans;
Lämmerzahl, Claus; Lobo, Alberto; Luo, Xinlian; Margolis, Helen; Ni,
Wei-Tou; Patón, Antonio Pulido; Peng, Qiuhe; Peters, Achim; Rasel,
Ernst; Rüdiger, Albrecht; Samain, Étienne; Selig, Hanns; Shaul,
Diana; Sumner, Timothy; Theil, Stephan; Touboul, Pierre; Turyshev,
Slava; Wang, Haitao; Wang, Li; Wen, Linqing; Wicht, Andreas; Wu, Ji;
Zhang, Xiaomin; Zhao, Cheng
Bibcode: 2009ExA....23..491A
Altcode: 2008arXiv0802.0582A
ASTROD I is a planned interplanetary space mission with multiple
goals. The primary aims are: to test general relativity with
an improvement in sensitivity of over three orders of magnitude,
improving our understanding of gravity and aiding the development of
a new quantum gravity theory; to measure key solar system parameters
with increased accuracy, advancing solar physics and our knowledge
of the solar system; and to measure the time rate of change of the
gravitational constant with an order of magnitude improvement and
the anomalous Pioneer acceleration, thereby probing dark matter and
dark energy gravitationally. It is an international project, with
major contributions from Europe and China and is envisaged as the
first in a series of ASTROD missions. ASTROD I will consist of one
spacecraft carrying a telescope, four lasers, two event timers and a
clock. Two-way, two-wavelength laser pulse ranging will be used between
the spacecraft in a solar orbit and deep space laser stations on Earth,
to achieve the ASTROD I goals. A second mission, ASTROD (ASTROD II)
is envisaged as a three-spacecraft mission which would test General
Relativity to 1 ppb, enable detection of solar g-modes, measure the
solar Lense-Thirring effect to 10 ppm, and probe gravitational waves
at frequencies below the LISA bandwidth. In the third phase (ASTROD
III or Super-ASTROD), larger orbits could be implemented to map the
outer solar system and to probe primordial gravitational-waves at
frequencies below the ASTROD II bandwidth.
Title: Helioseismology of Sunspots: A Case Study of NOAA Region 9787
Authors: Gizon, L.; Schunker, H.; Baldner, C. S.; Basu, S.; Birch,
A. C.; Bogart, R. S.; Braun, D. C.; Cameron, R.; Duvall, T. L.;
Hanasoge, S. M.; Jackiewicz, J.; Roth, M.; Stahn, T.; Thompson, M. J.;
Zharkov, S.
Bibcode: 2009odsm.book..249G
Altcode:
Various methods of helioseismology are used to study the subsurface
properties of the sunspot in NOAA Active Region 9787. This sunspot
was chosen because it is axisymmetric, shows little evolution during
20-28 January 2002, and was observed continuously by the MDI/SOHO
instrument. AR 9787 is visible on helioseismic maps of the farside of
the Sun from 15 January, i.e. days before it crossed the East limb.
Title: Commission 12: Solar Radiation and Structure
Authors: Martínez Pillet, Valentin; Kosovichev, Alexander;
Mariska, John T.; Bogdan, Thomas J.; Asplund, Martin; Cauzzi, Gianna;
Christensen-Dalsgaard, Jørgen; Cram, Lawrence E.; Gan, Weiqun; Gizon,
Laurent; Heinzl, Petr; Rovira, Marta G.; Venkatakrishnan, P.
Bibcode: 2009IAUTA..27..104M
Altcode:
Commission 12 encompasses investigations on the internal structure
and dynamics of the Sun, mostly accessible through the techniques of
local and global helioseismology, the quiet solar atmosphere, solar
radiation and its variability, and the nature of relatively stable
magnetic structures like sunspots, faculae and the magnetic network. A
revision of the progress made in these fields is presented. For some
specific topics, the review has counted with the help of experts
outside the Commission Organizing Committee that are leading and/or
have recently presented relevant works in the respective fields. In
this cases the contributor's name is given in parenthesis.
Title: German Data Center for the Solar Dynamics Observatory:
A model for the PLATO mission?
Authors: Burston, R.; Gizon, L.; Saidi, Y.; Solanki, S. K.
Bibcode: 2008CoAst.157..293B
Altcode:
The German Data Center for the Solar Dynamics Observatory
(GDC-SDO), hosted by the Max Planck Institute for Solar System
Research in Germany, will provide access to SDO data for the
German solar physics community. The GDC-SDO will make available
all the relevant Helioseismic and Magnetic Imager (HMI) data
for helioseismology and smaller se- lected Atmospheric Imaging
Assembly (AIA) data sets. This project commenced in August 2007
and is funded by the German Aerospace Center (Deutsches zentrum
fuer Luft- und Raumfahrt or DLR) until December 2012. An important
component of the GDC-SDO is the Data Record Management System (DRMS),
developed in collaboration with the Stan- ford/Lockheed Joint Science
Operations Center (JSOC). The PEGASUS workflow manage- ment system
will be used to implement GDC-SDO data analysis pipelines. This
makes use of the CONDOR High Throughput Computing Project for
optimal job scheduling and also the GLOBUS Toolkit to enable grid
technologies. Additional information about the GDC-SDO can be found at
http://www.mps.mpg.de/projects/seismo/GDC1/index.html. Here, we sug-
gest a similar structure and philosophy should be ideal for the PLATO
mission, which looks for planetary transits and stellar oscillations
and is being studied by ESA for an M-Mission slot in Cosmic Vision.
Title: Fourier analysis of gapped time-series
Authors: Stahn, T.; Gizon, L.
Bibcode: 2008CoAst.157..369S
Altcode:
In asteroseismology, gaps in the time series complicate the data
analysis and hamper the precise measurement of stellar oscillation
parameters, e.g. the frequencies, amplitudes, phases, and mode
lifetimes. In the Fourier domain the convolution of the stellar signal
with the Fourier transform of the temporal window function introduce
data correlations between the different frequencies. We developed a
method to derive Maximum Likelihood Estimates (MLE) of mode parameters
where these data correlations are explicitly taken into account. Using
simulated realisations of noisy time series with gaps, the MLE of
the mode parameters of solar-like oscillations obtained with our new
fitting method are more precise and less biased than the MLE determined
based on the unfounded assumption of uncorrelated frequency bins.
Title: HELAS Local Helioseismology Activities
Authors: Schunker, H.; Gizon, L.
Bibcode: 2008CoAst.156...93S
Altcode:
The main goals of the HELAS local helioseismology network activity are
to consolidate this field of research in Europe, to organise scientific
workshops, and to facilitate the distribution of observations and
data analysis software. Most of this is currently accomplished via a
dedicated website at http://www.mps.mpg.de/projects/seismo/NA4/. In
this paper we list the outreach material, observational data, analysis
tools and modelling tools currently available from the website and
describe the focus of the scientific workshops and their proceedings.
Title: Detecting solar g modes with ASTROD
Authors: Burston, R.; Gizon, L.; Appourchaux, T.; Ni, W. -T.; ASTROD
I ESA cosmic vision 2015-2025 Team
Bibcode: 2008JPhCS.118a2043B
Altcode: 2008arXiv0802.1614B
We present an up-to-date estimate for the prospect of using the
Astrodynamical Space Test of Relativity using Optical Devices (ASTROD)
[1, 2, 3, 4] for an unambiguous detection of solar g modes (f < 400
μHz) through their gravitational signature. There are currently two
major efforts to detect low-frequency gravitational effects, ASTROD and
the Laser Interferometer Space Antenna (LISA) [5]. Using the most recent
g mode surface amplitude estimates, both observational and theoretical,
it is unclear whether LISA will be capable of successfully detecting
these modes. The ASTROD project may be better suited for detection as
its sensitivity curve is shifted towards lower frequencies with the
best sensitivity occurring in the range 100 - 300 μHz.
Title: HELAS: local helioseismology data website
Authors: Schunker, Hannah; Gizon, Laurent; Roth, Markus
Bibcode: 2008JPhCS.118a2087S
Altcode: 2008arXiv0810.3414S
The Local Helioseismology Network Activity is part of the
European Helio-and Asteroseismology Network (HELAS). One
aspect of the network activity is to collate multipurpose
data sets and make them available to the community for local
helioseismic analysis. The first stage of the project is underway
whereby high quality and useful data sets have been selected and
acquired. The HELAS Local Helioseismology Network Activity website at http://www.mps.mpg.de/projects/seismo/NA4/
provides this data ready to download. Furthermore, the data is
supplemented with relevant documentation necessary for further analysis,
including details about the data reduction process that has already been
applied. The data primarily consists of Doppler velocity observations
but also includes observations of the line-of-sight magnetic field,
vector magnetic field measurements, intensity and travel time maps. The
website will be continuously updated with data thereby providing
convenient access to comprehensive data sets appropriate for use in
local helioseismology.
Title: The forward and inverse problems in time-distance
helioseismology
Authors: Jackiewicz, Jason; Gizon, Laurent; Birch, Aaron C.
Bibcode: 2008JPhCS.118a2033J
Altcode:
Time-distance helioseismology is a set of tools for peering into the
solar interior. In this paper we discuss and provide examples of the
steps that go into current high-resolution time-distance helioseismic
analyses. These steps include observations (cross covariances, travel
times), modeling of the seismic wavefield for a weakly inhomogeneous
solar model, and inversion of the travel times. The discussion
is framed in the context of studying quiet-Sun flows, although
the extension to other solar perturbations is straightforward and
analogous. The two-plus-one-dimensional (2+1D) inversion procedure
implemented here produces maps of vector flows in the near-surface
layers of the photosphere. We examine the flows obtained by
compromising, or 'trading off', between different observation times,
spatial resolutions, and noise levels. Also studied is the correlation
of the flows at different depths and over different time intervals.
Title: Solar Interior and Helioseismology
Authors: Gizon, L.
Bibcode: 2008ESPM...12..2.1G
Altcode:
The Sun vibrates in many different modes of acoustic oscillations
excited by convection. These modes are sensitive to the internal
properties of the Sun and thus inferences can be drawn about the
interior from surface observations of wave motions. Helioseismology is
a powerful tool to test and refine the physics of stellar interiors,
but also to search for clues regarding the origin and variability
of the Sun's magnetic field, possibly the most important unsolved
problem in solar physics. A selected overview of recent results
in helioseismology will be presented. Particular emphasis will be
placed on how to observe and model the effect of magnetic structures,
and active regions in particular, on wave propagation. According to
theory, the correlations in the seismic wave field contain all the
information needed to estimate the Green's function between two spatial
locations. This will be demonstrated using MDI/SOHO observations as
well as numerical simulations of small-amplitude MHD waves. A general
discussion of the helioseismological inverse problem will follow. This presentation would not be complete without mentioning the
Helioseismic and Magnetic Imager (HMI), an experiment to be launched
at the beginning of 2009 onboard the Solar Dynamics Observatory
(SDO). HMI/SDO will continuously provide full-disk Doppler images at
high spatial resolution, and will lead to important advances in local
helioseismology. While SDO is a significant technological step beyond
SOHO, the solar scientific community is to be inundated with a flood
of large-volume data. The establishment of the DLR German Data Center
for SDO will provide an advanced data management system to effectively
deliver rapid and simple access to users and developers in Germany and,
hopefully, throughout Europe.
Title: Helioseismology of Sunspots: Confronting Observations with
Three-Dimensional MHD Simulations of Wave Propagation
Authors: Cameron, R.; Gizon, L.; Duvall, T. L., Jr.
Bibcode: 2008SoPh..251..291C
Altcode: 2008arXiv0802.1603C; 2008SoPh..tmp...51C
The propagation of solar waves through the sunspot of AR 9787
is observed by using temporal cross-correlations of SOHO/MDI
Dopplergrams. We then use three-dimensional MHD numerical simulations
to compute the propagation of wave packets through self-similar
magnetohydrostatic sunspot models. The simulations are set up in
such a way as to allow a comparison with observed cross-covariances
(except in the immediate vicinity of the sunspot). We find that the
simulation and the f-mode observations are in good agreement when the
model sunspot has a peak field strength of 3 kG at the photosphere
and less so for lower field strengths. Constraining the sunspot model
with helioseismology is only possible because the direct effect of
the magnetic field on the waves has been fully taken into account. Our
work shows that the full-waveform modeling of sunspots is feasible.
Title: Observation and Modeling of the Solar-Cycle Variation of the
Meridional Flow
Authors: Gizon, Laurent; Rempel, Matthias
Bibcode: 2008SoPh..251..241G
Altcode: 2008arXiv0803.0950G; 2008SoPh..tmp...58G
We present independent observations of the solar-cycle variation
of flows near the solar surface and at a depth of about 60 Mm,
in the latitude range ± 45°. We show that the time-varying
components of the meridional flow at these two depths have opposite
sign, whereas the time-varying components of the zonal flow are in
phase. This is in agreement with previous results. We then investigate
whether the observations are consistent with a theoretical model of
solar-cycle-dependent meridional circulation based on a flux-transport
dynamo combined with a geostrophic flow caused by increased radiative
loss in the active region belt (the only existing quantitative
model). We find that the model and the data are in qualitative
agreement, although the amplitude of the solar-cycle variation of the
meridional flow at 60 Mm is underestimated by the model.
Title: High-Resolution Mapping of Flows in the Solar Interior:
Fully Consistent OLA Inversion of Helioseismic Travel Times
Authors: Jackiewicz, J.; Gizon, L.; Birch, A. C.
Bibcode: 2008SoPh..251..381J
Altcode: 2008arXiv0802.3810J; 2008SoPh..tmp...66J
To recover the flow information encoded in travel-time data of time
- distance helioseismology, accurate forward modeling and a robust
inversion of the travel times are required. We accomplish this using
three-dimensional finite-frequency travel-time sensitivity kernels
for flows along with a (2+1)-dimensional (2+1D) optimally localized
averaging (OLA) inversion scheme. Travel times are measured by
ridge filtering MDI full-disk Doppler data and the corresponding Born
sensitivity kernels are computed for these particular travel times. We
also utilize the full noise-covariance properties of the travel times,
which allow us to accurately estimate the errors for all inversions. The
whole procedure is thus fully consistent. Because of ridge filtering,
the kernel functions separate in the horizontal and vertical directions,
motivating our choice of a 2+1D inversion implementation. The inversion
procedure also minimizes cross-talk effects among the three flow
components, and the averaging kernels resulting from the inversion show
very small amounts of cross-talk. We obtain three-dimensional maps of
vector solar flows in the quiet Sun at horizontal spatial resolutions
of 7−10 Mm using generally 24 hours of data. For all of the flow
maps we provide averaging kernels and the noise estimates. We present
examples to test the inferred flows, such as a comparison with Doppler
data, in which we find a correlation of 0.9. We also present results
for quiet-Sun supergranular flows at different depths in the upper
convection zone. Our estimation of the vertical velocity shows good
qualitative agreement with the horizontal vector flows. We also show
vertical flows measured solely from f-mode travel times. In addition,
we demonstrate how to directly invert for the horizontal divergence
and flow vorticity. Finally we study inferred flow-map correlations
at different depths and find a rapid decrease in this correlation with
depth, consistent with other recent local helioseismic analyses.
Title: Fourier Analysis of Gapped Time Series: Improved Estimates
of Solar and Stellar Oscillation Parameters
Authors: Stahn, Thorsten; Gizon, Laurent
Bibcode: 2008SoPh..251...31S
Altcode: 2008arXiv0803.2261S; 2008SoPh..tmp...83S
Quantitative helioseismology and asteroseismology require very
precise measurements of the frequencies, amplitudes, and lifetimes
of the global modes of stellar oscillation. The precision of these
measurements depends on the total length (T), quality, and completeness
of the observations. Except in a few simple cases, the effect of
gaps in the data on measurement precision is poorly understood, in
particular in Fourier space where the convolution of the observable
with the observation window introduces correlations between different
frequencies. Here we describe and implement a rather general method to
retrieve maximum likelihood estimates of the oscillation parameters,
taking into account the proper statistics of the observations. Our
fitting method applies in complex Fourier space and exploits the phase
information. We consider both solar-like stochastic oscillations and
long-lived harmonic oscillations, plus random noise. Using numerical
simulations, we demonstrate the existence of cases for which our
improved fitting method is less biased and has a greater precision
than when the frequency correlations are ignored. This is especially
true of low signal-to-noise solar-like oscillations. For example, we
discuss a case where the precision of the mode frequency estimate is
increased by a factor of five, for a duty cycle of 15%. In the case of
long-lived sinusoidal oscillations, a proper treatment of the frequency
correlations does not provide any significant improvement; nevertheless,
we confirm that the mode frequency can be measured from gapped data
with a much better precision than the 1/T Rayleigh resolution.
Title: Structure and Evolution of Supergranulation from Local
Helioseismology
Authors: Hirzberger, Johann; Gizon, Laurent; Solanki, Sami K.; Duvall,
Thomas L.
Bibcode: 2008SoPh..251..417H
Altcode: 2008SoPh..tmp..106H
Supergranulation is visible at the solar surface as a cellular
pattern of horizontal outflows. Although it does not show a distinct
intensity pattern, it manifests itself indirectly in, for example,
the chromospheric network. Previous studies have reported significant
differences in the inferred basic parameters of the supergranulation
phenomenon. Here we study the structure and temporal evolution of a
large sample of supergranules, measured by using local helioseismology
and SOHO/MDI data from the year 2000 at solar activity minimum. Local
helioseismology with f modes provides maps of the horizontal divergence
of the flow velocity at a depth of about 1 Mm. From these divergence
maps supergranular cells were identified by using Fourier segmentation
procedures in two dimensions and in three dimensions (two spatial
dimensions plus time). The maps that we analyzed contain more than
105 supergranular cells and more than 103
lifetime histories, which makes possible a detailed analysis with high
statistical significance. We find that the supergranular cells have
a mean diameter of 27.1 Mm. The mean lifetime is estimated to be 1.6
days from the measured distribution of lifetimes (three-dimensional
segmentation), with a clear tendency for larger cells to live longer
than smaller ones. The pair and mark correlation functions do not
show pronounced features on scales larger than the typical cell size,
which suggests purely random cell positions. The temporal histories of
supergranular cells indicate a smooth evolution from their emergence
and growth in the first half of their lives to their decay in the
second half of their lives (unlike exploding granules, which reach
their maximum size just before they fragment).
Title: The seismic effects of a sunspot
Authors: Schunker, H.; Cameron, R.; Gizon, L.
Bibcode: 2008ESPM...12..3.5S
Altcode:
We simulate the helioseismic wave field by using the three-dimensional
Semi-spectral Linear MHD (SLiM) code and exciting small-amplitude waves
by sources distributed in the near-surface layers of a model solar
atmosphere.Our model atmosphere is realistic in the sense that it has
a standard sound-speed profile. Our source function is a realization
drawn from a random process specified by a statistical description of
solar convection. We obtain a quiet-Sun power spectrum of wave motions,
which is consistent with Doppler observations. In order to study wave
propagation through sunspots, we derive a simplified monolithic model
sunspot embedded in the quiet-Sun model atmosphere. The corresponding
wave field computed with SLiM is then compared with MDI observations
of f- and p-mode scattering by magnetic region AR9787. The comparison
is encouraging as the numerical simulation is able to reproduce wave
absorption and scattering phase shifts. As part of our analysis, we
show the advantage of computing a reference quiet-Sun wave field using
the same realization of the sources for the purpose of comparisons
and noise reduction.
Title: Preface
Authors: Gizon, Laurent; Cally, Paul; Leibacher, John
Bibcode: 2008SoPh..251....1G
Altcode: 2008SoPh..tmp..148G
No abstract at ADS
Title: FOREWORD: HELAS II International Conference
Authors: Gizon, Laurent; Roth, Markus
Bibcode: 2008JPhCS.118a1001G
Altcode:
Volume 118 (2008) of Journal of Physics: Conference Series provides
a written record of the talks and posters presented at the HELAS II
International Conference `Helioseismology, Asteroseismology and MHD
Connections'. The conference was held during the week 20-24 August 2007
in Göttingen, Germany, jointly hosted by the Max Planck Institute for
Solar System Research and the Faculty of Physics of the University
of Göttingen. A total of 140 scientists from all over the world
attended. The Scientific Organizing Committee consisted of Conny
Aerts, Annie Baglin, Jørgen Christensen-Dalsgaard, Thierry Corbard,
Jadwiga Daszyńska-Daszkiewicz, Stefan Dreizler, Yvonne Elsworth,
Laurent Gizon (Chairman), Wolfgang Glatzel, Frank Hill, Donald
Kurtz, Oskar von der Lühe, Maria Pia Di Mauro, Mário Monteiro,
Pere Pallé, Markus Roth, Philip Scherrer, Manfred Schüssler,
and Michael Thompson. HELAS stands for the European Helio- and
Asteroseismology Network, a Coordination Action supported by the sixth
Framework Programme of the European Union. It aims to bring together
researchers in the fields of solar and stellar oscillations. This
volume consists of 91 articles organized into sections that reflect the
scientific programme of the conference: 012001-07 Wave diagnostics
in physics, geophysics and astrophysics 012008-09 Perspectives on helio-
and asteroseismology 012010-17 Asteroseismology: Observations 012018-25
Asteroseismology: Theory 012026-32 Global helioseismology and solar
models 012033-38 Local helioseismology and magnetic activity 012039-44
Future observational projects in helio- and asteroseismology 012045-91
Poster papers. The overwhelming majority of papers discuss the
seismology of the Sun and stars. Papers in the first section provide a
broader perspective on wave phenomena and techniques for probing other
physical systems, from living beings to the universe as a whole. We
were extremely fortunate to have particularly distinguished experts
to cover these topics. Also available in the online edition are
(i) an interactive conference picture, (ii) the abstract book, and
(iii) material on the special session `Waves, Waves and Waves'. Additional articles related to both the HELAS II and the SOHO
19/GONG 2007 conferences can be found in a topical issue of Solar
Physics, volume 251, nos 1-2. Financial support was provided by
the HELAS Network, the Max Planck Institute for Solar System Research
(through Ulrich Christensen and Sami Solanki) and the University of
Göttingen (through Stefan Dreizler). We thank the local organizers,
and in particular Sabine Deutsch, for their outstanding efforts
in making the conference a success. We are also grateful to Graham
Douglas and Jacky Mucklow of IoP Publishing for their help in the
production of this volume. Laurent Gizon and Markus Roth Editors
Katlenburg-Lindau, Germany
Title: f-Mode Interactions with Thin Flux Tubes: The Scattering Matrix
Authors: Hanasoge, S. M.; Birch, A. C.; Bogdan, T. J.; Gizon, L.
Bibcode: 2008ApJ...680..774H
Altcode: 2007arXiv0711.2076H
We calculate the scattering effects associated with the interaction of
a surface gravity or f-mode with a thin magnetic flux tube embedded in
a realistically stratified medium. We find that the dominant scattered
wave is an f-mode with amplitude and phase of 1.17% and around 50°
relative to the incident wave, compared to the values of 0.13% and 40°
estimated from observations. The extent of scattering into high-order
acoustic p-modes is too weak to be accurately characterized. We
recover the result that the degree of scattering is enhanced as (1)
the frequency of the incident wave increases and (2) the flux tube
becomes magnetically dominated.
Title: Time-Distance Helioseismology: Sensitivity of f-mode Travel
Times to Flows
Authors: Jackiewicz, J.; Gizon, L.; Birch, A. C.; Duvall, T. L., Jr.
Bibcode: 2007ApJ...671.1051J
Altcode: 2007arXiv0708.3554J
Time-distance helioseismology has shown that f-mode travel times
contain information about horizontal flows in the Sun. The purpose
of this study is to provide a simple interpretation of these travel
times. We study the interaction of surface gravity waves with horizontal
flows in an incompressible, plane-parallel solar atmosphere. We show
that for uniform flows less than roughly 250 m s-1, the
travel-time shifts are linear in the flow amplitude. For stronger flows,
perturbation theory up to third order is needed to model waveforms. The
case of small-amplitude spatially varying flows is treated using the
first-order Born approximation. We derive two-dimensional Fréchet
kernels that give the sensitivity of travel-time shifts to local
flows. We show that the effect of flows on travel times depends
on wave damping and on the direction from which the observations
are made. The main physical effect is the advection of the waves by
the flow rather than the advection of wave sources or the effect of
flows on wave damping. We compare the two-dimensional sensitivity
kernels with simplified three-dimensional kernels that only account
for wave advection and assume a vertical line of sight. We find that
the three-dimensional f-mode kernels approximately separate in the
horizontal and vertical coordinates, with the horizontal variations
given by the simplified two-dimensional kernels. This consistency
between quite different models gives us confidence in the usefulness
of these kernels for interpreting quiet-Sun observations.
Title: Joint Discussion 17 Highlights of recent progress in the
seismology of the Sun and Sun-like stars
Authors: Bedding, Timothy R.; Brun, Allan S.; Christensen-Dalsgaard,
Jørgen; Crouch, Ashley; De Cat, Peter; García, Raphael A.; Gizon,
Laurent; Hill, Frank; Kjeldsen, Hans; Leibacher, John W.; Maillard,
Jean-Pierre; Mathis, S.; Rabello-Soares, M. Cristina; Rozelot,
Jean-Pierre; Rempel, Matthias; Roxburgh, Ian W.; Samadi, Réza; Talon,
Suzanne; Thompson, Michael J.
Bibcode: 2007HiA....14..491B
Altcode:
The seismology and physics of localized structures beneath the surface
of the Sun takes on a special significance with the completion in
2006 of a solar cycle of observations by the ground-based Global
Oscillation Network Group (GONG) and by the instruments on board the
Solar and Heliospheric Observatory (SOHO). Of course, the spatially
unresolved Birmingham Solar Oscillation Network (BiSON) has been
observing for even longer. At the same time, the testing of models of
stellar structure moves into high gear with the extension of deep probes
from the Sun to other solar-like stars and other multi-mode pulsators,
with ever-improving observations made from the ground, the success of
the MOST satellite, and the recently launched CoRoT satellite. Here
we report the current state of the two closely related and rapidly
developing fields of helio- and asteroseimology.
Title: The Linear Sensitivity of Helioseismic Ring Diagrams to
Local Flows
Authors: Birch, A. C.; Gizon, L.; Hindman, B. W.; Haber, D. A.
Bibcode: 2007ApJ...662..730B
Altcode:
Ring-diagram analysis is a technique of local helioseismology used
to infer plasma flows in the solar convection zone which generates
intermediate data products known as ring-fitting parameters. Knowing
the sensitivity of ring-fitting parameters to actual flows in the Sun is
important for interpreting these measurements. Working in plane-parallel
geometry, we compute the linear sensitivity of ring-fitting parameters
to small changes in the local power spectrum and then compute the
sensitivity of the power spectrum to time-independent weak local
flows. We combine these two results to obtain the three-dimensional
Frechet kernels that give the linear sensitivity of ring-fitting
parameters to both vertical and horizontal local mass flows. We
find that ring measurements are essentially only sensitive to flows
that are within the spatial region for which the ring diagram is
computed. In addition, we find that the depth dependence of the
sensitivity is essentially given by the mode kinetic energy density,
as has traditionally been assumed. We show that the exact form of
the sensitivity of ring measurements depends on the details of the
fitting procedure.
Title: SLiM: a code for the simulation of wave propagation through
an inhomogeneous, magnetised solar atmosphere
Authors: Cameron, R.; Gizon, L.; Daiffallah, K.
Bibcode: 2007AN....328..313C
Altcode: 2010arXiv1002.2344C
In this paper we describe the semi-spectral linear MHD (SLiM) code
which we have written to follow the interaction of linear waves through
an inhomogeneous three-dimensional solar atmosphere. The background
model allows almost arbitrary perturbations of density, temperature,
sound speed as well as magnetic and velocity fields. We give details of
several of the tests we have used to check the code. The code will be
useful in understanding the helioseismic signatures of various solar
features, including sunspots.
Title: Linear sensitivity of helioseismic travel times to local flows
Authors: Birch, A. C.; Gizon, L.
Bibcode: 2007AN....328..228B
Altcode: 2010arXiv1002.2338B
Time-distance helioseismology is a technique for measuring the
time for waves to travel from one point on the solar surface
to another. These wave travel times are affected by advection
by subsurface flows. Inferences of plasma flows based on observed
travel times depend critically on the ability to accurately model the
effects of subsurface flows on time-distance measurements. We present a
Born-approximation based computation of the sensitivity of time-distance
travel times to weak, steady, inhomogeneous subsurface flows. Three
sensitivity functions are obtained, one for each component of the
3D vector flow. We show that the depth sensitivity of travel times
to horizontally uniform flows is given approximately by the kinetic
energy density of the oscillation modes which contribute to the travel
times. For flows with strong depth dependence, the Born approximation
can give substantially different results than the ray approximation.
Title: A procedure for the inversion of f-mode travel times for
solar flows
Authors: Jackiewicz, J.; Gizon, L.; Birch, A. C.; Thompson, M. J.
Bibcode: 2007AN....328..234J
Altcode: 2007astro.ph..2345J
We perform a two-dimensional inversion of f-mode travel times
to determine near-surface solar flows. The inversion is based on
optimally localized averaging of travel times. We use finite-wavelength
travel-time sensitivity functions and a realistic model of the data
errors. We find that it is possible to obtain a spatial resolution
of 2 Mm. The error in the resulting flow estimate ultimately depends
on the observation time and the number of travel distances used in
the inversion.
Title: The Solar Orbiter mission and its prospects for helioseismology
Authors: Woch, J.; Gizon, L.
Bibcode: 2007AN....328..362W
Altcode: 2010arXiv1002.2278W
Solar Orbiter is intended to become ESA's next solar mission in heritage
of the successful SOHO project. The scientific objectives of the
mission, its design, and its scientific payload are reviewed. Specific
emphasis is given to the prospects of Solar Orbiter with respect to
helioseismology.
Title: Outstanding problems in local helioseismology
Authors: Gizon, L.; Thompson, M. J.
Bibcode: 2007AN....328..204G
Altcode: 2010arXiv1002.2347G
Time-distance helioseismology and related techniques show great promise
for probing the structure and dynamics of the subphotospheric layers
of the Sun. Indeed time-distance helioseismology has already been
applied to make inferences about structures and flows under sunspots
and active regions, to map long-lived convective flow patterns, and
so on. Yet certainly there are still many inadequacies in the current
approaches and, as the data get better and the questions we seek to
address get more subtle, methods that were previously regarded as
adequate are no longer acceptable. Here we give a short and partial
description of outstanding problems in local helioseismology, using
time-distance helioseismology as a guiding example.
Title: Measuring helioseismic travel times
Authors: Roth, M.; Gizon, L.; Beck, J. G.
Bibcode: 2007AN....328..215R
Altcode:
In time-distance helioseismology wave travel times are measured
from the cross-correlation between Doppler velocities recorded at
any two locations on the solar surface. We compare two different
methods to extract the travel times from the noisy cross-correlation
functions. The first method consists of fitting a 5-parameter analytic
function to the cross-correlation to obtain the phase travel time. The
second method consists of linearizing the distance between the observed
cross-correlation and a sliding reference cross-correlation (the only
parameter is the travel time). We find that the one-parameter fits
are more robust with respect to noise. Using SOHO data from the MDI
Structure Program for the years 1996-2003, we study in detail the
statistical properties of the noise associated with the travel-time
measurements for the two different fitting methods.
Title: Structure and evolution of supergranulation from local
helioseismology
Authors: Hirzberger, J.; Gizon, L.; Solanki, S. K.; Duvall, T. L.
Bibcode: 2007msfa.conf..103H
Altcode:
Maps of the horizontal divergence of the near-surface velocity field
have been calculated using local helioseismology and SOHO/MDI full-disk
Dopplergrams. These maps provide a continuous coverage for two to
three months each year with a cadence of 12 hours. Geometrical and
evolutional properties of individual supergranular cells have been
studied. Supergranular cells have sizes in a range around 650Mm2
(circular diameter of 28.77 Mm) with lifetimes of up to 4.5 days. We
also observe a clear trend for larger cells to have stronger divergence
values and larger lifetimes than smaller ones.
Title: Helioseismology at MPS
Authors: Gizon, L.; Cameron, R.; Jackiewicz, J.; Roth, M.; Schunker,
H.; Stahn, T.
Bibcode: 2007msfa.conf...89G
Altcode:
Research in solar and stellar seismology at the Max Planck Institute
for Solar System Research (MPS) is supported by the Junior Research
Group "Helio- and Asteroseismology" of the Max Planck Society since
September 2005. A presentation of the current topics of research is
given, with particular emphasis on local helioseismology.
Title: Helioseismology With Solar Orbiter: Science Objectives,
Observational Strategies And Requirements
Authors: Gizon, L.
Bibcode: 2007ESASP.641E..26G
Altcode: 2006ESASP.641E..26G
Solar Orbiter will offer novel perspectives for helioseismology. The
most interesting aspects of the mission reside in the unique vantage
points from which the Sun will be viewed. Not only will out-of-
the-ecliptic observations enable us to reach higher heliographic
latitudes into the solar convection zone but Solar Orbiter in
combination with Earth-side observations will also mean the advent of
stereoscopic helioseismology. The science objectives, observational
strategies and science requirements are discussed.
Title: f-mode sensitivity kernels for flows
Authors: Jackiewicz, J.; Gizon, L.; Birch, A.
Bibcode: 2006ESASP.624E..52J
Altcode: 2006soho...18E..52J; 2006astro.ph.12475J
We compute f-mode sensitivity kernels for flows. Using a two-dimensional
model, the scattered wavefield is calculated in the first Born
approximation. We test the correctness of the kernels by comparing an
exact solution (constant flow), a solution linearized in the flow, and
the total integral of the kernel. In practice, the linear approximation
is acceptable for flows as large as about 400 m/s.
Title: Time-varying component of the solar meridional flow
Authors: Gizon, L.; Rempel, M.
Bibcode: 2006ESASP.624E.129G
Altcode: 2006soho...18E.129G
No abstract at ADS
Title: European helio- and asteroseismology network HELAS
Authors: Roth, M.; Luhe, O. v. d.; Palle, P.; Thompson, M. J.;
Christensen-Dalsgaard, J.; Monteiro, M. J. P. F. G.; Gizon, L.;
Di Mauro, M. P.; Aerts, C.; Daszynska-Daszkiewicz, J.; Corbard, T.
Bibcode: 2006ESASP.624E.130R
Altcode: 2006soho...18E.130R
The Helio- and Asteroseismology Network (HELAS) is a Coordinated
Action funded by the FP6-Infrastructure-Programme of the European
Commission. Currently, HELAS consists of ten members. The objective
of HELAS is to co-ordinate European activities in helio- and
asteroseismology. HELAS will transfer knowledge and data analysis
techniques, and will prepare the European research community for
important missions in the immediate future.
Title: Sensitivity kernels for helioseismic travel times in spherical
geometry preliminary results
Authors: Roth, M.; Gizon, L.; Birch, A. C.
Bibcode: 2006ESASP.624E..43R
Altcode: 2006soho...18E..43R
No abstract at ADS
Title: Acoustic scattering by flux tubes: is the Born approximation
valid?
Authors: Gizon, L.; Hanasoge, S. M.; Birch, A. C.
Bibcode: 2006ESASP.624E..44G
Altcode: 2006soho...18E..44G
No abstract at ADS
Title: Three-dimensional numerical simulation of wave propagation
through a model sunspot
Authors: Cameron, R.; Gizon, L.
Bibcode: 2006ESASP.624E..63C
Altcode: 2006soho...18E..63C
No abstract at ADS
Title: Introductory - Overview: Local helioseismology
Authors: Gizon, L.
Bibcode: 2006IAUJD..17E..11G
Altcode:
Local helioseismology offers three-dimensional views of the Sun. Thanks
to high-resolution, uninterrupted observations from SOHO, it has become
possible to image convective structures, sunspots, and active regions
below the solar surface. I will summarize some of the recent advances
in this field and discuss the inverse problem with applications to
time-distance and ring-diagram analyses. I shall also mention recent
efforts for collaboration in Europe through the HELAS Coordinated
Action of the European Union.
Title: Probing Convection and Solar Activity with Local
Helioseismology
Authors: Gizon, L.
Bibcode: 2006ESASP.617E...5G
Altcode: 2006soho...17E...5G
No abstract at ADS
Title: Helas-European Helio- and Asteroseismology Network
Authors: Roth, M.; Lühe, O. v. d.; Pallé, P.; Thompson, M.;
Christensen-Dalsgaard, J.; Monteiro, M. J. P. F. G; Gizon, L.; Di
Mauro, M. P.; Aerts, C.; Daszynska-Daszkiewicz, J.; Corbard, T.
Bibcode: 2006ESASP.617E.157R
Altcode: 2006soho...17E.157R
No abstract at ADS
Title: Direct Measurement of Travel-Time Kernels for Helioseismology
Authors: Duvall, T. L., Jr.; Birch, A. C.; Gizon, L.
Bibcode: 2006ApJ...646..553D
Altcode:
Solar f-modes are surface gravity waves that propagate horizontally in a
thin layer near the photosphere with a dispersion relation approximately
that of deep water waves. At the power maximum near frequency ω/2π=3
mHz, the wavelength of 5 Mm is large enough for various wave scattering
properties to be observable. Gizon & Birch have calculated spatial
kernels for scattering in the Born approximation. In this paper, using
isolated small magnetic features as approximate point scatterers, a
linear-response kernel has been measured. In addition, the kernel has
been estimated by deconvolving the magnetograms from the travel-time
maps. The observed kernel is similar to the theoretical kernel for
wave damping computed by Gizon & Birch: it includes elliptical
and hyperbolic features. This is the first observational evidence
to suggest that it is appropriate to use the Born approximation to
compute kernels (as opposed to the ray approximation). Furthermore, the
observed hyperbolic features confirm that it is important to take into
account scattering of the waves coming from distant source locations (as
opposed to the single-source approximation). The observed kernel is due
to a superposition of the direct and indirect effects of the magnetic
field. A simple model that includes both monopole and dipole scattering
compares favorably with the data. This new technique appears to be
promising to study how seismic waves interact with magnetic flux tubes.
Title: Sensitivity of Solar F-Mode Travel Times to Internal Flows
Authors: Jackiewicz, J.; Gizon, L.; Birch, A. C.
Bibcode: 2006ESASP.617E..38J
Altcode: 2006soho...17E..38J; 2006astro.ph..8604J
We compute f-mode travel-time sensitivity kernels for flows. Using
a two-dimensional model, we show that it is important to account
for several systematic effects, such as the foreshortening and the
projection of the velocity vector onto the line of sight. Correcting
for these effects is necessary before any data inversion is attempted
away from the center of the solar disk.
Title: Helioseismology of the "Average" Supergranule
Authors: Birch, Aaron; Duvall, T. L.; Gizon, L.; Jackiewicz, J.
Bibcode: 2006SPD....37.0505B
Altcode: 2006BAAS...38..224B
We show time-distance travel times averaged over roughly ten thousand
supergranules. The statistical (realization) noise in these measurements
is substantially smaller than the noise associated with a single
supergranule. By both forward modeling and inversions we determine the
range of subsurface flows that are compatible with these travel times.
Title: Scattering of Acoustic Waves by a Magnetic Cylinder: Accuracy
of the Born Approximation
Authors: Gizon, L.; Hanasoge, S. M.; Birch, A. C.
Bibcode: 2006ApJ...643..549G
Altcode: 2008arXiv0803.3839G
With the aim of studying magnetic effects in time-distance
helioseismology, we use the first-order Born approximation to compute
the scattering of acoustic plane waves by a magnetic cylinder embedded
in a uniform medium. We show, by comparison with the exact solution,
that the travel-time shifts computed in the Born approximation are
everywhere valid to first order in the ratio of the magnetic to the gas
pressures. We also show that for arbitrary magnetic field strength,
the Born approximation is not valid in the limit where the radius of
the magnetic cylinder tends to zero.
Title: Tomography of the Solar Interior
Authors: Gizon, L.
Bibcode: 2006MPLA...21.1701G
Altcode: 2008arXiv0803.3844G
Solar oscillations consist of a rich spectrum of internal acoustic
waves and surface gravity waves, stochastically excited by turbulent
convection. They have been monitored almost continuously over the last
ten years with high-precision Doppler images of the solar surface. The
purpose of helioseismology is to retrieve information about the
structure and the dynamics of the solar interior from the frequencies,
phases and amplitudes of solar waves. Methods of analysis are being
developed to make three-dimensional images of subsurface motions and
temperature inhomogeneities in order to study convective structures
and regions of magnetic activity, like sunspots.
Title: Line Profiles of Fundamental Modes of Solar Oscillation
Authors: Gizon, L.
Bibcode: 2006CEAB...30....1G
Altcode:
I present MDI-SOHO measurements of f-mode line profile asymmetry at
high spatial wave numbers. The f-mode line asymmetry is pronounced
in the degree range 600--1200 and has opposite signs in velocity and
intensity power spectra.
Title: Local Helioseismology
Authors: Gizon, Laurent; Birch, Aaron C.
Bibcode: 2005LRSP....2....6G
Altcode:
We review the current status of local helioseismology, covering both
theoretical and observational results. After a brief introduction
to solar oscillations and wave propagation through in-homogeneous
media, we describe the main techniques of local helioseismology:
Fourier-Hankel decomposition, ring-diagram analysis, time-distance
helioseismology, helioseismic holography, and direct modeling. We
discuss local helioseismology of large-scale flows, the solar-cycle
dependence of these flows, perturbations associated with regions of
magnetic activity, and solar supergranulation.
Title: Time-Distance Helioseismology: Inversion of Noisy Correlated
Data
Authors: Couvidat, S.; Gizon, L.; Birch, A. C.; Larsen, R. M.;
Kosovichev, A. G.
Bibcode: 2005ApJS..158..217C
Altcode:
In time-distance helioseismology most inversion procedures ignore the
correlations in the data errors. Here we simulate the travel-time
perturbations of wavepackets that result from known distributions
of sound speed inhomogeneities. The forward and inverse problems are
carried out using recently developed Born approximation sensitivity
kernels. A realistic solar noise component, with the correct
statistics, is added to the data. We then apply a three-dimensional
inversion procedure based on an improved multichannel deconvolution
algorithm that includes the full covariance matrix of the simulated
data and constrains the solution both in the vertical and horizontal
directions. The validation of the inversion is achieved through
comparison of the inferred sound speed distributions with the exact
solutions. We show that including the covariance matrix matters for
sound speed inhomogeneities varying on a length scale smaller than
the correlation length. We also find that the inversion procedure is
improved by adding horizontal regularization.
Title: Direct Measurement of Wave Kernels in Time-Distance
Helioseismology
Authors: Duvall, T. L.; Birch, A. C.; Gizon, L.
Bibcode: 2005AGUSMSP23C..02D
Altcode:
Solar f-mode waves are surface-gravity waves which propagate
horizontally in a thin layer near the photosphere with a dispersion
relation approximately that of deep water waves. At the power
maximum near 3 mHz, the wavelength of 5 Mm is large enough for
various wave scattering properties to be observable. Gizon and Birch
(2002,ApJ,571,966) have calculated kernels, in the Born approximation,
for the sensitivity of wave travel times to local changes in damping
rate and source strength. In this work, using isolated small magnetic
features as approximate point source scatterers, such a kernel has been
measured. The observed kernel contains similar features to a theoretical
damping kernel but not for a source kernel. A full understanding of
the effect of small magnetic features on the waves will require more
detailed modeling.
Title: Time-Distance Helioseismology: Impact of Phase-Speed Filters
on Travel-Time Measurements
Authors: Couvidat, S.; Birch, A. C.; Gizon, L.
Bibcode: 2005AGUSMSP11B..01C
Altcode:
In time-distance helioseismology most procedures to compute travel-time
maps use Gaussian phase-speed filters to select different parts of the
wave propagation diagram. The mean phase speed of the filter is usually
derived using ray theory and a solar model, but there is no obvious rule
for choosing the width of the filter. We study how the filter widths
impact travel-time measurements and show what widths give the best
signal-to-noise ratio for measurements of sound-speed perturbations.
Title: Do Supergranules Tend to Align in the North-South Direction?
Authors: Zhao, J.; Gizon, L.
Bibcode: 2005AGUSMSP24A..04Z
Altcode:
We investigate the recent claim by Lisle, Rast, and Toomre (2004, ApJ)
that supergranules tend to align preferentially in the north-south
direction. We repeat their analysis with an extensive set of maps of
the horizontal divergence of flow fields derived from time-distance
helioseismology. We construct temporal averages of the divergence maps
for different east-west tracking velocities (vx) and measure the rms
values in the east-west (σx) and north-south (σy) directions from
these maps. In agreement with Lisle et al. (2004) we find that, near
the equator, the ratio σx/σy is maximum (and greater than one) for a
tracking velocity 120 m/s above the Carrington velocity. In addition,
we find that σx/σy displays a second local maximum at a tracking
rate close to the Carrington velocity. We argue that the variations
of the ratio σx/σy as a function of vx is a direct consequence and
additional evidence of the wavelike power spectrum of supergranulation
observed by Gizon, Duvall, and Schou (2003, Nature). Thus, a north-south
alignment of supergranules is not the only explanation nor a necessary
condition for a ratio σx/σy greater than one. Additionally, we also
study the variations of the ratio σy/σx as a function of north-south
velocity shifts (vy), and find that the ratio σy/σx also has two
maxima of similar amplitude separated by approximately 120 m/s.
Title: Validation of 3d Helioseismic Inversions of Travel-Times
Through Simulations of Artificial Data with the Correct Noise
Statistics
Authors: Couvidat, S.; Gizon, L.; Birch, A. C.
Bibcode: 2004ESASP.559..384C
Altcode: 2004soho...14..384C
No abstract at ADS
Title: Time-Distance Helioseismology: Noise Estimation
Authors: Gizon, L.; Birch, A. C.
Bibcode: 2004ApJ...614..472G
Altcode:
As in global helioseismology, the dominant source of noise in
time-distance helioseismology measurements is realization noise
due to the stochastic nature of the excitation mechanism of solar
oscillations. Characterizing noise is important for the interpretation
and inversion of time-distance measurements. In this paper we introduce
a robust definition of travel time that can be applied to very noisy
data. We then derive a simple model for the full covariance matrix of
the travel-time measurements. This model depends only on the expectation
value of the filtered power spectrum and assumes that solar oscillations
are stationary and homogeneous on the solar surface. The validity of
the model is confirmed through comparison with SOHO MDI measurements
in a quiet-Sun region. We show that the correlation length of the
noise in the travel times is about half the dominant wavelength of the
filtered power spectrum. We also show that the signal-to-noise ratio
in quiet-Sun travel-time maps increases roughly as the square root of
the observation time and is at maximum for a distance near half the
length scale of supergranulation.
Title: Helioseismology of Time-Varying Flows Through The Solar Cycle
Authors: Gizon, Laurent
Bibcode: 2004SoPh..224..217G
Altcode: 2005SoPh..224..217G
Flows in the upper convection zone are measured by helioseismology
on a wide variety of scales. These include differential rotation
and meridional circulation, local flows around complexes of magnetic
activity and sunspots, and convective flows. The temporal evolution
of flows through cycle 23 reveals connections between mass motions
in the solar interior and the large-scale characteristics of the
magnetic cycle. Here I summarize the latest observations and their
implications. Observations from local helioseismology suggest that
subsurface flows around active regions introduce a solar-cycle variation
in the meridional circulation.
Title: Comparison of Solar Subsurface Flows Assessed by Ring and
Time-Distance Analyses
Authors: Hindman, Bradley W.; Gizon, Laurent; Duvall, Thomas L., Jr.;
Haber, Deborah A.; Toomre, Juri
Bibcode: 2004ApJ...613.1253H
Altcode:
The solar near-surface shear layer exhibits a rich medley of flows
that are now being measured by a variety of local helioseismic
techniques. We present comparisons of the horizontal flows obtained
with two of these techniques, ring and time-distance analyses, applied
to Michelson Doppler Imager (MDI) Dynamics Program data from the years
1998 and 1999. The ring analyses use the frequencies of both f and
p modes in inversions to obtain flows within the near-surface shear
layer as a function of depth. The f-mode time-distance analyses make
velocity inferences just beneath the photosphere. After degrading
the spatial resolution of the time-distance analyses to match the
coarser resolution of the ring analyses, we find that the flows deduced
with the two methods are remarkably similar, with common inflow and
outflow sites as well as agreement in flow direction. The flows from
ring and time-distance analyses are highly correlated with each other
(correlation coefficients ~0.8) direct correspondence of features
in the flows is largely realized in both the quiet-Sun and magnetic
active regions.
Title: Measuring Stellar Differential rotation with asteroseismology
Authors: Gizon, Laurent; Solanki, Sami K.
Bibcode: 2004SoPh..220..169G
Altcode:
The variation of rotation with latitude is poorly known on stars
other than the Sun. Several indirect techniques, photometric and
spectroscopic, have been used to search for departure from rigid
rotation for sufficiently fast rotators. Here we investigate the
possibility of measuring stellar differential rotation for solar-type
stars through asteroseismology. Rotationally split frequencies of
global oscillation provide information about rotation at different
latitudes depending on the azimuthal order, m, of the mode of
pulsation. We present a method to estimate differential rotation
based on the realization that the m = ±1 and m = ±2 components
of quadrupole oscillations can be observed simultaneously in
asteroseismology. Rotational frequency splittings can be inverted
to provide an estimate of the difference in stellar angular velocity
between the equator and 45° latitude. The precision of the method,
assessed through Monte Carlo simulations, depends on the value of the
mean rotation and on the inclination angle between the rotation axis
and the line of sight.
Title: Solar-cycle variations in the spectrum of supergranulation
Authors: Gizon, Laurent; Duvall, Thomas L.
Bibcode: 2004IAUS..223...41G
Altcode: 2005IAUS..223...41G
Using local helioseismology, we construct maps of the horizontal
divergence of the velocity field near the solar surface and study
the spectrum of solar supergranulation during the period from 1996
to 2002. Supergranulation oscillates and propagates like a wave
interference pattern. The variations of the oscillation frequency with
latitude and time are less than 5%. We find significant solar-cycle
variations in the lifetime and the anisotropic distribution of power. We
also measure the time-dependent zonal and meridional flows that advect
supergranules.
Title: Probing flows in the upper solar convection zone
Authors: Gizon, Laurent
Bibcode: 2003PhDT.........9G
Altcode:
In order to better understand the origin and variability of
stellar magnetic fields it is necessary to understand mass flows
inside stars. With time-distance helioseismology local flows can
be inferred in the Sun by measuring the time it takes for seismic
waves to propagate between any two points on the solar surface. This
dissertation contains new observations of solar plasma flows and
a model for the interpretation of time- distance data. It also
discusses the prospects for stellar seismology. First, we present
new observations of the solar velocity field in the upper convection
zone. Using surface-gravity waves, we discover that supergranulation
exhibits wave- like properties, undergoing oscillations with periods
of 6 9 days. This points to a mechanism involving traveling-wave
convection and explains the observations of anomalously fast rotation
of the supergranulation pattern. Near the solar surface we detect
a large-scale 50 m/s flow converging toward active regions. Deeper
inside the convection zone, we detect, using acoustic waves, bands of
slower and faster meridional motion with a period of eleven years. The
data for this study were obtained with SOHO/MDI. Second, we present
a new and physically motivated general framework for calculations
of the sensitivity of travel times to small local perturbations to a
solar model, taking into account the fact that the sources of solar
oscillations are spatially distributed. We employ the first Born
approximation to model scattering from local inhomogeneities and we
use a clear and practical definition of travel-time perturbation which
allows a connection between observations and theory. After developing
the general theory we compute the sensitivity of surface-gravity-wave
travel times to local perturbations in the wave excitation and damping
rates. We show that the simple single-source picture, employed in most
time-distance analyses, is not correct as it does not reproduce all of
the features seen in the distributed-source sensitivity kernels. Last,
we show that future observations of stellar pulsations will provide us
with the possibility of determining the angular velocity of a Sun-like
star and the inclination angle between the direction of the rotation
axis and the line of sight. Measuring the inclination angle is useful
to determine the true masses of extra-solar planets detected from the
radial velocity shifts of their central star.
Title: Determining the Inclination of the Rotation Axis of a
Sun-like Star
Authors: Gizon, L.; Solanki, S. K.
Bibcode: 2003ApJ...589.1009G
Altcode:
Asteroseismology provides us with the possibility of determining the
angle, i, between the direction of the rotation axis of a pulsating
Sun-like star and the line of sight. A knowledge of i is important
not just for obtaining improved stellar parameters, but also in order
to determine the true masses of extrasolar planets detected from the
radial velocity shifts of their central stars. By means of Monte Carlo
simulations, we estimate the precision of the measurement of i and
other stellar parameters. We find that the inclination angle can be
retrieved accurately when i>~30deg for stars that rotate
at least twice as fast as the Sun.
Title: Comparison of near-surface flows assessed by ring-diagram
and f-mode time-distance analyses
Authors: Hindman, Bradley; Gizon, Laurent; Haber, Deborah; Duval,
Thomas, Jr.; Toomre, Juri
Bibcode: 2003ESASP.517..299H
Altcode: 2003soho...12..299H
The near-surface shear layer exhibits a rich medley of flows that are
now being measured by time-distance and ring analysis techniques. We
present comparisons of the flows obtained with the two techniques
using SOI-MDI Dynamics Program data from the years 1998 and 1999. The
time-distance analyses utilize f-mode data without depth inversion. The
flows deduced with the two methods are remarkably similar, with common
inflow and outflow sites as well as agreement in the general flow
directions. The direct correspondence of features in the flows is
realized in both quiet and active regions.
Title: Supergranulation supports waves
Authors: Gizon, L.; Duvall, T. L., Jr.
Bibcode: 2003ESASP.517...43G
Altcode: 2003soho...12...43G
Supergranulation on the surface of the Sun is a pattern of horizontal
outflows with a distinct scale of 30 Mm and an apparent lifetime of 1
day, outlined by a network of small magnetic features. The dynamics
of the supergranulation is poorly understood and there is as yet no
explanation for the observation that the supergranular pattern appears
to rotate faster than the magnetic features. In this paper we show
that supergranulation undergoes oscillations and supports waves with
periods of 6-9 days. The nature of supergranulation appears to be
travelling-wave convection. Waves are predominantly prograde, which
explains the apparent superrotation of the pattern. We also show that
supergranular flows have a net kinetic helicity, which is negative in
the northern hemisphere.
Title: erratum: Wave-like properties of solar supergranulation
Authors: Gizon, L.; Duvall, T. L.; Schou, J.
Bibcode: 2003Natur.421..764G
Altcode:
No abstract at ADS
Title: Wave-like properties of solar supergranulation
Authors: Gizon, L.; Duvall, T. L.; Schou, J.
Bibcode: 2003Natur.421...43G
Altcode: 2002astro.ph..8343G
Supergranulation on the surface of the Sun is a pattern of horizontal
outflows, outlined by a network of small magnetic features, with a
distinct scale of 30 million metres and an apparent lifetime of one
day. It is generally believed that supergranulation corresponds to
a preferred `cellular' scale of thermal convection; rising magnetic
fields are dragged by the outflows and concentrated into `ropes' at
the `cell' boundaries. But as the convection zone is highly turbulent
and stratified, numerical modelling has proved to be difficult and
the dynamics remain poorly understood. Moreover, there is as yet no
explanation for the observation that the pattern appears to rotate
faster around the Sun than the magnetic features. Here we report
observations showing that supergranulation undergoes oscillations and
supports waves with periods of 6-9 days. The waves are predominantly
prograde, which explains the apparent super-rotation of the pattern. The
rotation of the plasma through which the pattern propagates is
consistent with the motion of the magnetic network.
Title: Asteroseismic Determination of a Star
Authors: Gizon, Laurent; Solanki, Sami K.
Bibcode: 2003IAUJD..12E..19G
Altcode:
Asteroseismology provides us with the possibility of determining the
angle i between the direction of the rotation axis of a pulsating
Sun-like star and the line of sight. A knowledge of i is important
not just for obtaining improved stellar parameters but also in order
to determine the true masses of extra-solar planets detected from the
radial velocity shifts of their central stars. By means of Monte-Carlo
simulations we estimate the precision of the measurement of i and
other stellar parameters. We find that the inclination angle can be
retrieved accurately for sufficiently large i for stars that rotate
at least twice faster than the Sun.
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: A New Component of Solar Dynamics: North-South Diverging
Flows Migrating toward the Equator with an 11 Year Period
Authors: Beck, J. G.; Gizon, L.; Duvall, T. L., Jr.
Bibcode: 2002ApJ...575L..47B
Altcode:
Time-distance helioseismology analysis of Dopplergrams provides maps
of torsional oscillations and meridional flows. Meridional flow maps
show a time-varying component that has a banded structure that matches
the torsional oscillations with an equatorward migration over the solar
cycle. The time-varying component of meridional flow consists of a flow
diverging from the dominant latitude of magnetic activity. These maps
are compared with other torsional oscillation maps and with magnetic
flux maps, showing a strong correlation with active latitudes. These
results demonstrate a strong link between the time-varying component
of the meridional flow and the torsional oscillations.
Title: Prospects for detecting stellar activity through
asteroseismology
Authors: Gizon, L.
Bibcode: 2002AN....323..251G
Altcode:
Long and continuous space-based observations of stellar pulsations
will be made available in a few years. We investigate the signature
of localized magnetic activity in the spectrum of the global modes
of oscillation of a Sun-like star. Using Monte-Carlo simulations,
we estimate the precision that can be achieved on the measurement of
the even splitting coefficient a_2. It is found that, under certain
conditions, some information can be retrieved about the surface
distribution of stellar magnetic activity.
Title: Time-Distance Helioseismology: The Forward Problem for Random
Distributed Sources
Authors: Gizon, L.; Birch, A. C.
Bibcode: 2002ApJ...571..966G
Altcode:
The forward problem of time-distance helioseismology is computing
travel-time perturbations that result from perturbations to a solar
model. We present a new and physically motivated general framework
for calculations of the sensitivity of travel times to small local
perturbations to solar properties, taking into account the fact that the
sources of solar oscillations are spatially distributed. In addition to
perturbations in sound speed and flows, this theory can also be applied
to perturbations in the wave excitation and damping mechanisms. Our
starting point is a description of the wave field excited by distributed
random sources in the upper convection zone. We employ the first Born
approximation to model scattering from local inhomogeneities. We use
a clear and practical definition of travel-time perturbation, which
allows a connection between observations and theory. In this framework,
travel-time sensitivity kernels depend explicitly on the details of the
measurement procedure. After developing the general theory, we consider
the example of the sensitivity of surface gravity wave travel times
to local perturbations in the wave excitation and damping rates. We
derive explicit expressions for the two corresponding sensitivity
kernels. We show that the simple single-source picture, employed in
most time-distance analyses, does not reproduce all of the features
seen in the distributed-source kernels developed in this paper.
Title: Advances in Time-Distance Helioseismology
Authors: Duvall, T. L., Jr.; Beck, J. G.; Gizon, L.; Kosovichev, A. G.
Bibcode: 2002AAS...200.7902D
Altcode: 2002BAAS...34..780D
Time-distance helioseismology is a way to measure travel times between
surface locations for waves traversing the solar interior. Coupling
the travel time measurements with an extensive modeling effort has
proven to be a powerful tool for measuring flows and other wave speed
inhomogeneities in the solar interior. Problems receiving current
attention include studying the time variation of the meridional
circulation and torsional oscillation and active region emergence and
evolution. Current results on these topics will be presented.
Title: Local-area helioseismology as a diagnostic tool for solar
variability
Authors: Kosovichev, A. G.; Duvall, T. L.; Birch, A. C.; Gizon, L.;
Scherrer, P. H.; Zhao, Junwei
Bibcode: 2002AdSpR..29.1899K
Altcode:
Dynamical and thermal variations of the internal structure of the Sun
can affect the energy flow and result in variations in irradiance
at the surface. Studying variations in the interior is crucial for
understanding the mechanisms of the irradiance variations. "Global"
helioseismology based on analysis of normal mode frequencies, has helped
to reveal radial and latitudinal variations of the solar structure
and dynamics associated with the solar cycle in the deep interior. A
new technique, - "local-area" helioseismology or heliotomography,
offers additional potentially important diagnostics by providing
three-dimensional maps of the sound speed and flows in the upper
convection zone. These diagnostics are based on inversion of travel
times of acoustic waves which propagate between different points on the
solar surface through the interior. The most significant variations
in the thermodynamic structure found by this method are associated
with sunspots and complexes of solar activity. The inversion results
provide evidence for areas of higher sound speed beneath sunspot regions
located at depths of 4-20 Mm, which may be due to accumulated heat or
magnetic field concentrations. However, the physics of these structures
is not yet understood. Heliotomography also provides information about
large-scale stable longitudinal structures in the solar interior,
which can be used in irradiance models. This new diagnostic tool for
solar variability is currently under development. It will require both
a substantial theoretical and modeling effort and high-resolution
data to develop new capabilities for understanding mechanisms of
solar variability.
Title: The Linearized Forward Problem for Time-Distance
Helioseismology
Authors: Gizon, L.; Birch, A. C.
Bibcode: 2001AGUFMSH11B0711G
Altcode:
The forward problem of time-distance helioseismology is to compute
travel-time perturbations which result from perturbations to a
solar model. We present a physically motivated general framework
for calculations of the sensitivity of travel-times to small local
perturbations. The first Born approximation is used to model scattering
from local inhomogeneities. We take account of wave excitation by
distributed random sources. Travel-time sensitivity kernels depend
explicitly on the details of the measurement procedure. The method
is illustrated on an example. We consider the propagation of acoustic
waves in an infinite homogeneous medium. Waves are excited by random
pressure sources distributed on a thin horizontal sheet. We obtain 3D
travel-time sensitivity kernels for local perturbations in sound-speed,
damping rate and source strength.
Title: Travel-time sensitivity kernels for time-distance
helioseismology
Authors: Gizon, L.; Birch, A. C.
Bibcode: 2001ESASP.493..233G
Altcode: 2001sefs.work..233G
No abstract at ADS
Title: Time-distance helioseismology and the Solar Orbiter mission
Authors: Gizon, L.; Birch, A. C.; Bush, R. I.; Duvall, T. L., Jr.;
Kosovichev, A. G.; Scherrer, P. H.; Zhao, Junwei
Bibcode: 2001ESASP.493..227G
Altcode: 2001sefs.work..227G
No abstract at ADS
Title: Connection Between Phase-Sensitive Holography and Time-Distance
Helioseismology
Authors: Gizon, L.; Birch, A. C.; Jensen, J. M.
Bibcode: 2001AGUSM..SP31A22G
Altcode:
The temporal cross-correlation measured by phase-sensitive holography
(Braun & Lindsey, 2000) is a weighted average of cross-correlations
between pairs of points in the pupil. We derive a technique to compute
wave-based travel-time sensitivity kernels for phase-sensitive
holography. We finally comment on the spatial resolution of both
phase-sensitive holography and time-distance helioseismology.
Title: Heliotomography of the outer layers of the Sun
Authors: Kosovichev, A. G.; Duvall, T. L., Jr.; Birch, A. C.; Gizon,
L.; Scherrer, P. H.; Zhao, Junwei
Bibcode: 2001ESASP.464..701K
Altcode: 2001soho...10..701K
Heliotomography offers important diagnostics of the solar interior
by providing three-dimensional maps of the sound speed and flows in
the upper convection zone. These diagnostics are based on inversion
of travel times of acoustic waves which propagate between different
points on the solar surface through the interior. The most significant
variations in the thermodynamic structure found by this method
are associated with sunspots and complexes of solar activity. The
inversion results provide evidence for areas of higher sound speed
beneath sunspot regions located at depths of 4 - 20 Mm, which may be
due to accumulated heat or magnetic field concentrations. The results
reveal structures and flows associated with active regions and sunspots
at various stages of their evolution, and provide important constraints
for theories of solar dynamics and activity.
Title: Probing Surface Flows and Magnetic Activity with Time-Distance
Helioseismology
Authors: Gizon, L.; Duvall, T. L., Jr.; Larsen, R. M.
Bibcode: 2001IAUS..203..189G
Altcode:
Time-distance helioseismology, applied to surface gravity waves,
has been shown to be a useful tool to study horizontal flows near the
solar surface, and supergranulation in particular (Duvall & Gizon,
2000). Here, we present maps of horizontal flows and horizontal magnetic
fields, in both quiet and active regions. Travel-time sensitivity
kernels based on wave theory, as opposed to ray theory, are used in
the inversions.
Title: Seismic Tomography of the Near Solar Surface
Authors: Gizon, L.; Duvall, T. L., Jr.; Larsen, R. M.
Bibcode: 2000JApA...21..339G
Altcode:
No abstract at ADS
Title: Near-surface Horizontal Flows in Sunspots and Supergranules
Authors: Gizon, L.; Duvall, T. L., Jr.; Larsen, R. M.
Bibcode: 2000SPD....31.0108G
Altcode: 2000BAAS...32..802G
Surface gravity waves have been used to probe flows in the two
megameters beneath the photosphere using the techniques of time-distance
helioseismology. Realistic spatial kernels were calculated using the
Born approximation and used in an iterative deconvolution to obtain
an estimate of the subsurface horizontal flows. We detect an outward
flow outside sunspots: the moat flow. Penumbral outward flows are also
present, but appear to be smaller than the Evershed flow observed at
the surface. The effect of the Coriolis force on supergranular motion
has been measured.
Title: Time-Distance Helioseismology with f Modes as a Method for
Measurement of Near-Surface Flows
Authors: Duvall, T. L., Jr.; Gizon, L.
Bibcode: 2000SoPh..192..177D
Altcode:
Travel times measured for the f mode have been used to study flows near
the solar surface in conjunction with simultaneous measurements of the
magnetic field. Previous flow measurements of Doppler surface rotation,
small magnetic feature rotation, supergranular pattern rotation, and
surface meridional circulation have been confirmed. In addition, the
flow in supergranules due to Coriolis forces has been measured. The
spatial and temporal power spectra for a six-day observing sequence
have been measured.
Title: Surface Wave Time-Distance Helioseismology
Authors: Gizon, L.; Duvall, T. L., Jr.
Bibcode: 1999soho....9E..24G
Altcode:
The propagation of solar surface gravity waves (or f modes) is affected
by essentially two types of perturbations: the horizontal components of
the flows and of the magnetic field. We probe the first few megameters
below the photosphere using the time-distance technique, combined with
MDI/SOHO observations of surface waves. Two-dimensional theoretical
sensitivity kernels are computed in the Born approximation, to provide
a connection between the Sun's structure and the observed travel-time
anomalies. This research is supported by NASA contract NAG5-3077 at
Stanford University.
Title: The art of fitting p-mode spectra. II. Leakage and noise
covariance matrices
Authors: Appourchaux, T.; Rabello-Soares, M. -C.; Gizon, L.
Bibcode: 1998A&AS..132..121A
Altcode: 1997astro.ph.10131A
In Part I we have developed a theory for fitting p-mode Fourier spectra
assuming that these spectra have a multi-normal distribution. We showed,
using Monte-Carlo simulations, how one can obtain p-mode parameters
using ``Maximum Likelihood Estimators". In this article, hereafter
Part II, we show how to use the theory developed in Part I for fitting
real data. We introduce 4 new diagnostics in helioseismology: the
(m,nu ) echelle diagram, the cross echelle diagram, the inter echelle
diagram, and the cross spectrum ratio. These diagnostics are extremely
powerful to visualize and understand the covariance matrices of the
Fourier spectra, and also to find bugs in the data analysis code. The
diagrams are used to verify the computation of the leakage matrices,
and also to measure quantitatively these matrices. Cross spectrum ratios
are used to obtain quantitative information on the noise covariance
matrices. Numerous examples using the LOI/SOHO and GONG data are given.
Title: The art of fitting p-mode spectra. I. Maximum likelihood
estimation
Authors: Appourchaux, T.; Gizon, L.; Rabello-Soares, M. -C.
Bibcode: 1998A&AS..132..107A
Altcode: 1997astro.ph.10082A
In this article we present our state of the art of fitting helioseismic
p-mode spectra. We give a step by step recipe for fitting the spectra:
statistics of the spectra both for spatially unresolved and resolved
data, the use of Maximum Likelihood estimates, the statistics of
the p-mode parameters, the use of Monte-Carlo simulation and the
significance of fitted parameters. The recipe is applied to synthetic
low-resolution data, similar to those of the LOI, using Monte-Carlo
simulations. For such spatially resolved data, the statistics of the
Fourier spectrum is assumed to be a multi-normal distribution; the
statistics of the power spectrum is not a chi (2) with 2 degrees of
freedom. Results for l=1 shows that all parameters describing the p
modes can be obtained with negligible bias and with minimum variance
provided that the leakage matrix is known. Systematic errors due to
an imperfect knowledge of the leakage matrix are derived for all the
p-mode parameters.
Title: The art of fitting p-mode spectra
Authors: Appourchaux, T.; Gizon, L.
Bibcode: 1998IAUS..185...43A
Altcode:
We describe how to extract the p-mode parameters from complex Fourier
spectra using maximum likelihood estimators. We expect our description
to be quite general and didactic. We discuss problems such as the
choice of the optimal filters and the construction of the leakage and
covariance matrices. The LOI/SOHO Team makes use of these methods.
Title: Comments on the influence of solar activity on p-mode
oscillation spectra
Authors: Gizon, L.
Bibcode: 1998IAUS..185..173G
Altcode:
No abstract at ADS
Title: LOI low-degree rotational splittings
Authors: Appourchaux, T.; Rabello Soares, M. C.; Gizon, L.
Bibcode: 1998IAUS..185..167A
Altcode:
We present measurements of rotational splitting frequencies of
low-degree p-modes. The data which have been analyzed correspond to
one-year spatially resolved observations obtained with the Luminosity
Oscillation Imager onboard the SOHO spacecraft.
Title: LOI/SOHO constraints on oblique rotation of the solar core
Authors: Gizon, L.; Appourchaux, T.; Gough, D. O.
Bibcode: 1998IAUS..185...37G
Altcode:
The Sun is usually assumed to rotate about a single axis, tilted with
respect to the ecliptic normal by an angle of 7.25 degrees. Although
we have an excellent knowledge of the direction of the rotation axis of
the photospheric layers, we cannot exclude a priori that the direction
of the rotation axis could vary as a function of radius. We have tried
to check whether the assumption of rotation about a unique axis is
consistent with helioseismic data. We report on an attempt to measure
the directions of the pulsation axes of several low-degree modes of
oscillation in the LOI/SOHO Fourier spectra.
Title: On the direction of the rotation axis of the Sun
Authors: Gizon, L.; Appourchaux, T.
Bibcode: 1997BAAS...29R1121G
Altcode:
No abstract at ADS
Title: New IRIS constraints on the solar core rotation.
Authors: Gizon, L.; Fossat, E.; Lazrek, M.; Cacciani, A.; Ehgamberdiev,
S.; Gelly, B.; Grec, G.; Hoeksema, J. T.; Khalikov, S.; Palle, P. L.;
Pantel, A.; Regulo, C.; Schmider, F. -X.; Wilson, P. R.
Bibcode: 1997A&A...317L..71G
Altcode:
Four time series of IRIS data (4 to 6 months) have been used to obtain
improved measurements of the low degree (l=1,2,3) rotational splitting
frequencies. Assuming that the rotation law is known in the outer
layers of the Sun, we investigate the implications of IRIS splittings
for the central regions. Both a one-shell and a two-shell rotation
model have been considered in the solar core. A core rotating slightly
faster than the outer radiative envelope provides the best fit to the
data. Some evidence for the reliability of the observations is shown
by the visibility of differential rotation in the l=3 multiplets.