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Direct measurement of travel-time sensitivity kernels for helioseismology
No full textAmerican Astronomical Society logo American Astronomical Society logo iop-2016.png iop-2016.png A publishing partnership Direct Measurement of Travel-Time Kernels for Helioseismology T. L. Duvall, Jr.1, A. C. Birch2, and L. Gizon3 © 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A. The Astrophysical Journal, Volume 646, Number 1 Download Article PDF View article References 391 Total downloads 36 36 total citations on Dimensions. Turn on MathJax Get permission to re-use this article Share this article Share this content via email Share on Facebook Share on Twitter Share on Google+ Share on CiteULike Share on Mendeley Article information Abstract 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
Interaction of solar inertial modes with turbulent convection. A 2D model for the excitation of linearly stable modes
No full textInertial modes have been observed on the Sun at low longitudinal wavenumbers.
These modes probe the dynamics and structure of the solar convection zone down
to the tachocline. While linear analysis allows the complex eigenfrequencies
and eigenfunctions of these modes to be computed, it gives no information about
their excitation nor about their amplitudes.
We tested the hypothesis that solar inertial modes are stochastically excited
by the turbulent motions entailed by convection. We have developed a
theoretical formalism where the turbulent velocity fluctuations provide the
mechanical work necessary to excite the modes. The modes are described by means
of a 2D linear wave equation, relevant for the quasi-toroidal modes observed on
the Sun, with a source term, under the beta plane approximation. Latitudinal
differential rotation is included in the form of a parabolic profile that
approximates the solar differential rotation at low and mid latitudes.
We obtain synthetic power spectra for the wave's latitudinal velocity,
longitudinal velocity, and radial vorticity, with azimuthal orders between 1
and 20. The synthetic power spectra contain the classical equatorial Rossby
modes, as well as a rich spectrum of additional modes. The mode amplitudes are
found to be of the same order of magnitude as observed on the Sun (~ 1 m/s).
There is a qualitative transition between low and high azimuthal orders: the
power spectra for m < 5 show modes that are clearly resolved in frequency
space, while the power spectra for m > 5 display regions of excess power that
consist of many overlapping modes.
The general agreement between the predicted and observed inertial mode
amplitudes supports the assumption of stochastic excitation by turbulent
convection. Our work shows that the power spectra are not easily separable into
individual modes, thus complicated the interpretation of the observations.Comment: 19 pages, accepted for publication in Astronomy & Astrophysic
Detecting stellar activity cycles in
No full textContext. The 11 yr solar cycle is known to affect the global modes of solar acoustic oscillations. In particular, p mode frequencies increase with solar activity.
Aims. We propose a new method to detect the solar cycle from the p-mode autocorrelation function, and we validate this method using VIRGO/SPM photometric time series from solar cycles 23 and 24.
Methods. The p-mode autocorrelation function shows multiple wavepackets separated by time lags of ∼123 min. Using a one-parameter fitting method (from local helioseismology), we measure the seismic travel times from each wavepacket up to skip number 40.
Results. We find that the travel-time variations due to the solar cycle strongly depend on the skip number, with the strongest signature in odd skips from 17 to 31. Taking the noise covariance into account, the travel-time perturbations can be averaged over all skip numbers to enhance the signal-to-noise ratio.
Conclusions. This method is robust to noise, simpler to implement than peak bagging in the frequency domain, and is promising for asteroseismology. We estimate that the activity cycle of a Sun-like star should be detectable with this new method in Kepler-like observations down to a visual magnitude of mK ∼ 11. However, for fainter stars, activity cycles are easier to detect in the photometric variability on rotational timescales
Outstanding problems in local helioseismology
No full textTime‐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
The mean solar butterfly diagram and poloidal field generation rate at the surface of the Sun
No full textContext. The difference between individual solar cycles in the magnetic butterfly diagram can mostly be ascribed to the stochasticity of the emergence process.
Aims. We aim to obtain the expectation value of the butterfly diagram from observations of four cycles. This allows us to further determine the generation rate of the surface radial magnetic field.
Methods. We used data from Wilcox Solar Observatory to generate time-latitude diagrams of the surface radial and toroidal magnetic fields spanning cycles 21–24. We symmetrized them across the equator and cycle-averaged them. From the mean butterfly diagram and surface toroidal field, we then inferred the mean poloidal field generation rate at the surface of the Sun.
Results. The averaging procedure removes realization noise from individual cycles. The amount of emerging flux required to account for the evolution of the surface radial field is found to match that provided by the observed surface toroidal field and Joy’s law.
Conclusions. Cycle-averaging butterfly diagrams removes realization noise and artefacts due to imperfect scale separation and corresponds to an ensemble average that can be interpreted in the mean-field framework. The result can then be directly compared to αΩ-type dynamo models. The Babcock-Leighton α-effect is consistent with observations, a result that can be appreciated only if the observational data are averaged in some way
KIC 6951642: confirmed Kepler Doradus- Scuti star with intermediate to fast rotation in a possible single-lined binary system
No full textKIC 6951642 has been reported as a candidate hybrid pulsator of type-
Doradus- 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 4.8 yr. In the
low-frequency regime, we identified the frequencies of 0.721 d as well
as of 0.0087 d. We attribute the first frequency to stellar rotation and
the second one to stellar activity with a cycle. We also detected modes,
with the strongest mode located at 2.238 d, as well as three asymmetric
multiplets (with a mean spacing of 0.6750.044 d). In the
high-frequency regime, we detected frequencies of type- Scuti, with the
strongest mode located at 13.96 d, as well as seven asymmetric
multiplets (with a mean spacing of 0.6650.084 d). We subsequently
identified a few more frequencies that appear to be combinations of a or
mode and one of the higher cited frequencies not due to pulsations. We
propose that KIC 6951642 accommodates for a fast-rotating Dor-
Sct hybrid star with various rotationally split multiplets of and modes
and that it also displays a cycle lasting years of (possible) stellar activity
Detection of Rossby modes with even azimuthal orders using helioseismic normal-mode coupling
No full textContext. 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
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