Helioseismology of sunspots: defocusing, folding, and healing of wavefronts

Liang, Zhangqian; Gizon, Laurent; Schunker, Hannah; Philippe, T.

doi:10.1051/0004-6361/201321483

Cited by 10 publications

(27 citation statements)

References 20 publications

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“…The wave packets in our simulations are comparable to the Solar and Heliospheric Observatory's Michelson Doppler Imager (SOHO/MDI) observed cross-covariance functions presented by Liang et al (2013). The peak of wave power occurs at kR = 450 and ν = 2.1 mHz for the f modes, at kR = 420 and ν = 2.5 mHz for the p 1 modes, and at kR = 390 and ν = 3.1 mHz for the p 2 modes.…”

Section: Numerical Simulations Of Wave Propagationsupporting

confidence: 79%

“…A study by Liang et al (2013) has measured the traveltime shifts caused by the sunspot in AR 9787. In an effort to understand and interpret these measurements, we numerically simulated the interaction of surface-gravity (f) and acoustic (p 1 and p 2 ) wave packets with several sunspot models.…”

Section: A Numerical Approachmentioning

confidence: 99%

“…Following Gizon & Birch (2002) and Liang et al (2013), Δτ is the time, τ, that maximizes the function…”

Section: Measuring Travel Timesmentioning

confidence: 99%

“…We used the travel-time maps measured by Liang et al (2013) covering seven independent days of SOHO/MDI observations of AR9787 (Gizon et al 2009;Moradi et al 2010). To decrease noise, we further averaged these travel-time maps over a region behind the sunspot where scattering phase shifts are large.…”

Section: Estimating Travel-time Noisementioning

confidence: 99%

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Helioseismology of sunspots: how sensitive are travel times to the Wilson depression and to the subsurface magnetic field?

Schunker

Gizon

Cameron

et al. 2013

A&A

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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, p 1 , and p 2 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 p 1 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.

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Section: Numerical Simulations Of Wave Propagationsupporting

confidence: 79%

Section: A Numerical Approachmentioning

confidence: 99%

“…Following Gizon & Birch (2002) and Liang et al (2013), Δτ is the time, τ, that maximizes the function…”

Section: Measuring Travel Timesmentioning

confidence: 99%

Section: Estimating Travel-time Noisementioning

confidence: 99%

See 2 more Smart Citations

Helioseismology of sunspots: how sensitive are travel times to the Wilson depression and to the subsurface magnetic field?

Schunker

Gizon

Cameron

et al. 2013

A&A

Self Cite

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“…Using the value ∆δ ch = −0.44 estimated by Santos et al (2016), we have ε n ∼ 0.05 for a quadrupole mode, i.e. a larger value than proposed by Liang et al (2013) and used in the present paper. This may suggest that for large active regions the spectra calculations may have to be carried out in the nonlinear regime.…”

Section: Towards a Physical Model For Mode Interaction With An Activementioning

confidence: 61%

Asteroseismic Signature of a Large Active Region

Papini

Gizon

2019

Front. Astron. Space Sci.

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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 ( = 1) and quadrupole ( = 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 (2 + 1)-fold azimuthal degeneracy of the frequency spectrum of modes with harmonic degree . 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 (2 + 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 × (2 + 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.

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Comparison of Travel-Time and Amplitude Measurements for Deep-Focusing Time–Distance Helioseismology

Pourabdian¹,

Fournier²,

Gizon³

2018

Sol Phys

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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 deepfocusing 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 traveltime 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 traveltime measurements. We conclude that amplitude measurements are a useful complement to travel-time measurements in time-distance helioseismology.

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Helioseismology of sunspots: defocusing, folding, and healing of wavefronts

Cited by 10 publications

References 20 publications

Helioseismology of sunspots: how sensitive are travel times to the Wilson depression and to the subsurface magnetic field?

Helioseismology of sunspots: how sensitive are travel times to the Wilson depression and to the subsurface magnetic field?

Asteroseismic Signature of a Large Active Region

Comparison of Travel-Time and Amplitude Measurements for Deep-Focusing Time–Distance Helioseismology

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