Helioseismology of sunspots

Defocusing, folding, and healing of wavefronts

Z. C. Liang, Laurent Gizon, H. Schunker, T. Philippe

    Research output: Contribution to journalArticle

    Abstract

    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.

    Original languageEnglish (US)
    Article numberA129
    JournalAstronomy and Astrophysics
    Volume558
    DOIs
    StatePublished - Oct 31 2013

    Fingerprint

    helioseismology
    defocusing
    healing
    sunspots
    sunspot
    folding
    travel time
    travel
    wave packets
    rays
    scattering
    perturbation
    wavelength
    random wave
    acoustic wave
    wave field
    wave energy
    wavelengths
    alkalies
    plane waves

    Keywords

    • Sun: activity
    • Sun: helioseismology
    • Sun: interior
    • Sun: oscillations
    • Sun: surface magnetism
    • Sunspots

    ASJC Scopus subject areas

    • Astronomy and Astrophysics
    • Space and Planetary Science

    Cite this

    Helioseismology of sunspots : Defocusing, folding, and healing of wavefronts. / Liang, Z. C.; Gizon, Laurent; Schunker, H.; Philippe, T.

    In: Astronomy and Astrophysics, Vol. 558, A129, 31.10.2013.

    Research output: Contribution to journalArticle

    Liang, Z. C. ; Gizon, Laurent ; Schunker, H. ; Philippe, T. / Helioseismology of sunspots : Defocusing, folding, and healing of wavefronts. In: Astronomy and Astrophysics. 2013 ; Vol. 558.
    @article{a00b6e2fa61246d793693a32de349bd6,
    title = "Helioseismology of sunspots: Defocusing, folding, and healing of wavefronts",
    abstract = "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.",
    keywords = "Sun: activity, Sun: helioseismology, Sun: interior, Sun: oscillations, Sun: surface magnetism, Sunspots",
    author = "Liang, {Z. C.} and Laurent Gizon and H. Schunker and T. Philippe",
    year = "2013",
    month = "10",
    day = "31",
    doi = "10.1051/0004-6361/201321483",
    language = "English (US)",
    volume = "558",
    journal = "Astronomy and Astrophysics",
    issn = "0004-6361",
    publisher = "EDP Sciences",

    }

    TY - JOUR

    T1 - Helioseismology of sunspots

    T2 - Defocusing, folding, and healing of wavefronts

    AU - Liang, Z. C.

    AU - Gizon, Laurent

    AU - Schunker, H.

    AU - Philippe, T.

    PY - 2013/10/31

    Y1 - 2013/10/31

    N2 - 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.

    AB - 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.

    KW - Sun: activity

    KW - Sun: helioseismology

    KW - Sun: interior

    KW - Sun: oscillations

    KW - Sun: surface magnetism

    KW - Sunspots

    UR - http://www.scopus.com/inward/record.url?scp=84886499494&partnerID=8YFLogxK

    UR - http://www.scopus.com/inward/citedby.url?scp=84886499494&partnerID=8YFLogxK

    U2 - 10.1051/0004-6361/201321483

    DO - 10.1051/0004-6361/201321483

    M3 - Article

    VL - 558

    JO - Astronomy and Astrophysics

    JF - Astronomy and Astrophysics

    SN - 0004-6361

    M1 - A129

    ER -