Helioseismology of sunspots

A case study of NOAA Region 9787

Laurent Gizon, H. Schunker, C. S. Baldner, S. Basu, A. C. Birch, R. S. Bogart, D. C. Braun, R. Cameron, T. L. Duvall, Shravan Hanasoge, J. Jackiewicz, M. Roth, T. Stahn, M. J. Thompson, S. Zharkov

    Research output: Contribution to journalArticle

    Abstract

    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 mo 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 re 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.

    Original languageEnglish (US)
    Pages (from-to)249-273
    Number of pages25
    JournalSpace Science Reviews
    Volume144
    Issue number1-4
    DOIs
    StatePublished - Apr 1 2009

    Fingerprint

    helioseismology
    sunspots
    sunspot
    travel
    travel time
    acoustic wave
    acoustics
    inversions
    sun
    oscillation
    solar oscillations
    faculae
    subsurface flow
    limbs
    flow pattern
    wave packets
    wave propagation
    limb
    power spectra
    ridges

    Keywords

    • Helioseismology
    • Sun
    • Sunspots

    ASJC Scopus subject areas

    • Astronomy and Astrophysics
    • Space and Planetary Science

    Cite this

    Gizon, L., Schunker, H., Baldner, C. S., Basu, S., Birch, A. C., Bogart, R. S., ... Zharkov, S. (2009). Helioseismology of sunspots: A case study of NOAA Region 9787. Space Science Reviews, 144(1-4), 249-273. https://doi.org/10.1007/s11214-008-9466-5

    Helioseismology of sunspots : A case study of NOAA Region 9787. / Gizon, Laurent; Schunker, H.; Baldner, C. S.; Basu, S.; Birch, A. C.; Bogart, R. S.; Braun, D. C.; Cameron, R.; Duvall, T. L.; Hanasoge, Shravan; Jackiewicz, J.; Roth, M.; Stahn, T.; Thompson, M. J.; Zharkov, S.

    In: Space Science Reviews, Vol. 144, No. 1-4, 01.04.2009, p. 249-273.

    Research output: Contribution to journalArticle

    Gizon, L, Schunker, H, Baldner, CS, Basu, S, Birch, AC, Bogart, RS, Braun, DC, Cameron, R, Duvall, TL, Hanasoge, S, Jackiewicz, J, Roth, M, Stahn, T, Thompson, MJ & Zharkov, S 2009, 'Helioseismology of sunspots: A case study of NOAA Region 9787', Space Science Reviews, vol. 144, no. 1-4, pp. 249-273. https://doi.org/10.1007/s11214-008-9466-5
    Gizon L, Schunker H, Baldner CS, Basu S, Birch AC, Bogart RS et al. Helioseismology of sunspots: A case study of NOAA Region 9787. Space Science Reviews. 2009 Apr 1;144(1-4):249-273. https://doi.org/10.1007/s11214-008-9466-5
    Gizon, Laurent ; Schunker, H. ; Baldner, C. S. ; Basu, S. ; Birch, A. C. ; Bogart, R. S. ; Braun, D. C. ; Cameron, R. ; Duvall, T. L. ; Hanasoge, Shravan ; Jackiewicz, J. ; Roth, M. ; Stahn, T. ; Thompson, M. J. ; Zharkov, S. / Helioseismology of sunspots : A case study of NOAA Region 9787. In: Space Science Reviews. 2009 ; Vol. 144, No. 1-4. pp. 249-273.
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    AU - Duvall, T. L.

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