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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