Helioseismology of Sunspots: A Case Study of NOAA Region 9787

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 model 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 rely 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.

Modeling the Subsurface Structure of Sunspots

“…But progress has been made in addressing some of these inconsistencies (e.g. Braun and Birch, 2008;Birch et al, 2009;Gizon et al, 2009aGizon et al, , 2009b) and significant advances have also been made in the simulation of helioseismic wave propagation in magnetized plasmas (see e.g. Cameron, Gizon, and Daiffallah, 2007;Parchevsky and Kosovichev, 2007;Cameron, Gizon, and Duvall, 2008;Hanasoge, 2008;Moradi, Hanasoge, and Cally, 2009;Parchevsky and Kosovichev, 2009;Shelyag et al, 2009;Cameron et al, 2010).…”

“…The second goal is to extend the helioseismic analysis undertaken by Gizon et al (2009aGizon et al ( , 2009b of the sunspot in Active Region (AR) 9787, which was the topic of the Third HELAS (European Helio-and Asteroseismology Network) Local Helioseismology Workshop, held in Berlin on 12 -15 May 2009. This sunspot was observed during the period 20 -28 January 2002 by the SOHO/MDI instrument.…”

“…This flow is consistent with the moat flow. In another recent study, Gizon et al (2009aGizon et al ( , 2009b used f to p 4 ridge-filtered time -distance travel times to produce linear inversion for flows around AR 9787. These inversions showed an azimuthally averaged horizontal outflow in the first 4 Mm beneath the surface, reaching an amplitude of 230 m s −1 at a depth of 2.6 Mm and radial distance of some 5 Mm outside the outer spot boundary.…”

“…In this article, we provide a critical review of the existing sunspot models and an overview of numerical methods employed to model wave propagation through model sunspots. We then carry out a helioseismic analysis of the sunspot in Active Region 9787 and address the serious inconsistencies uncovered by Gizon et al (2009aGizon et al ( , 2009b. We find that this sunspot is most probably associated with a shallow, positive wave-speed perturbation (unlike the traditional two-layer model) and that travel-time measurements are consistent with a horizontal outflow in the surrounding moat.…”

Directional time–distance probing of model sunspot atmospheres

“…As highlighted by a number of detailed E-mail: hamed.moradi@monash.edu comparative studies and reviews on the matter (e.g., Gizon et al 2009;Moradi et al 2010;Moradi 2012), major challenges exist in the development of new helioseismic procedures that are robust in the presence of magnetism and capable of probing both subsurface magnetic structures and associated flows.…”