Helioseismology of Sunspots: A Case Study of NOAA Region 9787

TL;DR

This study applies various helioseismic methods to analyze the subsurface structure of a specific sunspot, revealing complex wave-speed profiles and highlighting current limitations in unified modeling of sunspot subsurface properties.

Contribution

It compares different helioseismic inversion techniques on the same sunspot, demonstrating discrepancies and challenges in interpreting subsurface wave-speed profiles.

Findings

Wave travel times are generally reduced by the sunspot.

Different inversion methods yield opposite signs for wave-speed perturbations.

Current methods cannot produce a unified model of sunspot subsurface structure.

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. (...) 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. (...) From this study of AR9787, we conclude that we are currently unable to provide a unified description of the subsurface structure and dynamics of the sunspot.