Comparative Results from Seismic Imaging and Hankel Analysis
Abstract
Studies of solar active regions using Hankel-decomposition techniques have clearly demonstrated over the past decade that sunspots and plage are efficient absorbers of acoustic waves, and that the wave speeds below sunspots are generally greater than that of the surrounding quiet Sun. These results have been consistently confirmed by more recent applications of both seismic-imaging and time-distance correlation analyses. The development of seismic imaging (or helioseismic holography), originally conceived ten years ago by C. Lindsey and D. Braun, and recently applied to SOHO/MDI data, has additionally provided important insights regarding the spatial (horizontal and vertical) structure of the acoustic perturbations first detected with Hankel analysis. Moreover, holography has enabled a number of remarkable new discoveries including the "acoustic moat," "acoustic glories," acoustic condensations up to 20 Mm beneath active regions, and the reflectivity of active regions above the quiet-Sun acoustic cut-off frequency. Recent application of phase-sensitive holographic procedures have also revealed a subsurface wave-speed perturbation extending tens of Mm beyond the sunspots, possibly associated with a convection cell driven by the local blockage of heat transport by the spots. A significant controversy still remains regarding the depth of the primary acoustic perturbation below sunspots. Modeling of scattering phase-shifts and seismic-imaging observations suggests that the perturbations are strongly concentrated within depths of a few Mm or less, in contrast to the deeper signatures implied by inversions of time-distance correlation measurements. Simple, but straightforward theoretical considerations, imply that the burden of proof rests with proponents of the latter scenario. Complexities introduced by the use of the photospheric signals within sunspots themselves also contribute to uncertainties in the interpretations. Careful control work and comparisons among diverse techniques, coupled with a continued theory-based effort in understanding and modeling local-helioseismic techniques and results, are crucial in providing a full understanding of the physics of solar active regions and their interaction with acoustic radiation.
- Publication:
-
SOHO-9 Workshop on Helioseismic Diagnostics of Solar Convection and Activity
- Pub Date:
- 1999
- Bibcode:
- 1999soho....9E..27B