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| Paper: |
Helioseismology: What are We Learning About the Sun? |
| Volume: |
154, Cool Stars, Stellar Systems and the Sun: Tenth Cambridge Workshop |
| Page: |
275 |
| Authors: |
Toomre, Juri |
| Abstract: |
Helioseismology studies the internal structure and dynamics of the Sun, utilizing very precise measurements of the frequencies of sound waves that propagate throughout the solar interior and are observed at the surface. Efforts to accurately and precisely measure the mode frequencies from a single observing site have met with fundamental limitations imposed by the day-night cycle. Such difficulties have recently been overcome as nearly uninterrupted Doppler imaged observations of the full solar disk have become available both from the ground-based six-station Global Oscillation Network Group (GONG) project and from the SOI Michelson Doppler Imager (SOI-MDI) aboard the SOHO spacecraft in continuous sunlight orbiting the Sun-Earth L_1 Lagrangian point. Thus helioseismology has entered a major new phase of intensive scientific study as coordinated scientific teams have started to analyze both the GONG and SOI data. We review some of the basic principles and motivations of helioseismology, and then discuss some of the preliminary scientific results obtained by the teams through inversion of the global-mode frequencies and their splittings, dealing with the structure of the solar interior and the physics of stellar models, and an assessment of the differential rotation profile with depth and latitude. The inversions confirm that the surface latitudinal variation of the rotation rate carries through much of the convection zone. At the base of the convection zone there is a currently unresolved adjustment layer with latitudinally independent rotation at greater depths. A shearing layer just below the surface is discernible at low to mid latitudes. Such global investigations are supplemented by local area analyses via ring diagrams and time-distance methods, which enable one to probe the subsurface variation of large-scale flows and thermal structures with depth beneath selected regions of the Sun. |
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