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A.G. Kosovichev (Stanford University), T.L. Duvall Jr. (Laboratory for Astronomy and Solar Physics, NASA GSFC)
Heliotomography (or time-distance helioseismology) is a relatively new tool for diagnostics of internal structures and dynamics of the Sun. It is based on inversion of travel times of acoustic wave packets propagating through the solar interior and bouncing back to the surface. The travel times provide information about the variations of temperature, magnetic fields and flow velocities along the wave paths. These properties of the solar interior are inferred from the travel times by tomographic inversions. Heliotomography has provided a three-dimensional view of the interior, not accessible by traditional helioseismology based on mode frequencies. This method has been applied to study both large-scale flows (meridional circulation, North-South asymmetry of solar rotation) and small-scale phenomena (supergranulation, sunspots, emerging magnetic flux). The results reveal very dynamical and complicated structures below the surface, associated with convection and magnetic fields, and shed new light on the formation and evolution of active regions and sunspots. We discuss the current limits for the temporal and spatial resolution and recent achievements. Most inversion results provide the results to a depth of 20 Mm. It has been demonstrated that with this method we can measure the solar flows to the base of the convection zone which is 200 Mm deep. However, resolving deep and small-scale features is very challenging, and requires concentrated efforts for developing both the measurement techniques and theoretical interpretations. We review the recent progress in developing a wave-theory approach to heliotomographic inversions, and perspectives for the diagnostics of the physical processes below the Sun's surface.
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