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Session 37 - First Results from the Solar and Heliospheric Observatory (SOHO).
Display session, Tuesday, June 11
We present a new method of 3D helioseismic diagnostics to study subphotospheric flow and thermal and magnetic structure associated with turbulent convection. The main difference from the previous studies by Duvall et al. (1996, Nature, 379, 235) and by Kosovichev (1996, ApJL, 461, L55) is that the new method can be applied for measuring solar properties in the shallow layer just beneath the surface. The shallow layer of superadiabatic convection, which is only few thousand kilometers deep, is the region of the greatest uncertainty in our knowledge of the Sun's interior. Recent numerical simulations have demonstrated substantial deviations of the structure of this layer from the mixing-length theory commonly used in modeling stellar structure and evolution. The uncertainty in the physics of turbulent convection also affects helioseismic inferences about the deep interior.
Our method of 3D diagnostics is based on measuring and inverting anomalies of the sound-wave travel time between two areas on the solar surface. Because of the stochastic nature of solar waves, these two areas must be sufficiently large to provide a good signal-to-noise ratio. In practice, the travel time can be measured from the cross-correlation function averaged over several thousand cross-correlations between individual points on the surface. Therefore, it is essential to have stable high-resolution series of Doppler images. Such data have been obtained from the Michelson Doppler Imager instrument on SOHO.
In this paper, we present some details of the cross-correlation time-distance analysis, and the technique to invert the travel-time measurements using the optical ray approximation. The travel time of the waves depends primarily on the wave group velocity and on the velocity of flow along the ray paths. The effects of the wave speed structure and of flows are separated by measuring the travel time of waves propagating in opposite directions along the same ray paths. The effects of magnetic fields are measured through anisotropy of the wave speed. We discuss the limits for observing small-scale features beneath the surface.
This research is supported by the SOI-MDI NASA contract NAG5-3077 at Stanford University.
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