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Session 18 - The Sun.
Display session, Monday, January 13
The strking stability of solar Dopplergrams measured by the Michelson Doppler Imager (MDI) instrument on the SOHO spacecraft, without an intervening atmosphere, substantially decreases the noise in the solar oscillations power spectrum compared with groundbased observations. This permits detection of lower amplitude oscillations, extending the range of measured normal mode frequencies. This is important for improving resolution and precision of helioseismic inferences about the Sun's internal structure and dynamics.
The MDI observations also reveal the asymmetries of oscillation spectral lines that until now have been largely hidden in noise. The line asymmetries agree with a theory of excitation of solar oscillations by acoustic sources localized in the upper convective boundary layer.
High-resolution MDI images make it possible to measure the travel time of acoustic waves propagating inside the Sun by comparing points on the surface as close as 2.4 Mm. This is sufficient to detect supergranulation flows beneath the surface. Coupled with tomographic inversion techniques, we can now study the 3-dimensional evolution of the flows near the photosphere.
The sound-speed profile inferred from normal modes frequencies shows a sharp variation at the edge of the energy-generating core, something not accounted for by the standard evolution theory. The analysis also confirms recent GONG results suggesting that helium is less abundant than theory predicts in a thin layer just beneath the convection zone. Inversion of the multiplet frequency splittings shows significant rotational shear in this thin layer. This shear flow probably generates turbulence that mixes the plasma in the upper radiative zone. This layer is likely to be the place where the solar dynamo operates. Continuous observation of the evolution of this transition layer during the entire 11-year activity cycle will be extremely important for understanding the mechanisms of solar activity.
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