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Session 73 - Solar Space Observations, SOHO & SERTS.
Display session, Friday, January 09
Exhibit Hall,

[73.11] Probing the Internal Structure of the Sun with the SOHO Michelson Doppler Imager

A. G. Kosovichev, R. Nigam, P. H. Scherrer, J. Schou (Stanford U.), J. Reiter (Tech. Univ. Munich), E. J. Rhodes Jr. (USC), T. Toutain (Nice Obs.)

The inference of the thermodynamic structure of the Sun from the observed properties of the solar normal modes of oscillation is a principal goal of helioseismology. We report the results of the first year of continuous observations of the Sun's internal structure using data from the Medium-l Program of the Michelson Doppler Imager (MDI) on board ESA/NASA spacecraft SOHO.

The data provide continuous coverage of the acoustic (p) modes of angular degree l from 0 to 250, and the fundamental (f) mode of the Sun from l=100 to 250. During two 2-month intervals, the high-degree modes, up to l=1000, have been observed. The great stability of solar Dopplergrams measured by MDI permits detection of lower amplitude oscillations, extending the range and precision of measured normal mode frequencies, and thus substantially increasing the resolution and precision of helioseismic inversions. We present new inversion results for the radial and latitudinal seismic solar structures with particular attention to the transition region between the radiative and convection zones and to the energy-generating core. We discuss evidence for convective overshoot at the base of the convection zone, and the significance of deviations in the core structure from the standard evolutionary model.

Comparing the f-mode frequencies with the corresponding frequencies of the standard solar models, we argue that the apparent photospheric solar radius (695.99 Mm) used to calibrate the models should be reduced by approximately 0.3 Mm. The discrepancy between the `seismic' and apparent photospheric radii is not explained by the known systematic errors in the helioseismic and photospheric measurements. If confirmed, this discrepancy represents a new interesting challenge to theories of solar convection and solar modeling.

Using f-mode frequency splitting we estimate the large-scale structure of the subsurface magnetic fields. The variations of the solar oscillation frequencies during the first year of MDI observations are also discussed.

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